Saturated absorption spectroscopy

The primary effect of this additional absorption loss in the laser cavity will be to reduce the laser output power. However, as the laser oscillation frequency is tuned towards the centre of the absorption line, the two holes burned in the population of absorbing atoms will start to overlap. At the line centre the population difference, and hence the absorption coefficient of the sample, is reduced by the saturation of atoms with zero axial velocity interacting with both components of the laser standing-wave field. This sharp drop in the absorption coefficient, which occurs about the centre of the absorption line, reduces the internal losses of the cavity and gives rise to a sharp increase in the power output of the laser. 

Saturated absorption spectroscopy of molecules. Fortunately molecular absorption lines are far more numerous than those of atoms and several accidental coincidences exist with known gas laser transitions. The first of these to be studied by saturated absorption spectroscopy was the coincidence between the 6328 X He-Ne laser and the R(127) rotational absorption line of the 11-5 vibration band in the 

This work was extended by Hansch et at. (1971) using a krypton ion laser, and the apparatus shown in Fig.13.13. In this experiment the iodine absorption cell was placed outside the laser cavity and a standing wave was obtained by arranging two laser beams, each of 10 mW power, to intersect at an angle of less than 2 mrad inside the 20 cm long absorption cell. From equation (13.40) the saturated absorption coefficient when both beams are present in the cell is given approximately by 

In this equation k (w) has been obtained by modifying equation (13.30). 

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Figure 4. Apparatus for saturated absorption spectroscopy of tritium.

B. Bobin, C.J. Bord6, J. Borde, C. Breant, Vibration-rotation molecular constants for the ground and (V3 = 1) states of SFg from saturated absorption spectroscopy. J. Mol. Spectrosc. 121, 91 (1987)... 

C.J. Borde, J.L. Hall Ultrahigh resolution saturated absorption spectroscopy . In Laser Spectroscopy, ed. by R.G. Brewer, H. Mooradian (Plenum, New York 1974) pp. 125-142... 

J.L. Hall Saturated absorption spectroscopy, in Atomic Physics, Vol.3, ed. by S. Smith, G.K. Walters (Plenum, New York 1973)... 

Fiq.l0.24a-c. Intracavity saturated absorption spectroscopy, (a) Experimental setup, (b) "Lamb peak" in the laser output, (c) derivative of a Lamb peak obtained by modulation of the laser frequency... 

J.L. Hall "Saturated Absorption Spectroscopy", in Atomic Physics,... 

Fig. 1 Apparatus for Doppler-free saturated absorption spectroscopy in a gas sample.

Fig. 2 Top Balmer spectrum of atomic hydrogen. Center Doppler profile of the Balmer-a line at room temperature and theoretical fine structure components. Bottom Doppler-free spectrum of Balmer-a, recorded by saturated absorption spectroscopy with a pulsed dye laser.

A large number of these accidental coincidences of molecular absorption lines with gas laser transitions are now known and some of the most important examples are listed by Hall (1973) in a review of saturated absorption spectroscopy. 

For saturated absorption spectroscopy experiments using the 3-39 pm line it was found convenient to transfer... 

In the previous three chapters we have discussed the properties of gas lasers and have shown how they can be designed for single frequency oscillation, and also how the output frequency may be tuned continuously over the bandwidth of the Doppler-broadened gain curve. Unfortunately this tuning range is relatively narrow and the application of these gas lasers to atomic and molecular spectroscopy is restricted to studies of the laser transitions themselves, or to accidental coincidences with molecular absorption lines. It would therefore seem that the new and powerful technique of saturated absorption spectroscopy was also of relatively limited applicability. 

Using saturated absorption spectroscopy, the hyperfine splittings of both the ground state and the excited levels... 

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