Optical frequency comb generation device and method based on lithium niobate thin film waveguide and double microrings in series

Abstract

The present invention discloses a device and method for generating an optical frequency comb based on a lithium niobate thin film waveguide connected in series with two microrings. The device comprises a first bus waveguide, a first ring waveguide, a second ring waveguide, and a second bus waveguide etched on a lithium niobate thin film. The first bus waveguide has one end as a pump light input and the other end as an auxiliary light input. The first bus waveguide and the first ring waveguide are mutually coupled, and the first and second ring waveguides are mutually coupled. The second ring waveguide forms a soliton cavity. A temperature control device is provided below the soliton cavity. By regulating the temperature of the soliton cavity, the resonance peak of the soliton cavity is shifted, causing the pump light to enter a soliton state. The auxiliary light then compensates for the soliton state of the pump light, generating a soliton optical frequency comb. The present invention combines thermal modulation with the auxiliary light to compensate for the thermal effects of the microcavity, improving soliton stability. Ultimately, a comb-like structure spectrum with stable performance, high coherence, and high flatness is output from the soliton cavity port.

Description

Technical Field

[0001] The present invention relates to the fields of optical integrated optoelectronics and nonlinear optics, and in particular to an optical frequency comb generating device and method based on a lithium niobate thin film waveguide with double microrings in series. Background Art

[0002] An optical frequency comb (OFC) is a coherent spectrum consisting of a series of discrete, equally spaced frequency components. It exhibits unprecedented precision in frequency and timing and is widely used in key fields such as precision measurement, microwave photonics, quantum optics, optical communications, and optical computing. Traditional OFCs are typically based on solid-state or fiber-mode-locked lasers, using optical fibers to form a resonant cavity-like loop structure. However, optical fibers are susceptible to external factors such as temperature and humidity, making them incapable of generating a stable frequency comb. Furthermore, the loop requires a gain medium, resulting in a bulky and costly system. In contrast, Kerr-based OFCs based on microring resonators offer a more compact, highly integrated solution with a wider repetition frequency range. They can be integrated on a single chip and offer advantages such as low cost, low loss, and low power consumption. Compared to Fabry-Perot (FP) cavities, which utilize the superposition of opposing propagating light fields within the cavity to generate standing waves and form resonances, microring resonators offer stronger confinement of light, achieving an extremely small mode volume while also achieving a very high quality factor.

[0003] Lithium niobate on insulator (LNOI) has garnered widespread attention due to its strong linear electro-optic effect and large second- and third-order nonlinear effects. In recent years, the emergence of thin-film lithium niobate (TFLN) has propelled LNOI research to a new level. By localizing the optical field within a very small thin-film lithium niobate waveguide, TFLN significantly enhances both the electro-optic and nonlinear effects over LNOI. Furthermore, due to the strong nonlinearity of the ultra-high-quality-factor LNOI microring resonator, it has become an excellent platform for generating optical frequency combs. Currently, the key to achieving highly coherent and flat microcavity optical frequency combs is to minimize optical losses and increase the efficiency of pump light conversion to optical solitons within the microcavity, while also improving soliton stability and duration. Summary of the Invention

[0004] In order to solve or partially solve the problems mentioned in the background technology, the present invention provides an optical frequency comb generation device and method based on a lithium niobate thin film waveguide with two microrings in series. The specific technical solution is as follows:

[0005] In one aspect of the present invention, an optical frequency comb generator based on a lithium niobate thin film waveguide connected in series with a double microring is provided, comprising a first bus waveguide, a first ring waveguide, a second ring waveguide, and a second bus waveguide etched on a lithium niobate thin film;

[0006] One end of the first bus waveguide is the pump light input end, and the other end is the auxiliary light input end. The first bus waveguide and the first ring waveguide are coupled to each other. The pump light and the auxiliary light input through the first bus waveguide are coupled into the first ring waveguide respectively. The first ring waveguide serves as a pump cavity. The pump light and the auxiliary light resonate in the pump cavity.

[0007] The first ring waveguide and the second ring waveguide are coupled to each other, and the pump light and auxiliary light in the first ring waveguide are coupled into the second ring waveguide respectively; the second ring waveguide is a soliton cavity, and a temperature control device is provided below the soliton cavity. The temperature control device is used to control the temperature of the soliton cavity to drift the resonance peak of the soliton cavity, so that the pump light enters a soliton state, and the auxiliary light compensates the soliton state of the pump light for power, thereby generating a soliton optical frequency comb;

[0008] The second ring waveguide and the second bus waveguide are coupled to each other, so that the soliton optical frequency comb generated on the second ring waveguide is coupled into the second bus waveguide and coupled out through one end of the second bus waveguide.

[0009] Furthermore, the first bus waveguide has straight waveguides on both sides and is curved in an arc shape in the middle, so as to form a curved coupling in the coupling area with the first ring waveguide, and / or the second bus waveguide has straight waveguides on both sides and is curved in the middle, so as to form a curved coupling in the coupling area with the second ring waveguide.

[0010] Furthermore, the first bus waveguide and the second bus waveguide are symmetrically arranged with respect to the coupling region of the first ring waveguide and the second ring waveguide.

[0011] Furthermore, the first bus waveguide and the second bus waveguide are single-slot ridge waveguide structures, and the first ring waveguide and the second ring waveguide are strip waveguide structures.

[0012] Furthermore, the first bus waveguide and the second bus waveguide meet the single-mode condition, and the dispersion of the first ring waveguide and the second ring waveguide is anomalous dispersion.

[0013] Furthermore, the total height of the lithium niobate film is 600nm, and the etching depth is 500nm; the width of the first ring waveguide and the second ring waveguide is 1800nm, and the cavity length is 188.496μm; the width of the first bus waveguide and the second bus waveguide is 750nm, and the slit width is 40nm.

[0014] Another aspect of the present invention provides a method for generating an optical frequency comb based on a lithium niobate thin film waveguide and a double microring in series, based on the apparatus described above. The method comprises:

[0015] S1. Input pump light and auxiliary light at both ends of the first bus waveguide, respectively. The pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, and the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity. The absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light.

[0016] S2. Adjust the soliton cavity temperature to blueshift the soliton cavity resonance peak and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, causing the optical frequency comb generated by the pump light to be in a chaotic state.

[0017] S3. Continue to adjust the temperature of the soliton cavity to make the soliton cavity resonance peak continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons, thereby obtaining a soliton optical frequency comb.

[0018] Furthermore, in step S3, the method further includes:

[0019] Continuously adjust the temperature of the soliton cavity until the shift of the first resonance peak of the soliton cavity passes through the central wavelength of the pump light;

[0020] The central wavelength position of the auxiliary light is adjusted multiple times to adapt to the temperature control of the soliton cavity until a stable optical soliton is achieved in the soliton cavity.

[0021] Furthermore, when executing step S2 and step S3, the method further includes: maintaining the central wavelengths of the pump light and the auxiliary light without shifting when adjusting the temperature of the soliton cavity.

[0022] The optical frequency comb generation device and method based on lithium niobate thin film waveguide and double microrings in series of the present invention have the following advantages:

[0023] 1. By using a dual-microcavity superposition method, the conversion efficiency limit of a traditional single microcavity is overcome. By refining the coupling spacing between the microring and the bus waveguide, the coupling distance between the dual microrings, and the detuning amount, the coupling and phase matching of the two microrings can be precisely controlled, and the resonant power of light in the cavity can be enhanced, allowing photons to interact for a long time in the cavity, thereby achieving higher coupling efficiency and lower loss.

[0024] 2. Two beams of light with opposite frequencies are input from the microcavity into the pump cavity and the soliton cavity respectively. The two cavities respectively provide pump energy and generate and output solitons. Combined with auxiliary optical thermal modulation, the thermal effect of the microcavity is compensated, and the stability of the soliton is improved. Finally, a comb structure spectrum with stable performance, high coherence and high flatness is output from the soliton cavity port.

[0025] 3. By using the method of superimposed bending coupling of two microcavities, the coupling efficiency is greatly improved, and the conversion efficiency limit of the traditional single microcavity can be overcome, achieving a breakthrough of nearly 100% pump-to-optical soliton energy conversion rate.

[0026] 4. The ridge slot waveguide combines the strong confinement of the light field by the strip waveguide and the simplicity of the ridge waveguide device process, strongly confining most of the mode field to the slit, that is, the high refractive index core area, thereby significantly reducing the effective mode area and improving the nonlinear performance of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of a lithium niobate thin film waveguide series double microring structure according to an embodiment of the present invention.

[0028] Figure 2 It is a series double-coupled microcavity structure.

[0029] Figure 3 Schematic diagram of the cross-section of the lithium niobate thin film optical waveguide structure.

[0030] Figure 4 Schematic diagram of simulation results of waveguide modes of lithium niobate thin films with different structures according to an embodiment of the present invention.

[0031] Figure 5 This is a partial cross-sectional structural diagram of a lithium niobate thin film waveguide series double microring structure according to an embodiment of the present invention.

[0032] Figure 6 This is a diagram showing experimental simulation data of a lithium niobate thin film waveguide according to an embodiment of the present invention;

[0033] Figure 6 (a) Comparison curve of effective refractive index data of lithium niobate film waveguides with different cross-sectional widths;

[0034] Figure 6 (b) is a comparison curve of the dispersion data of lithium niobate thin film waveguides with different cross-sectional widths;

[0035] Figure 7 This is an experimental system diagram of an optical frequency comb generator based on a lithium niobate thin film waveguide connected in series with two microrings according to an embodiment of the present invention.

[0036] Figure 8 Schematic diagram of degenerate four-wave mixing and non-degenerate four-wave mixing;

[0037] Figure 9 This is a schematic diagram of a method for generating an optical frequency comb based on a lithium niobate thin film waveguide connected in series with two microrings according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to better understand the purpose, structure and function of the present invention, the optical frequency comb generating device based on lithium niobate thin film slot waveguide and double microrings in series and the preparation method thereof are further described in detail below with reference to the accompanying drawings.

[0039] Traditional microcavity optical frequency combs generated by frequency tuning schemes have problems such as low pump conversion efficiency, large thermal effect, and poor soliton stability. Based on the above problems, an embodiment of the present invention provides a Kerr optical frequency comb device based on a thin-film lithium niobate material optical waveguide, which adopts a series double microring coupling structure.

[0040] A specific embodiment of the present invention provides an optical frequency comb generator based on a lithium niobate thin film waveguide and a double microring in series. Figure 1 As shown, it specifically includes a first bus waveguide 1, a first ring waveguide 3, a second ring waveguide 4 and a second bus waveguide 2 etched on a lithium niobate film.

[0041] The present invention uses a dual-pump thermal tuning method to generate a stable Kerr optical frequency comb. Specifically, one end of the first bus waveguide 1 is the pump light input end (M end), and the other end is the auxiliary light input end (T end). The first bus waveguide 1 is coupled with the first ring waveguide 3. The pump light and auxiliary light input through the first bus waveguide 1 are respectively coupled into the first ring waveguide 3. The first ring waveguide 3 serves as a pump cavity. The pump light and auxiliary light resonate in the pump cavity.

[0042] Furthermore, the first ring waveguide 3 and the second ring waveguide 4 are coupled to each other, and the pump light and auxiliary light in the first ring waveguide 2 are respectively coupled into the second ring waveguide 3; the second ring waveguide 3 is a soliton cavity, and a temperature control device 5 is provided below the soliton cavity. The temperature control device 5 controls the temperature of the soliton cavity to shift the resonance peak of the soliton cavity, so that the pump light enters a soliton state. The auxiliary light compensates the soliton state of the pump light for power, thereby generating a soliton optical frequency comb;

[0043] The second ring waveguide 4 is coupled to the second bus waveguide 2 so that the soliton optical frequency comb generated on the second ring waveguide 4 is coupled into the second bus waveguide 2 and coupled out through one end (end D) of the second bus waveguide.

[0044] In this embodiment of the present invention, a pump cavity is used to transfer pump light energy to a soliton cavity, which can accommodate Kerr solitons. Pump light is input from the pump light input port (M port) and coupled into the pump cavity through the first bus waveguide 1. Resonance occurs in the pump cavity, increasing the intracavity light intensity, effectively amplifying the pump light power. The light is coupled through the two cavities and ultimately output through the D port of the soliton cavity, thereby generating a soliton optical frequency comb. The pump cavity provides pump energy to the soliton cavity, which is responsible for the generation and output of solitons.

[0045] In addition, the microcavity optical frequency comb in the embodiment of the present invention is regulated by dual-pump thermal tuning, that is, thermal control is added to the soliton cavity to compensate for the soliton instability caused by the sudden drop in intracavity power when the pump light jumps from the blue detuning region to the red detuning region in the traditional frequency tuning scheme.

[0046] Furthermore, in an optional embodiment of the present invention, the series double micro-ring coupling structure is as follows Figure 2 As shown, Figure 2 (a) is a straight waveguide coupling form, that is, the first bus waveguide 1 and the second bus waveguide 2 are both straight waveguides. The preferred embodiment of the present invention is Figure 2 In the curved waveguide coupling form shown in (b), the first bus waveguide 1 has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling region with the first ring waveguide 3, and / or the second bus waveguide 2 has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling region with the second ring waveguide 4. The bus waveguide and the ring waveguide adopt the form of curved coupling in the coupling region, which can further increase the area and time of light coupling into the ring, thereby greatly enhancing the coupling efficiency, and can overcome the conversion efficiency limit of the traditional single microcavity, achieving a breakthrough in the pump-to-optical soliton energy conversion rate of nearly 100%.

[0047] Furthermore, in a preferred embodiment of the present invention, the first bus waveguide 1 and the second bus waveguide 2 are symmetrically arranged relative to the coupling regions of the first ring waveguide 3 and the second ring waveguide 3. This ensures that the optical signal achieves the same coupling effect and power conversion in the coupling between the two bus waveguides and the ring waveguide, and maintains the same coupling coefficient across different paths, thus avoiding output signal distortion and additional nonlinear effects caused by phase differences. Furthermore, by optimizing the coupling spacing parameter between the two microcavities and controlling the coupling strength, flexible control of pump-to-soliton conversion is achieved to meet diverse application scenarios.

[0048] Furthermore, in a preferred embodiment of the present invention, the first bus waveguide 1 and the second bus waveguide 2 are etched into a single strip-loaded slot-waveguide structure consisting of two parallel waveguides and a gap region in between. This is because the electric field has a sudden change at the interface between the high-refractive-index material and the low-refractive-index material, and the gap region will produce extremely strong confinement of light.

[0049] Furthermore, the present invention further verifies the advantages of the single-slit ridge waveguide through simulation experiments. Figure 3 Shows a schematic cross-sectional view of the lithium niobate thin film optical waveguide structure, n cladding 、n TFLN 、n box are the refractive indices of the cladding, lithium niobate film waveguide and buried oxide layer, Figure 4 The simulation results of the modes of lithium niobate film waveguides with different structures are shown. When the total thickness of the lithium niobate film and the waveguide width are limited to 600nm and 750nm respectively, and the slit width is set to 40nm, the effective mode areas of the three structures are 0.6607μm 2 , 0.6872μm 2 , 0.5394μm 2 Therefore, from the analysis of experimental data, it can be seen that the ridge slot waveguide combines the strong confinement of the light field of the strip waveguide and the simplicity of the ridge waveguide device process, which strongly confines most of the mode field to the slit, that is, the high refractive index core area, thereby significantly reducing the effective mode area and improving the nonlinear performance of the device.

[0050] Furthermore, in order to achieve a balance between dispersion and nonlinearity, and intracavity loss and gain, in an embodiment of the present invention, the bus waveguide width is set to satisfy the single-mode condition, and the ring waveguide width is set to satisfy the anomalous dispersion state, so as to generate an optical frequency comb.

[0051] Furthermore, in order to meet the anomalous dispersion conditions and better stimulate the nonlinear effect, the ring waveguide is usually set to be relatively wide to adapt to the transmission of light in the cavity to generate the optical frequency comb; and in order to meet the single-mode transmission, the bus waveguide width is set to be relatively narrow, which enhances the confinement of the light field distribution, reduces the loss caused by mode coupling, and improves the transmission characteristics of the device. Therefore, the cross-sectional structure of the optical frequency comb generation device based on lithium niobate thin film waveguide and double microrings in series provided by the embodiment of the present invention is as follows: Figure 4 shown.

[0052] Furthermore, the present invention designs the structural parameters of the lithium niobate thin film waveguide with double microrings in series based on simulation technology. Based on the simulation results, the effective refractive index of the lithium niobate thin film strip optical waveguide with different widths at a thickness of 600nm is as follows: Figure 6As shown in (a), when the width is about 750nm, the effective refractive index of the core waveguide is higher than the refractive index of the silica cladding, which is 1.44. At this time, the waveguide only supports one TE fundamental mode and one TM fundamental mode, achieving the condition of total reflection. Therefore, in the specific embodiment of the present invention, the width W2 of the first bus waveguide 1 and the second bus waveguide 2 is 750nm, the total height H of the lithium niobate is 600nm, and the etching depth D is 500nm. Figure 6 (a) The dotted line is the refractive index of the silica cladding

[0053] Furthermore, in order to meet the phase matching condition for triggering the four-wave mixing effect during the generation of the microcavity optical frequency comb, the dispersion curves of waveguides with different widths at a thickness of 600 nm are simulated to make the dispersion value greater than 0, such as Figure 6 (b) Taking into account both size and dispersion, the present invention selects a lithium niobate thin film waveguide with a width of 1800 nm, resulting in anomalous dispersion of the microcavity near the C-band. Therefore, in this embodiment, the width W1 of the first and second ring waveguides 3 and 4 is 1800 nm, the total height H of the lithium niobate is 600 nm, and the etching depth D is 500 nm. Furthermore, the cavity lengths of the pump and soliton cavities are 188.496 μm, and the free spectral range (FSR) is approximately 958.75 GHz.

[0054] Furthermore, in a specific embodiment of the present invention, the coupling distance between the first ring waveguide 3 and the second ring waveguide 4 is 0.25 μm. To achieve critical coupling, that is, the optical field resonance component in the bus waveguide can be coupled into the ring waveguide to the maximum extent, the coupling spacing between the ring waveguides and the bus waveguides is set to 0.7 μm, that is, the coupling spacing between the first bus waveguide 1 and the first ring waveguide 3 is 0.7 μm, and the coupling spacing between the second bus waveguide 2 and the second ring waveguide 4 is also 0.7 μm, so that it can achieve critical coupling while having a high quality factor.

[0055] Furthermore, the process of preparing the lithium niobate thin film waveguide in the embodiment of the present invention is as follows: using bonding technology, BCB (phenylcyclobutene) is spin-coated on the surface of the SOI substrate, and then a commercial lithium niobate substrate is attached to the SOI substrate, annealed at high temperature for curing, and the silicon substrate and silicon dioxide on the commercial lithium niobate substrate are removed in combination with an etching process to expose the lithium niobate thin film. Subsequently, the substrate with the lithium niobate thin film is subjected to process steps such as coating, electron beam exposure, development, fixing, and ICP etching to prepare the waveguide structure pattern, and a silicon dioxide isolation layer is grown on the waveguide layer by atomic layer deposition as an upper cladding layer, such as Figure 5 As shown, the device can remain stable at high temperatures and protect the device from external factors such as moisture.

[0056] Furthermore, in order to verify the effect of the optical frequency comb generator based on lithium niobate thin film slot waveguide and double microrings in series provided by the embodiment of the present invention, a Figure 7 The experimental system shown in the figure includes a pump light source input device and an auxiliary light source input device, wherein the pump light output by the continuous-wave pump (CWPump) in the pump light source input device is power-amplified by the first erbium-doped fiber amplifier (Erbium-doped Fiber Amplifier 1, EDFA1), directionally transmitted by the first circulator (Circulator 1), polarized by the first polarization controller (Fiber Polarization Controller 1, FPC1), and then coupled into the pump light input end (M end) at one end of the first bus waveguide 1; the auxiliary light output by the auxiliary light source (Continuous-wave Auxiliary, CW Aux.) in the auxiliary light source input device is power-amplified by the second erbium-doped fiber amplifier (Erbium-doped Fiber Amplifier 2, EDFA2), directionally transmitted by the second circulator (Circulator 2), polarized by the second polarization controller (Fiber Polarization Controller 2, FPC2) adjusts the polarization state and couples it into the auxiliary light input end (T end) at the other end of the first bus waveguide 1.

[0057] Furthermore, a coupler is connected to the output end (D end) of the soliton optical frequency comb. The output end of the coupler is connected to a first optical spectrum analyzer (OSA1) for observing and analyzing the output spectrum; a bandpass filter (BPF), a photodetector (PD), and an oscilloscope (Oscilloscope). A second optical spectrum analyzer (OSA2) is also connected to the other end (N end) of the second main waveguide 2 for further observing the spectrum on the second main waveguide.

[0058] In addition, the present invention also provides a method for generating an optical frequency comb based on a lithium niobate thin film slot waveguide and a double microring in series. Before introducing the optical frequency comb generation method in the present invention, its working principle is briefly introduced.

[0059] First, when light of a certain wavelength enters a microcavity (ring waveguide), self-phase modulation and cross-phase modulation will occur. The self-phase modulation (SPM) effect indicates that the refractive index of the microcavity medium changes due to the change in the input light field, thereby modulating the phase of the incident light field itself. The phase offset caused by SPM is shown in Equation (1). The phase offset is proportional to the light field size and the transmission distance.

[0060]

[0061] Where n2 is the nonlinear refractive index, L K is the effective propagation length, and k0 represents the wave number corresponding to the center frequency.

[0062] Cross-phase modulation (XPM) means that the input light field and the light field of different frequencies exist in the optical waveguide at the same time, affecting the refractive index of each other's light field, and nonlinear phase modulation is performed between the light fields. The phase offset caused by XPM is shown in (2).

[0063]

[0064] Both SPM and XPM manifest as a continuous nonlinear phase shift introduced into the microresonator during the propagation of the light field. Typically, the phase shift caused by XPM is about twice that of SPM. Furthermore, SPM and XPM describe the mutual influence of the phases between light fields and can alter the instantaneous frequency of the time-domain signal.

[0065] Furthermore, the microcavity will also produce four-wave mixing (FWM) effects, which involve the annihilation of two photons and the generation of two new frequency component photons. Depending on whether the frequencies of the two annihilated photons are the same, it can be divided into two cases: degenerate and non-degenerate FWM, as shown in the following example. Figure 8 As shown in (a) and (b), degenerate four-wave mixing involves the interaction of three photons of different frequencies, i.e., 2ω1=2ω2=ω3+ω4; while non-degenerate four-wave mixing involves the interaction of four photons of different frequencies, i.e., ω1+ω2=ω3+ω4.

[0066] When the external input power reaches the nonlinear power threshold of the microcavity, the pump light can generate two symmetrical new frequency sidebands under the action of degenerate four-wave mixing, and then further realize efficient frequency conversion through non-degenerate four-wave mixing effect, generate new frequencies, and broaden the optical comb spectrum.

[0067] Furthermore, the process whereby four-wave mixing (FWM) generates new frequency components around the pump light within the cavity is known as modulation instability. Specifically, when the continuous optical power within the cavity reaches a certain value, small fluctuations in power and energy occur, leading to fluctuations such as noise, which can introduce parametric amplification. When the nonlinear gain is greater than the total loss of the optical microcavity, a pair of frequency-symmetrical comb teeth are generated on either side of the pump light's frequency.

[0068] The optical frequency comb generation method based on lithium niobate thin film slot waveguide and double microrings in series provided by the embodiment of the present invention is implemented on the basis of the above principle. The present invention compensates for the thermal effect in the cavity by inputting auxiliary light that transmits in the opposite direction to the pump light. The principle is as follows: Figure 9 As shown, the blue vertical line and the red vertical line are the central wavelengths of the auxiliary light source (Pump) and the pump light source (Aux.), respectively. The peak position of the red curve is the first resonance peak of the central wavelength of the pump light in the soliton cavity, and the peak position of the blue curve is the second resonance peak of the central wavelength of the auxiliary light in the soliton cavity, that is, the pump light and the auxiliary light are both close to the resonance peak positions of the soliton cavity and the pump cavity. The input frequencies of the pump light injected from the M end (pump light input end) and the auxiliary light injected from the T port (auxiliary light input end) are kept unchanged. It should be noted that the structural parameters of the pump cavity and the soliton cavity in the embodiment of the present invention are the same. The pump light and the auxiliary light resonate after being coupled into the pump cavity, so that the optical power of both is amplified. The present invention uses thermal regulation to change the temperature of the soliton cavity, causing the microcavity resonance peak to drift, thereby achieving a change in the detuning amount. Specifically, the following steps are included:

[0069] S1, input pump light and auxiliary light at both ends of the first bus waveguide 1, respectively, the pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light; that is, the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity; Figure 9 State 1 in

[0070] In the embodiment of the present invention, the first resonance peak is the resonance peak of the pump light in the soliton cavity, and the second resonance peak is the resonance peak of the auxiliary light in the soliton cavity. The central wavelength of the auxiliary light can be smaller than or larger than the central wavelength of the pump light. It is only necessary to ensure that the absolute value of the detuning amount of the auxiliary light is smaller than the detuning amount of the pump light, and both are within the 3dB bandwidth of the resonance peak.

[0071] S2. Adjust the soliton cavity temperature to blueshift the soliton cavity resonance peak and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, causing the optical frequency comb generated by the pump light to be in a chaotic state.

[0072] In an embodiment of the present invention, the temperature of the soliton cavity is reduced through thermal regulation. Specifically, the blue shift of the soliton cavity resonance peak means that the first resonance peak of the pump light in the soliton cavity moves to the left (blue shift), so that the center wavelength of the pump light is closer to the first resonance peak, and the second resonance peak of the auxiliary light in the soliton cavity moves to the left (blue shift), so that the center wavelength of the auxiliary light is closer to the second resonance peak. Accordingly, the center wavelength of the pump light is closer to the second resonance peak and the center wavelength of the auxiliary light is closer to the second resonance peak, thereby triggering modulation instability, increasing the power in the cavity, and raising the temperature. The optical comb generated by the pump light is in a chaotic state, that is, state 2. At this time, both the pump light and the auxiliary light are in a blue-detuned position.

[0073] S3. Continue to adjust the temperature of the soliton cavity to make the soliton cavity resonance peak continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons, thereby obtaining a soliton optical frequency comb.

[0074] In this embodiment of the present invention, the microcavity temperature is further lowered, causing the soliton cavity's resonance peak to continue to blueshift. At this point, when the pump light enters the red detuning region of the soliton cavity's first resonance peak, the optical comb state transitions from a chaotic state to a soliton state, causing a sharp drop in the intracavity operating power and decreasing the amount of pump light coupled into the cavity. Simultaneously, because the absolute value of the pump light's detuning is smaller than that of the auxiliary light, the auxiliary light moves closer to the soliton cavity's second resonance peak, increasing the intracavity power. This compensates for the reduced optical power caused by the pump light, reduces the thermal sensitivity of the microcavity's resonance peak drift, and generates stable optical solitons, i.e., State 3.

[0075] Specifically, to ensure the soliton optical frequency comb is in the state described in step 3, the operation process of step 3 is as follows: continuously adjust the temperature of the soliton cavity until the offset of the first resonance peak of the soliton cavity passes the center wavelength of the pump light; after the offset of the first resonance peak passes the center wavelength of the pump light, repeatedly adjust the center wavelength of the auxiliary light to adapt to the temperature control of the soliton cavity until stable optical solitons are achieved in the soliton cavity. During the temperature control process, the output state of the D-port optical comb and the light transmittance of the N-port can be observed using a spectrometer until the offset of the resonance peak passes the pump light, verifying whether the auxiliary light can compensate for the power drop of the microcavity optical frequency comb from the modulation unstable state to the soliton state. In addition, it should be noted that when adjusting the soliton cavity temperature, the center wavelengths of the pump light and the auxiliary light are kept unchanged to simplify the adjustment process by controlling variables.

[0076] In a specific embodiment of the present invention, the central wavelengths of the pump and auxiliary light sources are determined based on the confirmed resonant wavelength of the microcavity. In a preferred embodiment, the central wavelength of the auxiliary light should be smaller than that of the pump light to prevent mode hopping and other phenomena, which could result in the auxiliary light failing to compensate for the thermal effects introduced by the pump light. After power amplification by an erbium-doped fiber amplifier (EDFA), directional transmission by a circulator, and polarization state adjustment by a polarization controller (FPC), the two opposing light sources enter the microcavity through the M and T ports, respectively. After resonating through the pump cavity, the light intensity increases, and the two amplified light beams enter the soliton cavity separately. The pump light, combined with the auxiliary light, provides thermal control, thereby generating an optical frequency comb.

[0077] An optical frequency comb generation device and method based on a lithium niobate thin film waveguide coupled with dual microrings in series, provided by embodiments of the present invention, utilizes a curved, coupled slit waveguide and a dual-coupling structure. By precisely controlling the coupling and phase matching of the two microrings, the resonance of light within the cavity is enhanced, allowing photons to interact for extended periods within the cavity, resulting in higher coupling efficiency and lower losses. Simultaneously, two beams of light with opposite frequencies are fed into the microcavity, one at the pump and the other at the soliton cavity. These two cavities provide pump energy and generate and output solitons, respectively. Combined with auxiliary optical thermal modulation, this compensates for the thermal effects of the microcavity, enhancing soliton stability. Through the thermo-optical and Kerr nonlinear effects of the microcavity, a stable comb-like structure with high coherence and flatness is ultimately output from the soliton cavity port. Results demonstrate that the dual-coupled microring resonator based on the lithium niobate thin film in series, with the auxiliary optical thermal modulation scheme, not only improves the pump-to-soliton conversion efficiency and the coupling efficiency between the two cavities, but also reduces optical losses, generating highly stable optical solitons.

[0078] The present invention has been further described above with the aid of specific embodiments. However, it should be understood that the specific descriptions herein should not be construed as limiting the essence and scope of the present invention. Any modifications made to the above embodiments by a person of ordinary skill in the art after reading this specification fall within the scope of protection of the present invention. The various specific technical features described in the above specific embodiments may be combined in any suitable manner unless there is any contradiction. To avoid unnecessary repetition, the embodiments of the present invention will not further describe various possible combinations.

[0079] If the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), such directional indications are only used to explain the relative positional relationship, movement, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

Claims

1. An optical frequency comb generator based on a lithium niobate thin film waveguide connected in series with two microrings, characterized in that: comprising a first bus waveguide, a first ring waveguide, a second ring waveguide and a second bus waveguide etched on a lithium niobate film; One end of the first bus waveguide is the pump light input end, and the other end is the auxiliary light input end. The first bus waveguide and the first ring waveguide are coupled to each other. The pump light and the auxiliary light input through the first bus waveguide are coupled into the first ring waveguide respectively. The first ring waveguide serves as a pump cavity. The pump light and the auxiliary light resonate in the pump cavity. The first ring waveguide and the second ring waveguide are coupled to each other, and the pump light and auxiliary light in the first ring waveguide are coupled into the second ring waveguide respectively; the second ring waveguide is a soliton cavity, and a temperature control device is provided below the soliton cavity. The temperature control device is used to control the temperature of the soliton cavity to drift the resonance peak of the soliton cavity, so that the pump light enters a soliton state, and the auxiliary light compensates the soliton state of the pump light for power, thereby generating a soliton optical frequency comb; The second ring waveguide and the second bus waveguide are coupled to each other, so that the soliton optical frequency comb generated on the second ring waveguide is coupled into the second bus waveguide and coupled out through one end of the second bus waveguide.

2. The device according to claim 1, characterized in that The first bus waveguide has straight waveguides on both sides and is curved in an arc shape in the middle, so as to form a curved coupling in the coupling region with the first ring waveguide, and / or the second bus waveguide has straight waveguides on both sides and is curved in the middle, so as to form a curved coupling in the coupling region with the second ring waveguide.

3. The device according to claim 1 or 2, characterized in that The first bus waveguide and the second bus waveguide are symmetrically arranged with respect to a coupling region of the first ring waveguide and the second ring waveguide.

4. The device according to claim 1 or 2, characterized in that The first bus waveguide and the second bus waveguide are single-slot ridge waveguide structures, and the first ring waveguide and the second ring waveguide are strip waveguide structures.

5. The device according to claim 4, characterized in that The first bus waveguide and the second bus waveguide meet the single-mode condition, and the dispersion of the first ring waveguide and the second ring waveguide is anomalous dispersion.

6. The device according to claim 5, characterized in that The total height of the lithium niobate film is 600nm, and the etching depth is 500nm; the width of the first ring waveguide and the second ring waveguide is 1800nm, and the cavity length is 188.496μm; the width of the first bus waveguide and the second bus waveguide is 750nm, and the slit width is 40nm.

7. A method for generating an optical frequency comb based on a lithium niobate thin film waveguide and a double microring in series, based on the device according to any one of claims 1 to 6, characterized in that: The method comprises: S1. Input pump light and auxiliary light at both ends of the first bus waveguide, respectively. The pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, and the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity. The absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light. S2. Adjust the soliton cavity temperature to blueshift the soliton cavity resonance peak and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, causing the optical frequency comb generated by the pump light to be in a chaotic state. S3. Continue to adjust the temperature of the soliton cavity to make the soliton cavity resonance peak continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons, thereby obtaining a soliton optical frequency comb.

8. The method according to claim 7, characterized in that In step S3, the method further includes: Continuously adjust the temperature of the soliton cavity until the shift of the first resonance peak of the soliton cavity passes through the central wavelength of the pump light; The central wavelength position of the auxiliary light is adjusted multiple times to adapt to the temperature control of the soliton cavity until a stable optical soliton is achieved in the soliton cavity.

9. The method according to claim 7 or 8, characterized in that When executing step S2 and step S3, the method further includes: maintaining the central wavelengths of the pump light and the auxiliary light unchanged when adjusting the temperature of the soliton cavity.

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