Quasi-continuous-wave diode laser bars with increased power (2 kW) and peak conversion efficiency (77%) at 203 K
Fig. 1: 1 cm laser bar assembled using a massive n-contact
Fig. 2: Power (black) and efficiency (red) vs. current for design 1 at 203 K
Fig. 3: Power (black) and efficiency (red) vs. current for design 2 at 203 K (solid) and design 3 at 288 K (dashed)
Quasi-continuous-wave (QCW) diode laser bars with the highest power conversion efficiency and operating powers are needed as optical pumps for pulsed solid-state laser systems. As reported at CLEO US in San Jose, USA [1], ASSL in Berlin Germany [2] and HEC-DPSSL in Prague, Czech Republic [3], FBH researchers have demonstrated during 2015 substantial increase in both power and efficiency, within the final stages of the CryoLaser project. The results were achieved by improving diode laser epitaxial layer design and packaging technology, with the very highest performance obtained by tailoring the designs for operation at a reduced temperature of 203 K. Such low temperatures are potentially economically deployable in high-energy-class solid-state laser systems that cool the amplifier crystals to around 176 K, by making use of a shared cooling system.
As a first stage, novel epitaxial layer designs were developed that operate with reduced electrical resistance, enabling high efficiency to be maintained at large operation powers. A careful optimization of semiconductor layer thickness and composition was performed, with designs adapted for different operation conditions. Three different epitaxial layer designs were established, the first one targeting maximum optical output power Pout at 203 K, the second one maximum conversion efficiency ηE at 203 K, and the third one maximum ηE at room temperature. These laser designs were processed into diode laser bars with 4mm resonator length and 69% fill factor, with facets passivated then dielectric coated ready for packaging.
Realizing the highest efficiency is only possible by also improving the performance of the bar packaging, modifying the construction for the lowest possible electrical resistance, as shown in Fig. 1, is essential. This novel package has a resistance of below 50 µW, substantially lower than for conventional assemblies (> 200 µW), which enables high conversion efficiency to be sustained to QCW currents of > 1000 A. Laser bars applying the three new epitaxial designs were assembled using the new bar packaging configuration, test results are presented in Figs. 2 and 3.
First, laser bars using the design optimized for peak power were shown to deliver an optical output of 2000 W per bar under QCW conditions (200 µs, 10 Hz) at 203 K [1], as illustrated in Fig. 2 (power limited by available current). The design optimized for highest conversion efficiency at 203 K enabled laser bars to operate with peak ηE = 77% at 400 W and ηE = 70% at 1000 W [2,3] under QCW conditions (1.2 ms, 10 Hz), as shown in Fig. 3. Finally, laser bars that used a design tailored for highest efficiency at room temperature achieved ηE = 62% at 1000 W [2,3] at 288 K. These are the highest powers and efficiencies reported to date for single QCW laser bars.
Publications:
[1] C. Frevert, P. Crump, F. Bugge, S. Knigge, A. Ginolas, G. Erbert, “Low-temperature Optimized 940 nm Diode Laser Bars with 1.98 kW Peak Power at 203 K”, Paper SM3F.8, Proc. CLEO, San Jose, USA (2015).
[2] A. Pietrzak, M.W. Woelz, R. Huelsewede, M. Zorn, O. Hirsekorn, J. Meusel, A. Kindsvater, M. Schröder, V. Bluemel, J. Sebastian, C. Frevert, F. Bugge, S. Knigge, A. Ginolas, G. Erbert, P. Crump, "Progress In The Development Of Kilowatt-class Diode Laser Bars For Pump Applications", ASSL ATh2A.7 (2015).
[3] C. Frevert, F. Bugge, S. Knigge, A. Ginolas, G. Erbert, P. Crump "Progress in development of kW-class diode laser bars for pump applications within the Cryolaser project", HEC-DPSSL conference, Stirin (2015) - not online.