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We develop a theory of modulation response of the coherent population trapping (CPT) resonance. We consider the simplest three-level atom, but take into account the polychromatic spectrum of the pumping and probing light produced by pure FM modulation of single frequency field. The analysis based on the density matrix equations rigorously includes the most important for applications range of modulation parameters where modulation frequency and deviation are comparable with or greater than the CPT linewidth, so the response of the atomic medium is not adiabatic. Some theoretical results, in particular the possibility of using a quadrature response to modulation to suppress light shift, are compared with the experiment carried out with a diode laser (VCSEL) and 87 Rb atoms.

A method of dynamic continuous-wave spectroscopy of coherent population trapping (CPT) resonances using phase modulation of the jump type is developed. The time evolution of the spectroscopic signal is investigated. A method for the formation of an error signal for frequency stabilization is proposed. We show that our approach has a reduced sensitivity to the lineshape asymmetry of the CPT resonance. The experimental results are in good qualitative agreement with theoretical predictions based on a mathematical model of a three-level Λ system in a bichromatic field. This method can be used in atomic frequency standards (including chip-scale atomic clocks).

We propose and demonstrate a simple technique for identifying the central Ramsey fringe of pulsed coherent population trapping resonance. An auxiliary optical field is applied during the free evolution time. It suppresses the nearby fringes but does not change the amplitude of central fringe practically, which marks it out clearly. The theory based on the density matrix equations for the Λ-system configuration of levels and Ramsey interrogation that takes into account the auxiliary optical field is presented and compared to the experiment with 87Rb atoms. We also propose a technique for improving the middle- and long-term stability of compact atomic clocks.