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).

We demonstrate coherent beam combining of four high brightness tapered amplifiers in pulsed, quasi continuous wave (QCW) operation, seeded by a 976 nm laser diode. The maximum power of 22.7 W was achieved with > 64 % combining efficiency in a close to diffraction limited beam. We discuss turn-on dynamics of tapered amplifiers operated in pulsed mode in detail.

Power scaling of end-pumped laser rod amplifiers is challenging due to beam distortions at high powers. It is difficult to understand these distortions since several physical effects contribute at the same time. We present a detailed investigation of the beam distortions by numerical simulation and experiments in different gain regimes (passive regime with no gain, small signal gain regime, saturated gain regime) in order to decouple effects caused by thermal lensing from aberrations induced by the transversal gain profile. Simulation and experiment are in good agreement and show the influence of the gain regime in details and reveal a significant impact of the transversal gain profile on the output beam quality.

Coherent beam combining aims at increasing the spatial brightness of lasers. It consists in maintaining a constant phase relationship between different emitters, in order to combine them constructively in one single beam. Different architectures are proposed in the literature, either based on lasers in a passive extended-cavity or amplifiers seeded by a master oscillator. We compare the different approaches for highbrightness tapered laser devices.

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