WO2011072073A2 - Routage photonique sous-canal, commutation et protection à mises à jour simplifiées de réseaux optiques wdm - Google Patents

Routage photonique sous-canal, commutation et protection à mises à jour simplifiées de réseaux optiques wdm Download PDF

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Publication number
WO2011072073A2
WO2011072073A2 PCT/US2010/059567 US2010059567W WO2011072073A2 WO 2011072073 A2 WO2011072073 A2 WO 2011072073A2 US 2010059567 W US2010059567 W US 2010059567W WO 2011072073 A2 WO2011072073 A2 WO 2011072073A2
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Prior art keywords
subchannel
signal
optical
client
signals
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WO2011072073A3 (fr
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Chris Barnard
Piotr Myslinski
Colin Wright
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Vello Systems Inc
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Vello Systems Inc
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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0287—Protection in WDM systems
    • H04J14/0293—Optical channel protection
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
    • H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
    • H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
    • H04B10/0793—Network aspects, e.g. central monitoring of transmission parameters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/572—Wavelength control
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0287—Protection in WDM systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L45/00—Routing or path finding of packets in data switching networks
    • H04L45/28—Routing or path finding of packets in data switching networks using route fault recovery
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L45/00—Routing or path finding of packets in data switching networks
    • H04L45/50—Routing or path finding of packets in data switching networks using label swapping, e.g. multi-protocol label switch [MPLS]
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
    • H04Q11/0062—Network aspects
    • H04Q11/0066—Provisions for optical burst or packet networks
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04Q—SELECTING
    • H04Q11/00—Selecting arrangements for multiplex systems
    • H04Q11/0001—Selecting arrangements for multiplex systems using optical switching
    • H04Q11/0062—Network aspects
    • H04Q2011/0079—Operation or maintenance aspects
    • H04Q2011/0081—Fault tolerance; Redundancy; Recovery; Reconfigurability

Definitions

  • This application relates generally to optical communications based on optical wavelength-division
  • WDM multiplexing
  • Optical WDM communication systems transmit multiple optical channels at different WDM carrier wavelengths through a single fiber.
  • the infrastructures of many deployed optical fiber networks today are based on lOGb/s per channel. As the demand for higher transmission speeds increases, there is a need for optical networks at 40Gb/s, 100 Gb/s or higher speeds per channel.
  • Optical fiber has been used as a communication means since about 1977. Over time, deployed baud rates on a single laser have increased from 45 MB/s to over 40 Gb/s. Various protocols have been transmitted across optical fiber,
  • FIG. 1A shows a deployed network 100 that uses OC-48
  • SONET add-drop multiplexers 120 at each node interconnected by a first fiber optic cable 125 for signals traveling in a clockwise direction, and a second fiber optic cable 135 for signals travelling in a counterclockwise direction.
  • lower-rate client traffic 110 can be added or dropped, or passed through that node.
  • SONET mappers are used to map the traffic to the STS-1 virtual containers [described in Telcordia Standard GR-253], and SONET multiplexers are used to direct the traffic to the add, drop, or passthrough ports.
  • a pair of multiplexers can be used on two separate line cards as shown to provide support for a Unidirectional Path Switched Ring (UPSR) , or a 2-fiber or 4- fiber Bidirectional Line-Switched Ring (BLSR) .
  • UPSR Unidirectional Path Switched Ring
  • BLSR 2-fiber or 4- fiber Bidirectional Line-Switched Ring
  • Telcordia Standard describes the SONET BLSR] .
  • the traffic from a SONET ADM can also be combined with other traffic using wave-division multiplexing (WDM) to increase the network capacity .
  • WDM wave-division multiplexing
  • FIG. IB shows a deployed network 150 that uses
  • Gigabit Ethernet switches 170 at each node interconnected by a first fiber optic cable 175 for signals traveling in a clockwise direction, and a second fiber optic cable 185 for signals travelling in a counterclockwise direction.
  • incoming Gigabit Ethernet traffic 160 is mapped to VLANs that are transmitted on the 10 GE line side.
  • traffic in each VLAN is selected to be added or dropped, or passed through that node.
  • STP Spanning-Tree Protocol
  • RPR Resilient Packet Ring
  • the traffic from a 10GE switch can also be
  • WDM networks transmit client traffic from multiple sources over an optical fiber network. The traffic is multiplexed on the fiber by
  • ITU International Telecommunication Union
  • Optical signals are transmitted using transponders or muxponders, and are demultiplexed with fixed optical add- drop multiplexers (FOADMs) , reconfigurable optical add-drop multiplexers (ROADMs) , and/or wavelength selective switches (WSS) .
  • FOADMs optical add- drop multiplexers
  • ROADMs reconfigurable optical add-drop multiplexers
  • WSS wavelength selective switches
  • FIG. 2 shows a currently deployed WDM transponder
  • Client traffic 210 is connected via a short-reach fiber interface to client transceivers 215.
  • client transceivers 215. are typically pluggable devices such as an XFP [MSA standard
  • the transmitted light signal can be combined with light signals from other WDM transponders on a single fiber with an optical multiplexer.
  • demultiplexer is used to separate the WDM signals 235 (on the incoming fiber) , which are then converted back into equivalent electrical signals by the receive circuitry 237 in the
  • transponder requires external means to select the particular wavelength that is being dropped, though this filter function can be integrated onto the transponder line card [see, eg, U.S. Patent No 6,525,857].
  • the electrical signal from the line receiver (utilizing clock recovery circuitry 239) can be processed digitally to optionally (1) extract performance monitoring information 241, (2) drop the channel overhead for remote network management 225, and (3) correct errors according to the Forward Error Correction (FEC) algorithm 243.
  • FEC Forward Error Correction
  • transponders may utilize clock recovery circuitry 239 to support different data rates and protocols.
  • the line side optics are designed to operate at 2.7 Gb/s, 10.7-11 Gb/s, or 43 Gb/s with the cost of the components increasing with bit rate.
  • the line receiver 237 is either a PIN photodiode or avalanche photodiode. In either case the receiver is not wavelength specific, so that an optical demultiplexer, or ITU channel filter, must be placed in front of the receiver to filter out the designated channel.
  • control plane circuitry and software 250 is employed to facilitate various transmit and receive functions of DWDM transponder 200, such as remote network management 225 (e.g., via the addition or removal of channel overhead) and the extraction of performance monitoring information 245.
  • control plane 250 is employed for configuration of transmission protocols 255 (in concert with clock recovery circuitry 218) and laser wavelengths 265 (to tune channels via transmitter 229) .
  • it can detect and handle faults involving the reception of both client-side (267a) and line-side (267b) signals.
  • FIG. 3 shows a currently deployed WDM muxponder 300.
  • This module maps lower-rate traffic 310 using a SONET
  • multiplexer [based on ITU standard G.709], Ethernet switch, or proprietary digital mapping and multiplexing 320.
  • the muxponder 300 has line-side WDM optics similar to the transponder 200 with a laser (in transmitter 329 ) set to a designated channel on the ITU grid and a receiver 337 that can detect any signal within the ITU channel plan.
  • transponder 200 and muxponder 300 can be designed to transmit signals from different sources and with different bit rates, the hardware limitations and costs typically limit the implementation to a specific set of protocols.
  • a 10 Gb/s transponder may transmit OC- 192 or STM-64 signals at 9.95 Gb/s, 10 GbE signals at 10.3125 Gb/s, FC-10 signals at 10.5 Gb/s, and OTU signals at 10.7 Gb/s. But it may not transmit data at significantly different data rates such as 2.5 Gb/s or 1.25 Gb/s.
  • a muxponder typically supports a subset of data rates and protocols that are determined by the capabilities of the digital and analog electronic circuits. The maximum data rate supported by the transponder and muxponder is typically limited by the analog circuits on the line side, such as the optical modulator (or bandwidth of the laser if direct
  • WDM network installations have been a compromise between price and functionality.
  • the cost of the high-speed optics increases with the line bit rate so that vendors typically partition their products into different data rates such as 2.5 Gb/s, 10 Gb/s, 40 Gb/s, and 100 Gb/s.
  • the price of WDM ports dictates that networks be deployed with as much bandwidth per port as possible. However, this has been offset by transponder prices increasing with bandwidth (e.g. 40G) so that most WDM lambdas have bandwidth assignments that are "right sized.”
  • WDM network channel plans usually use a subset of the wavelengths on the International Telecommunication Union, Telecommunication Sector (ITU-T) grid.
  • ITU-T International Telecommunication Union, Telecommunication Sector
  • Reference Standard G.692 which specifies a frequency grid anchored at 192.1 THz with interchannel spacings at integer multiples of 50 GHz and 100 GHz, is specified as the basis for selecting channel central frequencies.
  • the ITU channels will be referenced as 1-210 for 192.1 THz, 1-215 for 192.15 THz, etc.
  • the number of channels on the ITU grid is limited in most applications to the gain range available from erbium- doped fiber amplifiers (EDFAs) .
  • Gain-flattened EDFAs are now commercially available for either the C band ( ⁇ 191.8 THz to 196.3 GHz) and L band (-186.9 to 191.4 GHz).
  • C band ⁇ 191.8 THz to 196.3 GHz
  • L band -186.9 to 191.4 GHz.
  • a fully-loaded WDM network can transmit approximately 160 channels - 80 channels in the C band spaced at 50 GHz and 80 channels in the L band spaced at 50 GHz.
  • FIG. 4 shows a simplified diagram of a point-to- point WDM network 400 interconnecting two nodes - East Node 410 and West Node 420.
  • two fibers are used - one to transmit from east to west 402 and one to transmit from west to east 404 - but a single fiber can also be used.
  • Signals from different WDM lasers are combined via WDM Combiner 415 that can be wavelength-dependent (using ITU channel filters) or wavelength-independent (using a 1:N optical splitter).
  • a 1:N optical splitter can be based on fused fiber couplers. It has N input ports and one output port so that optical signals connected to the input ports are combined in the output fiber with a nominal power loss of 10*logl0 (N) dB for each signal. At the receive side the signals are
  • WDM Demultiplexer 417 using optical filters such as thin-film filters.
  • WDM signals can be transmitted over other
  • topologies such as a ring 500 shown in FIG. 5.
  • a single wavelength at each node has been used to simplify the example. This does not preclude the generalized case of an arbitrary number of wavelengths at each node.
  • the ring topology offers the advantage of having two diverse routes between any nodes on a ring so that any failure on one side of the ring can be protected with redundant traffic on the other side of the ring.
  • WDM equipment networks also support channel passthrough at a node - if an optical filter is not used to drop a
  • the signal continues around the ring to the next node.
  • Optical filters may be configured to selectively drop channels at a node. In this case the dropped wavelengths can be reused for other signals on the next ring segment.
  • This filter configuration is shown in FIG. 5 where all signals on ring 500 are directed to a filter (or plurality of filters
  • Node 1 510 is configured to drop 1-200
  • Node 2 520 is
  • Node 3 530 is configured to drop I- 220
  • Node 4 540 is configured to drop 1-230.
  • WDM signals may also be transmitted on a ring in a broadcast and select configuration [see, eg, U.S. Patent No 7,120,359] .
  • a portion of the powers of all signals is tapped off at a node and directed to a filter (or plurality of filters 615, 625, 635 and 645), to select the dropped channels.
  • This implementation uses a wideband tap coupler (616, 626, 636 and 646) that directs a fixed fraction of all signals to a drop port. In this case all signals continue around the ring 600 so that the dropped wavelength cannot be reused since it would have interference from the passthrough portion of the signal entering the node.
  • the return path of the signal on the protected side of the ring requires a separate
  • the broadcast and select configuration does not support channel re-use, it has the advantage that it supports drop and continue traffic, i.e. traffic that is dropped at more than one node. It also has the advantage that once the tap coupler is in place, adding a filter to drop more channels does not interrupt the passthrough channels. To date, broadcast and select architectures have been limited by the number of channels supported by the optical demultiplexers.
  • the drop filter may not provide enough isolation on the passthrough path. In that case, the drop filters can be cascaded to improve the isolation. Further note that the diagrams only show one channel dropped at each node. Typical installations cascade filters at each node, or use a parallel filter, to drop more than one channel at each node.
  • Another advantage of the broadcast and select architecture is that it does not reduce the bandwidth
  • All optical filters have a useable passband less than ideal because of the finite slope of the filter passband.
  • the useable bandwidth of cascaded filters decreases as more filters are inserted in the signal path.
  • the graph 700 in FIG. 7 shows the bandwidth narrowing effect for the case where commercially available WSS filters are cascaded in a network. Even though this WSS is designed for 100 GHz ITU channels, it has a useable passband of 68 GHz. Architectures that minimize the filter insertion in the optical path therefore have a higher available cumulative bandwidth .
  • WDM equipment is typically deployed in an equipment shelf that separates the add/drop functionality from the transponders or muxponders . This offers the service provider the benefits of paying as they grow, especially since the major expense can be the transponders and muxponders.
  • This partitioning of WDM equipment 800 is shown in FIG. 8. A practical implementation would use optical fiber patchcords (802a and 802b, and 804a and 804b) to connect the discrete modules (add/drop modules 810 and 820, and
  • module 9 - e.g., connecting corresponding module 910 to transponder 940 (instead of transponder 930) via corresponding fiber patchcords 902a and 902b.
  • module 920 is mistakenly connected to transponder 930 (instead of transponder 940) via fiber patchcords 904a and 904b. These mistaken connections may be difficult to detect, especially if there are two redundant paths between the terminals.
  • amplitude of the tapped portion of the optical signal can be measured within the network to provide power levels of the signals. But this method requires dedicated hardware at all monitoring points and it cannot detect third-party signals (i.e., "alien" signals that are generated by other equipment vendors, but which may be inserted into a WDM network as long as they are on the same ITU channel plan and do not interfere with other signals) .
  • third-party signals i.e., "alien" signals that are generated by other equipment vendors, but which may be inserted into a WDM network as long as they are on the same ITU channel plan and do not interfere with other signals
  • Another method that can be used to monitor signals in the network is to deploy optical spectrum analyzers at various points in a network. These can be accessed through the network management software. However, getting a full view of the network may require many of these and thus add
  • optical networks transmitting digital signals must maintain a minimum bit error rate (BER) .
  • BER bit error rate
  • Well- known factors affecting the WDM signal BER are received power levels, optical signal-noise ratio (OSNR) , chromatic
  • CD polarization-mode dispersion
  • PMD polarization-mode dispersion
  • nonlinear fiber penalties such as cross-phase modulation and four-wave mixing.
  • Network design rules determine the link budget (maximum distance and loss per fiber span) based on these penalties.
  • Guaranteeing the performance and determining the link budget for an installed network can be costly because determining the factors listed above may require expensive test equipment. Furthermore, the tests may have to be run while the network is out of service so that changes over time after a network is installed cannot be detected. There is therefore a need to measure the optical parameters for an installed network, with minimal service disruption, and minimal extra cost.
  • Optical networks often require protection against equipment failures or fiber cuts.
  • One good way of protecting traffic is to provide two or more redundant paths between the end points with a protection algorithm that selects traffic from one of the redundant paths.
  • Standard protection is to provide two or more redundant paths between the end points with a protection algorithm that selects traffic from one of the redundant paths.
  • UPSR SONET Unidirectional Path Switched Ring
  • BLSR Bidirectional Line-Switched Ring
  • Dedicated and shared protection both require means to (1) bridge traffic at the transmit end onto the redundant paths, and (2) select traffic at the receiver from one of the redundant paths .
  • Shared protection also requires a means to manage the passthrough traffic at the intermediate nodes. Examples of shared protection schemes can be found in U.S. Patent Nos. 7,046,619 and 7,499,647, and U.S. Pat. App . No. 2007/0025729.
  • FIG. 10 shows a typical network layout 1000 of a service provider.
  • the ring topology is commonly used in WDM networks because it provides the lowest-cost means of offering protected services.
  • a ring network requires that all nodes have at least two connections to separate neighboring nodes. Ring networks may have spurs hanging off them to serve a small number of isolated nodes that have only a single fiber span connected to another node. Ring networks may be interconnected as shown in FIG. 10 with traffic passing between the rings at one or more intersecting nodes (nodes A and B) .
  • a lxN Wavelength-Selective Switch can be used to direct traffic between N nodes with direct optical connections [see, eg, U.S. Patent No.
  • WSS-based filters are currently much more expensive than fixed filters. Ring interconnections can also be done with fixed optical filters, but those connections cannot be reconfigured remotely, so that network upgrades require technicians to go to the interconnecting sites and manually reconfigure the connections on the fixed filters.
  • many deployed optical networks have difficulty managing WDM traffic through on these paths so that the traffic may be segmented by electro-optical conversions at the intersecting nodes (A, B, F) . These electro-optical conversions add cost and complexity to the network while reducing reliability.
  • all-optical routing between rings and from spurs to rings requires that the network be engineered so that the link budget rules are met by the intra- ring signals, and that individual powers be managed at the intersecting nodes.
  • subchannels eg, subchannels of ITU channels
  • microwave modulators or comb generators with a single laser. Examples of optical comb generators are described in U.S.
  • Wavelength-Division-Multiplexed (WDM) Comb Generator Using a Single Laser and filed on July 17, 2008, which is
  • These subchannels are closely spaced relative to the source laser and are not independently tunable across a wide wavelength range, i.e. they are tuned in parallel as the source laser is tuned.
  • Lower-rate subcarriers support a simplified upgrade of an installed DWDM network.
  • a legacy 2.5 Gb/s network may have transmitters with a reach of 600 km.
  • dispersion compensators may have to be installed, since the reach of the 10 Gb/s transmitter may be only 80 km. Installing dispersion
  • subcarriers are used instead, with each subcarrier
  • WDM network equipment e.g., equipment 1100 shown in
  • FIG. 11 is typically installed in a shelf 1110 with one or two management cards 1120 (MGT) and various line cards 1125.
  • the equipment 1100 is typically managed with a client-server element management system (EMS) consisting of one or more clients, such as client 1130, and EMS Server 1140.
  • EMS client-server element management system
  • the EMS connects through a private or public IP network (via Router 1150) to the management cards 1120.
  • FIG. 12 illustrates how two management cards 1220a and 1220b in equipment shelf 1200 can be deployed in an active/standby configuration to improve network robustness.
  • the standby MGT 1220b takes over the management function if there is any hardware or software failure on the active MGT 1220a.
  • This configuration typically uses two ethernet planes (1235 and 1245) on the backplane so that any line card can communicate with either management card.
  • a handshaking protocol between the management cards is used to determine which is the active MGT at any given time.
  • the configuration and status databases on the operative MGT are constantly backed up on the backup MGT so that when a failure occurs the backup MGT can take over the management as quickly as possible, and without any service interruptions.
  • WDM equipment typically requires that the EMS have a management connection to all remote nodes for functions such as provisioning equipment, reporting faults, downloading software upgrades, and retrieving and reporting performance metrics.
  • the MGT also employs a management connection to remote nodes for end-to-end provisioning, controlling
  • OSC optical service channel
  • Control messages and status can be transmitted from the MGT card to the OSC card over the backplane, and then transmitted optically by the OSC to the remote node where it is routed to the remote MGT card over the remote backplane.
  • Adding the filters to add and drop the OSC channel add loss and cost to the network.
  • the OSC can be eliminated if channel overhead is inserted into the signals, but the typical channel overhead bandwidth (500 kb/s) is much lower than the typical OSC channel bandwidth (100 Mb/s) . There is therefore a need for improved in-band communications channels that provide the necessary bandwidth without adding cost.
  • Switching matrices are used in a telecommunications network to direct traffic from multiple inputs to multiple outputs.
  • An electrical crossbar switch has a matrix of
  • a crossbar has a matrix with M x N cross-points or places where the "bars" cross.
  • a given crossbar is a single layer, non-blocking switch. Collections of crossbars can be used to implement multiple layer switches.
  • a Clos network is a kind of multistage switching network, first formalized by Charles Clos in 1953 [see, eg, Charles Clos (March 1953), "A study of non-blocking switching
  • Clos network provides a practical multi-stage switching system that is not limited by the size of the largest feasible single crossbar switch.
  • the key advantage of Clos networks is that the number of crosspoints (which make up each crossbar switch) required can be much fewer than if the entire
  • WDM links can be used to interconnect large electro- optic switches, as illustrated in FIG. 13.
  • crossconnect switches based on MEMS [see, eg, U.S. Pat. No. 6,574,386] have also provided a means of switching at the optical layer, but these switches may need wavelength
  • crossconnect switches 1310 provide the connectivity required to support large traffic demands and WDM links 1320 provide the bandwidth between the switches.
  • this architecture shown in ring 1300 can be costly because O-E-0 conversions may be required at each switch and bandwidth is being used to send traffic to and from the centralized switches. Also, the cost of such switches increases with the number of ports and bandwidth per port so that a network based on switches that support traffic
  • Network functionality can be described by the 7- layer OSI model.
  • Optical networking equipment resides mainly at the lowest layer, the Physical Layer.
  • the Physical Layer can be divided into
  • transponders do not necessarily provide electrical mapping, multiplexing, or protection switching.
  • the electrical sublayers 1420 include:
  • mapping sublayer where client data is received and mapped to available bandwidth according to the mapping protocol used.
  • the protection switching sublayer which can provide protocol-based protection, e.g. UPSR or BLSR protection for SONET-mapped signals, or STP or RPR protection for ethernet-mapped signals.
  • protocol-based protection e.g. UPSR or BLSR protection for SONET-mapped signals
  • STP or RPR protection for ethernet-mapped signals.
  • Path trace and CRC checks can be inserted into the OTN frame for receive side monitoring of the signal source and signal quality respectively.
  • the lowest electrical sublayer provides forward- error correction encoding and correction.
  • the optical protection layer provides
  • the lowest optical sublayer provides multiple point-to-point connection between two points according to the provisions in the higher layers .
  • Managing a WDM network requires that the network management system (NMS) have a management link 1440 from the NMS server to all of the optical network elements.
  • the network connections can be provided by an external IP network, or with dedicated overhead channels that are provisioned on the optical network.
  • the overhead channel may be mapped directly to one of the deployed wavelengths, or it may be transmitted over the OTN overhead channel, e.g. GCCO in G.709, or in an unused section of the higher-layer protocol's overhead
  • the laser wavelength (or frequency) must be maintained within a certain accuracy so that there is no interference between neighboring channels, and there are no penalties from laser-filter misalignment.
  • the performance characteristics of the lasers employed in DWDM systems change with temperature and with time.
  • the frequency of emitted laser light changes due to ambient temperature variations (typically from -5degC to 65degC) and due to aging.
  • WDM laser frequencies are maintained to a first order by controlling the temperature of the laser by mounting the laser on a thermoelectric cooler (TEC) .
  • Etalons may also be integrated into the laser cavity to provide a second-order correction.
  • TEC thermoelectric cooler
  • Currently deployed WDM lasers have an accuracy that is adequate for 50 GHz spacing. There is currently a need for more accurate means of controlling laser frequencies to space the WDM channels as close together as possible.
  • Various embodiments of the current invention are disclosed herein, including techniques, apparatus, and systems for optical WDM communications that employ tunable lasers to generate respective subcarrier frequencies which represent subchannels of an ITU channel to which client signals can be mapped.
  • Client circuits can be divided and combined with one another before being mapped, independent of one another, to individual subchannels within and across ITU channels.
  • Novel techniques are employed (at the subchannel level/layer) to facilitate the desired optical routing, switching, concatenation and protection of the client circuits mapped to these subchannels across the nodes of a WDM network, resulting in a significant increase in the number of optical circuits in a fiber, and thus in the overall bandwidth and spectral efficiency of the WDM network.
  • Network architectures and subchannel transponders, muxponders and crossponders are disclosed that map client signals to a set of subchannel frequencies.
  • these architectures employ two levels of frequency mapping and two cascaded optical filters (one for filtering WDM channels and one for filtering subchannels) . Additional methods of multiplexing channels and subchannels by means of polarization multiplexing and related feedback control electronic systems are also disclosed.
  • Selectively mapping client signals to a subset of the subchannels facilitates network functions such as broadcast and select transmission, arbitrary concatenation, optical source routing, shared optical protection, and
  • Subchannel muxponders are disclosed that measure network characteristics such as optical signal to noise ratio, chromatic and polarization mode dispersion, power levels, and bit error rates. Highly accurate wavelocker circuits are also disclosed that enable the equipment to provide very dense subchannels with accurate control.
  • WDM network designs by adding a new sublayer to the WDM network architecture between the FEC encode layer and the wavelength assignment layer. Novel means of mapping,
  • multiplexing, switching, and managing sublayer services are described in a common format that scales from small 1GE and 2.5G access networks to large regional networks and long-haul networks with capacity scalable to 17 Tb/s.
  • Novel means of connecting spur traffic to a ring, and interconnecting optical rings without O-E-0 conversion, are also disclosed.
  • these techniques are designed so as to enable standard ITU-T G.692 based (and other legacy) hardware to be reused.
  • optical parameters such as power, OSNR, chromatic dispersion, and polarization-mode dispersion.
  • Multilayer routing protocols are disclosed that enable network operators to easily map services to available bandwidth, while maintaining full visibility of the deployed channels and available bandwidth. Means for adiabatically adjusting the network capacity are described to ensure minimal planned or indeliberate service interruption. A novel OSC routing protocol is described to manage such a network with minimal cost overhead. Other embodiments are disclosed that enable networks to be upgraded from being ITU-channel based to subchannel based.
  • FIGs . 1A and IB illustrate optical ring networks that utilize OC-48 SONET add/drop multiplexers and Gigabit Ethernet switches, respectively.
  • FIG . 2 is a block diagram of a WDM transponder.
  • FIG . 3 is a block diagram of a WDM muxponder.
  • FIG . 4 is a block diagram of a point-to-point WDM network employing two fiber-optic cables.
  • FIG . 5 is a block diagram of a WDM Ring network with drop filters and add couplers at each node.
  • FIG . 6 is a block diagram of a Broadcast and Select
  • FIG . 7 is a graph illustrating the effect of cascaded ROADMs or WSS filters on usable C-band bandwidth in an optical network.
  • FIG . 8 is a block diagram illustrating the
  • transponder modules and add/drop filters.
  • FIG . 9 is a block diagram illustrating common misconnections between WDM transponder modules and add/drop filters .
  • FIG . 10 is a block diagram illustrating
  • FIG . 11 is a block diagram illustrating how typical
  • WDM equipment is installed and managed in a shelf with
  • FIG . 12 is a block diagram illustrating a shelf configuration of WDM equipment designed for redundant
  • FIG . 13 is a block diagram illustrating a WDM Ring network employing large crossconnect switches at each node, interconnected via WDM links.
  • FIG . 14 is a block diagram illustrating electrical and optical sublayers of WDM equipment residing at the
  • FIG . 15 is a block diagram of one embodiment of a subchannel muxponder of the present invention.
  • FIG . 16 is a block diagram of one embodiment of multichannel clock recovery circuits in a subchannel muxponder of the present invention.
  • FIG . 17 is a block diagram of one embodiment of a subchannel muxponder of the present invention with an
  • FIG . 18 illustrates one embodiment of an overlay of the subchannels of the present invention on 100-GHz ITU channels and filters.
  • FIG . 19 illustrates the characteristics of one embodiment of cyclical filters of the present invention.
  • FIG . 20 illustrates the filtering of subchannels in one embodiment of the present invention, where an ITU filter is followed by cyclical filters.
  • FIG . 21 illustrates the filtering of subchannels in one embodiment of the present invention, where cyclical filters are followed by an ITU filter.
  • FIG. 22 illustrates one embodiment of an overlay of the subchannels of the present invention on 50-GHz ITU
  • FIG. 23 illustrates one embodiment of a pre-emphasis on the subchannels of the present invention to counteract penalties from the edge of ITU channel filters.
  • FIG. 24 illustrates one embodiment of cyclical filters of the present invention with cascaded interleavers .
  • FIG. 25 illustrates one embodiment of a cyclical filter of the present invention made with an array waveguide grating (AWG) .
  • AWG array waveguide grating
  • FIG. 26 is a graph illustrating the frequency offset from the optimum AWG design frequency (for an AWG cyclical filter of the present invention) .
  • FIG. 27 is a graph illustrating the shift in the AWG temperature setpoint (for an AWG cyclical filter of the present invention) with the ITU channel number.
  • FIG. 28 is a graph illustrating the shift in the AWG temperature setpoint (for an AWG cyclical filter of the present invention) with the ambient temperature.
  • FIGs . 29A and 29B are graphs illustrating changes in superimposed AWG transmission spectra (for an AWG cyclical filter of the present invention) of 4 subchannels for ITU channels 50 and 60 with changes in ambient temperature (65 degC) .
  • FIGs. 30A and 30B are graphs illustrating changes in superimposed AWG transmission spectra (for an AWG cyclical filter of the present invention) of 4 subchannels for ITU channels 30 and 40 with changes in ambient temperature (-5 degC) .
  • FIG . 31 is a graph illustrating superimposed AWG transmission spectra (for an AWG cyclical filter of the present invention) of 4 subchannels for 40 ITU channels
  • FIG . 32 is a graph illustrating superimposed AWG transmission spectra (for an AWG cyclical filter of the present invention) of 160 subchannels for 40 ITU channels (channels 20-60) at ambient temperature (-5 degC) .
  • FIG . 33 is a graph illustrating superimposed AWG transmission spectra (for an AWG cyclical filter of the present invention) of 160 subchannels for 40 ITU channels (channels 20-60) at ambient temperature (65 degC) .
  • FIG . 34 is a graph illustrating how a shift in channel spacing (for an AWG cyclical filter of the present invention) can be achieved by a change in ambient temperature.
  • FIG . 35 is a top view of a top enclosure for an AWG wafer (chip) embodying an AWG cyclical filter of the present invention .
  • FIG . 36 is a bottom view of a top enclosure for an
  • AWG wafer (chip) embodying an AWG cyclical filter of the present invention embodying an AWG cyclical filter of the present invention.
  • FIG . 37 is a component view of the thermo-mechanical design of an AWG wafer (chip) embodying an AWG cyclical filter of the present invention.
  • FIG . 38 is a schematic diagram of a high-precision electronic circuit to control the temperature of an AWG wafer (chip) embodying an AWG cyclical filter of the present
  • FIG. 39A illustrates one embodiment of a channel plan for subchannels of the present invention with
  • FIG. 39B is a block diagram of one embodiment of a receive circuit to demultiplex polarization-multiplexed subchannels of the present invention.
  • FIG. 40A is a block diagram of an existing
  • FIG. 40B is a block diagram of a novel embodiment of the feedback control electronics in the dithering scheme employed in the implementation of polarization multiplexing presented in FIG. 40A.
  • FIG. 40C is a block diagram of a novel embodiment of the feedback control electronics in the dithering scheme employed in the implementation of polarization multiplexing presented in FIG. 40A for f dith _ 2 substantially lower than f d i t h-
  • FIG. 40D is a block diagram of one embodiment of a polarization tracking scheme of the present invention with three dithering frequencies.
  • FIG. 40E is a block diagram of one embodiment of the feedback control electronics for the polarization multiplexed system presented in FIG. 40D.
  • FIG. 40F is a block diagram of one embodiment of the feedback control electronics for the polarization multiplexed system presented in FIG. 40D for f dit h-2 substantially lower than fdith-2 ⁇
  • FIG. 40G is a block diagram of one embodiment of polarization multiplexing feedback control electronics applied to a subchannel-based DWDM system of the present invention.
  • FIG. 40H is a block diagram of an alternative embodiment of polarization multiplexing feedback control electronics applied to a subchannel-based DWDM system of the present invention.
  • FIG . 401 is a block diagram of one embodiment of a polarization tracking scheme of the present invention without dithering lasers on the transmit end.
  • FIG . 40J is a block diagram of one embodiment of a polarization tracking scheme of the present invention that enables polarization matching of added signals to passthrough signals .
  • FIG . 41 is a block diagram of one embodiment of the mapping of client services to subchannels of the present invention .
  • FIG . 42 is a block diagram of one embodiment of mapping lower-rate client services to subchannels of the present invention.
  • FIG . 43 is a block diagram of one embodiment of mapping 40G client services to subchannels of the present invention .
  • FIG . 44 is a block diagram of a 4-node WDM network with one embodiment of subchannel muxponders of the present invention at Node 1 and lower-rate transponders or muxponders at Nodes 2, 3 and 4.
  • FIG . 45 is a block diagram of one embodiment of software-controlled 1x2 switches of the present invention to selectively direct traffic to the East or West side of a WDM ring network.
  • FIG . 46 is a block diagram of one embodiment of software-controlled 1x3 switches of the present invention to selectively direct traffic to the East or West side (or broadcast to both sides) of a WDM ring network.
  • FIG. 47 is a block diagram illustrating the
  • FIG. 48 is a block diagram illustrating the high- capacity transmission resulting from one embodiment of 10G subchannel muxponders of the present invention.
  • FIG. 49 is a block diagram of one embodiment of a monitor channel filter of the present invention in an optical network .
  • FIG. 50 is a block diagram of one embodiment of a circuit of the present invention to measure net dispersion of a fiber link due to subchannel delay times.
  • FIG. 51 is a graph illustrating the effect of subchannel spacing in the present invention on phase detector voltage .
  • FIG. 52 is a graph illustrating the measurement of polarization-mode dispersion based upon the delays between orthogonal subchannels of the present invention.
  • FIG. 53 is a graph illustrating the result of a software-controlled circuit of the present invention used to monitor the bit error rate (BER) in an optical network as a channel frequency is tuned.
  • BER bit error rate
  • FIG. 54 is a data structure of one embodiment of a diagnostic spreadsheet employed by the present invention that lists device registers as well as expected and actual values.
  • FIG. 55 is a block diagram of one embodiment of an element management system (EMS) of the present invention with distinct shelves for legacy and new products.
  • EMS element management system
  • FIG . 56 is a block diagram of one embodiment of an element management system (EMS) of the present invention managing a shelf running two software versions in parallel.
  • EMS element management system
  • FIG . 57 is a block diagram illustrating an
  • management data is optionally routed throughout an optical network.
  • FIG . 58 is a block diagram illustrating the addition of a subchannel management layer of the present invention to existing WDM management layers.
  • FIG . 59 is a block diagram illustrating the fiber interconnections in one embodiment of a 3-node optical ring network of the present invention with degree-2 nodes (i.e., which connect to 2 other nodes) .
  • FIG . 60 illustrates one embodiment of a simple routing table of the present invention for intra-node
  • FIG . 61 illustrates one embodiment of an
  • interconnect routing table of the present invention for the 3- node optical network illustrated in FIG . 59 .
  • FIG . 62 illustrates a linear representation of the fiber connections of the 3-node optical network illustrated in FIG . 59 .
  • FIG . 63 is a block diagram of a subchannel ring network of the present invention with subchannel routing.
  • FIG . 64 is a block diagram illustrating the fiber interconnections in one embodiment of a 4-node optical ring network of the present invention with degree-2 nodes (i.e., which connect to 2 other nodes) .
  • FIG . 65 illustrates one embodiment of a subchannel interconnect map of the present invention for the 4-node optical network illustrated in FIG . 64 with subchannel
  • FIG . 66 illustrates one embodiment of a subchannel bandwidth map of the present invention for the 4-node optical network illustrated in FIG . 64 with subchannel routing.
  • FIG . 67 illustrates the highlighted protected connection in the subchannel bandwidth map illustrated in FIG . 66 .
  • FIG . 68 illustrates 9 available subchannels (between
  • FIG . 69 illustrates various embodiments of
  • subchannel payloads of the present invention resulting from the mapping of client services to subchannels.
  • FIG . 70 is a block diagram illustrating the mapping of client services (lOxlG Ethernet switch cards) to a
  • FIG . 71 illustrates one embodiment of a bandwidth map for the subchannel services illustrated in FIG . 70 .
  • FIG . 72 illustrates one embodiment of a portion of a service status table of the present invention (listing
  • FIG . 73 illustrates one embodiment of the
  • FIG. 74 illustrates how the subchannel muxponder of
  • FIG. 73 can be deployed as a switchable subchannel crossponder of the present invention such that traffic can be redirected away from a span for node insertion.
  • FIG. 75 illustrates how the subchannel crossponder of FIG. 74 can be used to bridge traffic on two diverse spans to implement protection switching in one embodiment of the present invention.
  • FIG. 76 illustrates an alternative implementation of protection switching in the present invention.
  • FIG. 77 illustrates an alternative implementation of the bridge and switch functions that implement protection switching in the present invention.
  • FIG. 78 is a block diagram of one embodiment of a subchannel crossponder of the present invention.
  • FIG. 79 is a block diagram illustrating the
  • FIG. 80 is a block diagram illustrating the
  • FIG. 81 is a block diagram illustrating the
  • FIG . 82 is a block diagram of one embodiment of a distributed subchannel switching network of the present invention with up to N (number of subchannels) interconnects.
  • FIG . 83 is a block diagram of a logical mesh, provided by the distributed subchannel switching network illustrated in FIG . 82 , in which routing is controlled by tuning subchannel frequencies.
  • FIG . 84 is a graph illustrating a DWDM signal laser of the present invention beating at a fixed frequency with a narrow optical carrier while an oscillator laser frequency is scanned .
  • FIG . 85 is a graph illustrating a DWDM signal laser of the present invention beating at a fixed frequency over a broad spectrum while an oscillator laser frequency is scanned.
  • FIG . 86 is a block diagram of one embodiment of an optical frequency stabilization scheme of the present
  • FIG . 87 is a graph illustrating typical absorption lines of a hydrogen cyanide (H 13 C 14 N) gas cell.
  • FIG . 88 is a graph illustrating the dependence of the P16 absorption line spectral position on gas pressure for hydrogen cyanide (H 13 C 14 N) .
  • FIG . 89 is a graph illustrating the dependence of the PI 6 absorption line FWHM ("Full Width at Half Maximum") linewidth on gas pressure for hydrogen cyanide (H 13 C 14 N) .
  • FIG . 90 is a graph illustrating the absolute value
  • FIG . 91 is a graph illustrating the FWHM ("Full
  • FIG . 92 is a graph illustrating the spectral shape of the P16 absorption line of hydrogen cyanide (H 13 C 14 N) at a pressure of 13kPa (measured with a 1 pm scanning step at approximately 74 MHz) .
  • FIG . 93 is a graph illustrating calibration of the oscillator laser frequency setpoints by reference frequencies of the absorption cell.
  • FIG . 94 is a block diagram of one embodiment of an optical frequency stabilization scheme of the present
  • FIG . 95 is a block diagram of one embodiment of an optical frequency stabilization scheme of the present
  • FIG . 96 is a block diagram of one embodiment of an optical frequency stabilization scheme of the present
  • FIG . 97 is a block diagram of one embodiment of an optical frequency stabilization scheme of the present
  • FIG . 98 is a block diagram of one embodiment of an absolute wavelength stabilization scheme of the present invention for multiple DWDM signal lasers propagating in dedicated fibers.
  • FIG . 99 is a block diagram of one embodiment of an absolute wavelength stabilization scheme of the present invention for multiple DWDM signal lasers propagating in a single fiber.
  • FIG . 100 is a block diagram of one embodiment of a frequency monitoring scheme of the present invention where the DWDM spectrum in a network node (i.e., all incoming and outgoing optical spectra in all fibers of the node) are monitored with ultrahigh absolute accuracy.
  • FIG . 101 is a block diagram of one embodiment of a circuit of the present invention to measure the beat signal between a reference tunable laser and an array of signal lasers .
  • FIG . 102 is a block diagram of one embodiment of a circuit of the present invention to measure the beat signal between a reference tunable laser and an array of signal lasers with a polarization controller.
  • FIG . 103 is a block diagram of one embodiment of a circuit of the present invention to measure the beat signal between a reference tunable laser and an array of signal lasers with a polarization scrambler.
  • FIG . 104 is a block diagram of one embodiment of a subchannel muxponder of the present invention with integrated DWDM transceivers. IV. DETAILED DESCRIPTION OF THE CURRENT INVENTION
  • the current invention employs subchannels to increase the number of optical circuits in a single fiber, and leverages those subchannels to fully realize the benefits of the increased bandwidth by facilitating the desired optical routing, switching, concatenation and
  • subchannel muxponder implemented in a WDM network, one embodiment of a subchannel muxponder is described, followed by descriptions of the corresponding channel plans, filters and associated hardware and software used to map client circuits to (and extract them from) subchannels across various nodes of a WDM network.
  • subchannel muxponder 1500 maps data from client traffic onto four subchannels.
  • Data from the client traffic is terminated with client optics, 1501, typically pluggable devices such as an XFP, Xenpak, or SFP.
  • each subchannel's electrical signals can be processed digitally to optionally (1) extract performance monitoring information, (2) add channel overhead for remote network management, and (3) encode the data for forward error correction.
  • the 10 Gb/s deserializer converts the data to parallel streams of lower-rate data that are then processed by an FEC device.
  • the data is thereby mapped to a subchannel within an
  • Management overhead can optionally be inserted into one or more of the FEC frames. Moreover, in one
  • block 1502 can also monitor a client signal's overhead bytes to extract various data, such as a "destination tag" (e.g., an Ethernet address, IP address, VLAN ID, MPLS tag, etc.) .
  • a destination tag e.g., an Ethernet address, IP address, VLAN ID, MPLS tag, etc.
  • the header information can be relayed to the management software that uses the destination tag to determine the destination port.
  • the management software can then provision the subchannel frequency to the frequency assigned to that destination port.
  • Controlling wavelength switching in an optical network based on destination tags can, in one embodiment, proceed as follows:
  • Network operator provisions switching mechanism e.g. Virtual Local Area Network identifiers
  • Client receiver detects source address (SA) and destination address (DA)
  • NE broadcasts SA to other nodes over OSC or in-band overhead
  • ITU channel can be a fixed value (determined by fixed drop filter) or range of values if ROADM is being used.
  • the nodes distribute the SA/DA information to build up a network-wide distributed switch table
  • a signal appearing at a client port causes the source node to tune its subchannel frequency wavelength to the correct subchannel frequency
  • ITU channel filter ROADM add/drop/passthrough channels are tuned
  • Client Rx periodically checks the SA/DA and triggers a wavelength change when the DA changes [00191]
  • an optical modulator 1504 modulates a CW laser beam to produce a modulated laser beam that carries the respective lower speed electronic signals 1505.
  • Each tunable laser 1503 is set to an ITU G.692
  • the different electronic-to-optical conversion units 1506 are configured to have different lasers 1503 at different
  • the subchannel lasers 1503 can therefore be (a) assigned to different subchannels within different ITU G.692 windows, and (b) be transmitted to different receive nodes that have different ITU channel filters.
  • This embodiment can be distinguished from subcarrier multiplexing [such as was described in U.S. Patent No.
  • each subchannel has its own independently tuned and modulated laser, and each subcarrier can carry independent protocols. Moreover, there are no restrictions at the transmit side on the frequency spacing between
  • each subchannel can be transmitted in a different ITU channel.
  • each subchannel can be selectively chosen to be one of many different types of modulation such as Non-return to Zero, Duobinary, or Differential Quadrature Phase Shift Keying.
  • Modulation formats with a narrow spectral width such as duobinary and DQPSK, are favored because their spectra must pass through the narrow-band filter 1525 at the receive side.
  • duobinary modulation is used, a precoder and low-pass filter 1507 are inserted in the data path. The precoder is used such that the recovered signal is identical to the transmitted signal. For a duobinary signal of a
  • the low-pass filter passband is set to
  • the electrical baseband modulation signal swings from - ⁇ ⁇ to +V n (with the modulator biased at a minimum point) .
  • the modulation signal is then fed into the optical modulator 1504 to control the optical modulation which produces the optical WDM signal 1514.
  • each subchannel is then combined optically with a polarization combiner, 1:4 coupler, or subchannel multiplexing filter 1520.
  • the optical polarization of each signal is controlled so that two optical WDM channels next to each other in
  • the optical WDM channels in the same polarization are directed into beam combiners 1511 and 1512 to produce a combined signal with optical channels in the same polarization.
  • Two such beam combiners 1511 and 1512 are used, one for each polarization.
  • the combined signals from the beam combiners 1511 and 1512 are directed into a polarization-maintaining directional coupler 1513, to produce an output signal that combines all
  • transceiver are allowed to be set to arbitrary ITU G.692 channels and subchannels, the adjacent subchannel at a
  • the receiver may be transmitted from completely different source locations. In this case it is very difficult to maintain orthogonal polarizations between the adjacent signals. To minimize penalties in this case, an optional polarization scrambler 1518 can be inserted in the path to reduce the crosstalk penalties. Regardless of the means to control polarization of the adjacent subchannels, the system impact of adjacent-channel crosstalk must be quantified and accounted for during the network engineering. Typically, the crosstalk penalty leads to a slight increase in the required Optical Signal-to-Noise Ratio (OSNR) at the receiver.
  • OSNR Optical Signal-to-Noise Ratio
  • a variable optical attenuator (VOA) 1515 combined with a tap coupler and monitor photodiode 1516 can optionally be used to control the output power of the combined
  • the optical network transmits the subchannels from the transmit node through optical fiber waveguides to the receive node.
  • an optical amplifier and/or variable attenuator can be used to control the received power.
  • the ITU G.692 channel WDM demultiplexer 1521 is used to receive the light from the network and select the subchannels in a single ITU channel to be directed to the Line Input port of the subchannel transceiver.
  • an optional optical amplifier 1522 e.g. an EDFA
  • control circuit 1522 can be used to ensure that the amplified signal does not overload the photodetectors 1530. Furthermore, control
  • Attenuator VOA 1523 so that the optical power incident on each photodetector 1530 is kept very close to the ideal incident power of the photodetector 1530, thereby optimizing system performance.
  • EDFA 1522 is kept at high gain (hence low noise and high optical signal to noise ratio) and uses the VOA 1523 to optimize the received powers.
  • the EDFA 1522 and VOA 1523 at the receiver can be shared among the subchannels, dedicated to a single subchannel, or not used at all. These choices depend on the requirements for the reach of the subchannel muxponder and the cost targets.
  • the composite signal containing the subchannels is then directed to an Ultra-dense WDM filter 1525 that separates the subchannels to output paths 1526.
  • the cyclical filter 1525 described below requires that the subchannel spacing be equal to the ITU frequency spacing (in GHz) divided by an integer M. In the case when the bit rate per subchannel is on the order of 11 Gb/s, the channel spacing is typically set at 10 or 12.5GHz .
  • Each electronic signal path may include an electrical equalizer that is typically integrated into the photodetector' s transimpedance amplifier or deserializer.
  • the equalizer can mitigate the eye distortion, either due to static band-limiting effects caused by the electrical or optical pre-filtering in the optical transmitter module, or due to fiber chromatic dispersion.
  • FEC frame corrects the errors according to the FEC algorithm, demaps the data, and optionally provides performance
  • each subchannel has independent clock recovery and generation circuitry to support different data rates and protocols.
  • One embodiment of these circuits is shown in detail in FIG. 16.
  • Circuit 1600 in FIG. 16 shows one embodiment of independent clock circuits for the subchannel timing of the SERDES-FEC-SERDES block.
  • Each client-side clock recovery unit (CRU) 1630 has a multi-rate reference clock 1620 that can be set independently for each subchannel's reference clock rate.
  • a transmit digital phase-locked loop (PLL) 1610 for each subchannel multiplies the recovered client clock by a factor that provides the FEC rate clock.
  • the line side SERDES has a multi-rate reference clock 1640 for each
  • subchannel receiver CRU 1660 and a receive PLL 1650 is used to convert the subchannel FEC rate clock to the subchannel client rate.
  • the reference clocks 1620 and 1640 can also be used as the source clocks to transmit maintenance signals, such as an OTN AIS (Alarm Insertion Signal) when the client services are in an alarm state or out of service.
  • OTN AIS Alarm Insertion Signal
  • running each client service on an independent subchannel maintains the end-to-end synchronization of each client service.
  • this invention provides a distinct advantage for applications such as SONET and Synchronous Ethernet that require end-to-end synchronization of the client signals.
  • the subchannel muxponder e.g., subchannel muxponder
  • the 1500 from FIG. 15 supports capacity upgrades of ITU channel- based networks.
  • the transmission symbol rate (e.g., 10 Gbaud) per subchannel is equivalent to an existing low-data rate (e.g., 10 Gb/sec), which is already running on the incumbent infrastructure.
  • the subchannel muxponder can be used to increase the available bandwidth in an ITU channel by four times without changing or modifying the network.
  • control software to provision the subchannel laser wavelengths at different ITU channels enables the subchannel muxponder to perform optical routing based on the subchannel frequencies.
  • the subchannel muxponder is activated by turning on one laser at a time, recording the power on the tap photodiode of each laser and applying an offset to each laser to compensate for the power differences.
  • This balancing can be done at low output power with the VOA at or near full
  • FIG. 17 shows another embodiment of a subchannel muxponder 1700 with an electronic crossconnect .
  • crossconnect switch 1750 is added between the SERDES and external FEC blocks and provides further switching and routing functionality as described below. Note that the SERDES, FEC, and crossconnect functions can be integrated into a single VLSI device 1760.
  • FIG. 18 An example of a channel plan 1800 followed by the disclosed design is shown in FIG. 18.
  • the carriers 1810 are spaced 12.5 GHz apart centered around ITU channels 1820 spaced at 100 GHz.
  • tunable lasers are used to generate each subchannel, so that any client signal can be transmitted on any of the subchannels.
  • the receiver demultiplexing required in this network consists of two stages.
  • the first stage is comprised of fixed, tunable, or reconfigurable ITU-T G.692 channel filters that may be built with technologies such as (but not limited to) thin-film filters, Array Waveguide Gratings, MEMS arrays, or diffraction gratings.
  • the second stage of demultiplexing in this network consists of a narrow-band cyclical or tunable filter to select one of the subchannels within the ITU
  • Example of a cyclical filter are the Array Waveguide Grating and cascaded interleavers .
  • a common characteristic of cyclical filters in this network design is that the nth subchannel in each ITU window is directed to the nth output port, as shown in channel plan 1900 in FIG. 19, and implemented by cyclical filter 1910.
  • the cyclical filter is cascaded with an ITU channel filter as shown in FIG. 20.
  • a standard 100- GHz ITU channel filter 2010 is used as the first filtering stage. This design therefore supports an upgrade of an
  • the upgrade can be implemented gradually over time so that any port of the ITU channel filter 2010 in FIG. 20 can be used to drop a legacy ITU channel signal.
  • the same filtering of subchannels can also be achieved by placing the ITU channel filters 2110 after the cyclical filter 2120 as shown in the filtering architecture 2100 of FIG. 21.
  • the subchannels 2210 are centered around the ITU channels 2220 at 50 GHz spacing.
  • the 100 GHz carriers can be separated from the 50 GHz carriers with (1) Two separate cyclical filters, one for the 100 GHz grid and one for the 50 GHz grid, or (2) a cyclical filter that can be shifted between the two grids by temperature tuning for example, or (3) a cyclical filter with 8 ports, 4 for the 100 GHz channels, and 4 for the 50 GHz channels.
  • the outer channels i.e. SC-1 and SC-4
  • SC-1 and SC-4 can be attenuated by the edges of the ITU channel filter.
  • This attenuation and its subsequent bit-error rate penalty can be mitigated, as shown in channel plan 2300 in FIG. 23, by adding power and/or frequency pre-emphasis on the outer carriers. This is done by shifting the outer subchannels 2310 toward the ITU channel's center frequency 2320 and boosting their transmit powers by adjusting the laser transmit powers.
  • This pre-emphasis can be adjusted by software depending on the penalties on the edge subchannels .
  • the optical network in one embodiment uses a
  • transmitter module that combines a plurality of subchannels (in the examples shown herein, 4 subchannels are used, but any number greater than 1 could be employed) . Furthermore, the examples shown above are given for the case where the data mapped to the carrier is approximately 10 Gb/s, but the same methodology could be extended to any arbitrary rate per carrier .
  • the cyclical filter can be an interleaver [see, eg,
  • a 4-channel 12.5 GHz cyclical filter can be made by cascading a 12.5 GHz interleaver 2410 with two 25 GHz interleavers 2420a and 2420b. Note that centering the subchannels on the ITU grid requires that the subchannels and interleavers are offset from the ITU grid by half the subchannel spacing, which is 6.25 GHz in this example. In the case of an odd number of subchannels (eg, 5 subchannels) , the central subchannel is not offset from the ITU grid.
  • AWG array waveguide grating
  • FIG. 25 The principle of operation of cyclical AWG 2500 is shown in FIG. 25.
  • the AWG 2500 has a channel spacing of 12.5 GHz.
  • the four middle output ports can be used to select the four subchannels adjacent to the ITU grid.
  • the channel spacing of a cyclical AWG is based on wavelength and the ITU channel spacing is based on frequency. This discrepancy causes a frequency offset for channels that are further from the AWG optimum design frequency. This offset is illustrated in graph 2600 in FIG. 26.
  • Such use of an AWG may not yield sufficient accuracy for applications such as the subchannel demultiplexer that need frequency accuracy within a fraction of the accuracy required by ITU channel filters. In such cases, a new way of controlling the temperature of an optical filter provides more accurate operation of the filter within a wide range of optical frequencies and ambient temperatures.
  • the performance characteristics of the optical filters employed in DWDM systems change with temperature and over time. In particular, a central frequency of bandpass optical filters change due to ambient temperature variations (typically from -5degC to 65degC) , and due to aging.
  • the center frequency of an AWG is temperature dependent with a shift approximately equal to 1.5 GHz/°C.
  • the temperature of the filter is controlled by mounting it on a heater or thermoelectric cooler, and using control circuits to maintain the voltage reading on a thermistor inside the filter package.
  • the close spacing of the subchannels as described herein can be made more accurate with refinement of the AWG design and controls.
  • the change in subchannel offset with frequency can be compensated by shifting the AWG chip temperature when the ITU channel is changed (see, eg, top of FIG. 28) .
  • the ideal AWG chip temperature set point depends on the ambient or case temperature (see, eg, bottom of FIG. 28) . Therefore, the AWG channel spacing can be made more accurate by calibrating the performance over wavelength and ambient temperature for different control set points, and then adjusting the control set point in normal operations depending on the ITU channel and ambient temperature. Examples of this calibration are shown in both graph 2700 in FIG. 27 and graph 2800 in FIG. 28.
  • the vertical temperature scale in these figures is expressed in a change of a thermistor resistance.
  • the center frequency accuracy of the AWG subchannel spectra can be kept within the limits required by the system specifications.
  • the shift in AWG channel spacing with AWG chip temperature can also be used to select a different subset of carriers. This is shown in graph 3400 in FIG. 34, where the operating temperature has been shifted by approximately 30 degC to shift the channel spacing by 50 GHz. Applying this temperature shift enables one to use the same AWG to
  • a high precision and accuracy of the AWG chip temperature requires a well-designed mechanical enclosure.
  • a detailed design of such an enclosure is presented in FIGs . 35- 37.
  • top cover 3510 in FIG. 35 (a bottom view of which is illustrated in FIG. 36) .
  • This aluminum top cover 3510 is thermally well attached to the existing bottom aluminum heat spreader 3520.
  • polyurethane foam 3720 including space between the aluminum wafer cover and plastic top case cover. • A third thermistor 3730 is mounted on the bottom of the case 3710 to measure ambient temperature.
  • FIG. 38 shows a Thomson bridge implementation of the chip temperature sensing with 3 Ultra-precise resistors and the AWG chip thermistor. Note that the ADC is probing the Thomson bridge differentially .
  • wavelength-division multiplexing - where different frequencies of light represent different ITU transmission channels and their subchannels.
  • polarization multiplexing is employed.
  • Various polarization multiplexing schemes can be applied to the wavelength
  • multiplexing can be used as a means of increasing the
  • FIG. 39A An example of a channel plan 3900 for subchannels in this case is shown in FIG. 39A.
  • the first five subchannels (SC-1 to SC-5) are aligned along one polarization axis 3910 and the last five subchannels (SC-6 to SC-10) are aligned along the second polarization axis 3920.
  • the subchannel transmit laser can be combined with cascaded polarization combiners, similar to combiners 1511 and 1512 in FIG. 15.
  • the subchannels in orthogonal polarizations can be
  • a polarization coupler 3960 is used to separate the orthogonal polarizations.
  • a polarization controller 3970 is placed in front of the polarization coupler 3960 to align the subchannel polarization axes to the axes of the
  • the alignment circuit 3950 shown in FIG. 39B detects the strength of the subchannel signals in one polarization. This circuit 3950 assumes that a low-amplitude dither at a fixed frequency (250 kHz in this embodiment) is superimposed on the signal transmitters using the first polarization.
  • the control electronics and software monitor the strength of the received dither signal 3962 and adjust the polarization controller 3970 to maximize the signal.
  • a photodetector 3963 detecting the fraction of light in one of the outputs of the polarization
  • a band pass filter to filter a dither frequency
  • an adjustable gain 3964 where the gain adjustment is based on the optical signal power incoming from a line fiber to the receiver.
  • the gain-adjusting algorithm assures that the amplitude of the dither signal at the filter/gain stage output does not change significantly with significant changes of incoming optical power;
  • a clock recovery circuit 3965 to recover the dither frequency with substantially low time constant;
  • a lock-in amplifier 3966 which, synchronously with the recovered clock, detects the amplitude of the dither signal changing due to polarization changes of the incoming optical signal
  • This circuit can be implemented employing analog electronics circuitry, or the signal processing can be
  • both orthogonal polarizations can be dithered at different frequencies; two electronics dither processing circuits can be used in parallel, each optimized for one of the two dither frequencies, respectively.
  • this design of a subchannel muxponder has the advantage that it can double the spectral density by using polarization as an additional dimension. This embodiment, however, does not support routing of subchannels from
  • Polarization-multiplexed subchannels from different sites could be demultiplexed, though that would require one
  • FIG. 40A For a given wavelength, ⁇ , of a DWDM system, two independent data channels are being used: Tx-1 4001a and Tx-2 4002a. The output light of both lasers is linearly polarized and both polarizations are combined into a single fiber by a
  • polarization beam combiner (PBC) 4005a in such a way that the polarizations of Tx-1 4001a and Tx-2 4002a are linear and orthogonal on the transmit side.
  • components of the network such as nodes, wavelength
  • each channel Tx- 1 4001a and Tx-2 4002a changes due to birefringent effects of optical network components.
  • the state of polarization changes randomly on the receive end where the wavelength, ⁇ 4007a, is dropped .
  • Random changes of polarization act on both channels in such a way that the state of polarization at the receive end of Tx-1 light is still orthogonal to the state of
  • demultiplexing can be performed as long as random polarization of each (or in fact one) channel is changed to a linear polarization with a known orientation, and the channels are demultiplexed by a polarization beam splitter/combiner (PBC) 4015a.
  • PBC polarization beam splitter/combiner
  • a change from random polarization to a linear polarization can be performed by commercially available
  • Polarization Trackers such as polarization tracker 4010a. Since dropped polarization changes over time, a polarization tracker needs to follow these changes and correct incoming polarization accordingly. This is typically accomplished by a feedback loop 4020a which detects the polarization state at the output of the polarization tracker and provides a control signal 4022a to the tracker 4010a to assure that the
  • polarization is linear, and a proper polarization channel is directed to a proper receiver - i.e. the light from Tx-1 reaches the receiver Rx-1 4011a, and Tx-2 reaches Rx-2 4012a, respectively.
  • the modulation depth typically does not exceed a few percent of the average light intensity.
  • the amplitude of Tx-2 4002a is not modulated at all.
  • the polarization tracker dithers polarization at a frequency fdith-2 and a photodiode monitors light intensity in one arm of the polarization beam splitter.
  • the photodetector detects light from Tx-1 4001a and Tx-2
  • the signal 4014a from the photodetector 4013a is processed by feedback control electronics 4020a and input as a feedback signal 4022a to the polarization tracker 4010a.
  • the dithering of the tracker 4010a at fdith-2 is used to determine the direction in which to adjust the polarization in order to track it, if needed, to accomplish polarization demultiplexing; and (ii) the dithering of the Tx-1 4001a at fdith-i is used to maximize the amplitude of this dither in the Rx-1 arm of the PBC 4015a at the receive end and direct a proper transmitter signal to a proper receiver.
  • FIG. 40B presents a particular implementation 4000b of the feedback control electronics in the dithering scheme presented in FIG. 40A.
  • two electrical circuits are processing in parallel: (i) a Tx-2 dither at frequency fdith-i and (ii) a polarization tracker dither at frequency f dit h-2 ⁇
  • FIG. 40B The abbreviations in FIG. 40B include the following:
  • the feedback signal 4022b is input to the polarization tracker 4010b, it is amplified by a variable gain amplifier 4025b, where the amplifier gain is adjusted appropriately to accommodate for changes in optical input power of a dropped channel while keeping at a constant value the average voltage of the
  • variable gain amplifier can be placed at the output of a TZ 4030b.
  • the RMS detector 4035b (such as LTC1968 from Linear
  • a clock recovery circuit 4036b recovering dither frequency at fdith-i followed by a lock-in amplifier 4037b which transforms its AC input signal into DC.
  • a lock-in amplifier 4037b which transforms its AC input signal into DC.
  • 40C (and all other Figures discussed herein and illustrating embodiments of polarization multiplexing) can be implemented by using analog electronics or DSP in the digital domain.
  • the TZ output signal could be sampled by an ADC, and DSP processing output could drive a DAC and provide feedback voltage to the polarization tracker.
  • FIG. 40E A particular implementation 4000e of feedback control electronics for three dithering frequencies is presented in FIG. 40E.
  • both dithers at the receive end are detected in two parallel arms of the feedback control electronics, and after filtering they are subtracted.
  • FIG. 40G illustrates one of the possible embodiments (employing subchannels 4075g) based on the control electronics illustrated in FIG. 40B.
  • FIG. 40C electronics illustrated in FIG. 40C
  • FIG. 40H electronics illustrated in FIG. 40H.
  • FIG. 401 illustrates a novel embodiment 4000i of a polarization tracking scheme applied to the subchannel architecture of the present
  • the wavelength offset between subchannels in two orthogonal polarizations does not need to be exactly half of the subchannel spacing; the offset can be anywhere between half the channel spacing to zero offset.
  • the selection of the offset in any particular implementation may depend on the accuracy of the polarization tracker and the allowed total signal bandwidth .
  • the feedback control electronics is designed to maximize light intensity of one or more subchannels (or all subchannels in both polarizations) after the input signal is demultiplexed into separate single subchannels.
  • the particular embodiment 4000i presented in FIG. 401 shows a case of the feedback control electronics maximizing intensity of two subchannels - one for each polarization as an example. Note that instead of tapping the output of each cyclical filter, the signals can be taken from the receiver bias currents.
  • 40J shows a polarization recovery module 4010j that monitors the polarization of signals coming into a node and adjusts a polarization tracker 4020j so that the polarization of the signal at the output of the polarization tracker 4020j is aligned with a linear axis of a polarization maintaining coupler 4030j.
  • Polarization-maintaining fibers 4040j are used on the add side of the node to ensure that the polarization of the added signals is aligned with the passthrough signals.
  • the embodiment shown is for the case of a 1x2 WSS node.
  • Client data can be mapped to the subchannels as shown in FIG. 41, in which data from independent 10 Gb/s sources 4110 is mapped directly to each subchannel 4120.
  • an FEC encoding device 4125 (either an FPGA or ASIC ) is used to encode the data according to an error correction algorithm that improves the optical performance
  • Overhead (OH) data can also be inserted in the FEC overhead to enable the exchange of OAM&P (operations, administration, maintenance, and provisioning) data to be exchanged between the terminals.
  • OAM&P operations, administration, maintenance, and provisioning
  • a lOGb/s crossconnect 4115 may be inserted between the input data 4110 and the subchannels 4120.
  • This may be either a digital or analog electronic crossconnect, or optical crossconnect [see, eg, U.S. Patent No. 6,574,386]. It can also be a protocol-dependent switch such as an ethernet switch. This enables more flexible functionality as described below .
  • the design 4100 shown in FIG. 41 is flexible in that any 10G protocol (e.g., 10GE, OC-192, FC-10, etc.) can be connected to any input port 4110.
  • 10G protocol e.g., 10GE, OC-192, FC-10, etc.
  • Each port 4110 has
  • independent clock recovery and clock multiplication circuits can be assigned, via optional crossconnect switch 4115, to any subchannel 4120.
  • FEC encoding device 4125 After the data associated with each subchannel 4120 is encoded (and overhead data inserted) via FEC encoding device 4125, such data is then sent to be
  • tuning a laser to map a client circuit to a subchannel may require a relative long period of time, e.g., approximately a minute. Yet, if a subchannel laser has already been tuned to a particular frequency (subcarrier wavelength) , and a client circuit is being mapped to that subchannel via crossconnect switch 4115, then the process will typically require much less time, as the switching time of a switch such as crossconnect switch 4115 is typically much faster than the time required to tune or retune a laser.
  • FIG. 42 shows an embodiment 4200 where standard digital multiplexing and switching circuits 4212 are used to multiplex lower rate data 4210 up to 10 Gb/s.
  • any subchannel can carry either a native lOGb/s service or multiplexed lower-rate services.
  • FIG. 43 shows a third mapping method 4300 where data
  • the client data 4310 is inverse multiplexed (via inverse multiplexer 4313) to divide the data into 4 separate data streams 4314 (each of which can be assigned to a respective subchannel 4320, e.g., via
  • IMA Inverse Multiplexing for ATM
  • IMA is a standardized technology used to transport ATM traffic over a bundle of Tl or El cables using inverse
  • inverse multiplexing can be implemented in an Field-Programmable Gate Array (FPGA) or Application-Specific Integrated Circuit
  • Inverse multiplexing at the transmit side must be done in such a way that the original traffic stream can be recovered at the receiver.
  • Inverse multiplexed data streams have frame markers for the receiver to re-align the data.
  • the frame markers can be based on standard protocols, such as SONET, or they can be proprietary.
  • the receiver also has buffers to hold data before it is realigned. The buffers typically have enough memory to compensate for any skew in the network caused by variations in the propagation speed of the different inverse multiplexed data streams.
  • transponders have (1) the same laser tuning capability as the subchannel muxponder (2) a modulation format that is
  • the optical filtering may be ITU channel filtering only, as long as the subchannel muxponders are not using more than one subchannel per ITU channel.
  • FIG. 44 shows an application 4400 with a pair of subchannel muxponders C1-C2 at Node 1 and pairs of lower-rate transponders (C3-C4, C5-C6 and C7-C8) at Nodes 3, 4, and 5, respectively.
  • the subchannel muxponder has its transmit subchannels set as (1) ITU-210, SC-1, (2) ITU-220, SC-2, (3)
  • ITU-230, SC-3, and (4) unassigned A single ITU channel filter (from a FOADM or ROADM) at the satellite nodes (e.g., drop filter 4410 at Node 2) can be used to filter out the subchannels from the subchannel muxponder.
  • Each transponder at the satellite nodes must be set to transmit at one of the subchannel frequencies being received at Node 1.
  • the transponders at Node 1 are set to ITU 200, SC-
  • the transponders at Node 2 are set to ITU-200, SC-2; and the transponders at Node 3 are set to ITU-200, SC-3.
  • These subchannels are then dropped by the cyclical filter in the subchannel muxponders at Node 1. Note that this application 4400 requires that the lasers in the lower-rate transponders must have the same degree of accuracy as the subchannel muxponder lasers.
  • the embodiment shown in FIG. 46 can be used to selectively transmit the traffic to the east, west, or both directions.
  • the software-controlled switches 4520 in FIG. 45 and 4620 in FIG. 46 remove the east-west ambiguity and enable network operators to remotely reconfigure the direction of the traffic.
  • This embodiment 4600 is independent of the type of channel multiplexing and demultiplexing used to add and drop channels from the network. For example, it can be connected to ROADMs and WSS networks.
  • FIG. 47 gives an example of a legacy ITU channel-based WDM network 4700 that has been upgraded with subchannels 4710a and 4710b on ITU channels 192.1 GHz 4720a and 192.2 GHz 4720b, respectively.
  • the subchannels increase the capacity of those ITU channels by a factor of 4.
  • Having the subchannels 4710a and 4710b at the same bit rate as the legacy ITU services 4720c (on ITU channel 192.3 GHz 4720c) means that they have similar link budget rules as the ITU services. This upgrade can therefore be done without significant changes to the fiber plant and dispersion compensators since the subchannels 4710a and 4710b are
  • the network upgrade is further simplified by the high dynamic range (> 20 dB) of the subchannel transmitter (provided by the VOA 1515 in FIG. 15) and the subchannel receiver (provided by the EDFA 1522 and VOA 1523 in FIG. 15) .
  • Subchannel muxponders also provide additional functionality as described below.
  • FIG. 48 shows a network capacity of 7.0 Tb/s with 704 subchannels.
  • Subchannels are spaced at 12.5 GHz in both the Conventional (C-band) and Long-wavelength (L-band) bands.
  • this subchannel embodiment 4800 provides large capacity, it does so with a small granularity (10 Gb/s), while maintaining client signal synchronization.
  • Subrate multiplexing can also be combined with subchannels to provide even finer granularity as shown in FIG. 42.
  • subchannel implementation based on the cyclical filter described above provides the means of de-interleaving channels spaced at 50 GHz so that an external interleaver is not required in the embodiment shown in FIG. 48.
  • Optical amplifiers such as erbium-doped fiber amplifiers (EDFAs)
  • EDFAs erbium-doped fiber amplifiers
  • OSNR optical signal to noise ratio
  • OSNR is given by the ratio between the optical signal power to the ASE noise power in a given noise
  • Measuring OSNR with a high dynamic range requires that the optical receiver be able to monitor power with a high dynamic range. This can be accomplished by placing a resistor in series with the photodiode bias current and measuring the voltage across the receiver with a logamp connected to an analog to digital converter (ADC) .
  • ADC analog to digital converter
  • the receiver and filter in front of the receiver are calibrated in the factory.
  • the receiver can be calibrated by inserting light with a known power level and measuring the response of the logamp.
  • the OSNR of a signal can be measured each time the signal is enabled or tuned to a new channel or subchannel. This assumes that the noise level is independent of the signal level, which is typically the case in a multichannel DWDM network with gain-controlled and gain-flattened amplifiers.
  • the receiver first records the power before the channel is enabled, so that it is measuring the noise power, Pn (W) .
  • the receiver After the channel is enabled, the receiver records the signal plus noise power, given by:
  • OSNR (dB) 10*logl0((( Pt-Pn) *Bf) / (Pn*Br) ) where Br is the reference noise bandwidth.
  • This method has the advantage of providing software with only the added cost of placing the logamp and ADC after the receiver, calibrating the receiver and filter, and recording the OSNR.
  • the OSNR depends on the
  • transmitter launch power to first order.
  • OSNR when channels are added enables the management software to equalize the OSNR of the channels at the receiver.
  • the measured OSNRs provide an estimate as to how much the transmit powers need to be adjusted. For example, if one channel has OSNR 1 dB lower than the other channels, it can be equalized by increasing its launch power by approximately 1 dB .
  • FIG. 49 shows an embodiment 4900 of a method to track added channels 4910 and monitor out-of-service channels.
  • An unused channel on the edge of the amplifier gain spectrum is used for this purpose, e.g. at 1528 nm. Using a channel on the edge of the gain spectrum will not interfere with the usable channels, but there will still be some gain so that the edge channel propagates through the amplifier chain.
  • Placement of the monitors depends on the network configuration but it is ideally before 4920 and after 4930 each amplifier 4925 as shown in FIG. 49 .
  • the monitor 4920 before each amplifier 4925 can be used to estimate the OSNR contribution from each amplifier 4925 [see, eg, U.S. Patent No. 6,040,933] and the difference between the input and output powers can be used to estimate the gain of each amplifier
  • the gain at the monitor wavelength may be lower than the gain in the amplifier' s signal band; yet this difference can be calibrated in the factory for each amplifier 4925.
  • a signal laser can be tuned to the monitor channel and its power levels can be verified at each network point.
  • the power levels can be compared to the
  • the channel can be tuned to its designated frequency.
  • Standby channels and protection channels can also be periodically tuned to the monitor frequency for verification.
  • the noise of the amplifier 4925 can also be any noise
  • the output monitor photodiode e.g., 4930b
  • the filter e.g., 4930a
  • the passband of this filter can be calibrated so that the noise measured by the photodiode 4930b can be given relative to a reference noise bandwidth, such as 0.1 nm.
  • the amplifier 4925 has its gain and noise vs. wavelength stored in a calibration table, then the gain and noise at other signal wavelengths can be determined from the gain at the monitor wavelength.
  • the performance of an optical link depends on the net dispersion of the fiber and components in the path such as amplifiers and filters. System operators often do not have a record of the precise values of the dispersion of installed fibers. If the net dispersion of the link is outside of the allowed range of the transmitters being used, then dispersion compensators may have to be installed. If the dispersion of a link is not known precisely, then whether or not dispersion compensation is required may not be known a priori. Deploying a network with incorrect dispersion compensation can cause bit errors that are difficult to debug.
  • subchannels be in a maintenance mode because the measurement will disrupt traffic on the subchannels that are used. This is the case when a service is being installed.
  • the dispersion of a fiber link is given in units of ps/nm. This determines the delay in ps per nm spacing between two carriers.
  • the delay can be measured on a subchannel muxponder line card (employing a circuit such as circuit 5000 shown in FIG. 50) as follows:
  • 5020 can be programmed into most commercially available SERDES or FEC devices.
  • the SERDES or FEC devices for the two subchannels can be synchronized by simultaneously releasing them from reset.
  • phase difference between the two subchannels can be measured with a commercially available phase detector 5030 such as the AD8302 from Analog Devices. This assumes that the voltage V from the phase detector 5030 has been calibrated to provide a constant C in (ps/V) to be used in the calculation.
  • subchannels can be used to measure larger dispersion values, and the outside subchannels can be used to measure smaller dispersion values.
  • the AD8302 that provides an output of roughly 10 mV/ps of phase
  • the output of the phase detector 5030 will be 6 V for subchannel spacing of 37.5 GHz (or 0.3 nm) and 2 V for a subchannel spacing of 12.5 GHz (or 0.1 nm) .
  • the 6 V for subchannel spacing of 37.5 GHz (or 0.3 nm) and 2 V for a subchannel spacing of 12.5 GHz (or 0.1 nm) .
  • the 6 V for subchannel spacing of 37.5 GHz (or 0.3 nm) and 2 V for a subchannel spacing of 12.5 GHz (or 0.1 nm)
  • this measurement can only be done out-of-service, e.g. when the service is being installed.
  • the measurement process can be as follows:
  • Network management sends a command to the link (transmitter and receiver) to measure dispersion
  • Transmitter uses one subchannel for overhead and signals over that subchannel to the receiver that it is switching to dispersion measurement condition
  • Tdiff 1 (Txl + ⁇ ) - (Tx2 + ⁇ 2)
  • Tdiff2 (Txl + ⁇ 2) - (Tx2 + ⁇ )
  • Tdiff 1 - Tdiff2 2( ⁇ 1- ⁇ 2)
  • this method can be used to estimate the polarization-mode dispersion (PMD) seen by the signals.
  • PMD is a form of modal dispersion where two different polarizations of light in an optical fiber propagate at different speeds due to random imperfections and asymmetries.
  • PMD is a statistical effect, and it depends on alignment of the launched state of polarization (SOP) .
  • SOP launched state of polarization
  • the relative dispersion between two subchannels can be measured as described above. If the subchannel muxponder in FIG . 15 is used such that the carriers have orthogonal polarizations, then the delay between two wavelengths can be measured for parallel polarizations and orthogonal polarizations.
  • a distinct advantage of these measurement techniques is that they measure the cumulative CD and PMD seen by the signal in a single measurement. Another advantage is that these methods can measure the link without requiring a
  • a further advantage of these methods is that the results can easily be displayed by the network management software .
  • All transmitters have an acceptable dispersion window, i.e. range of dispersion values for which the transmit signal will have an allowed dispersion penalty. If the
  • the software can raise an alarm to the network operator to signal that dispersion compensation is required in the
  • the output power must be disabled to avoid interference on the other channels. Then, when the transmitter power is enabled, it should be turned on gradually to avoid any adverse effects on the live channels. While it is being enabled, the BER of the other channels should be monitored for increased BER.
  • FIG. 53 An embodiment of a sequence for tuning a channel is shown in graph 5300 in FIG. 53. This requires software communicating with all nodes to monitor the BER (e.g. at 5310) of all the live channels while turning on the transmit power (e.g.
  • a procedure for adding a channel with the techniques described above is as follows:
  • Assign the services (protocols) 6 Tune the transmitter to the EDFA monitor wavelength, turn on the power to a level that equals the other channels and check the OSNR contributions of the amplifiers. Record the OSNR at the receiver and raise an alarm if it is too low.
  • Electronic equipment such as the optical equipment described herein, can have several digital devices with registers containing configuration and status data. These registers may be implemented in custom-designed or off-the- shelf Application Specific Integrated Circuits (ASICs) , field programmable gate arrays (FPGAs) , and/or complex programmable logic devices (PLDs) . Registers may hold provisioning
  • ASICs Application Specific Integrated Circuits
  • FPGAs field programmable gate arrays
  • PLDs complex programmable logic devices
  • RTOS real-time operating system
  • RTOS software may provide commands to users to read certain registers, but typically it is difficult to get full visibility of all the digital data on a line card and to debug issues on the line card.
  • ASIC, FPGA, and CPLD registers on a line card compares the read values to expected values and reports the differences to a user for debugging, and/or uses the differences to provide debugging advice.
  • the interface is a spreadsheet, created for example with Microsoft Excel, that has been programmed to contain a list of all the devices on a line card with one spreadsheet tab per device.
  • FIG. 54 shows an example of such a spreadsheet 5400.
  • the first column 5410 lists the register numbers of a device
  • the second column 5420 lists the address
  • the fifth column 5450 lists the expected value of the data read from each register
  • the sixth column 5460 reads the actual value
  • the seventh column 5470 lists the difference between the expected and actual columns
  • the eighth column 5480 gives a
  • the spreadsheet 5400 is first generated by a person that transposes the functional specification for each device to the spreadsheet. Reading the data from the line card requires that the user have a communications link from their laptop to the line card. This can be done with a serial debug port or standard protocol such as telnet. The expected values can be generated from the functional specification, or by reading a device in a known good state and copying the read values from the "Actual" column to the "Expected” column.
  • the spreadsheet can also be programmed to hide data when the actual and expected values are equal.
  • This diagnostic spreadsheet 5400 therefore provides a network operator, field support engineer, or design engineer with a quick method to determine if a line card is correctly configured, or if there are any current alarms or hardware faults .
  • the new product release may have software that is not necessarily backwards compatible to the legacy product. This may be the case because the new product is enhanced with a new software architecture, and making the new software backwards compatible may require a lot of time and investment in engineering. To get a new product to market sooner, the new product may be deployed alongside the legacy product in two separate shelves
  • This implementation 5500 has the disadvantage that it requires extra shelf space for the new product even though there may be empty slots in the chassis of the legacy product.
  • the EMS 5650 managing shelf 5610 will display it as two separate shelves (with separate IP addresses) .
  • the legacy line cards 5627 can be upgraded remotely with software downloads to be managed by MGT-2 5635.
  • MGT-2 5635 Eventually all the legacy cards 5627 will be upgraded so that all cards are managed by the new software on MGT-2 5635, and MGT-1 5625 is not being used.
  • the software on MGT-1 5625 can be upgraded with redundancy enabled so that the chassis is managed with MGT-2 5635 and MGT-1 5635 is on standby to provide redundant management.
  • subchannel-based architecture the routing, switching, concatenation and protection of client circuits across nodes of an optical WDM network. It should be noted that the extent to which these methods are implemented in software and/or general-purpose or dedicated hardware is generally a matter of design choice.
  • WDM equipment often requires that the EMS have a management connection to all remote nodes for functions such as provisioning equipment, reporting faults, downloading software upgrades, and retrieving and reporting performance metrics.
  • the node management card (MGT) also needs a
  • OSC optical service channel
  • FIG. 57 and the advantages and limitations of various aspects of various aspects of various aspects of various aspects of various aspects of various aspects
  • management traffic is routed between nodes based on a metric that is inversely proportional to the bandwidth of each path. This requires each independent link to first establish end-to-end connectivity by handshaking with the remote end. After the connection is established, each link must publish its
  • OSPF routing tables on the MGT then select a route between each node. If required, load balancing can also be implemented on the management channels. In general, the routing algorithm and updates follow a standard OSPF implementation.
  • subchannels In addition to remote management applications, subchannels also facilitate secure communications that rely upon existing optical infrastructure, such as the subchannel muxponder shown in FIG. 17.
  • the embodiment shown in FIG. 17 can be used to provide secure communications by constantly redistributing the traffic among the carriers. This requires handshaking between the terminals to synchronize the mapping and remapping between the client traffic and the subchannels.
  • the channel overhead can be used to signal between the subchannel transmitters and receivers.
  • One embodiment of an algorithm for such subchannel distribution includes the following steps:
  • transmitter After timer expires, transmitter generates new random carrier distribution and signals that distribution to the receiver
  • Transmitter buffers starts buffering the traffic or hold off with Pause signals
  • data mapped onto the subchannels can be routed to any end terminal by selectively and independently tuning the wavelength of each subchannel, i.e. software tuning a laser's frequency to select a
  • channels, subchannels, and client services requires multi ⁇ layer routing software that uses subchannel mapping to direct services between endpoints.
  • FIG. 58 shows the addition of a new management sublayer, the Optical Subchannel Layer 5810,
  • This layer 5810 manages the subchannels within each ITU channel.
  • the network management system responsible for maintaining the assignments of client services to subchannels throughout an optical WDM network employs (in one embodiment) the following set of rules:
  • the total number of subchannels per ITU channel is fixed by the deployed hardware, but software should be flexible to allow future upgrades, e.g. from 4 to 10 subchannels
  • Routing is distance and OSNR aware so that a path with better optical performance is preferred
  • the number of nodes connected to a given node is as low as 1 for point-to-point applications, and as high as N-l, for mesh applications with an N-port WSS
  • FIG . 59 At each node the routing table numbers the node ports from PI to PN.
  • a ring node has 8 ports (East add, East drop, West add, West drop, East line in, East line out, West line in, and West line out) .
  • East add East drop
  • West add West drop
  • East line in East line out
  • West line in West line in, and West line out
  • this methodology can easily be extended to higher degree nodes by adding more ports to the routing description. Also note that the routing description and tables are
  • each node has 8 ports that can selectively direct the traffic in each ITU channel as follows:
  • P3 Line Input port that can drop channels to P5 or passthrough channels to P8
  • P4 Line Input port that can drop channels to P6 or passthrough channels to P7
  • bandwidth is managed with routing tables exchanged between network nodes.
  • a simple routing table that describes the possible connections is shown in FIG . 60.
  • a "1" in the table between input ports and output ports indicates that a connection between those ports is possible with a single hop, and a "0" indicates that a connection between those ports is not possible.
  • the OSC connection between the nodes is used to exchange the node connections.
  • the management cards at each node then exchange the routing information to build up a route connection table as shown in FIG . 61.
  • the first table 6110 in FIG . 61 only contains the intra-node connections for the 3 nodes shown in FIG . 59.
  • the connection of Nodel, Port 8 is made to Node 2, Port 3.
  • the table 6120 then gets filled in with new values which indicate that, since channels from Node 1 Ports 2 and 3 can be connected to Node 1, Port 8, they can also be connected to the same outputs as Node 2, Port 3. These connections are given the value of "2" in the table since they involve 2 hops.
  • the next version of the routing table 6130 shows the additions for the connection of Node 1, Port 7 to Node 3, Port 4.
  • the other cases shown in FIG. 61 demonstrate how the routing table is filled out.
  • Each table proceeds from top to bottom with the propagation of light around the ring.
  • the left- hand table 6210 of FIG. 62 represents the inner fiber of FIG. 59. Proceeding from the first line of the table 6210,
  • each node can demultiplex at least one ITU channel.
  • the control plane can map a plurality of client signals to tunable lasers at each node, and can route any client signal to any other node by tuning the transmitter laser of that signal to a subchannel within the ITU channel associated with the destination node.
  • FIG . 63 illustrates the concept with a simple network 6300 where a pair of single ITU-T G.692 channel filters are used at each node to drop the ITU channel
  • the ITU filter function can be realized by a variety of technologies, such as fixed thin-film filters or a ROADM. Since the carriers are tunable, the simplest means to add the signals to the ring is to use a wavelength-independent coupler (CPLR) as shown.
  • CPLR wavelength-independent coupler
  • the traffic map has two lOGb/s connections between every node in a protected full-mesh configuration.
  • the mapping of subchannels is given in the legend in the middle of the diagram. Note that each node has an unassigned subchannel available to carry more traffic.
  • channels e.g. SC-2 is used to connect Node 200 to Node 220, as well as Node 210 to Node 230.
  • FIG . 64 shows the simplified connection diagram 6400 for the 4-node ring example with the same numbering as the 3- node example in FIG. 59. After a subchannel is added at a node and connected to one direction, the OSC connection
  • Each node then classifies that subchannel as being passthrough, or dropped if a channel filter is used to drop that subchannel.
  • the connection between transmitter and receiver is updated in the routing table.
  • FIG. 65 and FIG. 66 we show two different means of displaying the connection map for the 4-node ring example shown in FIG. 64.
  • FIG. 65 shows a connection map 6500 displaying the connection between the end points of the subchannel muxponders at each node and FIG. 66 lists the state of each subchannel at each point in the network.
  • FIG. 65 lists the network hierarchy from nodes
  • FIG. 65 Available bandwidth in FIG. 65 is indicated by empty rows and columns. For example, the row associated with Node 1, Card 1, and Port 4 does not have a rectangle linking that port to a circuit, so that port is available for network upgrades.
  • FIG. 66 lists the state of each subchannel at each connection point for the network of FIG. 64. The first three columns of FIG. 66 list the fiber connections according to the convention illustrated in FIG. 62. There is one column in this table 6600 for each subchannel. The state of a subchannel is listed with the following nomenclature:
  • IS-P In Service Passthrough
  • table 6600 is divided into two halves.
  • the top half 6610 is for the counterclockwise fiber connection in FIG. 63 and the bottom half 6620 is for the clockwise fiber connection in FIG. 63.
  • UEQ unequipped
  • the available bandwidth is determined by the management software. For the purpose of simplicity in this example, we assume that it starts with 4 ITU channels and 4 subchannels per ITU channel. Furthermore, we assume that east and west drop filters are deployed at the first site that drops a subset of ITU channels, which then constricts a subset of the UEQ channels to be dropped at that node .
  • Subchannel 3 of Channel 230 is listed in the third row of that column to indicate the start of the connection at Port 1 of Node 1.
  • the subchannel passes through to Port 7 of Node 1, across the line fiber to Port 4 of Node 4, and is then dropped at Port 6 of Node 4. In the reverse direction
  • the Subchannel is added at Port 4 of Node 2 in the third column, second-to-last row. It then passes to Port 8 of Node 4, across the line fiber to Port 3 of Node 1, and is then dropped at Port 5 of Node 1. Note that this connection wraps around to the top of the second half 6620 of the table 6600.
  • This circuit has a corresponding protected
  • connection that is highlighted in FIG. 67. This connection uses the same subchannel, but it propagates around the other side of the ring, passing through Nodes 2 and 3 between the Node 1 and Node 3 terminals.
  • FIG. 66 indicates the assignment and use of
  • subchannels at any point in a fiber connection For example, the subchannels present at the output of Port 7 from Node 1 is listed in the fourth row, where the assignments are in bold characters. That row indicates that Subchannel 1 of Channel 210, Subchannels 2 and 3 of Channel 220, and Subchannels 2,3 and 4 of Channel 230 are In Service, and the other subchannels are unequipped.
  • the routing table indicates to the network operator which subchannels and channels are in use at each location of the network. With the fully-tunable subchannel lasers, any unassigned laser can then be mapped to any unused subchannel to provide the requested circuit. When a new circuit is requested, the table can indicate which subchannels are available, and which have the lowest cost of deployment. The routing information can also be passed to higher layers of software that monitor and control the subchannels.
  • the software can therefore provide to the network operator capacity lists and/or maps of in-service capacity, present but not deployed capacity, and unused capacity.
  • the tables can also have options to group subchannels by those that are (1) deployed and in service (2) installed, but not in service, and (3) available to be deployed.
  • This architecture also supports sub-rate
  • Subchannel mapping supports optical concatenation, e.g., four 10 Gb/s subchannels can be concatenated to carry a 40 Gb/s signal.
  • the same routing tables can be used to support an overlay of lower-layer protocol routing with subchannel optical routing.
  • the subchannel muxponder supports standard 10 Gigabit Ethernet data on the client side, and the 10 Gigabit Ethernet data comes from a 10x1 Gigabit Ethernet multiplexer.
  • FIG. 70 shows the overlay of lOxlGE services on one of the available subchannels of the 4-node network shown in FIG. 63.
  • the subrate muxponder in this case has 10 client SFP ports and an ethernet switch that maps the client ports to VLANs on the line side. The traffic in the VLANs is mapped to one of the available 10 Gb/s subchannels transmitted between the three nodes.
  • FIG. 71 shows the VLAN routing map 7100 for the subrate overlays displayed in FIG. 70. Similar to the tables above, the following notation is used:
  • IS-DP In Service Drop-Passthrough
  • IS-DP In Service Add-Passthrough
  • the last column 7110 of FIG. 71 lists a possible mapping of the sublayer service to the subchannels in the network shown in FIG. 63.
  • This architecture and routing method provides a means of interconnecting the rings and spurs shown in FIG. 10. For inter-ring traffic, a subset of the total number of ITU channels can be assigned to the inter-ring traffic and
  • ITU channels can be assigned to the intra-ring traffic.
  • Fixed or reconfigurable filters can then be used to direct the inter-ring traffic and intra-ring traffic.
  • the routing tables can be extended to cases where there are spur nodes, and interconnected rings.
  • the subchannel routing software provides multilayer routing where the first layer manages client services, the second layer manages subchannels, the third layer manages ITU channels, and the fourth layer manages fiber connections.
  • Additional tables can indicate the status of these services and connections.
  • the tables also provide route discovery for subchannels and services.
  • FIG. 72 shows an example of a status table 7200 that can be displayed by the network management software to the network operator.
  • the techniques described above can be used to determine the optical data listed.
  • the first table 7210 in FIG. 72 lists status of the subchannels transmitted from Node 1 in the example of FIG. 63. The columns list, in order from left to right, the local IP address of each line card, the transmitted subchannel, the service on each subchannel, a unique label used to identify the service, the destination node, the transmit port connecting the subchannel to the line fiber, identification of any subchannel that is protecting the traffic, the remote received power, OSNR, dispersion, PMD, and bit error rate. Similar entries are provided in the bottom table 7220 for the received subchannels.
  • visual cues can be used to alert the operator about network problems. For examples, metrics that are failing a requirement can be colored red, and metrics that are close to failing can be colored yellow. Moreover, in another embodiment, such cues could trigger automated actions including notifications of a problem or predefined corrective measures such as provisioning or de-provisioning a circuit.
  • transponder or muxponder can be remotely switched by software to be on either side of a ring, i.e. the transmitter can switch between transmitting in the clockwise direction or counter-clockwise direction; and the receiver can select a circuit from the clockwise direction or counter-clockwise direction .
  • FIG. 73 shows an example of a directionless
  • subchannel muxponder implementation 7300 The switching is accomplished by placing a red/blue filter 7310 at the
  • red/blue filter 7320 splits the C-band ITU channel bandwidth into two halves, "red” channels are ITU channels 200-390 and "blue” channels are ITU channels 400-600. Note that this division is arbitrary and dependent on the quality of the red/blue filter. Current filters may not have adequate isolation at the splitting frequency, so that 1 or more channels at the middle, e.g. channels 390-410, may be
  • a subchannel muxponder can set a subchannel transmitter to a blue frequency to transmit to one direction or to a red frequency to transmit to the other direction. Similarly, red or blue channels can be selected from either direction to be received .
  • the configuration 7400 of FIG. 74 enables a subchannel muxponder to be deployed as a switchable subchannel crossponder.
  • a "crossponder" in this context is a muxponder that can transmit and receive on two line ports.
  • One advantage of a subchannel crossponder is that it can redirect traffic away from a span for node insertion as shown in FIG. 74.
  • traffic can transmit and receive on both fiber spans 7410 or be redirected to only one of the spans 7420, or a new node can be inserted, effectively replacing one of the fiber spans 7430.
  • a subchannel crossponder can also be used to bridge traffic on two diverse spans for protection switching applications as described below. Subchannels can be
  • the subchannel network design offers a flexible means of protecting services. Traffic that propagates in one direction in a subchannel can be protected by the same or a different subchannel propagating around the ring in the opposite direction.
  • This architecture also supports shared optical protection. Regardless of the protection architecture deployed, a protected circuit requires two basic functions at the terminals - the bridge function and switch function. The signal to be protected has to be bridged onto two redundant paths at the transmit end, and one of the two signals from the redundant paths must be switched at the receive end to be selected as the working circuit.
  • FIG. 63 there are two duplex connections between every node - one in the clockwise direction, and one in the counter-clockwise direction.
  • the two subchannels can be used for independent services or for protecting against fiber cuts.
  • FIG. 75 An example of a simplified dedicated protection implementation 7500 is shown in FIG. 75 where the traffic between two nodes, Node 1 and Node 2, is protected with a working fiber pair 7510 and a protection fiber pair 7520.
  • a 1x2 switch selects traffic from one of the designated subchannels. For example, the switch
  • Node 1 for Client 1 selects the 1-200, SC-1 received signal in normal mode, and selects the 1-400, SC-3 received signal when the working fiber pair is cut.
  • the selection of subchannel can be done with the integrated crossconnect switch shown in FIG. 17, or with the use of external optical or electrical switches.
  • the crossconnect shown in FIG. 17 provides more connectivity, it also adds to the cost of the terminal equipment. If the transceiver module does not have a
  • equipment 7600 in FIG. 76 includes two optical splitters on the client side.
  • the top splitter 7610 is used to bridge the client data 7605 onto the first and third subchannels.
  • the signals from the first and third subchannels are connected to a second optical splitter 7620.
  • the software on the transport module selects which subchannel signal is sent back to the client 7605 by enabling one of the client transmit lasers. This function could also be accomplished with a 1x2 optical switch 7630 instead of the second optical splitter 7620.
  • the first option may be preferred because (1) cost of an optical splitter is lower than that of an optical switch and (2) the software controlling the switch is self-contained on the transport module - the second option requires external software
  • FIG. 77 Another embodiment for performing the bridge and switch is shown in FIG. 77.
  • the bridge function is done with an optical loopback 7710 on the client side between CL-1 and CL-3.
  • FIG. 78 shows an embodiment of a subchannel
  • crossponder 7800 that can be used to support dedicated or shared protection.
  • the first two carriers 7810a and 7810b are transmitted to one line fiber 7825 and the other two carriers 7810c and 7810d are
  • the receive side has a coupler 7840 to receive subchannels from both line fibers 7825 and 7835.
  • the integrated crossconnect switch 7850 can be used for dedicated protection (1) at the transmit side to bridge client traffic onto two subchannels, and (2) at the receive side to select traffic from one direction.
  • the switch 7850 can also be used for a shared protection application to regenerate the protection subchannel.
  • FIG. 79 shows an application 7900 that uses a subchannel crossponder at two distinct nodes connected by two fiber pairs.
  • the top fiber pair 7910 is designated as the working pair and the bottom pair 7920 is designated as the protection pair.
  • Subchannels SCI and SC2 are transmitted in the clockwise direction and SC3 and SC4 are transmitted in the counter-clockwise direction.
  • the integrated crossconnects 7930a and 7930b perform the bridge and switch functions for Nodes 1 and 2,
  • FIG. 80 shows an embodiment 8000 where the
  • subchannel crossponder is used in a shared protection
  • the subchannels assigned for the working circuits are SCI for the clockwise direction and SC3 for the counter-clockwise direction.
  • SC4 is allocated as a shared protection subchannel for failures in SCI; and SC2 is
  • FIG. 81 shows how the network shown in FIG. 80 is protected by a fiber cut 8130.
  • the fiber pair 8110 between Nodes 1 and 2 has been cut.
  • the control software determines that the fiber pair 8110 between Node 1 and Node 2 has been cut, e.g. by Loss of Signal alarms, it sends messages over the OSC (optical service channel) to each node in the ring.
  • the nodes adjacent to the cut are instructed to switch to protection so that the first client circuit at Node 1 switches from selecting the traffic from SC3 to selecting the traffic from SC2 on the other side of the ring, and the first client circuit at Node 2 switches from selecting the traffic from SCI to selecting the traffic from SC4 on the other side of the ring.
  • the protection subchannels are switched to passthrough mode.
  • SC4 is regenerated from Node 1
  • SC2 is regenerated from Node 2
  • the shared protection switching with subchannels requires real-time messaging software between the nodes over an overhead channel to coordinate the protection switching. It should be noted that, because one embodiment of the subchannel crossponder supports different protocols on each port, the protocol of the protection subchannel may have to be switched as well during the protection switch.
  • the protection bandwidth (SC2 and SC4 in this example) is normally unused so that it is available for low-priority traffic.
  • a low-priority circuit could be established between the second client port at Node 1 and the second client port at Node 2 transmitting on SC2 from Node 1 to Node 2, and transmitting on SC4 from Node 2 to Node 1.
  • Similar circuits could be established between Node 1 and Node 3, and between Node 3 and Node 2. Those circuits would be dropped in the event of a protection switch of high- priority traffic, since the protection switch then uses the protection bandwidth to maintain the high-priority circuit.
  • OSPR Optical Shared Protection Ring
  • BLSR SONET Bidirectional Line Switched Ring
  • crossconnect switch 1750 in the subchannel muxponder 1700 (shown in FIG . 17) or crossponder 7800 (shown in FIG. 78) can be used to provide 1:N shared protection against laser failures.
  • 3 client services can be mapped to the first 3 subchannels, and if any of the hardware used by one of the first 3 subchannels fails, the affected traffic can be bridged and switched to the fourth subchannel .
  • this architecture can support ring and mesh topologies with up to up NxM strict-sense nonblocking connections with low-cost fixed optical filters.
  • M 4 for 4 subchannels at 10 Gb/s
  • N on the order of 40 for C-band ITU channels spaced at 100 GHz
  • N on the order of 80 for C-band ITU channels spaced at 50 GHz
  • N on the order of 160 for C and L band channels spaced at 50 GHz.
  • this embodiment can therefore support a 640x640 full logical mesh of 10 Gb/s services, in a strict-sense nonblocking switching architecture.
  • JxMxN circuits can be supported by this network .
  • FIG. 82 shows the case of a large distributed optical switching network 8200. It is on a physical ring, but provides a logical mesh connection between all circuits as shown in network 8300 in FIG. 83.
  • a circuit can be
  • Spurs or other rings can be connected as well with ITU channel filters used to direct the traffic across the ring.
  • the ITU channel filter function can done with fixed or reconfigurable filters in a broadcast and select configuration or
  • the subchannel routing software described in this document can be used to determine which subchannels are available on each network segment, establish and monitor subchannel circuits, and reconfigure subchannels for network upgrades such as node insertions.
  • the subchannel muxponder can be used to upgrade an existing ITU network to provide M times the capacity while providing the additional functionality of subchannel routing, reconfiguration and restoration,
  • the network can also be operated in a hybrid manner, where subchannels coexist with standard ITU channels as ITU subchannels are subdivided into subchannels as the network grows. This provides a "pay as you grow" cost advantage.
  • the performance characteristics of the lasers employed in DWDM systems change with temperature and with time.
  • the frequency of emitted laser light changes due to ambient temperature variations (typically from -5degC to 65degC) and due to aging.
  • each laser has a set-point for its target frequency;
  • the laser absolute frequency is measured by a measurement means that has the required absolute frequency accuracy;
  • the control electronics and/or software calculate the offset between each laser' s actual frequency and its target
  • laser frequency and “laser wavelength” may be used interchangeably.
  • U.S. Patent No. 6, 282, 215 discloses a wavelocking scheme that employs one or more Fabry-Perot etalons placed inside each laser cavity. Yet, using at least one integrated etalon per laser adds cost to the system, and the locking error on each laser does not ensure that the lasers are all on the same frequency grid. Moreover, current integrated
  • wavelocker designs offer absolute frequency accuracy on the order of +/- lGHz or more. This level of absolute accuracy is adequate for WDM systems where channels are located on the ITU grid with spacing of 50 or 100 GHz, but this level of accuracy is not sufficient for DWDM systems such as those based on subchannels within an ITU with channel spacing on the order of 10 GHz.
  • FFT Fast Transform
  • a measured DWDM signal laser is referenced to a tunable laser (oscillator) that has a well- known and stable absolute frequency by measuring the beat frequency between the two lasers. This gives a frequency error equal to the frequency accuracy of the reference laser. If improved accuracy is required, the accuracy of the reference laser is improved by calibrating it in real-time to the absorption lines of a gas cell.
  • a tunable laser oscillator
  • this embodiment employs means of locking multiple DWDM lasers with a shared
  • the strength of the beat frequency signal between two lasers depends on the polarization alignment of the two lasers.
  • Polarization alignment between fiber-pigtailed lasers can be maintained with polarization-maintaining fibers (PMF) .
  • PMF polarization-maintaining fibers
  • most DWDM components have single-mode fiber (SMF) , which does not preserve the polarization.
  • SMF single-mode fiber
  • Some embodiments described below use PMF to control the polarization.
  • Other embodiments use a polarization scrambler for cases where PMF is not available.
  • , (e.g. fRF 500 MHz);
  • F HM for the oscillator and Af s _ FWH M for the signal laser are approximately the same and equal to:
  • the photocurrent of the photodetector detecting the laser frequency beating can be amplified by a trans-impedance amplifier (TZ) and filtered by an electrical bandpass filter (BPF) centered at a fixed or tunable frequency f RF with a fixed or tunable electrical bandwidth Af RF _ FWH M (Af RF _ FWH M being smaller than 2*f RF , but larger than Af FWH M) ⁇
  • the beat signal can be amplified at the input or output of the BPF.
  • the BPF can act also as an amplifier.
  • the beat signal can be detected as follows.
  • the BPF output is rectified, for example, by using a zero offset rectifier (i.e. a rectifier without a typical 0.7V voltage drop (offset) of a simple semiconductor silicon diode rectification); averaged, e.g., by an
  • ADC analog to digital converter
  • DSP digital signal processing
  • amplification and any other signal processing required can be performed after direct ADC conversion of the TZ output in the digital domain, e.g. within a microprocessor or a DSP.
  • the RFDBS output is maximized when the oscillator frequencies are set to f 0 -ieft and f 0 - r ight -
  • the described example is valid not only for DWDM signal lasers which are narrow in spectrum, as in FIG. 84, but also for modulated DWDM lasers at high bit rates (e.g. 10-40 Gb/s) with modulation formats such as Non-Return-to-Zero (NRZ) or Carrier-Suppressed-Return-to-Zero (CS-RZ) that have
  • the RFDBS output will have four distinct peaks and the formula to determine f 0 - set is slightly more complex than the prior formula given above for the NRZ modulation format.
  • Quadrature-Phase-Shifted-Keying QPSK
  • DQPSK Differential- Quadrature-Phase-Shifted-Keying
  • the RFDBS Output signal would in general show two broad spectral lines or more. They can be distinct and separated in spectrum as in FIG. 85 or overlap to some or large extent depending on the choice of detection parameters such as f RF , Af RF _ FWH M O _ FW HM/ bandwidth of the photodetector, gain and integration constant of the processing circuit, intensity of both signal and oscillator lasers and other design parameters for a particular implementation.
  • the RFDBS signal spectrum namely the RFDBS output versus the oscillator laser frequency
  • the absolute frequency of the signal laser, f s (e.g. defined at the center of its broad spectrum) can be determined.
  • FIG. 86 illustrates one possible implementation 8600 of this approach.
  • the linear polarization of the lasers is preserved in polarization maintaining fibers (PMF) 8610a and 8610b, and both lasers (Oscillator Laser 8615 and Signal Laser 8625) are combined in such a way that, at the output of PM Coupler 1 8630, their polarizations are aligned.
  • PMF polarization maintaining fibers
  • FIG. 86 includes the following steps:
  • the oscillator laser can be calibrated during each scan by a more precise external frequency reference.
  • a very stable and accurate frequency reference is available in the form of absorption cells filled with specific molecules in a gaseous state.
  • the molecules in the gas cell must contain only particular atomic isotopes with well-defined reference frequencies.
  • a mixture of molecular gases can be used to cover a much wider spectrum range than listed above.
  • a mixture of Hydrogen Cyanide (H 13 C 14 N) , Carbon Monoxide ( 12 CO) and Carbon Monoxide ( 13 CO) covers the spectral range 1520 nm to 1640 nm.
  • Cyanide (H 13 C 14 N) is shown in graph 8700 in FIG. 87.
  • the linewidths of the absorption lines also change with gas pressure as illustrated in graph 8900 in FIG . 89.
  • the lines are narrower in spectrum and their positions can be measured more accurately.
  • the gas pressure can be determined by measuring and analyzing the spectral widths of the absorption lines.
  • Graph 9000 in FIG . 90 shows the absolute frequency accuracy of Hydrogen Cyanide (H 13 C 14 N) absorption line positions over a typical operating temperature range of -5°C to +70°C.
  • the vertical axis is given in ITU channel numbers where, for example, channel 20 corresponds to the optical frequency 192.0 THz and channel 60 corresponds to 196.0 THz .
  • Hydrogen Cyanide (H 13 C 14 N) absorption lines It is apparent that lower gas pressures offer narrower absorption peaks, which provides more measurement accuracy.
  • Table 3 shows that when a cell with 1.0 kPa gas pressure is used, the maximum (worst line) uncertainty of the absorption peak spectral position is +/-llMHz (within 2 standard deviations) . The average error among all lines under these conditions is +/- 7.6 MHz. These uncertainties can be improved by 10-20% by calibrating out the temperature
  • a part of the oscillator laser light is transmitted through the absorption cell and the resulting optical output from the cell is detected by a photodiode, followed by a TZ, a low-pass filter (LPF) (possibly with amplification) and converted to the digital domain by an ADC.
  • LPF low-pass filter
  • amplification, LPF and other processing functions can be implemented in the digital domain, e.g. in a DSP, when the ADC is installed just after the TZ.
  • the CELL Output signal would stay mostly constant (corresponding to a fully transparent cell without any gas) except for the spectral regions which have gas absorption lines .
  • a typical spectral shape of an absorption line of a gas cell is presented in graph 9200 in FIG . 92 .
  • Cyanide (H 13 C 14 N) is measured at a quite high pressure of 13 kPa (100 Torr) . Consequently the line is quite broad, which reduces the accuracy with which the peak position can be determined .
  • the collected data can be analyzed by variety of methods in order to find an absorption dip position fCELL-I , where index "i" numbers consecutive dips in the whole
  • oscillator optical frequency setpoints f 0 -set-j / (where index "j' numbers consecutive sampling setpoints) recorded during the scan for each sampling point and located approximately
  • FIG. 93 shows an example of a calibration of the oscillator laser frequency setpoints by the reference
  • the oscillator laser frequency is being scanned over a wider range of optical frequencies, such that during scanning at least two absorption lines are measured, resulting in calibration of at least two frequency points of the oscillator during the scan.
  • the absorption lines measured should be placed on both sides of the measured signal wavelength.
  • FIG. 94 A practical implementation 9400 of calibration of the oscillator setpoints and the measurement of beat frequency spectrum is shown on FIG. 94.
  • a temperature sensor 9410 measuring the temperature of the absorption cell 9420 is optional and required only if a slight temperature dependence of the absorption line
  • FIG. 94 includes the following steps: 1 Select a particular signal laser frequency to be measured and/or controlled f s - ta rget;
  • a processing unit e.g. DSP
  • FIG. 95 illustrates an implementation 9500 in which the variations of the oscillator optical output power are measured and used as a reference signal to correct (normalize) both CELL Output and RFDBS Output spectra.
  • implementations described above may not be accurate when the signal laser is being modulated. This occurs when the signal laser light is modulated by an RF signal, e.g. at lOGb/s, resulting in a signal optical bandwidth, Af s _ FWH M / of several GHz, e.g. 16 GHz. Such a wide optical bandwidth would produce an approximately equally wide RFDBS Output spectrum, thereby reducing the accuracy of the measurement by an equivalent amount .
  • FIG. 96 illustrates an implementation 9600 in which this problem is avoided.
  • the signal laser 9625 light is split (tapped at 9660) before the RF modulation is applied (by data modulator 9670) and the frequency of the tapped light is determined by one of the methods described above .
  • Oscillator laser output is linearly polarized but the signal lasers polarization
  • both signal and oscillator lasers light are polarized linearly and both polarizations are aligned on the photodetector .
  • the implementation of these designs would be too costly or not practical .
  • the strength of the beat frequency depends on the polarization alignment between the two lasers. When the lasers are polarized orthogonally to each other the beat frequency would not be generated at all. This issue is addressed below in a practical design for random
  • FIG. 97 illustrates an implementation 9700 in which there are no restrictions for the state of polarization of the signal laser.
  • the oscillator laser 9715 light is transmitted through a polarization scrambler 9720 with preferably high scrambling speed (e.g. over 1 kHz) .
  • the polarization scrambler 9720 At the output of the polarization scrambler 9720, the
  • the scrambled oscillator light 9730 will beat with the randomly polarized and time-dependent signal light for half of the time on average, providing that:
  • the scanning speed of the oscillator laser is slow enough that, at each sampling point, both RFDSB and CELL Outputs would reach a steady state.
  • the implementation 9700 illustrated in FIG. 97 works equally well as all the previously described implementations in which both lasers polarizations were preserved and aligned.
  • the polarization scrambler 9720 can be introduced to any of the four implementations described above, and PM fibers in these implementations can be replaced by non-PM fibers, e.g. SMF-28.

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Abstract

La présente invention concerne de nouvelles techniques, un nouvel appareil et de nouveaux systèmes pour des communications WDM optiques. Des lasers syntonisables sont utilisés pour générer des fréquences de sous-porteuses respectives qui représentent les sous-canaux d'un canal ITU par rapport auquel des signaux client peuvent être mappés. Dans un mode de réalisation, des sous-canaux sont entrelacés par polarisation pour réduire la diaphonie. Dans un autre mode de réalisation, le multiplexage en polarisation est utilisé pour augmenter la densité spectrale. Des circuits clients peuvent être divisés et combinés à un autre avant d'être mappés, indépendamment les uns des autres, en sous-canaux individuels dans et à travers les canaux ITU. Un commutateur de point de croisement peut être utilisé pour commander le mappage client vers sous-canal, ce qui active la commutation de protection de sous-canal et la commutation de longueur d'onde sans à-coups. L'invention concerne des architectures en réseau et des transpondeurs à sous-canaux, des muxpondeurs et des crosspondeurs, ainsi que les techniques utilisées (sur le niveau/la couche de sous-canal) afin de faciliter le routage, la commutation, la concaténation et la protection optiques souhaités des circuits clients mappés sur ces sous-canaux sur les nœuds du réseau WDM.
PCT/US2010/059567 2009-12-08 2010-12-08 Routage photonique sous-canal, commutation et protection à mises à jour simplifiées de réseaux optiques wdm Ceased WO2011072073A2 (fr)

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US20110158642A1 (en) 2011-06-30

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