We report on a coherent beam combination of three high-brightness tapered amplifiers, which are seeded by a single-frequency laser at λ = 976 nm in a simple architecture with efficiently cooled emitters. The maximal combined power of 12.9 W is achieved at a combining efficiency of > 65%, which is limited by the amplifiers' intrinsic beam quality. The coherent combination cleans up the spatial profile, as the central lobe's power content increases by up to 86%. This high-brightness infrared beam is converted into the visible by second harmonic generation. This results in a high non-linear conversion efficiency of 4.5%/W and a maximum power over 2 W at 488 nm, which is limited by thermal effects in the periodically poled lithium niobate (PPLN).

Laser diodes, in general, are sensitive to optical feedback, especially with regards to maintaining single-frequency operation. Until now, however, the feedback sensitivity of high-power devices such as single-frequency DBR tapered laser diodes has not been investigated in quantitative detail. In this paper, we analyze the impact of very weak optical feedback between −105 dB and −40 dB on a high-power DBR tapered laser diode. The measurement set-up is validated using a typical DFB laser diode. The results are in good agreement with theory at low feedback levels.

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High brightness diode laser beam combining techniques are in demand for efficient high power nonlinear conversion. Coherent beam combining (CBC) is the only method that has the potential for brightness scaling by maintaining one single narrow spectral linewidth. CBC in a master oscillator power amplifier (MOPA) configuration using a small number of efficiently cooled tapered amplifiers is a promising approach for efficient brightness scaling in a simple architecture. We present the application of such a source based on CBC of three tapered amplifiers seeded by a DFB laser at λ = 976 nm for second harmonic generation (SHG). A maximum power of 2.1 W at 488 nm was generated by SHG in a MgO:PPLN bulk crystal limited by thermal effects. A clear benefit of the beam clean-up resulting from the CBC setup was documented leading to an improved nonlinear efficiency. As part of our ongoing studies into further brightness scaling in CBC architectures, we present an experimental analysis of the phase dynamics of tapered amplifiers in quasi continuous operation (QCW) at high currents. Furthermore, we are investigating different amplifier designs for improved beam quality at high powers and therefore improved combining efficiency.

A speckle-displacement-based wavemeter is combined with a spatial-fundamentalmode-pass filter to eliminate the influence of multi-mode operation on the directionality of the resulting output from a Distributed Bragg Reflector tapered Laser (DBR laser). The proposed setup is characterized theoretically and experimentally, and detections of modehops and side-mode suppression ratio in the optical output are demonstrated. The laser illuminates a rough surface at an oblique angle, and a camera observes the corresponding speckle pattern from an almost identical back-scattering direction. As the wavelength of the laser shifts, the speckle pattern responds with a corresponding displacement, which is approximately linear with respect to the shift within the detection area. The wavemeter tracks continuously the shifts of the speckles pattern by tracking the peak of the covariance function of sequentially acquired images. In this way, the speckle-displacement-based wavemeter achieves a spectral resolution of 10.4 MHz. Mode-hops in the laser do not cause any impeding decorrelation of the speckle patterns. Interestingly, the actual side-mode suppression ratio is related to the peak height and width of the absolute covariance function. A wavemeter, which is capable of measuring wavelength, mode-hops and side-mode suppression ratio, is highly useful for spectroscopy, quantum optics, nonlinear frequency conversion and other applications requiring stable single-frequency laser light, especially when using diode lasers.