Appendix D: The line shape in saturated absorption spectroscopy 309Fig. 8.4(b). This saturated-absorption signal comes from the velocity
class of atoms withv=0.
We assumed thatIIsatto obtain eqn D.3, but usually in exper-
iments the pump beam has an intensity close to saturation to give a
strong signal. The simple rate equation treatment becomes inaccurate
aroundIsatand a more sophisticated approach is required based on the
optical Bloch equations, as described in Letokhov and Chebotaev (1977).
Also, optical pumping out of the state that interacts with the radiation
into other Zeeman sub-levels, or hyperfine levels, can be the dominant
way of depleting the lower-level populationN 1 (v). However, the line
shape remains symmetrical aboutω 0 , so saturation spectroscopy gives
an accurate measurement of the atomic resonance frequency.