Characterization of the phase-noise induced by an optical frequency doubler

Summary form only given. State-of-the-art optical atomic clocks have attained short-term fractional frequency stabilities in the low 10 range [1, 2]. This has been made possible thanks to improved performances of the clock lasers, which are pre-stabilized using ultra-stable Fabry-Perot (FP) resonators [3] Many of the last-generation optical clocks rely on frequency doubling via second harmonic generation (SHG) to produce the laser clock frequency [4, 5], where the pump laser used for SHG is stabilized to the ultra-stable FP resonator. It is therefore necessary to ensure that the SHG setup does not degrade the beam fractional frequency stability.Here we employ and describe fiber-coupled PPLN waveguides to perform the frequency doubling from a commercially-available laser diode at a wavelength λ = 871 nm to the clock frequency of Yb' at λ = 435.5 nm. These modules are both compact, highly-efficient, and avoid the use of an enhancement cavity. We report SHG efficiencies up to 117.5 % /W.We measure the phase noise induced by the SHG with a Mach-Zender interferometer and a novel noise rejection technique and observe a relative phase noise as low as -40 dBrad 2/Hz at 1 Hz, which makes them compatible with the best up-to-date optical clocks and ultra-stable cavities.