Ramsay fringes

The interesting question of whether there exists a fundamental resolution limit, is discussed in Chap.13. Some recently developed techniques such as optical Ramsay fringes and trapping and cooling of atoms, illustrate how laser spectroscopy approaches this ultimate limit of spectral resolution. 

Some techniques of laser spectroscopy, such as the method of separated fields (optical Ramsay fringes, see Sect.13.1) or coherent transient spectroscopy (see Sect.11.4) allow one to distinguish between phase changing, velocity changing, or orientation changing collisions. 

Fig> 13 2> Signal power, ab-sorbed in the second field, as a function of detuning = 0) - o)q (Ramsay fringes for a narrow velocity distribution)... 

Measured as a function of the field frequency w, this signal exhibits an interference pattern called Ramsay fringes (Fig.13.2). The full halfwidth of the central fringe, which is = tt(v/L), decreases with the separation L between the fields. 

Fortunately several methods have been developed which overcome these difficulties and which allow ultranarrow Ramsay resonances to be obtained. One of these methods is based on Doppler-free two-photon spectroscopy, while another technique uses saturation spectroscopy but introduces a third interaction zone at a distance x = 2L downstream from the first zone to recover the Ramsay fringes. We briefly discuss both methods. 

Figure 13.5 illustrates the narrowing of the two-photon resonance with two separated interaction fields. The technique of two-photon Ramsay resonances has been succesfully applied to the ultrahigh resolution spectroscopy of Rydberg levels in the rubidium atom using a well-stabilized cw dye laser [13.6]. At a separation of 4.2 mm between the two fields, Ramsay fringes with... 

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. 

The capability of this combination of optical Ramsay fringes with saturation spectroscopy has been impressively demonstrated by BERQUIST et al. [13.8],... 

In the previous discussion the third field was used to detect the Ramsay fringes through resonances in the absorbed power. It is also possible to omit the third field. If two standing waves at x = 0 and x = L resonantly interact with the molecules, continuous coherent radiation may be observed at x = 2L, which is due to polarization transfer. The radiation intensity has a sharp peak at the center frequency u) 2 basic principle of this phenomenon is similar to that of photon echoes (see Sect.11.4.2). Due to a phase jump at the nonlinear interaction with the second field the Doppler phase, caused by the transverse velocity v, is exactly cancelled if the transfer times T 2 = v /(x2 x ) and T23 = v (x - X2) are equal [13.9]. 

---