Tunable dispersion slope compensation for 40-Gb/s WDM systems using broadband nonchannelized third-order chirped fiber Bragg gratings

We demonstrate tunable dispersion slope compensation using broadband nonchannelized third-order chirped fiber Bragg gratings (FBGs) for 40-Gb/s wavelength-division-multiplexed (WDM) systems. The slope is tuned by stretching as it has a third-order time delay variation with wavelength, and thus a second-order dispersion-wavelength curve. Calculation results include the examples of the slope tuning in addition to the interchannel dispersion differences and the slope tuning ranges with respect to the wavelength in a single grating. To verify the functionality of the third-order grating, we prepare a third-order grating with a dispersion variation of up to 400 ps/nm over the 3.5-nm usable bandwidth, and a dispersion slope tuning range of ∼65 ps/nm² is achieved. For 40-Gb/s systems, two new gratings are designed and inversely cascaded to eliminate the inherent errors originating from the deleterious higher-order dispersion components in a single grating. The slope tuning range is -20 to +20 ps/nm² over the 5-nm usable bandwidth. A ∼10-dB power penalty improvement is achieved and the BER floor is eliminated for the worst-case channel after transmission through a 170-km link using dispersion shifted fiber (DSF) with a 4×40-Gb/s RZ data stream.