Spatial filtering of large mode area fibre lasers using a self-Fourier cavity for high power applications

The spatial filtering properties of a self-Fourier cavity on a large mode area (LMA) fibre laser have been analysed and described. The spatial filtering is first analysed for a single LMA fibre laser coupled to a self-Fourier cavity and then for a 5 x 5 array of LMA fibre lasers. Results are described in terms of the scattering matrix, supermodes, eigenvalues, loss, and supermode discrimination. The laser array is shown to possess a single fundamental in-phase mode that can propagate with low loss and high discrimination against other transverse modes of oscillation. This modal discrimination can be utilized to force an array of high power fibre lasers to operate in a single fundamental transverse supermode, generating a high energy laser (HEL) array with a diffraction-limited output.