A low noise optical frequency synthesizer at 700-990 nm

Optical frequency synthesizers can generate single-frequency laser light with high precision and accuracy at any desired wavelength over a wide optical region. Here, we demonstrate such an optical frequency synthesizer, which yields coherent light at any wavelength within 700-990 nm with more than 500 mW of power. The relative fractional frequency instability and uncertainty between the output light and the reference light of the optical frequency synthesizer are 6 × 10^-19 at 1 s averaging time and 2 × 10^-21, respectively. This synthesis noise is two orders of magnitude better than the frequency stability and accuracy provided by optical clocks, supporting optical frequency synthesis from the most accurate optical clocks. When the optical frequency synthesizer is referenced to a cavity-stabilized laser at 1064 nm, the output of the optical frequency synthesizer is tested to have an average linewidth of 1 Hz and frequency instability of 1.5 × 10^-15 at 1 s, limited by the reference laser.