Classifications

• • 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
  • H04B10/29—Repeaters
  • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
  • H04B10/2912—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form characterised by the medium used for amplification or processing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J14/00—Optical multiplex systems
  • H04J14/02—Wavelength-division multiplex systems
  • H04J14/0221—Power control, e.g. to keep the total optical power constant
  • H04J14/02216—Power control, e.g. to keep the total optical power constant by gain equalization
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
  • H01S3/067—Fibre lasers
  • H01S3/06754—Fibre amplifiers
  • H01S3/06758—Tandem amplifiers
• • 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
  • H04B10/29—Repeaters
  • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
• • 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
  • H04B10/29—Repeaters
  • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
  • H04B10/293—Signal power control
  • H04B10/294—Signal power control in a multiwavelength system, e.g. gain equalisation
  • H04B10/2941—Signal power control in a multiwavelength system, e.g. gain equalisation using an equalising unit, e.g. a filter
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/094096—Multi-wavelength pumping
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2210/00—Indexing scheme relating to optical transmission systems
  • H04B2210/003—Devices including multiple stages, e.g., multi-stage optical amplifiers or dispersion compensators

Definitions

• the present invention relates generally to the field of optical communications and in particular to an erbium-doped fiber amplifier having gain tilt control with mid-stage attenuators.
• WDM wavelength-division-multiplexed
• Chraplyvy "Transmission of 32-WDM 10-Gb/s Channels Over 640 Km using Broad Band, Gain-Flattened Erbium-Doped Silica Fiber Amplifiers, " IEEE Photon. Tech. Lett., Vol. 9, No. 12, pp. 1652-1654, December 1997; A.K. Srivastava, Y. Sun, J.L. Zyskind, J.W. Sulhoff, C. Wolf, J.B. Judkins, J. Zhou, M. Zirngibl, R.P. Espindola, A.M. Vengsarkar, Y.P. Li, and A.R.
• Chraplyvy "Error Free Transmission of 64 WDM 10-Gb/s Channels Over 520 Km of TRUEWAVE Fiber", NEED PUBLICATION DATE; and A.K. Srivastava, Y.Sun, J.W. Sulhoff, C. Wolf, M. Zirngibl, R. Monnard, A.R. Chraplyvy, A.A. Abramov, R.P. Espindola, T.A. Strasser, J.R. Pedrazzini, A. M. Vengarkar, J.L. Zyskind, J. Zhou, D.A. Ferrand, P.F. Wysocki, J.B. Judkins and Y.P.
• rare earth-doped optical fiber amplifiers are used to amplify optical signals used in communications systems and networks. These rare earth-doped optical fiber amplifiers are found to be cost effective, exhibit low-noise, provide relatively large bandwidth which is not polarization dependent, display substantially reduced crosstalk, and present low insertion losses at relevant operating wavelengths. As a result of their favorable characteristics, rare earth-doped optical fiber amplifiers, e.g., erbium-doped fiber amplifiers (EDFAs), are replacing current optoelectronic regenerators in many optical lightwave communications systems and in particular, wavelength- division-multiplexed (WDM) optical communications systems and networks.
• WDM wavelength- division-multiplexed
• optical amplifiers having wide bandwidths are required. Accordingly, the gain of the amplifiers should be uniform over the entire WDM bandwidth so that the channels may be transmitted without impairment.
• gain equalization filters such as long period gratings (See, e.g., A.M. Vengsarkar, P.J. Jemaire, J.B. Judkins, V. Bhatia, T. Amsterdam, and J.E. Snipe, "Long-Period Fiber Gratings as Band-Rejection Filters", J.Lightwave Tech., Vol. 14, No. 1, pp. 58-65, January, 1996).
• the system power "flatness" may be affected by a number of factors such as spectral loss in the transmission or dispersion compensation fiber, spectral loss in passive components, variation in input signal power spectrum and Raman effect in the fiber (See, e.g., A.R. Chraplyvy and P.S. Henry, "Optical Power Limits In Multi-Channel Wavelength-Division-Multiplexed Systems Due to Stimulated Raman Scattering", Electron. Lett., Vol. 20, No.2, pp 58-59, January 1984).
• the deviation from the "ideal flatness" for wide band optical amplifier may be approximated to a linear tilt in the signal power spectrum. Consequently, methods and apparatus for controlling the tilt are desired to produce wide band optical amplifiers having desirable operating characteristics.
• the mid-stage attenuator mitigates channel power spectral tilt.
• an average inversion level of erbium-doped fiber can be adjusted, which further affects the gain tilt in the EDFA gain spectrum.
• Fig. 1(a) is a schematic of a two stage optical amplifier with a mid-stage variable optical attenuator according to the present invention
• Fig. 1(b) is a plot of the gain vs. wavelength of the optical amplifier of
• Fig. 2 is a schematic of an experimental setup for gain tilt control according to the present invention
• Fig. 3(a) is a plot of input power spectrum of 18 WDM channels with both +4dB and -2dB tilt;
• Fig. 3(b) is a plot of tilt corrected output spectra after amplification by amplifier according to the present invention
• Fig. 4 is a plot showing necessary attenuator loss to obtain a flat output spectrum for different signal tilts in the range of -4dB to 4dB;
• Fig. 5 is a plot showing necessary attenuator loss at constant gain operation for different signal tilts in the range of-4dB to 4dB.
• Fig. 1(a) illustrates the basic principle of our optical amplifier and inventive method.
• the amplifier shown there 100 is divided primarily into two stages and comprises optical isolators (01) 101, sections of erbium-doped optical fiber (EDF) 103, wavelength selective couplers (WSC) 105, gain equalization filter (GEF) 107, variable attenuator (VA) 109 and 980nm and 1480nm optical pumps 111 and 113, respectively.
• the amplifier exhibits broadband, large dynamic range, high power characteristics desirable for wavelength division multiplexed transmission of optical signals.
• optical signals enter the optical amplifier 100 through input port 110 and exit from output port 120, with the output port 120 being "downstream" of the input port 110.
• Optical isolators 101, attenuators 109, GEFs 107, and WSCs 105, are generally known in the art, some of which are commercially available. Furthermore, those skilled in the art know that it is conventional, but optional, to place optical isolators respectively upstream and downstream of an EDFA.
• Fig. 1(b) shows a plot of gain vs. wavelength for the optical amplifier of Fig. 1(a). As is shown, the amplifier exhibits uniform gain characteristics over 35nm of bandwidth (1526nm-1561nm). The gain spectrum may be kept flat for a range of input power levels by adjusting the variable attenuator 109. With an input power of -4dBm and the attenuator set to a minimum, the gain is 24dB with 12dB of gain compression with a noise figure of approximately 5dB.
• FIG. 2 An experimental setup for gain tilt control according to the present invention is shown schematically in Fig. 2. As is shown, two optical amplifiers are used therein. Specifically, a first erbium-doped fiber amplifier 210 is used to prepare an input signal spectrum with simulated power tilt for a second erbium- doped fiber amplifier 220. A waveguide grating router 230, was used to multiplex 18 WDM signals ( ⁇ i - ⁇ ig) that originated from external lasers (not shown). For our demonstrative purposes, the signal channels ranged from 1531.4 to 1558.6nm with 200GHz channel separation resulting in a total bandwidth of approximately 27nm.
• the signal power of the channels was then sent through an attenuator/power meter 240 which controlled the input power to the first (preparation) amplifier 210 which is constructed like amplifier 100 shown in Fig. 1.
• the attenuator 109 within a mid-stage of amplifier 100 may be tuned to obtain a total power tilt between -4dB and 4dB between the shortest and longest wavelength channels.
• the signal power spectral tilt input to the second (test) amplifier 220 was monitored by an optical spectrum analyzer 260 and a second attenuator/power meter 250 was used to adjust the total input power entering the test amplifier 220.
• positive tilt is the power tilt with low power in the short wavelength side and high power in the long wavelength side. Accordingly, negative tilt is the reverse situation.
• the input spectrum of the 18 WDM channels with both +4dB and -2dB tilt is shown in Fig. 3(a). With a suitable adjustment of the mid-stage variable optical attenuator in the test amplifier 220, the power spectrum tilt can be compensated. Shown in Fig. 3(b) are the tilt corrected output spectra after the test amplifier 220 for both 4dB and -2dB tilt in the input spectrum. As shown in that Figure, the tilt in the input spectrum can be completely mitigated by changing the mid-stage attenuator loss in both cases.
• the attenuator loss needed to obtain the flat output spectrum for different input signal tilts in the range of -4dB to +4dB when the total input power is fixed at 0.4dBm is shown in Fig. 4.
• the attenuator was set to 4.5dB to produce a flat output spectrum for a flat input spectrum.
• the compensation can be completed by adjusting the attenuator between 0 and 17 dB.
• a penalty results however, in that the output power decreases when attenuator loss is increased.
• the minimum loss in the attenuator is not sufficient to flatten the output power spectrum.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Plasma & Fusion (AREA)
• Optics & Photonics (AREA)
• Lasers (AREA)
• Optical Communication System (AREA)

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• 1999
  • 1999-06-18 EP EP99930403A patent/EP1005707A1/de not_active Withdrawn
  • 1999-06-18 WO PCT/US1999/013812 patent/WO1999066610A1/en not_active Ceased
  • 1999-06-18 CN CN99800964.4A patent/CN1310872A/zh active Pending
  • 1999-06-18 AU AU46951/99A patent/AU4695199A/en not_active Abandoned
  • 1999-06-18 JP JP2000555339A patent/JP2002518855A/ja active Pending

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