CN117433500B - Polarization mode multiplexing double optical path fiber optic gyroscope based on photonic chip - Google Patents

Abstract

The embodiment of the present invention discloses a polarization mode multiplexing double optical path optical fiber gyroscope based on a photon chip, comprising a photon integrated chip, a light source, a detector, a fiber ring, a modulator and a signal processing circuit, wherein the photon integrated chip comprises a substrate, on which a polarization selection grating, two beam splitters and four end couplers are integrated, the light source and the detector are each connected to one of the beam splitters through an end coupler, the pigtails at both ends of the fiber ring are each connected to another beam splitter through an end coupler, and the polarization selection grating connects the two beam splitters; the polarization selection grating transmits TM polarized light and reflects TE polarized light, or reflects TM polarized light and transmits TE polarized light. The present invention utilizes a photon integrated chip design including a polarization selection grating to realize the double optical path of polarization mode multiplexing of a fiber gyroscope, enhances the Sagnac rotation effect of the gyroscope without increasing the length of the fiber ring, and realizes the small size, low cost and high precision of the gyroscope.

Description

Polarization mode multiplexing double optical path fiber optic gyroscope based on photonic chip

Technical Field

The invention relates to the technical fields of integrated optics and fiber optic gyroscopes, and in particular to a fiber optic gyroscope based on a photonic chip that multiplexes polarization modes and doubles the optical path.

Background technique

A gyroscope is a sensitive device used to measure the rotation angular velocity of a carrier. When the three axes are installed orthogonally, it can be used to establish a spatial coordinate system for motion, which is the basis for motion measurement and control. Gyroscopes can be classified into mechanical gyroscopes, laser gyroscopes, fiber optic gyroscopes, micromechanical gyroscopes, etc. based on their working principles. The fiber optic gyroscope is based on the Sagnac interference principle and uses the interference of two forward and reverse light beams in a closed fiber loop to measure the rotation angular velocity. It has the characteristics of high precision, high reliability and high bandwidth.

Traditional fiber optic gyroscopes are made by welding discrete components such as polarizers and couplers one by one and rely on manual assembly. The gyroscopes produced in this way are large in size, high in cost, poor in consistency, and low in production efficiency. Integrated optical technology is used to integrate the core components used in fiber optic gyroscopes on the same photonic chip, greatly reducing the size of the device, and the large-scale tape-out process can achieve low cost and consistency of the device. Therefore, photonic integration of fiber optic gyroscopes is an important direction for the development of fiber optic gyroscopes.

The Sagnac phase ϕ _s of the fiber optic gyroscope can be expressed by equation (1):

(1)

Among them, L is the length of the fiber, D is the diameter of the fiber ring, λ is the operating wavelength, c is the speed of light in vacuum, and Ω is the rotation angular velocity. According to the interference principle shown in equation (1), when the gyroscope size (fiber ring diameter) and operating wavelength are determined, the longer the fiber length, the higher the gyroscope's sensitivity to rotational speed, that is, the higher the accuracy. However, the longer the optical fiber, the larger the gyroscope and the higher the cost. In response to this problem, scholars proposed to use optical path design methods to make the light pass through the fiber ring twice without increasing the length of the optical fiber, thereby doubling the optical path and achieving the goal of improving the accuracy of the gyroscope. Currently proposed solutions include using multi-core optical fibers or adding multiple discrete beam splitters. However, the additional components required for the current solution are high in cost and large in size, making it difficult to meet the development needs of miniaturization and low cost of gyroscopes.

Summary of the invention

The technical problem to be solved by the embodiments of the present invention is to provide a polarization mode multiplexing photonic chip to realize a fiber optic gyroscope with double optical path, allowing light to pass through the fiber ring twice, doubling the optical path, low cost and miniaturization. Improve gyro accuracy.

In order to solve the above technical problems, embodiments of the present invention propose a photonic chip-based polarization mode multiplexing double optical path fiber optic gyroscope, including a photonic integrated chip, a light source, a detector, a fiber ring, a modulator and a signal processing circuit. The photonic integrated chip includes a substrate on which a polarization-selective grating, two beam splitters and four end-face couplers are integrated. The light source and the detector are each connected to one of the beam splitters through an end-face coupler. The optical fiber ring The pigtails at both ends are each connected to the other beam splitter through an end face coupler, and the polarization selection grating connects the two beam splitters; the polarization selection grating transmits TM polarized light and reflects TE polarized light, or reflects TM polarized light and TE Polarized light transmission.

Furthermore, relative to the coupling polarization axis direction of the photonic integrated chip, the polarization angle of the pigtail at one end of the optical fiber ring is 0°, and the polarization angle of the pigtail at the other end is 90°.

Furthermore, the polarization selective grating adopts a grating structure composed of waveguides with periodic alternating refractive index, and the grating period L satisfies the following Bragg grating equation:

Among them, λ is the working center wavelength, L ₁ is the length of the small width area in the grating period, / is the effective refractive index of the TE/TM mode in the small-width waveguide, and / is the effective refractive index of the TE/TM mode in a waveguide with a large width in the period.

Further, the waveguide of the polarization selective grating is integrated on the substrate of the photonic integrated chip, and the material of the substrate is one or more of Si, SiO ₂ , Si ₃ N ₄ , and LNOI.

Further, the beam splitter adopts Y waveguide, directional coupler or multi-mode interferometer.

Further, the end coupler has an inverted tapered structure.

Furthermore, the light source uses a high polarization light source or a low polarization light source. If the polarization selection grating is designed to transmit TM polarized light, the light source enters the photonic integrated chip in the TM polarization state; if the polarization selection grating is designed to transmit TE polarized light, the light source The polarization state of TE enters the photonic integrated chip.

Furthermore, the light source is mounted on the side of the photon integrated chip by end-face coupling; or a high-polarization light source is first axially coupled to a polarization-maintaining fiber, and then the axial end face of the polarization-maintaining fiber is coupled to the photon integrated chip; or a low-polarization light source is first coupled to a polarization-maintaining fiber, a fiber polarizer is introduced into the polarization-maintaining fiber, and then the axial end face of the polarization-maintaining fiber is coupled to the photon integrated chip.

Furthermore, the detector is mounted on the side of the photonic integrated chip to complete the photoelectric conversion, and is connected to the signal processing circuit to complete the resolution of the gyroscope signal; or the signal light is first coupled to the receiving optical fiber, and then the signal light is guided to the detector through the receiving optical fiber.

Further, it is applied to multiple wavelength systems of 830nm, 850nm, 1310nm, and 1550nm.

The beneficial effects of the present invention are:

1. The present invention integrates a polarization selective grating, a beam splitter A, a beam splitter B and four end couplers on a photonic integrated chip, replacing the traditional fiber optic gyroscope optical device discrete production, splicing, and manual winding production methods, thereby improving the scale production level of the gyroscope, reducing the size of the gyroscope, reducing the cost of the gyroscope, and improving the consistency of gyroscope production.

2. The present invention uses a polarization-selective grating structure and a coupling method of polarization mode conversion to propagate light twice in the fiber ring, thereby doubling the optical path and improving gyro accuracy at low cost and small size.

3. The present invention realizes low-loss coupling of light sources, optical fibers and photonic integrated chips, has a compact structure and high stability, and is conducive to large-scale mass production of gyroscopes.

4. The present invention is suitable for multiple types of substrates such as silicon, silicon dioxide, silicon nitride, silicon oxynitride, and thin film lithium niobate, and is suitable for multi-working wavelength systems.

Description of drawings

Figure 1 is a schematic structural diagram of a fiber optic gyroscope based on a photonic chip that multiplexes polarization modes and doubles the optical path according to an embodiment of the present invention.

Figure 2 is a schematic diagram of the principle of a polarization selective grating according to an embodiment of the present invention.

Figure 3 is a simulation diagram of a polarization selective grating according to an embodiment of the present invention.

Figure 4 is a simulation diagram of the MMI beam splitter design according to the embodiment of the present invention.

FIG. 5 is an example diagram of an end-face coupler design simulation according to an embodiment of the present invention.

Explanation of reference numbers

Photonic integrated chip 1, polarization selective grating 2, light source 3, end coupler 4, beam splitter 5, fiber ring 6, detector 7, modulator 8, signal processing circuit 9, mode conversion waveguide 11, grating structure 12.

Detailed ways

It should be noted that, in the absence of conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

If there are directional indications (such as up, down, left, right, front, back...) in the embodiments of the present invention, they are only used to explain the relative positions between the components in a specific posture (as shown in the accompanying drawings). relationship, movement conditions, etc., if the specific posture changes, the directional indication will also change accordingly.

In addition, descriptions involving "first", "second", etc. in the present invention are for descriptive purposes only and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of indicated technical features. Therefore, features defined as "first" and "second" may explicitly or implicitly include at least one of these features.

Referring to Fig. 1, the polarization mode multiplexing double optical path optical fiber gyroscope based on the photon chip of the embodiment of the present invention comprises a photon integrated chip, a light source, a detector, a fiber ring, a modulator, and a signal processing circuit. The photon integrated chip comprises a substrate, on which a polarization selective grating, two beam splitters, and four end couplers are integrated.

There are 4 end-face couplers, one is coupled with the light source, and its mode field is designed to match the mode field of the light source; one is coupled with the detector, and the mode field matches the photosensitive surface of the detector; the other two are coupled with the polarization-maintaining pigtails of the fiber ring, and the mode Field design matches fiber mode field. The function of the end face coupler is to improve the coupling efficiency between the light source/optical fiber and the photonic integrated chip and reduce the loss of the optical path.

The beam splitting ratio of the two beam splitters is 1:1, and their function is to divide one incident light into two outgoing lights of equal amplitude. A polarization-selective grating is used between the two beam splitters. Through the design of the grating duty cycle and grating period, polarization splitting is achieved, whereby TM polarized light passes and TE polarized light is reflected, or TE polarized light passes and TM polarized light is reflected.

The main axes of the two pigtails of the fiber ring are respectively parallel and perpendicular to the coupling polarization axis of the photonic integrated chip. Therefore, the TM light passing through the fiber ring for the first time is converted into TE light, returns to the polarization selection grating and is reflected back to the fiber ring, and then passes through the fiber ring. After the second propagation of the optical fiber ring, the TE light is converted into TM light, enters the detector through the polarization selection grating, and completes signal demodulation. In this way, the interference light passes through the fiber ring twice in the form of TM and TE light, doubling the interference optical path.

Example: To facilitate the description of the embodiment of the present invention, a polarization selective grating is used that passes TM polarized light and reflects TE polarized light.

In the embodiment of the present invention, the light source is a high-bias TM output, and the end-face coupling is mounted on the side of the photonic integrated chip. The position of the light source is fine-tuned to align the light source emission position with the receiving end of the end-face coupler to ensure that the light incident on the photonic chip is maximized. TM polarized light is transmitted in the photonic integrated chip and enters the polarization selection grating through a beam splitter. The TM light passes through the polarization selective grating and is divided into two beams of equal amplitude by another beam splitter. One beam of light enters the fiber ring clockwise through the end face coupler, and the other beam enters the fiber ring counterclockwise through the end face coupler. The polarization angle of one end of the pigtail is 0°, and the polarization angle of the other end is 90°.

Due to this orthogonal polarization mode coupling, the light passing through the fiber ring is converted into the orthogonal polarization state, and the TM polarized light passing through the fiber ring is converted into TE polarized light. Both forward and reverse beams of light are combined by the beam splitter and enter the polarization selection grating in the TE polarization state. They are reflected by the grating and return to the beam splitter. They are again divided into two beams of forward and reverse transmission. After passing through the optical fiber ring, they are transmitted again. In one polarization conversion, TE polarized light is converted into TM polarized light, which undergoes combined beam interference through a beam splitter and carries phase information (including the modulation phase generated by the modulator to increase sensitivity and the Sagnac phase to be demodulated, which is linearly related to the rotation speed) The TM polarization state passes through the polarization selection grating and enters the beam splitter, where half of the light enters the detector to complete photoelectric conversion. The detector is connected to the signal processing circuit to complete the calculation of the rotational speed signal.

Each time the clockwise light passes through the fiber ring, an additional phase of 1/2 ( +/> ), each time the counterclockwise light passes through the fiber ring, an additional phase of −1/2(/> +/> ), where/> is the modulation phase. After two beam splittings before entering the ring, the incident light is divided into four beams of equal amplitude. The light I _cwcw that passes through the fiber ring clockwise twice carries the phase (/> +/> ); The light I _ccwccw that passes through the fiber ring counterclockwise twice carries the phase (/> ); for the light that passes clockwise for the first time and counterclockwise for the second time, I _cwccw , the additional phase offset is 0, and for the light that passes counterclockwise for the first time and clockwise for the second time, I _ccwcw , the additional phase offset is 0. When the beam splitter is an ideal 1:1 beam splitter and the loss is not considered, the light intensity I after forward and reverse light interference can be expressed as:

(2)

Among them, I ₀ is the input optical power. It can be seen from equation (2) that by stably controlling the input optical power and modulation depth through the control circuit, the rotating Sagnac phase ϕ _s can be calculated by measuring the light intensity after interference, and due to the double light process, with double the signal-to-noise ratio.

The polarization selective grating in the present invention is shown in Figure 2. The waveguide structure of the polarization selective grating is integrated on the substrate. The substrate material can be silicon, silicon dioxide, silicon nitride, silicon oxynitride, thin film lithium niobate ( Si, SiO ₂ , Si ₃ N ₄ , LNOI) and so on. Polarization selective gratings are composed of mode conversion waveguides and periodic grating structures.

The mode conversion waveguide is used to achieve matching between the straight waveguide modes on both sides of the grating structure and the Bloch mode in the grating structure to reduce insertion loss.

The grating structure consists of waveguides with periodic alternating refractive index. The grating period L is designed to satisfy the following Bragg grating equation:

(3)

Among them, λ is the working center wavelength, L ₁ is the length of the small width area in the grating period, / is the effective refractive index of the TE/TM mode in the small-width waveguide, and / is the effective refractive index of the TE/TM mode in a waveguide with a large width in the period. The simulation of polarized light in a polarization selective grating is shown in Figure 3. It can be seen from the simulation results that the TM light passes through the polarization selective grating with low loss, and the TE light is reflected by the grating.

The beam splitter in the present invention can use a Y waveguide, a directional coupler or a multi-mode interferometer (MMI). For the convenience of explanation, the MMI design is taken as an example. The MMI consists of a tapered waveguide transition area and a multi-mode interference area. Specifically, The parameters are determined according to the substrate, waveguide refractive index and process parameters. The design block diagram and numerical simulation example are shown in Figure 4. The end face coupler in the present invention has an inverted tapered structure, and the tapered parameters are determined according to the waveguide parameters and the light source mode field or optical fiber mode field to be selected. A simulation example is shown in Figure 5.

The present invention uses a polarization mode multiplexed photonic integrated chip to double the optical path of the fiber optic gyroscope. It doubles the interference effect without increasing the length of the fiber, thereby greatly improving the accuracy of the gyroscope. The use of photonic integrated chips to replace traditional discrete optical devices, and the integrated optical large-scale tape-out process, are conducive to the consistency of gyroscopes, large-scale mass production, reduction of gyroscope size, reduction of gyroscope costs, and improvement of manufacturing efficiency.

The invention can be applied to Si, SiO ₂ , Si ₃ N ₄ , LNOI and other integrated optical chip materials, and can be applied to multiple wavelength systems of 830nm, 850nm, 1310nm and 1550nm. The invention realizes the effective improvement of the accuracy of the fiber optic gyroscope with small size and low cost. The present invention can also provide strong support for the development of other optical fiber sensors such as optical fiber current transformers and optical fiber hydrophones.

Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art will understand that various changes, modifications, and substitutions can be made to these embodiments without departing from the principles and spirit of the invention. and modifications, the scope of the present invention is defined by the appended claims and their equivalents.

Claims (9)

1. The optical fiber gyro comprises a photon integrated chip, a light source, a detector, an optical fiber ring, a modulator and a signal processing circuit, and is characterized in that the photon integrated chip comprises a substrate, a polarization selection grating, two beam splitters and 4 end face couplers are integrated on the substrate, the light source and the detector are respectively connected with one of the beam splitters through one end face coupler, tail fibers at two ends of the optical fiber ring are respectively connected with the other beam splitter through one end face coupler, and the polarization selection grating is connected with the two beam splitters; the polarization selection grating transmits the TM polarized light, reflects the TE polarized light, or reflects the TM polarized light, and transmits the TE polarized light;

the polarization selection grating adopts a grating structure composed of waveguides with periodic alternating refractive indexes, and the grating periodLThe following bragg grating equation is satisfied:

wherein,λas the wavelength of the center of operation,L ₁ for the length of the area of small width in the grating period,//>is the effective refractive index of TE/TM mode in the waveguide with small width, and +.>//>Is the effective refractive index of the TE/TM mode in the waveguide with large width in the period.

2. The photonic-chip-based polarization-mode multiplexing double-path fiber optic gyroscope of claim 1, wherein the fiber optic ring has a fiber pigtail polarization angle of 0 ° at one end and a fiber pigtail polarization angle of 90 ° at the other end with respect to the coupling polarization axis direction of the photonic integrated chip.

3. The photonic chip-based polarization mode multiplexing double-optical-path fiber gyroscope according to claim 1, wherein the waveguide of the polarization-selective grating is integrated on the substrate of the photonic integrated chip, and the substrate is made of Si and SiO ₂ 、Si ₃ N ₄ One or more of LNOI.

4. The photonic chip-based polarization mode multiplexing double-optical path fiber gyroscope according to claim 1, wherein the beam splitter has a beam splitting ratio of 1:1, and the beam splitter adopts a Y waveguide, a directional coupler or a multimode interferometer.

5. The photonic chip-based polarization mode multiplexing double-optical path fiber gyroscope of claim 1, wherein the end-face coupler is of a back taper structure.

6. The photonic chip-based polarization mode multiplexing double optical path fiber gyroscope of claim 1, wherein the light source uses a high-polarization light source or a low-polarization light source, and if the polarization-selective grating is designed to transmit TM polarized light, the light source enters the photonic integrated chip in TM polarized state; if the polarization-selective grating is designed to transmit TE polarized light, the light source enters the photonic integrated chip in the TE polarization state.

7. The photonic chip-based polarization mode multiplexing double-optical-path fiber gyroscope according to claim 6, wherein the light source is attached to the side of the photonic integrated chip in an end-face coupling manner; or the high-polarization light source is firstly coupled to the polarization-maintaining optical fiber in a counter shaft mode, and then the end face of the polarization-maintaining optical fiber in the counter shaft mode is coupled to the photon integrated chip; or coupling the low-polarization light source to a polarization-maintaining optical fiber, introducing an optical fiber polarizer into the polarization-maintaining optical fiber, and coupling the polarization-maintaining optical fiber to the shaft end face to the photon integrated chip.

8. The photonic chip-based polarization mode multiplexing double-optical-path fiber gyroscope according to claim 1, wherein the detector is attached to the side of the photonic integrated chip to complete photoelectric conversion, and is connected with the signal processing circuit to complete resolution of a gyroscope signal; or the signal light is coupled to the receiving optical fiber, and then the signal light is led to the detector through the receiving optical fiber.

9. The photonic-chip-based polarization-mode multiplexing double-path fiber optic gyroscope of any of claims 1-8, applied to multiple wavelength systems of 830nm, 850nm, 1310nm, 1550 nm.