High-stability underwater frequency transmission based on low noise frequency doubled green laser

An underwater frequency transmission scheme based on low noise frequency doubled green laser was proposed and demonstrated. The 500 MHz microwave frequency signal was modulated by 1018 nm narrow-linewidth seed laser with low noise and seed laser frequency doubling to 509 nm, which verified that the additional noise introduced in the frequency-doubling process was low enough to realize the low noise electro-optic conversion of microwave frequency reference signals. Meanwhile, the transmission noise was compensated with electronic phase compensation technique. Then the frequency reference signal transmitted over 8 m underwater link with high precision was realized. The experimental results showed that the fractional frequency instability was 5.5 × 10⁻¹⁴ @ 1 s, 1.7 × 10⁻¹⁶ @ 1000 s, respectively. The total root-mean-square timing delay fluctuation was about 288 fs. By the scheme, the fractional frequency instability of underwater frequency transmission reached to 10⁻¹⁴ orders, which met the frequency transmission requirements of high-performance hydrogen clocks, etc. The method would have important application prospect in underwater space–time reference and distributed sensor network.