High-power fibre lasers: more modes, more possibilities

Fibre laser amplifiers are compact, high-brightness light sources that are becoming key enablers in fields such as directed energy, manufacturing, and remote sensing. One of the greatest challenges in scaling fibre lasers to extreme output powers is mitigating nonlinear optical effects and thermal effects in the fibre. In this talk, I would like to discuss two unconventional approaches to breaking through these limits.
In the first part, I will show that multimode fibre amplifiers can suppress detrimental nonlinear effects like transverse mode instabilities and stimulated Brillouin scattering. Challenging the long-held belief that multimode laser amplifiers are inherently limited by poor beam quality, we demonstrate that wavefront shaping of a low-power input seed laser enables high-power output beams that are either diffraction-limited (M^2 < 1.1) or structured (e.g., vortex and Bessel beams) at average powers of ~500 W.
High-power lasers typically generate substantial waste heat during operation. Although external cooling can effectively dissipate this heat, it can also introduce mechanical noise that destabilises the laser. In the second part, I will present radiation-balanced laser amplification as an alternative strategy, demonstrating watt-level laser output with zero net heating from a fibre amplifier suspended in air and discussing a pathway towards hundred-watt-level output.
Together, these approaches may enable further power scaling of stable fibre amplifiers and unlock functionalities beyond conventional single-mode amplifiers.