Highly efficient kW emission from long-resonator laser bars
Fig. 1: Power and power conversion efficiency as function of current for 4 mm long laser diode bars. Results are shown for reference commercial epitaxial design (black, dashed) and newly developed vertical structure (blue, solid) [3]
Fig. 2: Kilowatt-class bar with 6 mm resonator length soldered onto a low-resistance conductively cooled heatsink
Fig. 3: Power and voltage as a function of current for laser bars using further developed epitaxial design, with cavity lengths <i>L</i> = 4 and 6 mm
Fig. 4: Power-conversion efficiency as function of optical output power for laser bars displayed in Fig. 3. Inset: exemplary 6 mm long and 1cm wide laser bar [1]
Diode laser bars emitting in the 9xx nm range are the preferred light sources for a wide range of industrial and scientific high-power applications. In material processing applications, such as metal cutting and welding, laser bars are deployed as pump sources for high-power solid-state or fiber lasers. Recent research at the FBH into 10 mm aperture laser bars with long cavity lengths L enabled improved efficiency ηE at 1 kW output power. Recent results were presented at Photonics West in Feb 2017 and further advances in high-fill factor laser bars will be presented at this year’s Conference on Lasers and Electro-Optics / Europe [1].
Optimization of the epitaxial design for high-efficiency emission at the kW level targets structures with low losses ai, waveguides with low electrical resistance per unit area Rs/A, and improved temperature stability [2]. Fig. 1 compares results on 1 cm wide laser bars that are fabricated with a newly designed FBH structure (blue, solid lines, tailored for high ηE at high output power Pout) to bars fabricated using a commercially available reference design (black, dashed), taken from [3]. The bars have 4 mm long resonators and are identically fabricated and packaged. The new design maintains ηE > 60% at Pout = 1 kW under 0.2% duty cycle test conditions, significantly improved over the reference design.
In order to maintain these high efficiencies when operating in continuous-wave (CW) mode, the FBH has developed further improved designs that seek to maintain high efficiency to high powers in 1 cm bars with lengthened resonators L = 6 mm, cf. Fig. 2. We anticipate them to be highly beneficial since longer cavities reduce both the electrical and thermal resistance, as needed for efficient high-power applications. This, however, is commonly linked to increased threshold currents and reduced slope efficiencies dPout/dI via increased losses L×αi. In Fig. 3 we show test results from bars based on the newly developed epitaxial design with resonators lengths 4 and 6 mm, under QCW test conditions. The use of long resonators reduced the device voltage significantly and at the same time compromised the slope efficiency only mildly due to our low-loss epitaxial design. This then enables the highest reported efficiency for L > 4 mm to be achieved, of ηE = 62.9% at Pout = 1 kW, cf. Fig. 4.
In conclusion, progress in vertical designs (optimized for low loss and low power-saturation) taken in combination with advanced fabrication technology has enabled bars with 6 mm long resonators to deliver 1 kW optical output power with record 63% conversion efficiency, comparable to the best bars with 4 mm resonator. Long cavities will enable ultra-low thermal resistances and are therefore anticipated to be highly beneficial for kW-level CW operation [1,2,3].
References
[1] M. Karow et al., “Long-Resonator Laser-Diode Bars for Efficient kW Emission,” to be published Conf. on Lasers and Electro-Optics / Europe (CLEO 2017) Munich Germany, CB7 (2017).
[2] T. Kaul, G. Erbert, R. Platz, A. Maaßdorf, S. Knigge, P. Crump, “Studies of limitations to peak power and efficiency in diode lasers using extreme-double-asymmetric vertical designs”, 25th International Semiconductor Laser Conference (ISLC 2016), Kobe, Japan, Sep. 12-15, p. WD4 (2016).
[3] S.G. Strohmaier, G. Erbert, A.H. Meissner-Schenk, M. Lommel, B. Schmidt, T. Kaul, M. Karow P. Crump, “kW-class Diode Laser Bars”, Proc. SPIE 10085, Photonics West, San Francisco, USA, Jan 28 - Feb 02, 100850F (2017).