ULTRA-SENSITIVE INTRACAVITY SPECTROSCOPY WITH MULTIMODE LASERS

Abstract
Intracavity laser spectroscopy is characterized by extreme sensitivity of the emission spectrum to narrow spectral perturbations such as absorption, gain or light injection. Intracavity absorption spectra obey a modified Lambert-Beer law, where the length of the absorption cell is substituted by 1 = c.t, where c is the velocity of light, and t is the duration of the laser pulse. The time resolution of intracavity measurements is limited only by the sensitivity required for the detection of the extinction k, such that i ≥ 1/kc. With the minimum detectable absorption being 10^-5 cm^-1, e.g., the resolvable time is on the order of a microsecond. The ultimate sensitivity of intracavity spectroscopy with a cw laser is limited by one of two competing factors : spontaneous emission, and non-linear mode interaction, such as stimulated Brillouin scattering. Depending on laser parameters, minimum detectable extinction is in the range 10^-7-10^-12 cm^-1. Non-linear mode interaction can also give rise to distortion of the line shapes observed with intracavity spectroscopy. High sensitivity along with time resolution opens a wide field of practical application for intracavity spectroscopy such as pollution detection, detection of forbidden and non-linear transitions, combustion and plasma diagnostics, and the study of kinetics of molecules and radicals.