Today, infrared semiconductor lasers are used in a variety of applications in conjunction with a large range of different operating conditions. We report on improvements of different lasers, each tailored to the specific application.
For cw laser bars, we report on efficiency improvements to further increase the output power beyond today’s power limits for reliable operation with 250 W. For long term use under q-cw conditions, we show a very cost effective approach using a 1.5 mm cavity, capable to provide 500 W. For sensing applications we report on 200 μm wide emitters providing 130 W of pulsed power, based on monolithically stacked laser structures.
Proc. SPIE. 10086, High-Power Diode Laser Technology XV
KEYWORDS: High power lasers, Reliability, Laser applications, Resistance, Fiber lasers, Semiconductor lasers, Infrared lasers, Laser cutting, High power diode lasers, Gallium arsenide lasers, Broad area laser diodes, Laser systems engineering
Pump modules for fiber lasers and fiber-coupled direct diode laser systems require laser diodes with a high beam quality. While in fast axis direction diode lasers exhibit a nearly diffraction limited output beam, the maximum usable output power is usually limited by the slow axis divergence blooming at high power levels. Measures to improve the lateral beam quality are subject of extensive research. Among the many influencing factors are the chip temperature, thermal crosstalk between emitters, thermal lensing, lateral waveguiding and lateral mode structure.
We present results on the improvements of the lateral beam divergence and brightness of gain-guided mini-bars for emission at 976 nm. For efficient fiber coupling into a 200 μm fiber with NA 0.22, the upper limit of the lateral beam parameter product is 15.5 mm mrad. Within the last years, the power level at this beam quality has been improved from 44 W to 52 W for the chips in production, enabling more cost efficient pump modules and laser systems.
Our work towards further improvements of the beam quality focuses on advanced chip designs featuring reduced thermal lensing and mode shaping. Recent R&D results will be presented, showing a further improvement of the beam quality by 15%. Also, results of a chip design with an improved lateral emitter design for highest brightness levels will be shown, yielding in a record high brightness saturation of 4.8 W/mm mrad.
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 Quantel's 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
An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB)
microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind
catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field
μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start
from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects
are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD
dependence on temperature and power was analyzed in detail by
micro-Raman spectroscopy and real-time thermal
imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated
photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached
in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been
established. This understanding of the semiconductor physics behind COD is a key element for further improvement in
output power of AlGaInP diode lasers.
High-power laser bars with emission in the red spectral range (635 - 660 nm) are of great interest for several applications
such as display and projection solutions or pumping of Cr:LiSAF solid state lasers. Another field of application is a
medical use of red lasers. The German funded project ROLAS combines medical and technical aspects of photodynamic
therapy (PDT). One special approach under investigation is a laser bar based multi-port PDT system: To allow the
optimum treatment of widespread, complex-shaped tumors a PDT laser system with 8 independently operable fiber
outputs is designed, based on two laser bars with 652 nm emission and independently addressable emitters.
The necessity of individually addressable emitters leads to a more complex and allows a thermally less optimized
package design. In combination with conductive cooling - which is a must for most medical applications - the
possibilities for low-temperature operation of the laser bars are severely constricted. Especially for high-power laser bars
in the 635-660 nm range operation under the expected unfavourable thermal conditions constitutes an additional
challenge: These devices by principle exhibit a strong temperature dependence of their performance due to the
comparably weak carrier confinement in the InGaAlP material system.
In this paper, based on detailed measurements, an analysis of the temperature dependence of the laser bar performance is
carried out and the consequences for mounting and application of the laser bars are shown. The measurements illustrate
the significant progress that has been achieved during the last two years in terms of temperature stability by applying
specific design measures.
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.