FM spectroscopy of CPT resonances with AOM operating purely in the Raman-Nath diffraction regime as optic phase modulator

We report on further development of a new method for frequency control of the external cavity diode laser with the use of FM sideband heterodyne technique where AOM operating purely in the Raman-Nath diffraction regime is employed as an external phase modulator. It is shown that the AOM design can be considerably simplified and its size can be minimized if the Raman-Nath diffraction regime is the dominant diffraction regime of AOM. We report on the production of the AOM-RN designed primarily for operation in such a regime with 2 mm acousto-optic interaction length. We have detected CPT resonances by means of FM spectroscopy with AOM-RN as an external phase modulator. In combination with reduced light shift, the capacity of extremely high-speed laser frequency tuning given by FM sideband technique can promise further improvement of short-term stability of frequency standards utilizing CPT clock transitions.