Characterization of Frequency-Doubled 1.5- $\mu$ m Lasers for High-Performance Rb Clocks

We report on the characterization of two fiber-coupled 1.5-μm diode lasers, frequency-doubled and stabilized to Rubidium (Rb) atomic resonances at 780 nm. Such laser systems are of interest in view of their implementation in Rb vapor-cell atomic clocks, as an alternative to lasers emitting directly at 780 nm. The spectral properties and the instabilities of the frequency-doubled lasers are evaluated against a state-of-the-art compact Rb-stabilized laser system based on a distributed-feedback laser diode emitting at 780 nm. All three lasers are frequency stabilized using essentially identical Doppler-free spectroscopy schemes. The long-term optical power fluctuations at 780 nm are measured, simultaneously with the frequency instability measurements done by three beat notes established between the three lasers. One of the frequency-doubled laser systems shows at 780 nm excellent spectral properties. Its relative intensity noise <;10^-12 Hz^-1 is one order of magnitude lower than the reference 780-nm laser, and the frequency noise <;10⁶ Hz²/Hz is limited by the laser current source. Its optical frequency instability is <; 4 ×10^-12 at τ = 1 s, limited by the reference laser, and better than 1 × 10^-11 at all timescales up to one day. We also evaluate the impact of the laser spectral properties and instabilities on the Rb atomic clock performance, in particular taking into account the light-shift effect. Optical power instabilities on long-term timescales, largely originating from the frequency-doubling stage, are identified as a limitation in view of high-performance Rb atomic clocks.