Dichroic Two-Photon Rubidium Frequency Standard

We demonstrate an optical frequency standard based on the 5S(1/2 )-> 5D(5/2) two-photon transition of rubidium. The transition is interrogated in a Doppler-free arrangement by two lasers at 780 and 776 nm, with the sum frequency of the two lasers locked to the two-photon transition. We measure the fractional frequency stability of the frequency standard to be 1.5 x 10(-13) at 1 s, reaching 4.9 x 10(-14) at approximately 100 s, by comparison with an optical frequency comb. Performance limitations are presented and a clear pathway to an order-of-magnitude improvement is discussed. This platform has the potential to deliver a compact, robust standard for commercial and industrial applications.