Random temporal signal-based cavity ring-down spectroscopy: a noise-resilient approach for high-reflectance coating characterization

Accurate characterization of high-reflectance optical coatings is essential for applications such as laser systems and advanced optical instrumentation, where even parts-per-million (ppm) deviations in reflectance can critically affect system performance. Conventional measurement approaches, such as spectrophotometry or pulse-based cavity-ringdown spectroscopy (CRDS), face severe limitations at ultra-high reflectances-spectrophotometry typically saturates above 99.9%, while pulsed CRDS becomes increasingly sensitive to detector jitter, pulse instability, and long averaging times beyond 99.999%. In this work, we present and experimentally verify a time-resolved CRDS technique based on the random temporal signal (RATS) approach, which replaces deterministic laser pulses with a randomly modulated signal. Numerical simulations and experimental measurements using a four-mirror cavity (<i>R</i> > 99.99%) demonstrate that the RATS-based method achieves smoother decays and improved precision under identical acquisition times. The stochastic temporal signal enhances photon throughput while maintaining constant peak power, resulting in 50× higher signal-to-noise ratio and preventing mirror damage. The results confirm that the RATS-CRDS method can achieve comparable or superior accuracy to conventional pulsed CRDS in a fraction of the time, offering a robust and scalable approach for precision reflectometry of ultra-high reflectance coatings in industrial and space-optics applications.