Fig. 1. Photorefractive effect results in the uncertainty of soliton comb generation in X-cut LiTaO3. (a) Energy-level diagram illustrates the photorefractive effect inside the LiTaO3 material. Higher temperature contributes to a higher carrier conductivity and in turn suppresses the lifetime of carriers activated by photorefractive effect. (b) Principle diagram of photorefractive-induced EO modulation (i) and cavity-grating construction created by photovoltaic electric field (ii). (c) Measured transmission with a pump fixed in the red-detuned side of the resonance of an air-cladded X-cut LiTaO3 racetrack microresonator. The expected hysteresis can be attributed to the (i) situation in (b). (d) Measured scanning spectra near a resonance, with a scan rate up to 65 GHz/s and an on-chip laser power of near 32 mW. Its temporal evolution proves that the photorefractive optical grating evolves over time.

Fig. 2. Suppressed photorefractive effect by means of thermoelectric cooler (TEC)-controlled heating. (a) I-V curve with maximum ±100  V bias voltage in CPW with 6 μm gap (the inset image). Higher conductivity and less hysteresis level are observed with higher temperature. (b) Temporal variation of the splitting value at different temperatures with 3 min sweeping characterization. Missing sampling points, which are largely obscured by the background noise, do not impact the following analysis of the hysteresis phenomenon. The inset represents a splitting value extracted from a piezo-actuated probe laser (blue line) controlled by an electrical triangle wave (orange line). (c) Upper panel: close-up transmission of pump laser (gray) and probe laser (blue) under different temperature conditions. Lower panel: statistic correlation of maximum mode splitting with temperature from (b), manifesting a nonlinear decline in the splitting rendered by photorefractive grating.

Fig. 3. Soliton frequency comb generation in the TFLT-based racetrack microresonator at 230°C. (a) Fitted intrinsic linewidth histogram. (b) Simulated and measured dispersion data of TE modes and TM modes for efficient soliton generation. (c) The experimental setup via dual laser pumping. DUT, device under test; CTL, continuously tunable laser; EDFA, erbium-doped fiber amplifier; PC, polarization controller; PD, photodetector; CWDM, coarse wavelength division multiplexer; ESA, electrical spectrum analyzer; OSC, oscilloscope; OSA, optical spectrum analyzer. The inset is the scanning electron microscope (SEM) image of racetrack resonator for soliton generation. (d) The transmission spectrum of the pump (yellow) and the comb power (dark blue). With the help of auxiliary laser, the pump laser frequency is scanned from the blue-detuned regime to the red-detuned regime of the pump resonance to acquire the soliton steps in the red-detuned region. (e) Spectral snapshots recorded at five different stages marked in (d), corresponding to MI comb (state i), multi-soliton (state ii), breather soliton (state iii), two-soliton (state iv), and single-soliton (state v). (f) RF amplitude noise spectra corresponding to the five different states.

Fig. 4. Mode-locked frequency comb generation with its long-lived properties and device architecture for monolithic mode-locked state formation. (a) Measured wavelength-time mapping of the mode-locked state and normalized comb power versus time with fixed red-side pump laser and fixed blue-side auxiliary laser. The displayed jitters imply the transitions between different multi-soliton states. (b) Sliced spectral pictures at the time of 20 s and 160 s, which verify the long-term stability of mode-locked multi-soliton state via thermal-assisted technique. (c) SEM picture of LiTaO3 racetrack resonator with integrated spiral heater. (d) Measured temperature distribution of on-chip heater applied under 80 V DC voltage. (e) Transmission spectrum of the pump (yellow) and the comb power (dark blue); the typical stepwise comb power in the red-shaded region shows typical mode-locked state formation.

Fig. 5. Schematic outlook of fully integrated optoelectronic chip based on ferroelectric materials, compatible with coherent Kerr comb source, DC-stable ring filter (i), periodically poled components (ii), and high-speed EO modulation (iii).