Abstract:
An optical fiber transmission system includes an optical transmitter for generating light; a dispersive optical fiber link optically coupled to the optical transmitter, for transmitting the light generated by the optical transmitter; and a chirped optical fiber grating, optically coupled to the optical transmitter and to the dispersive optical fiber link, for providing at least a partial dispersion compensation to the light generated by the optical transmitter, before the light is transmitted in the dispersive optical fiber link; the chirped optical fiber grating including a polarization-maintaining optical fiber having a chirped refractive index variation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/GB97/02100, filed on Aug. 4, 1997, which claims priority to UK patent application No. 9617689.6 filed on Aug. 23, 1996, the contents of which are relied upon and hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to an optical fibre grating and to an optical fibre transmission system using such optical fibre grating. 
     Long distance transmission at 10 Gbit/s over standard telecommunications fibre is of great interest because of the large base of such fibres already installed in the ground currently supporting low bit-rate systems. 
     The low loss of these already installed fibres, together with the ready availability of erbium doped fibre amplifiers (EDFAs), make the 1.55 μm window an attractive wavelength region of operation. Unfortunately, however, the group velocity dispersion of these fibres is relatively large within this window, which severely limits the transmission distances achievable unless compensating techniques are employed. 
     Of the variety of methods which have been suggested thus far to solve this problem, linearly chirped fibre gratings as dispersion compensators are potentially very attractive, as they are compact, totally passive, and relatively simple to fabricate. 
     Progress in the development of fibre gratings has been rapid in the last few years, especially with the introduction of the phase mask technique, which provides a high degree of reproducibility in the gratings fabricated, as well as relaxing the tolerances on the fabrication set-up. In addition, longer fibre gratings are more readily realised with this approach than with the holographic technique. This is a crucial factor in dispersion compensation where the maximum compensatable distance is expected to scale directly with the grating length. Reports of experimental demonstrations of fibre compensation over standard fibre links, from 160 km, 220 km, to 270 km, were accomplished with gratings 4 cm to 12 cm long. With 10 cm long phase masks now commercially available, and even longer masks a likely prospect in the near future, fibre gratings capable of compensating over wide optical bandwidths and much longer distances should be feasible. 
     Recently, it has been experimentally demonstrated that dispersion compensation to 400 km of standard single mode fibre is possible, with a 3 dB power penalty, using a 10 cm long chirped fibre grating and an unchirped externally modulated transmitted. In separate experiments using a chirped externally modulated transmitter, transmission over 403 km with negligible penalty has been demonstrated and it has been shown that up to 537 km is possible with the use of two chirped gratings cascaded together. 
     SUMMARY OF THE INVENTION 
     This invention provides a chirped optical fibre grating formed by impressing a chirped substantially periodic refractive index variation on a polarisation-maintaining optical fibre. 
     This invention also provides an optical fibre transmission system comprising: 
     an optical transmitter; 
     a dispersive optical fibre link; and 
     a chirped optical fibre grating connected at or near to the input of the link to provide at least partial dispersion compensation to the light to be launched along the link, the grating being formed of polarisation-maintaining optical fibre and having a principal axis substantially aligned with a polarisation axis of light received from the optical transmitter. 
     The invention recognises that a dependence of system sensitivity on the input polarisation state to the grating can occur, due to polarisation mode dispersion (PMD) in linearly chirped dispersion compensating fibre gratings, and that this can give rise to a deterioration in the performance of optical transmission systems relying on such gratings for fibre dispersion compensation. 
     The invention addresses this problem by providing a polarisation-maintaining optical fibre grating, e.g. a chirped grating for dispersion compensation. The axes of the polarisation maintaining fibre of the grating can then be aligned with a polarisation axis of the light to be transmitted through a fibre transmission system. 
     The skilled man will understand that the term “polarisation maintaining fibre” refers to optical fibre having an optically asymmetric cross section, and typically a birefringence of greater than about 10 −4 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic graph showing the reflection and time delay characteristics of a conventional chirped optical fibre grating; 
     FIGS. 2 a  and  2   b  are schematic graphs illustrating the reflection and time delay characteristics for such a grating written into a fibre with a birefringence of 10 −5 ; 
     FIG. 3 schematically illustrates an optical transmission system incorporating polarisation mode dispersion compensation; and 
     FIG. 4 schematically illustrates another optical transmission system incorporating polarisation mode dispersion compensation. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The measured reflectivity and dispersion characteristics of a typical (conventional) chirped fibre grating are shown in FIG.  1 . To fabricate this example, a 10 cm long uniform phase mask from QPS Technology was used. Apodisation and chirping of the grating was accomplished during the writing process using the moving fibre-scanning beam technique (as described, for example, in M. J. Cole, W. H. Loh, R. I. Laming, M. N. Zervas and S. Barcelos, ‘Moving fibre/phase mask-scarining beam technique for enhanced flexibility in producing fibre grating with uniform phase mask’,  Electron. Lett ., vol 31, pp. 1488-1490, 1995), with a cosine apodisation profile being adopted to reduce excessive ripples in the refection/dispersion spectra. 
     This example grating exhibits a peak reflectivity of −50%, 3 dB bandwidth of 0.12 nm and 5400 ps/nm. dispersion. The grating is thus able to compensate for dispersion of 320 km of standard single mode fibre (17 ps/nm.km) (as described, for example, in W. H. Loh, R. I. Laming, N. Robinson, A. Cavaciuti, F. Vaninetti, C. J. Anderson, M. N. Zervas and M. J. Cole, ‘Dispersion compensation over distances in excess of 500 km for 10 Gbit/s systems using chirped fibre gratings’, submitted to IEEE Photonics Technology Letters). These measurements, although made with a polarised tunable laser, employed a polarisation insensitive receiver and are thus insensitive to and do not reveal the PMD in the grating. 
     All optical fibres exhibit a degree of birefringence B, due to slight core elipticity or anisotropic stress which gives rise to a difference between the refractive index and thus the mode propagation constants of the two principal axis, the fast and slow axes. The birefringence is defined as the difference between the mode indices n s  and n f  for the slow and fast axis 
     
       
         
           B=Δn=n 
           s 
           −n 
           f 
         
       
     
     For standard optical fibre B is typically in the range −10 −5 -10 −6  whilst in high birefringence, polarisation maintaining fibre it can be as high as −10 −3 . The effect of fibre birefringence on a chirped fibre grating is to split the reflection spectra for the fast and slow axis by Δλ=2BΛ=Bλ/n. Here Λ is the grating pitch, λ the grating central wavelength and n the average mode index. 
     This effect is indicated schematically in FIGS. 2 a  and  2   b , which are schematic graphs illustrating the reflection and time delay characteristics for such a grating written into a fibre with a birefringence of 10 −5 . 
     As a result of the PMD, the time delay (dispersion) curves are wavelength shifted for the two polarisations. 
     The PMD is given by PMD=ΔλD=2BΛD=BλD/n where D is the grating dispersion. 
     In this case the PMD would be −60 ps which is significant compared to the bit period (100 ps for a 10 Gbit/s non-return to zero, NRZ system). As a rule of thumb for reliable system operation, the total link PMD should be less than {fraction (1/10)}th of the bit-period. Taking into account the PMD of other components in the link fabricating the grating in high quality fibre with low B of −10 −6 , as in references  12  and  13 , will not be sufficient for long term reliable system operation since the input polarisation state to the grating may vary over time. The problem will be even worse for longer transmission distances where the dispersion and thus PMD of the grating will be larger. 
     It has thus been recognised that PMD in linearly chirped fibre gratings is likely to be a severe limitation to their future application. 
     This problem is overcome in the present embodiments of the invention by fabricating the grating in high birefringence polarisation maintaining fibre such as Fibrecore Bow-Tie fibre or Fujikura Panda fibre. 
     In a first embodiment shown schematically in FIG. 3, the polarisation state of a laser transmitter  10  is maintained through an external modulator  20  and a polarisation maintaining circulator  30  by the use of polarisation maintaining fibre pigtails  40  aligned to one of the principal axes of high birefringence optical fibre used to fabricate the dispersion compensating grating  50 . On reflection, light is output from the third port  60  of the circulator. Since only one polarisation mode of the grating  50  is excited, the PMD in the grating is eliminated. 
     The polarisation maintaining circulator  30  comprises input/output lenses  32  at each port, a polarisation beam splitter  34  and a Faraday rotator  36 . Light entering the circulator at the input port (shown on the left in FIG. 3) is arranged to be in a polarisation that passes through the polarisation beam splitter  34  towards the second port of the circulator (on the right in FIG.  3 ). The light is rotated by 45° in the rotator  36 . The grating  50  and the fibre pigtail at the second port of the circulator are arranged with their principal axes rotated by 45° with respect to the axes of the input to the circulator. Light reflected from the grating is then further rotated by 45° by the rotator, into the polarisation which is diverted by the polarisation beam splitter  34 . This light is therefore diverted to the third, output port  60  of the circulator. 
     Another embodiment could instead use a commercially available polarisation maintaining circulator such as the Model SPFC 210071000 from E-Tek Dynamics Inc, San Jose, Calif. 
     FIG. 4 schematically illustrates another embodiment in which two chirped gratings  100 ,  110  are formed in high birefringence polarisation maintaining fibre with axes aligned, and a 45° Faraday rotator  120  is placed in-between. 
     With a grating fibre birefringence of 10 −3 , the reflection spectra for the fast and slow axes are separated by −1 nm; thus for grating and signal bandwidths less than 1 nm, and with the signal wavelength positioned correctly (i.e. substantially aligned) with respect to one or other of the two spectra, the hi-bi fibre gratings are effectively transparent for one polarisation state of the light, along say, the fast axis, but reflecting for light on the other axis. 
     The principle of operation of this configuration is then simply explained as follows, assuming that in this case the transmitter wavelength is selected so that the fast axis is transmitting and the slow axis reflecting. 
     Light from the external modulator (not shown in FIG.  4 ), polarised along, say, the fast-axis, is first transmitted through the grating  100 , undergoes a 45° rotation by the Faraday rotator  120  and hence is launched into the slow-axis of grating  2 . It is thus reflected by grating  110 , and undergoes a further 45° rotation to now be launched into the slow-axis of grating  100 , where it is reflected. Passing once more through the Faraday rotator, it gets launched into the fast-axis of grating  110 , which is transparent to the signal, and hence the signal is finally transmitted out into the system. 
     Light initially launched along the slow axis of the grating  100  is simply reflected back from the grating  100 . 
     An output polariser  130  is used to block any output light in the unwanted polarisation, and therefore to improve further the performance of the system, because the polarisation rejection by the grating is not 100% efficient. 
     With this configuration, the total dispersion compensated is the sum of the dispersion of the two gratings  100  and  110 .