Michael Stoiber

Scalable high-power and high-brightness fiber coupled diode laser devices

The demand for high-power and high-brightness fiber coupled diode laser devices is mainly driven by applications for solid-state laser pumping and materials processing. The ongoing power scaling of fiber lasers requires scalable fibercoupled diode laser devices with increased power and brightness. For applications in materials processing multi-kW output power with beam quality of about 30 mm x mrad is needed. We have developed a modular diode laser concept combining high power, high brightness, wavelength stabilization and optionally low weight, which becomes more and more important for a multitude of applications. In particular the defense technology requires robust but lightweight high-power diode laser sources in combination with high brightness. Heart of the concept is a specially tailored diode laser bar, whose epitaxial and lateral structure is designed such that only standard fast- and slow-axis collimator lenses in combination with appropriate focusing optics are required to couple the beam into a fiber with a core diameter of 200 μm and a numerical aperture (NA) of 0.22. The spectral quality, which is an important issue especially for fiber laser pump sources, is ensured by means of Volume Holographic Gratings (VHG) for wavelength stabilization. In this paper we present a detailed characterization of different diode laser sources based on the scalable modular concept. The optical output power is scaled from 180 W coupled into a 100 μm NA 0.22 fiber up to 1.7 kW coupled into a 400 μm NA 0.22 fiber. In addition we present a lightweight laser unit with an output power of more than 300 W for a 200 μm NA 0.22 fiber with a weight vs. power ratio of only 0.9 kg/kW.

Tailored bar concepts for 10mm-mrad fiber coupled modules scalable to kW-class direct diode lasers

In this paper, laser modules based on newly developed tailored bars are presented. The modules allow efficient fiber coupling of more than 320 W into 10 mm-mrad or 160 W into 6 mm-mrad at one single wavelength. For further power scaling dense wavelength coupling concepts are presented which enable kW-class lasers with a beam quality of 10 mm-mrad.

Next generation 8xx nm laser bars and single emitters

Semiconductor lasers with emission in the range 790 - 880 nm are in use for a variety of application resulting in different laser designs to fulfill requirements in output power, operation temperature and lifetimes. The output power is limited by self heating and catastrophic optical mirror damage at the laser facet (COMD). Now we present data on bars fabricated with our new facet technology, which enables us to double the maximum facet load. We present q-cw laser bar with 80% fill factor with increased power level to 350W in long term operation at 200μs and 100Hz. The COMD limit of the bar is as high as 680W. Using Quantels optimized packaging stacks with 11 bars of 5mm widths are tested at up to 120A resulting over 66% power conversion efficiency at 1600W output power. Laser bars for continuous wave operation like 50% fill factor bars had an COMD limit of approx. 250W with conventional facet technology, the value is equivalent to 10W per 200μm emitter (conditions: 200μs). The new facet technology pushes the facet stability to 24W/emitter. The new process and an improved design enable us to shift continuous wave operation at 808nm from 100W to 150W/bar with lifetimes of several thousand hours at 30°C using DILAS mounting technology. Higher power is possible depending on lifetime requirements. The power conversion efficiency of the improved devices is as high as 62% at 200W cw. The next limitation of 8xxnm lasers is high temperature operation: Values of 60-80°C are common for consumer applications of single emitters. Therefore Osram developed a new epitaxial design which reduced the generation of bulk defects. The corresponding Osram single emitters operate at junction temperatures up to 95°C, a value which corresponds to 80°C heat sink temperature for lasers soldered on C-mount or 65°C case temperature for lasers mounted in TO can. Current densities of the single emitter broad area lasers are as high as 1.4kA/cm2 at 850nm emission wavelength.

Enhanced fiber coupled laser power and brightness for defense applications through tailored diode and thermal design

Advances in both diode laser design and packaging technology, particularly thermal management, are needed to enhance the brightness of fiber coupled diode lasers while maintaining the small size and light weight required for defense applications. The principles of design for high efficiency fiber coupling are briefly covered. Examples are provided of fielded and demonstrated 100 and 200 micron diameter fiber coupled packages ranging in output from a few hundred to kW-class units in fibers, to include sub-kg/kW capabilities. The demand for high-power and high-brightness fiber coupled diode laser devices is mainly driven by applications for solid-state and fiber laser pumping. The ongoing power scaling of fiber lasers requires scalable fiber-coupled diode laser devices with increased power and brightness. A modular diode laser concept combining high power, high brightness, wavelength stabilization and low weight, which is considerable concern in the SWaP trades needed to field defense systems, has been developed. In particular the defense technology requires robust but lightweight high-power diode laser sources in combination with high brightness. The heart of the concept is a specially tailored diode laser bar, with the epitaxial and lateral structures designed such that only standard fast- and slow-axis collimator lenses in combination with appropriate focusing optics are required to couple the beam into a fiber with a core diameter of 200 μm and a numerical aperture (NA) of 0.22. The spectral quality, which is an important issue especially for fiber laser pump sources, is ensured by means of Volume Holographic Gratings (VHG) for wavelength stabilization. This paper presents a detailed characterization of different diode laser sources based on the scalable modular concept. The optical output power is scaled from 180 W coupled into a 100 μm NA 0.22 fiber up to 800W coupled into a 400 μm NA 0.22 fiber. In addition we present a lightweight laser unit with an output power of more than 300 W for a 200 μm NA 0.22 fiber with a weight vs. power ratio of only 0.9 kg/kW.

Monolithically stacked high-power diode laser bars in quasi-continuous-wave operation exceeding 500 W

In this paper we report on quasi-continuous-wave (q-cw) operation of monolithically stacked laser diode bars. Monolithically stacked laser diode bars consist of more than one laser diode grown on top of each other. In between every two laser diodes a tunnel junction is included to ensure proper current injection to all lasers. In comparison to a standard laser operated at the same optical power level, the monolithic laser stack has a significantly reduced optical mirror load. Furthermore the required current is reduced drastically, which has positive consequences on both laser lifetime and diode driver costs. If one otherwise compares a monolithic integrated laser bar stack with a setup of three separate standard laser bars, the monolithic laser bar stack is characterized by very low costs per watt as well as high brilliance. By using monolithically stacked laser diode bars we were able to exceed an optical power of 500 W in q-cw mode and are moving to even higher output power levels. Typical wavelengths are in the range between 800 and 1000 nm.

Visible high power fiber coupled diode lasers

In this paper we report on further development of fiber coupled high-power diode lasers in the visible spectral range. New visible laser modules presented in this paper include the use of multi single emitter arrays @ 450 nm leading to a 120 W fiber coupled unit with a beam quality of 44 mm x mrad, as well as very compact modules with multi-W output power from 405 nm to 640 nm. However, as these lasers are based on single emitters, power scaling quickly leads to bulky laser units with a lot of optical components to be aligned. We also report on a new approach based on 450 nm diode laser bars, which dramatically reduces size and alignment effort. These activities were performed within the German government-funded project “BlauLas”: a maximum output power of 80 W per bar has been demonstrated @ 450 nm. We show results of a 200 μm NA0.22 fiber coupled 35 W source @ 450 nm, which has been reduced in size by a factor of 25 compared to standard single emitter approach. In addition, we will present a 200 μm NA0.22 fiber coupled laser unit with an output power of 135 W.

Extra bright high power laser bars

We present improvements of the lateral beam divergence and brightness of gain-guided mini-bars for emission at 976nm at highest brightness levels. The beam characteristics of devices in production are tailored for optimized fiber coupling to fiber diameter 200μm and numerical aperture 0.22, corresponding to a beam parameter product of 22 mm mrad. Cost-efficient coupling to this fiber requires a beam parameter product tailored to 15 mm mrad in lateral direction. The corresponding devices feature 5 emitters with cavity length 4mm and a lateral electrical contact width of 100μm per emitter. These bars can be driven to 44W before exceeding the beam parameter product of 15 mm mrad, corresponding to a slow axis beam divergence of 7° and a linear brightness of 2.9 W/mm mrad. In this work, we demonstrate results of two new mini-bar designs. The first design is intended for cost-reduction of divergence limited coupling systems and improves the output power at the slow axis divergence limit of 7° from 44W to 52W. The second structure, designed for coupling to fibers with beam parameter products as low as 11 mm mrad, exhibits significantly higher beam quality and excellent maximum linear brightness of 4.5 W/mm mrad, albeit at lower output power. All the structures exhibit power conversion efficiencies approaching 70%.

Diode laser modules based on new developments in tapered and broad area diode laser bars

In the last few years an increasing demand for high-brightness diode laser sources is observable, which is mainly driven by applications for fiber laser pumping and materials processing. A number of different approaches have been investigated in the past for the realization of such systems. In this paper we compare different concepts for high-brightness, high-power diode laser modules that are based on the new generation of tapered diode laser bars and new developments in broad area diode laser bars, respectively. One of the main advantages of tapered diode laser bars is the good beam quality in the slow-axis direction, which allows the design of high-power laser systems with a symmetric beam profile without the necessity of using sophisticated beam shaping systems. Such laser modules with multiple bars aiming for kilowatt output power can be realized with different incoherent coupling principles, including spatial multiplexing, polarization multiplexing and wavelength multiplexing. On the other hand, modules with a single or only a few tapered diode laser bars aim for very high brightness suitable for fiber coupling with fiber diameters down to 50 μm with a numerical aperture (NA) of 0.22. In this paper we present a detailed characterization of the new generation of tapered diode laser bars, including typical electro-optical data, measurements of beam quality and lifetime data. Tapered diode laser bars typically suffer from a broad spectrum which is extremely obstructive for pumping applications with small absorption bandwidths. To overcome this disadvantage we used volume bragg gratings (VBG) to improve the spectral quality of tapered diode laser bars. In addition to further improve the brightness of such diode laser systems we investigated external phaseplates to correct for smile and lens aberrations.