OPTICAL DISPERSION COMPENSATION

Abstract: A chirped optical fibre grating formed by impressing a chirped substantially periodic refractive index variation on a polarisation-maintaining optical fibre. 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 is arranged to be in a polarisation that passes through the polarisation beam splitter (34) towards the second port of the circulator. The light is rotated by 45 DEG 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 DEG with respect to the axes of the input to the circulator. Light reflected from the grating is then further rotated by 45 DEG by the rotator, into the polarisation which is deverted by the polarisation beam splitter (34). This light is therefore diverted to the third output (60) of the circulator. Another embodiment in which two chirped gratings (100, 110) are formed in high birefringence polarisation maintaining fibre with axes aligned, and a 45 DEG Faraday rotator (120) is placed inbetween.

Description: OPTICAL DISPERSION COMPENSATION This invention relates to optical dispersion compensation. 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 1.55μm 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 [1 , 2] 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 [3, 4], 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 [5, 6] than with the holographic technique [7]. 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 160km [8], 220km [9], to 270km [10, 11], were accomplished with gratings 4cm to 12cm long. With 10cm 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 400km of standard single mode fibre is possible, with a 3 dB power penalty, using a 10cm long chirped fibre grating and an unchirped externally modulated transmitted [12]. In separate experiments using a chiφed externally modulated transmitter, transmission over 403km with negligible penalty has been demonstrated and it has been shown that up to 537km is possible with the use of two chiφed gratings cascaded together [13]. This invention provides a chiφed optical fibre grating formed by impressing a chiφed 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 chiφed 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 chiφed 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 chiφed 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 " . The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic graph showing the reflection and time delay characteristics of a conventional chiφed optical fibre grating; Figures 2a and 2b are schematic graphs illustrating the reflection and time delay characteristics for such a grating written into a fibre with a birefringence of 10 "5 ; Figure 3 schematically illustrates an optical transmission system incoφorating polarisation mode dispersion compensation; Figure 4 schematically illustrates another optical transmission system incoφorating polarisation mode dispersion compensation The measured reflectivity and dispersion characteristics of a typical (conventional) chiφed fibre grating are shown in Figure 1. To fabricate this example, a 10cm long uniform phase mask from QPS Technology was used. Apodisation and chiφing of the grating was accomplished during the writing process using the moving fibre-scanning beam technique [14], 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 320km of standard single mode fibre (17 ps/nm.km) [13]. 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,. and n f for the slow and fast axis B = Δn = _vn 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 chiφed 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 Figures 2a and 2b, 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 -60ps which is significant compared to the bit period (lOOps for a lOGbit/s non-return to zero, NRZ system). As a rule of thumb for reliable system operation, the total link PMD should be less than 1/lOth 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 chiφed 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 Figure 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 Figure 3) is arranged to be in a polarisation that passes through the polarisation beam splitter 32 towards the second port of the circulator (on the right in Figure 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, California. Figure 4 schematically illustrates another embodiment in which two chiφed 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 Figure 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. PUBLICATION REFERENCES [ I ] F. Ouellette, ' Dispersion cancellation using linearly chiφed Bragg grating filters in optical waveguides' . Opt. Lett. , vol. 12. pp. 847-849., 1987. [2] K. 0. Hill, F. Bilodeau, B. Malo. T. Kitagawa, S. Theriault, D. C. Johnson and J. Albert. 'Chiφed in-fiber Bragg gratings for compensation of optical-fiber dispersion' , Opt. Lett. , vol. 19. pp. 1314-1316. 1994. [3] K. 0. Hill. B. Malo, F. Bilodeau, D. C. Johnson and J. Albert, 'Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask' , Opt. Lett. , vol. 62. pp. 1035-1037, 1993. [4] D. Z. Anderson. V. Mizrahi, T. Erdogan and A. E. White, 'Production of in-fiber gratings using a diffractive optical element' , Electron. Lett. , vol. 29, pp. 566-568. 1993. [5] J. Maπin and F. Ouellette, 'Novel writing technique of long and highly reflective in- fibre gratings' . Electron. Lett., vol. 30, pp. 811-812, 1994. [6] H. N. Rourke. S. R. Baker, K. C. Byron. R. S. Baulcomb, S. M. Ojha and S. Clements. 'Fabrication and characterisation of long, narrowband fibre gratings by phase mask scanning' , Electron. Lett. , vol. 30, pp. 1341-1342, 1994. [7] G. Meltz. W. W. Morey and W. H. Glenn, 'Formation of Bragg gratings in optical fibers by a transverse holographic method' , Opt. Lett. , vol. 14, pp. 823-825 , 1989. [8] D. Garthe, R. E. Epworth, W. S. Lee, A. Hadjifotiou, C. P. Chew, T. Bricheno, A. Fielding, H. N. Rourke, S. R. Baker, K. C. Byron, R. S. Baulcomb, S. M. Ohja and S. Clements, 'Adjustable dispersion equaliser for 10 and 20 Gbit/s over distances up to 160 km' , Electron. Lett. , vol. 30, pp. 2159-2160, 1994. [9] R. I. Laming, N. Robinson, P. L. Scrivener, M. N. Zervas, S. Barcelos, L. Reekie and J. A. Tucknott, 'A dispersion tunable grating in a 10 Gbit/s 100-220 km step index fiber link' . IEEE Photon. Technol. Lett., to be published. [10] P. A. Krug, T. Stephens, G. Yoffe, F. Ouellette, P. Hill and G. Dhosi. '270 km transmission at 10 Gbit/s in non-dispersion shifted fibre using an adjustably chiφed 120 mm fibre Bragg grating dispersion compensator' , Conf. Opt. Fiber Comm. , paper PDP27, 1995. [ I I] F. Ouellette. P. A. Krug, T. Stephens, G. Dhosi and B. Eggleton, 'Broadband and WDM dispersion compensation using chiφed sampled fibre Bragg gratings' . Electron. Lett. . vol. 31. pp. 899-900, 1995. [12] W. H. Loh. R. I. Laming, X. Gu. M. N. Zervas, M. J. Cole, T. Widdowson and A. D. Ellis. ' 10 cm chiφed fibre Bragg grating for dispersion compensation at 10 Gbit/s over 400 km of non-dispersion shifted fibre' . Electron. Lett.. Vol. 31 , 00. 2203-2204, 1995. [13] W. H. Loh. R. I. Laming. N. Robinson. A. Cavaciuti, F. Vanine ti, C. J. Anderson, M. N. Zervas and M. J. 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Claims: CLAIMS 1. A chiφed optical fibre grating formed by impressing a chiφed substantially periodic refractive index variation on a polarisation-maintaining optical fibre. 2. An optical fibre transmission system comprising: an optical transmitter; a dispersive optical fibre link; and a chiφed 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. 3. A system according to claim 2, comprising a substantially polarisation maintaining optical circulator having at least first, second and third successive circulatory input/ output ports, the first port being connected to receive light from the optical transmitter, the second port being connected to the chiφed optical fibre grating, and the third port being connected to supply light for transmission along the link. 4. A system according to claim 2, in which the dispersion compensation is provided by a series comprising: a first chiφed optical fibre grating formed of polarisation maintaining optical fibre connected to receive light from the optical transmitter and having one of its principal axes aligned with the light from the optical transmitter; a substantially 45° polarisation rotator; and a second chiφed optical fibre grating formed of polarisation maintaining optical fibre, the principal axes of the second grating being rotated by about 90° with respect to the principal axes of the first grating. 5. A system according to claim 4, comprising an output polariser connected to receive light from the second grating for launching along the link.

Inventors: LAMING RICHARD IAN, LOH WEI-HUNG, LAMING, RICHARD, IAN, LOH, WEI-HUNG
Applicants: LAMING RICHARD IAN, LOH WEI HUNG, PIRELLI CAVI E SISTEMI SPA, LAMING, RICHARD, IAN, LOH, WEI-HUNG, PIRELLI CAVI E SISTEMI S.P.A.
Application year: 1997
Main classification: G02B6
IPC codes: G02B6, H04B10