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We consider the effects of optical pumping on the conversion of laser-frequency modulation into intensity modulation by an atomic absorption line in a vapor of alkali atoms driven in a ?-configuration. It is found that, due to optical pumping in combination with the excited-state hyperfine structure, the absorption line shape is distorted substantially as the Fourier frequency 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 implications for locking lasers to atomic transitions and also for FM-AM noise conversion in atomic vapors.

The femtosecond laser is a key element of a coherent link between optical and microwave domains. Such a link has already enabled measurements of optical frequencies with an accuracy of a current time standard, the caesium beam microwave 'clock' (S.A. Diddams et al., Phys. Rev. Lett. vol. 84, pp. 5102-5105, 2000; T. Udem et al., ibid., vol. 86, p. 4996, 2001). Making use of a femtosecond laser, it is also possible to transfer the frequency stability of an optical 'clock' to radio frequencies. This may result in redefinition of a 'second' when optical frequency standards supersede their microwave counterparts in terms of accuracy (J.L. Hall et al., IEEE J. Quantum Electron., vol. 37, no. 12, pp. 1482-1492, 2001; L. Hollberg et al., ibid., vol. 37, no. 12, pp. 1502-1513, 2001). In this work we discuss the basic noise mechanisms affecting the frequency stability of a microwave signal produced with a femtosecond laser, including power-to-phase conversion in the photodetector and laser beam-pointing fluctuations.

This paper describes the fabrication of chip-scale alkali atom vapor cells, for use in highly miniaturized atomic frequency references, using silicon micromachining and anodic bonding technology. The cells consist of silicon cavities with internal volume ranging from a few mm/sup 3/ to less than 1 mm/sup 3/. The cells were filled with cesium and nitrogen buffer gas either by chemical reaction of cesium chloride and barium azide, or by direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption spectra were obtained from the cells, and coherent population trapping resonances with linewidths of about 1 kHz were measured.

The present work has three goals: first, to attract attention of theorists and experimentalists to the Doppler narrowing of the /spl Lambda/ resonance; second, to obtain an explicit analytical expression for the signal under arbitrary intensities of fields; and finally, to compare our theoretical results with experiments.