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%.
The performance of diode lasers in the visible spectral range has been continuously improved within the last few years, which was mainly driven by the goal to replace arc lamps in cinema or home projectors. In addition, the availability of such high power visible diode lasers also enables new applications in the medical field, but also the usage as pump sources for other solid state lasers. This paper summarizes the latest developments of fiber coupled sources with output power from 1.4 W to 120 W coupled into 100 μm to 400 μm fibers in the spectral range around 405 nm and 640 nm. New developments also include the use of fiber coupled multi single emitter arrays at 450 nm, as well as very compact modules with multi-W output power.
In modern diode lasers beam shaping of the highly asymmetric laser beam, which exits the front facet of the semiconductor laser material, is a crucial step towards cost efficient high brightness laser modules which in turn can be further combined towards kW-class diode lasers and can be efficiently fiber coupled. In order to scale up the power of a single laser module in an economic way, high fill factor laser bars are employed. The increased power density from such a laser bar requires improved cooling technologies. On the other hand the increased fill factor of the bar makes advanced beam shaping necessary to be able to achieve small focal spot sizes and couple the laser module efficiently into optical fibers. Finally, to be able to mass produce the laser modules, it is desirable to design the module in a way that allows automated packaging and optics alignment. In this talk, the beam shaping concepts developed at DILAS for high fill factor bars are presented. Starting from optical simulation and choice of optical elements the laser modules incorporating these bars are presented. The concepts developed enable very compact laser modules of up to 2kW of power at a single wavelength with beam qualities of less than 40mm x mrad. Optionally these modules can be wavelength stabilized via external feedback. The packaging technology developed enables the automated alignment of the optics and cooling is DI-water free. Based on the same concepts very compact free space and fiber coupled QCW packages are presented as well.
The demand for high brightness fiber coupled diode laser devices in the multi kW power region is mainly driven by industrial applications for materials processing, like brazing, cladding and metal welding, which require a beam quality better than 30 mm x mrad and power levels above 3kW. Reliability, modularity, and cost effectiveness are key factors for success in the market.
We have developed a scalable and modular diode laser architecture that fulfills these requirements through use of a simple beam shaping concept based on two dimensional stacking of tailored diode bars mounted on specially designed, tap water cooled heat sinks. The base element of the concept is a tailored diode laser bar with an epitaxial and lateral structure designed such that the desired beam quality in slow-axis direction can be realized without using sophisticated beam shaping optics. The optical design concept is based on fast-axis collimator (FAC) and slow-axis collimator (SAC) lenses followed by only one additional focusing optic for efficient coupling into a 400 μm fiber with a numerical aperture (NA) of 0.12.
To fulfill the requirements of scalability and modularity, four tailored bars are populated on a reduced size, tap water cooled heat sink. The diodes on these building blocks are collimated simply via FAC and SAC. The building blocks can be stacked vertically resulting in a two-dimensional diode stack, which enables a compact design of the laser source with minimum beam path length. For a single wavelength, up to eight of these building blocks, implying a total of 32 tailored bars, can be stacked into a submodule, polarization multiplexed, and coupled into a 400 μm, 0.12NA fiber. Scalability into the multi kW region is realized by wavelength combining of replaceable submodules in the spectral range from 900 – 1100 nm. We present results of a laser source based on this architecture with an output power of more than 4 kW and a beam quality of 25 mm x mrad.
We report on wavelength stabilized high-power diode laser systems with enhanced spectral brightness by means of Volume Holographic Gratings. 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 systems is an important method to increase the efficiency of diode pumped solid-state lasers. It also enables power scaling by dense wavelength multiplexing. To ensure a wide locking range and efficient 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 as well as its angular and spectral emission characteristics. In this paper we present detailed data on wavelength stabilized diode laser systems with and without fiber coupling in the spectral range from 634 nm up to 1533 nm. The maximum output power of 2.7 kW was measured for a fiber coupled system (1000 μm, NA 0.22), which was stabilized at a wavelength of 969 nm with a spectral width of only 0.6 nm (90% value). Another example is a narrow line-width diode laser stack, which was stabilized at a wavelength of 1533 nm with a spectral bandwidth below 1 nm and an output power of 835 W.
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.
We carried out a comprehensive study on single emitters with different antireflection (AR) coatings in the wavelength range between 780nm and 976nm, which have been exposed to optical feedback to investigate the reversible and irreversible impacts caused by back-reflected light. By observing the near-field pattern while varying the probe current, we got information about the influence on filamentation and on peak-power densities with and without external optical feedback. For GaAs-based laser diodes, the energy gap of GaAs makes a distinction at a wavelength of about 870nm. For shorter wavelengths, e.g. at 808nm, a substantial part of the feedback light is absorbed by the substrate and GaAs cap layers very close to the front facet leading to a significant heating of the outcoupling facet. For longer wavelengths, e.g. 976nm, this energy intrusion is not a local one at the front facet, but rather spreads along the whole cavity length.
We have carried out a comprehensive study on 976nm single emitters with different AR coatings (1%, 3%, 4%, and 5%), which have been exposed to optical feedback to investigate damages caused by back-reflected light and how to prevent them. By observing the near-field pattern while varying the probe current, we got information about the influence on filamentation and on peak-power densities with and without external optical feedback. For constant feedback strength, filamentation became more pronounced and more dynamic with increasing current. We observed bistable and chaotic “jumping” of high-intensity filaments. For usual operation currents and external feedback strengths of ≥4%, single emitters with low AR coating show COMDs; their positions correlate with excessive peaking in the near-field pattern. Finally we found that an increasing AR reflectivity depletes the influence of feedback light on the near-field pattern as well as on the emission spectra and lowers the risk of COMD.
In this paper we report on the further progress of fiber coupled high power diode lasers in the visible spectral range with regard to beam quality and spectral characteristics. Improved beam shaping concepts allow coupling of red and blue diode lasers into smaller fibers. For medical applications beam sources with narrow wavelength distribution in the blue spectral region were developed. Modules up to 100W in a 400μm NA0.22 fiber were realized. Progress in manufacturing technologies allows for coupling of more than 25W into a 200μm NA0.22 fiber in the blue wavelength range.
Diode lasers in the blue and red spectral range are the most promising light sources for upcoming high-brightness digital projectors in cinemas and large venue displays. They combine improved efficiency, longer lifetime and a greatly
improved color space compared to traditional xenon light sources. In this paper we report on high-power visible diode
laser sources to serve the demands of this emerging market. A unique electro-optical platform enables scalable fiber
coupled sources at 638 nm with an output power of up to 100 W from a 400 μm NA0.22 fiber. For the blue diode laser
we demonstrate scalable sources from 5 W to 100 W from a 400 μm NA0.22 fiber.
Fiber coupled diode laser devices are attractive light sources for applications in the area of solid-state laser pumping and materials processing. The ongoing improvement in the brightness of diode lasers, which means power per beam quality, makes more and more industrial applications accessible to diode lasers. For many applications in materials processing multi-kW output power with a beam quality of better than 30 mm x mrad is needed. Previously we have reported on a modular diode laser platform based on a tailored bar design (T-Bar) and have demonstrated an output power of up to 785 W out of a 200 μm NA 0.22 fiber at a single wavelength of 976 nm. We have now extended that tailored bar platform to different wavelengths in the range from 900 nm to 1100 nm. At each single wavelength efficient fiber coupling into a 200 μm NA 0.22 fiber will be demonstrated. One important concept for power scaling is coarse wavelength multiplexing with a spectral separation of typically about 40 nm. Combining of different wavelengths enables scalable multi-kW high-brightness diode laser units. Further power scaling can be achieved by dense wavelength multiplexing with a spectral separation of only about 5 nm. In this paper we report on a diode laser unit with 3.5 kW output power and a beam quality of 25 mm x mrad.
Bernd Köhler, Sandra Ahlert, Andreas Bayer, Heiko Kissel, Holger Müntz, Axel Noeske, Karsten Rotter, Armin Segref, Michael Stoiber, Andreas Unger, Paul Wolf, Jens Biesenbach
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.
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