Deriving of Single Intensive Picosecond Optical Pulses from a High-Power Gain-Switched Laser Diode by Spectral Filtering

Single 25 ps/16 W optical pulses were achieved by spectral filtering from a multiheterostructure gain-switched laser diode with its quasisteady-state modes suppressed by a factor of 103 as compared with the peak power. A significant transient spectrum broadening makes this possible provided that a very high dI/dt rate of the pumping current pulse is used. A simple numerical model is suggested which describes adequately both the spectral and transient features of the observed phenomenon. It follows from the model that single picosecond optical pulses can be obtained from any type of high power semiconductor laser

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

Laser diode structures with a saturable absorber for high-energy picosecond optical pulse generation by combined gain-and Q-switching

The performance of gain-switched Fabry-Perot asymmetric-waveguide semiconductor lasers with a large equivalent spot size and an intracavity saturable absorber was investigated experimentally and theoretically. The laser with a short (∼20 μm) absorber emitted high-energy afterpulse-free optical pulses in a broad range of injection current pulse amplitudes; optical pulses with a peak power of about 35 W and a duration of about 80 ps at half maximum were achieved with a current pulse with an amplitude of just 8 A and a duration of 1.5 ns. Good quality pulsations were observed in a broad range of elevated temperatures. The introduction of a substantially longer absorber section leads to strong spectral broadening of the output without a significant improvement to pulse energy and peak power

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

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

A low noise front end trans-impedance amplifier channel for a pulsed time-of-flight laser radar

Abstract A low noise front end trans-impedance amplifier for a pulsed time-of-flight laser range finder receiver is presented. The architecture is based on unipolar-to-bipolar pulse shaping immediately at the input of the receiver channel, where the received unipolar current pulse is converted to a bipolar current to be fed to the trans-impedance amplifier (TIA). An extensively wide dynamic range is achieved using the proposed pulse shaping scheme and TIA realization. As the timing point is located at the first zero crossing point of the bipolar signal, the non-idealities of the TIA have little effect on it. Simulations show the walk error of the TIA channel to be less than ±40ps in the dynamic range 1:100000. The input-referred noise current and bandwidth are 72nA and 415MHz, respectively

Timing and probability of crosstalk in a dense CMOS SPAD array in pulsed TOF applications

Abstract As the distance between neighboring devices in large CMOS single-photon avalanche diode (SPAD) arrays is reduced for improving the density, increased crosstalk becomes an important issue, limiting the maximum practical fill factor of the array. In this study, the temporal correlation of crosstalk events, as well as the crosstalk probability, and their dependence on parameters, such as the illumination wavelength and intensity, and the distance between SPADs, are investigated via measurement of a ~45%-fill factor CMOS SPAD array fabricated using 0.35-µm high-voltage CMOS technology. The SPADs have 24 µm × 24 µm square-shaped active areas, and all devices share a common deep-N-well cathode. On-chip time-to-digital converters with 65-ps resolution are used to measure the timing of crosstalk events in “coincidence measurements.” For the crosstalk measurements, the internal noise in one SPAD is used to produce crosstalk events in the neighboring devices. The measurement results indicate both optical and electrical crosstalk with the crosstalk events, having a specific temporal distribution. The crosstalk probability in the first two adjacent pixels is found to be 0.3% and 0.01%, with a distribution having full widths at half maximum (FWHMs) of 700 and 400 ps, respectively. In pulsed time-of-flight measurements, when one SPAD is triggered with external short-pulsed (FWHM of approximately 200 ps) illumination, extra correlated noise in the adjacent SPADs added to the crosstalk noise, increasing the correlated noise considerably. This additional noise was a secondary effect of the absorbed laser photons deep in the substrate

256 x TDC array with cyclic interpolators based on calibration-free 2x time amplifier

Abstract This paper presents the design of a CMOS time-to-digital converter (TDC) used in a direct time-of-flight laser radar line receiver consisting of an array of single-photon avalanche diodes (SPADs) and 256 TDCs. The TDCs use the Nutt interpolation method, where the start/stop fine interpolators are based on a cyclic converter architecture. The cyclic converters use a delay-line-based time interval amplifier to double the quantization residue after each quantization cycle. The time amplifier, and the entire TDC array, does not need any biasing nor does the TDC results need any post-processing to provide sub-100-ps precision and uniformity across the TDC array, therefore making the design very robust and appealing for large designs where hundreds of TDCs are needed on a single chip. Fabricated in a 0.35 μm CMOS technology, a single TDC channel occupies an area of 0.03 mm² and consumes a power of 1.17 mW @ 100 kHz measurement rate. The TDC has a measurement range of 640 ns, and with a resolution of 15 bits, the LSB is 20 ps. Linearity is ensured by the Nutt interpolation method while the rms single-shot precision of the TDC channel is 72 ps. Thus, when LIDAR applications are considered, the TDC is capable of providing a range of 96 m with a precision of 1 cm without any post-processing or calibration