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Optica Publishing Group
  • 2015 European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference
  • (Optica Publishing Group, 2015),
  • paper CB_5_4

High brightness narrow-stripe broad-area lasers with 7 W optical output at 910, 935 & 970 nm for coarse spectral beam combining

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Incoherent and spectral beam combining (SBC) of hundreds of high power broad area (BA) emitters enable direct diode laser systems with an optical output ranging from hundreds of Watts [1] to many kW’s [2]. However the brightness of these systems is ultimately limited by the in-plane beam parameter product BPP = ¼ W95% × Θ95% of their single emitters, where W95% and Θ95% are the width and angle with 95% power content of the emitted energy, respectively. Therefore reducing the BPP of 3…4 mm mrad of typical BA emitters, while sustaining the high output P ~ 10 W and conversion efficiency ~ 60% [3] is of major interest for high brightness applications. High power BA lasers using monolithically integrated microstructures in the waveguide to improve linear brightness up to P/BPP = 3.3 W/mm mrad at P = 8 W have been reported recently [4]. An alternative approach to increase the brightness of BA lasers is to narrow the contact stripe from typical 90...100 µm to 30 µm, to cut off higher-order lateral modes. These so-called narrow-stripe broad-area (NBA) lasers are a research topic within the BRIDLE project (www.bridle.eu), where low fill factor laser bars composed of five NBA emitters are incoherently combined and coupled into a fiber. Polarization multiplexing and low-cost, coarse SBC at operation wavelengths of 910 nm, 935 nm and 970 nm can potentially deliver an output power of > 1 kW from a fiber with a core diameter of 100 µm. We present the progress of NBA lasers at three different wavelengths suitable for such coarse SBC (910, 935 & 970 nm). To provide high power with high conversion efficiency, the series resistance is kept low, using long cavities (L = 6 mm) and extreme double asymmetric vertical waveguides with thin p-doped waveguide layers [5]. Facet passivation is used to ensure reliable operation at high power densities. The gain is provided by an InGaAs double quantum well. All devices are mounted and tested as explained in [5,6] under continuous wave (cw) conditions at 20°C. Each laser operates with an output power > 7 W. The conversion efficiencies at 7 W range from 47% at 910 nm to 53% at 935 nm & 970 nm (Fig.1a). The highest linear brightness of 4.8 W/mm mrad is achieved at 970 nm with a BPP = 1.46 mm mrad at an output of 7 W (Fig.1b). NBA lasers emitting at 910 nm and 935 nm show a degradation of BPP with increasing residual heat. As we assume that the lateral profile of the refractive index only depends on the residual heat, the number of modes in this thermal lateral waveguide depends mainly on the wavelength itself. We therefore attribute the increase in BPP to an increased number of lateral modes for shorter wavelengths (Fig.1c), which scales with -1 [7]. All lasers emit in TE-mode with a degree of polarization ≥ 98%, and are well suited for further power scaling using polarization multiplexing.

© 2015 IEEE

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