Self-referenced locking optical frequency comb generation system based on silicon nitride and lithium niobate hetero-integration

Abstract

This invention discloses a self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate. The system includes a mode-locked laser, an optical amplifier, and a frequency locking system integrated on the same silicon nitride chip. The mode-locked laser comprises a selectively erbium-doped silicon nitride amplifier and a heterogeneously integrated thin-film lithium niobate phase modulator; the optical amplifier comprises a selectively erbium-doped silicon nitride multimode amplifier; and the frequency locking system comprises a silicon nitride nonlinear waveguide and a heterogeneously integrated thin-film lithium niobate frequency doubler. This invention utilizes a silicon nitride platform to achieve low-loss passive devices, erbium-doped active devices, and third-order nonlinear functionality. The lithium niobate electro-optic modulator and frequency doubler are heterogeneously integrated onto the same silicon nitride chip using micro-transfer or bonding processes, avoiding the difficulty of monolithic integration of erbium-doped and undoped lithium niobate. The system can generate transoctave supercontinuum on-chip and realize… f ceo and f R The self-reference locking mechanism has advantages such as high integration, high reliability, and strong process compatibility.

Description

Technical Field

[0001] This invention relates to optical frequency combs, and in particular to a self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate. Background Technology

[0002] An optical frequency comb, or simply a frequency comb, is a set of optical frequency teeth in the frequency spectrum. The frequency spacing between the teeth is equal, and the absolute frequency of the teeth is also locked. Traditional optical frequency combs are based on mode-locked lasers, combined with functional units such as supercontinuum and frequency doubling to achieve self-reference locking of the frequency. Although microcavity optical frequency combs based on high Q values ​​have also verified the function of self-reference locking (e.g., D. Spencer, et al., “An optical-frequency synthesizer using integratedphotonics,” Nature 557, 81-85 (2018), and Z. Newman, et al., “Architecture for the photonic integration of an optical atomic clock,” Optica 6, 680-685 (2019)), the system is complex and requires a large number of discrete components to achieve locking. The previous invention ZL202210757429.X (Self-reference Locking Optical Frequency Comb Generation System Based on Thin-Film Lithium Niobate) proposed an on-chip self-reference optical frequency comb generation system based on thin-film lithium niobate, which requires the use of erbium-doped lithium niobate and undoped lithium niobate. Due to limitations in the fabrication process of erbium-doped lithium niobate, selective doping cannot be achieved on a single thin-film lithium niobate wafer; instead, erbium-doped lithium niobate chips must be joined end-to-end with undoped lithium niobate chips, increasing system complexity and reducing reliability. Therefore, a more integrated on-chip self-reference locking optical frequency comb generation scheme needs to be researched. Summary of the Invention

[0003] To address the shortcomings of the existing technologies, this invention provides a self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate. Using a silicon nitride chip as a base, and combining processes such as micro-transfer or bonding, devices such as lithium niobate modulators are heterogeneously integrated onto the silicon nitride chip, thereby realizing the generation of an on-chip self-locked optical frequency comb, which has broad application prospects.

[0004] The technical solution of the present invention is as follows: A self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate, characterized in that it includes: Along the signal transmission direction, it includes three levels: a mode-locked laser, an optical amplifier, and a frequency-locking system; The first-stage mode-locked laser is arranged on a silicon nitride chip 1 and includes a total reflection mirror 2, a first wavelength division multiplexer (WDM) 3, a first erbium-doped silicon nitride amplifier 4, a second WDM 5, a lithium niobate phase modulator 6, and a partial reflection mirror 7 along the direction of optical signal propagation. The total reflection mirror 2 reflects all the optical signal, and the partial reflection mirror 7 reflects part of the optical signal. Together with the total reflection mirror, they form the linear resonant cavity of the mode-locked laser. The optical signal is output at the output end of the partial reflection mirror and enters the second-stage optical amplifier for amplification. The first WDM 3 and the second WDM 5 are used to couple the pump light to the signal light, so that the pump light can enter the first erbium-doped silicon nitride amplifier 4 connected to the first and second WDMs; the first and second WDMs couple the light from the off-chip first pump laser 301 and the second pump laser 501 into the mode-locked laser through waveguides. The first erbium-doped silicon nitride amplifier 4 is a coiled silicon nitride waveguide. Erbium ions are injected into this coiled waveguide through a selective injection process, and the erbium ions are activated through processes such as annealing to give it gain characteristics. The first and second WDMs and the first erbium-doped silicon nitride amplifier constitute a bidirectional pumped on-chip optical amplifier structure, providing intracavity gain for the mode-locked laser. The lithium niobate phase modulator 6 is used for intracavity modulation. Its input and output terminals are silicon nitride waveguides, which are then coupled to a thin-film lithium niobate waveguide via tapered couplers. High-speed electrodes are fabricated on both sides of the thin-film lithium niobate waveguide. When an RF signal is applied to the electrodes, the phase modulation of the optical signal in the waveguide is achieved through the electro-optic effect of lithium niobate. When a DC signal is applied to the electrodes, the cavity length is finely adjusted. The lithium niobate thin film is heterogeneously integrated onto the silicon nitride chip through processes such as micro-transfer or chip bonding. The modulation frequency applied to the lithium niobate phase modulator 6 should match the fundamental frequency corresponding to the cavity length of the mode-locked laser; The second-stage optical amplifier is arranged on the silicon nitride chip 1 along the direction of optical signal propagation and includes a third WDM 8, a first mode converter 9, a second erbium-doped silicon nitride amplifier 10, a second mode converter 11 and a fourth WDM 12. The third WDM 8 and the fourth WDM 12 are used to couple the pump light with the signal light, so that the pump light can enter the second erbium-doped silicon nitride amplifier 10 connected to the third and fourth WDMs; the third and fourth WDMs couple the light from the off-chip third pump laser 801 and fourth pump laser 1201 into the optical amplifier through waveguides. The first mode converter 9 and the second mode converter 11 perform mode conversion between the single-mode silicon nitride waveguide at the input and output ends and the multimode wide waveguide in the second erbium-doped silicon nitride amplifier 10, while keeping the mode field still the fundamental mode. The second erbium-doped silicon nitride amplifier 10 is a coiled silicon nitride waveguide. Erbium ions are injected into this coiled waveguide through a selective injection process, and the erbium ions are activated through processes such as annealing to give it gain characteristics. In order to obtain higher output power, the waveguide is designed as a multimode waveguide with a large width. At the same time, the first and second mode converters ensure that the optical signal in the multimode waveguide is still in the fundamental mode. The second erbium-doped silicon nitride amplifier, together with the third and fourth WDMs and the first and second mode converters, constitute a bidirectional pumped on-chip optical amplifier structure, providing high-power optical amplification for the output light of the mode-locked laser. The third-level frequency locking system is arranged on the silicon nitride chip 1 along the direction of optical signal propagation and includes a dispersion compensator 13, a third mode-spot converter 14, a nonlinear waveguide 15, a fourth mode-spot converter 16, a first beam splitter 17, a second beam splitter 18, a first photodetector 19, a frequency multiplier 20, a bandpass filter 21, a second photodetector 22, and a controller 23. The dispersion compensator 13 is used to compensate for the linear chirp in the output signal of the second-stage optical amplifier, so that the optical signal is compressed into an ultra-short pulse in the time domain. The third mode converter 14 and the fourth mode converter 16 convert and match the optical field modes input to and output to the nonlinear waveguide 15; The nonlinear waveguide 15 is a silicon nitride waveguide with a cross-section designed to have a specific dispersion value, which can extend the spectrum of the input ultrashort optical pulse into a supercontinuum spectrum spanning octave bands. The first beam splitter 17 is used to split the supercontinuum spectrum into two parts, one of which is connected to the second beam splitter 18 and the other is connected to the frequency multiplier 20. The second beam splitter 18 splits the supercontinuum spectrum into two paths again, one as the chip output and the other connected to the first photodetector (PD) 19 for detecting the comb tooth spacing of the optical frequency comb (i.e., the repetition frequency of the optical frequency comb). The frequency multiplier 20 is a chirped polarized thin-film lithium niobate device. Its input and output terminals are silicon nitride waveguides, which are then coupled to the thin-film lithium niobate waveguides through tapered couplers. The thin-film lithium niobate waveguides are periodically polarized, and when a light signal of a specific wavelength passes through the device, it can be frequency multiplied. The lithium niobate thin film is heterogeneously integrated onto the silicon nitride chip through processes such as micro-transfer printing or chip bonding. The bandpass filter 21 is used to filter out the frequency-doubled optical signal and the original signal of the corresponding wavelength in the supercontinuum together for coherent detection; The second photodetector 22 is used to coherently detect the output light of the bandpass filter and measure the carrier envelope offset frequency. f ceo ; The controller 23 is used to control the output power of the first, second, third, and fourth pump lasers, perform radio frequency modulation and DC biasing on the lithium niobate phase modulator 6, detect the output of the photodetector 20, and adjust the power of the first and second pump lasers according to the detection signal to lock the carrier envelope offset frequency. f ceo .

[0005] The process flow of the self-reference locked optical frequency comb generation system is as follows: 1) A silicon oxide undercoat is prepared on a substrate material (such as silicon) wafer by thermal oxidation or deposition; 2) Deposit low-loss silicon nitride thin films and fabricate silicon nitride waveguides and devices by etching; 3) Cover the areas that do not need erbium doping with a mask, then dope erbium ions into the exposed silicon nitride waveguide area by ion implantation, and anneal to activate the erbium ions; 4) Deposit a low-loss silicon oxide cladding layer, the thickness of which should exceed the thickness of the silicon nitride waveguide; 5) The upper surface of the silicon nitride wafer is smoothed by grinding processes such as chemical mechanical polishing, so that the thickness of the silicon oxide cladding above the silicon nitride waveguide is 0-100 nm. 6) Fabricate waveguide devices on thin-film lithium niobate wafers, and if necessary, fabricate periodically polarized waveguide frequency multipliers; 7) Taking micro-transfer bonding as an example, a tether structure for micro-transfer is fabricated on a thin-film lithium niobate wafer, and the silicon oxide under the lithium niobate device is hollowed out by wet etching. Then, a polymer nozzle (such as PDMS) is used to extract the device (at which time the tether will be broken), align it and transfer it to the corresponding position on the silicon nitride wafer, and slowly remove the polymer nozzle. The lithium niobate device will be adsorbed onto the surface of the silicon nitride wafer by intermolecular forces. 8) Fabricate electrodes and heating electrodes (if any) on a silicon nitride and lithium niobate mixed wafer using a stripping method.

[0006] Preferably, all devices on the silicon nitride chip 1 operate in transverse electric TE fundamental mode.

[0007] Preferably, the silicon nitride waveguide thickness on the silicon nitride chip 1 is 100-400 nm.

[0008] Preferably, the first, second, third, and fourth pump lasers operate at wavelengths of 980 nm or 1480 nm and are located at the edge of the silicon nitride chip, and are directly coupled to the waveguide of the silicon nitride chip after mode matching.

[0009] Preferably, the lithium niobate phase modulator 6 and frequency multiplier 20 can also be replaced by other materials with similar characteristics to lithium niobate, such as lithium tantalate.

[0010] Preferably, the total reflection mirror 2 and the partial reflection mirror 7 adopt a ring mirror structure, and the reflectivity is controlled by adjusting the coupling ratio of the directional coupler in the ring mirror.

[0011] Preferably, the first, second, third, and fourth WDMs (3, 5, 8, 12) adopt a directional coupler structure, and wavelength division multiplexing is achieved by designing phase shift between the signal light and the pump light in the coupling region of the directional coupler.

[0012] Preferably, the erbium ion doping concentration of the first and second erbium-doped silicon nitride amplifiers (4, 10) should be greater than 1e19 cm⁻¹. -3 It is necessary to ensure that the region with the highest erbium ion concentration is at the center of the silicon nitride waveguide in order to maximize the overlap between the signal optical mode field and the erbium ion distribution.

[0013] Preferably, the dispersion compensator 13 can be a long silicon nitride waveguide with a specific dispersion value, or it can be an on-chip Michelson interferometer combined with a chirped grating.

[0014] Preferably, the nonlinear waveguide 15 can be a silicon nitride waveguide with a designed cross-section, or coupling waveguides can be further introduced on both sides of the waveguide to adjust the dispersion value at different wavelengths.

[0015] Preferably, the selection of the operating wavelength of the frequency multiplier 20 should be comprehensively considered, that is, ensuring that the comb teeth of the supercontinuum at that wavelength (e.g., 1600 nm) have high power to improve the frequency multiplication efficiency, and also ensuring that the comb teeth of the supercontinuum at the frequency multiplication wavelength (e.g., 800 nm) have high power to improve the frequency multiplication efficiency. f ceo Its detection capabilities.

[0016] Preferably, the center wavelength of the transmission spectrum of the bandpass filter 21 should be equal to the wavelength of the frequency-doubled light output by the frequency doubler 20.

[0017] Compared with the prior art, the present invention has the following advantages: 1) Utilizing the selective doping capability of silicon nitride waveguides, passive silicon nitride devices (such as wavelength division multiplexers and mirrors) and active devices (erbium-doped amplifiers) are fabricated simultaneously on silicon nitride chips. In scenarios requiring electro-optic effects and second-order nonlinear effects, thin-film lithium niobate devices are introduced through heterogeneous integration. This fully leverages the advantages of silicon nitride, such as low loss, selective doping, and third-order nonlinearity, achieving a high degree of integration. This solves the problem of monolithic integration of erbium-doped and undoped lithium niobate, as described in prior patent ZL202210757429.X, and separates the supercontinuum generation and frequency doubling processes, reducing the difficulty of system implementation.

[0018] 2) Except for the pump laser and photodetector, all optical devices are fabricated on a single silicon nitride chip, which has a high degree of integration, effectively reducing the difficulty of device packaging and improving the reliability of the system. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the self-reference locked optical frequency comb generation system of silicon nitride and lithium niobate heterointegration of the present invention.

[0020] In the diagram: 1-Silicon nitride chip, 2-Total reflection mirror, 3-First wavelength division multiplexer (WDM), 4-First erbium-doped silicon nitride amplifier, 5-Second WDM, 6-Lithium niobate phase modulator, 7-Partial reflection mirror, 8-Third WDM, 9-Mode converter, 10-Second erbium-doped silicon nitride amplifier, 11-Second mode converter, 12-Fourth WDM, 13-Dispersion compensator, 14-Third mode converter, 15-Nonlinear waveguide, 16-Fourth mode converter, 17-First beam splitter, 18-Second beam splitter, 19-First photodetector, 20-Frequency doubler, 21-Bandpass filter, 22-Second photodetector, 23-Controller, 301-First pump laser, 501-Second pump laser, 801-Third pump laser, 1201-Fourth pump laser.

[0021] Figure 2 This is a typical cross-sectional view of a silicon nitride and lithium niobate heterogeneous integrated device. Detailed Implementation

[0022] To enable those skilled in the art to better understand the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Please refer to Figure 1 , Figure 1 This is a schematic diagram of the self-reference locked optical frequency comb generation system of heterogeneous integration of silicon nitride and lithium niobate according to the present invention. The system is integrated on the same silicon nitride chip 1 and includes, in sequence along the optical signal transmission direction: a mode-locked laser (first stage), an optical amplifier (second stage), and a frequency locking system (third stage). All devices requiring electro-optic modulation or second-order nonlinear effects (lithium niobate phase modulator 6 and frequency multiplier 20) are heterogeneously integrated onto the silicon nitride chip 1 using a micro-transfer printing process.

[0024] The mode-locked laser employs a linear cavity structure, with its components arranged sequentially along the optical path as follows: a total reflection mirror 2, a first wavelength division multiplexer (WDM) 3, a first erbium-doped silicon nitride amplifier 4, a second WDM 5, a lithium niobate phase modulator 6, and a partial reflection mirror 7. The total reflection mirror 2 (with a reflectivity close to 100%) and the partial reflection mirror 7 (with a reflectivity of, for example, 90%) together constitute the linear resonant cavity. The partial reflection mirror 7 also serves as the laser output terminal. In this embodiment, both the total reflection mirror 2 and the partial reflection mirror 7 adopt a ring mirror structure, with the coupling ratio of the directional coupler within the ring mirror adjusted. The first erbium-doped silicon nitride amplifier 4 is a coiled silicon nitride waveguide, and the erbium ion doping concentration should be greater than 1e19 cm⁻¹. -3 The laser is activated by rapid annealing at 800~1000℃, supporting single-mode transmission (TE fundamental mode). The first pump laser 301 and the second pump laser 501 are semiconductor lasers with wavelengths of 980 nm or 1480 nm, arranged at the edge of the silicon nitride chip 1, and directly coupled into the on-chip waveguide through mode size matching. Pump light is injected from both ends of the amplifier 4 via the first WDM 3 and the second WDM 5, forming a bidirectional pump structure to improve gain efficiency and power output. The lithium niobate phase modulator 6 is integrated within the resonant cavity. Its input / output terminals are silicon nitride waveguides, thermally transitioning to thin-film lithium niobate waveguides via tapered couplers. When a frequency of [missing value] is applied to the electrodes... f R When receiving radio frequency signals, phase modulation is achieved through the linear electro-optic effect of lithium niobate, with the modulation depth designed to enable the mode-locked laser to operate in an active mode-locked state. Simultaneously, a DC bias voltage (0~10 V) is applied to the electrodes to fine-tune the intracavity optical path, thereby precisely controlling the repetition frequency. f R .

[0025] The pulsed light output from the mode-locked laser is output through partial reflector 7 and then enters the second-stage optical amplifier for power enhancement. The second-stage optical amplifier includes the following components along the optical path: third WDM 8, first mode converter 9, second erbium-doped silicon nitride amplifier 10, second mode converter 11, and fourth WDM 12.

[0026] The amplified pulsed light sequentially enters the dispersion compensator 13, nonlinear waveguide 15, beam splitter group, frequency multiplier 20, and detector unit. The dispersion compensator 13 compensates for the positive chirp accumulated in the second-stage amplifier. In this embodiment, the dispersion compensator 13 uses a silicon nitride waveguide with anomalous dispersion, designed to make the total chirp approximately zero, thereby compressing the pulse width to approximately 100 fs in the time domain. Alternatively, an on-chip Michelson interferometer combined with a chirped Bragg grating can be used to achieve tunable dispersion compensation. The compressed ultrashort pulse enters the nonlinear waveguide 15 via the third mode converter 14. The nonlinear waveguide 15 is a silicon nitride waveguide whose cross-section is designed with dispersion engineering to achieve near-zero anomalous dispersion and a high nonlinear coefficient near 1550 nm. The waveguide length is selected to be 5–10 mm, the input pulse peak power reaches tens of watt-hours, and the spectrum can be broadened to a supercontinuum spanning octaves. To further optimize dispersion flatness, coupled waveguides (not shown) can be introduced on both sides of the waveguide to form a dual-waveguide coupling structure. Dispersion fluctuations at specific wavelengths can be suppressed by adjusting the coupling spacing. The supercontinuum light enters the first beam splitter 17 after passing through the fourth mode-spot converter 16 (similar to converter 14). The first beam splitter 17 splits the light into two paths: one path enters the second beam splitter 18, and the other path enters the frequency multiplier 20. The second beam splitter 18 splits the input light into two paths again: 90% of the light is used as the chip output port (providing a self-reference-locked optical frequency comb), and 10% of the light is sent to the first photodetector 19 (a high-speed InGaAs PIN diode). The first photodetector 19 detects the repetition frequency. f R (i.e., the fundamental frequency of the pulse sequence), and outputs an electrical signal to controller 23. The optical signal entering frequency multiplier 20 generates frequency-doubled light. Frequency multiplier 20 uses a chirped periodically polarized thin-film lithium niobate (CPLN) waveguide, and the periodic polarization structure is designed to make the phase-matching bandwidth cover a relatively wide band in the supercontinuum. Specifically, the chirped period gradually changes from Λ=18 μm to 20 μm, which can satisfy quasi-phase matching in the range of 1500~1700 nm. The input / output terminals of frequency multiplier 20 are also connected to silicon nitride waveguide through tapered couplers. The light output from frequency multiplier 20 (including the frequency-doubled light and the unconverted fundamental frequency light) enters bandpass filter 21. The bandpass filter 21, for example, is an on-chip micro-ring resonator or a Bragg grating, with its transmission spectrum center wavelength set to 800 nm (the wavelength of the frequency-doubled light) and a bandwidth of approximately 5 nm. It filters out the frequency-doubled light near 800 nm and the comb component in the supercontinuum that was originally located at 800 nm. The two optical signals undergo coherent beat frequency conversion on the second photodetector 22 (a low-noise balanced detector), and the output frequency is equal to... f ceo The controller 23 receives the electrical signal output by the first photodetector 19. f R The signal and the output of the second photodetector 22f ceo Signal. Controller 23 generates control signals using a proportional-integral-derivative (PID) algorithm.

[0027] The frequency of an optical frequency comb can be expressed as f m = f ceo + mf R ,in m It is a positive integer. f ceo It is the carrier envelope offset frequency of the optical frequency comb. f R It is the repetition frequency of the optical frequency comb, equal to the modulation frequency, and 0 ≤ f ceo < f R When the optical frequency comb contains octaves, assuming for frequency... f m = f ceo + mf R Frequency multiplication yields 2 f m =2 f ceo +2 mf R Then compare this frequency with the frequency at the corresponding position in the optical frequency comb. f 2m = f ceo +2 mf R By performing coherent detection (beat frequency), we can obtain... f ceo = 2 f m - f 2m Thus, the carrier envelope offset frequency is extracted. f ceo By changing the power of the pump laser of the mode-locked laser, it is possible to achieve... f ceo The adjustment and control of the repetition frequency f can be achieved by applying a DC offset to the lithium niobate phase modulator in the mode-locked laser. R This locking mechanism enables complete self-reference locking of the optical frequency comb.

[0028] In this embodiment, the heterogeneous integration of the silicon nitride chip and the thin-film lithium niobate device is implemented according to the following steps, see [link to relevant documentation]. Figure 2 Through heterogeneous integration, lithium niobate devices are integrated on top of silicon nitride devices. The silicon nitride waveguide and the lithium niobate waveguide are coupled to achieve optical signal coupling through a tapered waveguide. Below the lithium niobate device, there can be a silicon nitride waveguide (in which case the silicon nitride waveguide should be relatively narrow to ensure that the optical field is mainly in the lithium niobate), or there may be no silicon nitride waveguide.

[0029] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate, characterized in that, It includes a three-level structure integrated on the same silicon nitride chip (1) and sequentially optically connected along the optical signal transmission direction: a mode-locked laser, an optical amplifier, and a frequency locking system; The mode-locked laser comprises a linear resonant cavity formed by sequentially connecting a total reflection mirror (2), a first wavelength division multiplexer (3), a first erbium-doped silicon nitride amplifier (4), a second wavelength division multiplexer (5), a lithium niobate phase modulator (6), and a partial reflection mirror (7) along the optical path. The total reflection mirror (2) and the partial reflection mirror (7) together constitute the two end faces of the linear resonant cavity, and the partial reflection mirror (7) also serves as the output end of the mode-locked laser. The first wavelength division multiplexer (3) and the second wavelength division multiplexer (5) each have a pump light input end for connecting an external first pump laser (301) and a second pump laser (501) to... The pump light is coupled into the first erbium-doped silicon nitride amplifier (4) to form a bidirectional pump structure; the first erbium-doped silicon nitride amplifier (4) is a silicon nitride waveguide doped with erbium ions by selected area ion implantation and activated by annealing, used to provide intracavity gain; the input and output ends of the lithium niobate phase modulator (6) are silicon nitride waveguides, which are transitioned to thin-film lithium niobate waveguides through tapered couplers. Electrodes are prepared on both sides of the thin-film lithium niobate waveguide. The electrodes are used to apply radio frequency signals to achieve phase modulation and apply DC signals to fine-tune the cavity length; the lithium niobate phase modulator (6) is heterogeneously integrated on the silicon nitride chip (1) through micro-transfer or chip bonding processes; The input of the optical amplifier is optically connected to the output of the partial reflector (7), and includes, in sequence along the optical path: a third wavelength division multiplexer (8), a first mode converter (9), a second erbium-doped silicon nitride amplifier (10), a second mode converter (11), and a fourth wavelength division multiplexer (12); the third wavelength division multiplexer (8) and the fourth wavelength division multiplexer (12) each have a pump light input terminal for connecting to an off-chip third pump laser (801) and a fourth pump laser (1201) to pump... The pump light is coupled into the second erbium-doped silicon nitride amplifier (10) to form a bidirectional pump structure; the first mode converter (9) and the second mode converter (11) are used to perform mode conversion between the single-mode silicon nitride waveguide and the multimode wide waveguide of the second erbium-doped silicon nitride amplifier (10) to maintain the fundamental mode transmission; the second erbium-doped silicon nitride amplifier (10) is a silicon nitride multimode waveguide doped with erbium ions by selected area ion implantation and activated by annealing, which is used to amplify the pulse light signal output by the mode-locked laser; The input of the frequency locking system is optically connected to the output of the fourth wavelength division multiplexer (12), and includes, in sequence along the optical path: a dispersion compensator (13), a third mode converter (14), a nonlinear waveguide (15), a fourth mode converter (16), and a first beam splitter (17); the first output of the first beam splitter (17) is optically connected to a second beam splitter (18), the first output of the second beam splitter (18) serves as the chip output, and the second output is optically connected to a first photodetector (19); the second output of the first beam splitter (17) is optically connected to a frequency multiplier (20), the output of the frequency multiplier (20) is optically connected to a bandpass filter (21), and the output of the bandpass filter (21) is optically connected to a second photodetector (22). The dispersion compensator (13) is used to compensate for linear chirp in the optical signal to compress the pulse width; the nonlinear waveguide (15) is a silicon nitride waveguide with a cross-section designed for dispersion to broaden the ultrashort pulse spectrum into a supercontinuum spanning multiple octaves; the frequency multiplier (20) is a chirped periodically polarized thin-film lithium niobate device with its input and output ends being silicon nitride waveguides, which are transitioned to thin-film lithium niobate waveguides via tapered couplers; the frequency multiplier (20) is heterogeneously integrated onto the silicon nitride chip (1) through micro-transfer or chip bonding processes; the center wavelength of the transmission spectrum of the bandpass filter (21) is equal to the wavelength of the frequency-doubled light output by the frequency multiplier (20), used to filter out the original signal of the corresponding wavelength in the frequency-doubled light and the supercontinuum, and coherently detected by the second photodetector (22) to output the carrier envelope offset frequency. f ceo Signal; The frequency locking system further includes a controller (23), the input of which is electrically connected to the output of the first photodetector (19) and the output of the second photodetector (22), and the output of which is electrically connected to the DC bias electrode of the lithium niobate phase modulator (6), the control terminal of the first pump laser (301), and the control terminal of the second pump laser (501), respectively; the controller (23) adjusts the frequency based on the repetition frequency detected by the first photodetector (19). f R The signal adjusts the DC bias of the lithium niobate phase modulator (6) to lock the repetition frequency, and adjusts the carrier envelope offset frequency detected by the second photodetector (22). f ceo The signal modulates the output power of the first pump laser (301) and / or the second pump laser (501) to lock the carrier envelope offset frequency, thereby achieving full self-reference locking of the optical frequency comb.

2. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The mode-locked laser, optical amplifier, passive devices, active erbium-doped amplifier devices, and nonlinear optical devices in the frequency-locked system are all fabricated on the silicon nitride chip (1). Devices requiring electro-optic modulation or second-order nonlinear effects are transferred and fixed on the silicon nitride chip (1) through heterogeneous integration.

3. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, All optical devices on the silicon nitride chip (1) operate in transverse electric TE fundamental mode, and the thickness of the silicon nitride waveguide is 100~400 nm.

4. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The first pump laser (301), the second pump laser (501), the third pump laser (801) and the fourth pump laser (1201) operate at wavelengths of 980 nm or 1480 nm independently. The pump lasers are arranged at the edge of the silicon nitride chip (1) and are directly coupled to the on-chip waveguide through a mode matching structure.

5. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The total reflection mirror (2) and the partial reflection mirror (7) adopt the structure of a ring mirror, and the reflectivity is controlled by adjusting the coupling ratio of the directional coupler in the ring mirror.

6. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The first wavelength division multiplexer (3), the second wavelength division multiplexer (5), the third wavelength division multiplexer (8) and the fourth wavelength division multiplexer (12) adopt the structure of a directional coupler. Wavelength division multiplexing is achieved by designing the phase shift of the signal light and the pump light in the coupling region of the directional coupler.

7. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The erbium ion doping concentration of the first erbium-doped silicon nitride amplifier (4) and the second erbium-doped silicon nitride amplifier (10) is greater than 1e19 cm -3 , and the maximum doping concentration is located at the center of the waveguide to maximize the overlap of the signal light mode field and the erbium ion distribution.

8. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The dispersion compensator (13) is a silicon nitride waveguide with anomalous dispersion value, or an on-chip Michelson interferometer combined with a chirped grating.

9. The self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate according to claim 1, characterized in that, The nonlinear waveguide (15) is further provided with coupling waveguides on both sides to adjust the dispersion value at different wavelengths in order to achieve a flatter dispersion curve.

10. A method for fabricating a self-reference locked optical frequency comb generation system based on heterogeneous integration of silicon nitride and lithium niobate, characterized in that, Includes the following steps: Step 1: Prepare a silicon oxide undercoat on the substrate wafer by thermal oxidation or deposition; Step 2: Deposit silicon nitride thin film, and fabricate silicon nitride waveguides and passive devices by photolithography and etching. The passive devices include wavelength division multiplexers, beam splitters, mode converters, dispersion-compensating waveguides, and nonlinear waveguides. Step 3: Use a mask to protect the areas that do not need to be doped, and then use erbium ions to dope the waveguide regions corresponding to the first erbium-doped silicon nitride amplifier (4) and the second erbium-doped silicon nitride amplifier (10) by ion implantation, and then anneal and activate them; Step 4: Deposit the silicon oxide cladding and use chemical mechanical polishing to smooth the upper surface, so that the remaining thickness of the silicon oxide cladding above the silicon nitride waveguide is 0~100 nm; Step 5: Fabricate lithium niobate waveguide devices on independent thin-film lithium niobate wafers and, as needed, fabricate periodic polarization structures. The lithium niobate waveguide devices include a phase modulator (6) and a frequency multiplier (20). Step 6: Using micro-transfer or chip bonding processes, the prepared thin-film lithium niobate device is transferred and fixed to the corresponding position on the silicon nitride wafer, so that the lithium niobate waveguide and the silicon nitride waveguide are optically coupled through a tapered coupler; Step 7: Fabricate metal electrodes on the hybrid wafer, the metal electrodes including high-speed modulation electrodes and DC bias electrodes on both sides of the lithium niobate phase modulator.

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