Blue fluorescence as a frequency offset reference in the rubidium 5S-5P-5D transition

In this work, we address the issue of the suitability of blue fluorescent light as a frequency offset reference during the ladder-level excitation 5S₁∕₂ → 5P₃∕₂ → 5D₃∕₂ of ⁸⁵Rb atoms. A simple pump–probe spectroscopy experiment is performed to generate non-degenerate blue light emission produced by the spontaneous decay route 5D₃∕₂ → 6P₃∕₂ → 5S₁∕2. By varying the relative intensity of the pump–probe combination, we can hover between regimes dominated by mechanisms such as double resonance optical pumping (DROP) and Autler–Townes splitting. The efficacy of the blue fluorescence (420 nm) as a frequency offset reference is pronounced under the DROP regime due to its narrow (∼natural) linewidth. To provide further insight into the stability performance, the pump laser connecting the 5P₃∕₂ → 5D₃∕₂ transition is stabilized to the peak of the blue light with the help of a probe stabilized to the saturation absorption signal of the 5S₁∕₂ → 5P₃∕₂ transition. For this purpose, we used the coherence-induced modulation transfer technique. The error frequency noise power spectra and the subsequent Allan variance calculation clearly show that probe fluctuations are transferred to the pump laser because of an existing two-photon coherent coupling.