A self-reference direct-measuring scheme for precision optical frequency ratio measurement

self-reference direct-measuring scheme for optical frequency ratio measurement is present in this work. The mathematical construction for the scheme is described in detail. An RF processing system was built according to the scheme. To examine the precision of the system, we measured the frequency ratio between the second harmonic and fundamental frequencies of a 1560-nm ultrastable laser. In 1 day’s measurement, a ratio of (2+1.2× 10^-22)± 1.4× 10^-21 was obtained. The corresponding fractional frequency instability was 5× 10^-19 at the averaging time of 1 s and less than 2× 10^-21 at 1000–10,000 s. This result suggested that the precision of the RF processing system was sufficient for comparison of state-of-the-art optical clocks at the level of 10^-18 .