A lithium niobate thin film modulator connected to an integrated optical gyroscope system

Abstract

The present invention discloses a lithium niobate thin film modulator connected to an integrated optical gyroscope system, belonging to the field of integrated optical gyroscopes; the device comprises a silicon substrate, upper and lower silicon dioxide claddings, an integrated lithium niobate thin film modulator, and a modulation electrode; the transmission optical path is divided into an input coupling optical path, a beam splitting optical path, a modulation optical path, a single-mode output optical path, and an output coupling optical path. The beam splitting optical path waveguide comprises a 1-to-2 multimode interference coupler structure and a curved waveguide structure, which evenly divides the light into two light beams with a power ratio of 1:1 for output; the modulation optical path waveguide comprises a beam splitting-modulation tapered waveguide, a modulation waveguide, and a modulation electrode, which achieves high modulation while having low absorption loss; the single-mode output optical path waveguide comprises a multimode-single-mode tapered waveguide and a single-mode output waveguide, which achieves low-loss single-mode output; the input and output coupling optical paths are designed with a double-layer tapered waveguide structure according to the size of the input and output coupling mode spots, which achieves low-loss end-face coupling. The present invention has the characteristics of low loss, integrability, and single-mode output.

Description

Technical Field

[0001] The invention belongs to the field of integrated optical gyroscopes, in particular to a lithium niobate thin film modulator connected to an integrated optical gyroscope system. Background Art

[0002] A gyroscope is an inertial sensor that measures the attitude angle and angular velocity of a vehicle. It is a key component of inertial navigation systems and inertial guidance systems. It is widely used in missile guidance, satellites, submarines, robots, automobiles, aircraft, and smart devices, and is of great significance to national security and national economic development.

[0003] In the PRIGM:AIMS guide published in 2015 by the DARPA Microsystems Technology Office, the U.S. Department of Defense's Advanced Research Projects Agency (DARPA) pointed out that the advantage of integrated optical waveguide gyroscopes is that they have no moving parts. Through optoelectronic integration, they are expected to solve the problems of miniaturization and low cost of fiber optic gyroscopes with their advantages of low cost, small size, light weight and low power consumption, thereby realizing chip-based high-precision autonomous navigation.

[0004] Currently, integrated optical waveguide gyroscopes (IWGs) are still at the basic research stage, both domestically and internationally. Various research institutions' IWG solutions vary in terms of structure, material selection, and processing techniques, and are still in the exploratory phase. Currently, no single material can achieve both ultra-low transmission loss (<0.1dB / m) and high-performance active devices, resulting in the widespread adoption of hybrid integration solutions.

[0005] As a key component of an integrated optical waveguide gyroscope system, the phase modulator integrates functions such as spectroscopic and phase modulation, impacting the modulation and demodulation accuracy of the gyroscope signal. Thanks to its large electro-optic coefficient and wide electromagnetic wave transmission window, lithium niobate (LNbO?) is widely used in electro-optical modulation and has become the most commonly used electro-optical modulator material.

[0006] Currently, the more mature modulators based on lithium niobate single crystal thin films are all used in optical communications with large modulation bandwidth and high modulation data rate. Further research is needed on lithium niobate thin film phase modulators used in the field of integrated optical gyroscopes.

[0007] Based on the above considerations, it is particularly important to study low-loss lithium niobate thin film phase modulators that can be connected to integrated optical gyroscope systems. Summary of the Invention

[0008] In response to the above problems, the present invention provides a lithium niobate thin film phase modulator connected to an integrated optical gyroscope system. The lithium niobate thin film phase modulator is suitable for the field of integrated optical resonant gyroscope technology and has the characteristics of low loss, integration and single-mode output.

[0009] The lithium niobate thin film phase modulator connected to the integrated optical gyroscope system includes a silicon substrate, a silicon dioxide lower cladding layer and a silicon dioxide upper cladding layer are respectively arranged on the silicon substrate from bottom to top, and an integrated etched lithium niobate thin film modulator and a lithium niobate end-face coupled spot converter are arranged between the silicon dioxide upper and lower cladding layers;

[0010] The lithium niobate thin film modulator includes a lithium niobate thin film ridge waveguide and a modulation electrode;

[0011] The lithium niobate thin film ridge waveguide includes beam splitting waveguides of different widths, modulation waveguides and single-mode output waveguides;

[0012] The beam splitting waveguide includes a multimode interference coupler (MMI) beam splitting structure, a bending structure and a beam splitting-modulation tapered waveguide;

[0013] The MMI beam splitting structure splits the input light into two beams with a power ratio of 1:1 and outputs them to a curved structure. The curved structure is used to increase the center-to-center spacing between the two waveguides to match the center-to-center spacing between the two input waveguides of the silicon nitride resonant cavity, a sensitive element in the integrated optical gyroscope system. Then, a beam splitting-modulation tapered waveguide is used to achieve a width transition from the beam splitting waveguide to the modulation waveguide.

[0014] The modulation waveguide is used to achieve optical phase modulation. Voltage is applied to the modulation electrodes on both sides of the modulation waveguide to change the optical phase when light propagates in the waveguide. A multimode-singlemode tapered waveguide is provided between the modulation waveguide and the singlemode output waveguide.

[0015] The single-mode output waveguide is used to ensure that the output light of the lithium niobate thin film modulator maintains a single-mode transmission condition.

[0016] The multimode-singlemode tapered waveguide is used to achieve width transition from a modulation waveguide to a singlemode waveguide.

[0017] The modulation electrodes are used for light modulation and are located on both sides of the modulation waveguide.

[0018] The lithium niobate end-face coupled spot mode converter is a two-layer tapered waveguide structure, including an input-coupled spot mode converter and an output-coupled spot mode converter.

[0019] The input-coupled spot mode converter is located between the input optical fiber and the input end of the lithium niobate thin film modulator, realizing low-loss spot mode conversion of the transmitted light from the large mode field of the optical fiber to the small mode field of the lithium niobate thin film modulator's beam splitting waveguide.

[0020] The single lower tapered waveguide end is connected to the input optical fiber end, and the upper and lower double tapered waveguide ends are connected to the input end of the lithium niobate thin film modulator.

[0021] The output-coupled spot mode converter is located between the output end of the lithium niobate thin film modulator and the silicon nitride resonant cavity input waveguide, realizing low-loss spot mode conversion of the transmitted light from the small mode field of the lithium niobate thin film modulator single-mode waveguide to the large mode field of the silicon nitride resonant cavity input waveguide.

[0022] The single lower tapered waveguide end is connected to the input waveguide end of the silicon nitride resonant cavity; the upper and lower double tapered waveguide ends are connected to the output end of the lithium niobate thin film modulator.

[0023] The working principle of the lithium niobate thin film phase modulator connected to the integrated optical gyroscope system is as follows:

[0024] Light is injected from the input optical fiber, passes through the single lower tapered waveguide end of the input-coupled mode spot converter, is converged by low-loss mode field, and is coupled from the upper and lower double-layer tapered waveguide ends to the input waveguide of the lithium niobate thin film modulator;

[0025] In a lithium niobate thin-film modulator, input light passes through an MMI beam-splitting structure, splitting one beam into two beams with a 1:1 power ratio. The two beams then pass through a curved structure and a beam-splitting-modulation waveguide before entering the modulation waveguide. When a modulation signal is applied to the modulation electrode, the light in the modulation waveguide undergoes a phase change. The modulated light then passes through a multimode-to-single-mode tapered waveguide and enters the single-mode output waveguide. During this process, the multimode light is dissipated into the cladding, ensuring that only the fundamental mode is transmitted in the output waveguide of the lithium niobate thin-film modulator.

[0026] The output light of the lithium niobate thin film modulator enters the upper and lower double-layer tapered waveguide ends of the output coupling mode spot converter, undergoes low-loss mode field expansion, and is coupled from the single lower tapered waveguide end to the input waveguide of the silicon nitride resonant cavity.

[0027] The beam splitting structure is designed, and the multimode interference coupler parameters are constrained based on the guided mode transmission analysis method, namely:

[0028]

[0029]

[0030] Among them, W ev is the effective width of each order guided mode after taking into account the Goos-Hansen shift; W MMI is the width of the multimode interference region; λ0 is the wavelength of light in free space; n co is the refractive index of the core ridge waveguide, n cl is the cladding refractive index; for TE mode, σ=0, for TM mode, σ=1; ΔW is the additional loss width in the approximate calculation; L MMI is the length of the multimode interference region; Δβ1 is the additional loss value of the multimode wave eigenmode propagation constant;

[0031] According to the relationship between the width and length of the multi-mode interference area, the length parameter is obtained by limiting the width parameter to complete the design of the beam splitting structure.

[0032] The modulation performance of the lithium niobate thin film modulator is comprehensively evaluated, and the parameter M is selected to express the relationship between the modulation performance and the modulation waveguide width, namely:

[0033] M=Δn 2 (au)·(1-AL(au))

[0034] Wherein, Δn is the refractive index change in the modulation waveguide, which is used to characterize the modulation efficiency of the modulator. The larger Δn is, the higher the modulation efficiency is. AL represents the absorption loss of the modulation electrode to the transmitted light. The larger AL is, the greater the electrode absorption loss is. For good modulation performance, high modulation efficiency and low absorption loss are required. Δn(au) and AL(au) are used to represent Δn and AL after data normalization, respectively, that is:

[0035]

[0036]

[0037] Among them, F1(w)=Δn represents the relationship between the modulation waveguide width w and the refractive index change Δn in the waveguide, and F2(w)=AL represents the relationship between the modulation waveguide width w and the electrode absorption loss AL. When the electrode parameters are fixed, the parameter M changes with the modulation waveguide width w. The larger the M parameter, the better the modulation performance of the modulator. The modulation waveguide width w with the maximum M parameter is found to achieve the optimal modulation performance.

[0038] The single-mode waveguide is designed based on the effective refractive index method, and the parameters of the single-mode output waveguide are constrained, so that:

[0039]

[0040]

[0041]

[0042] Where n1 and n2 are the refractive indices of the lithium niobate waveguide and the silicon dioxide waveguide, respectively. I and N II are the fundamental mode effective refractive indexes of the inner and outer ridges of the lithium niobate ridge waveguide, respectively. The effective refractive index of the first-order mode of the inner ridge slab waveguide is made smaller than the fundamental mode effective refractive index of the outer ridge slab waveguide, and the first-order mode cutoff is used as the single-mode transmission judgment condition, that is:

[0043]

[0044]

[0045] Wherein, k0 is the wave number in vacuum; H is the inner ridge height of the lithium niobate ridge waveguide; h is the outer ridge height of the lithium niobate ridge waveguide; W is the inner ridge width of the lithium niobate ridge waveguide; by limiting the inner and outer ridge height parameters of the lithium niobate ridge waveguide, the inner ridge width of the lithium niobate ridge waveguide that meets the single-mode transmission conditions is obtained according to the relationship.

[0046] The coupling performance of the lithium niobate end-face coupled spot converter was comprehensively evaluated. The electric field distribution in the optical fiber, lithium niobate film waveguide, and silicon nitride waveguide was measured by E f (x,y),E w (x,y),E n (x,y) means:

[0047]

[0048]

[0049]

[0050] Among them, w f w represents the waist radius of the electric field distribution of the optical fiber in the x and y directions; wx and w wy w represents the waist radius of the electric field distribution of the lithium niobate film waveguide in the x and y directions; nx and w ny represents the waist radius of the electric field distribution of the silicon nitride waveguide in the x and y directions;

[0051] The larger the mode field overlap integral, the higher the coupling efficiency. The mode field overlap integral η1 of the optical fiber and the lithium niobate thin film waveguide and the mode field overlap integral η2 of the silicon nitride waveguide and the lithium niobate thin film waveguide are studied, namely:

[0052]

[0053]

[0054] Among them, F3(w)=f(w wx ,w wy ) represents the width w of the lithium niobate film waveguide and the waist radius w of the optical mode field in the waveguide wx 、w wy When the width w of the lithium niobate film waveguide is less than a certain value, the optical mode field in the waveguide diffuses toward the cladding, and at this time it has a larger waist radius w. wx 、w wy , the optical mode field waist radius w of the optical fiber and silicon nitride waveguide f 、w nx and w nyis a known fixed value; by changing the width of the lithium niobate thin film waveguide, the mode field is diffused toward the cladding, with a larger mode field waist radius, so that the overlap integral reaches the maximum value and the optimal coupling efficiency is achieved.

[0055] Compared with existing products, the advantages of the present invention are:

[0056] 1) The present invention discloses a lithium niobate thin film phase modulator connected to an integrated optical resonant gyroscope system. Based on the requirements of the integrated optical resonant gyroscope system, the transmission waveguide of the lithium niobate thin film phase modulator is configured as three parts: a beam splitting waveguide, a modulation waveguide, and a single-mode output waveguide with different widths. According to the performance requirements of each waveguide part, different waveguide parameters can effectively reduce transmission loss. In particular, based on the resonance mechanism of the integrated optical gyroscope system, a single-mode output waveguide is designed to effectively eliminate multimode noise entering the subsequent resonant cavity, thereby realizing the application of a low-loss lithium niobate thin film modulator connected to the integrated optical resonant gyroscope system.

[0057] 2) The present invention provides a lithium niobate thin film phase modulator connected to an integrated optical resonant gyroscope system. Based on the integrated optical gyroscope system, a mode spot converter coupled to the input optical fiber end face is provided at the input end of the lithium niobate thin film phase modulator, and a mode spot converter coupled to the input waveguide end face of the silicon nitride resonant cavity is provided at the output end, thereby realizing low-loss mode spot conversion of the optical mode field at the input and output ends, solving the problems of mode field size mismatch and excessive coupling loss, so as to meet the integrated performance requirements of the connected integrated optical resonant gyroscope system and further realize the low-loss application of the lithium niobate thin film modulator connected to the integrated optical resonant gyroscope system. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 This is a schematic diagram of the cross-sectional structure of a lithium niobate thin film modulator connected to an integrated optical gyroscope system according to the present invention;

[0059] Figure 2 This is a schematic diagram of the overall optical path structure of a lithium niobate thin film modulator connected to an integrated optical gyroscope system according to the present invention;

[0060] Figure 3 This is a schematic diagram of the waveguide structure of the splitting optical path, modulation optical path, and single output optical path of the lithium niobate thin film modulator of the present invention;

[0061] Figure 4 This is a schematic diagram of the waveguide structure of the spot converter coupled with the input and output ends of the lithium niobate thin film modulator of the present invention;

[0062] Figure 5 This is a normalized data curve diagram showing the effect of the width of the lithium niobate thin film waveguide on the modulation performance of the present invention;

[0063] Figure 6A graph showing the relationship between the width of the lithium niobate film waveguide and the effective refractive index of the light mode in the waveguide according to the present invention;

[0064] Figure 7 A graph showing the relationship between the width of the lithium niobate thin film waveguide and the coupling efficiency of the present invention;

[0065] Among them, 1-silicon substrate, 2-silicon dioxide lower cladding, 3-silicon dioxide upper cladding, 4-modulation electrode, 5-multimode interference coupler (MMI) splitting structure, 6-bend structure, 7-beam splitting-modulation waveguide, 8-modulation waveguide, 9-single-mode output waveguide, 10-multimode-single-mode tapered waveguide, 11-input coupling spot mode converter, 12-output coupling spot mode converter, 13-input optical fiber, 14-silicon nitride resonant cavity input waveguide. DETAILED DESCRIPTION

[0066] 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.

[0067] Currently, the majority of relatively mature lithium niobate thin film modulators are used in the field of optical communications. Their designs are mostly aimed at improving performance such as high bandwidth and high modulation. However, for integrated optical gyroscope systems, low insertion loss is a more important performance. Therefore, based on the application requirements of accessing the integrated optical gyroscope system, the present invention proposes a low-loss lithium niobate thin film phase modulator accessed to the integrated optical gyroscope system. According to the working mechanism of the integrated optical gyroscope system, starting from reducing insertion loss, the functions of each part of the lithium niobate thin film modulator are decomposed. While realizing the splitting and modulation functions, a single-mode output waveguide and an end-face coupled mode spot converter are designed, and transmission loss, coupling loss and multi-mode noise are reduced at the same time. The lithium niobate thin film phase modulator is suitable for the field of integrated optical resonant gyroscope technology, and has the performance of low loss, integration and single-mode output.

[0068] The lithium niobate thin film phase modulator connected to the integrated optical gyroscope system, such as Figure 1 As shown, it includes a silicon substrate 1, a silicon dioxide lower cladding 2 is arranged on the silicon substrate 1, an integrated etched lithium niobate thin film modulator and a lithium niobate end-face coupled spot converter are arranged on the silicon dioxide lower cladding 2, and a silicon dioxide upper cladding 3 is arranged on the lithium niobate thin film modulator and the lithium niobate end-face coupled spot converter.

[0069] The silicon substrate 1 has a thickness of 0.525 mm and plays a supporting and protective role.

[0070] The thickness of the silicon dioxide lower cladding layer 2 is 4.7 μm, and the thickness of the silicon dioxide upper cladding layer 3 is 5 μm, which play the role of buffering and confining the optical mode field.

[0071] The lithium niobate thin film modulator includes a lithium niobate thin film ridge waveguide and a modulation electrode 4 , both of which have a thickness of 0.4 μm.

[0072] The lithium niobate thin film ridge waveguide includes a beam splitting waveguide, a modulation waveguide 8 and a single-mode output waveguide 9.

[0073] The splitting waveguide includes a multimode interference coupler (MMI) splitting structure 5, a bending structure 6 and a width-varying tapered waveguide 7 (splitting-modulation tapered waveguide); a width-varying tapered waveguide 10 (multimode-single-mode tapered waveguide) is arranged between the modulation waveguide 8 and the single-mode output waveguide 9.

[0074] The MMI beam splitting structure 5 is used to split the input light into two beams with a power ratio of 1:1 and output them to the bending structure;

[0075] The MMI beam splitting structure 5 includes an input waveguide, a multimode interference region and two output waveguides, wherein the multimode interference region is 65 μm long and 10 μm wide, the center distance between the two output waveguides of the multimode interference region is 5.2 μm, and the width of the two output waveguides of the beam splitting structure is 1.5 μm.

[0076] The curved structure 6 is used to increase the center distance between the two output waveguides so that the center distance between the two output waveguides of the lithium niobate thin film modulator matches the center distance between the two input waveguides of the silicon nitride resonant cavity, a sensitive element in the integrated optical gyroscope system.

[0077] The gradually tapered waveguide 7 (beam splitting-modulation tapered waveguide) is used to achieve a width transition from the beam splitting waveguide to the modulation waveguide. The waveguide width of the gradually tapered waveguide 7 is 1.5-2.4 μm.

[0078] The modulation waveguide 8 is used to realize optical phase modulation. Voltage is applied to the modulation electrodes on both sides of the waveguide to change the optical phase when light is transmitted in the waveguide. The width of the modulation waveguide 8 is 2.4 μm.

[0079] Comprehensively evaluate the modulation performance of lithium niobate thin film modulator and select the parameter M=Δn 2 (au)·(1-AL(au)) is used to express the relationship between the modulation performance and the modulation waveguide width. Finally, the modulation waveguide width is selected as 2.4 μm according to the fitting curve.

[0080] The single-mode output waveguide 9 is used to ensure that the output light of the lithium niobate thin film modulator maintains a single-mode transmission condition. The width of the single-mode output waveguide 9 is 0.7 μm.

[0081] Among them, most of the currently more mature lithium niobate thin film modulators are multi-mode waveguides. However, in an integrated optical resonator gyroscope, the presence of multiple modes in the waveguide will affect the accuracy of phase modulation and generate multi-mode noise. Therefore, in the present invention, in order to address the multi-mode noise of the integrated optical resonator gyroscope, the output waveguide of the lithium niobate thin film modulator is designed as a single-mode waveguide. According to the single-mode criterion of the effective refractive index method, only the fundamental mode is stably transmitted in the waveguide. When the higher-order mode leaks, the waveguide is a single-mode waveguide at this time. The relationship between the waveguide width w and the effective refractive index of the optical mode in the waveguide is studied. It is found that when the waveguide width w is less than 0.73μm, only the fundamental mode TE0 mode is stably transmitted in the waveguide. Finally, a single-mode waveguide width of 0.7μm is selected.

[0082] The gradually tapered waveguide 10 (multimode-singlemode tapered waveguide) is used to achieve a width transition from a modulation waveguide to a singlemode waveguide. The gradually tapered waveguide 10 has a waveguide width of 2.4-0.7 μm.

[0083] The modulation electrodes 4 are gold electrodes for light modulation and are located on both sides of the modulation waveguide 8. The spacing between the modulation electrodes 4 is 6.2-7.2 μm.

[0084] The lithium niobate end-face coupled spot mode converter includes an input-coupled spot mode converter 11 coupled to an optical fiber and an output-coupled spot mode converter 12 coupled to a silicon nitride waveguide. The lithium niobate end-face coupled spot mode converter has a two-layer tapered waveguide structure, with both upper and lower waveguide layers having a thickness of 0.2 μm.

[0085] The optical fiber-coupled input-coupled spot mode converter 11 is located between the input optical fiber 13 and the input end of the lithium niobate thin film modulator, connects the input optical fiber and the lithium niobate thin film modulator, and realizes low-loss spot mode conversion of the transmitted light from the large mode field of the optical fiber to the small mode field of the lithium niobate thin film modulator beam splitting waveguide.

[0086] The lower tapered waveguide width of the input spot converter 11 coupled to the optical fiber is 0.153-4 μm; the upper tapered waveguide width is 0.15-1.5 μm; the part connected to the input optical fiber has only the lower tapered waveguide with an end face size of 0.153 μm × 0.2 μm, and the part connected to the input end of the lithium niobate thin film modulator is an upper and lower waveguide with an upper end face size of 1.5 μm × 0.2 μm and a lower end face size of 4 μm × 0.2 μm.

[0087] The output spot mode converter 12 coupled to the silicon nitride waveguide is located between the output end of the lithium niobate thin film modulator and the silicon nitride resonant cavity input waveguide 14, connecting the lithium niobate thin film modulator and the silicon nitride resonant cavity, and realizing low-loss spot mode conversion of the transmitted light from the small mode field of the lithium niobate thin film modulator single-mode waveguide to the large mode field of the silicon nitride resonant cavity input waveguide.

[0088] The output spot converter 12 coupled with the silicon nitride waveguide has a lower tapered waveguide width of 4μm-0.225μm; the upper tapered waveguide has a width of 0.7-0.15μm; the output terminal of the lithium niobate thin film modulator is connected to the upper and lower waveguides, the upper end face size is 0.7μm×0.2μm, and the lower end face size is 4μm×0.2μm; the terminal part with the silicon nitride resonant cavity input waveguide has only the lower tapered waveguide, and the end face size is 0.225μm×0.2μm.

[0089] According to the fitting curve, the width of the coupled end face of the lithium niobate thin film spot converter at the optical fiber coupling end is determined to be 0.153 μm, and the width of the coupled end face of the lithium niobate thin film spot converter at the silicon nitride waveguide coupling end is determined to be 0.225 μm.

[0090] like Figure 2 As shown in the optical path diagram of the lithium niobate thin film modulator connected to the integrated optical resonant gyroscope system, the specific working principle is as follows:

[0091] A laser light source with a wavelength of 1550nm is connected to the input end of the lithium niobate thin film modulator through an input optical fiber with a mode field diameter of 6μm, achieving low coupling loss mode conversion.

[0092] The converted light field enters the lithium niobate thin film modulator and first passes through a 1-to-2 multimode interference coupler, splitting the light beam into two beams with a power ratio of 1:1. Low-loss beam splitting is achieved through a subsequent curved waveguide structure. The two beams of light after splitting enter the modulation optical path with the same electrode structure. First, a tapered waveguide with a different waveguide width is used to convert the waveguide width of the splitting optical path into the waveguide width of the modulation optical path with low loss. A modulation signal is applied to the electrodes on both sides of the waveguide, modulating the phase of the two light paths to achieve low-loss modulation.

[0093] The two modulated light beams pass through a multimode-fundamental mode tapered waveguide to dissipate the multimode in the optical waveguide, and ultimately only the fundamental mode is transmitted, achieving low-loss single-mode output; the light in the single-mode waveguide passes through the double-tapered mode spot converter at the output end of the lithium niobate thin film modulator, which converts the small mode field in the lithium niobate ridge waveguide into a large mode field, matching it with the waveguide mode field of the subsequent silicon nitride sensitive element, achieving low-loss coupled mode spot conversion.

[0094] Example:

[0095] The specific design process of the lithium niobate thin film phase modulator connected to the integrated optical resonant gyroscope system is as follows:

[0096] Step 1: Divide the lithium niobate thin film phase modulator connected to the integrated optical resonant gyroscope system;

[0097] The lithium niobate thin film modulator has both beam splitting and modulation functions. The beam splitting function is achieved through a 1-to-2 multimode interference coupler structure, and the modulation function is achieved through gold electrodes placed on both sides of the waveguide.

[0098] Based on the requirements of accessing the integrated optical gyroscope system, the lithium niobate thin film modulator should have single-mode output performance and a mode spot converter for coupling, so as to realize the low-loss access and application of the lithium niobate thin film modulator to the integrated optical gyroscope system.

[0099] Therefore, the lithium niobate thin film modulator is divided into four parts for design, namely the beam splitting optical path, the modulation optical path, the single-mode output optical path, and the coupling optical path. This realizes the beam splitting and modulation functions while reducing transmission loss, coupling loss, and multi-mode noise.

[0100] Step 2: Design the beam splitting path;

[0101] The beam splitting path is as follows Figure 3 As shown, the present invention uses a multimode interference coupler structure to split the input light into two optical outputs with a power ratio of 1:1. The principle is the self-imaging effect, that is, the incident light field is periodically imaged in the multimode waveguide. According to the optical path requirements of phase modulation, a multimode interference coupler structure with symmetrical interference is selected to ensure that the two light paths are transmitted symmetrically along the central axis and the phase changes are consistent. Based on the guided mode transmission analysis method, the multimode interference coupler parameters are constrained, namely:

[0102]

[0103]

[0104] Among them, W ev is the effective width of each order guided mode after taking into account the Goos-Hansen shift; W MMI is the width of the multimode interference region; λ0 is the wavelength of light in free space; n co is the refractive index of the core ridge waveguide, n cl is the cladding refractive index; for TE mode, σ=0, for TM mode, σ=1; ΔW is the additional loss width in the approximate calculation; L MMI is the length of the multimode interference region; Δβ1 is the additional loss value of the multimode wave eigenmode propagation constant;

[0105] According to the relationship between the width and length of the multi-mode interference area, the length parameter is obtained by limiting the width parameter to complete the design of the beam splitting structure.

[0106] Based on a laser light source with a wavelength of 1550nm, the width of the beam splitting waveguide is selected as 1.5μm. According to the width of the beam splitting waveguide, the interference zone width of the 1-to-2 multimode interference coupler is determined to be 10μm. Then, the structural design of the 1-to-2 multimode interference coupler is completed: the length of the multimode interference region is 65μm and the width is 10μm. The center spacing between the two waveguides outputted from the multimode interference region is 5.2μm, and the width of the two waveguides outputted from the beam splitting structure is 1.5μm.

[0107] In order to connect with the subsequent modulated optical path, the two waveguides output by the multimode interference coupler are connected to a waveguide with a curved structure at the rear end. The center spacing between the two output waveguides is adjusted to be equal to the input waveguide of the silicon nitride sensitive unit to achieve hybrid integrated termination. The waveguide width of the curved structure is 1.5μm, the bending radius is 100μm, and the center spacing of the waveguides is 393.6μm.

[0108] Step 3: Design the modulation optical path;

[0109] The overall schematic diagram of the modulation optical path is as follows Figure 3 As shown, the modulation principle of the present invention is based on the linear electro-optic effect of lithium niobate material. Gold electrodes are set on both sides of the optical waveguide propagating in the x-y direction. When a modulation signal is applied to the gold electrodes, the refractive index of the lithium niobate optical waveguide changes by Δn. Therefore, the magnitude of the refractive index change Δn is used to represent the modulation efficiency of the lithium niobate thin film modulator. The larger Δn, the higher the modulation efficiency. With the electrode parameters fixed, the relationship between the modulation waveguide width w and the refractive index change Δn in the waveguide is studied, that is, F1(w) = Δn. At the same time, the relationship between the modulation waveguide width w and the electrode absorption loss AL is studied, that is, F2(w) = AL. For the lithium niobate thin film modulator connected to the integrated optical gyroscope, the absorption loss must be as small as possible while the modulation efficiency is as high as possible. Since the refractive index change Δn and the electrode absorption loss AL have different dimensions, the data is normalized, that is, Δn(au) and AL(au):

[0110]

[0111]

[0112] The modulation performance of the lithium niobate thin film modulator is comprehensively evaluated, and the parameter M is selected to express the relationship between the modulation performance and the modulation waveguide width, namely:

[0113] M=Δn 2 (au)·(1-AL(au))

[0114] When the electrode parameters are fixed, the parameter M changes with the modulation waveguide width w. The larger the M parameter, the better the modulation performance of the modulator. The modulation waveguide width w with the maximum M parameter is found to achieve the optimal modulation performance.

[0115] According to the fitting curve, the modulation waveguide width is selected as 2.4 μm, as shown in Figure 5 shown.

[0116] A tapered gradient waveguide is used between the beam splitting waveguide and the modulation waveguide to achieve low-loss waveguide width conversion. The beam splitting-modulation tapered waveguide has a width of 1.5-2.4μm and a length of 100μm.

[0117] The electrode parameters are fixed, the electrode thickness is 0.4 μm, and the electrode spacing is 6.2-7.2 μm.

[0118] Step 4: Design single-mode output waveguide;

[0119] Schematic diagram of multimode-singlemode tapered waveguide and singlemode output waveguide Figure 3 As shown in the figure, based on the waveguide single-mode condition criterion, that is, only the fundamental mode can be stably transmitted in the waveguide, and the higher-order modes leak into the cladding. The relationship between the waveguide width w and the effective refractive index of the optical mode in the waveguide is studied. Based on the effective refractive index method, the parameters of the single-mode output waveguide are constrained, and the following is set:

[0120]

[0121]

[0122]

[0123] Where n1 and n2 are the refractive indices of the lithium niobate waveguide and the silicon dioxide waveguide, respectively. I and N II are the fundamental mode effective refractive indexes of the inner and outer ridges of the lithium niobate ridge waveguide, respectively. The effective refractive index of the first-order mode of the inner ridge slab waveguide is made smaller than the fundamental mode effective refractive index of the outer ridge slab waveguide, and the first-order mode cutoff is used as the single-mode transmission judgment condition, that is:

[0124]

[0125]

[0126] Wherein, k0 is the wave number in vacuum; H is the inner ridge height of the lithium niobate ridge waveguide; h is the outer ridge height of the lithium niobate ridge waveguide; W is the inner ridge width of the lithium niobate ridge waveguide; by limiting the inner and outer ridge height parameters of the lithium niobate ridge waveguide, the inner ridge width of the lithium niobate ridge waveguide that meets the single-mode transmission conditions is obtained according to the relationship.

[0127] When the waveguide width w is less than 0.73 μm, only the fundamental mode TE0 mode is stably transmitted in the waveguide. Figure 6 As shown in the figure, a multimode-to-singlemode tapered waveguide is designed to achieve the transition of waveguide width from the modulation optical path to the output optical path. The multimode-to-singlemode tapered waveguide has a width of 2.4-0.7μm and a length of 300μm; the singlemode output waveguide has a width of 0.7μm.

[0128] Step 5: Design the end-coupled spot converter

[0129] The present invention designs a spot mode converter coupled to the input optical fiber end face at the input end of the lithium niobate thin film phase modulator, and designs a spot mode converter coupled to the input waveguide end face of the silicon nitride resonant cavity at the output end, thereby realizing low-loss spot mode conversion of the optical mode field at the input and output ends, solving the problems of mode field size mismatch and excessive coupling loss, so as to meet the integrated performance requirements of the integrated optical resonant gyroscope system, and further realizing the low-loss application of the lithium niobate thin film modulator connected to the integrated optical resonant gyroscope system.

[0130] The mode spot converter based on a double-layer tapered waveguide structure achieves the conversion from a small mode spot to a large mode spot by changing the size of the waveguide. Specifically, when the width of the waveguide is less than a certain value, the light in the waveguide cannot be completely confined in the waveguide and diffuses into the cladding, thereby expanding the optical mode field. The mode field overlap integral of the coupled end faces of the two structures coupled by termination determines the coupling efficiency of the two structures. The electric field distribution in the optical fiber, lithium niobate film waveguide, and silicon nitride waveguide is expressed by E f (x,y),E w (x,y),E n (x,y) means:

[0131]

[0132]

[0133]

[0134] Among them, w f w represents the waist radius of the electric field distribution of the optical fiber in the x and y directions; wx and w wy w represents the waist radius of the electric field distribution of the lithium niobate film waveguide in the x and y directions; nx and w ny represents the waist radius of the electric field distribution of the silicon nitride waveguide in the x and y directions;

[0135] The larger the mode field overlap integral, the higher the coupling efficiency. The mode field overlap integral η1 of the optical fiber and the lithium niobate thin film waveguide and the mode field overlap integral η2 of the silicon nitride waveguide and the lithium niobate thin film waveguide are studied, namely:

[0136]

[0137]

[0138] Among them, F3(w)=f(w wx ,w wy) represents the numerical relationship between the width w of the lithium niobate film waveguide and the waist radius of the optical mode field in the waveguide. When the width w of the lithium niobate film waveguide is smaller than a certain level, the optical mode field in the waveguide diffuses toward the cladding, and at this time, the waist radius w is larger. wx 、w wy ; The optical mode field waist radius w of optical fiber and silicon nitride waveguide f 、w nx and w ny is a known fixed value; by changing the width of the lithium niobate thin film waveguide, the mode field is diffused toward the cladding, with a larger mode field waist radius, so that the overlap integral reaches the maximum value and the optimal coupling efficiency is achieved.

[0139] Finally, according to the fitting curve, Figure 7 As shown, the width of the end face coupled with the lithium niobate thin film spot converter at the optical fiber coupling end is determined to be 0.153 μm, and the width of the end face coupled with the lithium niobate thin film spot converter at the silicon nitride waveguide coupling end is determined to be 0.225 μm.

[0140] The lower tapered waveguide width of the spot mode converter coupled to the input optical fiber is 0.153-4 μm; the upper tapered waveguide width is 0.15-1.5 μm; the part connected to the input optical fiber has only the lower tapered waveguide, with an end face size of 0.153 μm × 0.2 μm, and the part connected to the input end of the lithium niobate thin film modulator is an upper and lower waveguide, with an upper end face size of 1.5 μm × 0.2 μm and a lower end face size of 4 μm × 0.2 μm.

[0141] The lower tapered waveguide width of the spot converter coupled to the silicon nitride waveguide at the output end is 4μm-0.225μm; the upper tapered waveguide width is 0.7-0.15μm; the output end of the lithium niobate thin film modulator is connected to the upper and lower waveguides, the upper end face size is 0.7μm×0.2μm, and the lower end face size is 4μm×0.2μm. The part connected to the input waveguide of the silicon nitride resonant cavity only has the lower tapered waveguide, and the end face size is 0.225μm×0.2μm.

[0142] The schematic diagram of the input and output end-face coupled mode spot converter is as follows: Figure 4 shown.

[0143] In this embodiment, based on the aforementioned dimensions, the lithium niobate thin-film modulator chip integrated into the integrated optical gyroscope has an overall length of 9.4 mm, a thickness of 0.6 mm, and a width of 1 mm. The designed performance parameters achieved include a 1:1 beam splitting ratio, 3 dB insertion loss, and a modulation half-wave voltage of 3.96 V.

[0144] Compared with existing products, this embodiment decomposes the structure of the lithium niobate thin film modulator and divides the optical path into parts such as the input coupling optical path, the beam splitting optical path, the modulation optical path, the single-mode output optical path, and the output coupling optical path according to the function of each part along the transmission light direction. Different waveguide parameters are designed according to the performance requirements of each part of the optical path to effectively reduce transmission loss. In particular, a single-mode output waveguide is designed based on the resonance mechanism of the integrated optical gyroscope system to effectively eliminate multi-mode noise entering the subsequent resonant cavity. In response to the integration requirements of the integrated optical gyroscope, an end-face coupled mode spot converter is designed to realize the application of low-loss lithium niobate thin film modulator connected to the integrated optical resonant gyroscope system.

[0145] 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 lithium niobate thin film phase modulator connected to an integrated optical gyroscope system, characterized in that: The invention comprises a silicon substrate, on which a silicon dioxide lower cladding layer and a silicon dioxide upper cladding layer are respectively arranged from bottom to top, and an integrated etched lithium niobate thin film modulator and a lithium niobate end-face coupled spot converter are arranged between the silicon dioxide upper and lower cladding layers; The lithium niobate thin film modulator includes a lithium niobate thin film ridge waveguide and a modulation electrode; the lithium niobate thin film ridge waveguide includes a beam splitting waveguide of different widths, a modulation waveguide and a single-mode output waveguide; The beam splitting waveguide includes an MMI beam splitting structure, a bending structure and a beam splitting-modulation tapered waveguide; The modulation waveguide is used to achieve optical phase modulation. Voltage is applied to the modulation electrodes on both sides of the modulation waveguide to change the optical phase when the light is transmitted in the waveguide. A multimode-singlemode tapered waveguide is set between the modulation waveguide and the single-mode output waveguide. The modulation electrodes are used for light modulation and are located on both sides of the modulation waveguide; The lithium niobate end-face coupled spot converter is a double-layer tapered waveguide structure, including an input-coupled spot converter and an output-coupled spot converter; The input-coupled spot mode converter is located between the input optical fiber and the input end of the lithium niobate thin film modulator, achieving low-loss spot mode conversion of the transmitted light from the large mode field of the optical fiber to the small mode field of the beam splitting waveguide of the lithium niobate thin film modulator. The single lower tapered waveguide end is connected to the input optical fiber end, and the upper and lower double tapered waveguide ends are connected to the input end of the lithium niobate thin film modulator. The output-coupled spot converter is located between the output end of the lithium niobate thin film modulator and the silicon nitride resonant cavity input waveguide in the integrated optical gyroscope system, realizing low-loss spot conversion of the transmitted light from the small mode field of the single-mode waveguide of the lithium niobate thin film modulator to the large mode field of the silicon nitride resonant cavity input waveguide; wherein, the single lower tapered waveguide end is connected to the silicon nitride resonant cavity input waveguide end; the upper and lower double-layer tapered waveguide ends are connected to the output end of the lithium niobate thin film modulator. The modulation performance of the lithium niobate thin film modulator is comprehensively evaluated, and the parameter M is selected to represent the relationship between the modulation performance and the modulation waveguide width, namely: M=Δn 2 (au)·(1-AL(au)) Wherein, Δn is the refractive index change in the modulation waveguide, which is used to characterize the modulation efficiency of the modulator. The larger Δn is, the higher the modulation efficiency is. AL represents the absorption loss of the modulation electrode to the transmitted light. The larger AL is, the greater the electrode absorption loss is. For good modulation performance, high modulation efficiency and low absorption loss are required. Δn(au) and AL(au) are used to represent Δn and AL after data normalization, respectively, that is: Wherein, F1(w)=Δn represents the relationship between the modulation waveguide width w and the refractive index change Δn, and F2(w)=AL represents the relationship between the modulation waveguide width w and the electrode absorption loss AL; When the electrode parameters are fixed, the parameter M changes with the modulation waveguide width w. The larger the M parameter, the better the modulation performance of the modulator. The modulation waveguide width w with the maximum M parameter is found to achieve the optimal modulation performance.

2. The lithium niobate thin film phase modulator connected to an integrated optical gyroscope system according to claim 1, characterized in that: The MMI beam splitting structure includes an input waveguide, a multimode interference region and two output waveguides; Based on the guided mode transmission analysis method and the relationship between the width and length of the multimode interference region, the length parameter is obtained by limiting the width parameter to complete the design of the MMI beam splitting structure; that is: Among them, W ev is the effective width of each order guided mode after taking into account the Goos-Hansen shift; W MMI is the width of the multimode interference region; λ0 is the wavelength of light in free space; n co is the refractive index of the core ridge waveguide, n cl is the cladding refractive index; for TE mode, σ=0, for TM mode, σ=1; ΔW is the additional loss width in the approximate calculation; L MMI is the length of the multimode interference region; Δβ1 is the additional loss value of the multimode wave eigenmode propagation constant; The MMI beam splitting structure splits the input light into two beams with a power ratio of 1:1 and outputs them to a curved structure. The curved structure is used to increase the center spacing between the two waveguides to match the center spacing between the two input waveguides of the silicon nitride resonant cavity. Then, the width transition from the beam splitting waveguide to the modulation waveguide is achieved through the beam splitting-modulation tapered waveguide.

3. The lithium niobate thin film phase modulator connected to an integrated optical gyroscope system according to claim 1, characterized in that: The single-mode output waveguide is used to ensure that the output light of the lithium niobate thin film modulator maintains a single-mode transmission condition; The single-mode waveguide is designed based on the effective refractive index method, and the parameters of the single-mode output waveguide are constrained, so that: Where n1 and n2 are the refractive indices of the lithium niobate waveguide and the silicon dioxide waveguide, respectively. I and N II are the fundamental mode effective refractive indexes of the inner and outer ridges of the lithium niobate ridge waveguide, respectively. The effective refractive index of the first-order mode of the inner ridge slab waveguide is made smaller than the fundamental mode effective refractive index of the outer ridge slab waveguide, and the first-order mode cutoff is used as the single-mode transmission judgment condition, that is: Wherein, k0 is the wave number in vacuum; H is the inner ridge height of the lithium niobate ridge waveguide; h is the outer ridge height of the lithium niobate ridge waveguide; W is the inner ridge width of the lithium niobate ridge waveguide; by limiting the inner and outer ridge height parameters of the lithium niobate ridge waveguide, the inner ridge width of the lithium niobate ridge waveguide that meets the single-mode transmission conditions is obtained according to the relationship.

4. The lithium niobate thin film phase modulator connected to an integrated optical gyroscope system according to claim 1, characterized in that: The multimode-singlemode tapered waveguide is used to achieve width transition from a modulation waveguide to a singlemode waveguide.

5. The lithium niobate thin film phase modulator connected to an integrated optical gyroscope system according to claim 1, characterized in that: Here's how it works: Light is injected from the input optical fiber, passes through the single lower tapered waveguide end of the input-coupled mode spot converter, is converged by low-loss mode field, and is coupled from the upper and lower double-layer tapered waveguide ends to the input waveguide of the lithium niobate thin film modulator; In the lithium niobate thin film modulator, the input light passes through the MMI beam splitting structure, which splits the light into two beams with a power ratio of 1:

1. The two beams pass through the bending structure and the beam splitting-modulation waveguide respectively and enter the modulation waveguide. When a modulation signal is applied to the modulation electrode, the light in the modulation waveguide undergoes a phase change. The modulated light then passes through the multimode-singlemode tapered waveguide and enters the single-mode output waveguide. The multimode light is dissipated into the cladding during this process, achieving only fundamental mode transmission in the output waveguide of the lithium niobate thin film modulator. The output light of the lithium niobate thin film modulator enters the upper and lower double-layer tapered waveguide ends of the output coupling mode spot converter, undergoes low-loss mode field expansion, and is coupled from the single lower tapered waveguide end to the input waveguide of the silicon nitride resonant cavity.

6. The lithium niobate thin film phase modulator connected to an integrated optical gyroscope system according to claim 1, characterized in that: The coupling performance of the lithium niobate end-face coupled spot converter was comprehensively evaluated. The electric field distribution in the optical fiber, lithium niobate film waveguide, and silicon nitride waveguide was measured by E f (x,y),E w (x,y),E n (x,y) means: Among them, w f w represents the waist radius of the electric field distribution of the optical fiber in the x and y directions; wx and w wy w represents the waist radius of the electric field distribution of the lithium niobate film waveguide in the x and y directions; nx and w ny represents the waist radius of the electric field distribution of the silicon nitride waveguide in the x and y directions; The larger the mode field overlap integral, the higher the coupling efficiency. The mode field overlap integral η1 of the optical fiber and the lithium niobate thin film waveguide and the mode field overlap integral η2 of the silicon nitride waveguide and the lithium niobate thin film waveguide are studied, namely: Among them, F3(w)=f(w wx ,w wy ) represents the width w of the lithium niobate film waveguide and the waist radius w of the optical mode field in the waveguide wx 、w wy When the width w of the lithium niobate film waveguide is smaller than the set range, the optical mode field in the waveguide diffuses toward the cladding, and at this time it has a larger waist radius w. wx 、w wy , the optical mode field waist radius w of the optical fiber and silicon nitride waveguide f 、w nx and w ny is a known fixed value; by changing the width of the lithium niobate thin film waveguide, the mode field is diffused toward the cladding, with a larger mode field waist radius, so that the overlap integral reaches the maximum value and the optimal coupling efficiency is achieved.

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