Spectroscopy saturated interference

In this section we will briefly discuss some variations of saturation, polarization, or multiphoton spectroscopy that either increase the sensitivity or are adapted to the solution of special spectroscopic problems. They are often based on combinations of several nonlinear techniques. 

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]. 

The probe beam is split by the plane-parallel plate Pll into two beams. One beam passes through that region of the absorbing sample that is saturated by the pump beam the other passes through an unsaturated region of the same sample cell. The two beams are recombined by a second plane-parallel plate P12. The two carefully aligned parallel plates form a Jamin interferometer [273], which can be adjusted by a piezoelement in such a way that without the saturating pump beam the two probe waves with intensities I and h interfere destructively. 

If the saturation by the pump wave introduces a phase shift p, the resulting intensity at the detector becomes 

The intensities h and h of the two interfering probe waves can be made equal by placing a polarizer PI into one of the beams and a second polarizer P2 in front of the detector. Due to a slight difference 5 in the absorptions of the two beams by the sample molecules, their intensities at the detector are related by 

The intensities I and I2 of the two interfering probe waves can be made equal by placing a polarizer PI into one of the beams and a second polarizer P2 in 

Inserting (7.73) into (7.72) yields, for the total intensity / at the minimum of the interference patterns, the Lorentzian profile. 

According to (7.66) the phase differences w) depends on the laser frequency w. However, it can always be adjusted to zero while the laser frequency is scanned. This can be accomplished by a sine wave voltage at the piezoelement which causes a modulation 

The probe beam is split by the plane parallel plate Pl into two beams. 

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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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