Abstract

Nonlinearities in high-power fibre lasers can be mitigated by increasing the mode effective area (A_eff). Fibre amplifiers designed to operate specifically in a single higher-order mode (HOM) are capable of scaling to effective areas of several thousand square microns, in a format that can be fusion spliced and coiled [1]. A UV-written long-period grating (LPG) converts the input fundamental mode to the desired HOM. Amplification takes place in the HOM, and at the output, reconversion to a diffraction limited beam is achieved with a second LPG matched to the input. The ultra-large area of the HOM fibre enables the generation of record peak powers with low nonlinearity in all-fibre fusion spliced amplifiers [1]. However, in high peak power pulsed systems, reconversion to the fundamental mode before exiting the fibre can add substantially to the nonlinearity in the amplifier [2]. Consequently, a low-nonlinearity, bulk-optic approach to beam conversion at the output of an HOM fibre amplifier would be advantageous for high peak power.

© 2015 IEEE