Introduction

High-finesse cavities are a powerful tool for sensitive laser spectroscopy, since the effective absorption path length can be 100-1000 times greater than commonly used multi-pass cells. A number of methods have been used to monitor absorption in the cavity, including measuring the cavity decay, the cavity transmission, and using radio-frequency (RF) techniques. Recently we have been using a cw technique to monitor absorption, first developed by J. Hall and co-workers¹ at JILA, in work that tests the veracity of the quantum-mechanical symmetrization postulate. The idea, first proposed by G. Tino ² in 1994, is to look for absorption at wavelengths that correspond to strictly "forbidden" rotational transitions of the oxygen molecule.

The search for limitations of the theoretical symmetrization postulate is an experiment that tests fundamental physics. However any progress in the instrumentation or the methods used to measure small quantities of gases is of interest to a large community of scientists and engineers in technical fields such as semiconductor fabrication, pollution detection, and process control. [See Figure]

J. Ye, L.-S. Ma, and J. L. Hall, "Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy," J. Opt. Soc. Am. B 15, 6 (1998).

G. M. Tino, "Proposed search for small violations of Bose statistics in molecular-spectra," Nuovo Cimento D 16, 523 (1994).

L. Gianfrani, R. W. Fox, L. Hollberg, "Cavity-enhanced absorption spectroscopy of molecular oxygen," submitted to J. Opt. Soc. Am B (1999).

Richard Fox, NIST, [email protected]