Advanced techniques of characterisation for high power fibre lasers and amplifiers

Abstract

We present advanced techniques to optimise splices between dissimilar fibres and to characterise High Power Fibre Lasers (HPFL). Developed in an industrial context of volume manufacturing and supported by academia, these techniques aim to improve HPFL efficiency and stability, understand causes of failure, increase the yield on the production line and deliver working prototypes in shorter times. An innovative high power test-kit has been purposely built to perform advanced characterisation on HPFLs and amplifiers. The high power test-kit can concurrently record data from a multitude of sensors connected to various parts of the laser whilst conditioning the pump power in continuous wave or pulsed regime. Concurrent data logging has allowed unveiling failure’s mechanisms and has given us precise clues on how to resolve instability and efficiency related problems. An innovative non-destructive technique has been invented to resolve, along the active fibre length and during high power operation, quantities such as pump absorption, atomic inversion, signal power and non-linear effects evolution. Splicing of the various components composing the optical systems plays a very important role, especially when splicing dissimilar fibres inside the laser cavity. An S² (Spatial and Spectral) test-kit has been developed with the intent of optimising splices. The S² test-kit allowed us to measure the modal excitation of multimode fibres in presence of SM-MM splices. We have experimentally discovered, repeatedly re-produced and theoretically justified the existence of an optimal splice between SM and diffusing MM fibres. Application of the so called optimal splice has proven to lead to measurable and positive effects in terms of HPFL’s performance. The splice optimisation criterion seems quite counterintuitive and contradicting some common beliefs related to fundamental mode excitation in multimode fibres. The optimal splice discovered in this work is now adopted in the mass production cycle of lasers at SPI Lasers.

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Identifiers

ORCID for Vincenzo Scarnera: orcid.org/0000-0001-8646-7063

ORCID for Michael Zervas: orcid.org/0000-0002-0651-4059

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