Fig. 1. Experimental setup of the frequency-stabilized Ti:sapphire OFC. The solid lines represent the light path, while the dashed lines represent the electrical path. PPKTP, periodically poled KTiOPO₄ crystal; OBPF, optical bandpass filter; PCF, photonic crystal fiber; PD, photo detector.

Fig. 2. (a) Frequency fluctuation of f_r when it is free running. (b) f_ceo detected at 532 nm with an RBW of 300 kHz. (c) Optical beat signals of the Ti:sapphire comb against a cavity-stabilized laser at 1064 nm (in dark wine), a 729 nm laser (Ca⁺ clock, in magenta), a 698 nm laser (Sr clock, in red), a 578 nm laser (Yb clock, in yellow), and an 532 nm laser (iodine optical reference, in green) with an RBW of 300 kHz.

Fig. 3. Logic block diagram of digital servo of f_ceo.

Fig. 4. Frequency jitters of (a) f_ceo and (b) f_r when phase-locked to an H maser. (c) The frequency instability of f_ceo (purple dots) and f_r (black squares).