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Progress in side-cooled stacks: 600 W CW output power at 780 nm in a narrow, symmetric beam

FBH research: 07.11.2022

Fig. 1: Ready-for-shipment stack module comprising 24 lasers, each equipped with its own individual fast axis collimation (FAC) lens. Common slow axis collimation (SAC) not shown.

Fig. 2: Optical in-pulse power and conversion efficiency as a function of driving current for a stack module emitting near 780 nm. Taken for 10 ms 10 Hz, 10 ms 50 Hz and CW.

Fig. 3: Emission spectra for two mode of operation, QCW at 10 ms 10 Hz with 70 A driving current and CW with 40 A driving current. The spectral width remains nearly the same and is 7…8 nm (95 % power content) for the two modes of operation.

High-power diode lasers are essential components in modern laser systems, either for direct use or as pump sources. For example, the largest market for laser systems is continuous wave (CW) metal cutting, served by high-brightness fiber or disk solid-state laser (SSL) systems. These systems are pumped using single emitters (fiber lasers) or diode laser bars (disk lasers) that have narrow lasing wavelengths, matching the absorption of the gain media.

For such challenging applications, SSLs need ever improved pump sources that provide light with the desired wavelength, optical power, and beam quality. Moreover, they have to deliver an ever-higher output power to yield higher performance and lower costs, in terms of €/W.

FBH scientists have supported these developments for many years, by developing and delivering innovative diode laser pump modules to SSL research groups. These modules are based on our highly efficient GaAs-based diode lasers and are fully manufactured in-house at FBH, spanning from design to ready-for-shipment laser fibered or direct applicable modules.

In recent work, our team has developed custom pulsed pump modules that emit with a wavelength of 780 nm and are based on efficient, high-power large aperture diode lasers (1200 μm wide, with 6 mm resonator length). As shown in Fig. 1, 24 of these lasers were integrated into innovative side-cooled stacks before being collimated. These laser stacks were originally designed for high-duty cycle (DC) of about 10…50 % quasi-continuous wave (QCW) operation. Each diode laser emits 60…70 W optical in-pulse power at a pulse condition of 10 ms and 10 Hz. However, the extraordinary cooling performance of the micro-channel free design (thermal impedance is 11 mK/W in QCW mode) has allowed us recently to demonstrate CW operation, at reduced peak optical power, but increased average power. Fig. 2 compares the QCW with the CW operation and shows that the peak power in CW operation is 600 W at 40 A driving current, which is just a 20 % decline, although the average power increases 8-fold. The thermal impedance in CW mode is 37 mK/W, which is a 3-fold increase compared to the 10 % DC case.

Moreover, the beam quality has been characterized by a caustic measurement. It showed no compromise over QCW conditions, with a nearly symmetric beam propagation ratio M2 for the vertical and lateral direction of 230 and 300, respectively, suitable for coupling into a 1 mm optical fiber. Finally, a spectral characterization has shown a spectral width of the emission of 7…8 nm, covering 95 % of the optical power.

These parameters provide excellent pumping performance for ultra-high repetition rate Tm:YAG lasers, as these crystals provide gain in the mid-IR wavelength range and are emerging as a critical gain material for future high-energy pulsed laser applications.

The results were achieved within the HECMIR project, funded by the Federal Ministry of Education and Research (BMBF) as part of the KMU-NetC program (FKZ -03VNE2068E).


M. Hübner, M. Wilkens, B. Eppich, A. Maaßdorf, D. Martin, A. Ginolas, P. S. Basler, P. Crump, “A 1.4 kW 780 nm pulsed diode laser, high duty cycle, passively side-cooled pump module”, Paper, Optics Express 29(7), 9749 (2021).

M. Hübner, M. Wilkens, A. Ginolas, B. Eppich, P. S. Basler, P. Crump, “Fiber coupling of a 1.4 kW diode laser stack module emitting near 780 nm as high duty-cycle pulsed pump source”, Paper HPLSE2021-2021-000048, Proc. of 4th International High Power Laser Science and Engineering Conference, Suzhou, China (2021).

M. Hübner, “Research progress in ultra-high repetition rate pulsed diode laser pumps”, Talk, Photonics Days, Berlin, Germany (2022).

M. Hübner, M. Wilkens, B. Eppich, P. S. Basler, A. Maaßdorf, D. Martin, A. Knigge, A. Ginolas, S. Kreutzmann, P. Crump, “High-brightness 1.4 kW 780 nm QCW laser pump module with low-loss coupling into 1 mm fiber up to 50% duty cycle”, Proc. Photonics West, doi: 10.1117/12.2608625 (2022).

M. Wilkens, M. Hübner and P. Crump, “Progress in efforts to increase power in GaAs-based high-power diode lasers and modules”, Invited Talk, IEEE RAPID, Florida, USA, https://doi.org/10.1109/RAPID54472.2022.9911575 (2022).