Residual Phase Noise Measurement of Optical Second Harmonic Generation in PPLN Waveguides

We report on the characterization, including residual phase noise and fractional frequency instability, of fiber-coupled periodically poled Lithium Niobate non-linear crystals. These components are devoted to frequency doubling 871-nm light from an extended-cavity diode laser to produce a 435.5-nm beam, corresponding to the ytterbium ion electric quadrupole clock transition. We measure the doubling efficiencies of up to 117.5%/W. Using a Mach-Zehnder interferometer and an original noise rejection technique, the residual phase noise of the doublers is estimated to be lower than -35 dBrad(2)/Hz at 1 Hz, making these modules compatible with up-to-date optical clocks and ultra-stable cavities. The influence of external parameters, such as pump laser frequency and intensity, is investigated, showing that they do not limit the stability of the frequency-doubled signal. Our results demonstrate that such compact, fiber-coupled modules are suitable for use in ultra-low phase noise metrological experiments, including transportable optical atomic clocks.