DILAS (Germany)

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.

Numerical calculations and experimental results show that, for the broad range of tensile strained p-AlxGa1−xAs/GaAs1−yPy/n-AlxGa1−xAs heterostructures widely used in commercial laser diodes emitting at 766–808 nm, polarization of emitted light may be extremely sensitive to external uniaxial stress due to the change of wave functions symmetry and possible optical transitions in the quantum well levels system. In some heterostructures with quantum well width of 10 nm and phosphorus content below 0.08, TM/TE polarization mode relation showcases a several times decrease and even dominant polarization mode switching under moderate compression of about 5–6 kbar in [100] and [110] directions.

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.

Abstract In the history of lasers, the diode laser is the most recent technology development for industrial applications. Driven by technology improvements, the diode laser has enormous potential in penetrating more and more industrial markets within a short time frame. It has replaced applications that were typically reserved for solid‐state lasers and has the capacity to actually generate better results due to its unique beam parameters. With Dilas' long‐term market history and experience, diode lasers are finding greater uses specifically for industrial usage. The latest generation series demonstrates this development and sets new standards in regard to highest reliability due to passive cooling technology. Within this article, new possibilities in the field of plastics welding, welding of thin sheet metals and soldering will be discussed.

We report on the EC BRIDLE project, which aims to develop a direct diode laser source (with more than 2kW of output power from a 100μm diameter optical fibre and an efficiency of 40%) for industrial sheet metal cutting applications. The project concepts are explored, from novel tailored diode laser mini-bars to advanced beam combining architectures.

Constant advancement in laser sources leads to commercial and industrial lasers with ever higher output powers and brilliance. The increasing capabilities of diode laser sources in particular produces extreme challenges for fibre launching. The difficulties arise due to the nature of the diode lasers, which are often designed with a numerical aperture (NA) exceeding the optical fibre’s NA and a spot size overfilling the fibre core so as to maintain the best possible brilliance. In addition to these properties, the spot imaged onto the fibre facet is typically rectangular. The combination of these properties result in an imperfect launch efficiency, forcing the connector built around the optical-fibre to cope with the radiation which is “lost” from the core of the fibre. Improvements in the Optoskand SMAQ connector are discussed, along with the presentation of results showing the increased power- and loss- handling capabilities when used with a variety of diode laser sources at 976nm. The sources used in the tests are optimised for an optical-fibre of core diameter (O) 200μm and NA of 0.22. The sources range in maximum power from 150W to 1000W with a coupling efficiency of between 80 and 90%. Additional complimentary results are shown for a O=400μm fibre guiding light of NA=0.12 where launch efficiency is 90 to 95%.

Diode lasers have a broad wavelength range, from the visible to beyond 2.2μm. This allows for various applications in the defense sector, ranging from classic pumping of DPSSL in range finders or target designators, up to pumping directed energy weapons in the 50+ kW range. Also direct diode applications for illumination above 1.55μm, or direct IR countermeasures are of interest. Here an overview is given on some new wavelengths and applications which are recently under discussion. In this overview the following aspects are reviewed: • High Power CW pumps at 808 / 880 / 940nm • Pumps for DPAL – Diode Pumped Alkali Lasers • High Power Diode Lasers in the range < 1.0 μm • Scalable Mini-Bar concept for high brightness fiber coupled modules • The Light Weight Fiber Coupled module based on the Mini-Bar concept Overall, High Power Diode Lasers offer many ways to be used in new applications in the defense market.

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 &mu;m (NA 0.22).