Effect of Grating Parameters on Mode-Locked External Cavity Lasers

This paper deals with numerical solutions of actively mode-locked fiber grating semiconductor lasers using a time-domain solution of coupled wave equations and rate equations. Simulation of linearly chirped tanh apodized fiber Bragg grating (FBG) utilized in hybrid soliton pulse source (HSPS) shows an extreme increase in the mode-locking frequency range of HSPS. Our model predicts transform-limited pulses over a frequency range of 1.6 GHz (1.8-3.4 GHz) for this grating around a system operating frequency of 2.5 GHz, with a pulsewidth of 46 ps required for a practical soliton transmission system, whereas it ranges to about 1.3 GHz (2-3.3 GHz) for the linearly chirped raised-cosine flat top and 850 MHz (2.1-2.95 GHz) for linearly chirped Gaussian apodized. Furthermore, in this study, the effects of FBG parameters, such as peak reflectivity, grating length, grating chirp, and modulation index, on output of mode-locked HSPS are also described for the first time. The numerical results indicated that although pulsewidths decreased with the increase in grating chirp, shorter grating lengths gave shorter pulses, and the modulation index and peak reflectivity of the grating did not significantly affect the pulsewidths.