LiNbO3 microcavity with pulley type coupling structure and design method

Abstract

The invention provides a design method of a LiNbO3 microcavity with a pulley type coupler structure, and aims to generate a broadband dissipation Kerr soliton micro comb and effectively inhibit stimulated Raman scattering. According to the invention, the dispersion engineering design and the dissipation engineering design are combined, the abnormal group velocity dispersion characteristic in the microcavity is optimized, and the coupling efficiency is improved. Through accurate control of geometric parameters, abnormal GVD, phase matching conditions and coupling strength, it is guaranteed that the SRS effect is reduced to the maximum extent in the microcavity with the high-quality factor Q value, and stable generation of DKS is promoted. The output efficiency, the spectral width and the stability of the frequency comb can be effectively improved, and the frequency comb is widely applied to the fields of precision spectroscopy, optical communication, millimeter wave and microwave generation and the like.

Description

Technical Field

[0001] The present invention belongs to the field of photonics and nonlinear optics, and in particular to a lithium niobate (LiNbO3) microcavity for generating dissipative Kerr solitons (DKS). Specifically, it includes the design of microcavity coupling structure and the suppression of stimulated Raman scattering (SRS) effect, so as to improve the output efficiency, spectrum width and stability of DKS. This technology is widely used in the fields of precision spectral measurement, communication, microwave and millimeter wave frequency generation. Background Art

[0002] In modern photonics and nonlinear optics research, Optical Frequency Comb (OFC), as an important bridge connecting light wave signals and microwave signals, has become a hot topic of research. OFC consists of equally spaced, discrete and coherent spectral lines with a wide spectrum range, showing great application potential in many fields. The application of OFC covers cutting-edge fields such as precision spectroscopy metrology, massively parallel communications, millimeter wave and microwave signal generation, astrophysical spectrometer calibration, ultra-large-scale cloud computing and high-precision ranging. Among them, the dissipative Kerr soliton (DKS) generated in a high-quality factor (Q value) microcavity through the Kerr nonlinear effect has become an important solution for many application scenarios due to its high repetition rate, wide spectrum, high integration and high stability.

[0003] As an important nonlinear optical material, LiNbO3 has been widely used in the field of integrated photonics in recent years due to its wide bandwidth, low loss and high electro-optic coefficient. In particular, thin-film lithium niobate chips have become an ideal platform for efficient generation of DKS due to their miniaturization, high integration and significant nonlinear coefficient. By enhancing the spatial confinement of the light field, lithium niobate microcavities can effectively enhance the nonlinear effect and thus stably generate DKS. However, existing designs still have significant bottlenecks in the coupling efficiency of the microcavity and the suppression of nonlinear optical noise, such as stimulated Raman scattering (SRS).

[0004] Under high power input, the SRS effect may occur in the lithium niobate microcavity. This nonlinear effect competes with the Kerr effect, transfers energy to the Raman mode, and suppresses the formation of DKS. The presence of SRS not only reduces the output efficiency of DKS, but also increases the system noise, limiting the spectral width and stability of the frequency comb. In addition, the traditional straight waveguide and microcavity coupling method is usually difficult to maintain low loss while ensuring efficient coupling, especially in high-Q lithium niobate microcavities, the complexity of coupling design is further exacerbated. In contrast, the pulley coupler (Pulley-Coupler, PC) has higher coupling flexibility and efficiency, and is particularly suitable for high-Q microcavity structures. However, although the existing pulley coupler design has made breakthroughs in optimizing coupling efficiency, it has failed to fully consider how to effectively suppress undesirable nonlinear effects such as SRS. How to minimize the SRS effect while ensuring high coupling efficiency has become an important issue that needs to be solved in current designs. Summary of the invention

[0005] Aiming at the problems of generating wide-spectrum DKS and suppressing SRS in lithium niobate microcavities, the present invention provides a new design method. By designing a structure that meets the requirements of anomalous group velocity dispersion (GVD) and combining it with PC, the SRS effect is effectively suppressed while achieving efficient coupling, and the performance of the Kerr effect in the microcavity is improved, thereby obtaining a more stable and wide-spectrum DKS output.

[0006] In order to achieve the above-mentioned object of the invention, the technical solution provided by the present invention is:

[0007] A LiNbO3 microcavity with a pulley coupling structure, comprising: a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR ( Figure 1 The unetched z-cut LiNbO3 waveguide and the rectangular calculation area are stacked on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR is stacked on the unetched z-cut LiNbO3 waveguide to form a micro-ring cavity tangential combination structure, see Figure 1 ;

[0008] The invention also includes a pulley coupler PC which suppresses SRS and promotes DKS generation, including a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR ( Figure 2 Left dotted box), etched z-cut LiNbO3 pulley waveguide PC ( Figure 2The unetched z-cut LiNbO3 waveguide and the calculation area are stacked on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR and the etched z-cut LiNbO3 pulley waveguide PC are stacked on the unetched z-cut LiNbO3 waveguide to form a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide, see Figure 2 ;in,

[0009] The effective refractive index of SiO2 The effective refractive index of z-cut LiNbO3 is given by the geometric dispersion and Sellmeier material dispersion formula (1). It is given by the geometric dispersion and Sellmeier material dispersion formula (2);

[0010]

[0011]

[0012] Where λ represents the wavelength in μm, A i and B i represents the Sellmeier coefficient;

[0013] Furthermore, the three-dimensional space size of the rectangular calculation area is determined by the tangential combination structure of the micro-ring cavity, which is divided into a global area and a local area, and a perfect matching layer is set to absorb electromagnetic waves at the boundary of the calculation area to prevent light reflection; the global grid size of the calculation area is set between 1 / 10 and 1 / 20 of the minimum wavelength; the local grid size of the calculation area is set between 1 / 20 and 1 / 40 of a more refined minimum wavelength.

[0014] Furthermore, the electromagnetic field distribution in the calculation area is represented by Maxwell's equations:

[0015]

[0016] Where E represents the electric field vector, H represents the electric field vector, ω represents the angular frequency, ω = 2πf, c represents the speed of light in free space, ε r Relative dielectric constant, μ r Represents the relative permeability. The mode characteristics are calculated according to the finite difference eigenmode (FDE) method:

[0017] Av=βv (4)

[0018] Where A represents the matrix of the system, which contains the geometric and material properties of the micro-ring cavity tangential combination structure. v represents the mode vector, which contains the electric and magnetic field distribution information. β represents the eigenvalue, i.e., the propagation constant. According to FDE, β in different modes is solved to obtain the effective refractive index n eff :

[0019]

[0020] Where k0 represents the wave number in free space, k0 = 2π / λ. When the light wave propagates in the microcavity and meets the resonance condition, the resonant frequency f m The geometrical dimensions and effective refractive index n of the micro-ring cavity tangential combination structure are eff Joint decision:

[0021]

[0022] Where m represents the azimuthal mode number and R represents the radius of the micro-ring cavity.

[0023] Further, according to the resonance condition, the resonance frequency f m The relationship with dispersion is expressed as:

[0024]

[0025]

[0026] Where m is the azimuth mode number, f m (ω m ) represents the resonant (angular) frequency of the mth longitudinal mode, β1 represents the first-order dispersion coefficient, β2 represents the second-order dispersion coefficient, i.e., the group velocity dispersion GVD, Δf m is the free spectral range (FSR), Δf m =(f m+1 -f m-1 ) / 2,Δ(Δf m ) = f m+1 -2f m +f m-1 .

[0027] Furthermore, the actual resonant frequency f μ The dispersion can also be described in terms of a Taylor expansion centered around the pump frequency f0:

[0028]

[0029] D int (μ) = f μ -(f0+D1 / 2π·μ) (10)

[0030] Where μ represents the relative mode number relative to the center (pump) mode (μ = 0), μ = m-m0. D1 / 2π represents the spacing between adjacent resonant frequencies, i.e., FSR, which is generally a constant. D2 / 2π represents the second-order dispersion associated with β2, i.e., GVD, which introduces a square with the relative mode number μ 2 Related frequency deviation. D3 / 2π, D4 / 2π, etc. represent higher order frequency dispersion. int (μ) represents the integrated dispersion, characterizing the actual resonant frequency f μ A measure of deviation from a linear distribution.

[0031] Furthermore, when the second-order dispersion coefficient β2>0, the actual resonant frequency f of the high-order mode μ Far from the ideal linear distribution, it shows normal dispersion, which is not conducive to the generation of DKS, but is more likely to enter the chaotic state or Turing ring state; when the second-order dispersion coefficient β2 < 0, the actual resonant frequency f of the high-order mode μ It is closer to the ideal value of linear distribution, showing anomalous dispersion, which is conducive to the generation of DKS and can easily enter the soliton state with high coherence and high stability.

[0032] Furthermore, the geometric dimensions of the micro-ring cavity tangential combination structure include the micro-ring cavity radius R, the etching depth h, the waveguide top width W, the lithium niobate film thickness H LN and the sidewall tilt angle θ, determine the integrated dispersion D int (μ) The shape, flatness and characteristic points of the curve. The shape is divided into opening upward and opening downward, corresponding to anomalous dispersion and normal dispersion respectively. Flatness refers to the rate of change of the integrated dispersion curve in the dispersion region. The flatter the curve, the smaller the dispersion change, and the more modes the frequency comb can cover. The characteristic points include dispersion wave points (Dispersion Wave Points, DW) and zero dispersion points (Zero Dispersion Point, ZD). The DW point is generated by the nonlinear interaction between the intracavity mode and the pump frequency, indicating the boundary at both ends of the DKS spectrum. The ZD point refers to the integrated dispersion curve crossing the zero value at a certain frequency, indicating the dividing point between anomalous dispersion and normal dispersion. Among them, the smaller W is, the integrated dispersion D int The shape of (μ) changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger h is, the more the integrated dispersion D int The shape of (μ) changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger R is, the more the integrated dispersion D int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency f0.

[0033] Furthermore, in the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide, the coupling strength coefficient S can be defined according to the first-order time perturbation theory by spatial mode overlap, inter-mode energy exchange efficiency and geometric characteristics of the coupling region:

[0034]

[0035] Among them, E R represents the electric field distribution of the etched z-cut LiNbO3 micro-ring cavity MR, E R represents the electric field distribution of the etched z-cut LiNbO3 pulley waveguide PC, and A represents the cross-sectional area of ​​the etched z-cut LiNbO3 pulley waveguide (see Figure 2 Dashed box PC part), ε-ε0 represents the mode perturbation introduced into the etched z-cut LiNbO3 micro-ring cavity MR by the etched z-cut LiNbO3 pulley waveguide PC.

[0036] Furthermore, in the coupling system, the relative mode phase matching between the microring mode and the waveguide mode also affects the coupling efficiency. Therefore, the coupling strength between the microring cavity and a single pulley waveguide is expressed as:

[0037] κ single =(2Sθsinc((m R -m WG )θ)) 2 (12)

[0038] Among them, m R represents the effective mode number of the etched z-cut LiNbO3 micro-ring cavity MR, m R =ω·n R / c×R R , R R represents the radius of the etched z-cut LiNbO3 micro-ring cavity MR, n R represents the effective refractive index of the etched z-cut LiNbO3 micro-ring cavity MR. WG represents the effective mode number of the etched z-cut LiNbO3 pulley waveguide PC, m WG =ω·n WG / c×L WG , L WG represents the length of the etched z-cut LiNbO3 pulley waveguide PC, n WGrepresents the effective refractive index of the etched z-cut LiNbO3 pulley waveguide PC. Wherein, sinc(u)=sin(πu) / (πu).

[0039] Furthermore, according to the coupling quality factor Q c To represent the energy loss due to coupling loss, Q c =ω0 / κ single , ω0 represents the resonant angular frequency of the coupled system.

[0040] Furthermore, the geometric parameters of the coupling system include the top width W of the micro-ring cavity. R 、W, the top width of the pulley waveguide WG , side wall inclination angles θ1 and θ2, coupling region angle and the coupling gap G, which determines the coupling quality factor Q c (Coupling strength and mode matching).

[0041] Furthermore, the competition between SRS and DKS formation is qualitatively described according to the ratio between the resonance enhancement power thresholds:

[0042]

[0043] in, and Respectively represent the power thresholds for FWM and SRS, and denote the loading quality factors of the pump mode and the Raman mode, respectively. The Raman mode is defined as the mode closest to the Raman gain peak f P The micro-ring resonant mode at -Δ, f P represents the pump frequency, Δ~18.94THz represents the Raman shift, g Kerr represents the FWM gain coefficient, represents the Raman gain function, δ = f R -(f P -Δ) represents the Raman frequency f R and the Raman gain peak f P -Δ detuning between.

[0044] Furthermore, when ζ < 1, the formation of DKS is better than that of SRS. This ratio is usually expressed as ζ>1, that is, SRS is easier to observe than DKS. To make ζ < 1, this not only introduces a high dissipation channel for the Raman mode but also maintains the pump mode.

[0045] Furthermore, in the optical microcavity system, the total load quality factor Q L Usually the microcavity intrinsic quality factor Qi (represents the loss of the microcavity itself, such as material absorption, scattering, etc.) and the coupling quality factor Q c (represents the energy loss caused by coupling between the microcavity and the waveguide) composition:

[0046]

[0047] Furthermore, due to f R and f P Close enough but far enough away from the material band gap, Therefore, as long as It can be achieved For critical coupling Pumping, as long as the LiNbO3 microcavity with PC structure meets If the conditions are met, the purpose of generating DKS and suppressing SRS can be achieved.

[0048] The present invention also provides a design method of a LiNbO3 microcavity with a pulley coupling structure, comprising the following steps:

[0049] Step S1: Setting the SiO2 substrate thickness H SiO2 , radius of micro-ring cavity R, thickness of lithium niobate film H LN , etching depth h, waveguide top width W, side wall inclination angle θ; set the bottom length x and thickness z of the micro-ring cavity tangential combination structure; set the grid size dx1=dy1=dz1 and dx2=dy2=dz2, FDE calculation area size V1 and V2;

[0050] According to formula (1), the refractive index of SiO2 substrate is set, and A is set i and B i represents the Sellmeier coefficient;

[0051] According to formula (2), the refractive index of the etched z-cut LiNbO3 micro-ring cavity MR and the unetched z-cut LiNbO3 waveguide is set, and A is set i and B i represents the Sellmeier coefficient, and the refractive index of z-cut LiNbO3 on the x-axis and y-axis is set to the normal refractive index n o , the refractive index in the z-axis is set to the extraordinary refractive index n e ;

[0052] Step S2: creating a micro-ring cavity tangential composite structure, including a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR, an unetched z-cut LiNbO3 waveguide, and calculation regions V1 and V2;

[0053] Step S3: setting the calculation wavelength range; in the MODE calculation mode, setting the number of trial modes, setting the initial effective refractive index, and setting the waveguide structure to a curved waveguide;

[0054] Step S4: Calculate the initial resonant frequency f m and the initial azimuth mode number m;

[0055] Step S5: According to the initial resonant frequency f m , recalculate the new resonant frequency f m′ , and calculate the difference between the new resonant frequency and the previous resonant frequency Δ = f m′ -f m , until Δ reaches the appropriate range and stops calculating, and finally confirms the new resonant frequency f m ;

[0056] Step S6: adding 1 to the initial azimuth mode number m, and repeating steps S4 and S5 until the azimuth mode number is m+2;

[0057] Step S7: Repeat steps S3, S4, S5 and S6 until all resonant frequencies within the set wavelength range including f are completed. m 、f m+1 、f m+2 and calculation of azimuth mode numbers m, m+1, m+2;

[0058] Step S8: Calculate the group velocity dispersion β2 of the corresponding wavelength range according to formula (8), and calculate the integrated dispersion D of the corresponding wavelength range according to formula (10). int (μ);

[0059] Step S9: Change the parameters, including the radius R of the micro-ring cavity, the etching depth h, and the waveguide top width W, and repeat steps S4 to S8 to compare the dispersion curve D int (μ) shape, flatness and feature points;

[0060] Step S10: Calculate the pump power threshold P according to the following formula: th :

[0061]

[0062] Among them, Q c Denotes the coupling quality factor, Q i represents the intrinsic quality factor, λ p represents the pump wavelength, n2 represents the nonlinear refractive index coefficient, n eff represents the effective refractive index, V represents the mode volume of the microcavity, V = 2πRA, and A represents the resonant mode of the microcavity;

[0063] Step S11: According to the integrated dispersion curve Dint The simulation results of (μ) and the calculation results of the pump power threshold, as well as the actual processing process, confirm that the dimensions of various geometric parameters are conducive to the generation of DKS, including the SiO2 substrate thickness H SiO2 , LiNbO3 film thickness H LN , micro-ring cavity radius R, side wall inclination angle θ, etching depth h, waveguide top width W;

[0064] Step S12: Further set the micro-ring cavity top width W R 、W, the top width of the pulley waveguide G , side wall inclination angles θ1 and θ2, coupling gap G, coupling area angle Set the length x and thickness z of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide; set the grid sizes dx1=dy1=dz1 and dx2=dy2=dz2, and the FDE calculation area sizes V3 and V4; set the refractive indexes of the two materials SiO2 and LiNbO3 according to step S1;

[0065] Step S13: creating a micro-ring cavity tangential combined structure;

[0066] Including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the micro-ring cavity tangential combination structure within the set wavelength range eff and electromagnetic field distributions E and H;

[0067] Step S14: creating a pulley-type waveguide tangential combination structure;

[0068] Including SiO2 substrate, etched z-cut LiNbO3 pulley waveguide PC, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the pulley waveguide tangential combination structure within the set wavelength range eff and electromagnetic field distributions E and H;

[0069] Step S15: creating a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide;

[0070] Including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, etched z-cut LiNbO3 pulley waveguide PC, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide within the set wavelength range eff and electromagnetic field distributions E and H;

[0071] Step S16: Calculate the coupling strength coefficient S according to formula (11);

[0072] Step S17: According to formula (12), calculate the coupling strength κ between the micro-ring cavity and a single pulley waveguide single ;

[0073] Step S18: Calculate the coupling quality factor Q c , plotting the Q within a set wavelength range c curve;

[0074] Step S19: Fix the SiO2 substrate thickness H determined in step S11 SiO2 , LiNbO3 film thickness H LN , micro-ring cavity radius R, etching depth h, micro-ring cavity top width W R Parameters, change the top width W of the pulley waveguide G , coupling gap G, side wall inclination angles θ1 and θ2, coupling area angle Repeat steps S13 to S18 and compare Q c Curves to determine coupling strength and mode matching;

[0075] Step S20: According to the Q c The critical coupling pump frequency and The calculation results and the actual processing technology are taken into account to determine the coupling area angle. Width of micro-ring cavity top W R 、W, the top width of the pulley waveguide G Detailed design to meet conditions, thereby inhibiting SRS and promoting DKS generation.

[0076] Compared with the prior art, the effects of the present invention include:

[0077] Through a new design method, the present invention is expected to effectively suppress undesirable nonlinear noise, especially the SRS effect, while maintaining a high Q value and improving coupling efficiency, thereby providing technical support for the generation of efficient, stable and wide-spectrum DKS. BRIEF DESCRIPTION OF THE DRAWINGS

[0078] Figure 1 It is a schematic diagram of the tangential combination structure of the micro-ring cavity in the present invention;

[0079] Figure 2 It is a schematic diagram of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide in the present invention;

[0080] Figure 3 Different integrated dispersion curves D obtained by changing the waveguide top width W in the present invention int (μ) simulation result diagram;

[0081] Figure 4 Different integrated dispersion curves D obtained by changing the etching depth h in the present invention int (μ) simulation result diagram;

[0082] Figure 5 Different integrated dispersion curves D obtained by changing the microcavity bending radius R in the present invention int (μ) simulation result diagram;

[0083] Figure 6 It is the electromagnetic field mode distribution diagram under different tangential combination structures in the present invention;

[0084] Figure 7 is the coupling quality factor Q under different microcavity sizes in the present invention c Simulation result graph of the curve. DETAILED DESCRIPTION

[0085] The specific implementation of the present invention is further described below in conjunction with specific examples.

[0086] like Figure 1 As shown, the basic LiNbO3 microcavity structure provided by the present invention comprises: a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR, an unetched z-cut LiNbO3 waveguide and a calculation area; wherein,

[0087] The effective refractive index of SiO2 The effective refractive index of z-cut LiNbO3 is given by the geometric dispersion and Sellmeier material dispersion formula (1). It is given by the geometric dispersion and Sellmeier material dispersion formula (2);

[0088]

[0089]

[0090] Where λ represents the wavelength in μm, A i and B i represents the Sellmeier coefficient;

[0091] The unetched z-cut LiNbO3 waveguide is superimposed on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR ( Figure 1 The dotted frame part) is superimposed on the unetched z-cut LiNbO3 waveguide to form a micro-ring cavity tangential combination structure, as shown in FIG. Figure 1 .

[0092] Furthermore, the three-dimensional space size of the rectangular calculation area is determined by the tangential combination structure of the micro-ring cavity, which is divided into a global area and a local area, and a perfect matching layer is set to absorb electromagnetic waves at the boundary of the calculation area to prevent light reflection; the global grid size of the calculation area is set between 1 / 10 and 1 / 20 of the minimum wavelength; the local grid size of the calculation area is set between 1 / 20 and 1 / 40 of a more refined minimum wavelength.

[0093] Furthermore, the electromagnetic field distribution in the calculation area is represented by Maxwell's equations:

[0094]

[0095] Where E represents the electric field vector, H represents the electric field vector, ω represents the angular frequency, ω = 2πf, c represents the speed of light in free space, ε r Relative dielectric constant, μ r Represents the relative permeability. The mode characteristics are calculated according to the finite difference eigenmode (FDE) method:

[0096] Av=βv (4)

[0097] Where A represents the matrix of the system, which contains the geometric and material properties of the micro-ring cavity tangential combination structure. v represents the mode vector, which contains the electric and magnetic field distribution information. β represents the eigenvalue, i.e., the propagation constant. According to FDE, β in different modes is solved to obtain the effective refractive index n eff :

[0098]

[0099] Where k0 represents the wave number in free space, k0 = 2π / λ. When the light wave propagates in the microcavity and meets the resonance condition, the resonant frequency f m The geometrical dimensions and effective refractive index n of the micro-ring cavity tangential combination structure are eff Joint decision:

[0100]

[0101] Where m represents the azimuthal mode number and R represents the radius of the micro-ring cavity.

[0102] Further, according to the resonance condition, the resonance frequency f m The relationship with dispersion is expressed as:

[0103]

[0104]

[0105] Among them, f m (ω m ) represents the resonant (angular) frequency of the mth longitudinal mode with azimuthal mode number m, β1 represents the first-order dispersion coefficient, β2 represents the second-order dispersion coefficient, i.e., group velocity dispersion GVD, Δf m is the free spectral range (FSR), Δf m =(f m+1 -f m-1 ) / 2,Δ(Δf m ) = f m+1 -2f m +f m-1 .

[0106] Furthermore, the actual resonant frequency f μ The dispersion can also be described in terms of a Taylor expansion centered around the pump frequency f0:

[0107]

[0108] D int (μ) = f μ -(f0+D1 / 2π·μ) (10)

[0109] Where μ represents the relative mode number relative to the center (pump) mode (μ = 0), μ = m-m0. D1 / 2π represents the spacing between adjacent resonant frequencies, i.e., FSR, which is generally a constant. D2 / 2π represents the second-order dispersion associated with β2, i.e., GVD, which introduces a square with the relative mode number μ 2 Related frequency deviation. D3 / 2π, D4 / 2π, ... represent higher order frequency dispersion. D int (μ) represents the integrated dispersion, characterizing the actual resonant frequency f μ A measure of deviation from a linear distribution.

[0110] Furthermore, when the second-order dispersion coefficient β2>0, the actual resonant frequency f of the high-order mode μ Far from the ideal linear distribution, it shows normal dispersion, which is not conducive to the generation of DKS, but is more likely to enter the chaotic state or Turing ring state; when the second-order dispersion coefficient β2 < 0, the actual resonant frequency f of the high-order mode μ It is closer to the ideal value of linear distribution, showing anomalous dispersion, which is conducive to the generation of DKS and can easily enter the soliton state with high coherence and high stability.

[0111] Furthermore, the geometric dimensions of the micro-ring cavity tangential combination structure include the micro-ring cavity radius R, the etching depth h, the waveguide top width W, the lithium niobate film thickness H LN and the sidewall tilt angle θ, determine the integrated dispersion Dint (μ) The shape, flatness and characteristic points of the curve. The shape is divided into opening upward and opening downward, corresponding to anomalous dispersion and normal dispersion respectively. Flatness refers to the rate of change of the integrated dispersion curve in the dispersion region. The flatter the curve, the smaller the dispersion change, and the more modes the frequency comb can cover. The characteristic points include dispersion wave points (Dispersion Wave Points, DW) and zero dispersion points (Zero Dispersion Point, ZD). The DW point is generated by the nonlinear interaction between the intracavity mode and the pump frequency, indicating the boundary at both ends of the DKS spectrum. The ZD point refers to the integrated dispersion curve crossing the zero value at a certain frequency, indicating the dividing point between anomalous dispersion and normal dispersion. Among them, the smaller W is, the integrated dispersion D int The shape of (μ) changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger h is, the more the integrated dispersion D int The shape of (μ) changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger R is, the more the integrated dispersion D int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency f0.

[0112] The second aspect of the present invention provides a pulley coupler PC, which suppresses SRS and promotes DKS generation. The key to the coupler design is to control the coupling strength, phase matching conditions and external losses to ensure effective soliton state excitation, rather than nonlinear effects dominated by the Raman effect. It is characterized in that it includes: SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, etched z-cut LiNbO3 pulley waveguide PC, unetched z-cut LiNbO3 waveguide and calculation area.

[0113] Furthermore, the unetched z-cut LiNbO3 waveguide is superimposed on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR ( Figure 2 The left dotted box) and the etched z-cut LiNbO3 pulley waveguide PC ( Figure 2 The right dotted frame part) is superimposed on the unetched z-cut LiNbO3 waveguide to form a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide, as shown in FIG. Figure 2 .

[0114] Furthermore, in the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide, the coupling strength coefficient S can be defined according to the first-order time perturbation theory by spatial mode overlap, inter-mode energy exchange efficiency and geometric characteristics of the coupling region:

[0115]

[0116] Among them, E R represents the electric field distribution of the etched z-cut LiNbO3 micro-ring cavity MR, E R represents the electric field distribution of the etched z-cut LiNbO3 pulley waveguide PC, and A represents the cross-sectional area of ​​the etched z-cut LiNbO3 pulley waveguide (see Figure 2 Dashed box PC part), ε-ε0 represents the mode perturbation introduced into the etched z-cut LiNbO3 micro-ring cavity MR by the etched z-cut LiNbO3 pulley waveguide PC.

[0117] Furthermore, in the coupling system, the relative mode phase matching between the microring mode and the waveguide mode also affects the coupling efficiency. Therefore, the coupling strength between the microring cavity and a single pulley waveguide is expressed as:

[0118] κ single =(2Sθsinc((m R -m WG )θ)) 2 (12)

[0119] Among them, m R represents the effective mode number of the etched z-cut LiNbO3 micro-ring cavity MR, m R =ω·n R / c×R R , R R represents the radius of the etched z-cut LiNbO3 micro-ring cavity MR, n R represents the effective refractive index of the etched z-cut LiNbO3 micro-ring cavity MR. WG represents the effective mode number of the etched z-cut LiNbO3 pulley waveguide PC, m WG =ω·n WG / c×L WG , L WG represents the length of the etched z-cut LiNbO3 pulley waveguide PC, n WG represents the effective refractive index of the etched z-cut LiNbO3 pulley waveguide PC. Wherein, sinc(u)=sin(πu) / (πu).

[0120] Furthermore, according to the coupling quality factor Q c To represent the energy loss due to coupling loss, Q c =ω0 / κ single, ω0 represents the resonant angular frequency of the coupled system.

[0121] Furthermore, the geometric parameters of the coupling system include the top width W of the micro-ring cavity. R 、W, the top width of the pulley waveguide WG , side wall inclination angles θ1 and θ2, coupling region angle and the coupling gap G, which determines the coupling quality factor Q c (Coupling strength and mode matching).

[0122] Furthermore, the competition between SRS and DKS formation is qualitatively described according to the ratio between the resonance enhancement power thresholds:

[0123]

[0124] in, and Respectively represent the power thresholds for FWM and SRS, and denote the loading quality factors of the pump mode and the Raman mode, respectively. The Raman mode is defined as the mode closest to the Raman gain peak f P The micro-ring resonant mode at -Δ, f P represents the pump frequency, 18.94 THz represents the Raman shift, g Kerr represents the FWM gain coefficient, represents the Raman gain function, δ = f R -(f P -Δ) represents the Raman frequency f R and the Raman gain peak f P -Δ detuning between.

[0125] Furthermore, when ζ < 1, the formation of DKS is better than that of SRS. This ratio is usually expressed as ζ>1, that is, SRS is easier to observe than DKS. To make ζ < 1, this not only introduces a high dissipation channel for the Raman mode but also maintains the pump mode.

[0126] Furthermore, in the optical microcavity system, the total load quality factor Q L Usually the microcavity intrinsic quality factor Q i (represents the loss of the microcavity itself, such as material absorption, scattering, etc.) and the coupling quality factor Q c (represents the energy loss caused by coupling between the microcavity and the waveguide) composition:

[0127]

[0128] Furthermore, due to f Rand f P Close enough but far enough away from the material band gap, Therefore, as long as It can be achieved For critical coupling Pumping, as long as the LiNbO3 microcavity with PC structure meets If the conditions are met, the purpose of generating DKS and suppressing SRS can be achieved.

[0129] The following example specifically illustrates the design method of the LiNbO3 microcavity with a pulley coupling structure of the present invention, including the following steps:

[0130] Step S1:

[0131] Set the SiO2 substrate thickness H SiO2 =5μm, radius of micro-ring cavity R = 60μm, thickness of lithium niobate film H LN =600nm, etching depth h=340nm, waveguide top width W=1.6μm, side wall inclination angle θ=60°;

[0132] The bottom length x of the micro-ring cavity tangential combination structure is set to be 10.6 μm and the thickness z is set to be 0.6 μm;

[0133] Set the grid size dx1=dy1=dz1=0.04μm and dx2=dy2=dz2=0.02μm, and the FDE calculation area size V1=8.6×4×0.6μm 3 and V2 = 4.3 × 2 × 0.6 μm 2 .

[0134] Set the refractive index of the SiO2 substrate according to formula (1) and set A according to Table 1 i and B i Represents the Sellmeier coefficient.

[0135] The refractive index of the etched z-cut LiNbO3 micro-ring cavity MR and the unetched z-cut LiNbO3 waveguide is set according to formula (2), and A is set according to Table 1 i and B i represents the Sellmeier coefficient, and the refractive index of z-cut LiNbO3 on the x-axis and y-axis is set to the normal refractive index n o , the refractive index in the z-axis is set to the extraordinary refractive index n e .

[0136] Table 1 Refractive index coefficients of SiO2 and LiNbO3 materials

[0137] Material <![CDATA[A1]]> <![CDATA[A2]]> <![CDATA[A3]]> <![CDATA[B1]]> <![CDATA[B2]]> <![CDATA[B3]]> <![CDATA[SiO2]]> 0.6961663 0.4079426 0.8974794 0.0684043 0.1162414 9.896161 <![CDATA[LiNbO3(n o )]]> 2.6734 1.229 12.614 0.01764 0.05914 474.6 <![CDATA[LiNbO3(n e )]]> 2.9804 0.5981 8.9543 0.02047 0.0666 416.08

[0138] Step S2: Create a micro-ring cavity tangential composite structure (see Figure 1 , including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, unetched z-cut LiNbO3 waveguide) and calculation areas V1 and V2.

[0139] Step S3: Set the calculation wavelength range to 900 to 2500 nm. In the MODE calculation mode, set the number of trial modes to 10, set the initial effective refractive index to 1, and set the waveguide structure to a curved waveguide.

[0140] Step S4: Calculate the initial resonant frequency f m and the initial azimuth mode number m.

[0141] Step S5: According to the initial resonant frequency f m , recalculate the new resonant frequency f m′ , and calculate the difference between the new resonant frequency and the previous resonant frequency Δ = f m′ -f m , until Δ>100Hz, then stop calculating and finally confirm the new resonant frequency f m .

[0142] Step S6: Add 1 to the initial azimuth mode number m, and repeat steps S4 and S5 until the azimuth mode number is m+2.

[0143] Step S7: Repeat steps S3, S4, S5 and S6 until all resonant frequencies (including f m 、f m+1 、f m+2 ) and the azimuth mode number (m, m+1, m+2).

[0144] Step S8: Calculate the group velocity dispersion β2 of the corresponding wavelength range according to formula (8), and calculate the integrated dispersion D of the corresponding wavelength range according to formula (10). int (μ).

[0145] Step S9: Combination Figure 3 , Figure 4 and Figure 5 , change the following parameters, including the radius R of the micro-ring cavity, the etching depth h, and the waveguide top width W, repeat steps S4 to S8, and compare the dispersion curve D int (μ) shape, flatness and feature points.

[0146] from Figure 3 It can be seen that when the fixed H LN = 600nm, h = 350nm and θ = 60°, and the radius of the micro-ring cavity is R = 60μm ( Figure 3 (a)), R = 80 μm ( Figure 3 (b)) and R = 100 μm ( Figure 3 (c)), when the waveguide top width increases from W = 1.25 μm to W = 2.2 μm, the smaller W is, the integrated dispersion D int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency.

[0147] from Figure 4 It can be seen that when H is fixed SiO2 =5μm, H LN = 600nm and θ = 60°, the radius of the micro-ring cavity is R = 60μm ( Figure 4 (a)~(c))、R=80μm( Figure 4 (d)~(f))、R=100μm( Figure 4 (g)~(i)), the waveguide top width is from W=1.45μm( Figure 4 (a), (d), (g)), W = 1.55μm ( Figure 4 (b), (e), (h)), W = 1.6μm ( Figure 4 (c), (f), (i)), when the etching depth increases from h = 300nm to h = 350nm, the larger h is, the integrated dispersion D int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency f0.

[0148] from Figure 5 It can be seen that the fixed H LN = 600nm, h = 350nm and θ = 60, and the width at the top of the waveguide is W = 1.6μm ( Figure 5 (a)), W = 1.7 μm ( Figure 5 (b)) W = 1.8 μm ( Figure 5 (c)), when the radius of the micro-ring cavity increases from R = 60 μm to R = 100 μm, the larger R is, the larger the integrated dispersion D is. int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency f0.

[0149] Step S10: Calculate the pump power threshold P according to the following formula: th :

[0150]

[0151] Among them, set to Q c=4×10 5 , Q i =5×10 5 ,λ p represents the pump wavelength, n2 represents the nonlinear refractive index, n2=0.91×10 -15 cm 2 W -1 , n eff Represents the effective refractive index, n at a wavelength of 1575nm eff =2.005171, V represents the mode volume of the microcavity, V = 2πRA, A represents the resonant mode of the microcavity, A = 1.1562×10 -12 m 2 .

[0152] Table 2 Pump power threshold P th Calculation results

[0153] <![CDATA[P th (mW)]]> W=1.45μm W=1.55μm W=1.6μm W=1.7μm W=1.8μm R=60μm 414.2 430.0 437.8 453.4 468.6 R=60μm 414.7 431.1 439.3 455.8 472.1 R=60μm 415.0 431.6 440.0 456.9 473.8

[0154] Step S11: According to the integrated dispersion curve D int The simulation results of (μ) and the calculation results of the pump power threshold, as well as the actual processing process, confirm that when the SiO2 substrate thickness H SiO2 =5μm, lithium niobate film thickness H LN When =600nm, micro-ring cavity radius R=60μm, side wall inclination angle θ=60°, etching depth h=330nm, waveguide top width W=1.45μm, W=1.55μm, W=1.6μm, W=1.7μm and W=1.8μm, better results are achieved, which is conducive to the generation of DKS.

[0155] Step S12: Further set the micro-ring cavity top width W R =1.6μm, pulley waveguide top width W G = 0.85 μm, side wall inclination angle θ1 = 60° and θ2 = 50°, coupling gap G = 1 μm, coupling area angle The length x of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide is set to 16.6 μm and the thickness z is set to 0.6 μm; the grid size dx1=dy1=dz1=0.04 μm and dx2=dy2=dz2=0.02 μm, and the FDE calculation area size V3=14.6×4×0.6 μm 3 and V4=8.3×2×0.6μm 3 The refractive indexes of the two materials SiO2 and LiNbO3 are set according to step S1.

[0156] Step S13: Create a micro-ring cavity tangential composite structure (including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, unetched z-cut LiNbO3 waveguide) and calculation areas V3 and V4. Calculate the effective refractive index n of the micro-ring cavity tangential composite structure within the set wavelength range. eff and electromagnetic field distribution E and H (see Figure 6 (a)).

[0157] Step S14: Create a pulley-waveguide tangential combination structure (including SiO2 substrate, etched z-cut LiNbO3 pulley-waveguide PC, unetched z-cut LiNbO3 waveguide) and calculation areas V3 and V4. Calculate the effective refractive index n of the pulley-waveguide tangential combination structure within the set wavelength range. eff and electromagnetic field distribution E and H (see Figure 6 (b)).

[0158] Step S15: Create a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide (see Figure 2 , including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, etched z-cut LiNbO3 pulley waveguide PC, and unetched z-cut LiNbO3 waveguide. When the coupling gap G is small, MR and PC will have overlapping parts, see Figure 2 The effective refractive index n of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide within the set wavelength range is calculated. eff and electromagnetic field distribution E and H (see Figure 6 (c)).

[0159] Step S16: Calculate the coupling strength coefficient S according to formula (11).

[0160] Step S17: According to formula (12), calculate the coupling strength κ between the micro-ring cavity and a single pulley waveguide single .

[0161] Step S18: Calculate the coupling quality factor Q c , plotting the Q within a set wavelength range c curve.

[0162] Step S19: Fix the SiO2 substrate thickness determined in step S11 Lithium niobate film thickness H LN , micro-ring cavity radius R, etching depth h, micro-ring cavity top width W R Parameters such as the pulley waveguide top width W are changed G , coupling gap G, side wall inclination angles θ1 and θ2, coupling area angle Repeat steps S13 to S18 and compare Q c Curves are used to determine the coupling strength and mode matching.

[0163] from Figure 7 As can be seen in (a), when the sidewall inclination angles θ1 = 60° and θ2 = 50°, the coupling gap G = 1 μm, and the coupling area angle Width of micro-ring cavity top W R =1.45μm, the top width of the pulley waveguide is from W G =950nm changes to W G =1000nm, Q c The critical coupling pump frequency in the curve changes from 200THz to 180THz. The smaller.

[0164] from Figure 7 As can be seen in (b), when the sidewall inclination angles θ1 = 60° and θ2 = 50°, the coupling gap G = 1 μm, and the coupling area angle Width of micro-ring cavity top W R =1.55μm, the top width of the pulley waveguide is from W G =975nm changes to W G =1025nm, Q c The critical coupling pump frequency in the curve changes from 200THz to 180THz. The smaller.

[0165] from Figure 7 As can be seen in (c), when the sidewall inclination angles θ1 = 60° and θ2 = 50°, the coupling gap G = 1 μm, and the coupling area angle Width of micro-ring cavity top W R =1.6μm, the top width of the pulley waveguide is from W G =965nm changes to W G =1015nm, Q c The critical coupling pump frequency in the curve changes from 200THz to 180THz. The smaller.

[0166] from Figure 7 As can be seen in (d), when the sidewall inclination angles θ1 = 60° and θ2 = 50°, the coupling gap G = 1 μm, and the coupling region angle Width of micro-ring cavity top W R =1.7μm, the top width of the pulley waveguide is from W G =1000nm changes to W G =1020nm, Q c The critical coupling pump frequency in the curve changes from 200THz to 180THz. The smaller.

[0167] Figure 7 As can be seen in (e), when the sidewall inclination angles θ1 = 60° and θ2 = 50°, the coupling gap G = 1 μm, and the coupling region angle Width of micro-ring cavity top W R =1.8μm, the top width of the pulley waveguide is from W G =1015nm changes to W G =1035nm, Q c The critical coupling pump frequency in the curve changes from 200THz to 180THz. The smaller.

[0168] Step S20: According to the Q c The critical coupling pump frequency and The calculation results, as well as the actual processing technology (the top width of the micro-ring cavity W R and the top width W of the pulley waveguide G will decrease by 100nm, and the coupling gap G will increase by 100nm), and the angle of the coupling area Width of micro-ring cavity top W R 、W, the top width of the pulley waveguide G A detailed design was carried out, and the data are shown in Table 3, so as to meet This condition inhibits SRS and promotes the generation of DKS.

[0169] Table 3 Design parameters of LiNbO microcavity with PC structure

[0170]

[0171] Through a new design method, the present invention is expected to effectively suppress undesirable nonlinear noise, especially the SRS effect, while maintaining a high Q value and improving coupling efficiency, thereby providing technical support for the generation of efficient, stable and wide-spectrum DKS.

Claims

1. A LiNbO3 microcavity with a pulley coupling structure, characterized in that: include: SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, unetched z-cut LiNbO3 waveguide and rectangular calculation area; the unetched z-cut LiNbO3 waveguide is superimposed on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR is superimposed on the unetched z-cut LiNbO3 waveguide to form a micro-ring cavity tangential combination structure; It also includes a pulley coupler PC for suppressing SRS and promoting the generation of DKS, including a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR, an etched z-cut LiNbO3 pulley waveguide PC, an unetched z-cut LiNbO3 waveguide and a calculation area; the unetched z-cut LiNbO3 waveguide is superimposed on the SiO2 substrate, and the etched z-cut LiNbO3 micro-ring cavity MR and the etched z-cut LiNbO3 pulley waveguide PC are superimposed on the unetched z-cut LiNbO3 waveguide to form a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide; wherein, The effective refractive index of SiO2 The effective refractive index of z-cutLiNbO3 is given by the geometric dispersion and Sellmeier material dispersion formula (1). It is given by the geometric dispersion and Sellmeier material dispersion formula (2); Where λ represents the wavelength in μm, A i and B i represents the Sellmeier coefficient; The electromagnetic field distribution in the rectangular calculation area is expressed by Maxwell's equations: Where E represents the electric field vector, H represents the electric field vector, ω represents the angular frequency, ω = 2πf, c represents the speed of light in free space, ε r Relative dielectric constant, μ r represents the relative permeability; the mode characteristics are calculated according to the finite difference eigenmode FDE method: Av=βv (4) Where A represents the matrix of the system, which contains the geometric and material properties of the micro-ring cavity tangential combination structure; v represents the mode vector, which contains the electric and magnetic field distribution information; β represents the eigenvalue, i.e., the propagation constant; β under different modes is solved according to FDE to obtain the effective refractive index n eff : Where k0 represents the wave number in free space, k0 = 2π / λ; When the light wave propagates in the microcavity and meets the resonance condition, the resonance frequency f m The geometrical dimensions and effective refractive index n of the micro-ring cavity tangential combination structure are eff Joint decision: Where m represents the azimuthal mode number and R represents the radius of the micro-ring cavity.

2. The LiNbO3 microcavity with a pulley coupling structure according to claim 1, characterized in that: The three-dimensional space size of the rectangular calculation area is determined by the tangential combination structure of the micro-ring cavity, which is divided into a global area and a local area, and a perfect matching layer is set to absorb electromagnetic waves at the boundary of the calculation area to prevent light reflection; The global grid size of the rectangular calculation area is set between 1 / 10 and 1 / 20 of the minimum wavelength; the local grid size of the rectangular calculation area is set between 1 / 20 and 1 / 40 of the more refined minimum wavelength.

3. The LiNbO3 microcavity with a pulley coupling structure according to claim 1, characterized in that: According to the resonance condition, the resonance frequency f m The relationship with dispersion is expressed as: Where m represents the azimuth mode number, f m (ω m ) represents the resonant (angular) frequency of the mth longitudinal mode, β1 represents the first-order dispersion coefficient, β2 represents the second-order dispersion coefficient, i.e., the group velocity dispersion GVD, Δf m is the free spectral range FSR, Δf m =(f m+1 -f m-1 ) / 2,Δ(Δf m ) = f m+1 -2f m +f m-1 .

4. The LiNbO3 microcavity with a pulley coupling structure according to claim 3, characterized in that: Furthermore, the actual resonant frequency f μ The dispersion can also be described in terms of a Taylor expansion centered around the pump frequency f0: D int (μ)=f μ -(f0+D1 / 2π·μ) (10) Where μ represents the relative mode number relative to the pump center mode (μ = 0), μ = m-m0; D1 / 2π represents the interval between adjacent resonant frequencies, i.e., FSR, which is generally a constant; D2 / 2π represents the second-order dispersion associated with β2, i.e., GVD, which introduces a square with the relative mode number μ 2 Related frequency shifts; D3 / 2π, D4 / 2π, L represents higher-order frequency dispersion; D int (μ) represents the integrated dispersion, characterizing the actual resonant frequency f μ A measure of deviation from a linear distribution; When the second-order dispersion coefficient β2>0, the actual resonant frequency f of the high-order mode μ Far from the ideal linear distribution, it shows normal dispersion, which is not conducive to the generation of DKS, but is more likely to enter the chaotic state or Turing ring state; when the second-order dispersion coefficient β2 < 0, the actual resonant frequency f of the high-order mode μ It is closer to the ideal value of linear distribution, showing anomalous dispersion, which is conducive to the generation of DKS and can easily enter the soliton state with high coherence and high stability.

5. The LiNbO3 microcavity with a pulley coupling structure according to claim 1, characterized in that: The geometrical dimensions of the micro-ring cavity tangential combination structure determine the integrated dispersion D int (μ) The shape, flatness and characteristic points of the curve; the geometric dimensions include the radius of the micro-ring cavity R, the etching depth h, the width of the waveguide top W, the thickness of the lithium niobate film H LN and sidewall inclination angle θ; The integrated dispersion D int The shape of the (μ) curve is divided into two types: opening upward and opening downward, corresponding to anomalous dispersion and normal dispersion, respectively; the integrated dispersion D int (μ) The flatness of the curve refers to the rate of change of the integrated dispersion curve within the dispersion region. The flatter the curve, the smaller the dispersion change, and the more modes the frequency comb can cover; The integrated dispersion D int (μ) The shape characteristic points of the curve include the dispersion wave point DW and the zero dispersion point ZD; The DW points are generated by the nonlinear interaction between the intracavity mode and the pump frequency, indicating the boundaries at both ends of the DKS spectrum; The ZD point refers to the point where the integrated dispersion curve crosses the zero value at a certain frequency, indicating the dividing point between anomalous dispersion and normal dispersion; The smaller W is, the greater the integrated dispersion D int (μ) The shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger h is, the more the integrated dispersion D int (μ) The shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the ZD point is farther away from the pump frequency f0. The larger R is, the more the integrated dispersion D int The (μ) shape changes from the normal dispersion region to the anomalous dispersion region. The less flat the curve is, the more the DW point expands outward, and the farther the ZD point is from the pump frequency f0.

6. The LiNbO3 microcavity with a pulley coupling structure according to claim 1, characterized in that: In the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide, the coupling strength coefficient S is defined according to the first-order time perturbation theory by the spatial mode overlap, the energy exchange efficiency between modes and the geometric characteristics of the coupling region: Among them, E R represents the electric field distribution of the etched z-cut LiNbO3 micro-ring cavity MR, E R represents the electric field distribution of the etched z-cut LiNbO3 pulley waveguide PC, A represents the cross-sectional area of ​​the etched z-cut LiNbO3 pulley waveguide, ε-ε0 represents the mode perturbation introduced into the etched z-cut LiNbO3 micro-ring cavity MR by the etched z-cut LiNbO3 pulley waveguide PC; The coupling strength between the micro-ring cavity and a single pulley waveguide is expressed as: k single =(2Sθsinc((m R -m WG )i)) 2 (12) Among them, m R represents the effective mode number of the etched z-cut LiNbO3 micro-ring cavity MR, m R =ω·n R / c×R R , R R represents the radius of the etched z-cut LiNbO3 micro-ring cavity MR, n R represents the effective refractive index of the etched z-cut LiNbO3 micro-ring cavity MR; m WG represents the effective mode number of the etched z-cut LiNbO3 pulley waveguide PC, m WG =ω·n WG / c×L WG , L WG represents the length of the etched z-cut LiNbO3 pulley waveguide PC, n WG represents the effective refractive index of the etched z-cut LiNbO3 pulley waveguide PC; wherein, sinc(u)=sin(πu) / (πu); And according to the coupling quality factor Q c To represent the energy loss due to coupling loss, Q c =ω0 / κ single , ω0 represents the resonant angular frequency of the coupled system.

7. The LiNbO3 microcavity with a pulley coupling structure according to claim 6, characterized in that: The geometric parameters of the pulley coupler PC include the top width W of the microring cavity R 、W, the top width of the pulley waveguide WG , W R Sidewall inclination angle θ1 and W WG Side wall inclination angle θ2, coupling area angle and the coupling gap G, which determines the coupling quality factor Q c .

8. The LiNbO3 microcavity with a pulley coupling structure according to claim 6, characterized in that: Furthermore, the competition between SRS and DKS formation is qualitatively described according to the ratio between the resonance enhancement power thresholds: in, and Respectively represent the power thresholds for FWM and SRS, and are the loading quality factors of the pump mode and the Raman mode, respectively. The Raman mode is defined as the mode closest to the Raman gain peak f P The micro-ring resonant mode at -Δ, f P represents the pump frequency, 18.94 THz represents the Raman shift, g Kerr represents the FWM gain coefficient, represents the Raman gain function, δ = f R -(f P -Δ) represents the Raman frequency f R and the Raman gain peak f P -Detuning between Δ; when When DKS is formed, it is better than SRS; This ratio is usually expressed as That is, SRS is easier to observe than DKS; by design To make This not only introduces a high dissipation channel for the Raman mode, but also maintains the pump mode; In an optical microcavity system, the total load quality factor Q L The intrinsic quality factor Q i and coupling quality factor Q c composition: Because f R and f P Close enough but far enough away from the material band gap, Therefore, as long as It can be achieved For critical coupling Pumping, as long as the LiNbO3 microcavity with PC structure meets If the conditions are met, DKS can be generated and SRS can be suppressed.

9. A method for designing a LiNbO3 microcavity with a pulley coupling structure according to any one of claims 1 to 8, characterized in that: Step S1: Setting SiO2 substrate thickness Micro-ring cavity radius R, lithium niobate film thickness H LN , etching depth h, waveguide top width W, side wall inclination angle θ; set the bottom length x and thickness z of the micro-ring cavity tangential combination structure; set the grid size dx1=dy1=dz1 and dx2=dy2=dz2, FDE calculation area size V1 and V2; According to formula (1), the refractive index of SiO2 substrate is set, and A is set i and B i represents the Sellmeier coefficient; According to formula (2), the refractive index of the etched z-cut LiNbO3 micro-ring cavity MR and the unetched z-cut LiNbO3 waveguide is set, and A is set i and B i represents the Sellmeier coefficient, and the refractive index of z-cut LiNbO3 on the x-axis and y-axis is set to the normal refractive index n o , the refractive index in the z-axis is set to the extraordinary refractive index n e ; Step S2: creating a micro-ring cavity tangential composite structure, including a SiO2 substrate, an etched z-cut LiNbO3 micro-ring cavity MR, an unetched z-cut LiNbO3 waveguide, and calculation regions V1 and V2; Step S3: setting the calculation wavelength range; In the MODE calculation mode, set the number of trial modes, set the initial effective refractive index, and set the waveguide structure to a curved waveguide; Step S4: Calculate the initial resonant frequency f m and the initial azimuth mode number m; Step S5: According to the initial resonant frequency f m , recalculate the new resonant frequency f m′ , and calculate the difference between the new resonant frequency and the previous resonant frequency Δ = f m′ -f m , until Δ reaches the appropriate range and stops calculating, and finally confirms the new resonant frequency f m ; Step S6: adding 1 to the initial azimuth mode number m, and repeating steps S4 and S5 until the azimuth mode number is m+2; Step S7: Repeat steps S3, S4, S5 and S6 until all resonant frequencies within the set wavelength range including f are completed. m 、f m+1 、f m+2 and calculation of azimuth mode numbers m, m+1, m+2; Step S8: Calculate the group velocity dispersion β2 of the corresponding wavelength range according to formula (8), and calculate the integrated dispersion D of the corresponding wavelength range according to formula (10). int (μ); Step S9: Change the parameters, including the radius R of the micro-ring cavity, the etching depth h, and the waveguide top width W, and repeat steps S4 to S8 to compare the dispersion curve D int (μ) shape, flatness and feature points; Step S10: Calculate the pump power threshold P according to the following formula: th : Among them, Q c Denotes the coupling quality factor, Q i represents the intrinsic quality factor, λ p represents the pump wavelength, n2 represents the nonlinear refractive index coefficient, n eff represents the effective refractive index, V represents the mode volume of the microcavity, V = 2πRA, and A represents the resonant mode of the microcavity; Step S11: According to the integrated dispersion curve D int The simulation results of (μ) and the calculation results of the pump power threshold, as well as the actual processing process, confirm that the dimensions of various geometric parameters are conducive to the generation of DKS, including the thickness of the SiO2 substrate. Lithium niobate film thickness H LN , micro-ring cavity radius R, side wall inclination angle θ, etching depth h, waveguide top width W; Step S12: Further set the micro-ring cavity top width W R 、W, the top width of the pulley waveguide G , side wall inclination angles θ1 and θ2, coupling gap G, coupling area angle Set the length x and thickness z of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide; set the grid sizes dx1=dy1=dz1 and dx2=dy2=dz2, and the FDE calculation area sizes V3 and V4; set the refractive indexes of the two materials SiO2 and LiNbO3 according to step S1; Step S13: creating a micro-ring cavity tangential combined structure; Including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the micro-ring cavity tangential combination structure within the set wavelength range eff and electromagnetic field distributions E and H; Step S14: creating a pulley-type waveguide tangential combination structure; Including SiO2 substrate, etched z-cut LiNbO3 pulley waveguide PC, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the pulley waveguide tangential combination structure within the set wavelength range eff and electromagnetic field distributions E and H; Step S15: creating a coupled tangential combination structure of the micro-ring cavity and the pulley waveguide; Including SiO2 substrate, etched z-cut LiNbO3 micro-ring cavity MR, etched z-cut LiNbO3 pulley waveguide PC, unetched z-cut LiNbO3 waveguide and calculation areas V3 and V4; calculate the effective refractive index n of the coupled tangential combination structure of the micro-ring cavity and the pulley waveguide within the set wavelength range eff and electromagnetic field distributions E and H; Step S16: Calculate the coupling strength coefficient S according to formula (11); Step S17: According to formula (12), calculate the coupling strength κ between the micro-ring cavity and a single pulley waveguide single ; Step S18: Calculate the coupling quality factor Q c , plotting the Q within a set wavelength range c curve; Step S19: Fix the SiO2 substrate thickness determined in step S11 Lithium niobate film thickness H LN , micro-ring cavity radius R, etching depth h, micro-ring cavity top width W R Parameters, change the top width W of the pulley waveguide G , coupling gap G, side wall inclination angles θ1 and θ2, coupling area angle Repeat steps S13 to S18 and compare Q c Curves to determine coupling strength and mode matching; Step S20: According to the Q c The critical coupling pump frequency and The calculation results and the actual processing technology are taken into account to determine the coupling area angle. Width of micro-ring cavity top W R 、W, the top width of the pulley waveguide G Detailed design to meet conditions, thereby inhibiting SRS and promoting DKS generation.

Images