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Controlling the light shift of the CPT resonance by modulation technique
Motivated by recent developments in atomic frequency standards employing the
effect of coherent population trapping (CPT), we propose a theoretical framework for the
frequency modulation spectroscopy of the CPT resonances. Under realistic assumptions we
provide simple yet non-trivial analytical formulae for the major spectroscopic signals such as
the CPT resonance line and the in-phase/quadrature responses. We discuss the influence of the
light shift and, in particular, derive a simple expression for the displacement of the resonance
as a function of modulation index. The performance of the model is checked against numerical
simulations, the agreement is good to perfect. The obtained results can be used in more general
models accounting for light absorption in the thick optical medium.