Coherent-population-trapping maser: Noise spectrum and frequency stability

We examine in this paper the spectral characteristics of a coherent population trapping (CPT) maser in order to evaluate the frequency stability achievable by this kind of passive frequency standard. The peculiar features of the coherent microwave emission associated with the coherent population trapping phenomenon are theoretically examined taking into account the impact of the various noise contributions on the physical principle of operation. The theoretical predictions are then compared with the experimental results obtained with a laboratory prototype of 87Rb CPT maser. The measured frequency stability is mainly limited in the short term by the system thermal noise and in the medium term by the thermal drift and turns out to be in agreement with the computations within a factor of 2.