Abstract:
The intrinsic optical loss exhibited by a circulator-based optical fiber chirped Bragg reflection grating optical fibre dispersion compensator is compensated by including a length of amplifying fibre in the path between the circulator and the Bragg grating, this amplifier fibre being optically pumped by pump power launched into the amplifier fibre from the far side of the Bragg grating.

Description:
FIELD OF THE INVENTION 
     This invention relates to compensation of chromatic dispersion (hereinafter simply referred to as dispersion) encountered in optical waveguides. 
     BACKGROUND TO THE INVENTION 
     In optical transmission systems, one of the ways of compensating for the dispersion produced by standard transmission fibre is to include in the transmission path one or more lengths of dispersion compensating fibre. This dispersion compensating fibre also exhibits dispersion, but the sign of the dispersion is the opposite of that exhibited by standard transmission fibre. Moreover the modulus of that dispersion is significantly larger, typically about four times larger, than that of standard transmission fibre, and in consequence the dispersion of long lengths of standard transmission fibre can be compensated by the use of significantly shorter lengths of dispersion compensating fibre. Amongst the drawbacks of this approach to dispersion compensation is the fact that dispersion compensating fibre is typically significantly more lossy and expensive than standard transmission fibre. Moreover its properties are generally considered not suitable for deployment of this fibre in the field, and so the effective span length is not the aggregate length, but only that of the standard transmission fibre. 
     An alternative approach to dispersion compensation is described in U.S. Pat. No. 4,953,939, this approach involving the use of a chirped Bragg reflection grating in a length of single mode optical fibre waveguide. Initially problems were encountered in the writing of long gratings section by section without unacceptably degrading the performance through the presence of stitch errors where individual sections fail to register quite correctly with their adjacent sections. However, with the advent of forms of active section alignment, such as described in U.S. Pat. No. 5,837,169, or in European Patent Application No. 0 878 721, it has been found feasible to create acceptable quality dispersion compensators employing chirped Bragg reflection gratings of between 2 and 3 metres in length. However these also exhibit significant loss. Typically, this loss may amount to between 5 and 11 dB, and amongst the factors contributing to this loss is the loss of the special fibre in which the Bragg reflection grating is formed, the loss incurred by the writing process employed for creating the grating, loss incurred by mode conversion by the grating into radiative cladding modes, loss arising from the fact that the grating is not fully saturated, and losses incurred by the propagation of the light twice through the circulator. 
     SUMMARY OF THE INVENTION 
     A primary object of the present invention is to provide a form of dispersion compensator of the circulator and Bragg reflection grating type that has reduced or eliminated loss. 
     According to a first aspect of the present invention there is provided a dispersion compensator having an optical amplifier optical pump optically coupled with one port of an optical circulator via an optical waveguide, which optical waveguide includes a chirped Bragg reflection grating and, between the grating and the circulator, a length of optically amplifying waveguide. 
     A superficial resemblance can be found between a dispersion compensator according to the present invention and the gain compensated optical amplifier of U.S. Pat. No. 5,636,301. Though both devices involve the use of circulators, optically amplifying waveguides and Bragg reflection gratings, the devices are in fact quite different devices. In particular the present invention is directed to dispersion compensation, whereas the gain compensated optical amplifier of U.S. Pat. No. 5,636,301 is not only not concerned with dispersion compensation, it is additionally incapable of functioning as a dispersion compensator. This is because it is specifically a device whose component Bragg reflection gratings have reflection wavebands that are spectrally separated by spectral guard bands, and accordingly any attempt to use the gain compensated optical amplifier of U.S. Pat. No. 5,636,301 for dispersion compensation of a signal would serve to punch spectral holes in that signal. 
     The optical coupling between the optical pump and the circulator may include a wavelength multiplexing coupler between the optical pump and the Bragg grating in order to divert any signal power not reflected by the Bragg grating away from entering the optical pump. 
     According to a second aspect of the present invention, there is provided a dispersion compensator having an optical amplifier optical pump optically coupled via a power splitter with two adjacent ports of a four-port optical circulator via respective optical waveguides optically in parallel, each of which optical waveguides includes a chirped Bragg reflection grating and, between the grating and the circulator, a length of optically amplifying waveguide. Alternatively, the power splitter may be dispensed with, and a separate pump used for pumping each of the optical amplifiers. 
     The amplification that is required in these dispersion compensators to offset their lossy components is typically significantly less than that typically required of optical amplifiers employed in transmission highways, and hence the pump power requirements, and consequential cost, are correspondingly smaller. 
     Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, from the drawings and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  4  are schematic depictions of four dispersion compensators embodying the present invention in alternative preferred forms. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a dispersion equaliser has a 3-port optical circulator  10  which has ports  10   a ,  10   b  and  10   c  arranged such that light launched into the circulator respectively by way of ports  10   a ,  10   b  and  10   c  emerges from the circulator respectively by way of ports  10   b ,  10   c  and  10   a . Ports  10   a  and  10   c  constitute respectively the input and output of the dispersion equaliser. Optically coupled with port  10   b  of the circulator is one end of a length of optically amplifying waveguide  11 , typically constituted by a length of erbium doped optical fibre waveguide. Optically coupled with the other end of the waveguide  11  is a length of optical waveguide  12  in which has been formed a chirped Bragg reflective grating  13 . The extent of the chirp of grating  13  is chosen to provide that grating with a reflection waveband that covers the spectral range over which the dispersion compensator is designed to operate. The rate of chirp, which is typically, but not necessarily, linear determines the amount of dispersion compensation that the compensator provides. Optically coupled with the other end of the waveguide  12  is an optical pump  14  for the amplifying waveguide  11 , for instance a laser diode. The optical couplings between the waveguides  11  and  12  and between waveguide  11  and port  10   b  of the circulator  10  are typically fused fibre splices. 
     The modulation depth of the individual grating elements of grating  13  will normally be chosen to make the grating only slightly less than 100% reflective over the signal waveband, and so very little of any signal power that is launched into port  10   a  of circulator  10  will reach pump source  14 . In the event that this residual power is thought to be potentially sufficient to upset the proper operation of the pump, it can be filtered out, for instance by means of a 2×2 wavelength multiplexing waveguide directional coupler. FIG. 2 depicts a dispersion equaliser identical with that of FIG. 1 except for the inclusion of such a coupler  25  spliced in between the source  14  and the grating waveguide  12 . 
     FIG. 3 discloses a dispersion compensator which, unlike the dispersion compensators of FIGS. 1 and 2, uses a 4-port optical circulator  30  instead of the 3-port circulators used in the dispersion compensators of FIGS. 1 and 2. This 4-port optical circulator  30  which has ports  30   a ,  30   b ,  30   c  and  30   d  arranged such that light launched into the circulator respectively by way of ports  30   a ,  30   b    30   c  and  30   d  emerges from the circulator respectively by way of ports  30   b ,  30   c ,  30   d  and  30   a . Ports  30   a  and  30   d  constitute respectively the input and output of the dispersion equaliser. Optically coupled with port  30   b  of the circulator is one end of a length of optically amplifying waveguide  31   b , typically constituted by a length of erbium doped optical fibre waveguide. Optically coupled with each one of ports  30   b  and  30   c  of the circulator is one end of a respective one of two lengths of optically amplifying waveguide  31   b  and  31   c , typically constituted by a length of erbium doped optical fibre waveguide. Optically coupled with the other ends of the waveguides  31   b  and  31   c  is a respective one of two lengths of optical waveguide  32   b  and  32   c  in each of which has been formed a respective one of two chirped Bragg reflective gratings  33   b  and  33   c . The other ends of the waveguides  32   b  and  32   c  are optically coupled with an optical pump  34   b , for instance a laser diode, by means of a power splitter, for instance a 2×2 waveguide 3 dB directional coupler  35 . Optionally, the pump power available for pumping the can be boosted by the provision of an additional pump  34   c  optically coupled with the free port of 3 dB coupler  35 . Alternatively, the power splitter may be dispensed with, as depicted in the dispersion compensator of FIG. 4, and the two amplifying waveguides  31   b  and  31   c  separately pumped by their associated pumps  34   b  and  34   c.    
     In this dispersion compensator of FIG. 3, the light that is launched into port  30   a  of circulator  30  undergoes a first amount of dispersion by virtue of its wavelength-dependent distributed reflection in grating  33   b , and then undergoes additional dispersion by virtue of its wavelength-dependent distributed reflection in grating  33   c.