Frequency-dependent optical pumping in atomic A-systems

We consider the effects of optical pumping on the conversion of laser-frequ ency modulation into intensity modulation by an atomic absorption line in a vapor of alkali atoms driven in a A-configuration. It is found that, due t o optical pumping in combination with the excited-state hyperfine structure , the absorption line shape is distorted substantially as the Fourier frequ ency of the FM is changed. The most significant effect of the distortion is a shift of the apparent line center, which depends on how the frequency of the modulation compares with the optical pumping rate. This shift has impl ications for locking lasers to atomic transitions and also for FM-AM noise conversion in atomic vapors.