Effects of modulation on signals obtained in saturated absorption spectroscopy experiments

An analysis of the useful signals detected in saturated absorption spectroscopy experiments is given. Both Gaussian and plane-wave fields are considered. The various cases of unmodulated, square-wave modulated and sinusoidally modulated saturating beams are examined. It is found that square-wave and sinusoidal modulations give different results. The first is identical to no modulation, and detection at the fundamental frequency records a fraction of the total signal which is also a square wave. The second gives fundamental or higher-harmonic signals each with a very different dependence on the saturation parameter. The signal at the fundamental saturates faster and more completely than that for square-wave modulation. The results suggest experimental methods for the determination of the absolute value of the saturation parameter.