Fig. 1. RF signal processing scheme for the optical frequency transfer. ⊕ means adding the two input frequencies, and ⊖ means subtracting the two input frequencies. DDS denotes the scaling of the frequency with the corresponding factors in the box.

Fig. 2. Optical set-up for the frequency transfer. FNC, fiber noise cancellation technique was applied to this fiber link; PPLN, periodically poled lithium niobate, which was used to generate the second harmonic of the OFC; EDFA, erbium-doped optical fiber amplifier, which was used to amplify the power of the OFC; HNLF, highly nonlinear fiber, which was used to expand the spectrum of the comb; PD, photodetector; AOM, acousto-optic modulator, which was used to shift the optical frequency; FC, fiber optic coupler, which was used to combine the two 1560 nm lasers.

Fig. 3. Measured result of the beating frequency between two 1.5 µm transfer lasers. (a) is the frequency difference between f_m and Δν/64, and (b) is the corresponding fractional Allan deviation related to ν₂.

Fig. 4. Frequency ratio of the two transfer lasers measured over the same OFC: (a) the measured value of f_diff2 (f_dm), (b) the corresponding Allan deviation of R_m.