Asymmetric waveguide design of laser diodes for pico- and nanosecond pulse generation in the eye safe spectral range: Linear and nonlinear electromagnetic effects

High energy optical pulse generation using semiconductor lasers has attracted significant attention recently, for applications such as high-precision laser radars, three-dimensional time imaging, spectroscopy, and lifetime studies. Depending on the resolution required, pulses of either several to several tens of nanoseconds or ~ 100 picoseconds in duration can be required. In the former case, from the laser dynamics point of view, the laser is operating in a steady state with a transient at the start of the pulse; in the latter case, the main techniques used are gain switching (pumping the laser with a current pulse of a nanosecond duration or somewhat shorter, but still significantly longer than the desired optical pulse), active or passive Q-switching (using a laser incorporating an active voltage-controlled modulator or a saturable absorber respectively), or a combination of these techniques. In our recent work (see [1,2] and references therein), we have used gain-switched Fabry-Perot asymmetric-waveguide semiconductor lasers with a large equivalent spot size d/Γ_α ≫ 1 μm (d being the active layer thickness and Γ_α, the active layer confinement factor). A saturable absorber can be monolithically integrated within the laser cavity to facilitate high-energy afterpulse-free optical pulse emission in a broad range of injection current pulse amplitudes by combined gain- and Q-switching. Optical pulses with a peak power of about 35 W and a duration of about 80 ps at half maximum, without a substantial afterpulse structure, were achieved with a current pulse with an amplitude of just 8 A and a duration of 1.5 ns. Good quality, after pulsing-free optical pulses were observed in a broad range of elevated temperatures. It has also been shown [3] that a similarly asymmetric laser structure, with a Quantum Well active layer, is advantageous for quasi-CW operation (that is, pumped by current pulses long enough for the output power transients to die out, but short enough to avoid heating), correspondingly emitting optical pulses with a similar duration, of a few nanoseconds to hundreds of nanoseconds long.