Abstract:
An optical dispersion compensation device includes a first optical compensation unit that applies non-linear dispersion compensation across a signal band, the first optical compensation unit being coupled to a second optical compensation unit that applies a degree of linear dispersion compensation across the signal band. The approach taken is to provide broadband dispersion compensation by applying dispersion slope compensation across the signal band to equalise residual dispersion slope and by applying a degree of linear compensation separately to affect the required linear dispersion compensation. Using these two degrees of freedom it is possible to set the desired dispersion slope and linear dispersion (whether positive or negative) to affect broadband dispersion compensation without needing to demultiplex the optical signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to the compensation of chromatic dispersion, hereinafter referred to as dispersion, in optical transmission systems. 
     Linear (first order) dispersion, D, is the measure of the rate of change of group delay, τ, with wavelength, λ. Linear dispersion is typically measured in picoseconds per nanometre (ps/nm). In the case of a transmission medium, for instance an optical fibre waveguide, whose waveguiding properties are uniform along its length, the linear dispersion exhibited by the medium is proportional to its length and so, for such a medium, it is convenient to define its linear dispersion per unit length, also known as its linear dispersion power. This is typically measured in picoseconds per nanometre per kilometer (ps/nm/km). 
     The value of the linear dispersion of a transmission path is generally itself a function of wavelength, and so there is a quadratic (second order) dispersion term, Q, also known as dispersion slope, which is a measure of the rate of change of linear dispersion with wavelength. This is typically measured in picoseconds per nanometre squared (ps/nm 2 ). 
     BACKGROUND TO THE INVENTION 
     In a digital transmission system the presence of dispersion leads to pulse broadening, and hence to a curtailment of system reach before some form of pulse regeneration becomes necessary. The problem presented by dispersion increases rapidly with increasing bit rate. This is because, on the one hand, increasing the bit rate produces increased spectral broadening of the pulses, and hence increased dispersion mediated pulse broadening; while on the other hand, increasing the bit rate also produces a reduction in the time interval between consecutive bits. In a WDM (wavelength division multiplexed) digital transmission system, it is not practical to minimise the problems of dispersion by choosing to employ a transmission medium exhibiting near-zero first order dispersive power because low first order dispersive power is associated with aggravated non-linear (e.g. four-wave mixing) distortion. A known solution to this problem is to employ ‘managed dispersion’ in which near-zero aggregate linear dispersion over a particular transmission path is achieved by the use of alternating sections respectively exhibiting positive linear dispersion and negative linear dispersion, for instance by the use of NDS and DC optical fibre waveguide. 
     Having regard to the manufacturing tolerances in practice encountered in the fabrication of NDS and DC fibre, achieving adequately low aggregate linear dispersion becomes increasingly difficult as the bit rate is increased. Consider for instance a 40 Gbit/s WDM transmission system with a reach of 400 km, and with the shortest and longest wavelength channels separated by 200 nm. The actual amount of linear dispersion in any particular channel that can be tolerated at the receiver will of course be dependent upon a number of system parameters, but typically may lie in the region of 100 ps/nm. A typical NDS fibre exhibits, at a wavelength of 1550 nm, a linear dispersive power of approximately 17 ps/(nmkm), and a quadratic dispersive power of approximately 0.058 ps/(nm 2 km). Currently DC fibre is fabricated to a tolerance of ±20% in respect of quadratic dispersive power. Therefore, for the 400 km span length, the uncertainty in linear dispersion compensation at the 1550 nm wavelength will amount to approximately 400 ps/nm (400×17×0.06 ps/nm). 
     Given the 200 nm wavelength range, the additional uncertainty at the wavelength extremities produced by the ±20% quadratic tolerance amounts approximately to a further 900 ps/nm (400×0.058×200×0.2 ps/nm). To this must be added any uncertainty arising from any imprecision in the knowledge of the length and dispersion of the transmission fibre. 
     The foregoing indicates that, even if the DC fibre were manufactured to tolerances tightened by an order of magnitude, those tolerances would still be large enough to cause difficulty in achieving an accurate enough compensation for the reliable provision of an operating point near the centre of the 100 ps/nm window. 
     Additionally, in ultra long haul optical transmission systems (&lt;3000 km) at 10 GBit/s or more, it is necessary to use high optical powers in the fibre in order to achieve sufficient signal to noise ratio at the receiver, A consequence of these high optical powers is that the nonlinear interactions caused by cross-phase modulation (XPM) and self-phase modulation are significantly enhanced. These two nonlinear mechanisms interact with chromatic dispersion to distort the pulse shape and cause timing jitter of the optical pulses. Whereas in a linear system it is possible to lump all the dispersion compensation in one place (e.g. at the receiver), in a nonlinear system, the distribution of the dispersion compensation can be critical. In particular it is necessary to control the net dispersion immediately following an optical amplifier, where the optical powers and hence nonlinearity are strongest. Simulations have shown the dispersion uncertainty at each amplifier site typically must be within 500 ps/nm, which is the equivalent 30 RM of NDS fibre. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, an optical dispersion compensation device comprises a first optical compensation unit that applies non-linear dispersion compensation across a signal band, the first optical compensation unit being coupled to a second optical compensation unit that applies a degree of linear dispersion compensation across the signal band. 
     According to a second aspect of the present invention, a method of providing dispersion compensation comprises the steps of passing an optical signal through a first optical compensation unit which applies non-linear dispersion compensation across a signal band, and also passing the optical signal through a second optical compensation unit, coupled to the first optical compensation unit, which applies a degree of linear dispersion compensation across the signal band. 
     The approach taken in the present invention is to provide broadband dispersion compensation by applying dispersion slope compensation across the signal band to equalise residual dispersion slope and by applying a degree of linear compensation separately to affect the required linear dispersion compensation. Using these two degrees of freedom it is possible to set the desired dispersion slope and linear dispersion (whether positive or negative) to affect broadband dispersion compensation. Importantly, it is not necessary to demultiplex the optical signal. 
     In a preferred embodiment, the first optical compensation unit comprises an array of non-linearly chirped fibre gratings, each having a different dispersion slope, and an optical switch for selectively coupling one of the chirped fibre gratings into an optical path. Likewise, the second optical compensation unit comprises an array of linearly chirped fibre gratings, each offering a different degree of linear dispersion, and an optical switch for selectively coupling one of the linearly chirped fibre gratings into an optical path. 
     It is preferred that the chirped fibre gratings in the first optical compensation unit each have a relatively high dispersion slope with relatively low linear dispersion. Furthermore, the respective dispersion slopes in the array of chirped fibre gratings are centred about a pivotal point positioned substantially at the centre of the signal band. 
     In the preferred embodiment, a four port optical circulator is used to couple light between the first optical compensation unit and the second optical compensation unit, wherein one port is coupled to the first optical compensation unit and an adjacent port is coupled to the second optical compensation unit. The remaining ports of the optical circulator are coupled to a respective one of an optical input and an optical output of the optical dispersion compensation device. 
     Dispersion compensation components within the first and second optical compensation units can be arranged either to be reflected or transmissive, according to requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: 
     FIG. 1 shows a first example of an optical dispersion compensation device in accordance with the present invention; 
     FIG. 2 shows the optical characteristics of chirped fibre gratings in the device of FIG. 1; 
     FIGS. 3A to  3 C show the effect of passing a chromatically dispersed signal through the device of FIG. 1; 
     FIG. 4 shows schematically an optical communications network having an optical dispersion compensation device in accordance with the present invention; 
     FIG. 5 shows a second example of an optical dispersion compensation device; and, 
     FIG. 6 shows a third example of an optical dispersion compensation device. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an example of an optical dispersion compensation device  1  having a four-port optical circulator  2 , two optical switches  3  and  4 , and two arrays of fibre gratings  5  and  6 . Light from an optical input  7  enters port ( 1 ) of the circulator  2  and is directed onto the first optical switch  3  on port ( 2 ). Depending on the switch state of the optical switch  3 , light is reflected by one of the fibre gratings Q 1  to Q 5  in the first array. Reflected light re-enters the circulator at port ( 2 ) and is directed onto the second optical switch  4  on port ( 3 ). Depending on the switch state of the optical switch  4 , light is reflected by one of the fibre gratings L 1  to L 5  in the second array (in this example L 1  is a mirror). Reflected light re-enters the circulator  2  at port ( 3 ) and is finally transmitted to an optical output  8  via port ( 4 ). 
     The fibre (Bragg) gratings Q 1  to Q 5  in the first array  5  are non-linearly (substantially quadratically) chirped to give the dispersions indicated in FIG.  2 . The first optical switch  3  is used to select the grating Q 1  to Q 5  that has a dispersion slope that most closely matches the net dispersion slope at the position of the device  1  in the network. The second optical switch  4  is then used to select a linear fibre (Bragg) grating L 1  to L 5  in the second array  6  to set the desired net dispersion at the position of the device (generally this will be slightly positive). Using these two degrees of freedom it is possible to set any desired dispersion slope and linear dispersion (positive or negative) to within the granularity of the two arrays  5  and  6 . 
     As shown in FIG. 2, the quadratic chirped gratings Q 1  to Q 5  introduce positive dispersion about a pivotal point in the centre of the band. The linear chirped gratings L 1  to L 5  introduce constant negative dispersion across the band. 
     FIGS. 3A to  3 C illustrate the chromatic dispersion compensation affected by passing an uncompensated signal (FIG. 3A) through the first array  5  (FIG. 3B) and subsequently the second array  6  (FIG.  3 C). 
     An example design would have a maximum dispersion slope term for Q 1  of 12 ps/nm 2 , and a linear dispersion of 20 ps/nm at the lowest end. Grating Q 2  would have a 6 ps/nm 2  dispersion slope and linear dispersion of 135 ps/nm at the lowest end. Grating Q 3  would have zero dispersion slope and a constant linear dispersion of 238 ps/nm. Grating Q 4  would have a dispersion slope of −6 ps/nm 2  and a linear dispersion of 135 ps/nm at the lowest dispersion end. Grating Q 5  would have a dispersion slope of −12 ps/nm 2 , and a linear dispersion of 20 ps/nm at the lowest dispersion end. Although five gratings Q 1  and Q 5  are shown in the first array  5 , a larger number could be used to provide intermediate dispersion slope values. The linear chirped gratings L 1  to L 5  in the second array  6  in this example have constant negative dispersions of 0 (a mirror), 119, 238, 357 and 476 ps/nm, respectively. 
     A fabrication process for broadband chirped fibre Bragg gratings which can be used to compensate for dispersion slope is disclosed in a paper: J A R Williams, L A Everall, I Bennion and N J Doran, “Fiber Bragg Grating Fabrication for Dispersion Slope Compensation”, IEEE Photonics Technology Letters, Vol 8, No 9, September 1996. 
     Instead of using broadband fibre gratings, dispersion compensation could be achieved by an array of single channel fibre gratings i.e., a separate grating for each wavelength. These gratings would be physically separated in the fibre and also spectrally separated with guard bands. Whilst this arrangement would require temperature drift compensation (either by mechanical design or temperature control), it affords two advantages: 
     (i) Single channel gratings typically have much less group delay ripple than long gratings, resulting in better performance; and, 
     (ii) The order of the gratings on the fibre can be reversed, such that the shortest wavelength channel is closest to the circulator. This avoids a problem known as “short wavelength loss”, whereby light at a shorter wavelength to the Bragg wavelength suffers a loss in transmission through out-coupling into cladding modes. 
     If individual gratings are used for the quadratic compensation component then they could all be linearly chirped, but with an increasing (or decreasing) amount of dispersion. This would make the gratings much simpler to fabricate. Equally the gratings could have a quadratic dispersion component as well to equalise the effect of quadratic dispersion within the signal bandwidth, which may be important at higher bit rates than 40 GBit/s. 
     It is possible to extend the hierarchy of this arrangement by having more ports in the optical circulator  2  coupled to grating arrays (not shown) offering cubic, quartic and higher dispersion terms in order to tailor the dispersion more closely to the actual fibre dispersion profile. This may be required if ELEAF (enhanged large effective area fibres) is used as the dispersion zero is close to the signal band and it suffers a significant slope on the dispersion slope. 
     Instead of chirped fibre gratings, the dispersion compensating component can be: 
     (i) A Bragg grating (fibre or planar waveguide type) in reflection; 
     (ii) A Bragg grating (fibre or planar waveguide type) in transmission (the grating couples light between two modes with different propagation constants). It is chirped such that different wavelengths of light spend different proportions of their transmission path between the two modes; 
     (iii) Dispersion Compensating fibre in transmission (or in reflection if a mirror is used); 
     (iv) Photonic crystal device (fibre or planar waveguide type) in transmission (or reflection if used with a mirror). Photonic crystal fiber can be formed from a array of capillaries, rather than a simple pre-form. The resultant fibre has a periodic array of ‘air holes’ or defects in its cross section, which allow guiding of light. [Reference: J C Knight, T A Birks, P St J Russell and DM Atkin, “All-silica single-mode optical fiber with photonic crystal cladding”, Optics Letters, Vol 21, No 19, pp 1547-1549, 1996]. More generally the guidance properties of photonic crystal waveguides can be modified by arrays of holes, and/or slits, and/or by arrays of regions with different refractive index to the surrounding core or cladding material. 
     (v) Higher order mode fibre. Light is launched into a higher order spatial mode of the fibre, which exhibits high wavelength dispersion. A mode converter is required to convert between the fundamental mode and the higher order mode and back again. This mode converter could be bulk optics or a fibre grating. Higher order mode compensators (fibre+mode-converter) are currently being fabricated by Lasercomm Inc (http://www.lasercomm-inc.com). [Reference: C D Poole, J M Wiesenfield, D J DiDiovanni and A M Vengsarker, “Optical Fibre-Based Dispersion Compensation Using Higher Order Modes Near Cutoff”, Journal of Lightwave Technology, Vol 12, No 10, pp 1746-1758, 1994]. 
     (vi) Planar waveguide component such as a Cascaded Mach-Zehnder [Reference: K Takiguchi, K Okamoto and K Moriwaki, “Planar Lightwave Circuit Dispersion Equalizer”, Journal of Lightwave Technology, Vol 14, No 9, pp 2003-2011, 1996, or arrayed waveguide device [Reference: H Tsuda, T Kurokawa, K Okamoto, T Ishii, K Naganuma, Y Inoue and H Takenouchi, “Second and third-order dispersion compensation using a high resolution arrayed waveguide grating”, Proceedings ECOC&#39;98 Madrid, pp 533-534, 1998]. 
     (vii) Etalon based filter [Reference: U.S. Pat. No. 5,557,468]. 
     Instead of using optical switches, it is possible to provide optical connectors (not shown) on ports ( 2 ) and ( 3 ) of the optical circulator  2 . An engineer could then select the most appropriate pair of gratings from the two arrays  5  and  6  and connect them to the circulator  2 . 
     An example of an optical communications network  20  is illustrated schematically in FIG. 4. A chromatic dispersion compensation device  21  is positioned between two remote terminals  22  and  23  in the optical network  20 , usually as part of an optical amplifier device  24  used to restore signal power for onward transmission of the optical signal. In this example, the dispersion compensation device  21  is coupled to an erbium doped fibre amplifier  25  and an appropriate pair of gratings in the two arrays (not shown) selected to most closely match the dispersion profile at the position of the device in the network. 
     Another example of a dispersion compensation device  10  is shown in FIG.  5 . In this example, both ends of each quadratic grating Q 1  to Q 3  in the first array  5  are used, thereby reducing the number of gratings in the array (the first optical switch  3  must be provided with adequate anti-reflection coatings). However, this design requires more ports in the second optical switch  11  because now the linear offset introduced by the quadratic gratings Q 1  to Q 3  could be positive or negative, depending on the particular optical transmission path selected by the first optical switch  3 . The sign of the linear offset is adjusted by using an appropriate end of a selected one of the linear grating L 2  to L 5 . 
     A further example of a dispersion compensation device is shown in FIG.  6 . In this example, the device  30  is configured in transmission and uses lengths of dispersion compensating fibre (for example photonic crystal fibre) with tailored dispersion characteristics in the first and second arrays  5  and  6 , respectively. The selection of the most appropriate fibres is made by selecting the switch state of the optical switches  12 ,  14 ,  16  and  18 . Again, other dispersion compensating components could be used instead. The hierarchy can of course be extended to multiple parallel and serial combinations.