Progress in the development of an optical frequency standard at 192.6 THz based on a two-photon transition of rubidium atoms at Laval University

We have developed frequency standards at 192.6 THz (1556.2 nm) based on the two-photon transitions in rubidium at 385.2 THz (778.1 nm). These standards use a high power DFB laser at 1556.2 nm and second harmonic generation (SHG) in a periodically poled lithium niobate (PPLN) crystal. The linewidth of the DFB is reduced to the kHz level using optical feedback from a confocal cavity. The SH light is used to injection-lock a 778.1 nm laser diode which allows to observe and lock the 1556.2 nm laser to the (5S_1/2,F_g=2-5D_5/2 ,F_e=4) two-photon transition ⁸⁷Rb. Allan variance measurements between identical standards show a beat stability 2.5×10^-13/τ^1/2 for observation times between 100 ms and 10 s and a level of 5.8×10^-14 for 100 s is obtained. These standards have been moved recently to the National Research Council, Ottawa, Canada in order to measure their absolute frequency