Design method of lithium niobate film phase modulator with high polarization extinction ratio

Abstract

The invention discloses a design method of a lithium niobate film phase modulator with a high polarization extinction ratio, and belongs to the field of integrated optical gyroscopes. The method comprises the following steps: firstly, designing a lithium niobate film phase modulator comprising a silicon substrate, a silicon dioxide cladding, a lithium niobate film optical waveguide core layer and a modulation electrode; the double-layer conical gradient waveguide meeting the TE mode light fundamental mode transmission condition couples light into a chip, and polarization of a transmittable light mode is limited by designing the width and the thickness of the waveguide; the effective refractive index, the bending radius, the waveguide width and the center distance are designed for the directional coupling unit, and polarization is generated through phase matching polarization beam splitting; the multimode interference coupler is polarized by designing the length and width of a multimode interference region; the bending beam splitting unit is polarized by designing bending radius leakage; finally, through an electric field generated by a modulation electrode, the light is subjected to phase change, and two beams of light with phase difference are output; according to the invention, a high polarization extinction ratio is realized on the basis of a basic beam splitting modulation function.

Description

Technical Field

[0001] The invention belongs to the field of integrated optical gyroscopes, and in particular is a design method for a lithium niobate thin film phase modulator with a high polarization extinction ratio. Background Art

[0002] After decades of research and development since the 20th century, optical gyroscopes have become a mainstream form of inertial instrumentation. They offer significant advantages, including solid-state design, high precision, and high reliability, and are used in a wide range of fields, including navigation and positioning, attitude control, and drilling exploration.

[0003] An optical gyroscope is mainly composed of a phase modulator, a coupler, a detector, an angular velocity sensor unit, and a light source. Among them, the phase modulator, as one of the core components, undertakes the key functions of light splitting and phase modulation.

[0004] With the emergence and development of the concept of integration, researchers are working to utilize advanced integration processes to integrate the various optical components of an optical gyroscope onto a single optical chip, enabling mass production of miniature integrated optical gyroscopes. To achieve on-chip integration of phase modulators, devices based on lithium niobate thin film materials are required. This type of modulator not only significantly reduces size and half-wave voltage, but also offers some cost savings.

[0005] In interferometric and resonant optical gyros, the performance of the phase modulator is crucial. A high polarization extinction ratio (PER) is a key performance metric for modulators and has a decisive impact on system accuracy and stability. By strictly controlling the polarization state of the optical signal, backscattered noise can be effectively suppressed, interferometric signal quality can be guaranteed, the resonant cavity's high quality factor can be maintained, and system stability can be enhanced.

[0006] Currently, in the integration process, limited by material properties, process accuracy, temperature sensitivity, integration complexity and cost factors, achieving and maintaining high-performance, low-cost, high polarization extinction ratio on-chip integrated modulators that meet the needs of high-precision gyroscopes is still the main technical challenge and research hotspot currently faced, which is of great significance for achieving high precision in integrated optical gyroscopes.

[0007] Based on the above considerations, it is particularly important to study lithium niobate thin film phase modulators that can achieve high polarization extinction ratio in structure. Summary of the Invention

[0008] In response to the demand for an on-chip integrated modulator with a high polarization extinction ratio for high-precision gyroscopes, the present invention provides a design method for a lithium niobate thin film phase modulator with a high polarization extinction ratio, which achieves the goal of high polarization extinction ratio performance of the on-chip integrated modulator and is conducive to the further high-precision development of integrated optical gyroscopes.

[0009] The method for designing a lithium niobate thin film phase modulator with a high polarization extinction ratio specifically includes the following steps:

[0010] I) The design includes a silicon substrate, a silicon dioxide cladding, a lithium niobate thin film optical waveguide core layer, and a lithium niobate thin film phase modulator of a modulation electrode.

[0011] The silicon substrate is located at the bottom layer of the entire phase modulator and plays a supporting and protective role.

[0012] The silicon dioxide cladding is located on the silicon substrate layer and has a double-layer structure as a whole, including a first silicon dioxide layer and a second silicon dioxide layer, which respectively cover the upper and lower layers of the lithium niobate thin film optical waveguide core layer.

[0013] The lithium niobate thin film optical waveguide core layer includes a lithium niobate flat layer and a lithium niobate ridge layer. The lithium niobate ridge layer is located on the lithium niobate flat layer, and the two together constitute a complete ridge optical waveguide core layer structure.

[0014] The specific structure consists of a TE fundamental mode double-layer tapered input waveguide, two curved tapered directional coupling unit waveguides, a polarization-specific multimode interference coupler unit waveguide, two curved beam splitting unit waveguides, two modulation arm units, and two TE fundamental mode double-layer tapered output waveguides, which realize polarization, beam splitting, and modulation functions. The TE fundamental mode double-layer tapered input and output waveguides both meet the TE mode optical fundamental mode transmission conditions.

[0015] The TE fundamental mode double-layer tapered gradient input waveguide transitions from a narrow to a wide single-layer structure to a double-layer structure, wherein the single-layer structure is a lithium niobate slab layer, and the double-layer structure is a complete ridge optical waveguide core layer structure consisting of a lithium niobate slab layer and a lithium niobate ridge layer; the narrow end of the TE fundamental mode double-layer tapered gradient input waveguide receives light output by the light source, and the wide end is connected to the front end of the curved gradient directional coupling unit waveguide I; the TE mode light continues to transmit through the curved gradient directional coupling unit waveguide I, and the TM mode light is directionally coupled into the curved gradient directional coupling unit waveguide I I is output to the chip; the rear end of the curved gradient directional coupling unit waveguide I is connected to one end of the polarization-specific multimode interference coupler unit waveguide, and the other end of the polarization-specific multimode interference coupler unit waveguide is connected to the front end of the two curved beam splitting unit waveguides; the rear ends of the two curved beam splitting unit waveguides are respectively connected to the two modulation arm units, and the two modulation arm units are respectively located in the middle of the four modulation electrodes, and then the two modulation arm units are respectively connected to the wide ends of the two TE fundamental mode double-layer tapered gradient output waveguides, and the narrow ends of the two TE fundamental mode double-layer tapered gradient output waveguides output light.

[0016] II) Design the width and thickness of the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide in the lithium niobate thin film phase modulator to achieve a high polarization extinction ratio.

[0017] First, the polarization extinction ratio formula is: PER = -10log(E TE / E TM )

[0018] Where PER is the defined polarization extinction ratio; E TE is the energy of the transverse electric polarization wave, that is, the transverse electric field component and the longitudinal magnetic field component, which is equivalent to TE polarization; E TM is the energy of the transverse magnetic polarization wave, that is, the longitudinal electric field component and the transverse magnetic field component, which is equivalent to TM polarization.

[0019] From this formula, it is necessary to reduce the loss of TE polarization mode light and increase the loss of TM polarization mode light in order to achieve a high polarization extinction ratio of the lithium niobate thin film phase modulator.

[0020] The characteristic equation of the mode light transmitted in the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide is:

[0021]

[0022]

[0023] Where n1 is the refractive index of the lithium niobate thin film waveguide core layer, w(h) is the equivalent width of the lithium niobate ridge layer considering the influence of the thickness h of the complete structure of the ridge waveguide core layer, k0 is the wave number of the light of a specific frequency in vacuum, θ i is the angle between the light propagating in the lithium niobate thin film optical waveguide core layer and the upper and lower surfaces of the ridge optical waveguide core layer, n2 is the refractive index of the silicon dioxide layer, F is the equivalent function of the right-hand side of the TE characteristic equation, and F′ is the equivalent function of the right-hand side of the TM characteristic equation.

[0024] Therefore, the solution of the characteristic equation is tan(n1w(h)k0cosθ i ) and F(w(h)).

[0025] The principle of the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide polarization is as follows: first, a set of minimum real number solutions are obtained according to the TE mode light characteristic equation to obtain the corresponding waveguide width and thickness. The structural design that satisfies this set of waveguide width and thickness can enable the TE mode light fundamental mode to be transmitted;

[0026] Then, this set of real number solutions is substituted into the TM mode light characteristic equation. When it is smaller than a set of minimum real number solutions obtained in the TM mode light characteristic equation, the TM mode light cannot propagate normally in this set of waveguide widths and thicknesses, thus achieving the polarization effect.

[0027] III), the effective refractive index and the bending radius of the curved gradient directional coupling unit waveguide are designed to achieve polarization beam splitting; and the width and center spacing of the waveguide are designed to improve the coupling power conversion efficiency and thus optimize the polarization beam splitting effect.

[0028] The lithium niobate ridge layer adopts two parallel curved circular arc waveguides as directional coupling units, the two circular arcs are concentric, the widths of the two circular arc ridge waveguides are different and gradually change, and the lithium niobate flat plate layer is below the two lithium niobate ridge waveguides, and the flat plate layers are connected to form a waveguide extending along the ridge layer waveguide.

[0029] The phase matching condition formula is: n I R I = n II R II .

[0030] Where n I is the effective refractive index of waveguide I, R I is the bending radius of waveguide I, n II is the effective refractive index of waveguide II, and R II is the bending radius of waveguide II.

[0031] It can be seen that the curved circular arc waveguide considers both the effective refractive index and the bending radius when calculating the phase matching condition, so it has greater tolerance, and the curved circular arc waveguide can shorten the overall required coupling length compared with the straight waveguide.

[0032] The effective refractive index and the bending radius of waveguide I and waveguide II are selected to satisfy the TM mode light phase matching condition:

[0033] n ITM R ITM = n IITM R IITM

[0034] Where n ITM is the TM mode light effective refractive index of waveguide I, R ITM is the bending radius of waveguide I, n IITM is the TM mode light effective refractive index of waveguide II, and R IITM is the bending radius of waveguide II.

[0035] Then the bending radius is brought into the TE mode light phase matching formula, and it is verified that the TE mode light phase matching condition is not satisfied: n ITE R ITM ≠ n IITE R IITM , so that the TM mode light can be directionally coupled into waveguide II from waveguide I, while the TE mode light cannot be directionally coupled and remains to propagate in waveguide I, achieving polarization beam splitting.

[0036] For lithium niobate thin film material, since it is an anisotropic material and its birefringence ordinary light refractive index n o and the extraordinary refractive index n e The difference is large. The effective refractive index of waveguide I and waveguide II will change with the change of bending angle. The formula is as follows:

[0037]

[0038] where n eff is the effective refractive index at the current position, θ j is the angle between the current propagation direction and the initial propagation direction, w0 is the width of the reference lithium niobate ridge waveguide, and w j is the width of the lithium niobate ridge waveguide at the current position.

[0039] From the above formula, we can see that the phase matching condition of TM mode light is always satisfied. When the bending radius remains unchanged, if and only if w j With θ j Change together, have So that n I With n II Remain unchanged.

[0040] The center distance between the waveguide I and the waveguide II affects the coupling power conversion efficiency, that is, the maximum coupling conversion rate of TM mode light from waveguide I to waveguide II and the shortest distance required to achieve the maximum coupling conversion rate.

[0041] For a directional coupler that meets the phase matching conditions, we have:

[0042]

[0043] Where η is the coupling power conversion efficiency, κ0 is the coupling coefficient under the reference spacing s0, which is usually the minimum allowable spacing, L is the coupling length, γ is the attenuation coefficient, which is determined by the waveguide mode field distribution, and s is the center-to-center distance between the two waveguides.

[0044] The TM mode optical coupling power conversion efficiency is determined by taking a set of coupling lengths L and center spacings s so that the coupling power conversion efficiency η(s) in the above formula reaches a maximum value.

[0045] IV) designing the length and width of the multimode interference region of the polarization-specific multimode interference coupler unit waveguide to provide a polarization effect;

[0046] The polarization-specific multimode interference coupler unit adopts a one-to-two structure with a splitting ratio close to 1:1, and its input waveguide and output waveguide are symmetrical up and down about the center line of the multimode waveguide to ensure a 1:1 ratio.

[0047] The polarization-specific multimode interference coupler satisfies the relationship

[0048]

[0049] where L MMI is the length of the multimode interference region, λ0 is the light wavelength, W MMI is the width of the multimode interference region; for TE mode light, σ = 0, and for TM mode light, σ = 1.

[0050] First, considering the low-loss transmission of TE mode light, σ = 0 is brought in, and the solution of L MMI and W MMI is obtained. Based on the obtained solution of W MMI , the right side of the formula is brought in, and σ = 1 is brought in, and it is verified that the obtained value is not equal to L MMI , and the more the difference, the greater the TM mode leakage in the waveguide, and the greater the optical loss.

[0051] V), the bending radius of the curved beam splitting unit waveguide is designed to have a polarizing effect;

[0052] The curved beam splitting unit is composed of two S-shaped curved arc waveguides, which fully separate the 2 beams obtained by the multimode interference coupler unit for subsequent modulation, and on the other hand, when the waveguide is bent, if the phase velocity of the guided mode exceeds the speed of light in the cladding, energy will be leaked in the form of radiation, satisfying the relationship:

[0053]

[0054] where, α TE / TM is a radiation loss coefficient, C TE / TM is an amplitude factor, which is related to the waveguide material, polarization mode and wavelength, γ TE / TM is an attenuation constant determined by the waveguide mode field distribution, and R is the bending radius, and C TM > C TE .

[0055] The curved beam splitting unit waveguide, because the amplitude factor of TM mode light is larger, under the same bending radius and attenuation constant conditions, the TM mode light leakage loss will be greater than the TE mode light leakage loss. By optimizing the bending radius, the polarizing effect is achieved. Considering the low-loss transmission of TE mode light in the curved beam splitting unit and the high-loss radiation leakage of TM mode light in the curved beam splitting unit, the bending radius R is selected and substituted into the formula, so that α TE << α TM .

[0056] VI), the modulation arm unit is designed to have a phase modulation function

[0057] The modulation arm unit consists of two straight waveguides and coplanar modulation gold electrodes located on both sides of the straight waveguides. The width of the two straight waveguides should meet the single-mode transmission conditions to avoid the generation of high-order mode light that affects the modulation and polarization effects.

[0058] The specific working principle of the lithium niobate thin film phase modulator with a high polarization extinction ratio is as follows:

[0059] The light emitted by the light source enters the lithium niobate thin film chip through the TE fundamental mode double-layer tapered gradient input waveguide. Subsequently, the light is polarized and split by the curved gradient directional coupling unit, and then enters the polarization-specialized multimode interference coupler unit and the curved beam splitting unit to achieve a one-to-two 1:1 splitting, generating two identical beams of light that enter the two modulation arm units respectively. At the same time, the four modulation electrodes generate an electric field. Due to the linear electro-optic effect of lithium niobate, the refractive index of lithium niobate will change under the action of the external electric field, which causes the phase of the light transmitted in the two modulation arms to change respectively, thereby outputting two beams of light with a certain phase difference, and finally outputting them through the TE fundamental mode double-layer tapered gradient output waveguide.

[0060] The phase calculation formula is:

[0061] in is the phase change, Δn eff is the effective refractive index change of lithium niobate, l el is the modulation electrode length.

[0062] Compared with the prior art, the present invention has the following beneficial effects:

[0063] The present invention discloses a design method for a lithium niobate thin film phase modulator with a high polarization extinction ratio. The method is based on lithium niobate thin film materials, fully utilizes the characteristics of lithium niobate materials, combines the basic structures of various parts of the lithium niobate thin film modulator with a high polarization design, and realizes the two basic functions of splitting and modulating phase. In addition, the method can further achieve low loss and a high polarization extinction ratio, and is applicable to fields requiring high polarization extinction ratio modulators. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 This is a flow chart of a design method for a lithium niobate thin film phase modulator with a high polarization extinction ratio according to the present invention;

[0065] Figure 2 A side view of the structure of a lithium niobate thin film phase modulator chip with a high polarization extinction ratio according to an embodiment of the present invention;

[0066] Figure 3 This is a cross-sectional diagram of an input port in an embodiment of the present invention;

[0067] Figure 4 Schematic cross-sectional view of a modulation arm in an embodiment of the present invention;

[0068] Figure 5 Schematic diagram of the intersection point solution of the characteristic equation of the transmittable mode light in an embodiment of the present invention;

[0069] Figure 6 1 is a light field transmission diagram of TM mode light in a directional coupler unit according to an embodiment of the present invention;

[0070] Figure 7 1 is a light field transmission diagram of TE mode light in a directional coupler unit according to an embodiment of the present invention;

[0071] Figure 8 1 is a light field transmission diagram of TE mode light in a multimode interference coupler unit in an embodiment of the present invention;

[0072] Figure 9 1. This is a diagram of the optical field transmission of TM mode light in a multimode interference coupler unit and a subsequent tapered waveguide in an embodiment of the present invention;

[0073] Figure 10 1 and 2 are light field transmission diagrams of TE and TM mode light in the first bending unit, respectively, in an embodiment of the present invention.

[0074] In the figure: 1-substrate, 2-first silicon dioxide layer, 3-lithium niobate thin film flat layer, 4-lithium niobate thin film ridge layer, 5-second silicon dioxide layer, 6-first electrode, 7-second electrode, 8-third electrode, 9-fourth electrode, 11-double-layer tapered gradient input waveguide, 12-directional coupler unit waveguide I, 13-directional coupler unit waveguide II, 14-multimode interference coupler unit waveguide, 15-first curved beam splitting unit waveguide, 16-second curved beam splitting unit waveguide, 17-first modulation arm unit, 18-second modulation arm unit, 19-first tapered gradient output waveguide, 20-second tapered gradient output waveguide.

[0075] In order to more clearly illustrate the core structure of the present invention, Figure 2 The second silicon dioxide layer 5 is made transparent to better show the lithium niobate thin film ridge waveguide structure. DETAILED DESCRIPTION

[0076] The following is a complete and detailed description of the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are also within the scope of protection of the present invention.

[0077] The present invention provides a design method for a lithium niobate thin-film phase modulator with a high polarization extinction ratio, particularly suitable for applications with stringent requirements for phase stability and polarization preservation, such as interferometric fiber gyroscopes. The input optical signal is first efficiently coupled into the chip via a TE fundamental mode double-layer conical spot converter. The optical signal then enters a directional coupling unit: the curved gradient directional coupling unit separates the input polarization light, allowing only high-purity TE polarization light to pass through and enter the subsequent polarization-specific multimode interference coupler unit; the polarization-specific multimode interference coupler unit stably divides the TE polarization light into two equal paths. These two light paths are respectively injected into the two modulation arms of the modulator. Under the influence of the modulation voltage applied by the push-pull electrode, the refractive index of the light in the two arms changes in opposite directions due to the linear electro-optic effect of lithium niobate, thereby introducing a controllable phase difference. Finally, the two modulated light paths enter the optical fiber ring at the modulator output. This design method achieves polarization, beam splitting, and modulation functions through innovative device design and coordinated integration.

[0078] The design method of the lithium niobate thin film phase modulator with high polarization extinction ratio is as follows: Figure 1 Specifically, it includes the following:

[0079] I) The design includes a silicon substrate, a silicon dioxide cladding, a lithium niobate thin film optical waveguide core layer, and a lithium niobate thin film phase modulator of a modulation electrode.

[0080] like Figure 2 As shown, the silicon substrate is located at the bottom layer of the entire phase modulator and plays a supporting and protective role.

[0081] The silica cladding has a double-layer structure, including a first silica layer and a second silica layer. Taking advantage of the fact that silica has a lower refractive index than lithium niobate, thus satisfying the total reflection condition, the silica cladding is respectively covered on the upper and lower layers of the lithium niobate optical waveguide layer. The entire structure is located on the silicon substrate layer, serving as the cladding of the lithium niobate optical waveguide.

[0082] The lithium niobate thin film optical waveguide core layer is as follows Figure 3 As shown, it includes a lithium niobate slab layer and a lithium niobate ridge layer. The lithium niobate ridge layer is located on the lithium niobate slab layer. The two together constitute the complete structure of the ridge optical waveguide core layer, which transmits light and ensures the high quality of the transmitted light field.

[0083] The lithium niobate thin-film optical waveguide core consists of a TE fundamental mode double-layer tapered gradient input waveguide, two curved gradient directional coupler unit waveguides, a polarization-specific multimode interference coupler unit waveguide, two curved beam splitter unit waveguides, two modulation arm units, and two TE fundamental mode double-layer tapered gradient output waveguides, achieving polarization, beam splitting, and modulation functions. The TE fundamental mode double-layer tapered gradient input and output waveguides both meet the TE mode optical fundamental mode transmission conditions.

[0084] The TE fundamental mode double-layer tapered gradient input waveguide transitions from a narrow to a wide single-layer structure to a double-layer structure, wherein the single-layer structure is a lithium niobate flat layer, and the double-layer structure is a complete ridge optical waveguide core layer structure composed of a lithium niobate flat layer and a lithium niobate ridge layer; the narrow end of the TE fundamental mode double-layer tapered gradient input waveguide receives the light output by the light source, and the wide end is connected to the front end of the curved gradient directional coupling unit waveguide I; the TE mode light continues to transmit through the curved gradient directional coupling unit waveguide I, and the TM mode light is directionally coupled into the curved gradient directional coupling unit waveguide II and then output from the chip; the rear end of the curved gradient directional coupling unit waveguide I is connected to one end of the polarization-specific multimode interference coupler unit waveguide, and the other end of the polarization-specific multimode interference coupler unit waveguide is connected to the front ends of two curved beam splitting unit waveguides; the rear ends of the two curved beam splitting unit waveguides are respectively connected to two modulation arm units, and the two modulation arm units are respectively located in the middle of the four modulation electrodes, such as Figure 4 As shown, the two modulation arm units are respectively connected to the wide ends of the two TE fundamental mode double-layer tapered gradient output waveguides, and the narrow ends of the two TE fundamental mode double-layer tapered gradient output waveguides output light.

[0085] II) Design the width and thickness of the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide in the lithium niobate thin film phase modulator to achieve a high polarization extinction ratio.

[0086] First, the polarization extinction ratio formula is: PER = -10log(E TE / E TM )

[0087] Where PER is the defined polarization extinction ratio in dB; E TE is the energy of the transverse electric polarization wave, that is, the transverse electric field component and the longitudinal magnetic field component, which is equivalent to TE polarization; E TM is the energy of the transverse magnetic polarization wave, that is, the longitudinal electric field component and the transverse magnetic field component, which is equivalent to TM polarization.

[0088] From the analysis of this formula: To realize the lithium niobate thin film phase modulator with a high polarization extinction ratio structure design, it is necessary to reduce the loss of TE polarization mode light while increasing the loss of TM polarization mode light.

[0089] The characteristic equation of the mode light transmitted in the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide is:

[0090]

[0091]

[0092] Where n1 is the refractive index of the lithium niobate thin film waveguide core layer, w(h) is the equivalent width of the lithium niobate ridge layer considering the influence of the thickness h of the complete structure of the ridge waveguide core layer, k0 is the wave number of the light of a specific frequency in vacuum, θi is the angle between the light propagating in the lithium niobate thin film optical waveguide core layer and the upper and lower surfaces of the ridge optical waveguide core layer, n2 is the refractive index of the silicon dioxide layer, F is the equivalent function of the right-hand side of the TE characteristic equation, and F′ is the equivalent function of the right-hand side of the TM characteristic equation.

[0093] Therefore, the solution of the characteristic equation is tan(n1w(h)k0cosθ i ) and F(w(h)).

[0094] The principle of the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide polarization is as follows: first, a set of minimum real number solutions are obtained according to the TE mode light characteristic equation to obtain the corresponding waveguide width and thickness. The structural design that satisfies this set of waveguide width and thickness can enable the TE mode light fundamental mode to be transmitted;

[0095] Then, this set of real number solutions is substituted into the TM mode light characteristic equation. When it is smaller than a set of minimum real number solutions obtained in the TM mode light characteristic equation, the TM mode light cannot propagate normally in this set of waveguide widths and thicknesses, thus achieving the polarization effect.

[0096] III) Design the effective refractive index and bending radius of the curved gradient directional coupling unit waveguide to achieve polarization splitting; and design the width and center spacing of the waveguide to improve the coupling power conversion efficiency and thus optimize the polarization splitting effect;

[0097] The curved tapered directional coupling unit takes into account the problem that common directional couplers use a straight waveguide structure design, resulting in small tolerance and a long overall required coupling length. The lithium niobate ridge layer uses two parallel curved arc waveguides as the directional coupling unit. The two arcs are concentric, and the two arc ridge waveguides have different and tapered widths. Below the two lithium niobate ridge waveguides is a lithium niobate slab layer, which is connected to form a waveguide that extends along the ridge waveguide.

[0098] The phase matching condition formula is: n I R I =n II R II ;

[0099] where n I is the effective refractive index of waveguide I, R I is the bending radius of waveguide I, n II is the effective refractive index of waveguide II, R II is the bending radius of waveguide II;

[0100] Compared with the straight waveguide directional coupler, the two coupled waveguides have infinite bending radius, so only the effective refractive index is considered when calculating the phase matching condition. The curved arc waveguide considers both the effective refractive index and the bending radius when calculating the phase matching condition, so it has a larger tolerance. At the same time, the curved arc waveguide can shorten the overall required coupling length compared to the straight waveguide.

[0101] The two parallel curved arc waveguides of the lithium niobate ridge layer are defined as waveguide I and waveguide II, respectively. The effective refractive index and bending radius of waveguide I and waveguide II are selected so as to satisfy the TM mode light phase matching condition:

[0102] n ITM R ITM =n IITM R IITM

[0103] where n ITM is the effective refractive index of the TM mode light in waveguide I, R ITM is the bending radius of waveguide I, where n IITM is the effective refractive index of TM mode light in waveguide II, R IITM is the bending radius of waveguide II.

[0104] Then, the bending radius is substituted into the TE mode optical phase matching formula to verify that the TE mode optical phase matching condition is not met: n ITE R ITM ≠n IITE R IITM , so that the TM mode light can be directionally coupled from waveguide I into waveguide II, while the TE mode light cannot be directionally coupled and remains propagating in waveguide I, realizing polarization splitting.

[0105] For lithium niobate thin film material, since it is an anisotropic material and its birefringence ordinary light refractive index n o and the extraordinary refractive index n e The difference is large. The effective refractive index of waveguide I and waveguide II will change with the change of bending angle. The formula is as follows:

[0106]

[0107] where n eff is the effective refractive index at the current position, θ j is the angle between the current propagation direction and the initial propagation direction, w0 is the width of the reference lithium niobate ridge waveguide, and w j is the width of the lithium niobate ridge waveguide at the current position.

[0108] From the above formula, we can see that the phase matching condition of TM mode light is always satisfied. When the bending radius remains unchanged, if and only if w j With θj Change together, have So that n I With n II Remain unchanged.

[0109] The center distance between the waveguide I and the waveguide II affects the coupling power conversion efficiency, that is, the maximum coupling conversion rate of TM mode light from waveguide I to waveguide II and the shortest distance required to achieve the maximum coupling conversion rate.

[0110] For a perfectly matched directional coupler, we have:

[0111]

[0112] Where η is the coupling power conversion efficiency, κ0 is the coupling coefficient under the reference spacing s0, which is usually the minimum allowable spacing, L is the coupling length, γ is the attenuation coefficient, which is determined by the waveguide mode field distribution, and s is the center-to-center distance between the two waveguides.

[0113] The TM mode optical coupling power conversion efficiency is determined by taking a set of coupling lengths L and center spacings s so that the coupling power conversion efficiency η(s) in the above formula reaches a maximum value.

[0114] IV) designing the length and width of the multimode interference region of the polarization-specific multimode interference coupler unit waveguide to provide a polarization effect;

[0115] The polarization-specific multimode interference coupler unit adopts a one-to-two structure with a splitting ratio close to 1:1, and its input waveguide and output waveguide are symmetrical up and down about the center line of the multimode waveguide to ensure a 1:1 ratio.

[0116] The polarization-specific multimode interference coupler satisfies the relationship

[0117]

[0118] Among them L MMI is the length of the multimode interference region, λ0 is the wavelength of light, W MMI is the width of the multimode interference region; for TE mode light, σ = 0, for TM mode light, σ = 1.

[0119] First, consider the low-loss transmission of TE mode light, substitute σ = 0, and find L MMI With W MMI The solution of W MMI Substitute the solution into the right side of the formula and insert σ=1 to verify that the obtained value is not equal to L MMI , and the greater the phase difference, the greater the TM mode leakage in the waveguide and the greater the optical loss.

[0120] The polarization-specific multimode interference coupler unit is designed specifically for TE mode light, enabling it to achieve a one-to-two 1:1 splitting with an excellent splitting ratio and transmittance. Simultaneously, the design prevents TM mode light from satisfying the multimode interference coupler equation, resulting in leakage loss in the multimode interference coupler unit and subsequent waveguides, thereby achieving a polarization effect.

[0121] V) Designing the bending radius of the curved beam splitting unit waveguide to have a polarizing effect;

[0122] The curved beam splitting unit is composed of two S-shaped curved waveguides. On the one hand, it fully separates the two splits obtained by the multimode interference coupler unit for subsequent modulation. On the other hand, when the waveguide is bent, if the phase velocity of the guided mode exceeds the speed of light in the cladding, it will cause energy to leak in the form of radiation, satisfying the relationship:

[0123]

[0124] Among them, α TE / TM is the radiation loss coefficient, C TE / TM is the amplitude factor, which is related to the waveguide material, polarization mode, and wavelength. TE / TM is the attenuation constant, which is determined by the waveguide mode field distribution, R is the bending radius, and C TM >C TE .

[0125] In the curved beam splitting unit waveguide, since the TM mode light amplitude factor is larger, under the same bending radius and attenuation constant conditions, the TM mode light leakage loss will be greater than the TE mode light leakage loss. By optimizing the bending radius, the polarization effect is achieved. Considering the low-loss transmission of TE mode light in the curved beam splitting unit and the high-loss radiation leakage of TM mode light in the curved beam splitting unit, the bending radius R is selected and substituted into the formula so that α TE <<α TM .

[0126] VI) Design the modulation arm unit to have phase modulation function

[0127] The modulation arm unit consists of two straight waveguides and coplanar modulation gold electrodes located on both sides of the straight waveguides. The width of the two straight waveguides should meet the single-mode transmission conditions to avoid the generation of high-order mode light that affects the modulation and polarization effects.

[0128] The specific working principle of the lithium niobate thin film phase modulator with a high polarization extinction ratio is as follows:

[0129] The light emitted by the light source enters the lithium niobate thin film chip through a double-layer tapered gradient input waveguide. After polarization splitting by the directional coupler unit, the light enters the multimode interference coupler unit and the bending unit to achieve a one-to-two 1:1 splitting, generating two identical beams of light that enter the two modulation arm units respectively. At the same time, the four modulation electrodes generate an electric field. Due to the linear electro-optical effect of lithium niobate, the refractive index of lithium niobate will change under the action of the external electric field, which causes the phase of the light transmitted in the two modulation arms to change respectively, thereby outputting two beams of light with a certain phase difference, and finally outputting them through the double-layer tapered gradient output waveguide.

[0130] The phase calculation formula is:

[0131] in is the phase change, Δn eff is the effective refractive index change of lithium niobate, l el is the modulation electrode length.

[0132] Example:

[0133] The specific manufacturing process of the lithium niobate thin film phase modulator with a high polarization extinction ratio is as follows:

[0134] Step 1: Determine a double-layer conical spot converter that is beneficial to spot conversion and polarization;

[0135] The characteristic equation of the transmittable mode light of the double-layer tapered gradient input waveguide and output waveguide is:

[0136] TE:2n1w(h)k0cosθ i -δ I -δ II =mπ

[0137] TM:2n1w(h)k0cosθ i -δ I ′-δ II ′=mπ

[0138] where θ i is the angle between the light propagating in the lithium niobate thin film optical waveguide core layer and the upper and lower surfaces of the ridge optical waveguide core layer, m = 0, 1, 2,….

[0139] Also because

[0140]

[0141]

[0142]

[0143]

[0144] Where n2 is the refractive index of the first silicon dioxide layer, which overlies the lithium niobate optical waveguide layer; n3 is the refractive index of the second silicon dioxide layer, which overlies the second lithium niobate optical waveguide layer. In this example, the first and second silicon dioxide layers are made of the same refractive index material, so n2 = n3, simplifying the above equation.

[0145] The above formula can also be obtained by the scalar Helmholtz equation:

[0146]

[0147] in is the transverse Laplace operator, ψ(x, y) is the transverse component distribution function of the electric or magnetic field, n(x, y) is the two-dimensional refractive index distribution function, which describes the spatial variation of the ridge, slab layer, substrate, and cover layer, and β is the propagation constant.

[0148] Therefore, the characteristic equation can be rewritten as:

[0149]

[0150]

[0151] Finally, we get:

[0152]

[0153]

[0154] Therefore, the solution of the characteristic equation is tan(n1w(h)k0cosθ i ) and F(w(h)), such as Figure 5 shown.

[0155] The polarization of the double-layer tapered gradient input waveguide and output waveguide is achieved by selecting a solution in which TE mode light can be transmitted normally but TM mode light cannot be transmitted, and limiting the width and thickness of the double-layer tapered gradient waveguide to achieve the polarization effect.

[0156] The double-layer tapered spot mode converter is designed to address the mode field mismatch challenge between lithium niobate thin-film waveguides and standard optical fibers and achieve polarization. This converter achieves efficient optical mode conversion through an optimized adiabatic tapered structure, minimizing end-face coupling losses. Geometric parameters are systematically optimized using the characteristic equation of the transmissive mode light and finite-difference eigenmode simulation. The reduced coupling width promotes efficient diffusion of the optical mode field into the waveguide's upper cladding, achieving optimal mode matching between the TFLN waveguide and the optical fiber. Polarization selection is also achieved by limiting the width. This optimized design achieves high coupling efficiency and a high polarization extinction ratio.

[0157] Optimally, the thickness of the lithium niobate film flat layer is 200±50nm, and the thickness of the lithium niobate film ridge layer is 200±50nm. The double-layer conical spot converter is divided into two sections. The first section consists of only the lithium niobate film flat layer, with a width varying from 250±50nm to 1500±100nm. The second section contains both the lithium niobate film flat layer and the ridge layer. The flat layer width varies from 1500±100nm to 4000±200nm, and the ridge layer width varies from 250±50nm to 700±50nm.

[0158] Step 2: Determine a directional coupler that is conducive to polarization;

[0159] The lithium niobate ridge layer of the directional coupler unit is composed of two parallel curved arc waveguides, the two arcs are concentric, and the widths of the two arc ridge waveguides are different and gradually change; under the two lithium niobate ridge waveguides is a lithium niobate flat layer, and the flat layers are connected to form an extension along the ridge waveguide.

[0160] The two parallel curved arc waveguides of the lithium niobate ridge layer are defined as waveguide I and waveguide II, respectively. While satisfying the phase matching condition of TM mode light, the two waveguides do not satisfy the phase matching condition of TE mode light, thereby enabling TM mode light to be directionally coupled from waveguide I into waveguide II, while the TE mode light cannot be directionally coupled and remains propagating in waveguide I, thereby realizing polarization splitting.

[0161] For lithium niobate thin film material, since it is an anisotropic material and its birefringence ordinary light refractive index n o and the extraordinary refractive index n e The difference is large. The effective refractive index of waveguide I and waveguide II will change with the change of bending angle. The formula is as follows:

[0162]

[0163] where n eff is the effective refractive index at the current position, θ j is the angle between the current propagation direction and the initial propagation direction, w0 is the width of the reference lithium niobate ridge waveguide, and w j is the width of the lithium niobate ridge waveguide at the current position.

[0164] From the above formula, we can see that the TM mode light phase matching condition is always established if and only if w j With θ j Change together, so that n I With n II At the same time, in order to make the difference between the left and right terms of the TE mode light phase matching condition as large as possible, w should be carefully selected. j The numerical value of .

[0165] The center distance between waveguide I and waveguide II affects the coupling power conversion efficiency, that is, the maximum coupling conversion rate of TM mode light from waveguide I to waveguide II and the shortest distance required to achieve the maximum coupling conversion rate. For a fully matched directional coupler, there are:

[0166]

[0167] Where η is the coupling power conversion efficiency, κ0 is the coupling coefficient under the reference spacing s0, which is usually the minimum allowable spacing, L is the coupling length, γ is the attenuation coefficient, which is determined by the waveguide mode field distribution, and s is the center-to-center distance between the two waveguides.

[0168] The center distance between waveguide I and waveguide II should theoretically be as small as possible. However, if the center distance is too small, TE mode light that is not desired to be directional coupled may also be coupled from waveguide I into waveguide II. Therefore, the optimal value should be carefully selected.

[0169] The directional coupler unit is used to efficiently separate TE and TM polarized light. A specific waveguide width is designed using a formula to achieve refractive index matching, completing the conversion of TM mode light from waveguide I to waveguide II. Finite-difference time-domain simulations verify its excellent performance.

[0170] Preferably, the thickness of the lithium niobate film planar layer is 200±50nm, and the thickness of the lithium niobate film ridge layer is 200±50nm. The ridge layer width of waveguide I is varied from 900±50nm to 800±50nm, and the ridge layer width of waveguide II is varied from 1500±50nm to 1400±50nm. The center-to-center distance between the two waveguides is 900±100nm.

[0171] The directional coupler unit has been optimized in parameters, and the TE mode transmission efficiency exceeds 98%, the TM mode suppression is less than 0.2%, the polarization extinction ratio is as high as 28dB, the on-chip loss is 0.1dB, and the light field is as Figure 6 and Figure 7 shown.

[0172] Step 3: Determine the multimode interference coupler and curved waveguide unit that are beneficial for beam splitting and polarization

[0173] The multimode interference coupler unit adopts a one-to-two structure with a splitting ratio close to 1:1, and its input waveguide and output waveguide are symmetrical up and down about the center line of the multimode waveguide to ensure 1:1 splitting.

[0174] The multimode interference coupler satisfies the relationship

[0175]

[0176] Among them L MMI is the length of the multimode interference region, λ0 is the wavelength of light, WMMI is the width of the multimode interference region. For TE mode light, σ = 0, and for TM mode light, σ = 1.

[0177] The multimode interference coupler unit is designed specifically for TE mode light, so that it can achieve one-to-two 1:1 splitting with an excellent splitting ratio and transmittance; at the same time, the design makes the TM mode light not satisfy the multimode interference coupler relationship, thereby reducing leakage loss in the multimode interference coupler unit and subsequent waveguides, achieving a polarization effect, and the light field is as follows Figure 8 and Figure 9 shown.

[0178] The curved beam splitting unit is composed of two S-shaped curved waveguides. On the one hand, it fully separates the two splits obtained by the multimode interference coupler unit for subsequent modulation. On the other hand, when the waveguide is bent, if the phase velocity of the guided mode exceeds the speed of light in the cladding, it will cause energy to leak in the form of radiation, satisfying the relationship:

[0179]

[0180] Among them, α TE / TM is the radiation loss coefficient, C TE / TM is the amplitude factor, which is related to the waveguide material, polarization mode, and wavelength. TE / TM is the attenuation constant, which is determined by the waveguide mode field distribution, R is the bending radius, and C TM >C TE .

[0181] In the bending beam splitting unit, since the TM mode light amplitude factor is larger, under the same bending radius and attenuation constant conditions, the TM mode light leakage loss will be greater than the TE mode light leakage loss. The bending radius can be optimized to achieve the polarization effect, and the light field is as follows Figure 10 shown.

[0182] Preferably, the length of the multimode interference coupling region of the multimode interference coupler unit is 65±5μm, the width is 10±2μm, the width of the tapered ridge waveguide before and after the multimode interference coupler is 0.70±0.05μm to 2.50±0.10μm, the center distance between the two tapered waveguides at the output end of the multimode interference coupler is 5.0±0.2μm, the ridge waveguide width of the curved beam splitting unit is 0.70±0.05μm, and the bending radius is 100±20μm.

[0183] The multimode interference coupler unit and curved waveguide unit achieve stable 1-to-2 optical power splitting and polarization. By formulating and optimizing their geometric parameters using finite-difference time-domain simulations, the optimal design achieves low loss and a precise 50:50 splitting ratio. This structure inherently provides TM mode suppression through the multimode interference coupler and the subsequent controlled leakage mechanisms in the tapered cross section and the curved waveguide, ensuring a high polarization extinction ratio.

[0184] In this embodiment, based on the aforementioned structural dimensions, the high polarization extinction ratio lithium niobate thin-film optical waveguide phase modulator chip has an overall width of 1200 μm, a thickness of 500 μm, and a length of 10 mm. Specific performance parameters obtained through simulation are summarized in the table below. They demonstrate a near 1:1 splitting ratio, a half-wave voltage of less than 3 V, a total on-chip optical loss of 3 dB, and an overall polarization extinction ratio of 45 dB for the first-stage directional coupling unit and 65 dB for the third-stage directional coupling unit.

[0185]

[0186]

[0187] The present invention uses rigorous formula calculations and optical simulations to guide the design, optimize key geometric parameters, and ensure that each functional unit and the overall architecture achieve high performance indicators. It is particularly suitable for the field of integrated optical systems that require ultra-high precision and stability.

[0188] This document describes the principles and implementation methods of the present invention with reference to specific examples. The above examples are intended only to help understand the methods and core concepts of the present invention. For those skilled in the art, variations may occur in the specific implementation and application based on the concepts of the present invention. In summary, this specification should not be construed as limiting the present invention.

Claims

1. A method for designing a lithium niobate thin film phase modulator with a high polarization extinction ratio, characterized in that: The specific steps include: I) Designing a lithium niobate thin film phase modulator including a silicon substrate, a silicon dioxide cladding, a lithium niobate thin film optical waveguide core layer, and a modulation electrode; In the lithium niobate thin film optical waveguide core structure, the TE fundamental mode double-layer tapered gradient input waveguide limits the fundamental mode transmission of TE mode light, its narrow end receives the light output by the light source, and the wide end is connected to the front end of the curved gradient directional coupling unit waveguide I; the TE mode light is polarized and then continues to transmit after being polarized by the directional coupler unit waveguide I, and the TM mode light is directionally coupled into the directional coupler unit waveguide II and then output from the chip; the rear end of the directional coupler unit waveguide I is connected to one end of the polarization-specialized multimode interference coupler unit waveguide, which realizes light splitting and specializes TE mode light transmission, and the other end is connected to the front ends of two curved beam splitting unit waveguides; the two curved beam splitting unit waveguides split light and leak polarization, and the rear ends are respectively connected to two modulation arm units, the two modulation arm units are respectively located in the middle of the four modulation electrodes, and then the two modulation arm units are respectively connected to the wide ends of two TE fundamental mode double-layer tapered gradient output waveguides, and the narrow ends of the two double-layer tapered gradient output waveguides output light; II) Design the width and thickness of the double-layer tapered gradient waveguide and output waveguide of the TE fundamental mode in the lithium niobate thin film phase modulator to limit the polarization of the transmittable light mode and enable it to achieve a high polarization extinction ratio; When a set of minimum real number solutions is obtained for the TE fundamental mode double-layer tapered gradient input waveguide according to the TE mode light characteristic equation, the corresponding waveguide width and thickness are obtained. When the solution is substituted into the TM mode light characteristic equation, it must satisfy the requirement that the solution is less than the minimum real number solution obtained in the TM mode light characteristic equation, thereby achieving the polarization effect; III) Designing the effective refractive index and bending radius of the curved gradient directional coupling unit waveguide to enable polarization by phase matching polarization splitting; and designing the width and center spacing of the waveguide to improve the coupling power conversion efficiency and thus optimize the polarization splitting effect; The phase matching condition means that the product of the effective refractive index of waveguide I and the bending radius of waveguide I is equal to the product of the effective refractive index of waveguide II and the bending radius of waveguide I. When the effective refractive index and bending radius of the two selected waveguides meet the TM mode light phase matching condition, the bending radius at this time is substituted into the TE mode light phase matching formula. If it does not meet the TE mode light phase matching condition and the difference between the left and right terms of the equation is greater than the set threshold, polarization splitting is achieved; IV) Design the length and width of the multimode interference region of the multimode interference coupler unit waveguide, use a one-to-two structure for light splitting, and utilize multimode interference conditions to generate polarization; V) Designing the bending radius of the curved beam splitting unit waveguide to split the light and utilize bending leakage to generate polarization; VI) Design the modulation arm unit to have phase modulation function.

2. The method according to claim 1, wherein: In step I), the silicon substrate is located at the bottom layer of the entire phase modulator, and the silicon dioxide cladding layer is located above the silicon substrate layer. The overall structure is a double-layer structure, including a first silicon dioxide layer and a second silicon dioxide layer, which respectively cover the upper and lower layers of the lithium niobate thin film optical waveguide core layer; The lithium niobate thin film optical waveguide core layer includes a lithium niobate slab layer and a lithium niobate ridge layer. The lithium niobate ridge layer is located on the lithium niobate slab layer, and the two together constitute the complete structure of the ridge optical waveguide core layer; it is divided into: a TE fundamental mode double-layer tapered gradient input waveguide, two curved gradient directional coupling unit waveguides, a polarization-specific multimode interference coupler unit waveguide, two curved beam splitting unit waveguides, two modulation arm units and two TE fundamental mode double-layer tapered gradient output waveguides; the double-layer tapered gradient input waveguide and output waveguide both meet the TE mode optical fundamental mode transmission conditions; The TE fundamental mode double-layer tapered gradient input waveguide transitions from a narrow to a wide single-layer structure to a double-layer structure, wherein the single-layer structure is a lithium niobate flat layer, and the double-layer structure is a complete ridge optical waveguide core layer structure consisting of a lithium niobate flat layer and a lithium niobate ridge layer.

3. The method according to claim 1, wherein: In the step II), achieving a high polarization extinction ratio of the lithium niobate thin film phase modulator requires reducing the loss of TE polarization mode light while increasing the loss of TM polarization mode light; The characteristic equation of the mode light transmitted in the TE fundamental mode double-layer tapered gradient input waveguide and output waveguide is: Where n1 is the refractive index of the lithium niobate thin film waveguide core layer, w(h) is the equivalent width of the lithium niobate ridge layer considering the influence of the thickness h of the complete structure of the ridge waveguide core layer, k0 is the wave number of the light of a specific frequency in vacuum, θ i is the angle between the light propagating in the lithium niobate thin film optical waveguide core layer and the upper and lower surfaces of the ridge optical waveguide core layer; n2 is the refractive index of the first silicon dioxide layer, which covers the upper layer of the lithium niobate optical waveguide layer; F is the equivalent function of the right-hand side of the TE characteristic equation, and F′ is the equivalent function of the right-hand side of the TM characteristic equation; Therefore, the solution of the characteristic equation is tan(n1w(h)k0cosθ i ) and F(w(h)).

4. The method according to claim 1, wherein In step III), the directional coupling unit waveguide is formed by two parallel curved arcs in the lithium niobate ridge layer, the two arcs are concentric, the widths of the two arc ridge waveguides are different and gradually change, and the two lithium niobate ridge waveguides are provided with a lithium niobate slab layer below, and the slab layers are connected to form a waveguide extending along the ridge waveguide; According to the phase matching condition formula, the curved arc waveguide considers both the effective refractive index and the bending radius when calculating the phase matching condition, thus having a larger tolerance. At the same time, the curved arc waveguide can shorten the overall required coupling length compared to the straight waveguide. The phase matching condition for TM mode light is that the product of the effective refractive index of TM mode light of waveguide I and the bending radius of waveguide I is equal to the product of the effective refractive index of TM mode light of waveguide II and the bending radius of waveguide I; For lithium niobate thin film material, since it is an anisotropic material and its birefringence ordinary light refractive index n o and the extraordinary refractive index n e The difference is large. The effective refractive index of waveguide I and waveguide II will change with the change of bending angle. The formula is as follows: where n eff is the effective refractive index at the current position, θ j is the angle between the current propagation direction and the initial propagation direction, w0 is the width of the reference lithium niobate ridge waveguide, and w j is the width of the lithium niobate ridge waveguide at the current position; From the above formula, we can see that the phase matching condition of TM mode light is always satisfied. When the bending radius remains unchanged, if and only if w j With θ j Change together, So that n I With n II Remain unchanged.

5. The method according to claim 4, wherein In step III), the center distance between the waveguide I and the waveguide II affects the coupling power conversion efficiency, that is, the maximum coupling conversion rate of TM mode light from waveguide I to waveguide II and the shortest distance required to achieve the maximum coupling conversion rate; For a curved tapered directional coupling unit that satisfies the phase matching condition, we have: Where η is the coupling power conversion efficiency, κ0 is the coupling coefficient under the reference spacing s0, which is usually the minimum allowable spacing, L is the coupling length, γ is the attenuation coefficient, which is determined by the waveguide mode field distribution, and s is the center-to-center distance between the two waveguides; The TM mode optical coupling power conversion efficiency is determined by taking a set of coupling lengths L and center spacings s so that the coupling power conversion efficiency η(s) in the above formula reaches a maximum value.

6. The method according to claim 1, wherein In step IV), the multimode interference coupler unit adopts a one-to-two structure with a splitting ratio close to 1:1, and its input waveguide and output waveguide are symmetrical about the center line of the multimode waveguide to ensure a 1:1 splitting ratio; The multimode interference coupler satisfies the relationship Among them L MMI is the length of the multimode interference region, λ0 is the wavelength of light, W MMI is the width of the multimode interference region; for TE mode light, σ = 0, for TM mode light, σ = 1; First, consider the low-loss transmission of TE mode light, substitute σ = 0, and find L MMI With W MMI The solution is based on the obtained W MMI Substitute the solution into the right side of the formula and insert σ=1 to verify that the obtained value is not equal to L MMI , and the greater the phase difference, the greater the TM mode leakage in the waveguide and the greater the optical loss.

7. The method according to claim 1, wherein In step V), the curved beam splitting unit is composed of two S-shaped curved waveguides. On the one hand, the two split beams obtained by the multimode interference coupler unit are fully separated to facilitate the subsequent modulation of the spacing of the arm electrode distribution. On the other hand, when the waveguide is bent, if the phase velocity of the guided mode exceeds the speed of light in the cladding, energy will be leaked in the form of radiation, satisfying the relationship: Among them, α TE / TM is the radiation loss coefficient, C TE / TM is the amplitude factor, which is related to the waveguide material, polarization mode, and wavelength. TE / TM is the attenuation constant, which is determined by the waveguide mode field distribution, R is the bending radius, and C TM >C TE ; In the curved beam splitting unit waveguide, since the TM mode light amplitude factor is larger, under the conditions of the same bending radius and attenuation constant, the TM mode light leakage loss will be greater than the TE mode light leakage loss; by optimizing the bending radius, a polarization effect is achieved. Considering the low-loss transmission of TE mode light in the curved beam splitting unit and the high-loss radiation leakage of TM mode light in the curved beam splitting unit, the bending radius R is selected and substituted into the formula so that α TE <<α TM .

8. The method according to claim 1, wherein The modulation arm unit in step VI) is composed of two straight waveguides and coplanar modulation gold electrodes located on both sides of the straight waveguides. The width of the two straight waveguides should meet the single-mode transmission conditions. The modulation electrodes are directly grown on the lithium niobate thin film flat layer, and the modulation electrodes are not connected to the silicon dioxide upper cladding on the left and right sides of the electrodes. The modulation electrodes generate an electric field, causing the phase of the light transmitted in the two modulation arms to change respectively, thereby outputting two beams of light with a certain phase difference.

9. The method according to claim 1, wherein The method is based on a lithium niobate thin film phase modulator device with the following characteristics: Light enters the lithium niobate thin film chip through the TE fundamental mode double-layer tapered gradient input waveguide. Subsequently, after polarization splitting by the curved gradient directional coupling unit, the light enters the polarization-specialized multimode interference coupler unit and the curved beam splitting unit to achieve a one-to-two 1:1 splitting, generating two identical beams of light that enter the two modulation arm units respectively. At the same time, the four modulation electrodes generate an electric field. Due to the linear electro-optic effect of lithium niobate, the refractive index of lithium niobate will change under the action of the external electric field, which causes the phase of the light transmitted in the two modulation arms to change respectively, thereby outputting two beams of light with a certain phase difference, and finally outputting them through the double-layer tapered gradient output waveguide.

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