Johann Luft

8-W high-efficiency continuous-wave semiconductor disk laser at 1000 nm

We demonstrate more than 8-W continuous-wave output power with good beam quality (M2<1.8) from an optically pumped semiconductor disk laser. The combination of low threshold density of 470 W/cm2 and high differential efficiency of 60% results in an optical-to-optical conversion efficiency of 46% for this high output level. Good epitaxial quality and low thermal resistance allow the scaling of output power with pump spot area.

Optically pumped semiconductor thin-disk laser with external cavity operating at 660 nm

The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.

More brilliance from high-power laser diodes

The introduction of high power diode laser systems in industry has boosted the interest in these devices for a wide range of applications. Besides printing and soldering, cutting and deep penetration welding are becoming more important. An overview about the developments, an update on todays high power laser activities and an outlook will be given, what characteristics laser bars will have to fulfil in the near future. For higher brightness, laser bars with lower fill factors, monolithic integrated laser junctions and tapered laser designs were investigated. High power diode laser (HPDL) bars with 25% - 50% fill factor were operated between 40 W and 80 W and lifetimes up to 100 000 hours could be extrapolated. Tapered laser bars with 50W output power and high wall plug efficiencies were developed. Wavelength multiplexing and polarisation coupling were used in order to reach multi-kilo-Watt diode laser emission. Examples for applications will be given.

Highly reliable 40-W cw InGaAlAs/GaAs 808-nm laser bars

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.

Vertical-External-Cavity Surface-Emitting Laser With Monolithically Integrated Pump Lasers

The monolithic integration of pump lasers and optically pumped vertical-external-cavity surface-emitting lasers is demonstrated. An innovative contacting scheme for the pump lasers offers high design flexibility and scalability. First devices at 1000 nm generate output powers of 2.5 W in pulsed and 0.65 W in continuous-wave operation.

High-efficiency high-power semiconductor disc laser

High efficiency, high power and excellent beam quality has been achieved in optically-pumped semiconductor disc lasers (OPS-disc laser) emitting at 1000nm. Minimizing the thermal resistance between active region and heat-sink, more than 5.5W of continuous wave (cw) output has been obtained at room-temperature. Even more remarkable, the laser characteristics corresponding to this power display differential efficiencies of better than 50% and optical conversion efficiencies of better than 40%. This combination of high power and high efficiency represents the best reported values so far. As such, a highly efficient beam converter has been realized, transforming low-brightness optical pump power into high-brightness laser emission.

Near-field photocurrent imaging of the optical mode profiles of semiconductor laser diodes

The potential of near-field photocurrent spectroscopy for direct imaging of mode profiles of submicron-sized waveguides in optoelectronic devices is demonstrated. The technique combines the submicron spatial resolution of near-field optics with tunable laser excitation, allowing for selective investigation of the waveguide properties of the device structure. Experiments on InGaAs/AlGaAs high-power laser diodes with different waveguide designs provide direct visualization of the effect of the waveguide design on (i) the number of guided modes and (ii) the spatial profile of both fundamental and higher-order modes. The technique thus provides a sensitive tool for nondestructive in situ analysis of waveguide properties in optoelectronic devices.

Green semiconductor disk laser with 0.7W cw output power

We demonstrate 0.7W cw output power at 520nm from an intracavity frequency doubled optically pumped semiconductor disk laser at room temperature. High beam quality and optical conversion efficiency of 10% has been achieved.

High-brightness high-power kW system with tapered diode laser bars

We report on a diode laser system, which is based on tapered diode laser bars and provides a combination of high power and high beam quality comparable to high power lamp pumped solid-state-rod lasers. Until now diode laser systems with output powers in the kW-range are based on broad area diode lasers. However, the output of these kilowatt laser systems usually is characterized by a strongly asymmetric beam profile, which is a consequence of the asymmetric beam parameter product (BPP) of broad area diode lasers with regard to the slow- and the fast-axis direction. Apparently the output of such a laser system can not be coupled efficiently into a fiber, which is required for a variety of applications. The symmetrization of the BPP of such a laser system requires complicated and expensive beam shaping systems. In contrast tapered diode laser bars allow the design of high power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Power scaling is realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. The total output power of the tapered diode laser system was 3230 W at a current of 75 A. Fiber coupling yielded 2380 W at 75 A for a fiber with a core diameter of 800 μm (NA 0.22) and 1650 W at 60 A for a 600 μm fiber (NA 0.22), respectively. Focusing with an objective with a focal length of 62 mm led to a beam diameter of 0.52 mm in the focal plane. Taking into account the total power of 2380 W behind the fiber the resulting intensity in the focal plane was 1.1 MW/cm2.

Selective excitation and photoinduced bleaching of defects in InAlGaAs/GaAs high-power diode lasers

Mounting-induced defects in semiconductor quantum-well (QW) lasers are investigated by photocurrent spectroscopy. The defects are located in the laser waveguides and give rise to an absorption band below the QW band gap with a maximum absorption cross section of σ=2×10−15 cm2. We observe a nonlinear fully reversible photobleaching of the defects and a resulting increase of QW photocurrent upon continuous wave irradiation of the devices, demonstrating a direct interaction between quantum-confined carriers and a defect level.