Tailoring the stability of a two-color, two-photon rubidium frequency standard

Rubidium two-photon frequency standards are emerging as powerful contenders for compact, durable devices with exceptional stability. To date, the field has focused on single-color excitation; here, we demonstrate the key advantages of a two-color excitation of a two-photon optical frequency standard based on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ transition of rubidium-87 using driving fields at 780 and 776 nm. Using the $5{P}_{3/2}$ intermediate state to resonantly enhance the transition, we show that frequency stabilities comparable to rubidium single-color two-photon frequency standards can be attained, notably with approximately tenfold lower optical power and tenfold lower rubidium vapor density. Optimization of the detuning from the $5{P}_{3/2}$ intermediate state and of the optical powers of the driving lasers has a dramatic effect on the frequency stability, achieving the best short-term stability of any two-photon rubidium frequency standard to date: $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ at $\ensuremath{\tau}=1$ s. We demonstrate that this level of performance is compatible with a compact geometry by fully self-referencing the frequency standard using an integrated fiber frequency comb to simultaneously stabilize the 780-nm laser's detuning from the $5{P}_{3/2}$ intermediate state, producing a frequency-stable microwave output. A comprehensive noise characterization underpins our observations of this two-color frequency standard, explaining the measured stability and showing that this frequency standard is initially shot-noise limited before becoming limited by light shifts in the long term. This work represents a major advance toward a low size, weight, and power frequency standard based on two-color excitation.