A Two-Photon Optical Atomic Clock based on Direct Comb Spectroscopy

We present progress on direct comb spectroscopy (DCS) of the 5S1/2 to 5D5/2 two-photon transition in Rubidium-87 as an optical atomic clock. The use of DCS eliminates the need for cw laser excitation of the clock transition while the broad spectrum provides light at multiple wavelengths that can be utilized for ac-Stark shift mitigation or removal of the Doppler background. Low size, weight, and power frequency standards are an enabling technology for many future systems, including next generation GPS. DCS benefits from efficient frequency doubling of ultrashort pulses from reliable laser sources at telecom wavelengths, and the potential for novel multiwavelength techniques. By carefully tailoring our comb bandwidth , we demonstrate narrow linewidth two-photon clock excitation linewidths comparable to those achieved with cw excitation and capable of supporting next generation high stability clocks. We also present experimental findings on the wavelength dependence of the ac-Stark shift around the two-photon transition, towards verifying the proper intensity ratio between a Stark shift mitigation beam and the frequency comb probe.