Optical Frequency Divider with 21 Digits

Abstract

Optical frequency divider (OFD) can convert an optical frequency from one optical clock to another or to the microwave region with an accurate frequency ratio. A low-noise, high precision OFD has been realized based on a Ti:sapphire mode-locked femtosecond laser. By comparing with the frequency ratio between the fundamental and the second harmonic of a 1064 nm laser, we demonstrate that the OFD has a fractional frequency division instability of 10−18 at 1 s and a fractional frequency division uncertainty of ${1.4\times 10^{-21}}$ . The remarkable numbers support frequency division of the best optical clocks in the world without degrading their performance.