Nonlinear Ramsay Fringes Using Three Separated Fields

2 Nonlinear Ramsay Fringes Using Three Separated Fields 

Another solution to restore the Ramsay fringes, which are generally washed out in the second field, is based on the introduction of a third field at a distance 2L downstream from the first field. The idea of this arrangement was first pointed out by BAKLANOV et al. [13.7]. The basic idea may be understood as follows. 

In Sect.10.2 it has been discussed in detail that the nonlinear absorption of a molecule in a monochromatic standing wave field leads to the formation of a narrow Lamb dip at the center o)q of a Doppler-broadened absorption line (see Fig.10.19). The Lamb-dip formation may be regarded as a two-step process the depletion of a small subgroup of molecules with velocity components v = 

0 Av by the pump wave (hole burning), and the successive probing of this depletion by a second wave. In the standing wave of the second zone the nonlinear saturation by the pump wave depends on the relative phase between the molecular dipoles and the field. This phase is determined by the starting point (0,Zj ) in the first zone and by the transverse velocity component v.  

The macroscopic polarization at (L,Z2) averages to zero because molecules with different velocities v arrive at Z2 from different points (0,z ). Note, however, that the population depletion An in the second field depends on the relative phase Acp and therefore on v. If the phases cp(z ) and cp(z2) of the two fields are made equal for z = Z2, the phase difference cp(z ) - p(z2) (p(z2 Z2) = qj(v2T) between the two fields at the intersection points depends 

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