Infrared & Visible Ultrastable Optical Frequency Standard

We report on the development of a three-wavelength optical frequency standard (OFS: $\omega, 2_{\omega}, 3\omega)$ , frequency stabilized to a narrow iodine hyperfine line around 514 nm. To bridge the frequency gap between the infrared and the green ranges of the optical domain (1542 nm $\rightarrow\ \pmb{514}\ \mathbf{nm}$ ), we use an efficient third harmonic generation process, based on two cascaded Lithium Niobate crystals, arranged in an original optical configuration. Up to 300 mW of green power is obtained from 800 mW of IR power, corresponding to an unprecedented demonstration of an optical conversion efficiency $\eta=\mathbf{P}_{3\omega}/\mathbf{P}_{\omega}\sim 36$ % in continuous wave regime. Iodine Doppler-free spectroscopy technique is used to frequency stabilize this OFS against a narrow hyperfine line at 514 nm yielding to a frequency stability $\sigma_{\mathrm{y}}(\tau)=2.8\ \mathbf{x}\ 10^{-14} \tau^{-1/2}$ , with a minimum value of $7.3\ \mathrm{x}10^{-15}$ after 200 s of integration time.