Intensity-noise suppression due to longitudinal mode coupling in multimode intracavity-doubled lasers
arXiv:2605.13745 · doi:10.1103/gf5b-vn5p
Abstract
Multimode intracavity frequency-doubled lasers can reach states of intensity-noise suppression orders of magnitude beyond the predictions of independent-mode partition statistics. I show that the \c{hi}(2) coupled-wave dynamics in the doubling crystal obey a dissipation inequality: a fourth-moment functional of the intracavity intensity decreases monotonically under each forward nonlinear pass, driving the mode ensemble toward a constant-intensity manifold. This per-pass result is a strict Lyapunov inequality, and round-trip simulations show attraction to the same low-M4, nearly constant-intensity state under the full cavity dynamics. I confirm the mechanism in an intracavity frequency-doubled Nd:YVO4/LBO laser, observing a 100x contrast between full and Fabry-Perot-filtered output noise at fixed detector bandwidth, well beyond the sqrt(N/N_f) statistical-averaging baseline. The argument depends only on the algebraic structure of the coupling: a coherent superposition of oscillators that share a quadratic dissipative channel. It therefore points to a general Lyapunov route to intensity-noise suppression in coupled oscillator systems, beyond the intracavity-doubled laser.
24 pages, 9 figures. Accepted for publication in Physical Review A