Abstract:
Optical signals used for telecommunications in time division multiplexed format are processed to remove channels for local demodulation and to replace the removed channels with locally modulated data. The received signals are subjected to a wavelength modulation so that signals in selected channels are given distinctive wavelengths. After wavelength modulation, the time division multiplexed signals are separated using wavelength sensitive splitters and the various channels are sent to their correct destinations. In one embodiment, one channel is selected and given a distinctive wavelength and all the non-selected channels retain their original wavelength. The channels with the original wavelength are provided to an output terminal for onward transmission whereas the selected channel is provided for local demodulation.

Description:
This application is a divisional of application Ser. No. 08/817,118, filed Apr. 8, 1997 now U.S. Pat. No. 6,0991,524, which itself is the national stage of PCT/GB96/01197, filed May 20, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the handling of optical telecommunications signals in digital form. More particularly it is concerned with optical signals in a time division multiplex format and on the separation of channels for supply to different terminal equipment. In many cases, one channel is removed and replaced by a new signal. 
     Optical telecommunication is particularly attractive because of the high speed of optical systems. In fact, optical systems have developed to the stage where it is difficult, sometimes impossible, to design electrical or electronic circuitry which is capable of matching the operating speeds of the fastest optical systems. In such fast systems it is appropriate or necessary that the signal processing be carried out by all optical equipment. 
     As mentioned above, this invention is particularly concerned with optical signals in time division multiplexed format. In such a format it is possible that each individual channel may be slow enough for processing in high speed electronic equipment but the multiplex may be too fast. For example, if four channels are multiplexed the bit rate of the multiplex will be four times the bit rate of each individual channel. Thus if each channel is operating at only 75% of the maximum speed available electronically the multiplex will be operating at 3 times the limit. In such a system it is clearly necessary that the multiplex be handled optically whereas the individual channels can be processed electronically. 
     It is appropriate to distinguish between two versions of time division multiplex format. These two versions are conveniently designated as “byte interleaved” and “bit interleaved”. The byte interleaved format is more familiar than the bit interleaved. Each “byte” comprises a plurality of bits, usually 8, and the byte represents a unit of transmission. In the case of digitised analogue signals, e.g. digitised audio or digitised video, each byte represents a single sample of the analogue signal. In the case of data transmission each byte usually represents a single symbol of the data, e.g. an alphanumeric character. In the byte interleaved version of time division multiplex format each slot of the multiplex relates to its own channel and it contains one byte relating to that channel. 
     The bit interleaved version of the time division multiplexed format is less familiar and each slot contains only one bit. The signals will normally consist of bytes as described above but each byte is spread over a plurality of slots (instead of the more usual version wherein the whole byte is contained in one slot). As mentioned above, a byte usually consists of 8 bits and, therefore, in bit interleaved format such a byte is spread over 8 slots. 
     “Electronic Letters” 30 (1994) 3rd February 1994 at pages 255 and 256 describes a laboratory experiment which demonstrated an all-optical time division multiplex to wavelength division multiplex conversion using four wave mixing in a semiconductor optical amplifier. The discussion is limited to demultiplexing and nothing is said about the removal and replacement of a channel. 
     SUMMARY OF THE INVENTION 
     This invention relates to techniques for the handling of high speed optical telecommunications signals. 
     This invention, which is more fully defined in the claims, relates to 
     (a) optical switching means for separating channels from optical signals in time division multiplex format, and replacing the removed channels by new signals modulated with local data, 
     (b) telecommunications stations which include the switching means, and 
     (c) telecommunications systems which include the stations. 
     The invention also includes methods of handling optical telecommunications signals in time divisional multiplex format. 
     The invention is based upon applying wavelength modulation to optical signals which are already modulated with data in a time division multiplex format. The modulation applies characteristic wavelengths to different channels of the multiplex. For example, to achieve the separation a primary wavelength is applied to all channels except selected channels and a complementary wavelength is applied to the selected channels. Having applied the wavelength modulation, the channels are separated by a suitable network of wavelength selective splitters so that signals having the primary wavelength go to one port and signals with the complementary wavelength go to a different port. This achieves the separation and the separated signals can, if desired, be converted into electrical form for further processing. The replacement is achieved by generating new signals at the primary wavelength in synchronisation with received time division multiplex. The wavelength selective splitters provide the new signals to the correct output terminal with appropriate synchronisation. 
     In preferred embodiments of the invention the wavelength modulation is achieved utilising clock signals generated in synchronisation with the received time division multiplex. The clock signals includes the wavelength modulation which defines the intended separation of the time division multiplex signals. The clock signals and the time division multiplex are combined preferably using an optical AND-gate. (An optical AND-gate has two input terminals, i.e. one for the time division multiplex and one for the clocks signals. The AND-gate produces an output signal when both of its inputs receive a signal. When an output is produced the output has the same wavelength as the clock signal. It will be appreciated that an AND-gate of this nature makes the appropriate combination of wavelength and data modulation). 
     This invention is particularly suitable for use in conjunction with signals which have a pulsed waveform. That is to say each timeslot potentially (depending on the data modulation) contains a signal pulse which has a low, preferably zero, intensity at the beginning of the slot. The intensity rises to a maximum within the slot, preferably at the middle of the slot, and then the intensity becomes low, preferably zero, at the end of the slot. It is emphasised that, in real transmissions, timing is unlikely to be prefect and the timing imperfections are often designated as “jitter”. It is emphasised that while it is desirable to make the timing as accurate as possible satisfactory communication is maintained provided that the intensities at the beginning and end of slots are sufficiently low and the intensities in the middle of the slots are sufficiently high. 
     Both the time division multiplex signals and the clock signals have the same basic pulsed waveform but the nature of the modulation is different in each case. In the case of the traffic signals all the pulses have the same wavelength and the modulation takes the form of the presence and absence of pulses. The presence of a pulses usually indicates a logical “one” and in that case the slot contains a pulse as described. Other slots relate to a logical “zero” and in this case there is no pulse in the relevant slot, e.g. the intensity remains low, (preferably zero) throughout the slot. In the case of clock signals there is a pulse as described in every time slot but the pulses have different wavelengths to define the destination of signals in that particular slot. Because a clock signal is separately generated at each location the clock signals should be subject to less jitter than the traffic signals. 
     With pulsed signals as described the function of the AND-gate can be defined as follows. When a pulse is received at both terminals the clock pulse is transmitted so that the output has the same wavelength as the clock pulse. When a clock pulse is received in the absence of a traffic pulse the AND-gate has no output and nothing is transmitted. (The possibility that there is no clock pulse need not be considered since there is a clock pulse in every timeslot). 
     The AND-gate is conveniently implemented as a loop mirror which contains a semi-conductor amplifier located therein preferably symmetrically. A loop mirror involves a waveguide which is fed from both ends simultaneously so that it contains counter propagating pulses. More specifically the loop mirror is fed by means of a splitter. The splitter receives a single pulse which is divided into two, preferably equal, pulses which are fed to opposite ends of the waveguide. In the simple case the loop is symmetrical and it has the effect that pulses received at the splitter are returned from whence they came, i.e. the device acts as a reflector or mirror. It is possible to place a semi-conductor amplifier within the loop without disturbing the symmetry and the loop will still act as a mirror. To make a loop serve as an AND-gate the clock pulses are fed to the splitter. The symmetry of the mirror can be modified by providing the traffic pulses to the semiconductor amplifier. When a traffic pulses passes through the amplifier, the symmetry is disturbed and the result is that clock pulses are transmitted. It can therefore be seen that the modified loop mirror functions as the required AND-gate. 
     The clock signals are conveniently generated from lasers which operate at the frame rate. As has been explained above, the frame rate is low enough for electrical control. Thus each laser initially produces one pulse per frame and these pulse are passively divided to give one pulses per slot for each laser. The use of differential delay lines times the individual pulses so that there is one pulse per slot. It is convenient to use synchronised lasers for the various wavelengths needed in the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a diagram illustrating a first embodiment of the invention; 
     FIG. 2 is a diagram illustrating one form of clock; 
     FIG. 3 is a diagram illustrating a preferred embodiment of the invention; 
     FIG. 4 is a diagram of a modified loop mirror suitable for use as the AND-gates shown in FIGS. 1 and 3; and 
     FIG. 5 illustrates a telecommunications system including 8 stations in accordance with invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a generalised form of the invention which is adapted to receive two distinct optical traffic streams each of which is in time division multiplex format. For this description it will be assumed that there are four channels in each of the traffic streams; i.e. each multiplex is made up of frames each containing four slots each slot relating to a different channel. Preferably there is one bit in each slot and it will be appreciated that this constitutes the bit interleaved version of a time division multiplex. The slot rate is clearly faster than the frame rate. In an example where there are four slots per frame the slot rate is four times the frame rate, e.g. a slot rate of 20 GHz corresponds to a frame rate of 5 GHz . The two input streams are synchronised to one another, i.e. individual slots occur at the same time. 
     The switching means shown in FIG. 1 comprises two optical AND-gates  13  and  14 . Gate  13  has a control port  15  to receive one of the input signals whereas gate  14  has a control port  18  to receive the other. The switch also comprises clock means  10  which generates a pair of complementary clock signals which are synchronised to the input signals and hence to one another. The clock signals use two different wavelengths, hereinafter distinguished as A and B, and they are complementary in that they are formed of pairs of pulses one of each wavelength. Gate  13  receives one of the clock signals at its input port  16  and gate  14  receives the complementary clock signal at its input port  19 . The output ports  17  and  20  of the gates  13  and  14  are connected to wavelength sensitive splitters  21  and  22  respectively. 
     The splitters  21  and  22  are connected to provide signals at wavelength A to terminal  25  and signals at wavelength B to terminal  26 . More specifically, the splitters  21  and  22  have A-outputs for wavelength A and B-outputs for wavelength 
     B. The two A-outputs are connected to the two inputs of a junction  23  having its output connected to the terminal  25  and the two B-outputs are connected to the two inputs of a junction  24  having its output connected to the terminal  26 . 
     The operation of the switch will now be described. The AND-gates  13  and  14  provide output only when both the control port and the input port receive optical energy. Under this circumstance the AND-gate passes the clock pulse to output and this output has the same wavelength as the clock pulse. Consider the operation when ports  15  and  18  both receive signal pulses with a clock pulse of wavelength A at port  16 . Since the clock signals are complementary, port  19  will receive a clock pulse of wavelength B. The result is that port  17  passes a signal pulse at wavelength A to terminal  25  via splitter  21  and junction  23  and port  20  passes a signal pulse at wavelength B to terminal  26  via splitter  22  and junction  24 . In the alternative configuration, port  1   6  receives wavelength B while port  19  simultaneously receives wavelength A. The operation is substantially the same as before but the signal received at port  15  acquires wavelength B so that it goes to terminal  26 . Similarly, the signal received at port  18  acquires wavelength A so that it goes to terminal  25 . Thus the primary and complementary clock signals control the switching means slot by slot so that a slot received at port  15  is directed to terminal  25  or  26  as required while the slot simultaneously received at port  18  is directed to the other terminal. 
     The overall performance of the switching means is primarily determined by the slot-by-slot operation described above. Because AND-gates are used the signals at ports  17  and  20  are (partly) controlled by the received traffic. The traffic is modulated by the presence or absence of pulses and this modulation is transferred to the output ports  17  and  20 . In addition the signals at each of the terminals  25  or  26  have only one wavelength because the splitters  21  and  22  are wavelength selective. As explained above, it is the traffic which controls the data modulation but it is the clock pulses which affect the pulse wavelength, shape and timing. It will be appreciated that the AND-gates are acting as optical regenerators. 
     The switching means shown in FIG. 1 redistributes two optical time division multiplexed signals to two terminals, the destination of any pair of input slots being independently controlled by the primary and complementary clock signals. A preferred form of clock will now be described with reference to FIG.  2 . This clock is suitable for use as the clock  10  of FIG.  1 . 
     The clock illustrated in FIG. 2 includes two lasers  30 A and  30 B. Laser  30 A operates at wavelength A whereas laser B operates at wavelength B. The lasers A and B have a common drive  29  so that they are synchronised. Conveniently, the drive  29  receives a sample of traffic (connections not shown) to facilitate synchronisation. More specifically, each laser produces one pulse per frame of the multiplexed signals. Using splitters, the primary pulses are divided to give one pulse (of each wavelength) per slot. It is emphasised that although each laser only produces one pulse per frame the duration of each pulse is less than the slot duration. On division, the pulses occur simultaneously but the divided pulses are fed to delay lines (32.1-32.4 for the primary clock and 33.1-33.4 for the complementary clock) so that they are separated by the slot interval. 
     As can be seen from FIG. 2, the clock comprises lasers  30 A and  30 B as well as a plurality (collectively indicated by the numeral  31 ) of 2×2 optical switches, i.e. one for each slot. This example assumes four slots per frame so there are four optical 2×2 switches  31 . 1 ,  31 . 2 ,  31 . 3  and  31 . 4 . which are associated with frames  1 ,  2 ,  3  and  4  respectively. The designation 2×2 specifies that the switch has two optical input ports and two optical output ports. One input signal goes to each of the outputs and when the switch inverts the output signals exchange output ports. Each of the switches  31 . 1  to  31 . 4  has one of its input ports connected to laser  30 A and its other input port is connected to laser  30 B. 
     The clock also comprises two sets of delay lines generally indicated by numerals  32  and  33 . Delay lines  32  provide the primary clock signal and delay lines  33  provide the complementary clock signal. With four slots per frame, each set of delay lines comprises four optical fibres of graded lengths. The grading takes the form of equal length steps each step corresponding to a transmission time equal to the bit period. Set  32  consists of fibres  32 . 1 ,  32 . 2 ,  32 . 3  and  32 . 4  providing the delays needed for slots  1 ,  2 ,  3  and  4  in the primary clock signal. Set  33  consists of fibres  33 . 1 ,  33 . 2 ,  33 . 3  and  33 . 4  providing the delays needed for slots  1 ,  2 ,  3  and  4  in the complementary clock signal. Each of the switches  31  has one output port connected to a fibre in set  32  and the other output port connected to the corresponding fibre of set  33 . Thus, in one configuration, switch  31 . 1  connects laser A to fibre  32 . 1  and laser B to fibre  33 . 1 ; in its other configuration switch  31 . 1  connects laser B to fibre  32 . 1  and laser A to fibre  33 . 1 . Similar connections apply to all the switches  31  and all the fibres  32  and  33 . 
     Because of their stepped lengths, the fibres  32  and  33  delay the pulses so that each clock signal has one pulse in each time slot, the wavelengths of the pulses being determined by the settings of the switches  31 . At their output ends, the fibres  32  are connected to fibre  11 . Because of the graded delays fibre  11  receives the pulses in sequence, i.e. the primary clock signal is assembled in fibre  11 . Similarly, the fibres  33  are connected to fibre  12  where the complementary clock signal is assembled. As stated, the clock of FIG. 2 is suitable for the clock  10  of FIG.  1  and it will be appreciated that it provides the primary and complementary clock signals as described with reference to FIG.  1 . Although the combination is not separately illustrated, connecting fibres  11  and  12  of FIG. 2 to AND-gates  13  and  14  of FIG. 1 provides the operation already described. 
     An alternative configuration, not shown in any drawing, distributes one stream of received traffic to two terminals and refills the empty slots with locally modulated signals. In this modification AND-gate  14  is not needed because there is no traffic for it. It is replaced by a modulator which modulates local data onto the complementary clock signal. The modulator is a modification (not separately illustrated) of the clock shown in FIG.  2 . In this modification each of the delay fibres  33  includes its own independent modulator which selectively passes (or fails to pass) clock pulses to modulate local data. Four different data channels can be modulated, i.e. one onto each of fibres  33 . It is emphasised that each modulator only operates at the frame rate because the complete clock signal is carried in four parallel fibres. This modulator is connected to a modified FIG. 1 as follows. Fibre is connected to input port  1   6  whereby the input signal is distributed to terminals  25  and  26  as described. There is no input for input port  18  so AND-gate  14  is not needed. Instead, fibre  12  of the (modified) FIG. 2 is connected to splitter  24  of FIG.  1 . Fibre  12  carries a locally modulated signal in time division multiplex format. The overall result is that the primary clock divides the input signal between terminals  25  and  26  in accordance with the settings of the switches  31 . The complementary clock divides the locally modulated data so as to fill the missing slots. 
     In a particularly important application only one slot is removed from the (single) input signal. This slot is provided at terminal  26  for demodulation. The other slots are provided to terminal  25  for onward transmission and the missing slot is replaced by a locally modulated signal produced from the complementary clock signals. This requires a combination of the clock of FIG. 2 (as shown) and FIG. 1 modified by the omission of gate  14 . A station including this combination is illustrated in FIG.  3 . 
     As shown in FIG. 3 the station comprises lasers  30 A and  30 B and four 2×2 switches  31 , each switch being connected to both lasers. Each switch  31  is also connected to a pair of delay fibres, one in set  32  and the other in set  33 . Thus, as described in greater detail with reference to FIG. 2, a primary clock signal is provided on fibre  11  and a complementary clock signal is provided on fibre  12 . 
     The station receives traffic, in time division multiplex format (four slots per frame), on transmission fibre  38  which is connected to control port  15  of AND-gate  13 . The output port  17  is connected to a wavelength sensitive splitter  21  having an output for wavelength A connected (via junction  23 ) to a terminal  25  and an output for wavelength B connected to terminal  26 . Terminal  25  is connected to demodulator  37  which makes received data available locally. Terminal  25  is connected to transmission fibre  39  for sending traffic to other stations (not shown in FIG.  3 ). This part of the station is as shown in FIG.  1  and it operates in the same way. 
     Fibre  12  (which carries the complementary clock signals) is connected, in sequence, to a band pass filter  34  (which passes wavelength A but excludes wavelength B), to a modulator  35 , to junction  23  and terminal  25  so that its modulated output is passed to transmission fibre  39 . Modulator  35  is controlled by local data source  36 . 
     The station illustrated in FIG. 3 can be used for two-way communication with a partner (not shown). This communication makes use of one channel out of the four of the multiplex format and it is desired to drop this channel for reception and to replace it for transmission. The other channels are regenerated for onward transmission. In order to provide this mode of operation, one channel is selected for “drop and replace”. The channel is selected by the settings of the four 2×2 switches. The switch corresponding to the selected channel is set to connect laser  30 B to the delay fibres  32  (and laser  30 A to the corresponding delay fibre of set  33 ). All the other 2×2-switches correspond to non-selected channels. All of these are set to connect laser  30 A to the delay fibres  32  (and laser  30 B to the delay fibres  33 . 
     In operation, the traffic is received at input port  15  via transmission fibre  38  and the non-selected pulses, i.e. pulses in the non-selected channels, coincide with clock pulses having wavelength A (because of the settings of 2×2-switches). Thus these pulses are regenerated as described above and the regenerated pulses have wavelength A whereby they are routed to terminal  25  for onward transmission on transmission fibre  39 . For these non-selected pulses the device acts as a regenerator and they pass through without further modification. 
     In the selected channel, the pulses are regenerated at wavelength B whereby they pass to terminal  26  and to demodulator  37  for local use. In the complementary clock signals, on fibre  12 , the selected slot contains wavelength A and these pulses get through band pass filter  34 . The non-selected slots contain wavelength B and these pulses are blocked by the band pass filter  34 . Thus modulator  35  receives one pulse per frame and this pulse is at wavelength A. Data from  36  is applied and the modulated signals are provided to junction  23  and thence to transmission fibre  39 . These pulses are timed to coincide with the gap where the selected regenerated pulse was removed for demodulation. Thus the device provides a “drop and replace” function for one channel which is used locally. The other channels are regenerated. 
     It is emphasised that gate  13  is the only active component which operates at the bit rate. The lasers  30 A and  30 B as well as the modulator  35  all operate at the frame rate. The switches  31  remain passive for most of the time. They are only actuated to change the operational configuration and this is infrequent. 
     It is emphasised that a station does not have to participate the whole time. Each station has a non-participatory mode in which it merely passes on, unchanged, signals which are regenerated. For the non-participatory mode no channel is set to wavelength B; i.e. all the clock pulses have wavelength A; i.e. all the 2×2-switches  31  are set to connect laser  30  to delay lines  32 . From the description given above, it will be appreciated that all the received traffic goes to terminal  25  for onward transmission. 
     FIG. 4 shows a loop mirror which is suitable for the AND-gate illustrated in FIG. 1 and 3. As shown in FIG. 4 fibres  11  and  17  are linked into a symmetrical splitter  51  which is connected to the opposite ends of a fibre loop  53 . Symmetrically placed in the loop  53  there is a travelling wave semi-conductor laser amplifier  52  which is also connected to receive traffic on fibre  38 . In use, clock pulses received on fibre  11  are split into two equal parts by the splitter  51  and caused to travel in opposite directions around the loop  53 . Since the amplifier  52  is symmetrically placed the split pulses tend to arrive back at the splitter  51  simultaneously and there is no output on fibre  17 . When a traffic pulse is received on fibre  38  the performance of the amplifier  52  is affected and the symmetry is spoiled. The result is that a clock pulse is provided on fibre  17 . It will be appreciated that the loop device shown in FIG. 4 meets the requirements of the AND-gates as described above. 
     A telecommunications system including 8 stations each as illustrated in FIG. 3 is shown in FIG.  5 . The stations, numbered  101  through to  108 , are connected into a loop for the anti clockwise propagation of signals as indicated in FIG.  5 . The system utilises 4 channels in a time division multiplexed format. Although there are only 4 channels it is possible to have 8 stations because each channel is used for two-way communication, i.e. each channel is used by 2 stations. As has been explained above, any one of the stations  101 - 108  can be temporarily configured to drop and insert any one of the 4 channels. This makes possible any of the pairings of the stations which, from time to time are needed for communication. 
     Consider the case where station  101  and station  104  are connected for two-way communication in the first channel. Station  104  selects channel  1  for drop and insert. That is primary clock pulses in channel  1  are given wavelength B and the complementary clock has wavelength A in channel  1 . The result is that channel  1  is removed for local use (by station  104 ) and it is replaced by locally generated signals. These signals are passed via stations  105 ,  106 ,  107  and  108  to station  101 . Station  101  adopts the same operational mode as station  104  so the signals inserted by station  104  are removed for reception at station  101 . At the same time these signals are replaced with data generated at station  101  and passed via stations  102  and  103  to station  104 . It can therefore be seen that stations  101  and  104  achieve two-way communication on channel  1  and, although this communication is transmitted via the other stations, the other stations do not interfere with it. It is also clear that the other stations can establish two-way communication, in any combination, using channels  2 ,  3  and  4  of the time division multiplex format. It will also be realised that any station which, temporarily, has no reception or transmission can adopted the non-participatory format described above. 
     With four channels it is clear that no more than eight stations can participate at any one time. Nevertheless, more stations than are shown in FIG. 5 can be connected in to the system because it is unlikely that any station will wish to transmit all the time. If more than eight stations are connected it is, of course, necessary that some of them will have to adopt the non-participatory configuration but all the stations can take turns to communicate. 
     It will be appreciated that the system as a whole may require some form of supervision which is not illustrated in the drawings.