Stabilizing diode laser to 1\xa0Hz-level Allan deviation with atomic spectroscopy for Rb four-level active optical frequency standard

Abstract

We achieve a compact ultra-stable 420 nm blue diode laser system by immediately stabilizing the laser on the hyperfine transition line of Rb atom. The Allan deviation of the residual error signal reaches 1 Hz-level Allan deviation within 1 s averaging time, and the fractional frequency Allan deviation is 1.4times 10^{-15}/sqrt{tau }, which shows the best result of frequency-stabilized lasers based on the atomic spectroscopy without Pound–Drever–Hall (PDH) system. The signal-to-noise ratio of the atomic spectroscopy is evaluated to be 3,000,000 from the Allan deviation formula, which is the highest record, to the best of our knowledge. The frequency noise suppression characterization is demonstrated and the maximal noise suppression can be near 40 dB at 6 Hz. As a good candidate of pumping source, the ultra-stable 420 nm diode laser is successfully used in our Rb four-level active optical frequency standard system. The method can be easily extended to other wavelengths ultra-stable lasers with a Allan deviation of 10^{-15} level retaining an atomic reference with low cost and low complexity while in the absence of an expensive and complicated PDH system.