WO2006043268A1 - Systeme et procede permettant d'augmenter la distance de transmission d'un signal optique - Google Patents

Systeme et procede permettant d'augmenter la distance de transmission d'un signal optique Download PDF

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Publication number
WO2006043268A1
WO2006043268A1 PCT/IL2005/001086 IL2005001086W WO2006043268A1 WO 2006043268 A1 WO2006043268 A1 WO 2006043268A1 IL 2005001086 W IL2005001086 W IL 2005001086W WO 2006043268 A1 WO2006043268 A1 WO 2006043268A1
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WIPO (PCT)
Prior art keywords
duobinary
logic
zero
signal
amplifier
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Ceased
Application number
PCT/IL2005/001086
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English (en)
Inventor
Moshe Avigdor
Roni Dadon
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TERASEA Ltd
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TERASEA Ltd
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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
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • 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
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H04B10/505—Laser transmitters using external modulation
    • H04B10/5055—Laser transmitters using external modulation using a pre-coder
    • 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
    • H04B10/516—Details of coding or modulation
    • H04B10/5167—Duo-binary; Alternative mark inversion; Phase shaped binary transmission

Definitions

  • the present invention relates to optical communication and more particularly to a system and method for increasing transmission distance and optical signal to noise ratio value using an optical duobinary signal.
  • DWDM Dense Wavelength Division Multiplexing
  • a duobinary signal is a partial response signal, also called Quadrature Partial Response (QPR), which overcomes Nyquist's limit for a digital communication link, by using a bit shaping based on the analog sum of the last two bits (the present and the previous transmitted bits).
  • QPR Quadrature Partial Response
  • Nyquist limit states that the number of bits you can safely transmit in a channel of bandwidth B Hz that has Inter-symbol interference is 2B per second using a "brick wall” filter having bandwidth of B Hz.
  • the spectrum of duobinary transmission uses approximately 2.8 GHz for a 10 Gbps transmission rate, while the conventional Non-Return to Zero (NRZ) modulation uses approximately 7.5 GHz for a 10 Gbps transmission rate (Nyquist theoretical limit in 5 GHz for 10 Gbps transmission).
  • NRZ Non-Return to Zero
  • Fig. 1 depicts a block diagram for the generation of duobinary modulation.
  • Fig. IA illustrates the theoretical implementation of a duobinary modulated link where the encoder and the coder are mounted on both sides of the link. The link transfers a three levels signal.
  • Fig. IB illustrates the theoretical implementation of a duobinary modulated link where the encoder and the coder are mounted on one side of the link. The link transfers two levels signal.
  • a partial response encoded sequence c(k) is related to the binary data sequence d(k) by the following encoding rule:
  • the encoding rule for a special code is fully described by m coefficients O 1n .
  • the output is an analog summation causing c(k) to be a three levels signal 0, 1, and 2 usually denote as -1; 0; 1.
  • the output sequence c(k) cannot contain a 1 followed by a -1, or a -1 followed by a 1.
  • a 1 and a -1 will always have a 0 between them.
  • the combinations ⁇ 1 0 1 ⁇ or a ⁇ -1 0 -1 ⁇ also never occur at the c(k). Only a ⁇ -1 0 1 ⁇ or a ⁇ 1 0 -l ⁇ can occur.
  • Two types of decoders are provided in order to decode the duobinary modulated data properly, as described in Fig. 1.
  • Properly decoding/encoding means minimum Bit Error Rate (BER) after the data has passed the link and recovered at the receiving end.
  • BER Bit Error Rate
  • the first type which is not being used anymore in optical communication, but works theoretically and may implement in other types of link, is illustrated in Fig. IA.
  • Fig. IB illustrates a differential encoder used as a precoder.
  • Such configuration avoids a recursive decoding. To avoid a change on both sides of the link, it is better to move this differential decoding to the transmitter and differentially precode the data.
  • MZM Mach Zehnder Modulator
  • Fig. 2 depicts graphs showing transmitted bandwidth comparison between NRZ and duobinary modulation.
  • the figure shows the differences between the NRZ modulation and a duobinary modulation.
  • the duobinary modulation bandwidth is about half the bandwidth needed for NRZ modulated signal.
  • the duobinary spectrum is the spectrum of the transmitted signal multiplied with
  • Fig. 3 shows the frequency response of Bessel filters of the 5 and the 9 order.
  • the 3 dB attenuation (6 dB for voltage response) frequency is at 2.5 GHz.
  • the response is identical up to 6 GHz and the voltage attenuation at this frequency is 28.5 dB. This explains why increasing the filter order will not increase attenuation of the high frequency harmonics of the data.
  • the maximum fundamental frequency component in NRZ data transmission is 5 GHz.
  • Fig. 4 shows the waveforms along the duobinary generation block and the difference in the behaviour of a duobinary communication system without and with a precoder, as was shown in Fig. IA and Fig. IB respectively.
  • Figs. 4a through Fig 4c depict the encoding process of a NRZ signal into duobinary signal
  • Fig. 4d depicts the decoding result of duobinary signal into NRZ signal using the decoding rules illustrated in Fig. IA;
  • Fig. 4e depicts the received signal, which includes one error, compare to the signal which was transmitted as indicated in Fig. 4c;
  • Fig. 4f depicts the sequence of errors being generated when decoding the signal of Fig. 4e. As can be seen the single error caused five consecutive errors;
  • Figs. 4g through Fig. 41 show the precoding, encoding and decoding process of converting NRZ signal into duobinary signal and back to duobinary, and
  • Figs. 4m and Fig. 4n depict the consequences when an error occurs while generating the transmitted signal illustrated in Fig. 41. Only one error is generated for one encoded/decoded error signal.
  • Fig 5 shows a practical block diagram of duobinary transmitter.
  • Fig. 5A illustrates a Duobinary transmitter using a linear/limiting amplifier
  • Fig. 5B illustrates a duobinary transmitter using a limiting amplifier.
  • LPF Low Pass Filter
  • the method of converting the electrical signal into optical signal is shown in Fig. 6.
  • the MZM is biased at V ⁇ and the driving voltage is amplified so that its Peak To Peak (PTP) value would be 2 V ⁇ 5 as indicated in the figure.
  • PTP Peak To Peak
  • Fig. 7 demonstrates the electrical and optical waveforms in a conventional duobinary transmitter.
  • the LPF forms a three level duobinary signal while narrowing the bandwidth of the transmitted signal.
  • the figure shows the effect of a 5 pole Bessel LPF with a typical 3 dB attenuation frequency at 2.8 GHz. Narrowing the bandwidth ( ⁇ half of the equivalent NRZ transmitter) increases the link tolerance to chromatic dispersion significantly. Since the dispersion is relative to the square of the transmitted bandwidth it turns out that the dispersion tolerance increases with duobinary modulation approximately 4 times, meaning the dispersion limited transmitting distance will increase from ⁇ 65 km to ⁇ 260 km.
  • FIG. 8 presents a prior art NRZ and duobinary optical eye pattern, illustrating the differences between the NRZ and duobinary optical power eye diagram.
  • a transmitted "0" in a conventional Duobinary signal has more optical power compared to the transmitted "0" in the NRZ format. Disclosure of the Invention
  • This invention is directed to duobinary modulation techniques, which increases the bandwidth efficient modulation format at channel rates of 10 Gbps and 40 Gbps.
  • duobinary modulation is more tolerant to dispersion effect in the fiber, it is more sensitive (compared to NRZ modulation) to optical noise results in lower values of Optical signal to noise ratio (OSNR).
  • OSNR Optical signal to noise ratio
  • a system for increasing transmission distance limited by Optical Signal to Noise Ratio comprising an optical duobinary transmitter including a duobinary precoder, a Low Pass Filter (LPF) producing a duobinary signal and a modulator; at least one amplifier and a voltage level suppressing logic "zero" voltage unit connected between said LPF and modulator for modifying the waveform of the signal fed into said modulator.
  • OSNR Optical Signal to Noise Ratio
  • the invention further provides a method for increasing transmission distance limited by low OSNR value, comprising providing a duobinary transmitter having logic "zero” and logic “one” levels, an optical duobinary transmitter including a duobinary precoder, a Low Pass Filter (LPF), a modulator, at least one amplifier and a voltage level suppressing logic “zero” voltage unit, and reducing the optical power level of the transmitted logic "zero” while maintaining the optical power level of logic "one” substantially the same.
  • LPF Low Pass Filter
  • Figs. 1 to 8 are respectively, block and waveform diagrams showing prior art ODB transmitters and transmission signals
  • Fig. 9 is a block diagram of an embodiment of an ODB transmitter and waveforms according to the present invention.
  • Fig. 10 is a circuit diagram of the transmitter of Fig. 9;
  • Figs. 11 and 12 are block and circuit diagrams illustrating another embodiment of an ODB transmitter and waveforms according to the present invention.
  • Figs. 13 and 14 are block and circuit diagrams of a further modification of the transmitter of Figs. 9 and 10;
  • Figs. 15 and 16 are block and circuit diagrams of still a further embodiment of an ODB transmitter and waveforms, according to the present invention.
  • Fig. 17 shows an NRZ and optical eye pattern resulting from use of the system and method according to the present invention, and
  • Fig. 18 is a graph showing the spectrum of a duobinary signal according to the present invention, as compared with the spectrum of a prior art duobinary signal.
  • the drawback in implementing the prior art duobinary transmitter is the fact that a logic "0" is not transmitted at zero optical power, or at least at the equivalent optical power when transmitting a "0" in an NRZ modulation.
  • the present invention is a way to increase the OSNR and ER by reducing the optical power level of the transmitted logic "zero" with a minimum effect on the optical power level of the transmitted logic
  • the invention has its affect in the electrical domain of the transmitter due to the fact that the optical bandwidth of the duobinary modulation has been optimized as an MZM-based duobinary modulation, does not induce optical frequency deviation (chirp).
  • a duobinary transmitter 2 which includes a precoder 4 having an NRZ data input 6 and data output 8 leading to a limiting amplifier 10 having a driver output impedance of 50 ⁇ .
  • the output from the limiting amplifier 10 is fed via a DC blocking capacitor 12 to a LPF 14, the output of which is directed to a controlled voltage level suppressing unit 16, e.g., two serially connected clamping diodes (Figs. 9 to 14), or an analog switch (Figs. 15 and 16).
  • the three-level signal exiting the unit 16 drives a MZM 18, which modulates the CW laser source 20.
  • the values of the optical power for "0" have been reduced and the voltage drop for the transmitted "1" and "-1" can be compensated by increasing the drive voltage.
  • Figs. 11 and 12 illustrate another embodiment of the duobinary transmitter of Figs. 9 and 10.
  • the first limiting amplifier 10 is interposed in circuit between the precoder 4 and the LPF 14 and a second linear/ limiting amplifier 22 is connected at the output of the unit 16.
  • the gain of the first stage should be at a value so that voltage excursion at zero level will not exist.
  • This implementation consists of a limiting amplifier as the first amplification stage, while the second amplification stage is a linear/limiting amplifier. This arrangement erases the voltage excursions at the zero level, as illustrated by the voltage waveforms.
  • FIGs. 13 and 14 A further modification is illustrated in Figs. 13 and 14 in which the linear/limiting amplifier 10 is interposed between the unit 16 and the MZM 18.
  • FIGs. 15 and 16 An embodiment showing a controlled "zero" voltage level suppressing unit 16 utilizing an analog switch is illustrated in Figs. 15 and 16.
  • the analog switch is controlled by a control circuit 24 receiving input data from the NRZ signal while being synchronized by the same input clock of the precoder 4, thus obtaining the same zero timing at the LPF 14 output.
  • the analog switch can be implemented by using a high speed transistor, PIN diode or any other equivalent electronic circuit.
  • the output of the LPF is a three levels signal.
  • the mid level (designated as "0" at the filter output waveform in Figs. 9, 11 and 13) including ripples caused by residual of the suppressed high frequency harmonics existing in the transmitted data.
  • the high frequency harmonics suppression is advantageously performed by a Bessel LPF.
  • Increasing the number of the filter's pole should suppress the residual value but actually is not effective due to the fact that a Bessel or a linear phase filter is used.
  • Such filters are required due to the fact that they keep the Group Delay (GD) value of the random data to a minimum required value.
  • GD Group Delay
  • the present invention utilizes a voltage level suppressing logic "zero" voltage unit, which will prevent ripples having amplitude values below a threshold voltage of the diodes (when clamping diodes are used) from propagating into the MZM. Likewise, the analog switch will erase any value of these ripples by shorting them to analog ground. The ripples caused by the filter will not appear in the optical domain.
  • the voltage drop across the clamping diodes will affect the transmitted power of the logical "1".
  • compensating the driving voltage for logical "1” will affect the driving voltage of the logical "0" significantly less, which means that the effect of suppressing of the voltage excursion of the driving voltage at the "0" level is maintained, in spite of changing the driving voltage for compensating the "1".
  • the voltage drop across the "1" is compensated by the second linear/limiting amplifier without any effect on the voltage corresponding to logical "0".
  • Fig. 18 shows that the spectrum of a duobinary output signal according to the present invention is not affected as compared to a combined output duobinary signal. This means that the increased dispersion tolerance which was gained by using a conventional duobinary modulation is not affected by using a duobinary modulation, having a voltage level control unit according to the present invention.
  • the present invention is applicable to any communication bit rate, e.g., bit rates between 10 Gbps to 40 Gbps provided the 3 dB attenuation of the LPF is adjusted to approximately 25% of the transmitted bit rate.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optics & Photonics (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un système permettant d'augmenter la distance de transmission limitée par le rapport signal optique - bruit (OSNR). Ce système comprend un émetteur optique duobinaire possédant un dispositif de précodage duobinaire, un filtre passe-bas (LPF) produisant un signal duobinaire et un modulateur, au moins un amplificateur et une unité de tension 'zéro' de logique de suppression de niveau de tension étant connectés entre le filtre passe-bas et le modulateur pour modifier la forme d'onde du signal amené au modulateur. L'invention concerne également un procédé permettant d'augmenter la distance de transmission limitée par le rapport OSNR.
PCT/IL2005/001086 2004-10-21 2005-10-16 Systeme et procede permettant d'augmenter la distance de transmission d'un signal optique Ceased WO2006043268A1 (fr)

Applications Claiming Priority (2)

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IL164753 2004-10-21
IL16475304A IL164753A0 (en) 2004-10-21 2004-10-21 A system and method for increasing transmission distance of and optical signal

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WO2006043268A1 true WO2006043268A1 (fr) 2006-04-27

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8295431B2 (en) 2007-06-21 2012-10-23 Non-Contacting Rotary Joint Non-contacting rotary joint
WO2018028924A1 (fr) * 2016-08-12 2018-02-15 Danmarks Tekniske Universitet Dispositif et procédé pour générer un signal polybinaire

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5892858A (en) * 1997-03-27 1999-04-06 Northern Telecom Limited Duobinary coding and modulation technique for optical communication systems
US6388786B1 (en) * 1997-08-15 2002-05-14 Nec Corporation Method for generating duobinary signal and optical transmitter using the same method
EP1416654A2 (fr) * 2002-10-31 2004-05-06 Samsung Electronics Co., Ltd. Transmission duobinaire optique

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5892858A (en) * 1997-03-27 1999-04-06 Northern Telecom Limited Duobinary coding and modulation technique for optical communication systems
US6388786B1 (en) * 1997-08-15 2002-05-14 Nec Corporation Method for generating duobinary signal and optical transmitter using the same method
EP1416654A2 (fr) * 2002-10-31 2004-05-06 Samsung Electronics Co., Ltd. Transmission duobinaire optique

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
GENE J M ET AL: "REDUCED DRIVING VOLTAGE OPTICAL DUOBINARY TRANSMITTER AND ITS IMPACT ON TRANSMISSION PERFORMANCE OVER STANDARD SINGLE-MODE FIBER", IEEE PHOTONICS TECHNOLOGY LETTERS, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, vol. 14, no. 6, June 2002 (2002-06-01), pages 843 - 845, XP001116465, ISSN: 1041-1135 *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8295431B2 (en) 2007-06-21 2012-10-23 Non-Contacting Rotary Joint Non-contacting rotary joint
WO2018028924A1 (fr) * 2016-08-12 2018-02-15 Danmarks Tekniske Universitet Dispositif et procédé pour générer un signal polybinaire

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