Low-loss, smile-insensitive external frequency-stabilization of high power diode lasers enabled by vertical designs with extremely low divergence angle and high efficiency

P. Crumpa, S. Kniggea, A. Maaßdorfa, F. Buggea, S. Hengesbachb, U. Wittec, H.D. Hoffmannc, B. Köhlerd, R. Hubrichd, H. Kisseld, J. Biesenbachd, G. Erberta and G. Tränklea

Published in:

Proc. SPIE, vol. 8605, Photonics West, San Francisco, USA, Feb. 02-07, 86050T (2013).

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Abstract:

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, ΘV95% ∼ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 µm stripes that use ELOD designs operate with ΘV95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 µm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.

a Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Straße 4, 12489 Berlin, Germany
b Chair for Laser Technology LLT, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany
c Fraunhofer-Institut für Lasertechnik, Steinbachstr. 15, 52074 Aachen, Germany
d DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, 55129 Mainz, Germany

Keywords:

diode laser, high power, narrow far field, volume holographic grating, frequency stabilization, bar smile insensitivity, high efficiency.