Saturation interference spectroscopy

The higher sensitivity of polarization spectroscopy compared with conventional saturation spectroscopy results from the detection of phase differences rather than amplitude differences. This advantage is also used in a method that monitors the interference between two probe beams where one of the beams suffers saturation-induced phase shifts. This saturated interference spectroscopy was independently developed in different laboratories [271, 272]. The basic principle can easily be understood from Fig. 2.43. We follow here the presentation in [271]. 

Contrary to the situation in polarization spectroscopy, where for slightly uncrossed polarizers the line shape of the polarization signal is a superposition of Lorentzian and dispersion profiles, with saturated interference spectroscopy pure Lorentzian profiles are obtained because the phase shift is compensated by the feedback control. Measuring the first derivative of the profiles, pure dispersion-type signals appear. To achieve this, the output of the lock-in amplifier that controls the... 

F.V. Kowalski, W.T. Hill, A.L. Schawlow, Saturated-interference spectroscopy. Opt. Lett. 

A further method of monitoring Doppler-free signals using transmitted beams is also possible. In this technique (saturated interference spectroscopy) [9.175,176], the change in refractive index for the atoms at the "hole" position is used to influence the light interference condition in a two-beam interferometer. If the set-up is initially adjusted for destructive interference an increase in light intensity will be observed at the line centre. 

Several researchers have demonstrated that Doppler-free signals can be obtained by observing light-induced changes in the refractive index or dispersion rather than the absorption, (14,15) although such saturated interference spectroscopy has so far found only limited use. 

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