We present a direct diode laser with an optical output power of more than 800 W ex 100 μm with an NA of 0.17. The system is based on 6 commercial pump modules that are wavelength stabilized by use of VBGs. Dielectric filters are used for coarse and dense wavelength multiplexing. Metal sheet cutting tests were performed in order to prove system performance and reliability. Based on a detailed analysis of loss mechanisms, we show that the design can be easily scaled to output powers in the range of 2 kW and to an optical efficiency of 80%.
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.
In this work we discuss the impact of visible light radiation on photodarkening generation in 1070-nm Yb-doped fiber lasers. Simultaneous photodarkening and photobleaching effects induced by 976 nm and 405 nm or 550 nm radiations respectively were investigated. We observed a significant photobleaching effect due to 405 nm radiation but not a complete recovery. A strong absorption of the 405 nm radiation by the excited ions (Excited-State Absorption) was also observed and found as a main limiting factor for the bleaching performance together with observation of photodarkening losses induced by ground-state absorption. To proper define the optimum bleaching wavelength we report, for the first time to the best of our knowledge, the Excited-State Absorption cross section in the visible range. The reported experiments allow to individuate the main parameters defining the optimum bleaching wavelength. In a final experiment, using optimized 550-nm wavelength bleaching radiation, we were able to operate a laser at 93% of its pristine power level compensating a power drop of about 45% in absence of bleaching. The method we present is an effective yet simple way to run laser using standard Al-silicate fibers with doping level over 1026 ions/m3 and high inversion.
This paper reviews and extends the work done on photodarkening by our project consortium and present our latest results
on bleaching and photodarkening mitigation in fiber lasers. We shows the need for a standard set-up to avoid
underestimation of photodarkening equilibrium losses and we suggest photodarkening losses scale with the square of Yb
doping level. Investigation on visible light emission suggest an interplay of visible light with the Yb excited level.
Finally we present an extensive investigation of photobleaching, both as post-irradiation and as simultaneous bleaching.
We show evidence photobleaching can effectively mitigate the impact of photodarkening on laser performance when
highly-doped Al-silicate fibers are used.
In this work we report on high-power diode laser modules covering a wide spectral range from 410 nm to 2200 nm.
Driven by improvements in the technology of diode laser bars with non-standard wavelengths, such systems are finding
a growing number of applications. Fields of application that benefit from these developments are direct medical
applications, printing industry, defense technology, polymer welding and pumping of solid-sate lasers.
Diode laser bars with standard wavelengths from 800 - 1000 nm are based on InGaAlAs, InGaAlP, GaAsP or InGaAs
semiconductor material with an optical power of more than 100 W per bar. For shorter wavelengths from 630 - 690 nm
InGaAlP semiconductor material is used with an optical power of about 5 W per bar. Extending the wavelength range
beyond 1100 nm is realized by using InGaAs on InP substrates or with InAs quantum dots embedded in GaAs for
wavelengths up to 1320 nm and (AlGaIn)(AsSb) for wavelengths up to 2200 nm. In these wavelength ranges the output
power per bar is about 6 - 20 W.
In this paper we present a detailed characterization of these diode laser bars, including measurements of power, spectral
data and life time data. In addition, we will show different fiber coupled modules, ranging from 638 nm with 13 W
output power (400 μm fiber, NA 0.22) up to 1940 nm with more than 50 W output power (600 μm fiber NA 0.22).
In this work we report on high-power diode laser modules with enhanced spectral brightness by means of volume
holographic gratings for wavelength stabilization.
High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the
center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping
applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser modules is an
important means for more efficient pumping of solid-state lasers with a narrow absorption bandwidth.
However, for efficient and reliable wavelength stabilization the parameters of the volume holographic grating and the
parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the volume
holographic grating, the reflectivity of the diode laser bar and the angular and spectral emission characteristics of the
diode laser bar. In addition, the lateral structure of the diode laser bar and the microoptical elements for beam shaping
have to be considered.
In this paper we present a detailed characterization of different diode laser systems with wavelength stabilization in the
spectral range from 790 - 1000 nm. The laser modules are divided into systems with and without fiber coupling. We will
present data for a wavelength stabilized single diode laser bar with an output power of 69 W at a wavelength of 808 nm.
Another example is a wavelength stabilized fiber-coupled diode laser module with an output power of 456 W for a fiber
with a core diameter of 400 μm (NA 0.22).
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.
High brightness becomes more and more important in diode laser applications for fiber laser pumping and materials
processing. For OEM customers fiber coupled devices have great advantages over direct beam modules: the fiber exit is
a standardized interface, beam guiding is easy with nearly unlimited flexibility. In addition to the transport function the
fiber serves as homogenizer: the beam profile of the laser radiation emitted from a fiber is symmetrical with highly
repeatable beam quality and pointing stability.
However, efficient fiber coupling requires an adaption of the slow-axis beam quality to the fiber requirements. Diode
laser systems based on standard 10mm bars usually employ beam transformation systems to rearrange the highly
asymmetrical beam of the laser bar or laser stack. These beam transformation systems (prism arrays, lens arrays, fiber
bundles etc.) are expensive and become inefficient with increasing complexity. This is especially true for high power
devices with small fiber diameters. On the other hand, systems based on single emitters are claimed to have good
potential in cost reduction. Brightness of the inevitable fiber bundles, though, is limited due to inherent fill-factor losses.
At DILAS a novel diode laser device has been developed combining the advantages of diode bars and single emitters:
high brightness at high reliability with single emitter cost structure. Heart of the device is a specially tailored laser bar
(T-Bar), which epitaxial and lateral structure was designed such that only standard fast- and slow-axis collimator lenses
are required to couple the beam into a 200&mgr;m fiber. Up to 30 of these T-Bars of one wavelength can be combined to
reach a total of > 500W ex fiber in the first step. Going to a power level of today's single emitter diodes even 1kW ex
200&mgr;m fiber can be expected.
In comparison with other laser systems diode lasers are characterized by a unique overall efficiency, a small footprint
and high reliability. However, one major drawback of direct diode laser systems is the inhomogeneous intensity
distribution in the far field. Furthermore the output power of current commercially available systems is limited to about
We report on a diode laser system with 11 kW output power at a single wavelength of 940 nm aiming for customer
specific large area treatment. To the best of our knowledge this is the highest output power reported so far for a direct
diode laser system. In addition to the high output power the intensity distribution of the laser beam is homogenized in
both axes leading to a 55 x 20 mm2 Top-Hat intensity profile at a working distance of 400 mm. Homogeneity of the
intensity distribution is better than 90%. The intensity in the focal plane is 1 kW/cm2.
We will present a detailed characterization of the laser system, including measurements of power, power stability and
intensity distribution of the homogenized laser beam. In addition we will compare the experimental data with the results
of non-sequential raytracing simulations.
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.
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 today's 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.
A major problem using laser diodes for longitudinal pumping of solid state lasers is the poor beam quality and the non symmetric beam profile of the diodes as the astigmatic beam emitted from the array of diodes exhibits a limited focusability in the direction of the slow axis of the diodes. To overcome this problem an optic which turns the astigmatic beam of a stacked diode array into a radially symmetric (stigmatic) beam has been designed. This symmetric beam is then focused into an axially water cooled disk laser to serve as a longitudinal pump source. Up to now different laser crystals have been investigated as laser source. Using Yb:YAG an output power of 60 W in qcw operation has been realized whereas Nd:YVO4 delivers a qcw power of 113 W. The optical to optical efficiencies are 18% and 32.3% respectively.
Diode pumping of solid state lasers promises many advantages compared with flashlamp pumping. Such as better efficiency, lower thermal stress, compact design, no high voltage and longer lifetime. In between several possibilities of pumping configurations direct coupling of laserdiodes and crystal seems to have the highest efficiency. Stacked arrays as pumping modules for Nd:YAG slab and rod lasers are investigated. The stack consists of 28 monolithic arrays mounted on copper microchannel coolers. The dimensions of the emitting area are 50 X 9 mm2 resulting in a maximum pump power density of 107 W/cm2. Using a slab crystal with dimensions of 3 X 16 X 65 mm3 and a doping concentration of 0.7 at% Nd, an output power of 140 W cw has been achieved. The optical efficiency has been 31%. With the same stacked array a rod of 50 mm length and 4 mm diameter has been pumped. The pump light has been concentrated into the rod by a cusp shape reflector. Using a stable resonator 170 W output power in free running operation and 100 W average power in high repetition rate Q-switch operation has been achieved.