Few-picosecond pulse generation featuring ultrafast spectral dynamics in gain-switched surface-grating DFB lasers via impulsive optical pumping

Abstract

To investigate the physics of picosecond gain-switching dynamics in single-mode lasers under femtosecond optical pumping at room temperature, we designed and fabricated first-order surface-grating GaAs distributed-feedback (DFB) lasers with five systematically varied grating periods (120-124 nm), corresponding to lasing wavelengths of 825.7-849.5 nm (1.502-1.459 eV). The 124-nm-period device, closest to the quantum-well gain peak among the investigated devices, exhibited the highest output power and spectral bandwidth. Among all devices, the 122-nm-period DFB laser (838.2 nm, 1.480 eV) generated the shortest pulses, despite lasing at a higher photon energy and lower output power than the device closest to the gain peak. All devices exhibited characteristic down-chirp behavior that increased with excitation power. The shortest pulses had a chirped pulse width of 6.6 ps and a chirp rate of 0.13 meV/ps, whereas spectrally resolved measurements revealed a minimum pulse width of 3.8 ps (2.3 ps after deconvolution of the detection time resolution) near the central photon energy of the pulse spectrum. Numerical simulations revealed temporally and spatially resolved dynamics of photons, carriers, gain, and refractive index, reproducing the experimental results qualitatively and quantitatively. Furthermore, a mechanism for generating the shortest pulses at photon energies above the gain peak was identified and attributed to higher differential gain, saturation gain, and a higher transparency carrier density in the high-energy region of the gain spectrum. These experimental and theoretical results elucidate the intrinsic dynamics of picosecond pulse generation in gain-switched DFB lasers and provide design guidance for short-pulse generation and computational tools applicable to both optical and electrical pumping.