NZ331214A

NZ331214A - Dark pulse tdma optical network

Info

Publication number: NZ331214A
Application number: NZ331214A
Authority: NZ
Inventors: Kevin Smith; Julian Kazimierz Lucek; Danny Robert Pitcher; Terence Widdowson; David Graham Moodie; Andrew David Ellis
Original assignee: British Telecomm
Priority date: 1996-02-26
Filing date: 1997-02-25
Publication date: 1999-10-28
Also published as: AU732674B2; JP2002516043A; WO1997031436A1; CA2245716A1; EP0883938A1; KR19990087351A; NO983894L; NO983894D0; AU1888297A

Description

<div class="application article clearfix" id="description"> New Zealand No 331214 International No PCT/GB97/00520 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates 26 02 1996,26 02 1996,26 06 19u96,26 06 19 96,02 10 1996,02 10u 1996, Complete Specification Filed 25 02 1997 Classification (6) H04B10/213, H04J14/08 Publication date 28 October 1999 Journal No 1445 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention Dark pulse tdma optical network Name, address and nationality of applicant(s) as in international application form BRITISH TELECOMMUNICATIONS public limited company, 81 Newgate Street, London EC1A 7AJ, United Kingdom WO 97/31436 1 PCT/GB97/00520 DARK PULSE TDHA OPTICAL NETWORK The present invention relates to an optical network for carrying TDMA (Time Division Multiple Access) signals and to transmitters and receivers for use in 5 nodes of such a network A network embodying the present invention might be used, for example, as a local area network (LAN) for interconnecting computer systems The increasing power of computer systems in terms of processor speeds and storage capacity has made it possible for conventional personal computers to handle 10 multimedia applications involving real time video and animation and computer graphics The high bandwidth data associated with such applications place heavy "" demands on the network and the performance of conventional LANs has failed to keep pace An optical network using synchronous TDMA potentially offers a far 15 higher bandwidth, and so might be used as a high speed LAN to replace a conventional LAN However, in existing optical networks, while signal transmission has been carried out in the optical domain, in practice some electronic circuits have been required for such functions as chanhel selection It has been recognised that such electronic components of the network infrastructure 20 constitute a bottleneck restricting the performance of the network "A High Speed Broadcast and Select TDMA network Using All-Optical Demultiplexing", L P Barry et al, ECOC '95 pp 437-440, describes an experimental OTDM network At the receivers in the network nodes, an optical clock signal is detected and a variable delay applied in the electrical domain to the detected clock 25 signal to select a particular TDMA channel After pulse shaping, the signal is taken back into the optical domain by driving a local optical source, a DFB laser, which produces an optical signal for use in a subsequent all-optical switching stage The paper by Prucnal et al, "Ultrafast all-optical synchronous multiplex access fibre networks", IEEE Journal on Selected Areas in Communications SAC-30 4, no 9, December 1986 proposes an alternative approach in which different delays, and hence different TDMA channels, are selected in the optical domain The optical signal is split between different, paths each having a different Pnnt""d from Mirnos -1 09/0S/1999 16 J9 04 picjo — $ — WO 97/31436 2 PCT/GB97/00520 characteristic delay and an electro optic gate in each path is controlled so that the signal passes only through the path having the desired delay According to a first aspect of the present invention, there is provided an optical network comprising 5 a) an optical transmission medium, and b) a plurality of nodes connected to the optical transmission medium, each of the plurality of node including a respective dark pulse generator which is coupled in-line with the optical transmission medium and in series with the others of the dark pulse generators and which is arranged to 10 generate dark pulses in an optical signal carried on the transmission medium In this specification, a 'dark pulse" is a temporal gap, or region of reduced intensity radiation, in an essentially continuous burst of optical radiation, or light beam An advantage of using dark pulses in place of bright pulses is that optical 15 signal generation is simplified, as will be discussed in the subsequent description Also, while pulse alignment remains important for dark pulse OTDM, to minimise cross talk, the extinction ratio necessary for successful dark pulse OTDM transmission is typically smaller than that required for bright pulse OTDM Preferably each node further comprises a variable delay stage which is 20 arranged to apply a variable delay to a network clock signal in the electrical domain and which is connected at its output to the dark pulse generator The inventors have found it to be particularly advantageous to use in combination dark pulse generation and channel selection in the electrical domain This further simplifies node structures, whilst enabling effective operation at high 25 bit rates, for example at 40 Gbit/s Preferably each node further comprises a clock receiver for receiving a network clock signal carried on the optical transmission medium, the clock receiver including a photoelectric detector for converting the clock signal to the electrical domain 30 Preferably the electro-optic modulator is an electro-absorption modulator <EAM) The present inventors have found that significant advantages can be achieved by combining channel selection in the electrical domain with the use of Puni" d from Ilimosi 08/05/1999 1G 39 04 pirje -4- WO 97/31436 3 PCT/GB97/00520 an electro-optic switch with a fast non-linearity to read the selected channel In particular, relatively high switching rates can be achieved without the power losses typically associated with all-optical channel selection It is found to be particularly advantageous to use an EAM The fast response time of such a device makes 5 possible a switching window as short as a few picoseconds The receiver as a whole is therefore capable of operating at bit rates of 40Gbit/s or higher Preferably the receiver includes means for separating the clock signal in the optical domain from the received TDMA datastream Preferably the said means for separating comprise a polarising beam splitter, in use the clock signal 10 being marked by a different polarisation state to the TDMA datastream Preferably a first output of the means for separating is connected to the optical input of the electro-optic modulator, in use TDMA data passing from the first output to the modulator, and a second output of the means for separating is connected to the detector, in use optical clock signals passing from the second 15 output to the detector Preferably an impulse generator is connected between the output of the variable delay stage and the control input of the electro-optic modulator The electro-optic modulator may require a drive signal having somewhat shorter pulses than those output by the delay stage In this case advantageously 20 some form of pulse shaping may be used, and in particular the output of the delay stage may be applied to an electrical impulse generator This may be a device using step recovery diodes to generate short electrical pulses from a sine wave Preferably the variable delay stage comprises a plurality of logic gates means connecting a first input of each gate to an input path for the clock signal, 25 control means connected to a second input of each gate, and means connecting outputs of the gates in common to an output path for the delayed clock signal the said means connecting inputs and outputs of the gates to respective input and output paths being arranged to provide paths of different respective lengths via different gates, in use the control means applying control signals to the gates to 30 select a path and a corresponding delay for the clock signal This preferred feature of the present invention uses an array of logic gates to provide an electronic channel selector suitable for an integrated construction and capable of quick reconfiguration This channel selector is not limited in Ptint-°d from Ilirnos i 09/05/1598 16 i9 04 p iqe -5- WO 97/31436 4 rCT/GB97/00520 applicability to receivers in accordance with the first aspect of the present invention, hut may be used with other receiver designs, or in node transmitters In particular, it may be combined with a local optical source in a receiver in which an all-optical switch was used in place of the electro optic modulator of the first 5 aspect of the invention Preferably at least one of the said means connecting inputs and outputs comprises a microstrip delay line Preferably the means connecting inputs and outputs comprise a pair of microstrip delay lines and the gates are connected between the pair of microstrip delay lines 10 Preferably adjacent connections to the gates on the microstrip delay line on the input side of the gates are separated by a path length corresponding to t,'2 and adjacent connections on the microstrip delay line on the output side of the gates are separated by a path length corresponding to t/2, in use the gates being controlled to vary the delay by multiples of t, where t corresponds to the channel 15 spacing in the time domain of the TDMA signal Preferably the optical transmission medium is an optica* bus, and more preferably hs an optical bus topology As set out in further detail in the description of the embodiments below, the use of dark pulse generation is found to be particularly well-adapted to a 20 network using a bus-topology This allows the dark pulse generators in the different nodes to be effectively coupled in series so as to build up an OTDM multiplex At the same time, the bus topology eliminates many of the timing problems associated with other topologies, such as star networks According to a second aspect of the present invention there is provided a 25 method of operating an optical network including a plurality of nodes connected to an optical transmission medium the method comprising a) at one of the plurality of nodes, imposing dark pulses representing a data stream on an optical signal which is carried on the optical transmission medium, and 30 b) at a subsequent node receiving the optical signal including the dark pulses imposed in step la) and imposing dark pulses on the optical signal in a different respective time slot, thereby creating an OTDM (optical time division multiplexed) signal Printed from Miriioc i 0°/05/19D9 16 39 04 page -6- WO 97/31436 5 PCT/GB97/00520 The present invention also encompasses an optical network incorporating a receiver in accordance with the preceding aspects and also LANs and other computer networks formed using such a network Systems embodying the present invention will now be described in further 5 detail, by way of example only, with reference to the accompanying drawings in which Figure 1 is a schematic of an optical network, Figure 2 is a diagram showing the structure of one of the nodes of Figure 1. 10 Figure 3 is a schematic of a transmitter for use in the network of Figure 1, Figure 4 is a schematic of a receiver for use in the network of Figure 1, Figure 5 is a circuit diagram for an electrical channel selector. Figure 6 is a diagram illustrating the use of the electrical channel selector with a local optical source, 15 Figure 7 is a detailed schematic of a receiver based on the topology of Figure 6, Figure 8 is a schematic of a pulse source Figure 9 illustrates a dark pulse generator incorporating one EAM Figure 10 is a graph representing a typical operational characteristic of an 20 EAM, Figure 11 is a representation of an optical output signal provided by the system in Figure 9, Figure 12 illustrates a system incorporating three EAMs, and Figure 13 is an eye diagram of an optical output signal provided by the 25 system in Figure 12, Figure 14 is a schematic of an optical fibre LAN incorporating dark pulse generators, and Figures 15a and 15b show fibre waveguides for use in the LAN of Figure 14 30 An optical network comprises a number of nodes N1 N2 N3 connected to an optical fibre bus 1 In the present example, the network is a local area network (LAN) and a number of personal computers PCI, PC2, PC3 are connected via the optical fibre bus to each other and to a network server 2 Printed from Mimosa 09/05/1999 16 J9 0 4 p^ge -7- WO 97/31436 6 PCT/G B97/00520 Although, for clarity only three nodes are shown, in practice the network may support many more nodes The network uses a structure termed by the inventors a re-entrant bus topology As seen in Fiqure 2 each node includes a transmitter 21 coupled to the bus 1 at two points and a receiver 22 coupled to the fibre bus 1 5 at a point downstream from the transmitter The transmitter 21 and receiver 22 are coupled to the respective personal computer by an electronic interface 23 The network operates using a synchronous TDMA (time division multiple access) protocol A clock stream is distributed to all users of the network thereby ensuring that each node is synchronised A clock pulse marks the start of each 10 frame The frame is precisely divided into time slots for example slots of lOps duration for a 100Gbit/s line rate In general each node has a tuneable transmitter and tuneable receiver and can thereby transmit and receive in any of the time-slots The granularity of the network, that is the relationship between the overall network bandwidth, and the bandwidth of individual channels, may be chosen to 15 be relatively high so that each user has access to a relatively low speed (say 155Mbit/s) channel from a fibre optic pipe which itself carries rates in excess of 100Gbit/s To minimise the costs of the electronic components required, the electronic speeds within each node are at most 2 5Gbit/s in this example The clock source is typically located at the network controller 3 associated with the 20 server 2 The clock produces a regular stream of picosecond duration optical pulses at a low repetition rate, say 1 55 or 250MHz, relative to the peak line rate of the optical pipe (100Gbit/s) Such a source may be provided by a mode locked laser or a gain-switched laser with external pulse compression As a guideline, for a 100Gbit/s LAN a pulse duration of around 2ps is required whereas for a 40Gbit/s 25 system around 5-7ps suffices A pulse source suitable for operation at 100Gbit/s or higher is disclosed and claimed in the present applicant's co-pendinq European Patent Application filed 16th February 1996 and entitled "Optical Pulse Source (applicant's ref A25146) The disclosures of that earlier application are incorporated herein by reference This pulse source may comprise a ridge-30 waveguide gain-switched distributed feedback semiconductor laser diode (DFB-SLD) having its output gated by an electro-absorption modulator Continuous wave (cw) light is injected into the optical cavity of the DFB-SLD A synchronised PL^nt-^d from Mimosa 09/05/1993 16 39 04 p^ne -9- WO 97/31436 7 PCT/GB97/00S20 RF drive is applied to the DFB-SLD and to the EAM This pulse source is shown schematically in Figure 8 Figure 3 shows the transmitter in one of the nodes At the transmitter, a fraction of the distributed clock stream is split-off and then encoded via an electro 5 optic modulator This may be, for example, a lithium niobate modulator such as that available commercially from United Technologies, model no APE MZM 15 3-T 1-1-B/C or an electro absorption modulator (HAM) A suitable EAM is described in the paper by D G Moodic et al published at pp 1370 1371 Electron Letts , 3 August 1995, Vol 31, no 16 The variable time delay in the transmitter then 10 places the modulated pulse stream into the correct time slot for onward transmission The data and clock streams must be distinguishable, and in this example polarisation is used to distinguish the clock from the rest of the frame In the transmitter, a polariser P eliminates the possibility of data channels breaking through and being modulated in the electro-optic modulator (EOmod) The polariser 15 need not be a separate device but might be integrated with the EO modulator For example, the United Technologies EAM referred to above is inherently polarisation-selective in operation The delay line provides the required delay and data pulses are inserted into the appropriate time-slot with a polarisation orthogonal to the clock stream This polarisation rotation may be done via a simple polarisation 20 rotator such as a retardation plate or, where polarisation maintaining fibre is used to implement the circuit, then rotation may be achieved by physically rotating the waveguide before reinserting it into the fibre optic pipe At the receiver, after tapping a fraction of the light from the optical pipe, the clock and the data are separated A polarising beam splitter (PBS) is used to 25 perform this function The clock and the data pulses are then forced to suffer a relative (programmable! optical delay using a variable time delay device This means that the clock pulse can be temporally overlapped with any data pulse slot and therefore used to demultiplex or read any channel After the channel is demultiplexed, it is converted back into the electrical domain using a receiver 30 operating at up to 2 5 Gbit/s the allocated bandwidth per user Figure 4 shows in detail the structure of the receiver and in particular shows how an electrical channel selector (ECS) is used to provide a signal which, after suitable amplification and shaping drives an electro-absorption modulator Ptint"td fiorn minor i G3/0S/19j3 16 39 04 p iq» -ci- WO 97/31436 8 PCT/GB97/00520 (EAM) The electrical channel selector (ECS) is shown in Figure 5 The optical LAN clock is first detected using a detector 52 which might be for example a PIN photodiode After amplification, the signal is filtered to generate a clean electrical sine wave The signal is then input to a delay stages 53 comprising a series of 5 electrical AND gates LG arranged in a linear array The array is implemented as a single low cost chip available commercially as NEL NLB6202 The AND gates control access to the microstrip delay lines The delay lines are accurately stepped in delays equal to the channel separation of the LAN For a system operating at 40Gbit/s, the channel delay t equals 25ps The AND gates are controlled via an 10 input from a demultiplexer 54 In this example the demultiplexer is an NL4705 device manufactured by NEL The demultiplexer converts an incoming serial delay select word generated by the PC connected to the node into an appropriate gating signal for the AND gate array and thereby selects the appropriate delay The electrical channel selector produces at its output a stepped sine wave 15 This may then be amplified and suitably shaped in order to generate the appropriate drive signal required for the next stage The next stage may be, for example, an EAM, or a laser diode If the pulses output by the ECS require shortening to drive the next component then an electrical impulse generator may be used A suitable coaxial step recovery diode comb generator is available 20 commercially as ELISRA series MW15900 Given that electronic clock recovery can be carried out with sub-picosecond temporal jitter and microstrip delay lines can be controlled to picosecond accuracy, it is potentially possible to use such an electrical channel selector at rates as high as 100Gbit/s Although the circuit of Figure 4 uses an EAM, the ECS might alternatively 25 be used in combination with a local optical source With such a source the ECS may be used either in the transmitter for programmable channel insertion (Figure 6), or in the receiver for channel dropping (Figure 7) In the case of channel dropping, the output of the local picosecond pulse laser is combined wi;n the data in an optical AND gate Advances in picosecond pulse lasers in recent years are 30 such that it is possible to generate stable picosecond duration optical pulses using semiconductor based active media One example of such a laser is a gain-switched DF8 laser fallowed by chirp compensation as described in our above-cited copending application This provides a simple reliable source of picosecond duration pulses at flexible repetition rates from MHz to 10s of GHz In the present Printed from Mimosa 03/05/19^9 16 39 04 plge -10- WO 97/31436 9 PCT/GB97/00520 example, such a source is driven by the output of the ECS after broadband amplification and using an impulse generator The resulting stream of optical pulses is then used directly to demultiplex the required channel in an optical AND gate The use of an optical AND gate as a demultiplexer is described in detail in 5 the present Applicant's earlier International Application no PCT/GB 95/00425, filed 28th February 1995 The wavelength of the source depends on the design of the optical AND gate but is not at all restricted to be the same as the data wavelength The optical AND gate may be an SLA NOLM or may be an integrated semiconductor-based device 10 The systems so far described have used what may be termed "bright pulses" to carry information Advantageously, dark pulses may be used instead A convenient system for generating dark pulses will now be described Initially the description covers the case of a system incorporating only one EAM Typically, however, more than one EAM would be utilised, as described in more detail below 15 In Figure 9, a 1555nm DFB laser source 110 is coupled into an EAM 120 with a power level of -2dBm The EAM has a maximum extinction ratio of 20dB and a mean absorption characteristic of 2 5dB/V A 10 GHz sinewave drive 142 is synchronised with and passively added to a lOGbit/s data sequence from a data source 144 via a power splitter 140 lused in reverse to combine the two signals) 20 A suitable power splitter is the Wiltron K240B, available from Anntsu Wiltron Both the sinewave and data sequence signal levels have a 2 5V peak-to peak amplitude The resulting signal comprises a sinewave with an offset voltage determined by the data signal, with the relative amplitudes arranged such that the maximum value of the cycle for a data 0 is below the minimum level for a data 1 25 It will be appreciated that this precise arrangement is not necessary, and it would be sufficient to ensure that the entire 10 GHz cycle remains in the low absorption region of the modulator for a data 1 The electrical signal is applied to the EAM 1 20 and the DC bias is adjusted to ensure that the entire cycle for a data 1 gives low extinction, whilst the troughs of the cycle for a data 0 give a high extinction 30 Consequently, dark-pulses are formed for data O's, whilst a low extinction is maintained for data Vs An EAM suitable for use in the system is the one described in, for example, "Generation of 6 3 ps optical pulses at a 10 GHz repetition rate using a Pnr i"pd 11 oin Mimosa 08/05/l')99 16 19 04 p-Kje. -11- WO 97A1436 10 FCT/GB97/00520 packaged EAM and dispersion compensating fibre", Electronics Letters, Volume 30, pp 1700-1701, which is incorporated herein by reference The absorption characteristic of this EAM is reproduced in Fiqure 10 In Figure 10 it can be seen that the EAM has an operating region of low extinction at positive or low negative 5 reverse biases, an operating region of high extinction at high reverse biases, and an exponentially varying operating region in between It is the exponentially varying operating region of the EAM which supports the generation of soliton-like dark pulses In theory, the dark pulses generated should resemble inverted SECH2 10 pulses (that is to say, inverted solitons) having the form P(t) = (Peak Power) x (1 - SECH(1 76(t/x))2) (equation 1) where t represents the pulse width at half its peak power In operation, the EAM 15 is electrically biased to remain in its low loss condition unless both data and sinewave are negative That is to say, the EAM transmits light unless both electrical signal components are negative Thus, dark pulses are generated corresponding to data zeros, as illustrated in Figure 11, which closely resemble the theoretical form of equation 1 20 Figure 12 shows a system according to the present invention implementing three EAMs In the system, three EAMs 400, 410 and 420 are optically cascaded, or are arranged to be in optically coupled alignment, with an optical light source 100 comprising a 1555nm DF8 laser Conveniently, the laser light is coupled onto the first EAM 400, using a standard telecommunications 25 optical fibre 105, the light having a power level of -2dBm In this example, optical amplifiers 405, 415 and 425, for example EDFAs follow each EAM to compensate for any losses incurred in the EAMe The amplifiers are only incorporated if necessary to compensate for optical loss incurred by the EAMs As for the system illustrated in Figure 9, each EAM is driven by an electrical signal comprising a 30 sinewave component and a data component aligned in data channel slots A, B or C Since three electrical drive signals arc required, electrical timing circuitry 450 is required to ensure that the data signals A, B and C are aligned correctly with the Printed from MimosT 08/05/1999 16 39 04 -12- WO 9 7/31436 PCT/GB97/00520 1 1 sinewave and are aligned also in the correct slot positions of the required OTDM signal which is output downstream of the third amplifier 425 The light source can be separate from the modulator, the light from the light source being roupled into the modulator via, for example, an optical fibre as 5 described above However in an alternative arrangement the light source and the EAMs are fabricated as an integrated device on a common semiconductor substrate Apart from convenience, this arrangement has the advantage that coupling loss between each modulator and between the light source and thp first modulator is reduced Also, amplification, if necessary could be provided by 10 integrating SLAs (semiconductor laser amplifiers) between one or more EAMs The skilled person will appreciate that any form of optical, acousto optic or electro-optic modulator having the necessary transmission and extinction or switching properties to provide dark pulses would be suitable for implementing the present invention 15 The electrical bias scheme described above for driving the EAM is particularly advantageous for two reasons Firstly, only one electrical signal is required to bias each EAM and secondly the electrical signal does not require any electrical processing Electrical processing would be required if using the method described in "Generation of 2 5Gbit/s soliton data stream with an integrated laser 20 modulator transmitter , Electronics Letters, Volume 30, pp 1880-1881 EAMs suffer some optical loss even when operating in their low optical loss regions The amount of optical loss of an EAM is partly determined by the length of the optical modulator section through which light from a light source travels Thus, in known modulator schemes comprising two modulators or multiple 25 modulator sections, which firstly generate an optical pulse stream using an electrical sinewave drive signal, and secondly modulate data onto the pulse stream using an electrical data signal both modulators or both modulator sections, incur an optical insertion loss In that the proposed system only implements one modulator (section) per data channel, the system intrinsically incurs a lower 30 insertion loss overhead, regardless of the type of modulator used, than other schemes incorporating more than one modulator, or modulator section, to generate one data channel Printed from iliino"i OS/O^/ID^S 16 39 04 p ige -15- WO 97/31436 12 PCT/GB97/00520 As described above, the optical radiation is in the form of a substantially continuous burst The duration of the burst depends on the application For the example of a trunk communications network where traffic is likely to be present at most times, the optical source might remain on all the time Alternatively, for a 5 less busy optical link, the source might be switched on only when transmission of data, or part thereof (for a packet swwihed network for example), is required Therefore, 'substantially continuous' might be interpreted as continuous during data transmission Alternatively, the cw light input into the first EAM can be substituted for 10 an op"cal clock, for example a sinewave or pulse stream Then, with the same general system arrangement used for dark pulse generation, each EAM can be used to modulate one time slot of the optical clock That is to say, each EAM is arranged either to transmit, or prevent transmission, of light (the peaks or bright pulse portions of the clock signal) depending on the data encoding requirements of 15 its designated data channel For example, for a 100Gbit/s optical clock pulse stream, ten EAMs may be cascaded to encode ten 10Gbit/s channels Also, one or more EAMs operating according to this arrangement may be used as data insert devices for one or more channels in an OTDM system The skilled person would easily be able to implement data modulation or an insert function by applying the 20 theory disclosed by the present description The skilled person will also appreciate that the level-shifted sinewave bias signal described above for generating dark pulses would be suitable for generating bright pulses when applied to a single electrical input EAM The electrical signal would in this case need to be arranged to maintain the EAM in its high optical 25 extinction state unless both data and sinewave components were positive Such an arrangement would obviate the naed for electrical signal processing to generate a suitable bias signal, and would thus be a simple and robust solution Thus, this arrangement could be used for generation of solitons or a conventional OTDM signal 30 Figure 14 shows dark pulse generators incorporated in a network using the re-entrant bus topology described previously It differs from the networks considered previously, in that the time-slot into which data is added at a node transmitter can be chosen in the electrical domain Each node transmitter has a Priced from tlimor i 08/05/1099 lb 39 04 pige -14- WO 97/31436 13 PCT/GB97/00520 respective CAM connected in line with the optical fibre and with the EAM s of other node transmitters By contrast, in the receivers, the EAM's do not need to be in-line, and preferably are connected to the optical fibre by an optical tap in order to optimise the power budget As described above, and in our co pending 5 European patent application no 96301277 8 filed 26 Feb 1996, the contents of which are incorporated herein by reference, the network clock may be distributed along the optical fibre bus using a different polarisation state to distinguish it from the optical data on the bus It is preferred however that the bus should comprise two co located optical fibre waveguides, with one of the two waveguides 10 dedicated to carrying the clock signal as described in our ro pending European patent application no 96304694 1 filed 26 June 1996, also incorporated herein by reference As shown in Figure 15a, the two waveguides may be provided by twin cores 151, 152 within a single optical fibre 153 Alternatively, as shown in Figure 15b, two or more optical fibres 51a 51d may be co-located within a single sheath 15 53 using e g a blown fibre cable construction including aramid yarn reinforcement 54 The optical signal for modulation by the EAM of the dark pulse generator is then transmitted down one of the waveguides while the network clock signal is transmitted down another of the waveguides The network clock does not need necessarily to be a short optical pulse, since in this embodiment the clock is not 20 modulated directly to produce the data channels A relatively broad optical pulse or modulation of a CW beam may be used to generate the clock The network clock is converted to the electrical domain and a variable delay applied in order to generate a control signal for the EAM in the appropriate respective time slot This signal is modulated with RZ data received for example, from a PC data interface 25 of the type previously described If the electrical data is originally in an NRZ format, then conversion to RZ with a low duty cycle is required in order to produce narrow dark pulses fririf-J fio.ilirio. i 09/05/lf»53 10 J J ri , p irJ -1-- WO 97/31436 14 PCT/GB97/0052D </div>

Claims

<div class="application article clearfix printTableText" id="claims">



CLAIMS 

1 An optical network comprising a) an optical transmission medium, and b) a plurality of nodes connected to the optical transmission medium, 5 each of the plurality of node including a respective dark pulse generator which is coupled in line with the optical transmission medium and in series with the others of the dark pulse generators and which is arranged to generate dark pulses in an optical signal carried on the transmission medium 

10
2 A optical network according to claim 1, in which each node further comprises a variable delay stage which is arranged to apply a variable delay to a network clock signal in the electrical domain and which is connected at its output to the dark pulse generator 

15
3 A optical network according to claim 2, in which each node futher comprises a clock receiver for receiving a network clock signal carried on the optical transmission medium, the clock receiver including a photoelectric detector for converting the clock signal to the electrical domain 

20
4 A optical network according to any one of the preceding claims, in which optical transmission medium is an optical bus and a plurality of node transmitters are coupled to an upstream portton of the optical bus 

25
5 An optical network according to claim 4, in which a node comprises a node transmitter coupled to an upstream portion of the optical bus, and a respective node receiver coupled to a downstream portion of the optical bus 

30
6 An optical network according to claim 5, in which each of the said dark pulse generators comprises a single optical modulator 

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WO 97/31436 

15 

PCT/GB97/0Q«0
7 An optical network according to claim 6, wherein each of said generators is an electro-absorption modulator
8 An optical network according to any one of the preceding claims in which each of the nodes includes a node receiver, each node receiver comprising 

5 i) an input for an optical clock signal, 

n) a detector for converting the clock signal to the electrical domain, ml a variable delay stage for applying a selected delay to the clock signal in electrical domain, and iv) a non linear electro optic modulator including an optical input 10 arranged to receive an optical TDMA datastream and an electrical control input connected to the output of the variable delay stage, in use the electro optic modulator outputting a TDMA channel selected by setting the delay of the variable delay stage 

15
9 An optical network according to any one of claims 2 to 8, in which the variable delay stage comprises a plurality of logic gates, 

means connecting a first input of each date to an input path for the clock signal, 

20 control means connected to a second input of each gate, and means connecting outputs of the gates in common to an output path for the delayed clock signal, the said means connecting inputs and outputs of the gates to respective inputs and output paths being arranged to provide paths of different respective lengths via different gates in use the control means applying 25 control signals to the gates to select a path and a corresponding delay for the clock signal
10 An optical network according to any one of the preceding claims, including a source of substantially continuous bursts of optical radiation coupled to the 

30 optical transmission medium upstream of the node transmitters
11 An optical network according to any one of the preceding claims, in which the network has a re entrant bus topology 

Prm^d tiorn MitiiOi i 09/05/1998 16 39 0 1 p iqe -17- 

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16 

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12 An optical network according to any one of the preceding claims, in which the optical transmission medium includes a plurality of co-located waveguides 

5
13 A network interconnecting a plurality of computer systems and comprising an optical network according to any one of claims 1 to 1 2
14 A method of operating an optical network including a plurality of nodes connected to an optical transmission medium, the method comprising 

10 a) at one of the plurality of nodes, imposing dark pulses representing a data stream on an optical signal which is carried on the optical transmission medium, and b) at a subsequent node, receiving the optical signal including the dark pulses imposed in step (a) and imposing dark pulses on the optical signal in a 15 different respective time slot, thereby creating an OTDM {optical time division multiplexed) signal
15 A method according to claim 13, in which the said optical signal received at one of the plurality of nodes in step |a) comprises a continuous burst of optical 

20 radiation
16 A method according to claim 13, in which the said optical signal comprises an optical clock signal onto which data is imposed by arranging the or each dark pulse generator to block peaks of the optical clock signal 

25
17 A method according to any one claims 14 to 16, including steps of applying a variable delay to a network clock signal in the electrical domain, and applying the resulting signal as a control input to an electro-optic modulator the electo-optic modulator thereby gpnerating the said dark pulses 

30 

END OF CLAIMS 

Printed from Mimosa 03/05/1998 16 39 04 parjf> -18- 



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