FBH research: 13.01.2020

Higher efficiency and narrower far field in kilowatt-class diode laser bars

Kilowatt-class laser bar
Fig. 1: Kilowatt-class laser bar, before (left) and after (right) mounting on a conductively cooled carrier.
Power, voltage and conversion efficiency of kW-class lasers
Fig. 2: Power, voltage and conversion efficiency of kW-class lasers, for baseline and high efficiency, high fill-factor designs.
Lateral far field of kilowatt-class laser bars, for baseline bars and bars with higher brightness (narrower far field
Fig. 3: Lateral far field of kilowatt-class laser bars, for baseline bars and bars with higher brightness (narrower far field). Left: far field as function of operating power. Right: Intensity as function of emission angle.

1 cm wide diode laser bars emitting in the 9xx nm-range are used as high-power light sources in a variety of industrial and scientific applications, with material processing (e.g. metal cutting and welding) being amongst the largest market segments. Specifically, the diode laser bars are used either directly or as pumps for solid-state and fiber lasers, and continuous improvement in performance is needed. FBH’s ongoing diode laser bar research and technology development has enabled rapid progress, with recent work performed in close cooperation with TRUMPF. Examples include the first demonstration of output power Popt = 2 kW per bar [1], the first bars with conversion efficiency > 60% at Popt = 1 kW [2] and the first demonstration of 1 kW continuous wave (CW) emission at 25°C [3].

In new studies published in 2019, FBH scientists have further improved both bar conversion efficiency hE and bar lateral far field angle qlat, at the Popt = 1 kW level, with details presented at the Conference on Lasers and Electro-Optics / Europe in Munich in June [4]. The new research is focused on the impact of variation in the lateral structure on bars fabricated using established highly efficient epitaxial layer designs. The bars in all cases are 10 mm wide with a resonator length of 4 mm and a high fill factor > 70%, mounted on passively cooled CCP heatsinks. Variations in emitter number, emitter width and lateral guiding mechanism were assessed, as shown in Fig. 1. For highest efficiency, FBH scientists made use of a novel lateral structure featuring 8 emitters, each 1093 µm wide, pushing the filling factor up to 87%. Fig. 2 depicts the measured light–current–voltage characteristic (Popt, I, V) of the new design compared to baseline material, in quasi-continuous wave (QCW) testing (200 µs pulses, 10 Hz repetition rate). Bars using the new design emit Popt = 1 kW with the highest reported conversion efficiency hE = 66%, significantly increased over the baseline (hE = 61%). High efficiency (hE = 64% at Popt = 1 kW) was also shown to be sustained for long-pulse QCW operation (2 ms, 20 Hz), where the heating is comparable to CW operation of bars on advanced microchannel-based coolers (same effective thermal resistance ~ 0.05 K/W). The use of wide stripes and resulting high fill factor were found to bring both lower series resistance and less power saturation, yielding higher efficiency at high power.

In parallel work, a second design was shown to reduce lateral far field angle qlat without compromising conversion efficiency. Baseline designs use deep-etched grooves next to each laser stripe to both limit current spreading (a known efficiency limit) and to stabilize the near field, at the cost of a broader lateral far field. Newly developed bars omit these etched grooves, instead using a deep implantation step to suppress lateral current spreading next to each stripe, maintaining high efficiency whilst narrowing lateral far field. Higher performance was demonstrated for exemplary bars containing 36 stripes, each 186 µm wide. As seen in Fig. 2, the new low far field design (solid red) and the baseline structure (solid black) hardly differ in their light–current–voltage characteristic. The lateral far field angle is, however, reduced by ~ 2° to qlat = 8.8° (at 95% power) at Popt = 1 kW, the narrowest reported value, with results shown in Fig. 3.

In conclusion, research into lateral structuring of high fill factor diode laser bars has enabled FBH scientists to demonstrate new performance records at the operation point of Popt = 1 kW. A design optimized for improved conversion efficiency pushed the efficiency up to hE = 66%, while a low far field design reduced the lateral divergence down to 8.8°, both at 1 kW emission power. These advances stress the suitability of FBH’s diode laser bars for applications requiring highly efficient and low divergent emission at the kW-level.

Publications

[1] C. Frevert, P. Crump, F. Bugge, S. Knigge, A. Ginolas, and G. Erbert “Low-temperature Optimized 940 nm Diode Laser Bars with 1.98 kW Peak Power at 203 K” Conference on Lasers and Electro Optics (CLEO) - Science and Innovations, San Jose, USA, May 10-15, p. SM3F.8 (2015).

[2] M.M. Karow, T. Kaul, S. Knigge, A. Maaßdorf, G. Erbert, S.G. Strohmaier and P. Crump Long-Resonator Laser-Diode Bars for Efficient kW Emission” Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. (CLEO/Europe-EQEC 2017), Munich, Germany, Jun. 25-29, ISBN: 978-1-5090-6736-7, cb-7.3 (2017).

[3] S.G. Strohmaier, G. Erbert, T. Rataj, A.H. Meissner-Schenk, V. Loyo-Maldonado, C. Carstens, H. Zimer, B. Schmidt, T. Kaul, M.M. Karow, M. Wilkens and P. Crump “Forward Development of kW-class Diode Laser Bars” Proc. SPIE 10514, High-Power Diode Laser Technology XVI, Photonics West, San Francisco, USA, Jan 27 - Feb 01, 1051409 (2018).

[4] M.M. Karow, D. Martin, P. Della Casa, G. Erbert, and P. Crump “Narrower Far Field and Higher Efficiency in 1 kW Diode-Laser Bars Using Improved Lateral Structuring” Conf. on Lasers and Electro-Optics/Europe and European Quantum Electronics Conf. (CLEO/Europe-EQEC 2019), Munich, Germany, Jun. 23-27, ISBN: 978-1-7281-0469-0, cb-5.4 (2019).