Multibeam-waist modes in an end-pumped Nd:YVO₄ laser

We studied both numerically and experimentally the transverse modes near degenerate resonator configurations such as g₁g₂ = 1/4, 1/2, 3/4 in a simple plano-concave Nd:YVO₄ laser with a small pump size. We found that there are stationary modes that show an additional beam waist besides the well-known waist on the fiat mirror end near g₁g₂ = 1/4, 3/4. When the specific modes of the three degeneracies propagate through a lens, they are capable of exhibiting multiple beam waists. The multibeam-waist mode possesses a small waist size without a ring structure in the near field on the flat mirror end; nevertheless, it has a far-field pattern with many concentric rings. The numerical results show good agreement with the experimental observations. Furthermore, by simultaneously considering the wavelike and the raylike character of the multibeam-waist mode, we found that it can be represented as a superposition of N consecutive round-trip electric fields of period-N solution in the degenerate empty cavity, where N = 2 for g₁g₂ = 1/2 and N = 3 for g₁g₂ = 1/4, 3/4.