Hysteresis-Free Piezoelectric Phase Controller for Optical Fibers at kHz Bandwidth for Coherent Beam Combination

Controlling the optical phase in a fiber is a common task, but coherent beam combination (CBC) brings a unique set of requirement and challenges [1]. A common implementation is CBC of tree-like optical fiber amplifiers to take advantage of the kW-level channel powers available to obtain 10s or 100s kW laser sources, requiring sub-wavelength path length stabilisation of each fiber chain. The stabilisation algorithm used determines the bandwidth required of the phase controller, with iterative schemes such as stochastic parallel gradient descent requiring 10s kHz bandwidth, or LOCSET at MHz. Another approach is to use neural networks for single-step phase correction [2], where the total phase error is detected and can be corrected in one step, which reduces the phase controller bandwidth requirements to 100s Hz with accurate phase changes.