Sub-Doppler dispersion and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy revised

Abstract

An expression for the peak-to-peak sub-Doppler optical phase shift of two counterpropagating modes of light, to which the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) dispersion signal is proportional, valid for arbitrary degree of saturation, is derived. For low degrees of saturation it agrees with the expression for weakly saturating (ws) conditions, [(1+S)−1/2−(1+2S)−1/2]α0/2, where S is the degree of saturation and α0 is the unsaturated peak absorption. However, the new expression, which can be written as 0.45S(1+S)−1α0/2, does not predict a distinct maximum as the ws expression does; instead it predicts an optical phase shift that increases monotonically with S and levels off to 0.45α0/2 for large S. This alters the optimum conditions for the sub-Doppler NICE-OHMS technique and improves its shot-noise-limited detectability. The new expression is based upon the same explicit assumptions as the ws expression but not the Kramers-Kronig relations, which are not valid for nonlinear responses, and is supported by experimental results up to S=100. The new expression is expected to be valid for all techniques measuring sub-Doppler dispersion signals.