Sharp focusing of superpositions of Hermite-Gaussian modes locked in fractionally degenerate cavity

We investigate, experimentally and numerically, sharp focusing of superpositions of Hermite-Gaussian modes obtained at transverse mode locking in an end-pumped laser with a fractionally degenerate cavity. Being focused with a lens, these beams create quasi-waists that are shorter and narrower as compared to the waist of a zero-order Gaussian mode. Comparison with the case of Laguerre-Gaussian superpositions showed that both mode families demonstrate similar sharp-focusing behavior in terms of the longitudinal and transverse scales, but the superpositions of Hermite-Gaussian modes have substantially non-zero on-axis intensity between the quasi-waists.