Abstract:
A sub-harmonic clock signal is provided in a series of soliton optical pulses that are transmitted at a given line rate in a soliton optical transmission system. The line rate defines time slots of equal duration. Each soliton optical pulse in every N time slots is modulated in a manner to make the pulse distinguishable from pulses in other time slots. The frequency of the sub-harmonic clock signal is equal to the line rate divided by N. This technique of providing a clock signal allows simple recovery of the clock signal using a PIN diode photo detector and a bandpass filter of appropriate bandwidth.

Description:
FIELD OF THE INVENTION 
     This invention relates to clock signals in optical systems, particularly to transmitting a clock signal in soliton optical transmission system. 
     BACKGROUND OF THE INVENTION 
     In very long distance optical fiber links, it is known to use a soliton type of optical signal to minimize the effects of chromatic dispersion on the signal due to the dispersive properties of the fiber. A soliton type of signal makes use of the way in which the refractive index of the fiber varies with signal intensity in order to offset the dispersive effects, thereby preserving the spectral form of the signal as it propagates along the fiber. 
     In a transmitter of the system, a group of data channels are time division multiplexed into a single channel, typically having a bit rate of 100 Gb/s, and the information in the single channel is transmitted over the fiber by the soliton signal. In the optical link, in-line optical amplifiers, such as erbium doped fiber amplifiers (EDFA), amplify the soliton signal to compensate for line losses of the link. Regenerators may also be used, especially in very long links, to recreate the original soliton signal, thereby removing effects from propagation and amplification, such as timing jitter, noise, and minimal spectral dispersion. At the end of the link, a receiver demultiplexes the data channels from soliton signal. 
     Both the regenerator and the receiver require a clock signal at the full line rate, 100 Gb/s, in order to perform their functions. Further, the regenerator would require a 100 Gb/s electrical clock signal in order to regenerate the soliton signal without time division demultiplexing it into the separate data channels. However, generating a 100 GHz electrical clock signal from a 100 Gb/s soliton signal presents a problem because opto-electronic convertors (i.e. PIN diodes) that can operate at such a frequency are not available now, nor are they likely to become available in the near future. Furthermore, all-optical solutions for generating a 100 GHz clock signal are unattractive because of their complexity, size, and lack of stability. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide method and apparatus for communicating a clock signal in a soliton optical transmission system. 
     The invention provides a stream of soliton optical pulses having a spectral line in their frequency spectrum at the line rate of the pulses divided by an integer (N). To this end, the stream of soliton pulses is modulated with the clock signal, which is an Nth sub-harmonic of the line rate of the soliton pulses. Accordingly, the average energy of the pulses in every Nth time slot is distinct from the rest of the pulses in the other time slots. This distinction in the average energy can be detected, thereby allowing recovery of the clock signal. For example, in a 100 Gb/s soliton system, the pulse in every fourth time slot would be modulated thereby corresponding to a 25 GHz clock signal. 
     An advantage of communicating the clock signal by modulating the stream of soliton pulses is that it does not use additional fiber bandwidth, hence it does not adversely effect the throughput of the soliton system. That is, it does not reduce the rate at which soliton pulses can be transmitted across the fiber optic link. 
     Conveniently, the frequency of the clock signal is within the frequency range of currently available PIN diodes. An advantage of using a sub-harmonic clock signal of such a frequency is that an electrical clock signal can be easily obtained. Furthermore, the electrical clock signal can be up-converted to the full line rate for use in a regenerator. Moreover, such a solution is more attractive than all-optical solutions because it is less costly and less complex. 
     According to the invention the average energy in the Nth time slot is made distinct in a way that does not effect the propagation properties of the soliton pulses, thereby maintaining all the benefits that they provide. That is, modulated pulses in the Nth time slot remain soliton pulses and therefore they propagate along the fiber link as such. Accordingly, the pulses in the Nth time slot are modulated to change either their width-to-amplitude aspect ratio, their position within the time slot or their polarization. Alternatively, binary data symbols carried by the stream of soliton pulses could be encoded such that the data symbol carried by the pulse in the N/2th time slot has a probability greater than 0.5 of being the converse of the data symbol in the previous N/2th time slot. This encoding would also create a spectral line in the frequency spectrum of the pulses at the line rate divided by N. 
     According to an aspect of the present invention there is provided a transmitter for transmitting optical soliton pulses and providing a clock signal via the optical soliton pulses in a soliton optical transmission system comprising an optical soliton pulse source for generating optical soliton pulses at a first rate, the first rate defining time slots of equal duration; and a modulator for modulating each optical soliton pulse in every Nth time slot in a manner such that each said optical soliton pulse is distinguishable from optical soliton pulses in other time slots, whereby the clock signal has a frequency equal to the first rate divided by N, where N is an integer greater than one. 
     According to another aspect of the present invention there is provided a transmitter for transmitting optical soliton pulses over an optical fiber in a soliton optical transmission system comprising an optical soliton pulse source for generating optical soliton pulses at a first rate, the first rate defining time slots of equal duration; a data source for providing data symbols at the first rate; a plurality of modulators for modulating the optical soliton pulses in dependence upon the data symbols provided by the data source; and a first modulator of the plurality of modulators for modulating each optical soliton pulse in every Nth time slot in a manner such that said each optical soliton pulse is distinguishable from optical soliton pulses in other time slots. 
     According to another aspect of the present invention there is provided a receiver for receiving optical soliton pulses arriving at a first rate from an optical fiber in a soliton optical transmission system comprising means for recovering a clock signal from the optical soliton pulses, the clock signal having a frequency equal to the first rate divided by an integer N, wherein the integer N is greater than one; and a demultiplexer for demultiplexing the optical soliton pulses into a number of streams of optical soliton pulses responsive to the recovered clock signal, the number of streams being an integer multiple of the integer N. 
     According to another aspect of the present invention there is provided a method of encoding a clock signal in a soliton optical transmission system, the method comprising the steps of generating a series of optical soliton pulses at a first rate, the rate defining time slots of equal duration; and modulating each optical soliton pulse in every Nth time slot in a manner such that said each optical soliton pulse is distinguishable from optical soliton pulses in other time slots, where N is an integer greater than one. 
     According to yet another aspect of the present invention there is provided a method of recovering a clock signal in a soliton optical transmission system from a series of optical soliton pulses transmitted at a line rate, the clock signal having a frequency equal to the line rate divided by an integer N, wherein the integer N is greater than one, the method comprising the steps of receiving the optical soliton pulses; converting the optical soliton pulses to an electrical signal; filtering the frequency of the clock signal from the electrical signal to provide a filtered clock frequency signal; and amplifying the filtered clock signal frequency signal to provide a recovered clock signal. 
     According to still another aspect of the present invention there is provided a signal in an optical soliton transmission system comprising a series of optical soliton pulses transmitted at a line rate, the line rate defining time slots of equal duration, and each optical soliton pulse in every Nth time slot being distinguishable from optical soliton pulses in other time slots to encode a clock signal having a frequency of the line rate divided by N, where N is an integer greater than one. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further understood from the following description with reference to the drawings in which: 
     FIG. 1 is a block diagram of a soliton transmission system in accordance with an embodiment of the present invention; 
     FIG. 2 is a frequency spectrum plot of the soliton pulses in the system of FIG. 1; 
     FIG. 3 is a block diagram of the transmitter of FIG. 1; 
     FIG. 4 is a block diagram of the receiver of FIG. 1; 
     FIG. 5 is a block diagram of the clock recovery circuit of FIG. 4; 
     FIG. 6 is a block diagram of a first embodiment of the fourth modulator in FIG. 3; 
     FIG. 7 is a diagram of the soliton pulses in FIG. 1 using the modulator of FIG. 6; 
     FIG. 8 is a block diagram of a second embodiment of the fourth modulator of FIG. 3; 
     FIG. 9 is a diagram of the soliton pulses in FIG. 1 using the modulator of FIG. 8; 
     FIG. 10 is a block diagram of a third embodiment of the fourth modulator of FIG. 3; 
     FIG. 11 is a diagram of the soliton pulses in FIG. 1 using the modulator of FIG. 10; 
     FIG. 12 is a block diagram of a fourth embodiment of the fourth modulator of FIG. 3; and 
     FIG. 13 is a block diagram of a second embodiment of the clock recovery circuit of FIG. 4 corresponding to the modulator of FIG.  12 . 
     In the drawings, similar features are shown with like reference numerals. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a soliton optical transmission system  10  in accordance with an embodiment of the present invention. The system  10  includes a transmitter  12 , a receiver  14 , first, second, and third amplifiers  16 ,  18 , and  20 . The transmitter is connected to the first amplifier  16  via an optical fiber link  22 . The first amplifier  16  is connected to the second amplifier  18  via an optical fiber link  24 . The second amplifier  18  is connected to the third amplifier  20  by a path  26 , which includes at least one optical fiber link segment, but may also have amplifiers and regenerators connected by additional segments of optical fiber link. The third amplifier  20  is connected to the receiver  14  via an optical fiber link  28 . 
     In operation, a signal s comprised of soliton optical pulses, each pulse representing a data symbol and the pulse in every fourth time slot modulated by a clock signal, is output from the transmitter  12  onto the link  22 . The first amplifier  16  receives the signal s, amplifies it, and outputs a signal s′ onto the link  24 . The signal s′ has the same information content as the signal s, but is slightly different due to noise, timing jitter and minimal spectral dispersion resulting from propagation and amplification. Similarly, the signal s′ is input to the second amplifier  18  and an amplified signal s″ with the same information content as the signal s′ is output onto the path  26 . The third amplifier  20  receives the signal s″ from the path  26  or an equivalent representation of the signal s in the case of additional amplifiers (not shown) in the path  26 . The signal s′″ is output by the third amplifier  20  onto the link  28  and is received by the receiver  14 . Although they are not shown, additional amplifiers connected by optical fiber links could connect the third amplifier  20  to the receiver  14 . The receiver  14  receives the signal s from the link  28  and recovers the clock signal information from the soliton pulses in the signal s. The receiver then uses this information to generate a clock signal, which it uses in detecting the data symbol represented by each soliton pulse. 
     FIG. 2 is a frequency spectrum plot of the soliton pulses in the system of FIG.  1 . The frequency spectrum  30  of the soliton pulses has a uniform distribution from zero to 100 GHz, assuming that the value of each data symbol is more or less random. A spectral line  32  is shown present at 25 GHz, due to the clock signal modulation of the pulses. The spectral line  32  is shown as an increase in power compared to the remainder of the spectrum  30 . 
     FIG. 3 is a block diagram of the transmitter  12  of FIG.  1 . The transmitter  12  includes a soliton pulse source  40 ; a 1:4 optical splitter  42 ; first, second, and third modulators  44   a,   44   b,  and  44   c  respectively; a fourth modulator  46 ; a data source  48 ; a 4:1 optical combiner  50 ; and an optical amplifier  52 . The soliton pulse source  40  is connected to the optical splitter  42  via a fiber link  54 . The splitter  42  is connected to the modulators  44   a,    44   b,    44   c,  and  46  by fiber links  56 ,  58 ,  60 , and  62  respectively. The combiner  50  is connected to the modulators  44   a,    44   b,   44   c,  and  46  by fiber links  64 ,  66 ,  68 , and  70  respectively. The data source  48  is connected to the modulators  44   a,    44   b,    44   c,  and  46  via fiber links  72 ,  74 ,  76 , and  78  respectively. The amplifier  52  is connected to the combiner  50  by a fiber link  80 . The fiber link  22  from FIG. 1 is shown connected to the output of the amplifier  52 . 
     In operation, time division multiplexed data signals d 1  to d 4  are output from the data source  48  into the modulators  44   a,    44   b,    44   c,  and  46  over links  72 ,  74 ,  76 , and  78  respectively. Each of the data signals d 1  to d 4  has a bit rate of 25 Gb/s. 
     A stream p of 2 ps duration soliton optical pulses is output from the soliton pulse source  40  at 25 GHz into the splitter  42  over the link  54 . The splitter  42  divides the stream p into four optical signals of approximately equal power level, and the four signals are input to the modulators  44   a,    44   b,    44   c,  and  46  over links  56 ,  58 ,  60 , and  62  respectively. The modulators  44   a,    44   b,    44   c,  and  46  modulate the pulses in their respective input signals according to the data symbols present in the data signals d 1  to d 4 , respectively. Each of the modulators  44   a,    44   b ,  44   c,  and  46  performs modulation as follows: if a logic true data symbol is present in the respective data signal d 1  to d 4  then the pulse is passed through the modulator; however, if a logic false data symbol is present in the respective data signal d 1  to d 4  then the pulse is not passed through the modulator. The fourth modulator  46  performs an additional modulation, which will be described later, in order that each pulse in the fourth time slot is distinguishable from pulses in the other time slots. Modulated pulse streams pd 1 , pd 2 , pd 3 , and p′d 4  are output by the modulators  44   a,    44   b,    44   c,  and  46  to the links  64 ,  66 ,  68 , and  70 , respectively. 
     The combiner  50  optically combines the pulse streams pd 1 , pd 2 , pd 3 , and p′d 4  with appropriate delays and outputs a TDM signal having a line rate of 100 Gb/s onto the link  80 . The amplifier  52  receives the TDM signal from the link  80 , amplifies it, and outputs the signal s onto the link  22 . According to the above, the signal s, which has a line rate of 100 Gb/s, is comprised of soliton pulses, each pulse representing a data symbol and the pulse in every fourth time slot modulated by a 25 GHz clock signal. 
     FIG. 4 is a block diagram of the receiver  14  of FIG.  1 . The receiver  14  includes an amplifier  90 , a 1:2 optical splitter  92 , a clock recovery circuit  94 , a 1:4 demultiplexer  96 , a set of opto-electronic convertors  98  and a set of decoders  100 . The receiver  14  is coupled to the soliton transmission system  10  by the fiber link  28 , which is connected to the input of the amplifier  90 . The amplifier  90  is connected to the splitter  92  via a fiber link  102 . Links  104  and  106  connect the output of the splitter  92  to the clock recovery circuit  94  and the demultiplexer  96 , respectively. An output of the clock recovery circuit  94  is connected to the decoders  100  by a link  110 . Another output of the clock recovery circuit  94  is coupled to the demultiplexer  96  via a link  108 . Outputs of the demultiplexer  96  are connected to the opto-electronic convertors  98  by links  112 ,  114 ,  116 , and  118 . Links  120 ,  122 ,  124 , and  126  connect the outputs of the opto-electronic convertors  98  to the inputs of the decoders  100 . Outputs of the decoders  100  are connected to links  128 ,  130 ,  132 , and  134 . 
     In operation, the receiver  14  receives the signal s from fiber link  28 , and the signal s is input to the amplifier  90 . The amplifier  90  amplifies the signal s and provides an amplified signal s′ to the splitter  92  by way of the fiber link  102 . The splitter  92  receives the amplified signal s′ from the link  102  and splits it into two signals, s 1  and s 2 , of approximately equal optical power. The splitter  92  feeds the signal s 1  to the clock recovery circuit  94  and the signal s 2  to the demultiplexer  96  over the links  104  and  106 , respectively. The clock recovery circuit  94  detects the difference in average energy at 25 GHz, this difference due to modulation performed on soliton pulses in the fourth timeslot, and outputs a clock signal c onto the links  108  and  110 . The demultiplexer  96  uses four different phases of the clock c to time division demultiplex the signal s 2  into the pulse streams pd 1 , pd 2 , pd 3 , and p′d 4  and output them onto the fiber links  112 ,  114 ,  116 , and  118 , respectively. These four different phases of the clock c are generated internally by the demultiplexer using four different delay elements, as is known in the art. The opto-electronic convertors  98  receive the pulse streams pd 1 , pd 2 , pd 3  and p′d 4 , converts them to respective electrical signals e 1  to e 4 , and output the signals e 1  to e 4  onto their respective links  120 ,  122 ,  124 , and  126 . The opto-electronic convertors  98  have a bandwidth that is less than 100 GHz, typically their bandwidth would be in the order of 25 GHz, therefore they output a pulse that is longer in duration than a corresponding input pulse. The decoders  100  receive the signals e 1  to e 4  from the links  120 ,  122 ,  124 , and  126 , as well as the clock signal c from the link  110 . The decoders  100  use the 25 GHz clock signal c to sample their respective signal, e 1  to e 4 , during the approximate pulse duration midpoint, in order to determine the logic symbol carried by a pulse. Data symbols determined by the decoders  100  from signals e 1  to e 4  are output as the data signals d 1  to d 4  over the links  128 , 130 ,  132 , and  134 , respectively. 
     FIG. 5 is a block diagram of the clock recovery circuit  94  of FIG.  4 . The clock recovery circuit  94  includes a PIN diode  150  an electrical amplifier  152  a 25 GHz bandpass filter  154 , and a limiting amplifier  156 . The optical fiber link  104  provides optical signal stimulus to the PIN diode  150  and an electrical link  160  connects the output of the PIN diode  150  to the amplifier  152 . The output of the amplifier  152  is connected to the filter  154  via an electrical link  162 . Another electrical link  164  provides connection between the output of the filter  154  and the input of the limiting amplifier  156 . The output of the limiting amplifier  156  is connected to the decoders  100  via the link  110 . The link  108  shown in FIG. 4 to provide a connection between the demultiplexer  96  and clock recovery circuit  94  has been omitted for clarity, but is also connected to the output of the limiting amplifier  156 . 
     It should be noted that the filter  154  is a bandpass filter with a high Q-factor, typically in the order of 1000. Such filters are commercially available. A preferred type is one that uses a dielectric resonator for achieving a Q-factor of 1000 or greater. Further, the limiting amplifier  156  has very high gain such that an input signal of very small amplitude will cause the limiting amplifier  156  to output a signal that has a large voltage swing. In this way, amplitude variations in the input signal are suppressed such that the limiting amplifier  156  outputs a signal with consistent amplitude, which is desirable for a clock signal. 
     In operation, the clock recovery circuit  94  receives the signal s as input via the link  104 . Optical energy from the signal s applied to the PIN diode  150  is converted to electrical energy. This energy is transmitted by an electrical signal se to the amplifier  152  by way of the link  160 . The signal se contains a spectrum of the signal s in the region around 25 GHz. The amplifier  152  receives the signal se and outputs an amplified version se′ onto the link  162 . The signal se′ is input to the filter  154  by the link  162 . The filter  154  allows frequencies at and very near its center frequency, 25 GHz, to pass through it and onto the link  164 . In this way, a clock frequency fc that corresponds to the distinct average energy encoded in the fourth time slot, is detected from the signal s. The clock frequency fc is input to the limiting amplifier  156 , and the amplifier  156  amplifies it to produce a clock signal c. The clock signal c is output from the limiting amplifier  156  over the link  110  (and  108  not shown). 
     FIG. 6 is a block diagram of a first embodiment of the fourth modulator  46  in FIG.  3 . The modulator  46  includes a 0.25 dB attenuator  170  and a modulator  44   d  that is the same as the modulators  44   a  to  44   c  that were described previously. The attenuator  170  is connected to the modulator  44   d  via an optical fiber link  172 . The fiber link  62  provides connection between the attenuator  170  and the splitter  42  shown in FIG.  3 . The data signal d 4  is input to the modulator  44   d  via the link  78 . The output of the modulator  44   d  is connected to the combiner  50  via the fiber link  70 . 
     In operation, the stream p of soliton pulses is input to the attenuator  170  by way of the fiber link  62 . The attenuator  170  provides a 0.25 dB attenuation to the stream p and outputs an attenuated stream of soliton pulses p′ onto the link  172 . The attenuated stream of soliton pulses p′ is input to the modulator  44   d  via the link  172 . The modulator  44   d  modulates the stream of attenuated pulses p′ according to the data content in the data signal d 4  in the same manner as modulators  44   a  to  44   c,  the operation of which having been previously explained. The modulated pulse stream p′d 4  is output onto the link  70  and is applied to the combiner  50 , as shown in FIG.  3 . Accordingly, each modulated soliton pulse in the fourth time slot is attenuated by 0.25 dB. This attenuation causes the average energy of the pulses in the fourth time slot to be distinct from the average energy of pulses in the other time slots. In this way, the 25 GHz sub-harmonic clock signal c is provided in the signal s by the transmitter  12  and is detectable by the receiver  14 . 
     It should be noted that while the modulator  46  attenuates each soliton pulse by 0.25 dB, thus providing amplitude modulation of each pulse in the fourth time slot, amplification could alternatively be performed to achieve the same desired result. That is, to cause the average energy of the pulses in every four time slots to be distinct from the average energy of the pulses in the other time slots. 
     FIG. 7 is a diagram of the soliton pulses, not drawn to scale, in the signal s of FIG. 1 that result from using the first embodiment of the modulator  46  of FIG.  6 . The signal s is depicted as a sequence of soliton pulses. The sequence is a result of the combination of modulated pulse streams pd 1 , pd 2 , pd 3 , and p′d 4 , as shown in FIG.  3 . Four time slots are shown and are labelled t 1  to t 4 . A soliton pulse  176  having normal amplitude is shown in time slot t 1 . An attenuated soliton pulse  174  having an attenuation of 0.25 dB is shown in time slot t 4 . The absence  178  of a pulse is shown in time slot t 1 . It should be apparent from FIG.  7  and from the description of operation of the modulators  44   a  to  44   d  that not all instances of a time slot will contain a soliton pulse. Rather, the present or absence of a pulse in a particular instance of a time slot is dependent on the data symbol in a respective data signal d 1  to d 4 , as previously described. 
     FIG. 8 is a block diagram of a second embodiment  46 ′ of the fourth modulator  46  in FIG.  3 . The modulator  46 ′ includes a pulse broadening filter  180  and the modulator  44   d.  The filter  180  is connected to the modulator  44   d  via an optical fiber link  182 . The fiber link  62  provides connection between the filter  180  and the splitter  42  shown in FIG.  3 . The data signal d 4  is input to the modulator  44   d  via the link  78 . The output of the modulator  44   d  is connected to the combiner  50  via the fiber link  70 . 
     In operation, the stream p of soliton pulses is input to the filter  180  by way of the fiber link  62 . The filter  180  widens (or broadens) input soliton pulses in the time domain and outputs a broadened stream of soliton pulses p′ onto the link  182 . Typically, the filter  180  broadens pulses in the stream p by 0.25 dB. The broadened stream of soliton pulses p′ is input to the modulator  44   d  via the link  182 . The modulator  44   d  modulates the stream of broadened pulses p′ according to the data content in the data signal d 4  as previously explained. The modulated pulse stream p′d 4  is output onto the link  70  and is applied to the combiner  50 , as shown in FIG.  3 . Accordingly, each modulated soliton pulse in the fourth time slot is broadened by 0.25 dB, or about 0.12 ps for a 2 ps wide pulse. This broadening causes the average energy of the pulses in the fourth time slot to be distinct from the average energy of the pulses in the other time slots. In this way, the 25 GHz sub-harmonic clock signal c is provided in the signal s by the transmitter  12  and is detectable by the receiver  14 . 
     It should be noted that while the modulator  46 ′ widens each soliton pulse by 0.25 dB, thus providing pulse width modulation of each pulse in the fourth time slot, pulse narrowing could alternatively be performed to achieve the same desired result. That is, to cause the average energy of the pulses in every four time slots to be distinct from the average energy of the pulses in the other time slots. 
     Further, it should be noted that the energy of a soliton pulse is proportional to the product of its peak power and pulse width. In addition, the peak power of a soliton pulse is proportional to the inverse of the square of its pulse width. Therefore, the energy of a soliton pulse is proportional to the inverse of its pulse width. Thus, if each pulse in the fourth time slot is widened by 0.25 dB, then the peak power of each such pulse will be decreased by 0.5 dB, and the resulting energy of each such pulse will be 0.25 dB less than energy of the other pulses. Furthermore, these soliton pulses of decreased energy will propagate as stable soliton pulses and therefore, the combination of these two effects is advantageous. 
     FIG. 9 is a diagram of the soliton pulses, not drawn to scale, in the signal s of FIG. 1 that result from using the modulator  46 ′ of FIG.  8 . The signal s is depicted as a sequence of soliton pulses. The sequence is a result of the combination of modulated pulse streams pd 1 , pd 2 , pd 3 , and p′d 4 , as shown in FIG.  3 . Four time slots are shown and are labelled t 1  to t 4 . A soliton pulse  186  having a normal pulse width is shown in time slot t 1 . A broadened soliton pulse  184  is shown in time slot t 4 . As previously explained, the presence or absence of soliton pulses in instances of time slots is dependent on the value of the data symbols in the data signals d 1  to d 4 . 
     FIG. 10 is a block diagram of a third embodiment  46 ″ of the fourth modulator  46  in FIG.  3 . The modulator  46 ″ includes a delay element  190  and the modulator  44   d.  The delay element  190  is connected to the modulator  44   d  via an optical fiber link  192 . The fiber link  62  provides connection between the delay element  190  and the splitter  42  shown in FIG.  3 . The data signal d 4  is input to the modulator  44   d  via the link  78 . The output of the modulator  44   d  is connected to the combiner  50  via the fiber link  70 . 
     In operation, the stream p of soliton pulses is input to the delay element  190  by way of the fiber link  62 . The delay element  190  delays the input soliton pulses and outputs a delayed stream of soliton pulses p′ onto the link  192 . Typically, the delay element  190  delays pulses in the stream p by 10 to 20 percent of the time slot width, or 1-2 ps in the case of 10 ps wide time slot. The delayed stream of soliton pulses p′ is input to the modulator  44   d  via the link  192 . The modulator  44   d  modulates the stream of delayed pulses p′ according to the data content in the data signal d 4  in the same manner as was previously explained. The modulated pulse stream p′d 4  is output onto the link  70  and is applied to the combiner  50 , as shown in FIG.  3 . Accordingly, each modulated soliton pulse in the fourth time slot is delayed by 1 to 2 ps. This delay causes the average energy of pulses in the fourth time slot to be distinct from the average energy of pulses in the other time slots in that the location of average energy in the time slot is distinct. In this way, the 25 GHz sub-harmonic clock signal c is provided in the signal s by the transmitter  12  and is detectable by the receiver  14 . 
     It should be noted that while the modulator  46 ″ delays each soliton pulse by 1-2 ps, thus providing timing modulation of each pulse in the fourth time slot, delay of pulses in the other time slots could alternatively be performed to achieve the same desired result. That is, to cause the average energy of the pulses in every four time slots to be distinct from the average energy of the pulses in the other time slots. 
     FIG. 11 is a diagram of the soliton pulses, not drawn to scale, in the signal s of FIG. 1 that result from using the modulator  46 ″ of FIG.  10 . The signal s is depicted as a sequence of soliton pulses. The sequence is a result of the combination of modulated pulse streams pd 1 , pd 2 , pd 3 , and p′d 4 , as shown in FIG.  3 . Four time slots are shown and are labelled t 1  to t 4 . A soliton pulse  196  in time slot t 1  is shown having a normal pulse position in that time slot. A delayed soliton pulse  194  is shown in time slot t 4 . Note that the delayed soliton pulse  194  starts much after the beginning of the time slot t 4  as compared to the soliton pulse  196 . Again, as previously explained, the presence or absence of soliton pulses in instances of time slots is dependent on the value of the data symbols in the data signals d 1  to d 4 . 
     FIG. 12 is a block diagram of a fourth embodiment  46 ′″ of the fourth modulator  46  of FIG.  3 . The modulator  46 ′″ includes a polarization rotator  200  and the modulator  44   d.  The polarization rotator  200  is connected to the modulator  44   d  via an optical fiber link  202 . The fiber link  62  provides connection between the polarization rotator  200  and the splitter  42  shown in FIG.  3 . The data signal d 4  is input to the modulator  44   d  via the link  78 . The output of the modulator  44   d  is connected to the combiner  50  via the fiber link  70 . 
     In operation, the stream p of soliton pulses is input to the polarization rotator  200  by way of the fiber link  62 . The polarization rotator  200  changes the polarization of the input soliton pulses and outputs a polarization rotated stream of soliton pulses p′ onto the link  202 . Typically, the polarization rotator  200  rotates the polarization of the pulses in the stream p by 90 degrees. The polarization rotated stream of soliton pulses p′ is input to the modulator  44   d  via the link  202 . The modulator  44   d  modulates the stream of polarization rotated pulses p′ according to the data content in the data signal d 4  in the same manner as was previously explained. The modulated pulse stream p′d 4  is output onto the link  70  and is applied to the combiner  50 , as shown in FIG.  3 . Accordingly, each modulated soliton pulse in the fourth time slot has a polarization that is 90 degrees different than the soliton pulses in the other time slots, t 1  to t 3 . This difference in polarization causes the average energy of the pulses in the fourth time slot to be distinct from the average energy of pulses in the other time slots in that the average energy has a distinct polarization. In this way, the 25 GHz sub-harmonic clock signal c is provided in the signal s by the transmitter  12  and is detectable by the receiver  14 . 
     It should be noted that while the modulator  46 ′″ rotates the polarization of each soliton pulse, thus providing polarization modulation of each pulse in the fourth time slot, polarization rotation of the pulses in the other time slots could alternatively be performed to achieve the same desired result. That is, to cause the pulses in every four time slots to be distinct from the pulses in the other time slots. 
     FIG. 13 is a block diagram of a second embodiment  94 ′ of the clock recovery circuit  94  of FIG.  4  and is to be used in conjunction with the fourth embodiment of the modulator  46 ′″ of FIG.  12 . The clock recovery circuit  94 ′ includes a polarization controller  210 , a polarizing filter  212  and a clock recovery block  214 . The output of the polarization controller  210  is connected to the input of polarizing filter  212  by a fiber link  216 . The output of the polarizing filter  212  is connected to the input of the clock recovery block  214 . The clock recovery block  214  is the same as clock recovery circuit  94  with one exception. The exception is that an additional output that indicates the amplitude of the 25 GHz sub-harmonic clock signal c is provided and connected to the polarization controller via a link  220 . This additional output is taken from the output of the filter  154  at the link  164  shown in the clock recovery circuit  94  of FIG.  5 . The fiber link  104  connects the input of the clock recovery circuit  94 ′ to the splitter  92  as shown in FIG.  4 . The output of the clock recovery circuit  94 ′ is connected to the decoders  100  via the link  110  and to the demultiplexer  96  via the link  108 , as shown in FIG.  4 . 
     In operation, the signal s 1  is input to the polarization controller by the link  104 . The polarization controller  210  compensates for variations in the polarization due fiber links in the system  10  such as links  22 ,  24 ,  26 , and  28  of FIG.  1 . The polarization controller  210 , provided with an indication of amplitude of the 25 GHz sub-harmonic clock signal c via the link  220 , operates to keep the amplitude of the clock signal c at a maximum and outputs a compensated signal s 1 ′ that is a polarization compensated version of the signal s 1 . Such polarization controllers are known in the art. The compensated signal s 1 ′ is input to the polarizing filter  212  via the link  216 . The polarizing filter  212  filters out light from an input signal according to the polarization of the light. The polarizing filter  212  has been selected to correspond to the polarization rotator  200  of FIG. 12, such that only light having a polarization as set by the polarization rotator  212  will pass through the polarizing filter  212 . Consequently, a filtered signal s 1 ″ being primarily the modulated pulse stream p′d 4 , as shown in FIG. 12, is output from the polarizing filter  212  onto the link  218 . This filtered signal s 1 ″ is input to the clock recovery block  214 , which operates in the same manner and has the same structure as the clock recovery circuit  94  of FIG.  5 . In short, the input signal s 1 ″ is converted to an electrical signal se, this signal is amplified and input to a 25 GHz bandpass filter that outputs a clock frequency fc, which is amplified by a limiting amplifier to provide a clock signal c on the links  108  and  110 . In this way, the 25 GHz clock signal c is recovered by from the signal s 1  by the clock recovery circuit  94 ′. 
     Other techniques of causing the average energy of the pulses in every fourth time slot are possible. For example, coding could be performed on the data signals d 1  to d 4 , whereby data symbols in every alternate fourth time slot have a higher probability of having complementary values, such that a fourth sub-harmonic (25 GHz) of the line rate (100 GHz) is generated in the signal s of optical soliton pulses. Clearly, this could be done at line rates other than 100 GHz and for any arbitrary (Nth) sub-harmonic frequency of the line rate. Furthermore, framing could be provided so that some of the time slots are dedicated to carrying data, while the remainder of the time slots, for example every fourth time slot, carry the sub-harmonic clock signal. 
     Numerous modifications, variations, and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.