Wolfgang Pittroff

Mounting of high power laser diodes on boron nitride heat sinks using an optimized Au/Sn metallurgy

High power diode lasers have become more and more important to industrial and medical applications. In contrast to low power applications, long cavity lasers or laser bars are used in this field and mounting quality influences considerably laser performance and life time. In this paper we focus on the solder metallurgy and stress-induced laser behavior after mounting. The laser chips have been bonded fluxless epi-side down on translucent cubic boron nitride (T-cBN) using Au/Sn solder. The laser behavior has been tested with different chip metallizations preserving the eutectic solder composition or forming the Au rich /spl zeta/-phase during reflow. The resulting additional stress in the lasing region has been independently indicated by polarization measurements of the emitted light. A metallization scheme has been developed which forms the highly melting /spl zeta/-phase during soldering within a wide process window. This procedure yields better results then using eutectic Au/Sn which has a higher hardness than the /spl zeta/-phase. Laser diodes up to a cavity length of 2000 /spl mu/m and an aperture of 200 /spl mu/m have successfully been mounted on T-cBN. State of the art laser data, excellent thermal stability, high yield and reliability have been obtained.

3-W high-brightness tapered diode lasers at 735 nm based on tensile-strained GaAsP QWs

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.

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide LRW (500 μm ≤ LRW ≤ 1250 μm for L = 2 mm and 500 μm ≤ LRW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M2 were measured. From the experiments, it can be stated that the lasers with a small LRW reach higher output powers compared to those with larger LRW. Concerning the beam quality the length LRW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.

650-nm InGaP Broad Area Lasers With 5000-h Reliable Operation at 600 mW

Reliable operation of 650-nm broad area laser diodes with InGaP quantum-wells embedded in AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers is reported. Mounted on chemical vapor deposition (CVD)-diamond heat spreader and standard C-mounts, the 100-mum stripe width lasers showed reliable operation over 5000 h at 15 degC and 600 mW

Conductively Cooled 1 kW—QCW Diode Laser Stacks Enabling Simple Fiber Coupling

A new mini-bar design densely packed with 13 emitters in a 1.6 mm aperture has been developed. These mini-bars deliver >200 W output. With a very low divergence of 95% power enclosed in 25° they are suitable for fast axis collimation (FAC) lenses with large working distance. The mini-bars are mounted on compact CuW carriers, which are further integrated into a custom stack assembly where they are conductively cooled from both sides. This completely AuSn soldered stack consists of only coefficient of thermal expansion matched materials and was 100 times temperature cycled from -40°C to 85°C, without any change in the PUI characteristic, the spectrum and the far field. The beam propagation factor M 2 of a 12-layer stack is ¿130 in the vertical direction (after FAC) and ¿210 in the lateral direction. Thus, the stack can be coupled to a 1.2 mm multi-mode fiber (M 2 = 260) with a coupling efficiency of 90% using only low-cost lenses.

Limitations to peak continuous wave power in high power broad area single emitter 980 nm diode lasers

980nm broad area diode lasers with stripe widths in the 90–100µm range are commercially important as pumps for fiber lasers, as well as for direct applications [1]. Recent work on 910–990nm sources has shown that peak continuous wave (CW) powers in the 20–25W range are achievable in such structures [2,3] and reliable operation for ≫ 4000 hours was reported at 20W [4]. Further improvements in peak reliable power and beam quality of diode laser sources are required to enable greater brightness from the overall laser system.

Mounting of laser bars on copper heat sinks using Au/Sn solder and CuW submounts

GaAs high power diode laser bars were mounted on CuW submounts using an optimized Au(80)Sn(20) solder. During soldering the dissolution of an additional Au layer placed on the die caused the growth of the gold rich /spl zeta/-phase in the solder and thus an isothermal solidification. The laser bars on CuW submounts were mounted on passively cooled Cu heat sinks using a Pb(37)Sn(63) solder. After mounting, the degradation of 800 nm and 960 nm laser bars operated at 30 W and 45 W, respectively, was examined. Degradation rates less than 10/sup -5/ h/sup -1/ have been obtained. Measurements of stress distribution and failure analysis after aging have been performed.

Cryogenic ultra-high power infrared diode laser bars

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

Tensile-strained GaAsP-AlGaAs laser diodes for reliable 1.2-W continuous-wave operation at 735 nm

/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.

Al-free 950-nm BA diode lasers with high efficiency and reliability at 50° C ambient temperature

Tensile-strained GaAsP quantum wells embedded in AlGaAs large optical cavity structure were investigated at an emission wavelength of 735 nm. 1.2-W continuous-wave operation for 100-/spl mu/m stripe width diode lasers over 1000 h is reported. Experiments with different stripe widths showed a high stability at an output power of 12-mW//spl mu/m stripe widths with degradation rates below 5/spl middot/10/sup -5/ h/sup -1/, i.e., lifetimes larger than 5000 h could be expected.