Gain-switched pulse generation of 5.3†ps from 30†GHz-modulation-bandwidth 1270†nm DFB laser diode.

We generated gain-switched pulses via electrical pulse excitations in a 1270 nm distributed feedback (DFB) laser diode (LD) with a direct-modulation bandwidth of 30 GHz. The measurements revealed short-pulse widths of 5.3 and 8.8 ps with and without chirp compensation, via a single-mode optical fiber. The 5.3 ps pulses exhibited a spectral width of 0.40 nm (spectral bandwidth of 71 GHz), yielding a time-bandwidth product of 0.38. Although the gain-switched pulses in DFB LDs inherently contain linear and nonlinear chirp, optimized pumping conditions enable generation of nearly transform-limited ps pulses after linear chirp compensation.

Direct generation of sub-picosecond pulse via multi-section gain switching.

We have directly generated optical pulses having a duration of 0.56 ps with a peak power of 25 W by gain switching of multi-section semiconductor lasers in which the optimized lengths of the absorption and gain regions were 50 and 200 µm, respectively. Even though the experiment was conducted via impulsive optical pumping at a low temperature, we observed that the multi-section gain switching suppresses the low-energy tail and chirping inherent to conventional gain switching in single-section lasers and is useful in direct short-pulse generation.

Picosecond tunable gain-switched blue pulses from GaN laser diodes with nanosecond current injections.

We investigated the gain-switching properties of GaN-based ridge-waveguide lasers on free-standing GaN substrates with low-cost nanosecond current injection. It was observed that the output pulses with intense injection consisted of an isolated short pulse with a duration of around 50 ps at the high-energy side and a long steady-state component at the lower energy side independent of the electric pulse duration. The energy separation between the short pulse and steady-state component can be over 30 meV, favoring short-pulse generation with the spectral filtering technique. The duration of the steady-state component can be tuned freely by controlling the duration and voltage of the electric pulse, which is very useful for generating pulse-width-tunable optical pulses for various applications.

Transient gain analysis of gain-switched semiconductor lasers during pulse lasing.

We analyzed the transient gain properties of three gain-switched semiconductor lasers with different materials and cavity structures during pulse lasing. All the semiconductor lasers were pumped with impulse optical pumping, and all the generated gain-switched output pulses were well described by exponential functions in their rise parts, wherein the transient gains were derived according to the rate-equation theoretical model. In spite of the different laser structures and materials, the results consistently demonstrated that a higher transient gain produces shorter output pulses, indicating the dominant role of higher transient gain in the generation of even shorter gain-switched pulses with semiconductor lasers.