EP1000478A1 - Traitement de donnees optiques a l'aide de modulateurs d'absorption - Google Patents

Traitement de donnees optiques a l'aide de modulateurs d'absorption

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Publication number
EP1000478A1
EP1000478A1 EP98935210A EP98935210A EP1000478A1 EP 1000478 A1 EP1000478 A1 EP 1000478A1 EP 98935210 A EP98935210 A EP 98935210A EP 98935210 A EP98935210 A EP 98935210A EP 1000478 A1 EP1000478 A1 EP 1000478A1
Authority
EP
European Patent Office
Prior art keywords
data stream
eam
optical
modulator
port
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP98935210A
Other languages
German (de)
English (en)
Inventor
David Graham Moodie
Andrew John Gloag
Andrew David Ellis
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
British Telecommunications PLC
Original Assignee
British Telecommunications PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by British Telecommunications PLC filed Critical British Telecommunications PLC
Publication of EP1000478A1 publication Critical patent/EP1000478A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/08—Time-division multiplex systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/08—Time-division multiplex systems
    • H04J14/083—Add and drop multiplexing

Definitions

  • the present invention relates to optical communications and in particular, but not exclusively, to optical time division multiplexing (OTDM) .
  • OTDM optical time division multiplexing
  • WDM optical wavelength division
  • OTDM time division multiplexing
  • WDM provides the simplest implementation of optical multiplexing.
  • the provision of very high capacity links using WDM would require a large number of closely-spaced channels. With large scale networks this implies the use of dispersion-shifted transmission fibre to minimise dispersion penalties. It has been reported that the operation of WDM networks with more than a few channels over fibre with a low dispersion can lead to significant system degradation due to four-wave mixing and other non-linearities, even over fibre spans of less than 50 kilometres.
  • a generalised OTDM network node consists of a set of building blocks: traffic enters the node through a demultiplexing unit, which drops out every jth bit. The dropped out channel may be detected at the network node or may be transported further to a remote location.
  • the remaining channels of data are forwarded to the multiplexing unit, where one or more channels may be inserted to replace the dropped optical data streams.
  • the inserted channel may originate from a transmitter local to the node or from a remote location. Clock recovery is necessary to provide a signal to drive both the demultiplexer and sychronisation unit to ensure that inserted channels go into a vacant timeslot in the multiplexed data stream.
  • the present invention is particularly concerned with clock recovery, synchronisation, demultiplexing and multiplexing, wherever these functions are carried out.
  • the present invention provides a method of processing an optical data stream modulated with a return to zero format, the method comprising: (i) splitting the optical data stream into first and second data streams; (u) coupling said first data stream into a first input port of an electro absorption modulator (EAM); (in) processing said first data stream in the EAM; (iv) coupling the processed first data stream out of a first output port of the EAM;(v) coupling said second data stream into a second input port of the EAM; and (vi) processing said second data stream in the EAM.
  • EAM electro absorption modulator
  • the invention provides a method of processing an optical time division multiplexed (OTDM) data stream, the method comp ⁇ s ⁇ ng:( ⁇ )spl ⁇ tt ⁇ ng the OTDM data stream into first and second data streams;( ⁇ )coup! ⁇ ng said first data stream into a first input port of an electro absorption modulator (EAM);( ⁇ ) processing said first data stream in the EAM;( ⁇ v)coupl ⁇ ng the processed first data stream out of a first output port of the EAM;(v) coupling said second data stream into a second input port of the EAM;(v ⁇ )process ⁇ ng said second data stream in the EAM; and (vn) coupling the processed second data stream out of a second output port of the EAM
  • the present invention provides a method of processing an optical data stream modulated with a return to zero format, the method comprising: (i) coupling said first data stream into a first input port of an electro absorption modulator (EAM); (n) processing said optical data stream in
  • the present invention provides use of an electroabsorption modulator as a gating element in the processing of optical signals, the arrangement being such that optical signals are passed bidirectionally through the modulator.
  • Fig. 1 shows schematically an experimental set up of a two channel demultiplexer
  • Fig. 2 shows schematically an experimental set up for simultaneous clock recovery and demultiplexing
  • FIG. 3 shows schematically an experimental set up of a three-node OTDM network, demonstrating "drop and insert” multiplexing
  • Fig. 4a shows a 40Gb ⁇ t/s OTDM input data stream used in conjunction with the Fig. 1 arrangement
  • Fig. 4b shows a 10Gb ⁇ t/s demultiplexed channel, channel 1 , from port A of the arrangement shown in Fig. 1 ;
  • Fig. 4c shows another demultiplexed channel, channel 3, from port B of the arrangement shown in Fig. 1 ;
  • Fig. 5 shows the bit error rate of the two 10Gb ⁇ t/s demultiplexed channels shown in Figs 4b and 4c;
  • Fig. 6a shows the 40Gb ⁇ t/s OTDM input data stream used with the Fig. 2 arrangement
  • Fig. 6b shows a 1 0Gb ⁇ t/s demultiplexed channel, channel 1 , from port A of the Fig. 2 arrangement;
  • Fig. 6c shows the 10Ghz recovered electrical clock from the Fig. 2 arrangement
  • Fig. 6d shows the RF spectrum of the recovered clock from the Figure 2 arrangement
  • Fig.7 shows the bit error rate of the 10 Gbit/s demultiplexed channel with and without clock recovery
  • Fig. 8a shows another 40Gb ⁇ t per second OTDM input data stream used in the Fig. 3 arrangement
  • Fig. 8b shows a transmission window, indicating that channel 3 is to be dropped, from the Fig. 3 arrangement
  • Fig. 8c shows a 40Gb ⁇ t/s OTDM data stream with channel 3 dropped in accordance with the Fig. 3 arrangement
  • Fig. 8d shows a 40Gb ⁇ t/s OTDM data stream with channel 3 reinserted according to the Fig. 3 arrangement
  • Fig. 8e shows a 10Gb ⁇ t/s demultiplexed data stream as present at the port D in the Fig. 3 arrangement;
  • Fig. 9 shows the bit error rate for each 10Gb ⁇ t/s demultiplexed channel from the Fig. 3 arrangement
  • Fig. 10 shows schematically an arrangement to perform a simultaneous channel drop and photodetection of the dropped channel
  • Fig. 1 1 shows schematically an arrangement for demultiplexing several channels of a bitstream and for simultaneously photodetecting the dropped channels.
  • Electro absorption modulators In OTDM systems it is known to use electro absorption modulators for demultiplexing. Electro absorption modulators (EAM) have the advantage of being compact devices which can be driven by simple, low voltage sinusoidal drive electronics.
  • a 10 Gbit/s channel can be selected from a 40 Gbit/s data stream, by smusoidally driving an EA modulator at the channel clock frequency of 10 GHz to create the desired transmission window
  • Gbit/s channels can be demultiplexed simultaneously from a 40 Gbit/s OTDM data stream, using a single EA modulator. This will improve network reliability and simplify its management, whilst also representing a substantial cost saving.
  • FIG. 1 The experimental set-up is shown in Figure 1 .
  • a 10 GHz pulse stream was generated using a CW-DFB laser, with a wavelength of 1 557 nm, incident on an EA modulator.
  • the modulator was harmonically driven, enabling the duty cycle to be reduced, producing 5.5 ps pulses, as described in the paper by Marcenac et al, "80 Gbit/s OTDM using electroabsorption modulators", ECOC 1 997.
  • the output was then amplified and coupled into a second EA-modulator, allowing the extinction ratio to be increased to greater than 45 dB.
  • the 40 Gbit/s data stream which is shown in Figure 4, was split using a 3 dB coupler 10 , 50% being fed in a clockwise direction through the EA modulator 1 2 , and detected at port A. The remaining 50% was fed in an anti-clockwise direction back through the same EA modulator, and monitored at port B Isolators 14 and 1 6 were used to prevent any back reflections from creating interference effects. Alternatively a 3-port circulator could be used in place of the second coupler 1 7, decreasing the overall loss.
  • Each facet of the EA-modulator had an anti-reflective coating, and a measured residual reflectivity of -30.5 dB. This was due to uncoated lens-ended fibre pigtails, and resulted in incoherent interference, as discussed by Gimlett et al in Journal of Lightwave Technology, 1 898, 7(6), pp888-895, between the reflected and required signals, causing an additional crosstalk penalty at each of the output ports. This interference effect can be minimised using the polarisation controllers, by ensuring that the reflected wave is orthogonally polarised to the demultiplexed signal.
  • Figure 5 shows the results of BER measurements made on the two 10 Gbit/s demultiplexed channels.
  • a 10 Gbit/s optimised data signal is used as a back-to-back reference of the receiver sensitivity.
  • With the polarisation states set to minimise incoherent interference no additional penalty was observed when compared to the same modulator operating as a traditional uni-direction demultiplexer. This was true for both ports A and B. With the signals aligned for maximum incoherent interference the system still operated error free, albeit with a small (0.8 dB) penalty.
  • Coupler 1 0 could have a split of other than 50/50.
  • the illustrated embodiment splits the data stream 8 before the fist pass through the modulator 1 2. While this approach has many advantages, it is not essential to configure the system in this way. It would be feasible to pass the datastream 8 through the modulator 1 2 before splitting it and returning the split part back through the modulator. For this approach to be worthwhile, the initial pass through the modulator would select several channels (which in unreturned form would provide output 22) from which a further selection of one or more channels would be made on the return pass, the selected channel or channels forming output 24.
  • PLL techniques rely on a phase detector to determine the frequency mismatch between the incoming data stream and a locally generated clock, and appropriate optical phase detection methods include, loop mirrors, gain modulation and four- wave-mixing (FWM) within semiconductor optical amplifiers (SOAs) .
  • FWM gain modulation and four- wave-mixing
  • SOAs semiconductor optical amplifiers
  • FIG. 2 The experimental set-up is shown in Figure 2.
  • a 40 Gbit/s OTDM data stream was generated as before by passively multiplexing a high extinction ( > 45 dB), low duty cycle (4 ps), 10 Gbit/s data stream, realised from two cascaded harmonically driven EA modulators and a lithium niobate amplitude modulator which had a low frequency response that limited the word length to a
  • the 40 Gbit/s data stream was split using a 3 dB coupler 10, 50% being fed in a clockwise direction through the EA modulator 1 2 and detected at port A, after passing through a second 3 dB coupler 17. The remaining 50% was fed in an anti-clockwise direction using a fibre delay line, back through the same EA modulator, and detected with photodetector 30 at port B.
  • a fibre delay line back through the same EA modulator, and detected with photodetector 30 at port B.
  • the gating function is timed so that the leading (or trailing ) edge of a pulse is "cut" by the EAM window.
  • the idea is to choose a point on the pulse edge where the slope is great ( eg the greatest slope) so that a small difference in the phase of the pulse relative to the window makes a significant change in the power of the gated part of the pulse.
  • the effect of this change in power is detected by the photodiode 30.
  • a small phase change in one sense will cause the time-averaged power detected by the photodiode to drop, while a similar phase change of the opposite sense will cause the time averaged power to rise.
  • the power entering the demultiplexer varies, so the phase locked loop will need to make allowances for this variation.
  • the photodiode output can be used in a control loop to ensure clock synchronization between the gating pulses applied to the EAM and the incoming data stream.
  • the sychronized clock can of course be used for other purposes in the node or elsewhere.
  • the EA modulator was the same as that used in the first embodiment.
  • the device was again sinusoidally driven by a voltage controlled oscillator (VCO) 20 at 10 GHz to produce a 22.5 ps rectangular transmission window, enabling a single 10 Gbit/s channel to be optimally demultiplexed at port A.
  • VCO voltage controlled oscillator
  • the output of port B was detected on a low speed photodiode 30 and used as an error signal.
  • the error signal was fed through standard PLL controller electronics which were used to drive the VCO 20, thus creating a closed loop.
  • the EA modulator 1 2 acts as an electro-optic sampler, the time averaged intensity of whose output is a function determined by the phase / frequency difference between the 40 Gbit/s OTDM data stream and the locally generated 1 0 GHz electrical clock.
  • variable fibre stretcher 22 By adjusting the phase of the 40 Gbit/s signal fed in the anti-clockwise direction using the variable fibre stretcher 22, it is possible to offset the error signal by a fraction of the clock period, to enable locking to a point in the data stream (here a point of high slope, such as half way up the leading or trailing edge of a pulse) therefore making the system extremely stable. This removes the requirement for a dither signal by achieving independent control between the error signal used in clock recovery, and the demultiplexed signal.
  • the photodiode 30 was a pin diode with an InGaAs absorber layer, which provided good gain and low noise.
  • the diode had a bandwidth of 1 25MHz, chosen to be considerably higher than the bandwidth of the electrical filters in the PLL electronics ( 1 -2 MHz) 32 and lower than that required to "see” the individual pulses of the data stream ( i.e. slow relative to the bit rate of the data stream).
  • Figure 6a shows the 40 Gbit/s OTDM data stream.
  • Figure 6b shows a 10 Gbit/s demultiplexed channel
  • Figure 6c shows the 10 GHz recovered electrical clock. Jitter analysis of the recovered clock was performed using an RF spectrum analyser, by measuring the noise spectral density and the total power of each signal harmonic as described in the paper by Taylor et al, Applied Physics Letters, 1 986, 49 (1 2) ,pp681 - The RF spectrum of the recovered clock is shown in Figure 6d; the phase noise pedestal is 50 dB down from the peak.
  • the rms timing jitter was calculated to be ⁇ 1 50 fs, making this technique suitable for use in an OTDM network at > 100 Gbit/s as in the paper by Kamatani referenced above.
  • Figure 7 shows the results of BER measurements made on the 10 Gbit/s demultiplexed channels, with and without clock recovery.
  • a 10 Gbit/s optimised data signal is used as a back-to-back reference to indicate receiver sensitivity
  • the system operated error free, with minimal penalty and no indication of an error floor.
  • the example of this embodiment shows that clock recovery and demultiplexing of a 10 Gbit/s channels from a 40 Gbit/s OTDM data stream can be achieved simultaneously using a single electroabsorption modulator.
  • the system showed excellent stability and used reliable low speed inexpensive components to achieve clock recovery.
  • the recovered clock had a phase noise-mduced timing jitter of ⁇ 1 50 fs, indicating operation up to and beyond 100 Gb ⁇ t/s is possible.
  • This technique reduces the number of high-speed components necessary in an OTDM network node, therefore increasing reliability and simplifying management, whilst also reducing costs.
  • demultiplexing, 'drop and insert' (D&l) multiplexing, clock recovery and synchronisation are the key functions required within an OTDM network node.
  • D&l demultiplexing, 'drop and insert'
  • clock recovery is achieved to ensure correct synchronisation of all data channels.
  • EA Electroabsorption
  • EA modulators as the core element in a 3-node OTDM network.
  • Each node is self contained and performs its own clock recovery.
  • Node 1 uses two EA modulators to generate a 40 Gbit/s OTDM data stream, node 2 performs the D&l function on a single channel, whilst simultaneously recovering the 10 GHz clock by exploiting the bi-directiona ty of the EA modulator in the phase locked loop configuration of the second embodiment.
  • Node 3 uses an EA-Modulator based demultiplexer with electrical clock recovery to extract a 10 Gbit/s channel.
  • OTDM data stream was generated as before by passively multiplexing a high extinction ( > 45 dB), low duty cycle (4 ps), 1 0 Gb ⁇ t/s data stream, realised from two cascaded harmonically driven EA modulators and a lithium niobate (L1N1O 3 ) amplitude modulator This was driven by a 2 -1 pseudo-random 10 Gbit/s pattern generator, the word length of which was limited by the modulator's poor low frequency response.
  • the resulting OTDM data stream 8 was amplified and fed into the input of the 'drop' section of the OTDM node 2. Node 2 is based on the arrangement of the second embodiment.
  • the 40 Gbit/s input data stream was split using a 3 dB coupler 1 0, 50% being fed in a clockwise direction through the EA modulator, and detected at port A via a 3 dB coupler 1 7. ( Again, a 3 port circulator would be preferred at this point in place of coupler 1 7.)
  • the remaining 50% was fed in an anti-clockwise direction using a fibre delay line, back through the same EA modulator, and detected at port B.
  • a symmetrical coupler at this point: it will often be more attractive to pass more than 50% of the input signal to port A ,e.g. 60, 70 or 80 %.
  • the EA modulator 1 2 is the same as that used in the first and second embodiments.
  • the device was reverse biased at 1 V and sinusoidally driven using a voltage controlled oscillator (VCO) at 1 0 GHz, producing a 75 ps rectangular transmission window.
  • VCO voltage controlled oscillator
  • FIG 8 shows the 40 Gbit/s OTDM input data stream, (b) shows the transmission window for the 'drop' process, (c) shows the result after channel 3 has been dropped.
  • Simultaneous clock recovery was achieved by detecting the output at port B using a slow photodiode 30, as described in relation to the second embodiment.
  • the resulting electrical signal is a time averaged intensity which is related to the phase difference between the 40 Gbit/s OTDM data stream and the locally generated 10 GHz electrical clock. This was fed through standard PLL electronics and used to control the VCO driving the modulator. In this way a closed loop is formed, with any fluctuations in phase being constantly corrected for. By locking to the leading edge of the data stream, it was possible to extract an extremely clean and stable clock signal with an rms. timing jitter of ⁇ 1 50 fs.
  • Figure 9 shows the results of BER measurements made on each of the 1 0 Gb ⁇ t/s demultiplexed channels.
  • An optimised 10 Gbit/s channel is used as a receiver sensitivity reference.
  • Each of the undropped 10 Gbit/s channels operated error free with no indication of an error floor, but incurred a 1 dB penalty due to an imperfect transmission window at the 'drop' stage. The inserted channel however suffered from no or minimal penalty.
  • a high extinction 10 GHz pulse, pulse duration 5ps was passed through the EA modulator and detected on an optical power meter. The pulse was tuned through the transmission window of the EA modulator using a variable electrical delay line The results are shown in the inset of Figure 9, which indicates an extinction ratio of " 14 dB.
  • the example of this embodiment shows that the main OTDM processing functions of nodes for an OTDM network can be realised using electroabsorption modulators as the core elements.
  • Each node can be self contained, performing its own clock recovery and synchronisation.
  • Drop and Insert functionality has been demonstrated for the first time with an EA modulator by completely removing a 10 Gbit/s channel from a 40 Gbit/s OTDM data stream and then subsequently inserting a different 10 Gbit/s channel into the vacant time slot.
  • two synchronised modulators may be necessary at each node, one to perform the drop function, and the other to perform the demultiplexing operation.
  • the EAM which performs that processing can also, in many instances, provide a further useful function as a photodetector. This further function can, in general, be achieved whether or not the EAM is being used to process signals which enter it from different directions.
  • the EAM is driven so that it transmits all but one of the incoming OTDM channels and the EAM is in a highly absorbing state for the remaining dropped channel.
  • the absorbed channel is converted into a photocurrent by the electro absorption process within the modulator.
  • the dropped channel may be detected as an electrical output from the EAM.
  • This technique avoids the need for a further EAM (connected in parallel) to demultiplex the dropped channel.
  • This technique could be combined with the clock recovery technique used in node 2 of the Fig. 3 third embodiment (counter propagating clock recovery technique) to give simultaneous clock recovery, channel drop and photodetection of the dropped channel.
  • the optical set up could be left as in Fig. 3, but an arrangement of electrical filters like that shown in Fig. 10 would be required.
  • the counter-propagating signal used for clock recovery will also generate a photocurrent in the EAM and thus could potentially degrade the quality of the electrical dropped channel signal.
  • Demultiplexing could be achieved using an array of EAMs optically connected in parallel (as is conventional but without the need for any separate photodiodes) or in series. In either case a saving in the number of high speed opto electronic components could be achieved.
  • Electro absorption modulators have been used for in line control, i.e. cleaning up digital (not necessarily OTDM) signals.
  • An example of this is described in the paper by G Aubin et al, "40Gb ⁇ t/s OTDM sohton transmission over transoceanic distances," El. Lett. Vol.32, 24, pp 21 88-21 89, 1 996, where an EAM was driven at 40GHz to provide in-line control (e.g. reshaping and re-timing) in a so ton transmission system. Clock recovery is needed to achieve this in line control.
  • the counter propagating clock recovery technique described here in relation to the second and third embodiments could be used in such an application to reduce the number of high speed opto electronic components required.
  • Wavelength conversion has been demonstrated using an EAM, see the paper by N Edagawa, et al, "Novel wavelength converter using an electro absorption modulator: conversion experiments at up to 40Gbit/s", OFC '97 Technical Digest, pp 77-78, 1 997[2].
  • One incoming high power optical signal modulates the absorption of the DC biased modulator due to carrier saturation, thus causing a weaker CW incoming optical signal to be modulated effectively converting the wavelength of the signal. If required, the signal could be monitored by using the photocurrent generated in the EAM.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne un procédé pour le traitement d'un flux de données optiques, modulé avec format de retour à zéro, qui consiste à scinder ce flux en des premier et second flux de données; à coupler le premier flux dans une première borne d'entrée d'un modulateur à électroabsorption; à traiter ce premier flux dans ledit modulateur; à coupler le premier flux ainsi traité hors d'une première borne de sortie du modulateur; à coupler le second flux dans une seconde borne d'entrée du modulateur; et à traiter ledit second flux dans ce modulateur. L'invention concerne également des systèmes de transmission à multiplexage temporel optique et des noeuds pour ces systèmes, dans lesquels on utilise comme porte un modulateur à électroabsorption pour les données optiques traversant le modulateur en question dans des directions opposées. L'invention concerne aussi les possibilités de démultiplexage, d'extraction/insertion, d'extraction/de synchronisation d'horloge, décrites comme autant d'options pour le processus relevant du modulateur.
EP98935210A 1997-07-28 1998-07-24 Traitement de donnees optiques a l'aide de modulateurs d'absorption Withdrawn EP1000478A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GBGB9715936.2A GB9715936D0 (en) 1997-07-28 1997-07-28 Optical communications
GB9715936 1997-07-28
PCT/GB1998/002212 WO1999005812A1 (fr) 1997-07-28 1998-07-24 Traitement de donnees optiques a l'aide de modulateurs d'absorption

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EP1000478A1 true EP1000478A1 (fr) 2000-05-17

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AU (1) AU8455898A (fr)
CA (1) CA2295825A1 (fr)
GB (1) GB9715936D0 (fr)
WO (1) WO1999005812A1 (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2825474B1 (fr) * 2001-06-05 2003-10-24 France Telecom Demultiplexeur temporel optique
DE10344314A1 (de) * 2003-09-21 2005-04-28 Fraunhofer Ges Forschung Optischer Multi-Kanal-Demultiplexer auf der Basis eines einzelnen optischen Schalters

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Publication number Priority date Publication date Assignee Title
DE69324391T2 (de) * 1992-02-03 1999-11-04 Kokusai Denshin Denwa Co Ltd Vorrichtung zur optischen Wellenformung
JPH0990298A (ja) * 1995-09-21 1997-04-04 Oki Electric Ind Co Ltd 光時分割多重分離装置

Non-Patent Citations (1)

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Title
See references of WO9905812A1 *

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GB9715936D0 (en) 1997-10-01
CA2295825A1 (fr) 1999-02-04
WO1999005812A1 (fr) 1999-02-04
AU8455898A (en) 1999-02-16

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