Optical clocks with unprecedented accuracy of 10−18 promise innovations in many research areas. Their applications rely to a large extent on the ability of precisely converting the frequency from one optical clock to another, or particularly to the frequencies in the fiber telecom band for long-distance transmission. This report demonstrates a low-noise, high-precision optical frequency divider, which realizes accurate optical frequency conversion and enables precise measurement of optical frequency ratios. By measuring against the frequency ratio between the fundamental and the second harmonic of a 1064-nm laser instead of a second copy of the same system, we demonstrate that the optical frequency divider has a fractional frequency division instability of 6 × 10−19 at 1 s and a fractional frequency division uncertainty of 1.4 × 10−21. The remarkable numbers can support frequency division of the best optical clocks in the world without frequency-conversion-caused degradation of their performance.

Figure. Diagram of performance measurement. (a) Schematic diagram. (b) Experimental diagram. We used the OFD to connect the optical frequencies of a cavitystabilized laser at 1064 nm (f1064–1) and the light with frequency f532 which is second-harmonic generated from an independent 1064-nm laser (f1064–2). As a result, f532 = f1064–1/Rx. The uncertainty of Rx is obtained by measuring the beating frequency (fb) between f1064–1 and f1064–2 on an RF frequency counter with a self-referenced time base of f1064–1/k. fs laser, femtosecond laser; PCF, photonic crystal fiber; PPKTP, periodically poled KTP; SHG, second-harmonic generation; BS, beam splitter; PD, photo detector; OBPF, optical band pass filter; fb1, the beat note between f1064–1 and the comb; fb2, the beat note between f532 and the comb; f0, the carrier-envelope offset frequency of the comb.

Optical frequency divider with division uncertainty at the 10−21 level.pdf