Atomic frequency offset locking in a Λ type three-level Doppler broadened Cs system

We here present a comparative study of frequency stabilities of pump and probe lasers coupled at a frequency offset generated by coherent photon-atom interaction. Pump-probe spectroscopy of the Λ configuration in D ₂ transition of cesium is carried out to obtain sub-natural (∼2 MHz) electromagnetically induced transparency (EIT) and sub-Doppler (∼10 MHz) Autler-Townes (AT) resonance. The pump laser is locked on the saturated absorption spectrum (SAS, ∼13 MHz) and the probe laser is successively stabilized on EIT and AT signals. Frequency stabilities of pump and probe lasers are calculated in terms of Allan variance σ(2,τ) by using the frequency noise power spectrum. It is found that the frequency stability of the probe stabilized on EIT is superior (σ ∼ 2 × 10^-13) to that of SAS locked pump laser (σ ∼ 10^-12), whereas the performance of the AT stabilized laser is inferior (σ ∼6× 10^-12). This contrasting behavior is discussed in terms of the theme of conventional master-slave offset locking scheme and the mechanisms underlying the EIT and sub-Doppler AT resonances in a Doppler broadened atomic medium.