Fiber Bragg gratings (FBGs) with Gaussian apodization profiles and zero dc index change are studied extensively and optimized for optical filtering in 40-Gb/s single-channel and WDM systems with channel spacing of 100 and 200 GHz, for a single filter and for a cascade of optical filters. In the single-filter case, the optimized FBG leads practically to the same performance for single-channel and WDM systems, due to its low crosstalk introduction. The optimized filter is nonflat-top, with delay distortion below 7 ps, -3~dB bandwidth between 60 and 90 GHz,and is very robust to a variation of the grating length and the dc effective index change. For a cascade of optical filters and a WDM system with channel spacing of 100 GHz, the FBG must be designed carefully due to a tradeoff between crosstalk and accumulated distortion. This tradeoff leads to 1.2 dB of sensitivity degradation relative to the single-filter case, and the optimized filter approaches the flat-top response with -3~dB bandwidth between 110 and 130 GHz and delay distortion of about 3 ps. For a 200-GHz channel spacing, that tradeoff is lessened and the FBG optimization is mainly ruled by the distortion accumulation.
João L. Rebola and Adolfo V. T. Cartaxo, "Performance Optimization of Gaussian Apodized Fiber Bragg Grating Filters in WDM Systems," J. Lightwave Technol. 20, 1537- (2002)