Coherent combining of fiber and solid-state lasers

Summary form only given. Actively phase-locked coherent beam combining (CBC) in a master-oscillator power-amplifier (MOPA) architecture has allowed laser developers to replace physical constraints on laser power scaling with more traditional engineering constraints of complexity and cost [1]. This has enabled scaling of cw coherent sources to unprecedented powers and brightnesses.Fig. 1 shows a canonical system-level architecture for an actively phase-locked CBC MOPA array. A master oscillator (MO) seeds a plurality of N laser channels, each of which contains a modulo-2 piston phase actuator and a coherence-preserving laser amplifier (or a chain of amplifiers) to boost the channel power to the limit of the laser technology. The high power outputs from all N channels are geometrically combined so that they copropagate, either using one or more beamsplitters or by tiling side-by-side. The combined output beam is sampled optically to generate error signals for servo-based phase-locking of all N channels to within a small fraction of a wave.In this talk, we review recent advances in key technologies for CBC, including active control methods for locking phase, polarization, and spatio-temporal alignments; geometric beam combination using either tiled apertures or diffractive optics; and high-coherence, multi-kW laser amplifiers suitable for combining. In particular, we review the engineering challenges, design, and recent test results from two specific CBC systems: an array of N = 7 Nd:YAG zigzag slab MOPAs tiled side-by-side with 100 kW combined power (Fig. 2a) [2]; and an array of N = 5 Yb-doped fiber MOPAs diffractively combined to 1.9 kW (Fig. 2b) [3]. While current CBC fiber powers are lower than those from slabs, it appears likely that fiber-based systems will provide substantial benefits in size, weight, efficiency, and packaging. Recent development of high-coherence Tmdoped fiber amplifiers at the kW level [4] provides a technology roadmap toward retina-safe CBC- laser systems, a key consideration for propagation through atmosphere.