The detection of narrow and high-amplitude optical resonances in atomic cells is of crucial importance for the development of compact and high-stability lasers or vapor-cell optical references. In this work, using monochromatic light and appropriate polarization settings for the counterpropagating pump and probe beams, we demonstrate the detection of enhanced-absorption sub-Doppler resonances on the
D1 line of the
Rb atom. We report a theoretical model showing that the Zeeman coherence induced between magnetic sublevels---especially in the $F\ensuremath{\rightarrow}{F}^{\ensuremath{'}}=F\ensuremath{-}1$ systems, with
F and ${F}^{\ensuremath{'}}$ being the hyperfine quantum numbers of the ground and excited states, respectively---plays a key role in the observed enhanced-absorption process. The theoretically predicted behavior matches the experimental observations well. The impact of light polarization, laser intensity, magnetic field, and cell temperature on the Doppler-free resonance features is experimentally investigated. For application, two identical diode lasers are frequency-stabilized onto enhanced-absorption sub-Doppler resonances. The fractional frequency stability of the laser beatnote is measured to be $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at
1s and $8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ at
104s. These results demonstrate the potential of this scheme for the implementation of a compact or even chip-scale optical frequency reference, which might find applications in instrumentation, navigation, and metrology.
Abstract Locking laser frequencies to Doppler-free spectra produced by the interaction of light with alkali atoms in a vapor cell is a promising approach for realizing compact atomic optical frequency reference. In this work, we employ counter-propagating monochromatic laser beams with a polarization angle of 45° to interact with 87 Rb atoms in a 3 mm cubic vapor cell, inducing coherent resonance. By detecting the laser polarization rotation, we obtain a Doppler free spectral line, which is used as a reference for laser frequency locking, thus realizing an optical reference. We build two nominally identical systems and measure their beatnote, yielding a fractional frequency stability of 5.3×10 -13 at 1 s. This study demonstrates that the proposed scheme is a competitive approach for realizing high performance compact atomic optical reference.