Compact Rb optical frequency standard with 10 -15 stability.

We achieved a low-cost and small-sized Rb optical frequency standard based on ⁸⁵ Rb 5S _1/2 → 6P _3/2 transition with 10 ^-15 stability, which is comparable with that of the best 532 nm I ₂ optical frequency standards. In this system, we directly lock the 420 nm diode laser on the 5S _1/2 F = 3 → 6P _3/2 F ^' = 4 hyperfine transition line without an additional Pound-Drever-Hall pre-locking system. The signal-to-noise-ratio reaches as high as 350 000 when the averaging time is at 1 s. Eventually by the fluctuation of the residual error signal after locking, the preliminary stability of the optical frequency standard reaches 1.2×10 ^-14 /τ, decreasing to 2.1 × 10 ^-15 at 80 s. It shows potential in stability performance, experimental cost, and system volume compared with the 532 nm I ₂ optical frequency standard as a wavelength standard. It also opens a door for the achievement of wavelength standards by using higher excited states of alkalies.