Fig. 1. Fabrication technique of TFLN photonic integrated chip based on femtosecond laser micromachining^{[12, 70, 72-73]}. (a) Schematic of the cross-section structure of REI-doped TFLN wafer [bottom inset: the picture of an Er³⁺-doped TFLN (Er³⁺: TFLN) wafer and an undoped TFLN wafer taken by a digital camera]; (b) schematic of the PLACE fabrication process of the REI-doped TFLN photonic device; (c) overview of the optical micrograph of an ultra high-Q LN microdisk fabricated by the PLACE technique; (d) transmission spectrum of the microdisk in Fig. (c) obtained by ring-down measurement, the intrinsic Q-factor of the microdisk is measured to be 1.23×10⁸

Fig. 2. Home-built ultra high-speed high-resolution femtosecond laser lithography fabrication system. (a) Wafer stage of the fs laser lithography fabrication system; (b) microscope image of the real-time femtosecond laser writing process for Cr patterning; (c) a 4 inch wafer with a Cr mask of a 11 × 68 array of MZI optical modulators; (d) microscope images of one of the MZI on the wafer in Fig. (c) after the CMP process

Fig. 3. Demonstration of the C-band on-chip wavelength tunable microlaser by constructing high Q Er³⁺∶TFLN microdisk^[47]. (a) Optical micrograph of an Er³⁺∶TFLN microdisk; (b) Lorentzian fitting of the transmission curve indicates the Q-factor of the microdisk reaches 1.8×10⁶; (c) spectrum of the Er³⁺-doped laser when the pump power is around 0.92 mW (upper left inset is enlarged spectrum around 1560 nm, shows the multimode lasing spectrum; upper right inset is micrograph of the strong green upconversion fluorescence of the microdisk); (d) dependence of the intensity of all emission lines on the absorbed pump power at 976 nm (experimental data are marked by circles, and the linear fitting in solid line indicates a lasing threshold lower than 400 μW)

Fig. 4. Modes trimming and clustering in a weakly perturbed high-Q whispering gallery microresonator^[78]. (a) Cosine of the chord angle of the eigenmodes at different wavelengths [black dots and colored dotted line represent the quasi-steady-state solution and the approximate result given by Eq. (1), respectively]; observed mode patterns of (b) triangle mode, (c) triangle mode with first excited state, (d) star mode, (e) square mode, (f) pentagon mode, (g) hexagonal mode, (h) heptagon mode, and (i) octagon mode

Fig. 5. Characterization of single-mode laser with ultra-narrow linewidth in Er³⁺-doped microdisk^[49]. (a) Spectrum of the up-conversion fluorescence and the pump light [upper insets are optical micrographs of the square modes of the upconversion fluorescence near the 550 nm wavelength (left) and the pump light (right)]; (b) laser output power at increasing pump power; (c) spectra of the microlaser output powers at different pump power levels (upper left inset is optical micrograph of the lasing mode with a square pattern at 1546 nm wavelength, upper right inset is theoretically calculated intensity distribution of the square lasing mode); (d) illustration schematic of the experimental setup for the linewidth measurement; (e) spectrum of the detected beating signal; (f) frequency noise is measured to be 196.5 Hz²·Hz^-1 in the high-frequency range, indicating that the fundamental linewidth is 436.6 Hz; (g) linewidth glows linearly with the output power of the microlaser

Fig. 6. Wavelength tuning performance of the ultra-narrow linewidth mcirolaser^[49]. (a) Lasing wavelength at different pump powers; (b) lasing wavelength drifts with pump power; (c) lasing wavelength at different direct current (DC) biases; (d) lasing wavelength drifts with DC bias

Fig. 7. Dual-wavelength microlasers^[55]. (a) Spectrum of the microlasers with a fine structure of two peaks, showing a wavelength interval of 8 pm [insets are transmission spectrum of the microlaser near the lasing wavelength (left) and the pumping wavelength (right)]; (b) optical micrographs of polygon modes of the up-conversion fluorescence (top), pump laser (bottom left), and output laser (bottom right); (c) output power of the microlaser as a function of the pump power; (d) spectrum of the microlasers with a large wavelength interval of 70 pm [insets are optical micrographs of polygon-shaped patterns of the up-conversion fluorescence (left) and lasing modes (right)]

Fig. 8. Microwave signal synthesis^[55]. (a) Detected microwave signal (inset is optical micrograph of the probe contracted with the electrode on the center of the microdisk); (b) phase noise of the microwave signal carrier; (c) electro-optic tuning of the microwave signal; (d) microwave signal synthesized by the dual-wavelength laser of a wavelength interval of 70 pm

Fig. 9. Monolithically integrated high-power narrow-bandwidth microdisk laser^[82]. (a) Optical micrograph of the fabricated integrated microdisk laser; (b) spectrum of the integrated microdisk laser with 409 μm diameter, exhibiting a single-frequency lasing at 1552.82 nm; (c) green upconversion fluorescence of the integrated microdisk laser, showing a hexagon pattern; (d) laser output power versus pump power dropped to the cavity; (e) spectrum of the integrated microdisk laser with 1 mm diameter, confirming a single-frequency lasing at 1551.68 nm wavelength (inset is micrograph of the upconversion fluorescence); (f) Lorentz fitting of the detected beating signal (black dots) featuring a laser linewidth of 0.11 MHz

Fig. 10. Laser diode-pumped compact hybrid lithium niobate microring laser^[52]. (a) Illustration of the microlaser device, which consists of a commercial CoS laser diode and an Er³⁺∶TFLN microring; (b) overview picture of the microlaser device taken by a digital camera; (c) top-view optical micrograph of the interface between the CoS laser diode and the array of Er³⁺∶TFLN microrings; (d) spectra of the microring laser with increasing pump power; (e) on-chip laser power drifts with input pump power; (f) on-chip laser power drifts with driving electric power

Fig. 11. Er³⁺∶TFLN single-mode laser based on Sagnac loop reflectors^[90]. (a) Optical microscope image of an Er³⁺∶TFLN FP resonator (the arrows in bottom right corner illustrate the LN crystallographic axes X, Y, and Z), and the green upconversion fluorescence of the Er³⁺∶TFLN FP resonator pumped by the 980 nm LD; (b) zoomed-in optical microscope image of a 3 dB directional coupler (the upper inset), the coupling region (the bottom-left inset), and a Sagnac loop reflector (the bottom-right inset); (c) enlarged spectrum around wavelength 1544 nm, the lasing peak is fitted with a Lorentzian line shape (red) (inset is the infrared image of the output port of the Er³⁺∶TFLN FP resonator); (d) on-chip laser power of Er³⁺∶TFLN FP resonator laser drifts with absorbed pump power

Fig. 12. On-chip wavelength-tunable narrow-linewidth laser diode based on self-injection locking^[96]. (a) Illustration scheme of the narrow linewidth self-injection-locked laser, which is composed of a commercial CoS laser diode and a high-Q LN microring laser; (b) close-up optical micrograph of the interface between the CoS laser diode and LN microring; (c) Lorentz fitting (red curve) reveals a Q-factor of 6.91×10⁵ at the wavelength of 975.36 nm; (d) comparison of the laser linewidth for the free-running DFB case and the case where the DFB is self-injection-locked to a LN microring cavity; (e) spectrum of the 980 nm narrow linewidth self-injected locking laser emission; (f) dependence of the narrow linewidth laser wavelength on the applied electrical power

Fig. 13. Lasing characterization of the Yb³⁺∶TFLN microring when pumped by a 980 nm wavelength laser^[100]. (a) Evolution of the lasing modes at different pump powers; (b) Yb³⁺∶TFLN microring lasing power versus the increasing pump powers (threshold and the slope efficiency deduced from the linear fitting are 10 mW and 1.77×10^-3%, respectively); (c) spectrum around the laser emission at 1025.62 nm and featuring a linewidth of 0.035 nm; (d) lasing spectrum features 14 longitudinal modes when the pump power is 20 mW

Fig. 14. Meter-scale length LN waveguide OTDL^[13]. (a) Schematic design of a beam splitter connected with two waveguides of different lengths (upper waveguide serves as the OTDL); (b) digital camera picture of an OTDL with a total length of 109.26 cm; (c) SEM image of the bend section of the OTDL in Fig. (b); (d) optical micrograph of the fabricated beam splitter in the OTDL device; (e) near-field distributions of the modes at the output port 2 (inset is cross-sectional SEM image of the ridge waveguide); (f) propagation loss as a function of the length of the OTDL (inset is near-field distributions of the TE modes at the output ports 1 and 2)

Fig. 15. Electro-optically tunable ODL with a continuous tuning range of 220 fs in TFLN^[101]. (a) Micrograph of a TFLN unbalanced MZI with a tunable ODL arm; (b) measured losses derived from the measured transmission spectra of unbalanced MZIs without microelectrodes in the arm-length differences of 10 cm, 20 cm, and 30 cm, showing a linear dependence on the length; (c) linear wavelength shift (marked by black squares with blue linear fitting curve) and time delay (marked by black circles with red linear fitting curve) of the device in Fig. (a) as a function of the DC voltage (measured tuning efficiency is 2.42 pm/V, indicating a time tuning efficiency of 3.18 fs/V)

Fig. 16. The monolithic Er³⁺-doped waveguide amplifier^[48]. (a) Schematic of the device; (b) optical micrograph of the straight waveguide of the on-chip amplifier; (c) optical micrograph of the curved waveguide of the on-chip amplifier; (d) schematic of the experimental setup of the gain measurement

Fig. 17. Cladded Er³⁺∶TFLN waveguide configuration^[53]. (a) Cross-sectional schematic of the waveguide fabricated on the Z-cut Er³⁺∶TFLN wafer with a cladding layer of Ta₂O₅; (b) top-view microscope image of the air-clad TFLN waveguide; (c) SEM image of the Ta₂O₅-clad TFLN waveguide cross-section; simulated mode distribution of the fundamental TE modes for the (d) air-clad and (e) Ta₂O₅-clad waveguides at the pump and signal wavelengths, respectively (power confinement factor Г is labelled in each panel)

Fig. 18. Optical gain measurement of the Er³⁺∶TFLN waveguide amplifiers^[53]. (a) Experimental setup for optical gain measurement (PC is polarization controller, OSA is optical spectrum analyzer, the digital camera photograph of the excited Er³⁺∶TFLN waveguide chip is shown in the center); (b) internal net gain measured from the air-clad (blue squares) and Ta₂O₅-clad (red circles) amplifiers varies with the incident pump light intensity

Fig. 19. Schematic of the fabrication process for robust low-loss optical interconnection of passive and active LN photonics using stitch-chips^[69]. (a) Prepare non-doped and REI-doped TFLN samples; (b) non-doped and REI-doped TFLN samples are flip-chip on a polished plate glass; (c) non-doped and REI-doped TFLN samples are stitched seamlessly using customized fixtures and the UV glue is applied on the bottom of the stitched chips to bond the two samples; (d) subsequent laser welding is operated in the boundary of the two TFLN samples and quartz substrate to achieve durable bonding; (e) completed stitch-chip of passive and active TFLN; (f) photonic structures fabricated using the PLACE technique on the stitch-chips

Fig. 20. Four-channel waveguide amplifiers fabricated on the monolithically integrated active/passive TFLN chip^[69]. (a) Illustration of the device design; (b) digital picture of the four-channel waveguide amplifiers; (c) mode (insets) and intensity distribution of the 1550 nm wavelength signal in the four-channel Er³⁺-doped waveguides; (d) photo of the four-channel waveguide amplifier array when pumped by a 976 nm diode laser; gain characterization of the four Er³⁺-doped LN waveguides array for the signal wavelengths of (e) 1550 nm and (f) 1530 nm

Table 1. Typical parameters for the on-chip microlasers fabricated by PLACE technique