A three-axis integrated optical gyroscope with a full-thin-film lithium niobate chip

Abstract

The present invention discloses a full-thin-film lithium niobate chip for a three-axis integrated optical gyroscope, belonging to the field of optoelectronic integration technology. It includes an input port and six output ports, wherein the input port and any output port form a branch, each branch is provided with a grating coupler, an on-chip polarizer and a phase modulator, each branch is connected to the output port through a 1×n beam splitter, and a pattern spot converter is placed on the input port and the six output ports. The light input by the present invention can realize the functions of beam splitting, polarization, modulation, coupling, etc., and can be coupled with a light source and a detector chip by termination, so as to construct a three-axis integrated optical gyroscope system. The coupling efficiency between the pattern spot converter and the optical fiber can reach 90%, the coupling efficiency between the grating coupler and the photodetector can reach 50%, and the modulation efficiency of the three phase modulators on the chip can reach 1.5V·cm. After forming a complete gyroscope system, the accuracy difference between the three axes is ±5%.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic integration technology, and in particular to a full-thin-film lithium niobate chip for a three-axis integrated optical gyroscope. Background Art

[0002] Interferometric optical gyroscopes (IGOs) are inertial attitude measurement devices that have been widely used due to their high precision, reliability, and fast response speed. In recent years, with the increasing subdivision of application scenarios such as drones, autonomous driving, and resource exploration, IGOs ​​are required to be miniaturized and lightweight while maintaining high performance. Furthermore, for many applications, at least three gyroscopes are typically used simultaneously to obtain three-axis attitude information. Therefore, on-chip implementation of three-axis gyroscopes is inevitable, but no mature solutions have been reported.

[0003] Interferometric optical gyroscopes contain five types of optical devices: light sources, detectors, couplers, modulators, and fiber rings. Among them, on-chip integration of passive devices is relatively easy to achieve. However, on-chip multi-material integration of multiple active and passive devices is still difficult to achieve due to its technological immaturity. Therefore, using the same material platform to integrate various functional devices has become an important way to solve this problem.

[0004] Integrated optical chips are currently a research hotspot. Lithium niobate material is called "optical silicon" because of its excellent electro-optical effect coefficient and first-order nonlinear coefficient. This material can be used to prepare phase modulators, beam splitters and other passive optical structures required for gyroscopes on the same chip, realizing the integration of discrete devices. It can further integrate all the devices required for a three-axis integrated optical gyroscope on the chip, realizing the key technology exploration of the monolithic three-axis integrated gyroscope. No similar solutions have been reported so far. Summary of the Invention

[0005] The present invention aims to provide an all-thin-film lithium niobate chip for a three-axis integrated optical gyroscope. On this thin-film lithium niobate platform, the 1×n beam splitter, grating coupler, phase modulator, on-chip polarizer, and spot size converter required for the three-axis integrated optical gyroscope are simultaneously fabricated. The components are interconnected via thin-film lithium niobate optical waveguides, ensuring that the functions of each component meet the requirements of the integrated gyroscope (the coupling efficiency between the spot size converter and the optical fiber / chip can reach 90%, the polarization extinction ratio of the polarizer is ≥80dB, the coupling efficiency between the grating coupler and the photodetector can reach 50%, and the modulation efficiency of the modulator is ≤1.5V·cm).

[0006] To achieve the above objectives, the present invention provides a full-thin-film lithium niobate chip for a three-axis integrated optical gyroscope, comprising an input port and six output ports, wherein the input port and any output port form a branch, each of the branches is provided with a grating coupler, an on-chip polarizer and a phase modulator, each of the branches is connected to the output port via a 1×n beam splitter, and the input port and the six output ports are all provided with a pattern spot converter.

[0007] First, a thin-film lithium niobate wafer of appropriate thickness is selected, and its bottom flatness, thin-film uniformity and other parameters are tested. After confirming the wafer selection, the next step is pattern transfer, followed by a series of steps such as electron beam exposure / photolithography and dry etching to realize the generation of thin-film lithium niobate waveguide devices and structures on the wafer. Subsequently, the sputtering electrode layer, upper cladding deposition, chemical mechanical polishing, gold wire bonding, optical fiber coupling and other steps are carried out to complete the preparation and coupling of the full lithium niobate chip.

[0008] Preferably, the pattern spot converter is a multi-layer tapered structure, which is used to achieve coupling with the optical fiber / chip.

[0009] Preferably, the on-chip polarizer is a multi-stage cascade structure, and is used to be placed before the light input modulation area.

[0010] Preferably, the grating coupler adopts a fan-shaped structure.

[0011] Preferably, the phase modulator adopts a push-pull electrode structure, including a ground electrode and a signal electrode, and the ground electrode is located on both sides of the signal electrode.

[0012] Preferably, the end face of the output port is coupled to a broadband laser light source, each of the grating couplers is coupled to a photodetector, and the six output ports are coupled to optical fibers and connected to three optical fiber rings.

[0013] Preferably, the spot size converter, the 1×n beam splitter, the on-chip polarizer, and the phase modulator are all made of thin-film lithium niobate material.

[0014] Therefore, the present invention adopts a three-axis integrated optical gyroscope with a full-thin-film lithium niobate chip of the above structure, which has the following beneficial effects:

[0015] (1) On a unified thin-film lithium niobate optical platform, the integration of multiple functional devices for a three-axis integrated optical gyroscope is achieved, while miniaturizing and integrating each device and ensuring the performance of each integrated optical device.

[0016] (2) The present invention has achieved a preliminary exploration of a chip for a three-axis integrated optical gyroscope, in which the coupling efficiency of the pattern converter is ≥90%, the coupling efficiency between the grating coupler and the photodetector can reach 50%, the theoretical polarization extinction ratio of the cascade polarizer is greater than 80dB, and the theoretical modulation efficiency of the thin-film lithium niobate modulator is 1.5V·cm. After the three-axis optical gyroscope is constructed, the accuracy difference between each axis is ±5%, which lays a certain foundation for the full on-chip integration of active and passive components of the future three-axis optical gyroscope and explores a new route for the miniaturization and lightweight development of optical gyroscopes.

[0017] (3) The present invention can be adapted to the existing thin-film lithium niobate chip processing technology, and the processing parameters of each device therein can be continuously iteratively optimized to eventually form a PDK, which has the prospect of large-scale and batch production to reduce the overall cost of the three-axis integrated optical gyroscope system.

[0018] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the device distribution of a three-axis integrated optical gyroscope with a full-thin-film lithium niobate core;

[0020] Figure 2 This is a schematic structural diagram of a thin-film lithium niobate three-layer conical spot converter used in a full-film lithium niobate core for a three-axis integrated optical gyroscope of the present invention;

[0021] Figure 3 This is a schematic structural diagram of a thin-film lithium niobate grating coupler in a full-thin-film lithium niobate core for a three-axis integrated optical gyroscope according to the present invention;

[0022] Figure 4 This is a schematic diagram of the working process of a thin-film lithium niobate grating coupler in a full-thin-film lithium niobate core for a three-axis integrated optical gyroscope according to the present invention;

[0023] Figure 5 This is a schematic diagram of a cascade structure of polarizers on thin-film lithium niobate sheets in a full-thin-film lithium niobate core for a three-axis integrated optical gyroscope according to the present invention;

[0024] Figure 6 A schematic diagram of a push-pull electrode thin-film lithium niobate phase modulator in a full-thin-film lithium niobate core for a three-axis integrated optical gyroscope according to the present invention;

[0025] Figure 7 This is a schematic diagram of the optical path system of a three-axis integrated optical gyroscope formed by connecting a three-axis integrated optical gyroscope full-thin-film lithium niobate chip in a three-axis integrated optical gyroscope full-thin-film lithium niobate core to a broadband laser light source, a photodetector, and an optical fiber ring;

[0026] Reference numerals

[0027] 1. 1×n beam splitter; 2. Grating coupler; 3. On-chip polarizer; 4. Phase modulator; 5. Pattern converter; 6. Ground electrode; 7. Signal electrode; 8. Broad-spectrum light source for gyroscope; 9. Photodetector; 10. Fiber ring. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0029] Unless otherwise defined, the technical or scientific terms used in the present invention shall have the usual meanings understood by persons of ordinary skill in the field to which the present invention belongs. The words "first", "second" and similar terms used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0030] Example

[0031] like Figure 1-5 As shown, the present invention provides a full-thin-film lithium niobate chip for a three-axis integrated optical gyroscope, including one output port and six output ports, wherein every two output ports form a branch, each branch is provided with a grating coupler 2, an on-chip polarizer 3 and a phase modulator 4, each branch is connected to the output port through a 1×n beam splitter 1, and a pattern spot converter 5 is placed on the input port and the six output ports.

[0032] The spot converter 5 is a three-layer tapered structure, and is used to achieve coupling with the optical fiber / chip.

[0033] The on-chip polarizer 3 is a multi-stage cascade structure and is used to be placed before the light input modulation area.

[0034] The grating coupler 2 adopts a fan-shaped structure.

[0035] The phase modulator 4 adopts a push-pull electrode structure, including a ground electrode and a signal electrode, wherein the ground electrodes are located on both sides of the signal electrode.

[0036] The end face of the output port is coupled to a broadband laser light source 8 , each grating coupler is coupled to a photodetector 9 , and the six output ports are coupled to optical fibers and connected to three optical fiber rings 10 .

[0037] The spot size converter 5, the 1×n beam splitter 1, the on-chip polarizer 3, and the phase modulator 4 are all made of thin-film lithium niobate material.

[0038] Working principle: First, a thin-film lithium niobate wafer of appropriate thickness is selected, and its bottom flatness, film uniformity and other parameters are tested. After confirming the wafer selection, the next step is pattern transfer and then electron beam exposure / photolithography, dry etching and other steps to realize the generation of thin-film lithium niobate waveguide devices and structures on the wafer. Subsequently, the electrode layer is sputtered, the upper cladding layer is deposited, chemical mechanical polishing, gold wire bonding, optical fiber coupling and other steps are carried out to complete the preparation and coupling of the full lithium niobate chip.

[0039] The all-lithium niobate chips of the three-axis integrated optical gyroscope all use x-cut lithium niobate material, and the waveguides are all ridge waveguide structures. The waveguides used to construct and connect devices are all single-mode waveguides.

[0040] A three-layer inverted cone-shaped pattern converter is placed on the single input port and six output ports of the all-lithium niobate chip, thereby achieving efficient coupling with the broadband laser chip / fiber, with a coupling efficiency of ≥90%.

[0041] The all-lithium niobate chip detects and receives the gyroscope's return light through a photodetector mounted on the surface of the grating coupler. The coupling efficiency of the grating coupler can reach 50%.

[0042] The all-lithium niobate chip places an on-chip polarizing structure on the optical waveguide entering the modulator and performs multi-stage cascading to improve the polarization extinction ratio of the light entering the modulation area, ensuring a polarization extinction ratio of ≥80dB.

[0043] The all-lithium niobate chip features an active structure consisting of three identical lithium niobate phase modulators. The electrodes are placed directly on the lithium niobate layer. To balance modulation efficiency with light absorption losses from the electrodes, and to relatively reduce the half-wave voltage and improve modulation efficiency, a push-pull electrode structure is employed. The modulation efficiency of the phase modulators is ≤1.5V·cm.

[0044] The all-lithium niobate chip, by terminating the broadband laser diode die and the surface-mounted photodetector chip, can form a complete hybrid integrated optical gyroscope driver chip, which is further connected to three identical optical fiber rings to complete the construction of the entire three-axis integrated optical gyroscope optical path system. The gyroscope accuracy difference between the three axes is ±5%.

[0045] Figure 1This is a device distribution diagram of an all-lithium niobate chip for a three-axis integrated optical gyroscope according to the present invention. The chip is composed of five on-chip waveguide devices: a spot size converter, a 1×n beam splitter, a grating coupler, an on-chip polarizer, and a phase modulator. All of the above devices are composed of thin-film lithium niobate waveguides. In the chip according to the present invention, input light enters through one port of the 1×3 beam splitter, passes through two 1×2 couplers, and then passes through the cascaded on-chip polarizers into the phase modulator. The phase-modulated light then passes through the spot size converter and enters the subsequent optical fiber ring. After completing transmission in the optical fiber ring, the light returns to the all-lithium niobate chip and is finally coupled to the photodetector through the grating coupler, completing the entire optical transmission process of the three-axis gyroscope.

[0046] Figure 2 The waveguide spot converter has a three-layer inverted tapered structure, wherein the length of each section L1=L2=L3=100μm~1mm, the length is inversely proportional to the loss and does not affect the coupling efficiency. The tapered waveguide width parameters are W1=5μm~100μm, W2=1μm~5μm, W3=0.5μm~1μm, and W4=0.01μm~0.5μm. The coupling efficiency between the spot converter and the optical fiber can reach more than 90%;

[0047] Figure 3 is a structural diagram of the grating coupler, Figure 4 This is a schematic diagram of the coupling process between the grating coupler and the photodetector. The fan-shaped structure is used, and its size matches the size of the photosensitive surface of the photodetector. The coupling efficiency can reach 50%.

[0048] Figure 5 For the on-chip polarizer, the extinction ratio of the light in the progress modulation area is improved through a multi-stage cascade method to ensure the gyro accuracy;

[0049] Figure 6 The figure is a schematic diagram of the structure of the phase modulator. The phase modulator adopts a push-pull electrode structure, that is, the ground electrode between the two arms uses the same electrode. Through this design, the half-wave voltage of the modulator is reduced, and the modulation efficiency is improved, ensuring ≤1.5V·cm;

[0050] Figure 7 The complete optical path of the three-axis integrated optical gyroscope system composed of the all-lithium niobate chip is as follows: the chip of the present invention couples a wide-spectrum light source at the input port, couples the detector component through a grating coupler, and couples the optical fiber ring at the output port to form an on-chip hybrid integrated three-axis optical gyroscope, thereby realizing the closed-loop optical path of the three-axis optical gyroscope.

[0051] The size of the three-axis gyroscope formed by this chip mainly depends on the configured fiber optic ring. The use of a fiber optic ring wound with a thin-diameter optical fiber can greatly reduce the size of the three-axis optical gyroscope.

[0052] Therefore, the present invention adopts the above-mentioned all-thin-film lithium niobate chip for a three-axis integrated optical gyroscope, in which all devices are based on thin-film lithium niobate materials, and on-chip optical interconnection between devices is achieved through thin-film lithium niobate waveguides. Light input from a single port of the chip can achieve functions such as beam splitting, polarization, modulation, and coupling during transmission within the chip. The light source and detector chip can be coupled by termination, and can be used to construct a three-axis integrated optical gyroscope system. The all-thin-film lithium niobate chip has a coupling efficiency of up to 90% between the pattern converter and the optical fiber, a coupling efficiency of up to 50% between its grating coupler and the photodetector, and a modulation efficiency of up to 1.5V·cm for each of the three phase modulators on the chip. After forming a complete gyroscope system, the accuracy difference between the three axes is ±5%.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A three-axis integrated optical gyroscope with a thin-film lithium niobate chip, characterized by: The device comprises an input port and six output ports, wherein the input port and any output port form a branch, each branch is provided with a grating coupler, an on-chip polarizer, and a phase modulator, each branch is connected to the output port via a 1×n beam splitter, and the input port and the six output ports are each provided with a pattern spot converter; The pattern converter is a multi-layer tapered structure, wherein the length of each section L1=L2=L3=100μm~1mm, and the length is inversely proportional to the loss. The tapered waveguide width parameters are W1=5μm~100μm, W2=1μm~5μm, W3=0.5μm~1μm, and W4=0.01μm~0.5μm, respectively, for achieving coupling with the optical fiber / chip; The on-chip polarizer is a multi-stage cascade structure, and is used to be placed before the light input modulation area; The grating coupler adopts a fan-shaped structure, the size of which matches the size of the photosensitive surface of the photodetector, and the coupling efficiency reaches 50%; The phase modulator adopts a push-pull electrode structure, including a ground electrode and a signal electrode, wherein the ground electrodes are located on both sides of the signal electrode, and the ground electrodes between the two arms use the same electrode; The end face of the output port is coupled with a broadband laser light source, each of the grating couplers is coupled with a photodetector, and the six output ports are coupled with optical fibers and connected to three optical fiber rings. By terminating the broadband laser diode die and the surface-mounted photodetector chip, a complete hybrid integrated optical gyroscope driver chip is formed, which is further connected to three identical optical fiber rings to complete the construction of the entire three-axis integrated optical gyroscope optical path system. The gyroscope accuracy difference between the three axes is ±5.

2. The all-thin-film lithium niobate chip for a three-axis integrated optical gyroscope according to claim 1, characterized in that: The spot size converter, the 1×n beam splitter, the on-chip polarizer, and the phase modulator are all made of thin-film lithium niobate material.

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