Towards diffraction limited output via wavefront shaping in a record peak power, single-frequency, pulsed 1.5μm multimode fibre amplifier

In this study, we explore active beam shaping in a pulsed single-frequency highly multimode fibre amplifier operating at 1.55 μm. A 10 kHz, 186 ns pulsed signal is amplified to a peak power of 7.7 kW with a gain of 18.5 dB, achieved in an Er/Yb co-doped fibre diode pumped at 915 nm. By adjusting the wavefront of the seed using a spatial light modulator (SLM) before entry into the multimode fibre, the transverse eigenmodes of the fibre can be manipulated to produce a high-quality output while preserving the gain narrow linewidth of the signal. This approach distributes laser power across multiple modes, mitigating nonlinear effects, particularly stimulated Brillouin scattering (SBS), transverse mode instability (TMI), and modulation instability – while maintaining beam quality. We will present our latest results, where an optimized input wavefront can generate a diffraction limited output, enhancing applications in defence, LIDAR, and remote sensing. This method also mitigates the acoustic interactions within the fibre that typically cause SBS, thus maintaining high-quality, single-frequency output even under high-power conditions. Our ongoing work focuses on further power scaling and wavefront shaping refinement while preventing self-lasing at increased pump levels. This research represents a significant advancement in the practical application of fibre amplifiers in single-frequency pulsed regimes.