Study On Tunable Dispersion-Slope Compensator In High-Speed Optical Communication System

In the future reconfigurable optical networks, dispersion-slopecompensation must be tunable, because dispersion-slope varies with thevariation of optical fiber length, optical nonlinearity, signal power, opticalfiber type and the temperature of environment. Fiber Bragg Grating (FBG) is acompetitive solution to the tunable dispersion-slope compensation modulebecause of its low loss, low optical nonlinearity, small footprint and low cost.In order to compensate dispersion-slope with FBG, group delay curvemust be a third-order function of wavelength, which means thatdispersion-slope varies at different wavelength. To satisfy this requirement,the chirp of the FBG must vary along the FBG length. In this paper, by usingthe nonlinearly chirped FBG, dispersion-slope varies at different wavelength.Thus, dispersion-slope compensation can be tunable by varying the workingpoint of the FBG.The key point of fabrication of this kind of nonlinearly chirped FBG ishow to introduce the nonlinear chirp. In this paper, the introduction ofnonlinear chirp in some reflection band is realized by chirping the samplingperiod of Sampled Fiber Bragg Grating (SFBG). By properly designing chirpin the sampling period, the grating produce an equivalent nonlinear chirpprofile within reflection bands with non-zero Fourier orders. Compared withFBG with chirp in the grating period, SFBG with chirp in the sampling periodis more advantageous in that it is flexible in designing and fabrication, that itdoes not need chirped phase masks, and that the sampling period is easier tocontrol than the grating period. Compared with chirped FBG with complexdifferential stretching or hearting, it is more advantageous in that it has simplestructure and small footprint, and that it is easier to be sealed.In this paper, a novel design method of tunable dispersion-slopecompensator based on SFBG is presented. According to universal type ofcubic group delay curve, concrete design method based on equivalent-chirpmethod has been given to produce desirable SFBG. Especially, analyticalexpression solution to the simplest type of cubic group delay curve has beengiven. For some more complicated group delay curve, the solution can begained by some numerical methods in the same way. In some situations, thesolution of SFBG structure must be adjusted by reconstructionalgorithm.Furthermore in this paper, fabrication results of SFBG based onabove-mentioned design method are given. These results not only validate thecorrectness of the design method, but also fulfill the desirable requirements.At the same time, some conclusions of the results are helpful to some otherresearches about SFBG.