Progress towards in-line detection along dominant decay path for the 5S1/2 - 5D5/2 two-photon rubidium clock

Clocks built upon the 5S1/2-5D5/2 Doppler free two-photon transition in rubidium are promising candidates for navigation, communications, and other extra-laboratory applications. The most common current method to stabilize to the transition is through fluorescence detection of a decay path yielding 420 nm photons. However, this is not the dominant decay path, leading to small signals that require a high voltage photo-multiplier tube and larger optical powers. The dominant decay path, through the 5P3/2 state, produces ~12x more fluorescence at 776 nm, unfortunately, only 2 nm de-tuned from the 778 nm excitation laser. Detection at 776 nm would allow for a lower intensity probe beam without the loss of signal-to-noise ratio, potentially leading to a reduction of ac-Stark shifts. Moreover, detection at 776 nm allows the use of a multi-pixel photon counter, eliminating the need for a high voltage photo-multiplier tube. We present progress towards implementing a clock detecting the 5D5/2-5P3/2 decay at 776 nm with in-line geometry using high OD, sharp cut-off filters and a multi-pixel photon counter.

Approved for public release; distribution is unlimited. Public Affairs release approval # AFRL-2021-4449