Description
Power scaling of fibre lasers and amplifiers whilst maintaining excellent beam quality is important for defence, micro-machining, wind lidar, and scientific discoveries. Single spatial fibre mode systems are reaching limits of power-scaling due to limitations imposed by thermal and non-linear effects; the dominant effect for narrow linewidth light being stimulated Brillouin scattering. Entering the multimode fibre regime can reduce these nonlinear limitations by spreading power out spatially and across many spatial modes at the cost of a poor beam quality speckle output. Wavefront shaping techniques using a spatial light modulator on the input of an amplifier can reshape this output speckle to dramatically improve the beam quality. The aim is to maximise the amount of power in a diffraction-limited spot, thereby reducing the background speckle intensity.
Wavefront shaping in passive systems with phase-only modulation and single polarisation control has shown strong promise, achieving upwards of 70% of power in a focussed spot. This work utilises the same premise to shape the 1550 nm seed laser in a master oscillator fibre amplifier configuration but has struggled to achieve focussing efficiencies beyond 30%. To diagnose this discrepancy, the system is analysed in the passive regime by replacing the 1550 nm seed with a 1645 nm seed, which is not absorbed in the gain medium. This can be accomplished without any major alterations to the setup, thus enabling a direct comparison between the linear and nonlinear regimes. Preliminary results already demonstrate focussing efficiencies exceeding 50% in the passive configuration, possibly suggesting that the nonlinear gain strongly influences the wavefront shaping process.
| I am the presenting author | Yes |
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