Fig. 1. Simulation and experiment results of transmission and reflection. (a) Different schemes of reflection or backscattering for injection locking. (b) The photograph of the Sagnac ring and the ring structure are lighted by green. (c) Simulated transmission (red line) and reflection (blue line) of the Sagnac ring. (d) Simulated transmission (red line) and reflection (blue line) of the resonance around 1550 nm which is shadowed with light blue in (c). (e) Simulated reflection under different intracavity reflection index and coupling index. (f) Tested transmission (red line) and reflection (blue line) of a normal ring resonant. (g) Tested transmission (red line) and reflection (blue line) of a Sagnac ring resonant, the shadowed areas in (f), (g), and (i) mark resonances with relatively low reflection (lower than 0.1). (h) Tested integrated dispersion of the Sagnac ring. The inset shows the resonances at 1544.5 nm and 1548.6 nm, respectively. (i) Extracted peak reflection and loaded quality factor of the Sagnac ring.

Fig. 2. Self-injection lasers employing the Sagnac ring. (a) The photograph of the SIL laser. (b) The setup of the test link for SIL lasers. DFB, distributed feedback laser diode; ISO, optical isolator; OSA, optical spectrum analyzer; AOM, acoustic-optic modulator; PC, polarization controller; PD, photodetector; OSC, oscilloscope. The tested phase noise (c) and frequency noise (d) of SIL lasers locked to different resonances and the free-running DFB laser. (e) Optical spectra of SIL lasers locked to different resonances.

Fig. 3. Microcomb generation due to forward–backward field coupling. (a) The transmission of the Sagnac ring of resonances around 1554.5 nm. (b) The generated microcomb. (c) The simulated intracavity power trace as the pump laser sweeping. (d) The simulated microcomb.

Fig. 4. Localized microcomb states in the Sagnac ring. (a) The solution trace in the Sagnac ring with normal dispersion. The single period pulse shapes (b) in time domain and optical spectra (c) of different dark pulse states under conditions marked in (a). (d) Generated localized comb states locked to different resonances. (e) Tested frequency noise curves of different comb lines of the dark pulse locked to the resonance around 1550.49 nm. The inset shows the corresponding intrinsic linewidths. (f) The measured frequency noise curves of −2 comb line of different combs depicted in (d).

Fig. 5. Comparison of Sagnac rings and add-drop rings with Sagnac loops at the drop port.

Fig. 6. Parameter of the Sagnac ring. (a) The structure of the Sagnac ring. (b) The histogram of estimated loaded quality factors of racetrack rings. (c) The histogram of estimated loaded quality factors of Sagnac rings.

Fig. 7. (a) The experiment setup of the measurement. DFB, distributed feedback laser diode; ISO, optical isolator; Ref. Laser, reference laser; PD, photodetector; OSC, oscilloscope. (b) The beat note spectrum of the hybrid laser with a reference laser during the driving current sweep of the DFB laser.