Dual frequency calibration to build a portable vapor cell optical clock with improved stability and without a frequency comb
This article theoretically proposes a new dual interferometer technique to accurately calibrate two laser frequencies simultaneously using four-wave mixing in an alkali metal vapor cell. The two frequency-calibrated lasers are mixed to create a beat signal at radio frequency to build a portable optical atomic clock (OAC) without an optical frequency comb (OFC). Removal of the OFC improves the portability of OAC, while the dual interferometer setup enhances the one second stability to $1.3\times 10^{-15}$, which is better than the current portable OAC. Thermal noise in the OAC is minimized by choosing the double-lambda atomic scheme with co-propagating laser fields. Using D2 transition of Rb-87, the standard quantum limited frequency sensitivity and stability of the OAC are estimated as $3.2\;\sqrt{\mbox{Hz}}$, and $1.3\times10^{-15}\sqrt{\mbox{Hz}^{-1}}$, respectively. After considering broadening effects due to $357\,$K temperature and collisions, the optimum stability of the OAC is reduced to $3.3\times10^{-15}\sqrt{\mbox{Hz}^{-1}}$ for a laser with $1\,$KHz linewidth and $0.54$ mW power at the input of the vapor cell.