Dispersion compensation in optical fibre transmission

Abstract: An optical fibre transmission system comprises an optical signal source operable to generate an optical signal at a predetermined bit rate and at a signal wavelength; an optical fibre transmission link connected at a first end to the signal source, the link having dispersion characteristics at the signal wavelength; an optical amplifier serially disposed in the link; and a signal receiver connected at the second end of the link; in which a grating is connected in the link, the grating being chirped by an amount providing at least partial compensation of the dispersion characteristics of the link, the compensation being such as to provide a signal, at the second end of the link, compatible with the sensitivity requirements of the receiver at the second end of the link.

Description: This invention relates to dispersion compensation in optical fibre transmission. Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55µm (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity multi-wavelength optical networks. More importantly, most fibre which has already been installed for telecommunication links (ie. standard non-dispersion shifted fibre) exhibits a dispersion zero around 1.3µm and thus exhibits high (about 17ps/nm.km (picosecond per nanometre-kilometre)) dispersion around 1.55µm. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelength to 1.55µm where dispersion-compensation becomes a necessity. Several techniques have been demonstrated including laser pre-chirping (reference 1 - below), mid-span spectral-inversion (phase-conjugation) (reference 2 - below), the addition of highly-dispersive compensating fibre (reference 3 - below) and chirped fibre gratings (references 4 to 7 - below). Chirped fibre gratings are of particular interest, since they are compact, low-loss and offer high negative-dispersion of arbitrary and tunable profile. In separate experiments 450fs (femtosecond) pulses have been successfully reconstructed after transmission through 245m of fibre (reference 4 - below), and gratings with dispersion equivalent to 20km and 1km of standard fibre have been fabricated (references 5 and 6 - below). Whilst more recently a grating has been employed to compensate the dispersion of 160km of standard fibre in a 100bits -1 (gigabits per second) externally modulated experiment (reference 7 - below) although no information of the grating strength was given in this case. It is a constant aim to improve dispersion compensation techniques in optical fibre transmission systems. Martin et al. in "Novel writing technique of long in-fiber Bragg gratings and investigation of the linearly chirped component", Conference on Lasers and Electro Optics (CLEO), US, New York, IEEE, 1994, page 242 describe a scheme for investigating the linear component of a chirp of a fibre grating by studying the effect of applying a linear temperature gradient to the fibre. In one example the temperature gradient is applied to accentuate the chirp. In another example the temperature gradient is applied to compensate the chirp. A first aspect of the invention provides a method of forming a chirped optical fiber grating, comprising the steps of: providing an optical fiber; impressing a non-linear grating onto a portion of said optical fiber to define a non-linear variation in refractive index; and applying a temperature gradient to said portion of said optical fiber characterised in that said temperature gradient provides a variation that acts against said non-linear variation in refractive index so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. This invention also provides an optical transmitter for use with an optical fibre transmission link, the transmitter comprising a light source capable of direct or indirect modulation and a chirped grating to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source, the grating being formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation induced by the temperature gradient acting against the non-linear variation of the impressed grating so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. This invention also provides a dispersion compensating device for an optical fibre transmission link, the device comprising a chirped grating connectable at a light input end of the transmission link, the grating being formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation induced by the temperature gradient acting against the non-linear variation of the impressed grating so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. This invention also provides a chirped optical fibre grating formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation induced by the temperature gradient acting against the non-linear variation of the impressed grating so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. This invention also provides an optical fibre transmission system comprising: an optical fibre transmission link and a chirped grating connected at an input end of the link to provide compensation against the dispersion characteristics of the link, the grating being formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation induced by the temperature gradient acting against the non-linear variation of the impressed grating so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. Preferably the grating is a reflection fibre grating. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a dispersion compensating optical fibre transmission system; Figure 2 schematically illustrates the spectrum of a DFB (distributed feedback) laser transmitter; Figures 3a to 3c schematically illustrate the reflectivity spectra of a fibre grating as written (Figure 3a), with a temperature gradient set to add to the existing chirp (Figure 3b) and with a temperature gradient set to reverse the existing chirp (Figure 3c). Figures 4a to 4c schematically illustrate the time delay of the gratings of Figures 3a to 3c respectively; Figures 5a and 5b schematically illustrate sampling oscilloscope traces of an approximately 10ps, 0.318nm spectral halfwidth signal after propagation through 50km of standard fibre without compensation (Figure 5a) and with compensation (Figure 5b); Figure 6 schematically illustrates bit error rate (BER) curves for the system of Figure 1; Figure 7 schematically illustrates a transmission penalty at a 10 -9 BER as a function of span length with and without dispersion compensation; and Figures 8a to 8f schematically illustrate eye diagrams showing the different results obtained without (Figures 8a to 8c) and with (Figures 8d to 8f) dispersion compensation. Referring now to Figure 1, in this embodiment a chirped fibre grating 10 was incorporated into a 2.5Ghits -1 directly-modulated system operating at 1536nm. (However, in other embodiments and in the description below, an indirectly modulated transmitter could be used instead). As a consequence of the direct modulation the output of the DFB laser transmitter 20 was chirped and exhibited a 3dB optical bandwidth of 0.1nm and a 10dB (decibel) bandwidth of 0.24nm, ie equivalent to a 10Gbit/s modulation signal. The transmitter 20 was supplied with data from a commercial multiplexer (not shown) from a Phillips SDH (synchronous digital hierarchy) 2.5Gbits -1 system. The multiplexer combines 16 channels of data at 140Mbits -1 (megabits per second) up to a line rate of 2.5Gbits -1 . In the absence of data on any channel, the multiplexer generates random data. Random data was input to several of the channels whilst, on the test channel, pseudorandom data at 140Mbits -1 with a 2 23 -1 pattern length (generated by a BER test set 110) was employed. However, in a real application, it will of course be appreciated that real input data would be supplied to the transmitter instead of the pseudorandom data from the BER test set. The transmitter 20 consisted of a directly-modulated DFB laser with wavelength centred at 1536nm and whose chirped output had a 3dB bandwidth of 0.108nm and 10dB bandwidth of 0.24nm. The spectral characteristics of the transmitter output are illustrated schematically in Figure 2. As a consequence of this chirp (and the fibre dispersion) a penalty was observed for transmission distances in standard fibre in excess of a few tens of km. The transmitter was followed by a single-stage, 980nm-pumped erbium-doped power-amplifier 30 giving an output power of +12dBm (decibels relative to 1 milliwatt) which was transmitted through standard fibre having lengths of 100, 143 and 200km. In the latter case (as illustrated in Figure 1), a dual-stage 980nm-pumped line amplifier 40 giving an output power of +13dBm was incorporated between two series connected 100km lengths of fibre 50, 60. The output of the link was coupled via a variable attenuator 70 to a commercial, Phillips, receiver and demultiplexer 80, the output of which was in turn passed to the BER test set 110 for BER measurement (by comparison with the test data supplied to the transmitter 20 by the BER generator 110). Dispersion-compensation of the link was provided by incorporating the chirped fibre grating 10 between the transmitter 20 and power amplifier 30. Since the grating 10 operates in reflection, an optical circulator 90 was included to convert it to a transmission device. The grating was connected to the circulator using so-called NTT FC/PC compatible connectors (not shown). However, to ensure successful operation, index matching liquid (not shown) was inserted in the connection to minimise reflections. Power levels in the link are such that it is operating in the so-called linear regime, thus the dispersion compensation could in theory be performed at any location in the link. However it is advantageous to incorporate the grating in its present location (before the fibre lengths 50, 60) since the input signal to the power amplifier is still relatively large and thus a relatively insignificant noise penalty is incurred. In addition, since this amplifier 30 is then operating in saturation the output power will be effectively unaltered. Alternatively, if the dispersion compensation had been included immediately prior to the receiver a penalty would have been incurred due to its insertion loss. The fibre grating was written using standard techniques with a frequency-doubled excimer laser in a germania-boron co-doped fibre (0.1 NA (numerical aperture), 1µm λ cutoff (cutoff wavelength)). The grating was about 20mm in length with an approximately Gaussian strength profile and about 70% peak reflectivity. In its "as-written" state it had some residual chirp and a measured bandwidth of about 0.2nm. The grating was further chirped to a 3dB bandwidth of about 0.3nm by applying a linear temperature gradient. The temperature gradient could be applied to either add to or reverse the existing chirp. Surprisingly superior performance was obtained when the temperature gradient was applied to reverse the existing chirp of the grating. This was due to the slightly non-linear characteristics of the existing chirp. In other words, the chirped optical fibre grating is formed by applying a temperature gradient to a portion of optical fibre on which a non-linear grating is impressed, the variation induced by the temperature gradient acting against the non-linear variation of the impressed grating, and in particular where the temperature gradient at least negates the non-linear variation of the impressed grating, thereby generating a grating having a noti-linear variation in the opposite sense to the impressed grating. Figures 3a to 3c show the grating spectral response as written (Figure 3a), with the temperature gradient set to add to (Figure 3b) and reverse (Figure 3c) the existing chirp. A slight dip is noted in Figure 3b. Figures 4a to 4c show the time delay of the gratings, measured using an standard interferometric set up, corresponding to the respective cases illustrated in Figures 3a to 3c. (Since the measurements were performed on different instruments there is a slight mismatch in indicated wavelengths. Also, all three measurements were taken from the same end of the grating and thus in the grating of Figure 4b the grating was tested by the interferometer in the opposite direction to its direction of use in the embodiment of Figure 1). As stated, case b (i.e. as shown in Figures 3b and 4b) did not tend to give stable link performance and thus a temperature profile100 indicated in Figure 1 (case c, i.e. as indicated in Figures 3c and 4c) was employed. The centre wavelength of the chirped grating was also tuned to match the laser wavelength of 1536nm. Once chirped, the grating reflectivity reduced and thus the circulator-grating combination exhibited an insertion loss of 3.5dB, but owing to its location at the output of the transmitter 20 and before the amplifier 30, this had a negligible effect on the link's power-budget. Separate measurements involving compensation for the propagation of about 10ps pulses over 50 and 100km using this grating showed structure in the compressed pulses, indicating phase-distortion of the pulses and thus non-perfect compensation of the dispersion. However, owing to the non-transform-limited data (chirped source) an improvement in system performance was nevertheless obtained. Figure 5a shows a sampling oscilloscope trace of an approximately 10ps, 0.318nm spectral halfwidth pulse after propagation through 50km of standard fibre. The pulse can be seen to have broadened to about 281ps. After recompression with the grating, Figure 5b, the pulse width is seen to be reduced to about 39ps. However, structure can be seen, particularly on the leading edge of the pulse which might be detrimental at higher bit rates. Bit-error-rate (BER) curves for the system are shown in Figure 6. Data are given for back-to-back and direct transmission through 100, 143 and 200km of standard fibre. Dispersion-equalised curves, with the chirped grating included, are given for back-to-back and transmission through 100 and 200km of fibre. In the case of direct transmission, a back-to-back sensitivity of -32.7dBm at a 10 -9 BER is observed. At this error rate a penalty of 1.3dB was found at 100km, increasing to 3.2 and 8.5dB at 143 and 200km, respectively. The increase in penalty with distance is shown again in Figure 7. With the grating incorporated, the back-to-back sensitivity is actually improved by 1.2dB, since the grating compresses the chirped-source pulses. The grating virtually eradicates the penalty at 100km (0.5dB) and significantly reduces the penalty at 200km to only 3dB. No floor in the error-rate curves was observed when using the grating. The increase in penalty with distance in this case can be compared with the direct result in Figure 7, where it can be seen that the grating dispersion is equivalent (but opposite in sign) to around 60km of standard fibre. This result is in substantial agreement with the delay data illustrated in Figure 4c. Receiver eye diagrams are shown for various points in the system in Figure 8. Although the interpretation of eye diagrams is always subjective, the skilled man will appreciate that the beneficial effect of using the grating 10 can be seen. In summary, dispersion-compensation using a chirped fibre grating has been successfully demonstrated in a 200km standard-fibre transmission experiment using a 2.5Gbits -1 1.55µm directly-modulated transmitter. The about 20mm long, 0.3nm chirped grating 10 effectively compensated for about 60km of standard fibre (i.e. fibre having a dispersion zero around 1.3µm and about 17ps/nm.km dispersion around 1.55µm), as anticipated. These results demonstrate that a non-uniformly chirped grating could provide significant improvements in current, directly modulated commercial systems. Thus, an approximately 20mm (millimetre) long grating with substantially linear chirp, to give a 0.3nm 3dB bandwidth, substantially negates the dispersion of about 60km of standard fibre. This allowed transmission through 200km of standard fibre with a 3dB penalty, which compares with an approximately 8.5dB penalty without the compensation. In summary, therefore, embodiments of the invention make use of a grating is connected (in a dispersion compensating fashion e.g. using an optical circulator) in an optical fibre link, where the grating is chirped by an amount providing at least partial compensation of the dispersion characteristics of the link, to provide an output signal from the link compatible with the sensitivity requirements of the receiver at the second end of the link. In particular, in embodiments of the invention, the optical signal at the output end of the link can be made to have a dispersion causing a penalty in sensitivity at the receiver of less than 8.5 decibels at a bit error rate of 10 -9 in a link longer than 200 kilometres. Publication References 1. B. Wedding, B. Franz and B. Junginer, "Dispersion supported transmission at 10Gbit/s via up to 253km of standard single-mode fibre". Proc. ECOC, Sept 12-16, 1993, paper TuC4.3. 2. R.I. Laming, D.J. Richardson, D. Taverner and D.N. Payne, "Transmission of 6ps linear pulses over 50km of standard fibre using midpoint spectral inversion to eliminate dispersion." IEEE Jnl. of Quantum Electronics, Vol, 30, 1994, pp.2114-2119. 3. M. Onishi, H. Ishikawa, T. Kashiwada, K. Nakazato. A. Fukuda, H. Kanainori and M. Nishimura, "High performance dispersion-compensating fiber and its application to upgrading of 1.31µm optimized system", Proc. ECOC, Sept 12-16, 1993, paper WeC8.5. 4. R. Kashyap, S.V. Chernikov, P.F. McKee and J.R. Taylor, "30ps chromatic dispersion compensation of 400fs pulses at 100Gbits/s in optical fibres using an all fibre photoinduced chirped reflection grating", Electronics Letters. Vol. 30, No. 13, pp. 1078-1080. 1994. 5. K.O. Hill, F. Bilodeau, B. Malo, T. Kitagawa, S. Theriault, D.C. Johnson, J. Albert and K. Takiguchi, "A periodic in-fibre Bragg gratings for optical fibre dispersion compensation", Proc. OFC94, PD2, pp. 17-20. 6. J.A.R. Williams, 1. Bennion, K. Sugden and N.J. Doran, "Fibre dispersion compensation using a chirped in-fibre grating", Electr. Lett., Vol.30(12), 1994, pp.985-987. 7. D. Garthe, W.S. Lee, R.E. Epworth, T. Bricheno and C P Chew, "Practical dispersion equaliser based on fibre gratings with a bit-rate length product of 1-6 TB/s.km" Proc. ECOC Vol. 4 (Postdeadline papers), pp.11-14, Sept.25-29, 1994, 8. B. Malo, K.O. Hill, S. Theriault, F. Bilodeau, T. Kitagawa. D.C. Johnson, J. Albert, K. Takiguchi. T. Kataoka and K. Hagimoto, "Dispersion compensation of a 100km. 10Gbit/s optical fiber link using a chirped in-fiber Bragg grating with a linear dispersion characteristic", ibid., pp. 23-26.

Claims: A method of forming a chirped optical fiber grating (10), comprising the steps of: providing an optical fiber; impressing a non-linear grating onto a portion of said optical fiber to define a non-linear variation in refractive index; and applying a temperature gradient (100) to said portion of said optical fiber characterised in that said temperature gradient provides a variation that acts against said non-linear variation in refractive index so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. A chirped optical fiber grating (10) formed by applying a temperature gradient (100) to a portion of optical fiber on which a non-linear grating is impressed, characterised in that the variation induced by the temperature gradient acts against the non-linear variation of the impressed grating so as to generate a grating having a non-linear variation having an opposite sense to that of the impressed grating. An optical transmitter for use with an optical fiber transmission link, the transmitter comprising a light source (20) capable of direct or indirect modulation and a chirped grating (10) according to claim 2 to provide compensation for the dispersion characteristics of the link over the range of wavelengths of the modulated light source. A dispersion compensating device for an optical fiber transmission link, the device comprising a chirped grating (10) according to claim 2 connectable at a light input end of the transmission link. An optical fiber transmission system comprising: an optical fiber transmission link; and a chirped grating (10) according to claim 2 connected at an input end of the link to provide compensation against the dispersion characteristics of the link.

Inventors: COLE MARTIN, LAMING RICHARD IAN, REEKIE LAURENCE, COLE, MARTIN, LAMING, RICHARD IAN, REEKIE, LAURENCE
Applicants: PIRELLI & C SPA, PIRELLI & C. S.P.A.
Application year: 1996
Main classification: H04B10
IPC codes: H04B10