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-1 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.
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.
KEYWORDS: Semiconductor lasers, High power diode lasers, Reliability, High power lasers, Solid state lasers, Solid state electronics, Fiber lasers, Diodes, Materials processing, Printing, Data conversion, Fiber couplers
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
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.
We report results of multi-cell life tests performed on nearly (500) laser diodes representing our new generation of very
efficient high power broad area 9xx nm lasers. The acceleration model showed a steep power dependence of the failure
rate with an exponent of nearly 6. Improvement in the facet passivation process resulted in significantly less power
acceleration of failures. Analysis of the life test on upgraded lasers showed median lifetime of 1,500,000 hours at
operating conditions of 8W and 350°C. Optical powers as high as 17.8W for thermally limited CW operation and 32W
for 20 μs pulsed operation were recorded. The CW life test was complemented by a life test performed at power cycling
conditions (1Hz repetition rate, 50% duty cycle).
Laser diodes and bars with high efficiency, power, and reliability are critical for a wide variety of applications including direct material processing and pumping high power and efficient fiber lasers and solid state lasers. We present progress towards the 80% power conversion efficiency goal of the DARPA Super High Efficiency Diode Sources (SHEDS) program. Currently, laser bars using JDSU SHEDS technology achieve as high as 72.7% total power conversion efficiency at room temperature and 80W operating power.
A new generation of very efficient high power laser diodes has been developed. The design was optimized for efficient operation of a long cavity device necessary to reduce electrical and thermal resistance. CW operation of a 100 μm wide laser at 25C yielded slope efficiency as high as 1.14W/A and 64% electrical-to-optical conversion efficiency. Optical power as high as 13.5 W for thermally limited CW operation and 17.3 W for pulsed operation were also recorded.
We have investigated the degradation behavior of high power diode-laser bars at 80 nm with single quantum and double quantum well structures in continuous wave operation. The 1 cm bars have a fill factor of 50%. Laser diodes with different resonator lengths from 300 micrometers to 2000 micrometers have been investigated. Different bars were mounted on actively cooled submounts and operated at comparable current densities and heat load.
The maximum useful optical power of laser bars is limited due to thermal and lifetime constraints to typical values of 50 W/cm cw or 120 W/cm qcw. A promising new approach is the so-called microstack laser in which several laseractive areas are integrated vertically in the same monolithic structure. In order to drive these structures in series with high efficiency low-resistance tunnel-junctions have to be realized. By optimizing the MOVPE growth process tunnel- junctions with a specific differential resistivity of 2.5 X 10-4 (Omega) cm2 could be obtained, which are suitable for the monolithic inter-connection of laser structures.
Solid state laser pumping is an important application for high power semiconductor lasers. The higher electro-optical conversion efficiency (50%), and narrower spectral emission (2 - 5 nm) of laser diodes allow for more efficient pumping compared to flash lamps, and consequently lead to superior thermal and optical properties of the solid state laser. A further performance increase and thus price reduction, especially for pulsed and qcw operation, is conceivable using a promising approach proposed almost 20 years ago which we dubbed 'micro-stack lasers.' It consists in vertically integrating multiple active laser junctions in one -- structure using degenerately doped tunnel junctions to electrically connect the intermediate reverse junctions. By stacking 2 to 4 emitters in this way, the output power of semiconductor lasers could theoretically be increased by a factor of 2 to 4 as the reliable output power is mainly limited by the power density at the laser facets. Modern growth technology can provide the necessary 10 to 20 micrometer thick high quality epitaxial layers. The limiting factors rather originate from the additional electrical and thermal resistance, and from current spreading towards the deeper junctions.
Christian Hanke, Lutz Korte, Bruno Acklin, Johann Luft, Stefan Groetsch, Gerhard Herrmann, Zeljko Spika, Marcel Marchiano, Bernhard DeOdorico, Jens Wilhelmi
The fundamental question whether aluminum-free semiconductor lasers in the 808 nm band are significantly more reliable than Al-containing lasers is still open. We have fabricated and tested high-power InGaAlAs/GaAs-lasers which show excellent reliability data at and above 40 W cw. The laser structure consists of an InGaAlAs-double-quantum well (DQW) as active layer embedded in a large optical cavity (LOC) waveguide structure. The layers were grown in a low pressure MOVPE (LP- MOVPE) reactor using high quality precursors. Asymmetrically coated bars with a width of 1 cm containing 25 groups of 200 micrometer wide emitters were mounted junction down on actively cooled heatsinks. At a heatsink temperature of 18 degrees Celsius the slope-efficiency is 1.1 - 1.2 W/A. Due to the low series-resistance of 2.2 m(Omega) and the low internal losses in the range of 1.7 cm-1 the overall efficiency at 40 W cw reaches 50%. Lifetime studies over 33 0000 h accumulated device hours show that the laser bars with a resonator length of 900 micrometer can be operated at 40 W with high reliability. The mean degradation rate is -0.11%/kh. This result emphasizes that Al-containing lasers can also have a very high reliability usually claimed for Al-free lasers. As a consequence of these encouraging results we will start further lifetime tests at 50 to 60 W.
The use of optical interconnections for communicating between chips or boards of a computer has been hampered by the lack of practical packaging schemes for free-space optics. In order to solve this problem, the concept of "planar optics" was proposed. This approach is based on the use of computer aided design and standard fabrication techniques adapted from semiconductor processing. Microoptical components are placed on the surfaces of a single optical substrates. Optoelectronic chips are integrated using hybrid techniques such as flip chip solder bump bonding. In this review article, we present the idea of planar optics, show several demonstration experiments, and discuss various aspects related to manufacturability. For our considerations, emphasis will be put on interconnection applications.
KEYWORDS: Neurons, Photodetectors, Optoelectronics, Very large scale integration, Analog electronics, Optical computing, Silicon, Chemical elements, Tolerancing, Optical interconnects
A systems is described which finds solutions to the 6-city TSP using a Kohonen-type network. The system shows robustness with regard to the light intensity fluctuations and weight discretization which have been simulated. Scalability to larger size problems appears straightforward.
A GaAlAs Fabry-Perot device has been grown which displays thermally stable switching with a threshold of 1 mW and a contrast ratio of 10:1. With the help of theoretical analysis and experimentation we probe the issues which are relevant to using this class of device in a practical system context.
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