Abstract:
Polarization mode dispersion in an optical signal transmitted through a waveguide of a communications system is compensated by separating the dispersed signal into components corresponding to principal polarization states. The components are delayed by respective delays differing by a delay increment which is controlled to correspond to the dispersion delay and the delayed components are recombined to provide a dispersion compensated optical output signal. Each of the delays is provided by an chirped Bragg reflector forming part of a delay line, the Bragg reflectors comprising optical fibres with chirped intracore index gratings. Transducers or temperature controllers acting on one of the fibres allows dimensional control of the grating periodicity such that the position of Bragg reflection is variable. Wavelength division multiplexed optical signals are compensated using sampled gratings which allow a common Bragg reflection position for each wavelength.

Description:
FIELD OF THE INVENTION 
     This invention relates to compensating for polarisation mode dispersion in an optical signal of an optical communications system and to an optical element for providing a variable differential delay primarily but not exclusively for use in such polarisation mode dispersion compensation. 
     BACKGROUND TO THE INVENTION 
     Existing communications systems typically rely for transmission over long distances upon the use of nominally single mode optical fibres which carry optical signals and provide transmission of signal data at 10 Gb/sec or more over distances of the order of 100 km. Although such fibres are nominally single mode, propagation of optical signals is generally characterised in such fibres by two orthogonally polarised HE 11  modes for which slightly different group velocities exist in the presence of birefringence. 
     For a given span of optical fibre, the difference in transmission time for these modes is termed polarisation mode dispersion. 
     For the given span of optical fibre, it is possible to define a pair or orthogonal principal polarisation states such that an optical pulse launched into the fibre in only one of the principal polarisation states will be received at the other end of the fibre without polarisation mode dispersion being evident, the principal polarisation states therefore representing the fast and slow axis modes of propagation. In practical systems however, it is difficult to control the launch state to always correspond to one of the principal polarisation states so that an optical signal typically comprises the sum of fast and slow mode components. 
     Environmental factors affecting the optical fibre produce variation over time in the birefringence effects causing polarisation mode dispersion and the resulting dispersion is observed to vary relatively slowly for fibres in buried cables and more quickly for fibres contained in overhead cables. 
     It is known from U.S. Pat. No. 5473457, Ono, to analyze a received optical signal in a manner which permits the principal states of polarisation to be determined and the received pulse separated into fast and slow mode components, the fast mode component then being subject to a compensating delay by means of transmission of both components through a polarisation maintaining optical fibre of predetermined length and high polarisation dispersion to provide a differential delay. This technique however has the disadvantage of making available only a fixed amount of compensation and therefore does not allow variable compensation of polarisation mode dispersion suitable for a practical communications system. A further disadvantage is that a delay element providing optical delay by transmission via a fibre will typically require a relatively long length of fibre in the range 10 to 100 meters. 
     It is known from WO 97/50185 to compensate for polarisation mode dispersion by splitting the received optical signal at the receiver into two polarisation states and to apply switched delays of different length to the separated components, thereby providing a variable delay. A disadvantage of this system is that the delay is not continuously and smoothly variable and also requires a relatively complex optical switching configuration. 
     The inventor of the present invention has previously disclosed in U.S. Pat. No. 4953939 the use of a chirped Bragg grating reflector in combination with a directional coupler to introduce a delay which is wavelength dependent because the periodicity of the Bragg grating varies with position along the fibre so that different wavelengths are reflected from different positions along the fibre. The inventor has also disclosed in U.S. Pat. No. 5602949 a technique for manufacturing a suitable chirped fibre by producing strain in a non-chirped fibre for this purpose. GB-A-2316761 discloses a method of directly writing a chirped grating into a fibre for the purpose of compensating chromatic dispersion. 
     There remains a need to provide an improved method of providing a continuously variable optical delay and for compensating for polarisation mode dispersion in optical fibres. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus of compensating for polarisation mode dispersion in a manner which adapts continuously to environmental changes affecting birefringence of the optical fibre. 
     It is a further object of the present invention to provide an improved optical element for providing a variable delay in an optical signal. 
     According to the present invention there is disclosed apparatus for compensating for polarisation mode dispersion in an optical signal transmitted via an optical waveguide of an optical communications system, the apparatus comprising; 
     separating means operable to separate the optical signal into first and second components having a dispersion delay therebetween; 
     delay means operable to delay the first and second components by respective delays differing by a delay increment; 
     recombining means operable to recombine the delayed first and second components to form an output signal; and 
     control means operable to control the delay increment such that in the output signal the dispersion delay is substantially compensated by the delay increment; 
     wherein the delay means comprises at least one delay line having a chirped Bragg reflector extending longitudinally of the delay line, wherein said respective delay results from a time taken for said respective component to traverse the delay line to and from a reflection position at which Bragg reflection occurs, and wherein the control means is operable to variably regulate at least one parameter determining the conditions under which Bragg reflection occurs to variably define the reflection position relative to the Bragg reflector. 
     Preferably the parameter controlled by the control means is a characteristic of spatial periodicity as a function of position of a chirped refractive index grating formed in an optical medium of the Bragg reflector. 
     Conveniently the control means produces strain in the optical medium of the Bragg reflector, thereby providing a shift in the position of the Bragg reflection which is sensitive to a control signal to an actuator producing the strain. 
     Conveniently the Bragg reflector is formed as a refractive index grating in an optical fibre and the control means may comprise an electromechanical actuator deforming the fibre or a temperature controller allowing dimensions of the fibre to be regulated by thermal effects. 
     The optical fibre may comprise a sampled grating having a series of pass bands giving corresponding positions of Bragg reflection for different wavelengths of a wavelength division multiplexed optical signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of part of an optical communications system including a polarisation mode dispersion compensator; 
     FIG. 2 is a schematic diagram showing in greater detail the polarisation mode dispersion compensator of FIG.  1  and including a differential delay system; 
     FIG. 3 is a schematic diagram showing a differential delay system for use in the compensator of FIG.  2  and comprising a differential delay element formed by a pair of chirped Bragg grating fibres; 
     FIG. 3 a  is a schematic graphical representation of grating pitch as a function of position for the differential delay element of FIG. 3; 
     FIG. 4 a  is a schematic graphical representation of the spectral profile of a grating fibre; 
     FIG. 4 b  is a schematic graphical representation of the refractive index modulation of a fibre grating; 
     FIG. 4 c  is a schematic graphical representation of differential delay as a function of wavelength in a chirped fibre grating; 
     FIG. 5 a  is a schematic graphical representation of part of the spectral response of a sampled grating fibre; 
     FIG. 5 b  is a schematic graphical representation of the refractive index profile of the sampled grating fibre of FIG. 5 a;    
     FIG. 5 c  is a schematic graphical representation of the delay characteristic as a function of wavelength for the sampled grating of FIGS. 5 a  and  5   b;    
     FIG. 6 is a schematic representation of an alternative polarisation mode dispersion compensator using a single optical circulator; 
     FIG. 7 is a schematic representation of a differential delay element with a single grating fibre; 
     FIG. 7 a  is a schematic graphical representation of the variation of grating pitch with position for the element of FIG. 7; 
     FIG. 8 is a schematic representation of a differential delay element having an electro-mechanical transducer; 
     FIG. 9 is a schematic representation of a further differential delay element having a thermal transducer; 
     FIG. 10 is a schematic representation of a further differential delay element having fused fibres; 
     FIG. 10 a  is a schematic graphical representation illustrating the variation of pitch with position for the fibre of FIG. 10; 
     FIG. 11 is a schematic representation of a further alternative differential delay element having a polarisation maintaining fibre; 
     FIG. 11 a  is a schematic graphical representation of the grating pitch as a function of position for the separate modes conducted via the fibre of FIG. 11; 
     FIG. 11 b  is a schematic representation of the fibre of FIG. 11; 
     FIG. 12 is a schematic representation of an alternative embodiment of the polarization mode dispersion compensator of FIG. 1; and 
     FIG. 13 shows a further alternative embodiment. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In FIG. 1, an optical fibre  1  provides a transmission path for propagation of an optical signal from a polarised light emitting transmitter  2  to a receiver  3 , this transmission path including an erbium doped optical fibre amplifier  4  and, adjacent the receiver  3 , a PMD (polarisation mode dispersion) compensator  5 . 
     The optical fibre  1  is a nominally circularly symmetric single mode fibre extending over a substantial distance, which in the present example is 100 km. Over a distance of this length the departures from perfect circular symmetry of that fibre, for example as a result of bending strain, are liable to be of a sufficient magnitude for the fibre to function as a concatenation of birefringent elements of random relative orientation. Moreover that orientation is liable to change with time. 
     When polarised light of any particular wavelength is transmitted through a single element exhibiting uniform birefringence, that light is, in general, resolved into two components (modes) propagating with two specific different velocities, and so possessing different transit times of propagation through that element. For each of two particular orthogonal states of polarisation (SOPs), known as the principal SOPS, the light is not resolved into different components, but propagates at a single velocity with a single transit time, i.e. propagates as a single (polarisation) mode. These principal SOPs are aligned with the principal axes of birefringence of the element. For light launched into the element with either one of these two principal SOPs, the SOP of the light remains unchanged in its passage through the element. For light launched into the element with any other SOP, that light is resolved in its passage through the element into two orthogonal components aligned with the principal axes of the element and propagating with different velocities. As a result of the velocity difference, the relative phase of the two components at the far end of the element is generally not the same as that at the launch end, and so the light emerging at the far end generally emerges with an SOP that is different from that with which it entered the element. This characteristic can be conceptualised as the SOP of the light evolving in a cyclic manner in its passage through the element. 
     When polarised light is transmitted through a concatenation of elements, each exhibiting uniform birefringence, but whose principal axes are not all co-aligned, then, even if that light is not resolved into two components by the first element of the concatenation, it will be so resolved by a later element. Then each of those two elements will itself be resolved into two further components by an element further along the concatenation, and so on. It can be demonstrated that for any such concatenation there exists a specific pair of orthogonal SOPs having the property that light launched with either SOP into the concatenation propagates through it with a single transit time. The transit is faster for one of the SOPs than for the other, and the difference in transit time is the polarisation mode dispersion of the concatenation. For neither one of this specific pair of orthogonal SOPs is the launch SOP maintained in the passage of the light through the concatenation, and the light emerges at the far end with an SOP that is in general different from that with which it was launched. The emergent SOP for one of the single transit time launch SOPs is orthogonal to the emergent SOP for the other single transit time launch SOP. For any launch SOP that is not one of the single transit time launch SOPs, the emergent light is composed of two components (polarisation modes), generally of unequal amplitude, which have propagated through the concatenation with different transit times, respectively the previously mentioned fast and slow single transit times of the concatenation. 
     By analogy with the single uniform birefringence element situation, the two single transit time launch SOPs for the concatenation are often referred to as the principle SOPs of the concatenation. Having regard to the fact that for such a concatenation the single transit time launch SOPs are, in general, different from the corresponding emergence SOPs, reference in this specification will be made to launch principal SOPs (LPSOPS) and to emergence principal SOPs (EPSOPs). From consideration of principles of reciprocity, it will be evident that the LPSOPs for one direction of propagation through the concatenation are the EPSOPs for the other, and vice versa. 
     As shown in FIG. 2, the input  6  of the PMD compensator  5  is connected, within the compensator to a polarisation controller  7  operable to introduce a controlled amount of change of SOP from that received at input  6  so that delivered to a polarisation beam splitter  8 . The amount of change of SOP induced by polarisation controller  7  is regulated by a control signal  9  from a controller  10  such that the polarisation controller maps the (time varying) EPSOPs of transmission path  1  substantially on to the principal SOPs of the polarisation beam splitter  8  so that the splitter operates to separate the polarisation modes emerging from the transmission path  1 , delivering from one output of the polarisation beam splitter  8  substantially exclusively the slow mode component  11 , and from the other output substantially exclusively the fast mode component  12 . 
     Optical taps  13  and  14  are arranged to sample the slow and fast mode components  11  and  12  respectively and detectors  15  and  16  provide conversion of optical tap signals to electrical inputs of a dispersion measurement signals to electrical inputs of a dispersion measurement circuit  17 . The optical signal conducted by the fibre  1  consists of a series of optical pulses at 10 Gb/sec, the dispersion measurement circuit  17  being arranged to detect a dispersion interval t between the arrival of corresponding pulses in the slow and fast mode components  11  and  12  respectively. In the context of the dispersion measurement circuit  17  therefore, the term “dispersion” is used herein to mean “differential delay”. 
     The measurement circuit  17  outputs a signal  18  representative of the value of t to the controller  10 , thereby completing a control loop enabling the polarisation controller  7  to be regulated such that the value of signal  18  is a maximum. 
     The slow and fast mode components  11  and  12  are input to a differential delay system  19  which introduces unequal delays differing by an amount equal to the dispersion interval t and applied in an opposite sense so as to synchronise the data pulses in the respective components. 
     Dispersion compensated outputs  20  and  21  from the differential delay system  19  are combined in an optical recombiner  22  to form output optical signal  23  which is directed to the receiver  3  or may alternatively be routed via a further waveguide to a remote receiver or node of the communications system. 
     The differential delay system  19  is responsive to a control signal  24  from the controller  10  which is dependent upon the magnitude of the dispersion interval t and effects a continuously variable differential delay T. The differential delay system  19  is therefore controlled to achieve a value of T=t. 
     A first example of a differential delay system  19  will now be described with reference to FIG.  3 . The differential delay system  19  comprises first and second chirped Bragg grating fibres  25  and  26  which respectively constitute Bragg reflectors which introduce delays into the transmission of the slow and fast mode components  11  and  12 . The slow mode component  11  is conducted by a waveguide  27  to a first optical circulator  28  which directs the incoming signal into a first end  29  of the first grating fibre  25  via a further waveguide  30 . For an optical signal of a single wavelength λ, the grating fibre  25  reflects the signal at reflection position  31 , at which position the grating pitch p is given by equation 1;              p   =         L                 λ                    2      n               (   2   )                                
     where L (generally L=1) is the order of the grating and n is the refractive index of the grating fibre (generally approximately 1.5). 
     The first chirped Bragg grating  25  has a chirp characteristic A which is variable by means of a dimensional control element  32 , responsive to the control signal  24  to continuously vary the overall length of the fibre  25  and to correspondingly increase or decrease the grating pitch. The variation of grating pitch with length is illustrated graphically in FIG. 3A which illustrates for the purpose of the present example a linear chirp characteristic A of fibre  25  represented as a graph of grating pitch as a function of position along the length of the fibre. The dimensional control element  32 , by lengthening or shortening the fibre  25 , effects an increase or decrease in the grating pitch at each position along the length of the fibre. In doing so, the reflection position  31  is movable away from or towards the first end  29  of the fibre  25  respectively, thereby increasing or decreasing the delay time taken for the optical signal to travel between the first optical circulator  28 , the reflection position  31  and to return to the circulator  28 . In FIG. 3 a , a linear chirped characteristic B of the fibre  25  illustrates the characteristic of the fibre in the absence of any perturbation being applied by the dimensional control elements  32  and therefore also corresponds to the characteristic of the second grating fibre  26 . In the graph of FIG. 3 a , the characteristic A is elevated above characteristic B and therefore corresponds to an elongation of the fibre, the reflection position  31  of the first grating fibre corresponding to this elongation. 
     In FIG. 3 a , the horizontal broken line C indicates the grating pitch at which Bragg reflection occurs in the fibres  25  and  26  for light at wavelength λ. From equation 1 it is apparent that any change in wavelength λ will correspondingly result in a change in reflection position  31 , for example an increase in wavelength will move the reflection position further away from the first end  29  for a chirped grating in which the grating pitch increases with position relative to the first end. 
     After being returned to the circulator  28 , the slow mode component  11  is routed by the circulator into a further waveguide conducting the dispersion compensated output  20 . 
     The fast mode component  12  is conducted by a waveguide  33  to a second optical circulator  34  which diverts the fast mode component via a further waveguide  35  to the first end  36  of the second grating fibre  26 . It should be noted that FIG. 3 is only schematic and that the optical paths of waveguides  30  and  35  are intended to be identical. 
     The fast mode component  12  is reflected at reflection position  37  in the second fibre  26  and is then returned to the second optical circulator  34  which diverts the delayed fast mode component into a further waveguide to form the dispersion compensated output  21 . Because of the difference x in position of the reflection positions  31  and  37  it is apparent that the differential delay, T, between the slow and fast modes is therefore given by;              T   =       2      xn     c             (   3   )                                
     Where n is the refractive index of the fibre core and c is the speed of light. 
     In FIG. 3, the first and second grating fibres  25  and  26  together with the dimensional control element  32  together comprise a differential delay element  38  suitable for compensating PMD. The delay element  38  may however also have other uses in optical communication systems where a differential delay is required to be introduced between optical signals or components separated from a single optical signal. 
     In the PMD compensator of FIGS. 1,  2  and  3 , the length of the grating fibres  25  and  26  is of the order of 1 cm and is suitable for correcting PMD occurring in a 10 Gb/sec data transmission with PMD having a maximum value of the order of 100 picosec. Approximately 1% strain is required to produce an extension of 100 microns providing the above delay. It is therefore apparent that such an arrangement has the advantage of being compact and requiring minimal physical movement to achieve the required range of delay. The delay is continuously variable and achievable with a rapid response time. 
     The optical signal will in practice have a finite bandwidth which must be taken into consideration when selecting the chirp characteristics of the grating fibres  25  and  26 . Firstly, the reflection characteristics of the grating fibre have a finite bandwidth as illustrated in FIG. 4 a  which schematically shows a spectral profile of the reflection properties of a grating fibre with a linear chirp characteristic and for which the refractive index modulation defining the grating is uniform along the lengths of the grating as shown in FIG. 4 b . The delay introduced by the grating fibre is linearly dependent on wavelengths within the pass band of the grating fibre as shown in FIG. 4 c . The effect of this wavelength dependence may be utilised to provide compensation for chromatic dispersion in the optical signal. The amount of chromatic dispersion increases with the amount of chirp present in the grating fibre, i.e. with the gradient of the chirp characteristic, so that in general a degree of chromatic dispersion compensation may be selected by design choice of the magnitude and sign of the gradient of chirp characteristic of the fibre. 
     When the optical signal comprises a plurality of wavelength division multiplexed signal components, it is preferable for each of the signal components to be subject to substantially the same value of differential delay T. This may be accomplished by modulating the envelope of the refractive index modulation in the fibre grating as illustrated in FIG. 5 b  which illustrates the refractive index n as a function of distance along the grating fibre. Whereas in FIG. 4 b  the refractive index is modulated approximately sinusoidally with uniform envelope, the modulation of FIG. 5 b  consists of periods of sinusoidal modulation of fixed level envelope separated by intervals of zero modulation in a periodic manner. Such fibre gratings are referred to as sampled gratings, or alternatively long period superstructure Bragg gratings, as described for example in Electronics Letters, Sept. 15, 1994, Vol. 30, No. 19 B. J. Eggleton et al, “Long Period Super Structure Bragg Gratings in Optical Fibres”. Such sampled gratings have a frequency response characterised by a series of periodically spaced pass bands as illustrating schematically in FIG. 5a which, by appropriate modulation of the refractive index envelope in FIG. 5 b  are selected to coincide substantially with the wavelengths of the multiplexed components of the optical signal. As illustrated in FIG. 5 c , each pass band has substantially the same delay characteristic as a function of wavelength when reflected by Bragg reflection in fibres  25  and  26  described above. 
     A further embodiment will now be described with reference to FIG. 6 which illustrates a PMD compensator using a single optical circulator  42 . The PMD compensator  40  of FIG. 6 is connected to an optical communications system in the same manner as PMD compensator  5  in FIG.  1  and receives an input  6 , an output optical signal  23  subsequently being transmitted to a receiver  3  after compensation for PMD. The PMD compensator  40  has a single optical circulator  42  which is a polarisation insensitive circulator which directs optical signals from the input  6  into a waveguide  41  which directs in turn the signal into a principal state separator  39 , corresponding to the separator  39  of FIG.  2 . Slow and fast mode components  11  and  12  are output from the principal state separator  39  to a differential delay element  38  which may correspond to the element  38  shown in FIG. 3 or may comprise an alternative element described hereafter. 
     After introducing a differential delay between the slow and fast mode components  11  and  12 , these components are returned by reverse propagation via the principal state separator  39  into waveguide  41  and are directed by the circulator  42  to form the output optical signal  23  which has been PMD compensated. 
     An advantage of the arrangement of FIG. 6 is that only a single circulator  42  is required and the provision of a separate optical recombiner  22  of FIG. 2 is no longer required since optical recombination is effected by means of the polarisation beam splitter  8 , acting as a Y coupler for the returning components  11  and  12 . 
     The embodiment of FIG. 6 may be configured to provide compensation in the case of single wavelength optical signals as discussed above with reference to FIG. 4 or wavelength division multiplexed signals as referred to above with reference to FIG.  5 . 
     An alternative differential delay element will now be described with reference to FIG. 7 using corresponding reference numerals to those of preceding figures where appropriate. The differential delay element  45  of FIG. 7 comprises a single chirped Bragg grating fibre  25  with a dimensional control element  32  regulated by a control signal  24  in the manner described above with reference to FIG. 3. A slow mode component  11  is input to the grating fibre  25  and is subject to Bragg reflection with a variable delay dependent upon the value of signal  24 . 
     A fast mode component  12  is input to a waveguide  46  terminated in a simple reflector represented schematically by mirror  47 . The waveguide  46  thereby provides a constant delay and the grating fibre  25  provides a variable delay so that the difference in delay between modes  11  and  12  is variable. This arrangement is clearly simpler in construction than the arrangement of FIG.  3  and would be suitable in systems where chromatic dispersion compensation was not required. The delay element  38  of FIG. 3 however would be preferable if a relatively large amount of chromatic dispersion compensation was required since equal amounts of chromatic dispersion compensation are available to both slow and fast mode components  11  and  12 . 
     In the differential delay elements  38  and  45  referred to above, reference has been made to a dimensional control element  32 . Practical implementations of such dimensional control element will now be disclosed. 
     FIG. 8 illustrates a first example in which the first grating fibre  25  is clamped at its first end  29  relative to a support  48 . The dimensional control element  32  consists of an electrically powered actuator  49  operable to apply tension to the fibre  25  by displacement of a second end  50  of the fibre relative to the support  48 . 
     FIG. 9 illustrates an alternative dimensional control element  32  in which fibre  25  is maintained at a variably controllable temperature by means of a temperature control circuit  51  connected to a thermal actuator  52 . The thermal actuator  52  may simply be a resistive coating applied to the fibre. 
     In each of the differential delay elements of FIGS. 8 and 9, a second grating fibre  26  remains uninfluenced by the dimensional control element  32 , thereby providing the reference against which the variable delay induced in the first fibre  25  creates a differential delay. 
     FIG. 10 shows an alternative arrangement in which the first and second grating fibres  25  and  26  are fused together in side by side configuration so as to co-extend along a support  48  to which the resulting fused fibre assembly  53  is clamped at a first end  54 . 
     A dimensional control element  32  is arranged to apply a bending moment to the fused fibre assembly  53  relative to the support  48  to effect bending in a plane common to the respective axes of the fibres  25  and  26 . In FIG. 10, bending in the direction of arrow  55  for example will result in longitudinal extension of the first fibre  25  and longitudinal compression of the second fibre  26 , thereby increasing the pitch of the grating in first fibre  25  and decreasing the pitch in fibre  26  at each position along the length of the respective fibres. This is illustrated graphically in FIG. 10 a  in which curve B is the chirp characteristic for the unstrained fibres  25  and  26  and curves A 1  and A 2  are the chirped characteristics when strained in direction  55 . 
     The dimensional control element  32  in FIG. 10 may be any suitable electrically operable actuator such as a solenoid or piezoelectric device controlled by the control signal  24 . 
     A further alternative differential delay element  56  will now be described with reference to FIG. 11 using corresponding references to those of previous figures where appropriate for corresponding elements. 
     In the differential delay element  56  of FIG. 11, wave guides  57  and  58  conducting the slow and fast mode components  11  and  12  are coupled by means of a coupler  59  into a single polarisation maintaining fibre  60  such that the slow and fast mode components  11  and  12  are separately and orthogonally propagated as illustrated schematically in FIG. 11 b . A dimensional control element  32  is arranged to apply a bending moment in the same manner described above with reference to the fused fibre assembly  53  of FIG.  10 . The polarisation maintaining fibres  60  is selected to be highly birefringent with respect to the orthogonal modes, thereby presenting different refractive indices n 1  and n 2  which, by equation 1, results in reflection occurring for a given wavelength at values of grating pitch p 1  and P 2  where;                p   1     =           L                 λ       2        n   1                       and                   p   2       =       L                 λ       2        n   2                   (   4   )                                
     Applying a bending moment differentially changes n 1  and n 2  thereby shifting the axial positions along the grating fibre  60  at which reflection occurs for the slow and fast mode components  11  and  12 . 
     FIG. 11 a  shows graphically the effect on the respective characteristics caused by bending, movement of the respective characteristic curves being indicated by arrows. 
     FIG. 12 illustrates schematically an alternative polarization mode dispersion compensator  70  which may be used as compensator  5  in the system of FIG.  1 . 
     The compensator  70  will be described using corresponding reference numerals to preceding figures where appropriate for corresponding elements. The compensator  70  differs from the compensator of FIG. 2 in that the dispersion measurement circuit  17  of FIG.  2  measuring the input to the differential delay system  19  is replaced by a measurement circuit  71  which measures the output of a receiver  72  whose optical input has been subjected to compensation by differential delay system  19 . The measurement circuit  71  provides a feedback signal  73  to the controller  10  which controls both the polarisation controller  7  and the differential delay system  19 . 
     The compensator  70  of FIG. 12 has therefore essentially a feedback configuration whereas the compensator of FIG. 2 has essentially a feed forward configuration. It will be apparent that, when the output of the compensator of FIG. 2 is connected to a receiver, the arrangement of FIG. 2 requires two detection stages, i.e. detection in the receiver and detection by the detectors  15  and  16 . The arrangement of FIG. 12 however includes a single detection stage, i.e. detection in the receiver  72 . 
     The PMD compensator  70  of FIG. 12 has an input waveguide  6  connected to a polarization controller  7  operable to introduce a controlled amount of change of polarization state of the optical signal as received. The output of the polarization controller is diverted by an optical circulator  74  into a polarization beam splitter and recombiner  8  connected to output waveguides  75  and  76  such that the optical signal is divided into a slow mode component output in waveguide  75  and a fast mode component output in waveguide  76 , these components having polarization states corresponding substantially to the instantaneous emergence principal polarization states. 
     After applying a differential delay by operation of the differential system  19  under the control of controller  10 , the components are returned to the recombiner  8  and the recombined output returned to the circulator  74 . The returned compensated output is directed to the receiver  72  by the circulator  74  where the compensated output optical signal is detected. 
     The measurement circuit  71  derives from the electronic output of the receiver  72  a measure of the amount of signal degradation associated with polarization mode dispersion and any one of a number of techniques may be used. For example, the measurement circuit  71  may comprise an eye monitor responsive to the electrical signal output of the receiver  72  to obtain a measure of signal eye opening. Since the signal eye closes in the presence of dispersion, the eye monitor output provides an appropriate feedback signal  73  for control, the term dispersion here being used in the sense of pulse spreading. 
     FIG. 13 shows a further alternative PMD compensator  80  in which reference numerals corresponding to those of preceding figures are used where appropriate for corresponding elements. In the compensator  80 , a polarization mode dispersion monitor  81  receives an optical tap signal  82  and measures the amount of polarization mode dispersion to obtain the required feedback signal  73 . The PMD monitor may be of any one of a number of types and may for example comprise a monitor as disclosed in co-pending application US08/795684 filed Feb. 5, 1997. 
     The optical output following differential delay and recombination may be input to a receiver (not shown) or may alternatively form an output signal for onward transmission in the optical communication system of FIG.  1 . The compensator  80  of FIG. 13 is therefore particularly suited to providing PMD compensation at a node of a network at which no receiver is located. 
     Any one of the differential delay elements of FIGS. 7,  8 ,  9 ,  10  and  11  may be used in a PMD compensator such as described above with reference to FIG. 2,  6 ,  12  or  13 , using either a sampled or non-sampled chirped Bragg grating fibre according to whether wavelength multiplexed division transmission is required. 
     In each case the chirped characteristic may be selected to provide a controlled amount of chromatic dispersion compensation if required. 
     With the exception of the embodiment of FIG. 7, each of the above differential delay elements includes reflection of the slow and fast mode components  11  and  12  by Bragg reflection in a correspondingly chirped grating, thereby providing an inherent insensitivity to any fluctuation in wavelength of the optical signal since both fast and slow mode components will be subject to equivalent changes in delay due to wavelength fluctuation. In the case of the FIG. 7 embodiment, one of the components is reflected by a simple reflector so that the point of reflection in the Bragg grating fibre  25  is sensitive to changes in wavelength in accordance with equation  1 . As an alternative embodiment therefore, a differential delay element as shown in FIG. 7 may be utilised without actuation of dimensional control element  32  simply by controlled variation of the transmitted wavelength of the optical signal, the wavelength being controlled by a feedback signal such that the position at which Bragg reflection occurs provides a differential delay equal and opposite to the amount of polarisation mode dispersion measured in measurement circuit  17 . The feedback signal may conveniently be transmitted optically in the fibre  1  in a direction opposite to that of the optical signal transmission. 
     The electromagnetic solenoid actuator  49  referred to above may alternatively be replaced by a piezoelectric actuator. The thermal actuator  52  referred to above may alternatively comprise Peltier heating/cooling elements arranged to raise or lower the temperature of the fibre in a controlled manner in response to the signal  24 . 
     In the above described embodiments, the formation of the Bragg grating may alternatively be effected in a waveguide other than an optical fibre. 
     The above described embodiments refer to a linear chirp characteristic in the Bragg grating. Non-linear chirp characteristics may alternatively be utilised. 
     The embodiments described with reference to FIG. 3,  8  and  9  may alternatively include an additional dimensional control element operable on the second chirped Bragg grating fibre, the control actuation being preferably in an opposite sense to control of the first fibre. 
     Other variations falling within the scope of the appended claims will be apparent to those skilled in the art in the light of the above disclosure.