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Advances in fiber coupled high power diode laser modules at 793nm and 976nm for defense applications
This approach minimizes development costs due to its flexibility, while reducing overall product cost by using similar building blocks for a variety of products and utilizing a high grade of automation. The modules can be operated with industrial grade water, lowering overall system and maintenance cost.
Stackable macro coolers are used as the smallest building block of the system. Each cooler can hold up to five diode laser bars. Micro optical components, collimating the beam, are mounted directly to the cooler. All optical assembly steps are fully automated.
Initially, the beams from all laser bars propagate in the same direction. Key to the concept is an arrangement of deflectors, which re-arrange the beams into a 2-D array of the desired shape and high fill factor. Standard multiplexing techniques like polarization- or wavelengths-multiplexing have been implemented as well.
A variety of fiber coupled modules ranging from a few hundred watts of optical output power to multiple kilowatts of power, as well as customized laser spot geometries like uniform line sources, have been realized.
We also report on a new approach based on 450 nm diode laser bars, which dramatically reduces size and alignment effort. These activities were performed within the German government-funded project “BlauLas”: a maximum output power of 80 W per bar has been demonstrated @ 450 nm. We show results of a 200 μm NA0.22 fiber coupled 35 W source @ 450 nm, which has been reduced in size by a factor of 25 compared to standard single emitter approach. In addition, we will present a 200 μm NA0.22 fiber coupled laser unit with an output power of 135 W.
In this paper we will summarize several concepts for adapting beam properties of diode lasers by using specific optical components. For building very compact laser modules of up to 2 kW we already presented a concept based on beam shaping of high fill factor bars. In this paper we will focus on further tailoring the beam properties of these very compact laser modules in the wavelength range from 808 nm up to 1020 nm. Fiber coupling of such modules into an 800 μm NA0.22 fiber yielded 1.6 kW without using polarization coupling. Another example is the generation of a 2.5 kW homogenized line with 40 mm length and a width of 4 mm.
We present results on the improvement of T-Bars tailored for optimized coupling into fibers with a diameter of 200 μm with NA 0.22 corresponding to a beam parameter product of 22 mm·mrad. Cost efficient coupling to this fiber requires a tailored beam parameter product smaller than 15.5 mm·mrad in slow axis direction corresponding to a slow axis beam divergence of 7° (full angle, 95% power content) for five 100 μm wide emitters. The improved T-Bars fulfil this requirement up to an output power of 52 W with a brightness of 3.1 W/mm·mrad and a power conversion efficiency achieving 69%. This progress in the T-Bar performance together with modifications in the module design led to the increase of the reliable output power from 135 W in 2009 to 360 W in 2017 for a T-Bar module with one baseplate. We will also give a review of the main development steps and further R and D improvements.
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.
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