Time-domain traveling-wave simulations for generating near-transform-limited 1-ps pulses using gain-switched distributed-feedback lasers

We performed time-domain traveling-wave simulations of gain-switched distributed-feedback (DFB) lasers under impulsive pumping. As the impulsively injected carrier density increased, the temporal width of the directly generated gain-switched pulses initially decreased but eventually saturated at 3.6 ps, whereas the spectral width and chirp rate increased quasi-linearly without saturation. Linear chirp compensation using single-mode fibers (SMFs) with optimized lengths enabled pulse compression down to 1.18 ps. Further reduction was obtained by combining an SMF with a high-pass filter (HPF) to suppress low-energy spectral components, yielding pulses with a width of 1.15 ps and nearly Fourier-transform-limited pulses with a width of 1.23 ps, with corresponding transform-limited widths of 1.07 and 1.21 ps, respectively. Residual trailing structures, consisting of weak secondary pulses following the main peak, were partially suppressed by the HPF. These results establish a simple and robust route for generating near-transform-limited ultrashort pulses from gain-switched DFB lasers.