Fiber optic gyroscope

Abstract

The utility model discloses a fiber-optic gyroscope. The whole structure adopts a plurality of mixed bent waveguides with continuously variable curvature parts, so that insertion loss caused by mode field mismatch of the straight waveguides and the bent waveguides at the joints is reduced, high-order mode excitation is reduced, the whole size of the fiber-optic gyroscope is reduced, the bent waveguides increase the polarization extinction ratio of the fiber-optic gyroscope, and the fiber-optic gyroscope is more compact in structure. And the waveguide modes except the quasi TE00 mode are filtered out. According to the utility model, the single-mode single-polarization work of the lithium niobate waveguide is realized by utilizing waveguide mode leakage, and the single-TE mode lithium niobate waveguide except for the bent waveguide of the folded modulator and the coupler on the whole fiber-optic gyroscope forms a waveguide polarizer with a high polarization extinction ratio; therefore, on-chip complete single-mode single-polarization work of the system can be realized without additionally designing a polarizer, the complexity of the system is effectively reduced, the stability of the system is improved, and the size of the system is reduced. And stray light is absorbed by the stray light absorber, so that the measurement precision of the fiber-optic gyroscope system is improved.

Description

Technical Field

[0001] The utility model relates to the technical field of inertial sensors, in particular to an optical fiber gyroscope. Background Art

[0002] An optical fiber 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, all-solid state, and high reliability. Optical fiber gyroscopes have become the mainstream solution for medium- and high-precision gyroscopes, and the product accuracy covers 0.0005° / h to 10° / h. High-precision optical fiber gyroscopes are mainly applied in fields such as space technology and scientific research, while low-cost medium- and low-precision optical fiber gyroscopes are mainly applied in civilian fields such as navigation and positioning, attitude control of unmanned aerial vehicles, and robots.

[0003] An optical fiber gyroscope consists of optical devices such as a light source, a coupler, a phase folding modulator, a detector, a polarizer, and an optical fiber loop. In the traditional optical fiber gyroscope solution, each core optical device is 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, for application scenarios that require small size and high performance, such as unmanned aerial vehicles, robots, cube satellites, autonomous driving vehicles, and underwater vehicles, the optical fiber gyroscope based on discrete optical devices is too large in volume. Using discrete optical elements to form a Sagnac reciprocal interferometer requires additional optical interconnection technology, which will lead to parasitic interface reflection, increased insertion loss, and polarization misalignment, resulting in a decline in system performance and reliability. Generally speaking, discrete core optical devices make it difficult to improve the comprehensive performance of the optical fiber gyroscope in terms of size, weight, power consumption, and cost (Size, Weight, and Power, Cost, SWaP-C), thus causing the traditional optical fiber gyroscope to gradually lose its competitive advantage.

[0004] In order to solve the above problems encountered in the development of traditional optical fiber gyroscopes, the concept of integrated optical gyroscopes emerged as the times require. The connotation of an integrated optical fiber gyroscope is to use an integrated optical chip to partially or completely replace multiple discrete optical devices in a traditional optical fiber gyroscope. 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 optical fiber gyroscopes. Integrated optical gyroscopes inherit the high-precision advantages 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 an interferometric closed-loop fiber optic gyroscope, accurate phase modulation is a prerequisite for the chip to work well in a high-precision inertial system. Currently, bulk lithium niobate folded modulators have been widely used in fiber optic gyroscope systems due to their excellent electro-optic performance. However, commercially available bulk lithium niobate folded 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 core and the cladding, resulting in a large size of the optical mode. The weak optical confinement requires the metal electrodes to be placed far away from the optical waveguide, thus reducing the electro-optic modulation efficiency. Therefore, the current size of the bulk lithium niobate modulator is still very large, even several centimeters long. Summary of the Invention

[0006] The present invention provides an optical fiber gyroscope, which effectively improves stability, reliability and integration, and at the same time reduces size and cost.

[0007] The utility model provides an optical fiber gyroscope, comprising: an external light source, a first optical fiber-lithium niobate waveguide mode converter, a first optical coupler, a second optical coupler, a folded modulator, a second optical fiber-lithium niobate waveguide mode converter, a third optical fiber-lithium niobate waveguide mode converter, a fourth optical fiber-lithium niobate waveguide mode converter, a photodetector and a stray light absorber; a first end of the first optical fiber-lithium niobate waveguide mode converter is connected to the external light source, and a second end of the first optical fiber-lithium niobate waveguide mode converter is connected to a first branch of the first optical coupler; a fundamental waveguide of the first optical coupler and a fundamental waveguide of the second optical coupler are connected through two 90° partially variable-curvature hybrid bending waveguides; a first branch of the second optical coupler is connected to a first end of the second optical fiber-lithium niobate waveguide mode converter, and a second branch of the second optical coupler is connected to a first end of the third optical fiber-lithium niobate waveguide mode converter; two arms of the folded modulator are two branches of the second optical coupler, and both arms of the folded modulator form a folded structure through two 180° partially variable-curvature hybrid bending waveguides; a second end of the second optical fiber-lithium niobate waveguide mode converter is connected to one end of an external polarization-maintaining optical fiber loop; a second end of the third optical fiber-lithium niobate waveguide mode converter is connected to the other end of the polarization-maintaining optical fiber loop; a second branch of the first optical coupler is connected to a first end of the fourth optical fiber-lithium niobate waveguide mode converter, and a second end of the fourth optical fiber-lithium niobate waveguide mode converter is connected to the photodetector; the stray light absorber is located on both sides of 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; a single TE mode lithium niobate waveguide except for the hybrid bending waveguides and optical couplers of the folded modulator forms a waveguide polarizer; the waveguide polarizer is composed of a lithium niobate waveguide with shallow etching and / or narrow width.

[0008] Specifically, the waveguide polarizer includes, from top to bottom, an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, an SiO2 substrate layer and an Si substrate layer, and the ridge waveguide is formed by etching away a 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 <2.5μm, the etching depth <0.4μm, and the working wavelength range is 1210-1410nm.

[0010] Specifically, the partially variable-curvature hybrid bending waveguide is jointly composed of a linearly variable-curvature circular arc waveguide and a constant-curvature circular arc waveguide.

[0011] Specifically, the width of the 90° partially variable-curvature hybrid bent waveguide < 2.5 μm, the proportion of the angle of the linearly variable-curvature arc waveguide in the total bending angle < 95%, and the minimum radius of the 90° partially variable-curvature hybrid bent waveguide > 80 μm.

[0012] Specifically, the width of the 180° partially variable-curvature hybrid bent waveguide < 5 μm, the proportion of the angle of the linearly variable-curvature arc waveguide in the total bending angle < 95%, and the minimum radius of the 180° partially variable-curvature hybrid bent waveguide > 50 μm.

[0013] Specifically, the folding modulator is a lithium niobate electro-optic folding modulator, which uses an x-cut thin-film lithium niobate wafer, and the electrode structure of the folding modulator adopts a horizontal design, which is 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 fiber-lithium niobate waveguide mode converter, the second fiber-lithium niobate waveguide mode converter, the third fiber-lithium niobate waveguide mode converter, and the fourth fiber-lithium niobate waveguide mode converter all adopt an inverse taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the polarization-maintaining fiber loop.

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

[0016] One or more technical solutions provided in the present utility model have at least the following technical effects or advantages:

[0017] 1. The overall structure adopts a hybrid bent 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 bent waveguide at the connection, and reduces the excitation of high-order modes, and is beneficial to reducing the overall size of the fiber optic gyroscope. Moreover, the bent waveguide increases the polarization extinction ratio of the fiber optic gyroscope and filters out quasi-TE 00Waveguide modes other than the mode. The utility model realizes single-mode and single-polarization operation of the lithium niobate waveguide by means of waveguide mode leakage. The single TE mode lithium niobate waveguide on the entire fiber optic gyroscope, except for the bent waveguide and coupler of the folded modulator, forms a waveguide polarizer with a high polarization extinction ratio, so that the on-chip complete single-mode and single-polarization operation of the system can be realized without an additionally designed polarizer, effectively reducing the complexity of the system, improving the stability of the system and reducing the volume. By absorbing stray light through a stray light absorber, the measurement accuracy of the fiber optic gyroscope system is improved. By integrating the core optical devices of the fiber optic gyroscope on the thin-film lithium niobate photon platform, the problems of parasitic interface reflection, increased insertion loss and polarization misalignment 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 system volume and lowering the cost.

[0018] 2. The folded modulator adopts a 180° hybrid bent waveguide, and the folded method increases the modulation length. Under the same half-wave voltage, that is, when the voltage-length product V π L is the same, the driving voltage is reduced.

[0019] In summary, while ensuring the accuracy of the closed-loop fiber optic gyroscope, the utility model can effectively improve its stability, reliability and integration, and at the same time reduce the size, power consumption and cost. Brief Description of the Drawings

[0020] Figure 1 It is a schematic structural diagram of the fiber optic gyroscope provided by an embodiment of the utility model;

[0021] Figure 2 It is a schematic cross-sectional view of the stray light absorber 10 in the fiber optic gyroscope provided by an embodiment of the utility model;

[0022] Figure 3 It is a schematic cross-sectional view of the waveguide polarizer in the fiber optic gyroscope provided by an embodiment of the utility model;

[0023] Figure 4 It is the quasi-TM of the waveguide polarizer in the embodiment of the utility model based on the x-z plane waveguide of the x-cut thin-film lithium niobate wafer at a wavelength of 1310 nm 00 Leakage mode loss (dB / cm) and quasi-TE 10 The existence of the mode changes with the waveguide width and etching depth;

[0024] Figure 5 It is the quasi-TM of the waveguide polarizer in the embodiment of the utility model based on the x-y plane waveguide of the x-cut thin-film lithium niobate wafer at a wavelength of 1310 nm 00 Leakage mode loss (dB / cm) and quasi-TE 10The presence of the mode varies with the waveguide width and the etching depth;

[0025] Figure 6 It is a hybrid bending waveguide with a 90° partial variable curvature for the fundamental waveguide connecting the first 3 dB optical coupler 2 and the second 3 dB optical coupler 3 in the embodiment of the present invention; wherein, (a) is a top view structural schematic diagram of the hybrid bending waveguide; (b) is the variation of the curvature of the hybrid bending waveguide with the waveguide path length.

[0026] Figure 7 It is a hybrid bending waveguide with a 180° partial variable curvature used in the folding modulator 4 in the embodiment of the present invention; wherein, (a) is a top view structural schematic diagram of the hybrid bending waveguide; (b) is the variation of the curvature of the hybrid bending waveguide with the waveguide path length.

[0027] Figure 8 It is a cross-sectional structural schematic diagram of the folding modulator 4 in the 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 9 It is the product of the half-wave voltage length V π L and the transmission loss vary with the metal spacing;

[0029] Figure 10 It is a structural schematic diagram of the Y-type coupler in the embodiment of the present invention;

[0030] Figure 11 It is a structural schematic diagram of the 1×2 multimode interference coupler in the embodiment of the present invention. Specific implementation manner

[0031] The embodiment of the present invention provides an optical fiber gyroscope, which effectively improves the stability, reliability and integration degree, and at the same time reduces the cost.

[0032] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the specification drawings and specific implementation manners.

[0033] See Figure 1, the fiber optic gyroscope provided by the embodiment of the present utility model includes: an external light source, a first fiber-lithium niobate waveguide mode converter 1, a first optical coupler 2, a second optical coupler 3, a folded modulator 4, a second fiber-lithium niobate waveguide mode converter 7, a third fiber-lithium niobate waveguide mode converter 8, a fourth fiber-lithium niobate waveguide mode converter 9, a photodetector, and a stray light absorber 10; the first end of the first fiber-lithium niobate waveguide mode converter 1 is connected to the external light source through an optical fiber. The second end of the first fiber-lithium niobate waveguide mode converter 1 is connected to the first branch of the first optical coupler 2; the fundamental waveguide of the first optical coupler 2 and the fundamental waveguide of the second optical coupler 3 are connected by two 90° partially variable curvature hybrid bending waveguides, avoiding relative placement to achieve filtering of the interference cancellation mode; the first branch of the second optical coupler 3 is connected to the first end of the second 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 fiber-lithium niobate waveguide mode converter 8; the two arms 5 and 6 of the folded modulator 4 are the two branches of the second optical coupler 3, and the two arms 5 and 6 of the folded modulator 4 both form a folded structure through two 180° partially variable curvature hybrid bending waveguides; the second end of the second fiber-lithium niobate waveguide mode converter 7 is connected to one end of an external polarization maintaining fiber loop; the second end of the third fiber-lithium niobate waveguide mode converter 8 is connected to the other end of the polarization maintaining fiber loop; the second branch of the first optical coupler 2 is connected to the first end of the fourth fiber-lithium niobate waveguide mode converter 9, and the second end of the fourth fiber-lithium niobate waveguide mode converter 9 is connected to the photodetector; the stray light absorber 10 is located on both sides of the first fiber-lithium niobate waveguide mode converter 1, the second fiber-lithium niobate waveguide mode converter 7, the third fiber-lithium niobate waveguide mode converter 8, and the fourth fiber-lithium niobate waveguide mode converter 9. The fiber optic gyroscope provided by the embodiment of the present utility model forms a waveguide polarizer with a single TE mode lithium niobate waveguide except for the hybrid bending waveguides of the folded modulator 4 and the first optical coupler 2 and the second optical coupler 3. The waveguide polarizer is composed of a shallow etched and narrow width lithium niobate waveguide. The rest of the overall structure is of this waveguide structure. The TM mode of this waveguide has high leakage loss, and the single-mode and single-polarization operation of the lithium niobate ridge waveguide is achieved by waveguide mode leakage to achieve a high polarization extinction ratio, that is, it has the function of polarization. Moreover, this waveguide polarizer does not occupy additional dimensional space and can polarize within the entire range of the fiber optic gyroscope, efficiently providing the polarization function. 11 is the direction of the input of the external light source, and 12 is the direction of the light output to the external detector.

[0034] In this embodiment, the stray light absorber 10 is made of a metal material. The cross-sectional schematic diagram of the stray light absorber 10 is as Figure 2As shown, the thin-film lithium niobate in the partial regions on both sides of the four fiber-lithium niobate waveguide mode converters is etched into the silica substrate layer, and then metal is evaporated on these regions. When the stray light in the lithium niobate waveguide flat layer transmits to the vicinity of this absorber, the stray light is absorbed by the metal, achieving the effect of absorbing the stray light and improving the measurement accuracy of the fiber optic gyro system.

[0035] As Figure 3 shown, the waveguide polarizer in the embodiment of the present utility model from top to bottom is respectively 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 the partial lithium niobate layer outside the waveguide. The total thickness of the lithium niobate is 300 nm, and the refractive index of the low refractive index cover layer is 1 - 1.7.

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

[0037] The folded modulator 4 in the embodiment of the present utility model adopts coplanar electrodes, utilizes the largest electro-optic coefficient of the lithium niobate crystal, and modulates the TE mode in the waveguide in the x - z plane. Therefore, the single-mode single-polarization situation is a single TE mode. The single-mode single-polarization conditions under different structural parameters are investigated at the working wavelength of 1310 nm. The design objective of the embodiment of the present utility model is to optimize the waveguide structure so that it has the largest possible quasi-TM 00 mode loss and the TE polarization only has a quasi-TE 00 mode. The quasi-TM 00 leakage mode loss (dB / cm) and the existence situation of the quasi-TE 10 mode of the x - z plane waveguide based on the x-cut thin-film lithium niobate wafer at the wavelength of 1310 nm vary with the waveguide width and etching depth as Figure 4 shown. According to Figure 4 the results in, the waveguide dimensions in the embodiment of the present utility model are selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, the TM 00 mode transmission loss is about 332 dB / cm, and the TE 10 mode is in the cutoff region and does not exist. Therefore, only the TE 00 mode can stably transmit at this time, and its calculated transmission loss is 2.6×10 -10 dB / cm. The waveguide is a single-mode single-polarization waveguide. The quasi-TM 00 leakage mode loss (dB / cm) and the existence situation of the quasi-TE 10 mode of the x - y plane waveguide based on the x-cut thin-film lithium niobate wafer at the wavelength of 1310 nm vary with the waveguide width and etching depth as Figure 5 shown. According to Figure 5As shown in the results, in the embodiment of the present utility model, the waveguide size can be selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, for the TM 00 mode, since the transverse leakage transmission loss is 125 dB / cm, and for the TE 10 mode, it is in the cutoff region and does not exist. Therefore, at this time, only the TE 00 mode can stably transmit, 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 utility model.

[0038] The structure of the partially variable-curvature hybrid bent waveguide in the embodiment of the present utility model will be specifically described. This partially variable-curvature hybrid bent waveguide is jointly composed of a linearly variable-curvature circular arc waveguide and a constant-curvature circular arc waveguide. The linearly variable-curvature circular arc waveguide reduces the insertion loss caused by the mode field mismatch between the straight waveguide and the bent waveguide at the junction, and reduces the excitation of higher-order modes.

[0039] In this embodiment, the width of the 90° partially variable-curvature hybrid bent waveguide < 2.5 μm, the proportion of the angle of the linearly variable-curvature circular arc waveguide in the total bending angle < 95%, and the minimum radius of the 90° partially variable-curvature hybrid bent waveguide > 80 μm. The width of the 180° partially variable-curvature hybrid bent waveguide < 5 μm, the proportion of the angle of the linearly variable-curvature circular arc waveguide in the total bending angle < 95%, and the minimum radius of the 180° partially variable-curvature hybrid bent waveguide > 50 μm.

[0040] Specifically, Figure 6 (a) is a top view structural schematic diagram of the 90° partially variable-curvature hybrid bent waveguide. The angle of the linearly variable-curvature of this bent waveguide accounts for 30% of the total bending angle. The starting point and the ending point of this bent waveguide are consistent with the fixed-curvature bent waveguide with a radius of 500 μm, the effective radius is 500 μm, and the minimum radius R min takes a value of 401.8 μm. Figure 6 (b) is the variation of the curvature of this hybrid bent waveguide with the waveguide path length. In the angular range of [0°, 13.5°] of the bent waveguide, the linear variation range of the curvature is [0, 1 / R min , in the angular range of [13.5°, 76.5°], the curvature takes a value of 1 / R min , and in the angular range of [76.5°, 90°], the linear variation range of the curvature is [1 / R min , 0], and the maximum curvature 1 / R min is 2488.8 m -1 .

[0041] Four 180° partially variable-curvature hybrid bent waveguides are used inside the folded electro-optic device to achieve the folded placement of the modulation region.Figure 7 (a) is a top - view structural schematic diagram of a hybrid bent waveguide with a 180° partial variable curvature. The angle of the linear variable curvature of this hybrid bent waveguide accounts for 30% of the total bending angle. The starting point and the ending point of this hybrid bent waveguide are consistent with those of a constant - curvature bent waveguide with a radius of 260 μm, the effective radius is 260 μm, and the minimum radius R min takes a value of 250.8 μm. Figure 7 (b) shows the variation of the curvature of this hybrid bent waveguide with the waveguide path length. In the angular range of [0°, 27°] of the bent waveguide, the linear variation range of the curvature is [0, 1 / R min . In the angular range of [27°, 153°], the curvature value is 1 / R min . In the angular range of [153°, 180°], the linear variation range of the curvature is [1 / R min , 0], and the maximum curvature 1 / R min is 3987.4 m -1 . The linearly - transformed curvature can reduce the mode - field mismatch at the junction of the straight waveguide and the bent waveguide.

[0042] The structure of the folded modulator 4 is specifically described. The folded modulator 4 is a lithium - niobate electro - optic folded modulator, which realizes phase modulation by using the electro - optic characteristics of the lithium - niobate material and utilizes the largest electro - optic coefficient of the lithium - niobate crystal. An x - cut thin - film lithium - niobate wafer is used, that is, the x - axis of the lithium - niobate crystal axis is perpendicular to the thin - film lithium - niobate plane. The refractive index matrices corresponding to different waveguide cross - sections in the y - z plane transmission direction of the thin - film lithium - niobate are different, and the total thickness of the lithium - niobate thin film is 300 nm. The electrode structure of the folded modulator 4 adopts a horizontal design, which is 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 by a factor of two.

[0043] In order to reduce the polarization error caused by the polarization cross - coupling effect in the fiber optic gyroscope, according to the birefringence difference of the lithium - niobate waveguide, the length difference between the two arms 5 and 6 of the folded modulator 4 is 900 μm.

[0044] As Figure 8 shown, the cross - sectional structure of the folded modulator 4 from left to right is: metal electrode, waveguide, metal electrode, waveguide, metal electrode. The metal electrode and the lithium - niobate (LiNbO3) flat layer are separated by a 100 - nm - thick SiO2 to achieve the effect of reducing metal absorption. The radio - frequency electric - field distribution in the cross - section of the lithium - niobate waveguide is mainly along the horizontal direction, so that the electrode spacing can be reduced to increase the overlap factor between the radio - frequency electric field and the waveguide quasi - TE 00 optical mode field, thereby improving the modulation efficiency of the electro - optic folded modulator 4.

[0045] To improve the electro-optical modulation efficiency and reduce the transmission loss of waveguide modes, the embodiments of the present utility model reasonably design the electrode spacing. The product of half-wave voltage and length V π L and the relationship between the transmission loss and the metal spacing are as shown in Figure 9 shown. According to Figure 9 the results shown in, the embodiments of the present utility model aim at a loss lower than 0.1 dB / cm. In the embodiments of the present utility model, the electrode spacing can be selected as 7.5 μm. At this time, V π L is 2.57 V·cm, and the waveguide transmission loss is 0.01 dB / cm. By setting a reasonable electrode spacing, the embodiments of the present utility model avoid the additional absorption loss caused by the influence of the quasi-TE 00 fundamental mode in the lithium niobate waveguide being absorbed by the metal electrode.

[0046] The structure of the fiber optic gyroscope provided by the embodiments of the present utility model is further described. The first fiber-lithium niobate waveguide mode converter 1, the second fiber-lithium niobate waveguide mode converter 7, the third fiber-lithium niobate waveguide mode converter 8, and the fourth fiber-lithium niobate waveguide mode converter 9 all adopt an inverse-taper structure to achieve the mode field matching between the lithium niobate waveguide at the end face and the polarization-maintaining fiber loop. And all the fiber-lithium niobate waveguide mode converters are located on the same side, which is convenient for coupling with an external light source, a photodetector, and a fiber loop using a single fiber array (FA). The end face on this side is ground and polished to reduce the end face reflection.

[0047] The structures of the first optical coupler 2 and the second optical coupler 3 are specifically described. The first optical coupler 2 and the second optical coupler 3 are Y-type couplers and / or 1×2 multimode interferometer couplers.

[0048] To reduce the parasitic reflection generated by the device, as shown in Figure 10 shown, the Y-type coupler includes: a fundamental waveguide 201, a tapered region 202, a first branch 205-1, and a second branch 205-2; the fundamental waveguide 201 leads out the first branch 205-1 and the second branch 205-2 from two ports respectively 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 gradually widens 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 by widening the wider side 203 of the tapered region 202, the overlapping integral of the modes on both sides is increased, ensuring that the Y-type coupler has a large forward transmittance.

[0049] The structure of the 1×2 multimode interferometer in the embodiments of the present utility model is as shown in Figure 11As shown, a partial etching method is adopted to form channels 301 on both sides of the port for light to leak into the flat plate, thereby reducing reflection. The edge of the entire flat plate area 302 is a tapered structure, so that the light leaking into the flat plate leaks out through the taper. In this embodiment, the width of the multimode region 303 is 10.2 μm, and the length of the multimode region 303 is 93 μm.

[0050] The lithium niobate waveguide in the embodiment of the present invention is a single-mode single-polarization waveguide. After losing 3 dB of light through the first 3 dB optical coupler 2, this mode enters the second 3 dB optical coupler 3 and is evenly divided into two beams. After the two beams of light are phase-modulated by the two arms 5 and 6 of the folded modulator 4, they are respectively coupled into the optical fiber loop through the second fiber-lithium niobate waveguide mode converter 7 and the third fiber-lithium niobate waveguide mode converter 8; after the two beams of light enter the optical fiber loop, they propagate in opposite directions along the clockwise and counterclockwise directions respectively, and each enters another arm of the folded modulator 4. Due to the Sagnac effect of the optical fiber loop, when the optical fiber loop rotates in a plane, a phase difference related to the rotation angular velocity will be generated between the two beams of light. Since the two beams of light are coherent. When the two beams of light return to the second 3 dB optical coupler 3 again through the folded modulator 4, interference will occur; the interference optical signal returns to the first 3 dB optical coupler 2 and then enters the fourth fiber-lithium niobate waveguide mode converter 9 from another branch, and is output to the optical fiber array on the right along the direction of arrow 12, so that the output light is received by the photodetector. When the angular velocity changes, the interference light intensity changes accordingly, and the angular velocity of the rotation of the optical fiber loop can be demodulated through this change; since part of the light leaks into the waveguide flat plate layer during the processes of optical fiber coupling, transmission, beam combination, etc., stray light absorbers 10 are arranged in the lithium niobate flat plate layer near the first to fourth fiber-lithium niobate waveguide mode converters. Utilizing the light absorption characteristics of the metal, the free propagation of stray light in the waveguide flat plate layer and its entry into the photodetector are avoided.

[0051] In summary, the embodiment of the present invention utilizes the excellent electro-optic modulation performance of thin-film lithium niobate to simultaneously achieve polarization generation, low-loss optical transmission, beam splitting, and modulation on the chip. While ensuring the accuracy of the closed-loop fiber optic gyroscope, the embodiment of the present invention realizes a closed-loop fiber optic gyroscope with higher integration, lower driving voltage, and lower cost through special structural design and the like.

[0052] The details not described in the embodiments of the present invention are all well-known technologies in the technical field. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. An optical fiber gyroscope, characterized in that, Including: An external light source, a first fiber - lithium niobate waveguide mode converter, a first optical coupler, a second optical coupler, a folded modulator, a second fiber - lithium niobate waveguide mode converter, a third fiber - lithium niobate waveguide mode converter, a fourth fiber - lithium niobate waveguide mode converter, a photodetector, and a stray light absorber; the first end of the first fiber - lithium niobate waveguide mode converter is connected to the external light source, and the second end of the first fiber - lithium niobate waveguide mode converter is connected to the first branch of the first optical coupler; the fundamental waveguide of the first optical coupler and the fundamental waveguide of the second optical coupler are connected by 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 fiber - lithium niobate waveguide mode converter, and the second branch of the second optical coupler is connected to the first end of the third fiber - lithium niobate waveguide mode converter; the two arms of the folded modulator are the two branches of the second optical coupler, and both arms of the folded modulator form a folded structure through two 180° partially variable - curvature hybrid - bending waveguides; the second end of the second fiber - lithium niobate waveguide mode converter is connected to one end of an external polarization - maintaining fiber loop; the second end of the third fiber - lithium niobate waveguide mode converter is connected to the other end of the polarization - maintaining fiber loop; the second branch of the first optical coupler is connected to the first end of the fourth fiber - lithium niobate waveguide mode converter, and the second end of the fourth fiber - lithium niobate waveguide mode converter is connected to the photodetector; the stray light absorber is located on both sides of the first fiber - lithium niobate waveguide mode converter, the second fiber - lithium niobate waveguide mode converter, the third fiber - lithium niobate waveguide mode converter, and the fourth fiber - lithium niobate waveguide mode converter; the single - TE - mode lithium niobate waveguides except for the hybrid - bending waveguides and optical couplers of the folded modulator form a waveguide polarizer; the waveguide polarizer is composed of a lithium niobate waveguide with shallow etching and / or narrow width.

2. The fiber optic gyroscope according to claim 1, characterized in that, The waveguide polarizer, from top to bottom, is respectively an air layer, a low - refractive - index capping 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 capping layer is 1 - 1.

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3. The fiber optic gyroscope according to claim 1 or 2, characterized in that, The width of the waveguide polarizer < 2.5μm, the etching depth < 0.4μm, and the working wavelength range is 1210 - 1410nm.

4. The fiber optic gyroscope according to claim 1, wherein The partially variable - curvature hybrid - bending waveguide is jointly composed of a linearly variable - curvature circular - arc waveguide and a constant - curvature circular - arc waveguide.

5. The fiber optic gyroscope according to claim 4, characterized in that, The width of the 90° partially variable - curvature hybrid - bending waveguide < 2.5μm, the proportion of the angle of the linearly variable - curvature circular - arc waveguide in the total bending angle < 95%, and the minimum radius of the 90° partially variable - curvature hybrid - bending waveguide > 80μm.

6. The fiber optic gyroscope according to claim 4, wherein, The width of the 180° partially variable - curvature hybrid - bending waveguide < 5μm, the proportion of the angle of the linearly variable - curvature circular - arc waveguide in the total bending angle < 95%, and the minimum radius of the 180° partially variable - curvature hybrid - bending waveguide > 50μm.

7. The fiber optic gyroscope according to claim 1, characterized in that, The folded modulator is a lithium niobate electro-optic folded modulator, which uses an x-cut thin film lithium niobate wafer, and the electrode structure of the folded modulator adopts a horizontal design, which 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 fiber optic gyroscope according to claim 1, wherein, The first fiber-lithium niobate waveguide mode converter, the second fiber-lithium niobate waveguide mode converter, the third fiber-lithium niobate waveguide mode converter, and the fourth fiber-lithium niobate waveguide mode converter all adopt an inverse taper structure to achieve mode field matching between the lithium niobate waveguide and the polarization-maintaining fiber loop at the end face.

9. The fiber optic gyroscope according to claim 1, wherein, The first optical coupler and the second optical coupler are Y-type couplers and / or 1×2 multimode interferometer couplers.

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