Short-term stability improvements of an optical frequency standard based on free Ca atoms

Compared to optical frequency standards featuring trapped ions or atoms in optical lattices, the strength of a standard using freely expanding neutral calcium atoms is not ultimate accuracy but rather short-term stability and experimental simplicity. Recently, a fractional frequency instability of $4 \times 10^{-15}$ at 1 second was demonstrated for the Ca standard at 657 nm~[1]. The short cycle time ($\sim$2 ms) combined with only a moderate interrogation duty cycle ($\sim$15 \%) is thought to introduce excess, and potentially critically limiting technical noise due to the Dick effect---high-frequency noise on the laser oscillator is not averaged away but is instead down-sampled by aliasing. We will present results of two strategies employed to minimize this effect: the reduction of clock laser noise by filtering the master clock oscillator through a high-finesse optical cavity~[2], and an optimization of the interrogation cycle to match our laser's noise spectrum.\\[4pt] [1] Oates et al., \emph{Optics Letters}, \textbf{25}(21), 1603--5 (2000)\\[0pt] [2] Nazarova et al., \emph{J.\ Opt.\ Soc.\ Am.\ B}, \textbf{5}(10), 1632--8 (2008)