Fig. 1. Diagrammatic sketch of (a) fold cavity and (b) muti-cavity.

Fig. 2. Schematic of the optical synthesizer. PD, photodiode; SG, signal generator; ÷n, frequency divider; BP, band-pass filter; AOM, acoustic-optical modulator.

Fig. 3. Experimental setup of an ultrastable laser with two reference cavities. U, ultrastable laser; CO, collimator; PBS, polarization beam splitter; HWP, half-wave plate; P, polarizer; PD, photodiode; ÷2, frequency divider; SG, signal generator; LP, low pass filter; PS, RF signal power splitter; red line, optical signal; black line, electronic signal.

Fig. 4. Relative frequency instability of lasers and their noise floors, where ν1 and ν2 are frequencies of two independent ultrastable lasers, ν1&2 is the synthesized laser with a frequency of (ν1+ν2)/2−νSG1, discrimination is obtained from variation of (ν1−ν2)−2(ν1−ν1&2), and normalized by the optical frequency (192 THz).