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. (paper)