Boris S. Ryvkin

Asymmetric-Waveguide Laser Diode for High-Power Optical Pulse Generation by Gain Switching

A semiconductor laser with a strongly asymmetric waveguide structure and a relatively thick (~0.1 mum) active layer, resulting in an extremely large equivalent spot size, is proposed and analyzed for the purpose of generating high-power single-optical pulses by gain switching. An improvement in obtainable single-pulse energies of about an order of magnitude over conventional laser structures is predicted.

Performance improvement by a saturable absorber in gain-switched asymmetric-waveguide laser diodes

A simplest saturable absorber, in the form of an unpumped section, is introduced into a Fabry-Perot semiconductor laser with a strongly asymmetric broadened waveguide structure incorporating a relatively thick (80 nm) active layer. This allows for suppression of trailing oscillations and a decrease in the optical pulse width compared to the uniformly biased structure. Single optical pulses of ~80 ps full width at half maximum (FWHM) and ~35 W peak power (~3 nJ pulse energy, E(opt)), practically without trailing edge oscillations, were experimentally achieved under room temperature conditions by absorber-assisted gain-switching, using pumping current pulses of ~1.3 ns FWHM and ~17 A amplitude. The laser emission has a narrow (13 degrees FWHM in the transverse direction) far field.

On Laser Ranging Based on High-Speed/Energy Laser Diode Pulses and Single-Photon Detection Techniques

This paper discusses the construction principles and performance of a pulsed time-of-flight (TOF) laser radar based on high-speed (FWHM ~100 ps) and high-energy (~1 nJ) optical transmitter pulses produced with a specific laser diode working in an “enhanced gain-switching” regime and based on single-photon detection in the receiver. It is shown by analysis and experiments that single-shot precision at the level of 2W3 cm is achievable. The effective measurement rate can exceed 10 kHz to a noncooperative target (20% reflectivity) at a distance of > 50 m, with an effective receiver aperture size of 2.5 cm2. The effect of background illumination is analyzed. It is shown that the gating of the SPAD detector is an effective means to avoid the blocking of the receiver in a high-level background illumination case. A brief comparison with pulsed TOF laser radars employing linear detection techniques is also made.

High-Energy Picosecond Pulse Generation by Gain Switching in Asymmetric Waveguide Structure Multiple Quantum Well Lasers

A multiple quantum well laser diode utilizing an asymmetric waveguide structure with a large equivalent spot size of ~3 μm is shown to give high energy (~1 nJ) and short (~100 ps) isolated optical pulses when injected with <;10 A and ~1-ns current pulses realized with a MOS driver. The active dimensions of the laser diode are 30 μm (stripe width) and 3 mm (cavity length), and it works in a single transversal mode at a wavelength of ~0.8 μm. Detailed investigation of the laser behavior at elevated temperatures is conducted; it is shown that at high enough injection currents, lasers of the investigated type show low temperature sensitivity. Laser diodes of this type may find use in accurate and miniaturized laser radars utilizing single photon detection in the receiver.

Quantum well laser with an extremely large active layer width to optical confinement factor ratio for high-energy single picosecond pulse generation by gain switching

It is theoretically shown that gain switching quantum well AlGaAs/InGaAs laser with a very large ratio of active layer thickness to optical confinement factor (~8 µm) can result in single, high-energy (1 nJ) short (~100 ps) single optical pulses at wavelengths ~1 µm. Calculations predict that high electrical-to-optical conversion efficiency can be achieved in such structures, which require an injection current pulse of a modest amplitude of ~10 A and a duration of 1.5–2 ns. A narrow asymmetric waveguide design is proposed as a way of implementing such a structure while maintaining good far-field properties of the single emitted mode.

3 nJ, 100 ps laser pulses generated with an asymmetric waveguide laser diode for a single-photon avalanche diode time-of-flight (SPAD TOF) rangefinder application

An asymmetric waveguide laser diode with a thick active layer operated with enhanced gain switching is shown to be able to produce ?100 ps, ?25 W optical pulses in fundamental transverse mode with ?15 A, ?1.5 ns injection current pulses. A pulsed time-of-flight distance measurement demonstration utilizing this laser diode and a SPAD detector indicates centimetre-level precision and compensated accuracy from uncooperative targets at tens to hundreds of metres in a measurement time of a fraction of a second.

Vertical cavity surface emitting lasers with the active layer position detuned from standing wave antinode for picosecond pulse generation by gain switching

A vertical cavity surface emitting laser with the active layer position detuned from the standing wave antinode is proposed for the purpose of high energy picosecond pulse generation by gain switching and analysed using numerical simulations and a fully analytical model. An optimum value of the confinement factor for high-energy pulse generation is predicted and its origins discussed.

Heating-induced carrier accumulation in the optical confinement layer and the output power in broadened symmetric and narrow asymmetric waveguide laser diodes

We analyze the thermal effects in carrier accumulation (leakage) in the optical confinement layer of high-power λ=1.06μm semiconductor lasers. The experimental data for the symmetric broadened-cavity lasers are analyzed to extract the information on the current dependence of the internal loss and laser temperature. These data are used to predict the thermal behavior and output power-current dependence of a proposed asymmetric nonbroadened construction operating at the same wavelength, and a significant improvement is predicted.

Theory of direct and indirect effect of two-photon absorption on nonlinear optical losses in high power semiconductor lasers

The effect of the transverse laser structure on two-photon absorption (TPA) related effects in high-power diode lasers is analysed theoretically. The direct effect of TPA is found to depend significantly on the transverse waveguide structure, and predicted to be weaker in broad and asymmetric waveguide designs. The indirect effect of TPA, via carrier generation in the waveguide and free-carrier absorption, is analysed for the case of a symmetric laser waveguide and shown to be strongly dependent on the active layer position. With the active layer near the mode peak, the indirect effect is weaker than the direct effect due to the population of TPA-created carriers being efficiently depleted by their diffusion and capture into the active layer, whereas for the active layer position strongly shifted towards the p-cladding, the indirect effect can become the dominant power limitation at very high currents. It is shown that for optimizing a laser design for pulsed high power operation, both TPA related effects and the inhomogeneous carrier accumulation in the waveguide caused by diffusive current need to be taken into account.

Narrow versus broad asymmetric waveguides for single-mode high-power laser diodes

We investigate numerically the effect of the optical confinement layer thickness on the far field properties (far field shape and input efficiency) and confinement factor of an asymmetric-waveguide high power laser diode. A strong correlation is found between the confinement and input efficiency. It is shown that the far field properties of lasers with narrow asymmetric structures tend to be superior to those of broad waveguide ones with a similar confinement factor.