Investigation of novel fibre laser designs and power-scaling concepts for the development of a continuous-wave green source

Abstract

A novel method for power scaling continuous-wave ytterbium-doped frequency-doubled fibre lasers is investigated. The limiting effects on output powers and the requirement on polarisation control are discussed. A ring fibre laser configuration using a Faraday mirror without the requirement of polarisation-maintaining fibre is proposed. The requirement for a seed source to prevent self-pulsing in fibre lasers during alignment is presented. It lead to the construction of a 1075.0nm-central-wavelength 85pm-linewidth ASE source with 2.97W output. The fibre feedback was tackled using large bulk end-caps processed in the lab. A first ring fibre laser configuration without a frequency-doubling scheme was built: a wavelength-tuneable linearly-polarised source was obtained with an 32.0W average power over the 1047.1-1085.8nm range, a linewidth inferior to 0.4nm, a 70.2% slope efficiency and a polarisation extinction ratio below 100:1. Injection-locking of a similar cavity was tested with the ASE source. With an 1.86W injected signal, 17.4W were extracted at 1075.0nm. Then, power-scaling of the ring fibre laser pumped by 276W power was studied first without and then with empty enhancement cavity in used for frequency-doubling. Thermal effects, beam distortion, fibre recoupling efficiency reduction and unwanted polarisation rotation were diagnosed and tackled. Hybrid end-caps were produced and improved recoupling efficiencies by 16%. Power instabilities are reported with the investigation of their cause. The nonlinear crystal was placed in the enhancement cavity at last.

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ORCID for Florian Leroi: orcid.org/0000-0001-6665-4689

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