Günther Grönninger

Advances in performance and beam quality of 9xx-nm laser diodes tailored for efficient fiber coupling

The impact of new direct-diode and fiber laser systems on industrial manufacturing drives the demand for highbrightness diode laser pump sources suitable for simple fiber coupling with high efficiency. Within the German funded project HEMILAS laser mini-bars with different bar geometries and small fill factors were investigated. We present results on 9xx nm bars with tailored beam parameter products for simplified coupling to fibers with core diameters of 200μm and 300μm with a numerical aperture of 0.22 and compare beam quality parameters, brightness, conversion efficiency, and thermal performance of different bar designs. Optimized epitaxy structures yield conversion efficiency maxima above 66%. The slow axis divergence angle of mini-bars with a fill factor of 10% featuring five 100μm wide and 4mm long emitters based on this epitaxy structure stays below 7°, which corresponds to a beam parameter product of 15mm mrad, up to very high output power of over 45W. This result was achieved for mounting on actively cooled submounts using hard solder. A similar bar with 5mm cavity length and using soft soldering reached an output power of 60W at the same beam parameter product. At 4mm cavity length, no COMD failures were observed up to currents exceeding the thermal rollover and the maximum output cw power was 95W.

Brilliant low fill factor diode laser bars at 9xx nm for fiber coupling

New semiconductor multi emitters combine the advantages of laser bars and strength of single emitters for fiber coupling applications. We introduce a new technology to drive the device at highest power density at the laser facet. The new technology enables us to reduce the fill factor while maintaining output power per laser bar at reliable and efficient operation. The overall output power and slow axis beam parameter product is scaled with the numbers of emitters and may be coupled into a single fiber with low effort in beam shaping or fiber combining. We demonstrate that multi emitters can operate at same power level as the same number of single emitters. In this paper we present data on highly efficient and reliable 9xx nm laser bars designed for a defined fiber diameter and numerical aperture. For comparison single emitters and short bars with different fill factors were investigated. Efficiencies above 60% were reached with 4mm cavity length and fast axis far field angles of 45° (95%). Stable operation at powers up to 70W from short bars with five 100μm wide emitters was reached. Slow axis divergence is below 7° up to power levels of 38W and is suitable for coupling into 200μm NA 0.22 fibers with only slow axis and fast axis collimation without beam rearrangement.

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.

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.

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%.

Tailored 9xx nm laser bars for fiber coupling

Status on low fill factor 9xx nm laser bars will be presented. Conversion efficiency peaks above 66% and slow axis divergence of less than 7° was reached up to 45W for 10%-fill factor half-bars.

IR lasers optimised for high-temperature operation in frequency-doubled green module for mobile laser projection

The above presented data of a green laser module regarding output power of 100mW based on an 850nm pump laser, 1060nm VECSEL and intra-cavity second-harmonic generation prove an excellent performance of the whole module with the overall WPE of as high as 10% at 40°C. It is the highest value of all cutting edge technologies enabling green lasing achieved up to date with capability of high-speed modulation in MHz range. Reliability and WPE of a pump die at high temperatures are of crucial importance for this kind of device. We demonstrate reliable operation up to 90°C junction temperature of the pumping IR laser diode at current density of 1.4kA/cm2, which is also used as a single emitter delivering 0.5–1W output power after being mounted in TO-can under the same junction temperature and current load.

Low fill factor diode laser bars with high brilliance from 808 nm to 1020 nm for fibre coupling

In recent years high brilliance semiconductor diode lasers are gaining attraction for the industrial laser market. The emerging generation of multi kilowatt fibre lasers require brilliant pump sources and additionally fibre coupled direct diode lasers increasingly compete with solid state lasers in many industrial areas. This paper investigates diode lasers, which combine several advantages from single emitters and high fill factor laser bars when considering fibre coupling applications. New facet technologies enable the laser to be driven at higher powers per emitter allowing a reduction in fill factor without penalty in total output power per laser bar. These low fill factor bars enable reliable and efficient operation at output power densities comparable to single emitters. The resulting beam parameters allow coupling into a single fibre with low effort in beam shaping or fibre combining.