Optical gyroscope integrated chip based on thin film lithium niobate photon integrated platform

Abstract

The invention discloses an optical gyroscope integrated chip based on a thin film lithium niobate photon integrated platform. The whole structure adopts a plurality of mixed bending waveguides with continuously variable curvature parts, so that insertion loss caused by mode field mismatch of the straight waveguides and the bending waveguides at the joints is reduced, high-order mode excitation is reduced, the whole size of the chip is reduced, the polarization extinction ratio of the chip is increased by the bending waveguides, and waveguide modes except a quasi TE00 mode are filtered out. Single-mode single-polarization work of the lithium niobate waveguide is achieved through waveguide mode leakage, the single-TE mode lithium niobate waveguide, except for a bent waveguide of a folded modulator and a coupler, on the whole chip forms a waveguide polarizer with a high polarization extinction ratio, and therefore on-chip single-mode single-polarization work can be achieved without additionally designing a polarizer; the complexity of the chip is effectively reduced, the stability of the chip is improved, and the size is reduced.

Description

Technical Field

[0001] The present invention relates to the field of inertial sensing technology, and in particular to an optical gyroscope integrated chip based on a thin-film lithium niobate photon integrated platform. Background Art

[0002] Fiber optic gyroscope is a high-performance inertial sensor based on the optical Sagnac interference effect. As an angular velocity sensor, it has the advantages of high precision, high sensitivity, full solid state, and high reliability. Since Vali and Shorthill et al. of the University of Utah successfully developed the first fiber optic gyroscope in 1976, after more than 40 years of development, fiber optic gyroscope has become the mainstream solution for medium and high precision gyroscopes, with product accuracy ranging from 0.0005° / h to 10° / h. High-precision fiber optic gyroscopes are mainly used in space technology, military applications, scientific research and other fields, while low-cost medium and low precision fiber optic gyroscopes are mainly used in civilian fields such as navigation and positioning, attitude control of unmanned aerial vehicles and robots.

[0003] Fiber optic gyroscopes are composed of optical devices such as light sources, couplers, phase folding modulators, detectors, polarizers, and fiber rings. The core optical devices of traditional fiber optic gyroscope solutions are all discrete. The cost of discrete optical devices accounts for more than 70% of the total cost of the gyroscope. On the one hand, it is difficult to reduce the system cost. On the other hand, the fiber optic gyroscope based on discrete optical devices is too large for small and high-performance application scenarios such as drones, robots, cube satellites, self-driving cars, and underwater vehicles. The use of discrete optical components to form a Sagnac reciprocal interferometer requires additional optical interconnection technology, which will lead to parasitic interface reflections, increased insertion loss, and polarization misalignment, resulting in reduced system performance and reliability. In summary, the discrete core optical devices make it difficult to improve the size, weight, power consumption, and cost (SWaP-C) of the fiber optic gyroscope, which has caused the traditional fiber optic gyroscope to gradually lose its competitive advantage.

[0004] In order to solve the above-mentioned difficulties encountered by traditional fiber optic gyroscopes in their development, the concept of integrated optical gyroscopes came into being. The connotation of integrated fiber optic gyroscopes is to use integrated optical chips to partially or completely replace multiple separate optical devices in traditional fiber optic gyroscopes. In recent years, with the development of integrated photonics, integrated optical chips with various functions have made breakthrough progress, providing a basis for the realization of integrated fiber optic gyroscopes. Integrated optical gyroscopes inherit the advantages of high precision of traditional optical gyroscopes, and have the advantages of small size, low cost and high reliability that traditional optical gyroscopes do not have.

[0005] On the other hand, for interferometric closed-loop fiber gyroscopes, accurate phase modulation is a prerequisite for the chip to work well in high-precision inertial systems. At present, bulk lithium niobate folding modulators have been widely used in fiber gyroscope systems because of their excellent electro-optical performance. However, commercial bulk lithium niobate folding modulators are based on lithium niobate waveguides formed by titanium diffusion or proton exchange. These waveguides usually have a low refractive index contrast of about 0.02 between the waveguide center and the cladding, which results in a large size of the optical mode. Weak light confinement requires that the metal electrodes be placed far away from the optical waveguide, thereby reducing the electro-optical modulation efficiency. Therefore, the size of the current bulk lithium niobate modulators is still large, even several centimeters long. Summary of the invention

[0006] The present invention provides an optical gyroscope integrated chip based on a thin-film lithium niobate photon integrated platform, thereby effectively improving stability, reliability and integration, while also reducing and lowering costs.

[0007] The present invention provides an optical gyroscope integrated chip based on a thin-film lithium niobate photonic integrated platform, comprising: a first optical fiber-lithium niobate waveguide mode converter, a first optical coupler, a second optical coupler, a folding modulator, a second optical fiber-lithium niobate waveguide mode converter, a third optical fiber-lithium niobate waveguide mode converter and a fourth optical fiber-lithium niobate waveguide mode converter; the first end of the first optical fiber-lithium niobate waveguide mode converter is used to be connected to an external light source, and the second end of the first optical fiber-lithium niobate waveguide mode converter is connected to a first branch of the first optical coupler; the base waveguide of the first optical coupler is connected to the base waveguide of the second optical coupler through two 90° partially variable curvature hybrid bending waveguides; the first branch of the second optical coupler is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter, and the second branch of the second optical coupler is connected to the third optical fiber. -The first end of a lithium niobate waveguide mode converter is connected to the first end of the lithium niobate waveguide mode converter; the two arms of the folding modulator are two branches of the second optical coupler, and the two arms of the folding modulator form a folded structure through two 180° partially variable curvature hybrid bending waveguides; the second end of the second optical fiber-lithium niobate waveguide mode converter is connected to one end of an external polarization-maintaining optical fiber ring; the second end of the third optical fiber-lithium niobate waveguide mode converter is connected to the other end of the polarization-maintaining optical fiber ring; the second branch of the first optical coupler is connected to the first end of the fourth optical fiber-lithium niobate waveguide mode converter, and the second end of the fourth optical fiber-lithium niobate waveguide mode converter is used to be connected to an external photodetector; the single TE mode lithium niobate waveguide other than the hybrid bending waveguide and optical coupler of the folding modulator forms a waveguide polarizer; the waveguide polarizer is composed of a shallowly etched and / or narrow-width lithium niobate waveguide.

[0008] Specifically, the waveguide polarizer comprises, from top to bottom, an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, a SiO2 substrate layer and a Si substrate layer. The ridge waveguide is formed by etching away part of the lithium niobate layer outside the waveguide; the refractive index of the low refractive index cover layer is 1-1.7.

[0009] Specifically, the width of the waveguide polarizer is less than 2.5 μm, the etching depth is less than 0.4 μm, and the operating wavelength range is 1210-1410 nm.

[0010] Specifically, the partially variable curvature hybrid curved waveguide is composed of a linear variable curvature circular arc waveguide and a constant curvature circular arc waveguide.

[0011] Specifically, the width of the 90° partially variable curvature hybrid curved waveguide is less than 2.5 μm, the angle of the linear variable curvature arc waveguide accounts for less than 95% of the total bending angle, and the minimum radius of the 90° partially variable curvature hybrid curved waveguide is greater than 80 μm.

[0012] Specifically, the width of the 180° partially variable curvature hybrid curved waveguide is <5 μm, the angle of the linear variable curvature arc waveguide accounts for <95% of the total bending angle, and the minimum radius of the 180° partially variable curvature hybrid curved waveguide is >50 μm.

[0013] Specifically, the folding modulator is a lithium niobate electro-optical folding modulator, which uses an x-cut thin-film lithium niobate wafer, and the electrode structure of the folding modulator adopts a horizontal design, distributed on both sides and in the middle of the two branches of the second optical coupler to form a push-pull structure.

[0014] Specifically, the first optical fiber-lithium niobate waveguide mode converter, the second optical fiber-lithium niobate waveguide mode converter, the third optical fiber-lithium niobate waveguide mode converter and the fourth optical fiber-lithium niobate waveguide mode converter all adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the polarization-maintaining optical fiber ring.

[0015] Specifically, the first optical coupler and the second optical coupler are Y-type couplers and / or 1×2 multi-mode interferometer couplers.

[0016] Specifically, the Y-type coupler includes: a base waveguide, a tapered region, a first branch and a second branch; the base waveguide leads out the first branch and the second branch through two ports through the tapered region; the width of the tapered region widens according to a quadratic function; the gap between the first branch and the second branch gradually widens in the form of a rising cosine curve function.

[0017] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0018] 1. The overall structure adopts a hybrid curved waveguide with multiple parts of continuously variable curvature, which reduces the insertion loss caused by the mode field mismatch between the straight waveguide and the curved waveguide at the junction, reduces the excitation of high-order modes, and is conducive to reducing the overall size of the chip. The curved waveguide increases the polarization extinction ratio of the chip and filters out the quasi-TE 00 The present invention utilizes waveguide mode leakage to realize single-mode single-polarization operation of lithium niobate waveguide. The single TE mode lithium niobate waveguide on the entire chip except for the curved waveguide and coupler of the folded modulator forms a waveguide polarizer with a high polarization extinction ratio, so that single-mode single-polarization operation can be achieved on the chip without the need to design an additional polarizer, which effectively reduces the complexity of the chip, improves the stability of the chip, and reduces the volume. By integrating the core optical device of the fiber optic gyroscope on the thin-film lithium niobate photonic platform through the present invention, the parasitic interface reflection, increased insertion loss and polarization misalignment problems caused by the additional optical interconnection required for discrete optical components can be effectively overcome, further improving the reliability of the fiber optic gyroscope system, reducing the chip volume and reducing the cost.

[0019] 2. The folded modulator uses a 180° hybrid curved waveguide, and the folding method increases the modulation length. When the half-wave voltage is the same, that is, when the voltage-length product V π When L is the same, the driving voltage is reduced.

[0020] In summary, the present invention can effectively improve the stability, reliability and integration of the closed-loop fiber optic gyroscope while ensuring its accuracy, while reducing its size, power consumption and cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the structure of an optical gyroscope integrated chip based on a thin-film lithium niobate photonic integrated platform provided by an embodiment of the present invention;

[0022] Figure 2 A schematic cross-sectional view of a waveguide polarizer in an optical gyroscope integrated chip based on a thin-film lithium niobate photonic integrated platform provided by an embodiment of the present invention;

[0023] Figure 3 The waveguide polarizer in the embodiment of the present invention is based on the quasi-TM of the x-cut thin film lithium niobate wafer xz plane waveguide at a wavelength of 1310nm. 00 Leakage Mode Loss (dB / cm) and Quasi-TE 10 How the presence of modes varies with waveguide width and etch depth;

[0024] Figure 4 The waveguide polarizer in the embodiment of the present invention is based on the quasi-TM of the xy plane waveguide of the x-cut thin film lithium niobate wafer at a wavelength of 1310nm. 00Leakage Mode Loss (dB / cm) and Quasi-TE 10 How the presence of modes varies with waveguide width and etch depth;

[0025] Figure 5 The hybrid curved waveguide with a 90° portion of variable curvature of the base waveguide connecting the first 3dB optical coupler 2 and the second 3dB optical coupler 3 in the embodiment of the present invention; wherein (a) is a schematic diagram of the top view of the hybrid curved waveguide; (b) is a variation of the curvature of the hybrid curved waveguide with the length of the waveguide path;

[0026] Figure 6 180° partially variable curvature hybrid curved waveguide used in the folded modulator 4 in the embodiment of the present invention; (a) is a schematic diagram of the top view of the hybrid curved waveguide; (b) is the variation of the curvature of the hybrid curved waveguide with the length of the waveguide path;

[0027] Figure 7 It is a schematic diagram of the cross-sectional structure of the folded modulator 4 in an embodiment of the present invention, the z-axis of the lithium niobate crystal axis is parallel to the waveguide plane, and the electrode structure adopts a horizontal design;

[0028] Figure 8 is the half-wave voltage length product V of the folding modulator 4 in the embodiment of the present invention π The relationship between L and transmission loss as a function of metal spacing;

[0029] Fig. 9 Schematic diagram of the structure of a Y-type coupler in an embodiment of the present invention;

[0030] Fig.10 is a schematic structural diagram of a 1×2 multimode interferometer coupler in an embodiment of the present invention;

[0031] Fig.11 is a schematic cross-sectional view of the stray light absorber 10 in an embodiment of the present invention. DETAILED DESCRIPTION

[0032] The embodiment of the present invention provides an optical gyroscope integrated chip based on a thin-film lithium niobate photonic integrated platform, thereby effectively improving stability, reliability and integration, while also reducing and lowering costs.

[0033] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0034] See also Figure 1The optical gyroscope integrated chip based on the thin-film lithium niobate photonic integrated platform provided in an embodiment of the present invention includes: a first optical fiber-lithium niobate waveguide mode converter 1, a first optical coupler 2, a second optical coupler 3, a folding modulator 4, a second optical fiber-lithium niobate waveguide mode converter 7, a third optical fiber-lithium niobate waveguide mode converter 8 and a fourth optical fiber-lithium niobate waveguide mode converter 9; the first end of the first optical fiber-lithium niobate waveguide mode converter 1 is used to be connected to an external light source through an optical fiber, and the external light source is coupled to the integrated chip along the direction of arrow 11. The second end of the first optical fiber-lithium niobate waveguide mode converter 1 is connected to the first branch of the first optical coupler 2; the base waveguide of the first optical coupler 2 is connected to the base waveguide of the second optical coupler 3 through two 90° partially variable curvature hybrid bending waveguides to avoid relative placement and realize filtering of interference destructive mode; the first branch of the second optical coupler 3 is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter 7, and the second branch of the second optical coupler 3 is connected to the first end of the third optical fiber-lithium niobate waveguide mode converter 8; the double arms 5 and 6 of the folding modulator 4 are two branches of the second optical coupler 3, and the double arms 5 and 6 of the folding modulator 4 are formed into a folded structure through two 180° partially variable curvature hybrid bending waveguides; the second end of the second optical fiber-lithium niobate waveguide mode converter 7 is connected to the outer polarization-maintaining fiber ring. One end is connected; the second end of the third optical fiber-lithium niobate waveguide mode converter 8 is connected to the other end of the polarization-maintaining optical fiber ring; the second branch of the first optical coupler 2 is connected to the first end of the fourth optical fiber-lithium niobate waveguide mode converter 9, and the second end of the fourth optical fiber-lithium niobate waveguide mode converter 9 is used to connect to an external photodetector; the chip provided by the embodiment of the present invention forms a waveguide polarizer with a single TE mode lithium niobate waveguide other than the hybrid bending waveguide of the folding modulator 4 and the first optical coupler 2 and the second optical coupler 3. The waveguide polarizer is composed of a shallowly etched, narrow-width lithium niobate waveguide, and the rest of the chip is the waveguide structure. The TM mode of the waveguide has high leakage loss, and the waveguide mode leakage is used to realize the single-mode single-polarization operation of the lithium niobate ridge waveguide to achieve a high polarization extinction ratio, that is, it has a polarization function. Moreover, the waveguide polarizer does not occupy additional size space and can be polarized within the range of the entire chip, and efficiently provides a polarization function. 11 is the direction of the external light source input, and 12 is the direction of the light output to the external detector.

[0035] like Figure 2 As shown, the waveguide polarizer in the embodiment of the present invention is composed of an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, a SiO2 substrate layer and a Si substrate layer from top to bottom. The ridge waveguide is formed by etching away part of the lithium niobate layer outside the waveguide. The total thickness of the lithium niobate is 300nm, and the refractive index of the low refractive index cover layer is 1-1.7.

[0036] Specifically, the width of the waveguide polarizer is less than 2.5 μm, the etching depth is less than 0.4 μm, and the operating wavelength range is 1210-1410 nm.

[0037] The folded modulator 4 in the embodiment of the present invention uses coplanar electrodes and utilizes the largest electro-optic coefficient of lithium niobate crystal to modulate the TE mode in the waveguide in the xz plane. Therefore, the single-mode single-polarization condition is a single TE mode. The single-mode single-polarization condition under different structural parameters is investigated at an operating wavelength of 1310 nm. The design goal of the embodiment of the present invention is to optimize the waveguide structure so that it has as large a quasi-TM mode as possible. 00 Mode loss and TE polarization only has quasi-TE 00 Quasi-TM mode based on x-cut thin-film lithium niobate wafer xz planar waveguide at a wavelength of 1310nm 00 Leakage Mode Loss (dB / cm) and Quasi-TE 10 The existence of the mode varies with the waveguide width and etching depth as shown in Figure 3 As shown. Figure 3 As shown in the results, the waveguide size in the embodiment of the present invention is selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, TM 00 Mode transmission loss is about 332dB / cm, TE 10 The mode is in the cutoff region and does not exist, so at this time there is only TE in the waveguide 00 It can transmit stably, and the calculated transmission loss is 2.6×10 -10 dB / cm, the waveguide is a single-mode single-polarization waveguide. Quasi-TM based on x-cut thin-film lithium niobate wafer xy plane waveguide at a wavelength of 1310nm 00 Leakage Mode Loss (dB / cm) and Quasi-TE 10 The existence of the mode varies with the waveguide width and etching depth as shown in Figure 4 As shown. Figure 4 As shown in the results, the waveguide size in the embodiment of the present invention can be selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, TM 00 The transmission loss of the mode due to lateral leakage is 125dB / cm, TE 10 The mode is in the cutoff region and does not exist, so at this time there is only TE in the waveguide 00 It can transmit stably, and the calculated transmission loss is 3.2×10 -10 dB / cm, the waveguide is a single-mode single-polarization waveguide. The above two single-mode single-polarization waveguides constitute the waveguide polarizer in the embodiment of the present invention.

[0038] The structure of the partially variable curvature hybrid curved waveguide in the embodiment of the present invention is specifically described. The partially variable curvature hybrid curved waveguide is composed of a linear variable curvature arc waveguide and a constant curvature arc waveguide. The linear variable curvature arc waveguide reduces the insertion loss caused by the mode field mismatch between the straight waveguide and the curved waveguide at the junction, and reduces the excitation of high-order modes.

[0039] In this embodiment, the width of the hybrid curved waveguide with a 90° partially variable curvature is less than 2.5 μm, the angle of the linear variable curvature arc waveguide accounts for less than 95% of the total bending angle, and the minimum radius of the hybrid curved waveguide with a 90° partially variable curvature is greater than 80 μm. The width of the hybrid curved waveguide with a 180° partially variable curvature is less than 5 μm, the angle of the linear variable curvature arc waveguide accounts for less than 95% of the total bending angle, and the minimum radius of the hybrid curved waveguide with a 180° partially variable curvature is greater than 50 μm.

[0040] Specifically, Figure 5 (a) is a top view of a hybrid curved waveguide with a 90° partially variable curvature. The angle of linear variable curvature of the curved waveguide accounts for 30% of the total bending angle. The starting point and end point of the curved waveguide are consistent with those of a constant curvature curved waveguide with a radius of 500 μm. The effective radius is 500 μm and the minimum radius R min The value is 401.8μm. Figure 5 (b) is the variation of the curvature of the hybrid curved waveguide with the length of the waveguide path. The curved waveguide is in the angle range of [0°, 13.5°], and the curvature linearly varies in the range of [0, 1 / R min ], in the angle range of [13.5°, 76.5°], the curvature is 1 / R min , in the angle range of [76.5°, 90°], the curvature linear variation range is [1 / R min , 0], maximum curvature 1 / R min 2488.8m -1 .

[0041] Four 180° partially variable curvature hybrid curved waveguides are used inside the folded electro-optic device to achieve the folded placement of the modulation area. Figure 6 (a) is a schematic diagram of the top view of a hybrid curved waveguide with a 180° partially variable curvature. The angle of linear variable curvature of the hybrid curved waveguide accounts for 30% of the total bending angle. The starting point and end point of the hybrid curved waveguide are consistent with those of a constant curvature curved waveguide with a radius of 260 μm. The effective radius is 260 μm and the minimum radius R min The value is 250.8μm. Figure 6 (b) is the variation of the curvature of the hybrid curved waveguide with the length of the waveguide path. The curved waveguide is in the angle range of [0°, 27°], and the curvature linearly varies in the range of [0, 1 / R min], in the angle range of [27°,153°], the curvature is 1 / R min , in the angle range of [153°,180°], the curvature linear variation range is [1 / R min , 0], maximum curvature 1 / R min 3987.4m -1 The curvature of the linear transformation can reduce the mode field mismatch at the junction of the straight waveguide and the curved waveguide.

[0042] The structure of the folding modulator 4 is specifically described. The folding modulator 4 is a lithium niobate electro-optical folding modulator, which uses the electro-optical properties of lithium niobate materials to achieve phase modulation and uses the largest electro-optical coefficient of lithium niobate crystals. An x-cut thin-film lithium niobate wafer is used, that is, the x-axis of the lithium niobate crystal is perpendicular to the plane of the thin-film lithium niobate. The refractive index matrix corresponding to the waveguide cross-section in different transmission directions in the yz plane of the thin-film lithium niobate is different, and the total thickness of the lithium niobate film is 300nm. The electrode structure of the folding modulator 4 adopts a horizontal design, distributed on both sides and in the middle of the two branches of the second optical coupler, that is, the corresponding crystal axis x-axis is perpendicular to the chip plane, forming a push-pull structure to reduce the driving voltage exponentially. The two arms 5 and 6 of the folding modulator 4 are set with a length difference of 900μm according to the birefringence difference of the lithium niobate waveguide, thereby reducing the polarization error caused by the polarization cross-coupling effect in the optical fiber gyroscope.

[0043] like Figure 7 As shown, the cross-sectional structure of the folded modulator 4 is from left to right: metal electrode, waveguide, metal electrode, waveguide, metal electrode. The metal electrode and the lithium niobate (LiNbO3) plate layer are separated by 100nm thick SiO2 to achieve the effect of reducing metal absorption. The RF electric field distribution in the cross section of the lithium niobate waveguide is mainly in the horizontal direction, so the electrode spacing can be reduced to increase the RF electric field and waveguide quasi-TE 00 The overlap factor between the optical mode fields is increased, thereby improving the modulation efficiency of the electro-optical folding modulator 4.

[0044] In order to improve the efficiency of electro-optic modulation and reduce the transmission loss of the waveguide mode, the embodiment of the present invention reasonably designs the electrode spacing. π The relationship between L and transmission loss as the metal spacing changes is as follows Figure 8 As shown. Figure 8 The results shown in FIG. 1 show that the loss of the embodiment of the present invention is lower than 0.1 dB / cm. In the embodiment of the present invention, the electrode spacing can be selected to be 7.5 μm. At this time, V π L is 2.57V.cm, and the waveguide transmission loss is 0.01dB / cm. The embodiment of the present invention avoids the quasi-TE in the lithium niobate waveguide by setting a reasonable electrode spacing. 00 The fundamental mode will be affected by the absorption of the metal electrode and add additional absorption loss.

[0045] The structure of the optical gyroscope integrated chip provided by the embodiment of the present invention is further described. The first optical fiber-lithium niobate waveguide mode converter 1, the second optical fiber-lithium niobate waveguide mode converter 7, the third optical fiber-lithium niobate waveguide mode converter 8 and the fourth optical fiber-lithium niobate waveguide mode converter 9 all adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide and the polarization-maintaining optical fiber ring at the end face, and all optical fiber-lithium niobate waveguide mode converters are located on the same side of the chip, which is convenient for coupling with an external light source, a detector and an optical fiber ring using a single optical fiber array (FA). The end face of this side is polished to reduce end face reflection.

[0046] The structures of the first optical coupler 2 and the second optical coupler 3 are described in detail. The first optical coupler 2 and the second optical coupler 3 are Y-type couplers and / or 1×2 multi-mode interferometer couplers.

[0047] In order to reduce the parasitic reflections generated by the device, such as Fig. 9 As shown, the Y-type coupler includes: a base waveguide 201, a tapered region 202, a first branch 205-1 and a second branch 205-2; the base waveguide 201 leads to the first branch 205-1 and the second branch 205-2 through two ports through the tapered region 202; the width of the tapered region 202 changes from 1 μm to 3.5 μm according to a quadratic function; the gap between the first branch 205-1 and the second branch 205-2 is gradually widened from 0.5 μm to 10 μm in the form of a rising cosine curve function. In addition, the gap between the two output ports 204-1 and 204-2 is increased to 0.5 μm, and the overlap integral of the modes on both sides is increased by widening the wider side 203 of the tapered region 202, thereby ensuring that the Y-type coupler has a large forward transmittance.

[0048] The structure of the 1×2 multi-mode interferometer in the embodiment of the present invention is as follows: Fig.10 As shown, a partial etching method is used to form channels 301 on both sides of the port for light to leak into the flat plate, thereby reducing reflection, and the edge of the entire flat plate area 302 is a pointed cone structure, so that the light leaking into the flat plate leaks out through the pointed cone. In this embodiment, the width of the multimode area 303 is 10.2μm, and the length of the multimode area 303 is 93μm.

[0049] In order to absorb stray light, it also includes: a stray light absorber 10; the stray light absorber 10 is located on both sides of the first optical fiber-lithium niobate waveguide mode converter 1, the second optical fiber-lithium niobate waveguide mode converter 7, the third optical fiber-lithium niobate waveguide mode converter 8 and the fourth optical fiber-lithium niobate waveguide mode converter 9.

[0050] In this embodiment, the stray light absorber 10 is made of metal material. The cross-sectional view of the stray light absorber 10 is shown in FIG. Fig.11As shown. The thin-film lithium niobate in some areas on both sides of the four optical fiber-lithium niobate waveguide mode converters is etched onto the silicon dioxide substrate layer, and then metal is evaporated in these areas. When the stray light in the lithium niobate waveguide slab layer is transmitted to the vicinity of the absorber, the stray light is absorbed by the metal, achieving the function of absorbing stray light.

[0051] The lithium niobate waveguide in the embodiment of the present invention is a single-mode single-polarization waveguide. After the mode passes through the first 3dB optical coupler 2 and loses 3dB light, it enters the second 3dB optical coupler 3 and is evenly divided into two beams. After the two beams are phase modulated by the two arms 5 and 6 of the folding modulator 4, they are coupled into the optical fiber ring through the second optical fiber-lithium niobate waveguide mode converter 7 and the third optical fiber-lithium niobate waveguide mode converter 8 respectively; after entering the optical fiber ring, the two beams propagate in the clockwise and counterclockwise directions respectively, and each enters the other arm of the folding modulator 4. Due to the Sagnac effect of the optical fiber ring, when the optical fiber ring rotates in a plane, the two beams will generate a phase difference related to the angular velocity of rotation. Since the two beams are coherent. When the two beams return to the second 3dB optical coupler 3 again through the folding modulator 4, interference will occur; after the interference light signal returns to the first 3dB optical coupler 2, it enters the fourth optical fiber-lithium niobate waveguide mode converter 9 from another branch, and is output to the optical fiber array on the right side of the chip along the direction of arrow 12, so that the output light is received by the external detector. When the angular velocity changes, the interference light intensity changes accordingly, and the angular velocity of the fiber ring rotation can be demodulated through this change; since part of the light leaks into the waveguide slab layer during the process of fiber coupling, transmission, and beam combining, a stray light absorber 10 is set in the lithium niobate slab layer near the first to fourth optical fiber-lithium niobate waveguide mode converters, and the metal's absorption characteristics of light are used to prevent stray light from freely propagating in the waveguide slab layer and entering the detector.

[0052] In summary, the embodiments of the present invention utilize the excellent electro-optical modulation performance of thin-film lithium niobate to simultaneously realize full-chip polarization, low-loss optical transmission, beam splitting, and modulation. While ensuring the accuracy of the closed-loop fiber gyroscope, the embodiments of the present invention utilize special structural designs to achieve a closed-loop fiber gyroscope with higher integration, lower driving voltage, and lower cost.

[0053] The parts not described in detail in the embodiments of the present invention are all known technologies to those skilled in the art. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. An optical gyroscope integrated chip based on thin-film lithium niobate photonic integrated platform, It is characterized in that include: A first optical fiber-lithium niobate waveguide mode converter, a first optical coupler, a second optical coupler, a folding modulator, a second optical fiber-lithium niobate waveguide mode converter, a third optical fiber-lithium niobate waveguide mode converter, and a fourth optical fiber-lithium niobate waveguide mode converter; The first end of the first optical fiber-lithium niobate waveguide mode converter is used to connect to an external light source, and the second end of the first optical fiber-lithium niobate waveguide mode converter is connected to the first branch of the first optical coupler; the base waveguide of the first optical coupler is connected to the base waveguide of the second optical coupler through two 90° partially variable curvature hybrid bending waveguides; the first branch of the second optical coupler is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter, and the second branch of the second optical coupler is connected to the first end of the third optical fiber-lithium niobate waveguide mode converter; the two arms of the folding modulator are the two branches of the second optical coupler, and the two arms of the folding modulator are connected through two 1 A hybrid bending waveguide with a partially variable curvature of 80° forms a folded structure; the second end of the second optical fiber-lithium niobate waveguide mode converter is connected to one end of an external polarization-maintaining optical fiber ring; the second end of the third optical fiber-lithium niobate waveguide mode converter is connected to the other end of the polarization-maintaining optical fiber ring; the second branch of the first optical coupler is connected to the first end of the fourth optical fiber-lithium niobate waveguide mode converter, and the second end of the fourth optical fiber-lithium niobate waveguide mode converter is used to be connected to an external photodetector; a single TE mode lithium niobate waveguide other than the hybrid bending waveguide and optical coupler of the folded modulator forms a waveguide polarizer; the waveguide polarizer is composed of a shallowly etched and / or narrow-width lithium niobate waveguide.

2. The optical gyroscope integrated chip based on the thin-film lithium niobate photonic integrated platform as claimed in claim 1, It is characterized in that The waveguide polarizer is composed of an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, and a SiO 2 The substrate layer and the Si substrate layer are formed into a ridge waveguide by etching away part of the lithium niobate layer outside the waveguide; the refractive index of the low-refractive-index cap layer is 1-1.

7.

3. The optical gyroscope integrated chip based on the thin film lithium niobate photonic integrated platform as claimed in claim 1 or 2, It is characterized in that The waveguide polarizer has a width of less than 2.5 μm, an etching depth of less than 0.4 μm, and an operating wavelength range of 1210-1410 nm.

4. The optical gyroscope integrated chip based on the thin film lithium niobate photonic integrated platform as claimed in claim 1, It is characterized in that The partially variable curvature hybrid curved waveguide is composed of a linear variable curvature circular arc waveguide and a constant curvature circular arc waveguide.

5. The optical gyroscope integrated chip based on the thin-film lithium niobate photonic integrated platform as claimed in claim 4, It is characterized in that The width of the 90° partially variable curvature hybrid curved waveguide is less than 2.5 μm, the angle of the linear variable curvature arc waveguide accounts for less than 95% of the total bending angle, and the minimum radius of the 90° partially variable curvature hybrid curved waveguide is greater than 80 μm.

6. The optical gyroscope integrated chip based on the thin film lithium niobate photonic integrated platform as claimed in claim 4, It is characterized in that The width of the 180° partially variable curvature hybrid curved waveguide is less than 5 μm, the angle of the linear variable curvature arc waveguide accounts for less than 95% of the total bending angle, and the minimum radius of the 180° partially variable curvature hybrid curved waveguide is greater than 50 μm.

7. The optical gyroscope integrated chip based on the thin-film lithium niobate photonic integrated platform as claimed in claim 1, It is characterized in that The folding modulator is a lithium niobate electro-optical folding modulator, which adopts an x-cut thin-film lithium niobate wafer, and the electrode structure of the folding modulator adopts a horizontal design and is distributed on both sides and in the middle of the two branches of the second optical coupler to form a push-pull structure.

8. The optical gyroscope integrated chip based on the thin-film lithium niobate photonic integrated platform as claimed in claim 1, It is characterized in that The first optical fiber-lithium niobate waveguide mode converter, the second optical fiber-lithium niobate waveguide mode converter, the third optical fiber-lithium niobate waveguide mode converter and the fourth optical fiber-lithium niobate waveguide mode converter all adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the polarization-maintaining optical fiber ring.

9. The optical gyroscope integrated chip based on the thin film lithium niobate photonic integrated platform as claimed in claim 1, It is characterized in that The first optical coupler and the second optical coupler are Y-type couplers and / or 1×2 multi-mode interferometer couplers.

10. The optical gyroscope integrated chip based on the thin film lithium niobate photonic integrated platform as claimed in claim 9, It is characterized in that The Y-type coupler comprises: a base waveguide, a tapered region, a first branch and a second branch; the base waveguide leads out the first branch and the second branch respectively through two ports through the tapered region; the width of the tapered region widens according to a quadratic function; the gap between the first branch and the second branch gradually widens in the form of a rising cosine curve function.

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