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 conditions, , where S is the degree of saturation and is the unsaturated peak absorption. However, the new expression, which can be written as , does not predict a distinct maximum as the expression does; instead it predicts an optical phase shift that increases monotonically with S and levels off to 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 expression but not the Kramers–Kronig relations, which are not valid for nonlinear responses, and is supported by experimental results up to . The new expression is expected to be valid for all techniques measuring sub-Doppler dispersion signals.
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