AlGaAs/GaAs asymmetric-waveguide, short cavity laser diode design with a bulk active layer near the p-cladding for high pulsed power emission

It is shown theoretically that a GaAs/AlGaAs laser diode design using an asymmetric waveguide structure and a bulk active layer, located close to the p-cladding, can provide high output power in a single, broad transverse mode for short-wavelength (< 0.9 m, matching the spectral range of high efficiency of silicon photodetectors) pulsed emission in the nanosecond pulse duration region, typically << 100 ns. The dependences of the laser performance on the thickness of the active layer and the cavity length are analysed. It is shown that the relatively thick bulk active layer allows the of short cavity lengths (<1 mm), for achieving high pulsed power while maintaining a relatively low series resistance and a narrow far field

Asymmetric-waveguide, short cavity designs with a bulk active layer for high pulsed power eye-safe spectral range laser diodes

It is shown, by calculations calibrated against the authors’ recent experimental data, that an eye-safe wavelength range InGaAsP/InP high pulsed power laser design using a bulk active layer, which has a large refractive index step with respect to the optical confinement layer and is located close to the p-cladding, can provide substantial performance improvement compared to the best results achieved so far for this operating regime and wavelength. The dependence of the laser performance on the design parameters such as the thicknesses of the active layer and the waveguide, as well as the cavity length, are analysed. It is shown that the relatively thick bulk active layer in such InGaAsP/InP lasers allows the use of short cavity lengths (~1 mm or even shorter), for achieving high pulsed power while maintaining a low p-cladding series resistance (making for high efficiency) and a narrow far field (making for high brightness). A single-asymmetry structure with the asymmetric active layer location but symmetric optical confinement layer/cladding refractive index steps gives performance only marginally inferior that of a double-asymmetric one including asymmetric refractive index steps

Single Photon Lidar in Mobile Laser Scanning: The Sampling Rate Problem and Initial Solution via Spatial Correlations

Single photon lidars (in solid state form) offer several benefits over pulsed lidars, such as independence of micro-mechanical moving parts or rotating joints, lower power consumption, faster acquisition rate, and reduced size. When mass produced, they will be cheaper and smaller and thus very attractive for mobile laser scanning applications. However, as these lidars operate by receiving single photons, they are very susceptible to background illumination such as sunlight. In other words, the observations contain a significant amount of noise, or to be specific, outliers. This causes trouble for measurements done in motion, as the sampling rate (i.e. the measurement frequency) should be low and high at the same time. It should be low enough so that target detection is robust, meaning that the targets can be distinguished from the single-photon avalanche diode (SPAD) triggings caused by the background photons. On the other hand, the sampling rate should be high enough to allow for measurements to be done from motion. Quick sampling reduces the probability that a sample gathered during motion would contain data from more than a single target at a specific range. Here, we study the exploitation of spatial correlations that exist between the observations as a mean to overcome this sampling rate paradox. We propose computational methods for short and long range. Our results indicate that the spatial correlations do indeed allow for faster and more robust sampling of measurements, which makes single photon lidars more attractive in (daylight) mobile laser scanning

High-energy sub-nanosecond optical pulse generation with a semiconductor laser diode for pulsed TOF laser ranging utilizing the single photon detection approach

Bulk and quantum well laser diodes with a large equivalent spot size of da/Γa ≈ 3 µm and stripe width/cavity length of 30 µm/3 mm were realized and tested. They achieved a pulse energy and pulse length of the order of ~1 nJ and ~100 ps, respectively, with a peak pulse current of 6–8 A and a current pulse width of 1 ns. The 2D characteristics of the optical output power versus wavelength and time were also analyzed with a monochromator/streak camera set-up. The far-field characteristics were studied with respect to the time-homogeneity and energy distribution. The feasibility of a laser diode with a large equivalent spot size in single photon detection based laser ranging was demonstrated to a non-cooperative target at a distance of a few tens of meters

A solid-state block-based pulsed laser illuminator for SPAD-based time-of-flight 3-D range imaging

A laser diode illuminator for single photon detection (SPAD) based pulsed time-of-flight 3-D range imaging is presented. The illuminator supports the block-based illumination scheme. It consists of a 16-element custom designed common anode quantum-well (QW) laser diode bar working in the enhanced gain switching regime and lasing at ~810nm. The laser diode elements are separately addressable and driven with GaN drivers producing 5...10A/~2ns current pulses from a supply voltage of 90V. Cylindrical optics is used to produce a total illumination field-of-view of 40ºx10º (FWHM) in 16 separately addressable blocks. With a laser pulsing frequency of 256kHz and laser pulse energy of ~8.5nJ, the average optical illumination power of the transmitter is 2.2m

High Power 1.5um Pulsed Laser Diode with Asymmetric Waveguide and Active Layer Near p-cladding

We report first experimental results on a high-power pulsed semiconductor laser operating in the eye-safe spectral range (wavelength around 1.5 lm) with an asymmetric waveguide structure. The laser has a bulk active layer positioned very close to the p-cladding in order to eliminate current-induced nonuniform carrier accumulation in the p-side of the waveguide and the associated carrier losses. Moderate doping of the n-side of the waveguide is used to strongly suppress nonuniform carrier accumulation within this part of the waveguide. Highly p-doped InP p-cladding facilitates low series resistance. An as-cleaved sample with a stripe width of 90 lm exhibits an output power of about 18 W at a pumping current amplitude of 80 A. Theoretical calculations, validated by comparison to experiment, suggest that the performance of lasers of this type can be improved further by optimization of the waveguide thickness and doping as well as improvement of injection efficiency.publishedVersionPeer reviewe

Fluorescence-suppressed time-resolved Raman spectroscopy of pharmaceuticals using complementary metal-oxide semiconductor (CMOS) single-photon avalanche diode (SPAD) detector

In this work, we utilize a short-wavelength, 532-nm picosecond pulsed laser coupled with a time-gated complementary metal-oxide semiconductor (CMOS) single-photon avalanche diode (SPAD) detector to acquire Raman spectra of several drugs of interest. With this approach, we are able to reveal previously unseen Raman features and suppress the fluorescence background of these drugs. Compared to traditional Raman setups, the present time-resolved technique has two major improvements. First, it is possible to overcome the strong fluorescence background that usually interferes with the much weaker Raman spectra. Second, using the high photon energy excitation light source, we are able to generate a stronger Raman signal compared to traditional instruments. In addition, observations in the time domain can be performed, thus enabling new capabilities in the field of Raman and fluorescence spectroscopy. With this system, we demonstrate for the first time the possibility of recording fluorescence-suppressed Raman spectra of solid, amorphous and crystalline, and non-photoluminescent and photoluminescent drugs such as caffeine, ranitidine hydrochloride, and indomethacin (amorphous and crystalline forms). The raw data acquired by utilizing only the picosecond pulsed laser and a CMOS SPAD detector could be used for identifying the compounds directly without any data processing. Moreover, to validate the accuracy of this time-resolved technique, we present density functional theory (DFT) calculations for a widely used gastric acid inhibitor, ranitidine hydrochloride. The obtained time-resolved Raman peaks were identified based on the calculations and existing literature. Raman spectra using non-time-resolved setups with continuous-wave 785- and 532-nm excitation lasers were used as reference data. Overall, this demonstration of time-resolved Raman and fluorescence measurements with a CMOS SPAD detector shows promise in diverse areas, including fundamental chemical research, the pharmaceutical setting, process analytical technology (PAT), and the life sciences.Peer reviewe

Strong Doping of the n-Optical Confinement Layer for Increasing Output Power of High- Power Pulsed Laser Diodes in the Eye Safe Wavelength Range

Abstract—An analytical model for internal optical losses at high power in a 1.5 μm laser diode with strong n-doping in the n-side of the optical confinement layer is created. The model includes intervalence band absorption by holes supplied by both current flow and two-photon absorption, as well as the direct two-photon absorption effect. The resulting losses are compared with those in an identical structure with a weakly doped waveguide, and shown to be substantially lower, resulting in a significant improvement in the output power and efficiency in the structure with a strongly doped waveguid