Alexis Schmitt

Modeling of the performance of high-brightness tapered lasers

A comprehensive model has been developed to study the operating characteristics of high-power high-brightness lasers consisting of a ridge-waveguide section coupled to a tapered region. The model, based on the Beam Propagation Method (BPM), includes a non-linear gain coefficient, current spreading due to junction voltage, and thermal effects taking into account for the first time to our knowledge a longitudinal gradient in the device temperature. We first demonstrate that during operation unwanted radiation that does not couple into the lateral mode of the waveguide, systematically propagates into the tapered region, and leads to the deterioration of the beam quality. To deflect and scatter this radiation, the use of specific cavity-spoiling elements, consisting of grooves etched down through the active region, appears necessary. We also study the role of the ridge section length in the operation of the device. A long ridge-waveguide region, providing both a well defined fundamental mode in the ridge- waveguide, and a gain saturation in the tapered region, improves the beam stability, but can lead, on the other hand, to optical self-focusing. Thermal effects are also investigated. We show how thermal lensing induces a lateral quadratic phase curvature and therefore alters the astigmatism of the device.

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.

Improved beam quality for high-power tapered laser diodes with LMG (low-modal-gain) epitaxial layer structures

In high-power, high-brightness laser diodes, beam filamentation is one of the main physical effects that limit the device performance. Due to the interaction between the optical power and the carrier density in the active region of broad area devices, spatial hole-burning leads to an inhomogeneous optical index that causes the degradation of the optical beam profile. We show, that epitaxial layer structures with low optical confinement are much more insensitive to beam filamentation because of their reduced differential gain. Experimentally we find, that the beam quality of tapered laser oscillators can be improved by an order of magnitude, when epitaxial layer structures with reduced modal gain are used for the device fabrication. Two mm long tapered devices with a 200 micrometer wide output facet show near diffraction limited farfield profiles up to output powers of more than 2 W cw.

High-brightness highly reliable InGaAlAs/GaAs laser bars with reduced fill factor and 60% efficiency

We have investigated high-power diode laser bars from 808 nm to 980 nm. The presentation is focussed upon the development of suitable laser bars for improved beam quality at increased output power. For better beam shaping structures with reduced fill factor of 30% were developed. They were operated in continuous wave mode at power levels of up to 60W. Moreover industrial applications require lifetimes of more than 10,000 hours. We present data yielding an extrapolated lifetime of up to 100,000 hours at 40W with 60% wall-plug efficiency at 980nm cw.

Highly reliable and efficient laser bars and cost efficient packaging

High power diode lasers convince by their very efficient conversion of electrical into optical energy. Besides high efficiencies and record absolute power levels, reliability in all possible operation modes and cost become increasingly important. We present diode laser bars in the 940nm range with wall plug efficiencies of about 65% at an emission power of 100W and with excellent reliability. The test had been performed on a stack with 5 bars at an output power of 100W per 1cm bar and after about 4000hrs test time, lifetimes of more than 40 000hour were estimated. The efficiency of these bars was at the beginning and at the end of this test about 65%. Operation modes between cw operation and q-cw (200μsec pulses) were evaluated and it will be shown, that pulses in the range of 1Hz are the hardest conditions, which can cause catastrophic failures. Using submounts with matched thermal expansion coefficient, this failure was prevented and lifetimes similar to cw-operation were reached. In order to reduce costs of laser power, we developed a laser package that offers high power at good reliability and provides a collimated beam for about 5

Breakthroughs in laser bar component packaging enable a new generation of applications for self-cooled laser diode arrays

/W, as a cost target in mass production conditions. This was achieved by using packaging concepts that were developed for high power semiconductor devices. These results will further enhance the applicability of diode lasers in industrial application.

High-Power InAlGaAs Laser Diodes with High Efficiency at 980 nm

Laser Diode Arrays continue to gain momentum as versatile, cost effective, reliable solution for a wide variety of existing and emerging illumination and pumping applications. In order to meet these growing demands, designers find themselves faced with three challenges: reducing system size, improving user serviceability, and managing cost. We developed a compact laser package platform that offers high output power, good reliability, and different beam collimation options. Both active cooling and passive cooling is possible with this new packaging concept. It has the footprint of the TO263 package and is based on packaging concepts that were developed for high power semiconductor devices and high volume opto semiconductor products like Light Emitting Diodes. High efficiency and high power laser bars are critical to various pumping and material processing applications. Wavelength multiplexing is an option to increase output power from laser systems. Typical wavelengths used are 808nm, 940nm and 980nm. We discuss the results of wavelength multiplexing of 880nm high power lasers.

Current status of high-power diode laser bars for pump and direct applications

Within the last few years, high power laser diodes with remarkable improvements concerning output power, efficiency, and reliability have been investigated in the wavelength range between 780 nm and 1064 nm. The discussion, whether laser diodes fabricated from Al-free material systems can surpass the performance of devices made from the conventional InAlGaAs-material system is still ongoing. In our contribution to this discussion we present 980 nm high-power InAlGaAs-laser diodes and laser diode bars with high conversion efficiencies grown by MBE. Broad area laser diodes with 100 micrometer aperture show an output power as high as 9.2 W cw at room temperature corresponding to a COMD level of 17 MW/cm2. Up to this output power the conversion efficiency remains above 46%. The highest efficiency of nearly 60% is reached at 2.5 W of output power. Reliability tests are ongoing and predict a lifetime of at least 20.000 h at a power level of 1.5 W cw. Laser diode bars of 1 cm width comprising 25 of these oscillators have been fabricated. Similar to single emitters these devices achieve a conversion efficiency of 58% at 62 W of cw output power. In terms of conversion efficiency and output power these results are among the best reported for both, Al-containing and Al-free laser diodes and laser diode bars. They can be attributed to the material quality, the facet coating technology, and the design of our devices. Clearly, they show the competitiveness of the material system used here.

High-efficiency and high-brightness 980-nm AlGaAs/InGaAs Diode Lasers

In this paper, the current status of high-power diode laser bars for pump and direct applications are proposed. The Nd:YAG with absorption lines at 808 nm and the Yb:YAG with absorption lines at 940 nm are actually most important for high power diode laser pump applications. Compared to flash lamp pumped solid state lasers, diode pumped systems have advantages in efficiency, lifetime and beam quality. Beside the current status of industrial bar products, top values from research labs in important laser characteristics are quite above the actual level. Today, for some material processing applications, the tapered laser structure seems to be most favourable, concerning beam quality and reproducible production.

Module support for an electronic module, has electrically insulating support body with upper side and lower side, and cooling channel runs in support body

High-brightness and high-efficiency laser bars have been fabricated in the AlGaAs/InGaAs material system. The electro-optical conversion efficiency is close to 60 % at 60 W optical power for bars with broad area lasers. Laser bars comprising 25 tapered emitters have been fabricated as well. Their total output power is higher than 25 W and the average beam quality factor M2of each individual emitter is 2.18 at 10 W total power.