We report progress in the development of GaAs-based laser diodes with ultra-wide stripe widths of W = 1200 μm emitting at a wavelength of λ = 915 nm. In order to restrict ring oscillations and higher order modes in these ultra-wide devices we utilise periodic current structuring with a period of 29 μm and width of 20 μm. We compare the performance of a device with current structuring realised through contact layer implantation of the device after epitaxial growth, termed a 'Contact Implant' laser, and a device with buried current structuring close to the active region of the device realised using two step epitaxial regrowth and Buried-Regrown-Implant-Structure (BRIS) technology, termed a 'BRIS' laser. Quasi-Continuous Wave (QCW) measurement of the devices show that both the 'Contact Implant' and 'BRIS' laser achieve a very high peak output power of Popt = 200 W at a power conversion efficiency of ηE = 59% and ηE = 52%, respectively, with a peak efficiency of around 70%. QCW beam-quality measurements show that the 'BRIS' laser has a much reduced 95% power content far-field angle of 9°, compared to 12.7° for the 'contact implant' laser, at a power of Popt = 100 W. Under Continuous Wave (CW) operation the 'contact implant' laser reaches an output power of Popt = 68 W at ηE = 57% and the 'BRIS' laser reaches Popt = 53 W at ηE = 50%, but with a reduced far-field angle of 11.9° at Popt = 40 W for the 'BRIS' laser.
Innovation in diode laser design and technology, assembly techniques and optical design are used to realize high-brightness pump modules for application in pumping of fiber lasers. In a first demonstration, monolithically grating-stabilized diode lasers with wavelength around 970 nm are integrated into prototype modules that deliver 500 W of continuous wave TE-polarized optical power at a conversion efficiency ⪆ 55% within a spectral width of 1.2 nm (95% power) in a narrow beam, suitable for low-loss coupling into a 200-µm core fiber. An especially simple opto-mechanical configuration is developed, without need for external volume Bragg gratings.
The performance characteristics of two stack modules (emitting near 780 nm) each consisting of 24 wide-aperture (1200 μm) diode laser chips is presented and the results are discussed. The stack modules are constructed using diode lasers from two different epitaxial design iterations. Compared to the first iteration, the second iteration was optimized for higher conversion efficiency and optical in-pulse power (lower losses), without compromising the beam characteristics. The stack modules make use of an established (field-proven) FBH design that utilizes innovative edge-cooling of both sides of the diode stack with large-channel (micro-channel free), water-cooled, thermally-expansion-matched heatsinks. We investigate here their performance up to high duty cycles and results for pulse width up to 10 ms at high duty cycle (50 %) operation is presented. Test of the completed modules show that the iteration 2 (power-optimized) chips deliver about 15 % more optical power without compromising the beam propagation ratio. Specifically, the stack module with first iteration chips delivers approx. 1.4 kW whereas the stack module with the optimized chips delivers approx. 1.6 kW. For the stack module that uses the first chip iteration a fiber coupling to a 1 mm core fiber was demonstrated with approx. 90 % coupling efficiency and loss channels are discussed. Finally, very high duty cycle operation (50 %) is demonstrated for the first time, using an iteration 1 stack module.
Mid-infrared (MIR) solid state lasers based on thulium and holmium-doped crystals are of increasing interest in applications in medicine, material processing and particle physics. Thulium-doped lasers can be efficiently pumped at wavelengths around 780 nm and diode laser pumps with high conversion efficiency and high intensity are sought at this wavelength. Diode lasers integrated in laser stacks suitable for high duty cycle pumping are of particular interest for high energy class applications, especially when realizable without need for the additional cost and reliability hazard of microchannel cooling. However, high efficiency and reliable power is more challenging to realize at 780 nm than around 940…980 nm, due to limitations on the capability of the available semiconductor materials. Progress is therefore presented here in the design, realization and test of 780 nm pump sources suitable for high energy class pump applications, using GaAs-based TM-polarized diode lasers. We show how power per device can be increased from 4 W for conventional single emitters (90…100 μm) up to 60 W at high duty cycle (10%) and long pulse length (10 ms) for high brightness large aperture emitters (with 1200 μm aperture, equivalent to around 500 W per bar), at the cost of reduced operating efficiency (from 60 to 50%). We show progress in integrating these large aperture emitters into novel passively (macro-channel) edge-cooled stacks, that are then suitable for use in pumping high energy class Th:YAG laser systems.
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