New pump module concepts had to be developed for space borne applications, because a simple transformation of terrestrial solutions to space requirements was often not useful. A planar approach has been chosen, which prevents inherent draw-backs of stacks.
High power 9xxnm QCW- pump modules are very interesting for high- and ultra-high-energy laser systems. Main relevant issues beside price and power conversion efficiency are long term stability of the mounting scheme and stable fiber coupling. We present a design based on diode laser stacks with lateral heat removal. A single stack element consists of a diode laser, which is soldered on both sides to CuW carriers using AuSn. Life test over 1000 h showed no degradation. DCB coolers are subsequently soldered onto both outer sides of the stack. The thermal resistance of a single stack element is about 1.7 K/W. For >3 J pulse energy the stack contains 28 elements. ≥60% power conversion efficiency of the used 940 nm diode laser chips at 120 W output power allows ≥20% duty cycle without substantial heating (maximum measured output power >200 W). The light is collimated in vertical direction for each stack element. We choose a size for the FAC which allows staggering the beams of two stacks. The diode laser chips have an aperture width of 1.2 mm and a lateral divergence <14° (95 % power) at 120 W. Fiber coupling is performed by cylindrical lenses in both directions. For 6 J pump energy two stacks are used, coupled into 1.9 mm diameter fiber with a high optical coupling efficiency of >90 %. The principle design is very flexible to match other demands in fiber size and output power.
GaAs-based high power diode lasers are the most efficient source of optical energy, and are in wide use in industrial applications, either directly or as pump sources for other laser media. Increased output power per laser is required to enable new applications (increased optical power density) and to reduce cost (more output per component leads to lower cost in $/W). For example, laser bars in the 9xx nm wavelength range with the very highest power and efficiency are needed as pump sources for many high-energy-class solid-state laser systems. We here present latest performance progress using a novel design approach that leverages operation at temperatures below 0°C for increases in bar power and efficiency. We show experimentally that operation at -55°C increases conversion efficiency and suppresses thermal rollover, enabling peak quasi-continuous wave bar powers of Pout > 1.6 kW to be achieved (1.2 ms, 10 Hz), limited by the available current. The conversion efficiency at 1.6 kW is 53%. Following on from this demonstration work, the key open challenge is to develop designs that deliver higher efficiencies, targeting > 80% at 1.6 kW. We present an analysis of the limiting factors and show that low electrical resistance is crucial, meaning that long resonators and high fill factor are needed. We review also progress in epitaxial design developments that leverage low temperatures to enable both low resistance and high optical performance. Latest results will be presented, summarizing the impact on bar performance and options for further improvements to efficiency will also be reviewed.
Compared to diode lasers emitting in the near infrared, the development of high power diode lasers in the red spectral range is more challenging due to the applicable compound semiconductors, the limited stability of the laser facets, and the small barrier heights for electrons and holes. For CW applications, their mounting requires excellent heat removal or expansion matched submounts. For QCW operation with small duty cycles and about 2 W per 100 μm stripe width emitter, like for the pumping of Q-switched alexandrite (Cr3+:BeAl2O4) lasers at 654 nm, a compromise is the application of aluminum nitride as heat sink. The presented broad area (BA) lasers are based on a GaInP single quantum well embedded in AlGaInP waveguide layers. The structure provides a vertical far field angle of 31° (FWHM). The material data can be compiled as follows: transparency current density jT = 220 A/cm2, internal efficiency ƞi = 0.83, internal losses αi = 1.0 cm-1. BA lasers with a stripe width of 100 μm and a length of 1.5 mm were fabricated, facet coated including a passivation procedure, and mounted on AlN submounts. In QCW operation (100 μs, 35 Hz) at 15°C, the devices had threshold currents of about 600 mA, slope efficiencies up to 1.3 W/A and conversion efficiencies of 0.36. A maximal output of 6.3 W was measured. At lower temperatures of -10°C the maximal peak power was determined to 7.6 W, i.e. a facet load of 76 mW/μm. The devices showed reliable operation over 1,000 h at a peak power of 2.7 W.
For the pumping of solid state lasers with high peak power pulses up to the TW range QCW diode laser stacks with pulse
lengths between 200μs and 2ms are used. To realize long-term stable pump modules we already presented high power, high brightness 100W QCW diode laser bars [1] having a lateral aperture of 1.7mm only, a length of 4mm and a vertical divergence of 14° FWHM. Based on these we have developed a mounting scheme for stacks with > 1kW output power using these new kind of diode lasers.
Due to the geometric dimensions of the chip we successfully realized a stack with a passive cooling scheme on both
sides. Furthermore, we only used expansion matched materials such as CuW and Al2O3 ceramics, as well as AuSn
solder processes for fixing the parts together. As a result the stack is very insensitive against environmental influences.
Due to the small vertical divergence we were able to use fast axis collimators with large focal lengths, which relax the
lens adjustment tolerances.
At the conference we will present results for diode laser stacks with an output power of more than 1kW at duty-cycles up
to 10% and an efficiency of about 50%. The beam parameter product for such diode laser stacks result in < 50mm•mrad
for the vertical direction and in < 75mm•mrad for the lateral direction. These beam parameter values enable the coupling
of the pump module to an optical fiber having a 1.2mm core diameter and a NA of 0.22.
Furthermore, the low vertical fill factor of the stack radiation allows the combination of two stacks by beam deflection
mirrors without significantly degrading beam quality, hence doubling the power coupled into the same fiber.
We demonstrate 940nm diode lasers with more than 100W QCW output power having an aperture width 5 to 10 times
smaller than commonly used 10mm bars. We used a super-large vertical waveguide structure to reduce the facet load.
The waveguide design results in a very small vertical divergence of only 14° FWHM (24° including 95% of power). The
threshold current of a device with 1mm wide aperture is about 8A and the slope efficiency is above 70%. The lateral far
field width is below 10°, including 95% of power, and the wall plug efficiency is around 50% at 100W output power.
Tensile strained GaAsP quantum wells embedded in AlGaAs waveguide structures are used to realize high power, high brightness short wavelength tapered laser diodes. At 735nm these laser diodes show up to 3W nearly diffraction limited output power with a wall plug efficiency of about 40%. Single spectral mode behavior is observed at output power levels up to 1W. From aging test a high realiability with lifetime exceeding 5000 can be derived comparable to results obtained from broad area laser diodes with the same aperture width. There are only small changes of the beam quality during aging. In conclusion it is shown that well designed tapered laser are a step forward to high efficiency, diffraction limited light soruces in the Watt-range which can be easily fabricated in high volumes.
KEYWORDS: Semiconductor lasers, Waveguides, Quantum wells, Aluminum, Cladding, Resonators, Lab on a chip, Near field optics, Gallium, Broad area laser diodes
In this paper we report on Al-free InGaAs/InGaAsP/InGaP broad area laser diodes emitting at 950 nm and on 810 nm- laser diodes with Al-free GaAsP quantum wells in AlGaAs waveguides. 2 mm long diode lasers show a high wall plug efficiency above 50% at output powers of about 3 W. The beam characteristics of these diode lasers benefit from small confinement factors. Results depending on stripe width and resonator length are given.
We report device properties and results of lifetime tests for Al-free InGaAs/InGaAsP/InGaP broad-area (BA) laser diodes, emitting at 950 nm. The epitaxial layers were grown by metal organic vapor phase epitaxy (MOVPE). The mounted diode lasers have a high wallplug efficiency around 60%, for a resonator length of 2 mm, and about 50% for 4 mm long devices due to low threshold current densities of jth equals 110 . . . 140 A/cm2, high slope efficiencies of 75% and the typical low series resistance of the Al-free material. The lasers were mounted on copper heatsinks, episide-down as well as episide- up. Lifetime tests were performed with a facet load of 15 mW/micrometers at temperatures between 25 degrees Celsius and 70 degrees Celsius and with a facet load of 20 mW/micrometers at 25 degrees Celsius. All diodes survived 3000 h with degradation rates lower than 6 X 10-5h-1 at 50 degrees Celsius and 1 X 10-4h-1 at 70 degrees Celsius as well as 2000 h with a low degradation rates of 2 X 10-5h-1 at 20 mW/micrometer. As far we know, the results belong to the best ones reported until now for Al-free BA laser diodes.
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