Passive and Active Fiber Laser Array Beam Combining.

Abstract

The beam combination of fiber lasers can achieve higher output average power and energy than a single fiber amplifier. This thesis explores fiber laser combining as a path towards increasing powers and energies from fiber-based systems. In this work two general areas are addressed: we explore passive coherent self-locking of cw fiber laser arrays, and active coherent phasing of ultrashort-pulse fiber laser arrays. Passive self-locking is technologically attractive since it does not require any active phase-control of parallel channels in the array and, therefore, could potentially lead to much simpler coherently self-locked fiber arrays. However, the array-size scalability of passive self-locking has not been well understood. In this work we (i) experimentally validate the new numerical model developed in collaboration with Prof. H. Winful group, which describes accurately the complex phenomena occurring in such arrays, and (ii) experimentally confirm the array-size scaling limitations, predicted by this theoretical model. Extensive experimental characterization of 16-channel self-locked fiber laser array agrees well with the theoretically predicted decrease of power combining efficiency with array size, occurring as a result of increasing scarcity of coherently-combined supermodes. In pulsed coherent combining, we demonstrate femtosecond pulse combining with up to 4 parallel fiber amplifier channels using LOCSET single-detector-feedback phase-locking technique. We achieve good combining efficiency and negligible distortions in the combined pulses. Extrapolating from experimental results we explore array-size scalability in relation to modulation amplitude, power amplitude, and phase error, and show that multi-channel pulse combining with LOCSET feedback can be scalable to very large combined-array sizes. Using the established 4-channel pulse coherent combining experimental test bed, we demonstrate femtosecond pulse simultaneous combining and synthesis by coherently spectrally combining 3 fiber chirped pulse amplifiers. We explore different phase-feedback strategies using linear detection schemes when sufficient partial spectral overlap is present, and using two-photon-absorption detector when this spectral overlap becomes vanishingly small. This demonstration shows a path towards simultaneous power scaling and pulse shape and spectral shape control in coherently-combined femtosecond-pulse laser arrays. For example, this could be used to overcome gain narrowing effects in a rare-earth doped fiber amplifier.