<title>Sensitivity limits of laser intracavity spectroscopy</title>

Abstract

Intracavity laser spectroscopy is one of the most sensitive techniques for absorption measurements. The main feature of this technique is that the narrow line absorber is placed in the cavity of a laser with large homogeneously broadened gain. Laser gain compensates only broadband cavity loss, such as mirror transmission, but is not affected by narrow line intracavity absorption (ICA). The laser light passes through the absorber many times and ICA is accumulated in its spectrum like in a multipass cell. The ultimate sensitivity of the emission spectrum of a multimode laser to ICA is limited by the laser pulse duration, nonlinear mode coupling, or by quantum noise. Multimode lasers which have been applied to ICA measurements, such as dye lasers, solid state lasers, and diode lasers show different sensitivity limits. These limits are determined by specific laser parameters and can be optimized individually for each laser. The highest sensitivity limit, 3 x 10 exp -10/cm, has been achieved with CW dye lasers so far. It is determined by nonlinear coupling of laser modes in the gain media. Quantum-limited sensitivity of this laser, which corresponds to 10 exp -12/cm, allows the measurement of the absorption of single atoms in the cavity.