The effect of oxygen defects on the gradual degradation rates of power and nonradiative carrier recombination in ~800 nm laser diodes was studied experimentally. While intentional introduction of oxygen at low levels (<5×10^15 cm^-3) was observed to degrade lasing performance prior to aging, no variation in gradual degradation rate of lasing power was observed. This suggests that degradation in these devices is not due to nonradiative recombination at low levels of point defects. Simulation of our data indicates that the power degradation may arise from increased intracavity absorption.
The size and weight of a fiber laser source are important metrics. The diodes used to pump this device and the cooling associated with it are the largest contributors to the size and weight of the fiber laser source. A bar-based approach has the potential for a lighter more compact diode since the source heat removal is done from a more compact heat sink. In this paper we describe results obtained from the development of a bar-based fiber coupled diode source, resulting in over 500 W of power out of the fiber, with 49% efficiency, wavelength locked at 976 nm, from a 231 μm core 0.22 NA fiber. The resulting volume metric is 0.25 cm3 /W and the weight metric is 0.36 g/W, which includes the diode heatsink.
A new type of laser diode bar offering over 1000W peak power at near-infrared wavelengths (770nm to 1100nm) has been developed. Multi-bar arrays with bar-to-bar pitches as low as 350μm are assembled creating individual units with over 50kW peak power. Data will be presented showing performance at various operating conditions and pulse modes. Scaled assemblies with common electrical and thermal manifolds offering over 1MW of peak optical power within a 58mm x 63mm emission area will also be shown. The impact of beam shaping and beam conditioning with micro-optics will be presented. To power these devices, a new breed of drive and pulser electronics have been developed to operate over 1,000A with the voltage necessary to drive 1MW-class scaled assemblies. This presentation will show how an integrated approach to mating drive electronics and the diode arrays lead to optimal performance and significant size, weight, and cost advantages.
Light absorption at the facet of a high power diode laser can lead to severe heating and catastrophic optical damage. In this work, a combination of high resolution thermoreflectance imaging and a detailed heat transport model of the diode chip are used to measure facet absorption in diode lasers. This approach permits a direct measurement of the effectiveness of passivation layers in improving facet robustness and device lifetime. The ability to quantify facet absorption is an essential step toward enabling rapid development of alternative passivation technologies and improving the reliability and maximum output power of diode laser systems.
Long time being used in Datacom, in computer mice and cell phone, last decade VCSEL developments show that this laser source is an alternative to the long established edge emitter laser diodes (EELD), with attractive high power performance, better reliability, lower manufacturing costs and design flexibility. This paper will discuss the benefits of VCSEL compared to EELD and the importance to minimize parasitic inductance when used in LiDAR application were short pulses, high peak power are required1. A theoretical analysis will be presented to show the advantage of multijunction VCSEL with better Power Conversion Efficiency (PCE) and Brightness than single junction VCSEL.
Advancements in VCSEL and edge emitter laser diode technology, associated micro-optics and drive electronics has enabled use of direct diode lasers as illumination sources for solid-state lidar. This paper will discuss advancements in integrated, low cost solid-state laser diode based illumination sources that offer pulse widths below 5ns and peak powers over 1kW with very small form factors. Data will be presented on state-of-the-art near-infrared (NIR) and short-wave infrared (SWIR) sources. The impact on beam size and shape upon adding micro-optics to the assembly will also be shown.
Maximizing brightness and peak power out of the laser diode array is essential in the development of high powered solid state and fiber laser systems. Recent developments have enabled single 1 cm laser diode bars capable of producing over 500W peak power at wavelengths between 770nm and 1100nm. New technologies in bar cooling and optical beam shaping have allowed scaled laser pump diodes to achieve peak powers over 1MW. Novel manifold designs have allowed 100kW to 1MW stacks maintaining a brightness of over 11kW/cm2. The latest performance from high brightness pump diodes operating under a variety of pulse conditions will be discussed. Additionally, discussion will be provided regarding a novel method of powering and controlling diodes with megawatt-class powers in MIL applications.
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 (ΔΤAR) is determined at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations. Interestingly, our study shows that ΔΤAR 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, ΔΤAR 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.
Enhancements of laser diode epitaxy in conjunction with process and packaging improvements have led to the availability of 1cm bars capable of over 500W peak power at near-infrared wavelengths (770nm to 1100nm). Advances in cooler design allow for multi-bar stacks with bar-to-bar pitches as low as 350μm and a scalable package architecture enabled a single diode assembly with total peak powers of over 1MegaWatt of peak power. With the addition of micro-optics, overall array brightness greater than 10kW/cm2 was achieved. Performance metrics of barbased diode lasers specifically engineered for high peak power and high brightness at wavelengths and pulse conditions commonly used to pump a variety of fiber and solid-state materials are presented.
Military, industrial, and medical applications have expressed interest in using ~1550nm laser diodes as efficient laser sources for reduced eye safety concerns, especially where free-space propagation is concerned. In addition, covert military applications, including pointers and illuminators, benefit from the spectral insensitivity of common sensor types to ~1.5um wavelengths. While more efficient than other sources, high-power broad-area laser diodes can sometimes require complex packaging and beam shaping/combining optics in order to meet requirements for brightness, uniformity, stray light, and environmental insensitivity. This paper will highlight and discuss a range of developments exploring laser diode components near 1.5um for free-space military applications, including broadarea high-power illuminators and highly-collimated pointers. Options for tailored beam conditioning and performance results will also be presented.
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
characterised.
Operation of 808-nm laser diode pumps at elevated temperature is crucial to many applications. Reliable operation at high power is limited by high thermal load and low catastrophic optical mirror damage (COMD) threshold at elevated temperature range. We demonstrate high efficiency and high power operation at elevated temperatures with high COMD power. These results were achieved through device design optimization such as growth conditions, doping profile, and materials composition of the quantum-well and other layers. Electrical-to-optical efficiency as high as 62 percent was obtained through lowered threshold current and lowered series resistance and increased slope efficiency. The performance of single broad-area laser diodes scales to that of high power single bars on water-cooled copper micro-channel heatsinks or conductively-cooled CS heatsinks. No reduction in bar performance or significant spectral broadening is seen when these micro-channel coolers are assembled into 6-bar and 18-bar cw stacks for the highest power levels.
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