We present recent development of single lateral mode 1050 nm laser bars. The devices are based on an InGaAs/AlGaAs single quantum well and an asymmetric large optical cavity waveguide structure. By optimizing the AlGaAs composition, doping profiles, and QW thickness, the low internal loss of 0.5 cm<sup>-1</sup> and high internal quantum efficiency of 98% are obtained. A standard bar (10% fill factor; 4mm cavity length) reaches 72% peak electro-optical efficiency and 1.0 W/A slope efficiency at 25°C. To achieve high single lateral mode power, the current confinement and optical loss profile in lateral direction are carefully designed and optimized to suppress higher order lateral modes. We demonstrate 1.5W single lateral mode power per emitter from a 19-emitter 10mm bar at 25°C. High electro-optical efficiency are also demonstrated at 25°C from two separate full-bar geometries on conduction cooled packaging: 20 W with <50% electro-optical efficiency from a 19-emitter bar and 50 W with <45% electro-optical efficiency from a 50-emitter bar.
Proc. SPIE. 9733, High-Power Diode Laser Technology and Applications XIV
KEYWORDS: High power lasers, Materials processing, Reliability, Solid state lasers, Fiber lasers, Semiconductor lasers, Printing, Diodes, Fiber couplers, High power diode lasers, Data conversion, Solid state electronics
Key applications for 780-830nm high power diode lasers include the pumping of various gas, solid state, and fiber laser media; medical and aesthetic applications including hair removal; direct diode materials processing; and computer-to-plate (CtP) printing. Many of these applications require high brightness fiber coupled beam delivery, in turn requiring high brightness optical output at the bar and chip level. Many require multiple bars per system, with aggregate powers on the order of kWs, placing a premium on high power and high power conversion efficiency. This paper presents Coherent’s recent advances in the production of high power, high brightness, high efficiency bars and chips at 780-830nm. Results are presented for bars and single emitters of various geometries. Performance data is presented demonstrating peak power conversion efficiencies of 63% in CW mode. Reliability data is presented demonstrating <50k hours lifetime for products including 60W 18% fill factor and 80W 28% fill factor conduction cooled bars, and <1e9 shots lifetime for 500W QCW bars.
A novel, 9XX nm fiber-coupled module using arrays of highly reliable laser diode bars has been developed. The module is capable of multi-kW output power in a beam parameter product of 80 mm-mrad. The module incorporates a hard-soldered, isolated stack package compatible with tap-water cooling. Using extensive, accelerated multi-cell life-testing, with more than ten million device hours of test, we have demonstrated a MTTF for emitters of >500,000 hrs. In addition we have qualified the module in hard-pulse on-off cycling and stringent environmental tests. Finally we have demonstrated promising results for a next generation 9xx nm chip design currently in applications and qualification testing
We report on our progress developing long wavelength high power laser diodes based on the InGaAsP/InP alloy system emitting in the range from 1400 to 2010 nm. Output power levels exceeding 50 Watts CW and 40% conversion efficiency were obtained at 1470 nm wavelength from 20% fill factor (FF) bars with 2 mm cavity length mounted on water cooled plates. Using these stackable plates we built a water cooled stack with 8 bars, successfully demonstrating 400 W at 1470 nm with good reliability. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. For lasers emitting in the range from 1930 to 2010 nm we achieved output power levels over 15 W and 20 % conversion efficiency from 20% FF bars with 2 mm cavity length on a conductively cooled platform. Life testing of the 1470 nm lasers bars over 14,000 hours under constant current mode has shown no significant degradation.
The scalability of semiconductor diode lasers to multi-kilowatt power levels has increasing importance in direct diode material processing applications. These applications require hard-pulse on-off cycling capability and high brightness achieved using low fill-factor (FF) bars with a tight vertical pitch. Coherent uses 20%FF bars operated at <60W/bar packaged on water-cooled packages with a 1.65mm vertical pitch in the Highlight D-series, which achieves <8kW of power in a < 1mm x 8mm beam line at a working distance of ~ 280mm. We compare thermal measurement results to thermal fluid flow simulations to show the emitters are cooled to low junction temperatures with minimal thermal crosstalk, similar to single emitter packaging. Good thermal performance allows for scaling to operation at higher power and brightness. We present accelerated life-testing results in both CW and hard-pulse on-off cycling conditions.
The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.
Developments in Nd-based lasers pumped on the <sup>4</sup>I<sub>9/2</sub>→<sup>4</sup>F<sub>3/2</sub> transition have led to
demands for increased power, brightness, and spectral stability from diode pump sources.
We describe the development of fiber coupled diode pump sources that generate >120W
of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain
spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to
stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs
between facet reflectivity and VBG reflectivity was conducted in order to determine an
optimal combination that balances output power and locking range.
In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber
lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X
wavelengths. We describe the development of fiber coupled products with >80W from a
200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber
Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the
past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser
pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm
fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved
for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence
while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO,
diamond composite and other advanced materials. We present the results of residual bonding stress from various solders
and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of
threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for
a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability
under on-off life-testing and thermal cycling tests.
State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled
diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar
pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a
unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of
the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm<sup>2</sup> peak
power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in
the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability
results for these diode laser stacks are also reported.
We present kW QCW vertical and horizontal arrays composed of 200W bars (peak power) at 8xxnm wavelength. We
also present an unique Bar-on-Submount design using the electrically insulating submounts, which can provide a
platform for simple and flexible horizontal array construction. The p-n junction temperature of the arrays under QCW
operation is modeled with FEA software, as well as measured in this research. Updated reliability test results for these
kW arrays will be also reported. As the examples, we present the performance of the vertical arrays with > 57% Wall-Plug-Efficiency and the horizontal arrays with < 23 degree fast axis divergence (FWHM), both with 808nm wavelength.
The available wavelength for such arrays ranges from 780nm to beyond 1 um. Coherent also have the capability to
produce the array with wide and relatively uniform spectrum for athermal pumping of solid-state lasers, by integrating
diode lasers bars with different wavelength into single array.
We describe the performance of diode laser bars mounted on conductive and water cooled platforms using low smile processes. Total smile of <1μm is readily achieved on both In and AuSn based platforms. Combined with environmentally robust lensing, these mounts form the basis of multiple, high-brightness products.
Free-space-coupled devices utilizing conductively-cooled bars delivering 100W from a 200μm, 0.22NA fiber at 976nm have been developed for pumping fiber lasers, as well as for materials processing. Additionally, line generators for graphics and materials processing applications have been produced. Starting from single bars mounted on water-cooled packages that do not require de-ionized or pH-controlled water, these line generators deliver over 80W of power into a line with an aspect ratio of 600:1, and have a BPP of <2mm-mrad in the direction orthogonal to the line.
We present the reliability of high-power laser diodes utilizing hard solder (AuSn) on a conduction-cooled package
(HCCP). We present results of 50 W hard-pulse operation at 8xx nm and demonstrate a reliability of MTTF > 27 khrs
(90% CL), which is an order of magnitude improvement over traditional packaging. We also present results at 9xx nm
with a reliability of MTTF >17 khrs (90% CL) at 75 W. We discuss finite element analysis (FEA) modeling and time
dependent temperature measurements combined with experimental life-test data to quantify true hard-pulse operation.
We also discuss FEA and measured stress profiles across laser bars comparing soft and hard solder packaging.
Ongoing optimization of epitaxial design within Coherent device engineering has led to a family of high power-conversion-efficiency (PCE) products on conductively cooled packages (CCP) and fiber array packages (FAP). At a 25°C heat sink temperature, the PCE was measured at 71.5% with 75W CW output power on 30% fill-factor (FF) bars with passive cooling. At heat sink temperatures as high as 60°C the PCE of these bars is still maintained above 60%. Powered by such high efficiency 9xx nm diodes, Coherent FAP products have consistently exceeded 55% PCE up to 50W power levels, with 62% PCE demonstrated out of the fiber. High linear-power-density (LPD) operation of 100μm x 7-emitter bars at LPD = 80 mW/μm was also demonstrated. Bars with 7-emitter were measured up to 140W QCW power before catastrophic optical mirror damage (COMD) occurred, which corresponds to a COMD value of 200mW/μm or 2D facet power density of 29.4 MW/cm<sup>2</sup>. Leveraging these improvements has enabled high power FAPs with >90W CW from an 800μm-diameter fiber bundle. Extensive reliability testing has already accumulated 400,000 total real-time device hours at a variety of accelerated and non-accelerated operating conditions. A random failure rate <0.5% per kilo-hours and gradual degradation rate <0.4% per kilo-hours have been observed. For a 30% FF 50W CW 9xx nm bar, this equates to >30,000 hours of median lifetime at a 90% confidence level. More optimized 30% FF 9xx nm bars are under development for power outputs up to 80W CW with extrapolated median lifetimes greater than 20,000 hours.
The University of Chicago polarimeter, Hertz, is designed for observations at the Caltech Submillimeter Observatory in the 350 micrometer atmospheric window. Initial observations with this instrument, the first array polarimeter for submillimeter observations, have produced over 700 measurements at 3(sigma) or better. This paper summarizes the characteristics of the instrument, presents examples of its performance including polarization maps of molecular clouds and regions near the Galactic center, and outlines the opportunities for improvements with emphasis on requirements for mapping widely extended sources.