The two-dimensional (2D) temperature profile of a high-power junction-down broad-area diode laser facet subject to back-irradiance (BI) is studied via CCD-based thermoreflectance (TR) imaging and finite element modeling. The temperature rise in the active region (ΔΤ<sub>AR</sub>) is determined at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations. Interestingly, our study shows that ΔΤ<sub>AR</sub> rises sharpest not when the back-irradiance is boresight-aligned with the active region but rather when it is centered in the absorbing substrate approximately 5 μm away from the active region, a distance roughly equal to half of the back-irradiance spot FWHM (9 μm). At this critical location, ΔΤ<sub>AR</sub> is found to increase by nearly a factor of three compared to its increase without back-irradiance. This provides insight on an important location for back-irradiance that may be correlated with catastrophic optical damage (COD) for diode lasers fabricated on absorbing substrates, and also suggests a thermal basis for truncated lifetime and deegraded performance for diode lasers experiencing backirradiance.
We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which
were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (130 °C) environment without
any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power
and a nice pulse shape at 130 °C. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of
waste heat, we demonstrate over 40-millisecond long-pulse operation of the 8XX-nm CS bars at 130 °C and 100 A.
Pumping the bar with 5-Hz frequency 15-millisecond rectangular current pulses, we generate over 60 W peak power at
100 A and 130 °C. During the pulse duration, the pulse shape of the CS bars is well-maintained and the power almost
linearly decays with a rate of 1.9% peak power per millisecond at 130 °C and 100 A. Regardless of the pulse shape, this
laser bar can lase at very high temperature and output pulse can last for 8 ms/2ms at 170 °C/180 °C (both driven by 60 A
current pulses with 5-Hz frequency, 10 millisecond pulse width), respectively. To the best of our knowledge, this is the
highest operating temperature for a long-pulse 8XX-nm laser bar. Under the condition of 130 °C and 100 A, the laser bars
do not show any degradation after 310,000 10-millisecond current pulse shots. The performance of stack arrays at 130 °C
and 100 A are also presented. The development of reliable high-temperature diode laser bar paves the way for diode
laser long-pulse pumping within a high-temperature environment without any cooling.
This paper gives an overview of recent development of high-efficiency 50-W CW TE/TM polarized 808-nm diode laser
bar at Lasertel. Focused development of device design and MBE growth processes has yielded significant improvement
in power conversion efficiency (PCE) of 50-W CW TE/TM polarized 808-nm laser bars. We have achieved CW PCEs of
67 % to 64 % at heat-sink temperature of 5 °C and 25 °C, respectively. Ongoing life-testing indicates that the reliable
powers of devices based on the new developments exceed those of established, highly reliable, production designs.
In this paper we present the use of high power diode arrays, spectrally stabilised using chirped Volume Bragg Gratings
as a pump source for a Nd:YAG based laser. The temperature dependant performance of a series of different stabilised
diodes, and the side pumped Nd:YAG slab resonator was measured over a 55°C temperature range. The best performing
stabilised LDAs exhibited Q-switched output energy consistent over 80% of the temperature range, and drop off by 40%
at the higher temperature extremes. Beam parameters of the laser such as divergence were found to drop in combination
with input energy. Factors such as spectral drifting of the diodes are also considered and the effect on the resonator is
Tight control of blood glucose levels has been shown to dramatically reduce the long-term complications of diabetes. Current invasive technology for monitoring glucose levels is effective but underutilized by people with diabetes because of the pain of repeated finger-sticks, the inconvenience of handling samples of blood, and the cost of reagent strips. A continuous glucose sensor coupled with an insulin delivery system could provide closed-loop glucose control without the need for discrete sampling or user intervention. We describe an optical glucose microsensor based on absorption spectroscopy in interstitial fluid that can potentially be implanted to provide continuous glucose readings. Light from a GaInAsSb LED in the 2.2-2.4 μm wavelength range is passed through a sample of interstitial fluid and a linear variable filter before being detected by an uncooled, 32-element GaInAsSb detector array. Spectral resolution is provided by the linear variable filter, which has a 10 nm band pass and a center wavelength that varies from 2.18-2.38 μm (4600-4200 cm<sup>-1</sup>) over the length of the detector array. The sensor assembly is a monolithic design requiring no coupling optics. In the present system, the LED running with 100 mA of drive current delivers 20 nW of power to each of the detector pixels, which have a noise-equivalent-power of 3 pW/Hz<sup>1/2</sup>. This is sufficient to provide a signal-to-noise ratio of 4500 Hz<sup>1/2</sup> under detector-noise limited conditions. This signal-to-noise ratio corresponds to a spectral noise level less than 10 μAU for a five minute integration, which should be sufficient for sub-millimolar glucose detection.
The performances of a pin versus a pn structure from GaInAsSb materials operating at room temperature are compared both from a theoretical point of view and experimentally. Theoretically, it is found in materials limited by generation-recombination currents, pn junctions have a higher D* than pin junctions. The thinner depletion region of pn junctions results in a lower responsivity but a higher dynamic resistance, giving an overall higher D* compared to a pin structure. A series of five p+pn+ Ga<sub>0.80</sub>In<sub>0.20</sub>As<sub>0.18</sub>Sb<sub>0.82</sub> detector structures latticed matched to GaSb substrates and with 2.37 μm cut off wavelength were grown by molecular beam epitaxy and processed into variable size mesa photodiodes. Only the doping of the absorbing (p) region was varied from sample to sample, starting with nominally undoped (~1x10<sup>16</sup> cm<sup>-3</sup> pbackground doping due to native defects) and increasing the doping until a p+n+ structure was attained. Room temperature dynamic resistance-area product R0A was measured for each sample. A simple method is presented and used to disentangle perimeter from areal leakage currents. All five samples had comparable R0A's. Maximum measured R0A was 30 Ω-cm<sup>2</sup> in the largest mesas. Extracted R0A's in the zero perimeter/area limit were about ~50 Ω-cm<sup>2</sup> (20-100 Ω-cm<sup>2</sup>) for all samples. Within uncertainty, no clear trend was seen. Tentative explanations are proposed.
In this study, we examine processes limiting the performance of 4 micron superlattice pin photodiodes for different temperature and mesa size regimes. We show that the performance of large mesa photodiodes at low temperature is most severely limited by a trap-assisted tunneling leakage current (x300), while small mesa sizes are additionally limited by perimeter leakage (x20). At room temperature, large mesa photodiodes are limited by the diffusion current, and small
mesa photodiodes are further limited by the perimeter leakage (x100). To reduce or eliminate the impact of perimeter leakage, we have tried passivating the mesa sidewalls with SiN, an approach that was only minimally successful. We have also laid the groundwork for another approach to elimination of perimeter leakage currents, namely, elimination of the sidewalls altogether through planar processing techniques. Planar processing schemes require the deposition of a
thick, wide bandgap semiconductor or "window layer" on top of the homojunction. We compare the performance of two otherwise identical InAs/GaSb superlattice homojunction detectors, except one with a GaSb window layer, and one without. We show that inclusion of the thick GaSb window layer does not degrade detector performance.
A focal plane array detector sensitive from 2.0-2.5 μm and consisting of 32, 1.0 mm x 50 μm pixels, all functional, is demonstrated. Mean room-temperature R<sub>0</sub>A is found to be 1.0 Ωcm<sup>2</sup>, limited by sidewall leakage. The focal plane array is fabricated from an MBE-grown homojunction <i>p-i-n </i>GaInAsSb grown on an <i>n</i>-type GaSb substrate. Back-illumination geometry is compared to front-illumination geometry and is found to be favorable, particularly the improved responsivity (1.3 A/W at 2.35 μm corresponding to 68% quantum efficiency) due to reflection of light off the metal contact. Further, back-illumination is the most convenient geometry for mounting the array onto a compact blood glucose sensor because both contacts can be mounted on one side, while detector illumination occurs on the other.