An integrated gyroscopic resonator cavity structure coupled to a polarization beam splitter

Abstract

The application relates to an integrated gyro resonant cavity structure coupled with a polarization beam splitter, which adopts a reflective resonant cavity composed of a straight waveguide and a ring waveguide, and the input and output of an optical field are completed through a coupling area at the upper end of the reflective resonant cavity; a polarization beam splitter structure is coupled with the lower area of the ring waveguide; the polarization beam splitter structure adopts a waveguide directional coupler, which is used for coupling the resonant cavity cavity waveguide and the ring waveguide in a partial area, and inducing corresponding mode light field leakage to the cavity waveguide; clockwise and counterclockwise incident light fields are input from the two ends of the straight waveguide, coupled into the ring waveguide through the waveguide, circularly transmitted in the ring waveguide, coupled out to the straight waveguide again, and gyro output spectrum lines are obtained through a detector. The application can reduce the output crosstalk of two polarization modes and the influence of external environmental factors, and is beneficial to improving the detection precision and working stability of the resonant integrated gyro.

Description

Technical Field

[0001] This invention relates to the field of resonant integrated optical gyroscope technology, and more particularly to an integrated gyroscope resonant cavity structure for a coupled polarization beam splitter. Background Technology

[0002] Optical gyroscopes, as sensors in the field of inertial navigation, are used to sense the attitude and position information of a carrier relative to space, and can be widely used in aerospace, deep-sea exploration, weaponry, and other fields. Optical gyroscope sensors are gradually developing towards higher sensing accuracy, stronger anti-interference capabilities, smaller size, and lower cost. Currently, the more mature optical gyroscope sensors are mainly interferometric fiber optic gyroscopes and laser gyroscopes. However, due to the latch-up effect in laser gyroscopes, a mechanical dithering frequency-biasing structure is required to avoid this phenomenon, and the internal components are relatively complex. Furthermore, interferometric fiber optic gyroscopes typically require several kilometers of fiber optic cable to meet the accuracy requirements of gyroscope navigation, thus lacking a significant advantage in integrated gyroscope research. Resonant optical gyroscopes, on the other hand, sense angular velocity changes by measuring the frequency difference of the clockwise and counterclockwise light fields, eliminating the need for several kilometers of fiber optic cable for sensing, giving them an advantage in integrated research. With the subsequent development of integrated optics theory and the continuous advancement of micro-nano fabrication technology, using waveguide resonant cavities to replace fiber optic rings has become a research direction for resonant integrated gyroscopes. By selecting a suitable substrate and depositing a core dielectric, and then using etching methods to fabricate waveguides of the required size, a waveguide resonant cavity structure can be constructed, which is expected to enable on-chip integration of gyroscope devices.

[0003] However, integrated gyroscopes built with waveguide resonant cavities still face limitations in accuracy improvement. On one hand, they are constrained by the high transmission loss within the optical waveguide; on the other hand, coherent noise interference still exists within the resonant cavity. Among these, polarization noise caused by polarization mode crosstalk is a major long-term noise limiting gyroscope accuracy. When an optical field propagates within a rectangular waveguide, it excites two orthogonal polarization modes (TE mode and TM mode), meaning there are fast and slow optical axes within the waveguide. Due to angular alignment errors in the waveguide coupling region and at the coupling point between the waveguide and the pigtail, even when a specific polarization mode of optical field is input, the resonant cavity output is still an overlap of the two polarization modes, and is easily affected by external factors such as temperature. Therefore, the presence of polarization noise ultimately affects the gyroscope's optical field output spectrum and angular velocity sensing accuracy, requiring corresponding measures to suppress it. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes an integrated gyroscope resonant cavity structure for a coupled polarization beam splitter that can reduce output crosstalk between two polarization modes and the influence of external environmental factors, thereby improving the detection accuracy and operational stability of resonant integrated gyroscopes.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution:

[0006] An integrated gyroscope resonant cavity structure with a coupled polarization beamsplitter is disclosed. The gyroscope resonant cavity structure employs a reflective resonant cavity composed of a straight waveguide and a ring waveguide. The straight waveguide and the ring waveguide complete the input and output of the optical field through the coupling region at the upper end of the reflective resonant cavity. A polarization beamsplitter structure is coupled in the lower region of the ring waveguide. The polarization beamsplitter structure employs a waveguide directional coupler to couple a portion of the external waveguide of the resonant cavity with the ring waveguide, inducing the leakage of the corresponding mode optical field to the external waveguide.

[0007] The clockwise and counterclockwise incident light fields are input from both ends of the straight waveguide, respectively. They are coupled into the inner loop of the ring waveguide and transmitted through the waveguide. They are then coupled out again to the straight waveguide, and the gyroscope output spectrum is obtained through the detector.

[0008] Furthermore, the waveguide directional coupler uses silicon nitride as the core layer with a refractive index of 1.98; the cladding is silicon dioxide with a refractive index of 1.45; the ring resonator has an input center wavelength of 1550 nm, and the ratio of cavity length to light source wavelength is 7.7 × 10⁻⁶. 4 .

[0009] Furthermore, the waveguide directional coupler includes waveguide a and waveguide b, with waveguide a having a width of 1.7 μm and a height of 73.6 nm, and waveguide b having a width of 1.6 μm and a height of 97.9 nm; the polarization beam splitter structure has a polarization effect of 12 dB on the optical field.

[0010] Moreover, the straight waveguide, the ring waveguide, and the waveguide directional coupler are all fabricated using a deposition and etching process.

[0011] The advantages and positive effects of this invention are as follows:

[0012] 1. This invention proposes an integrated gyroscope resonant cavity structure for a coupled polarization beam splitter. Based on the principle of waveguide directional couplers, it utilizes the difference in phase matching conditions between the TE and TM modes to perform energy exchange between the guided modes within the two waveguides, achieving polarization mode beam splitting. This structure can reduce output crosstalk between the two polarization modes and the influence of external environmental factors, which is beneficial to improving the detection accuracy and operational stability of the resonant integrated gyroscope.

[0013] 2. This invention relates to an integrated gyro waveguide resonant cavity structure for a coupled polarization beam splitter. The design of this invention simplifies the waveguide structure, eliminating the need for fabricating tilted gratings inside the waveguide for polarization. Furthermore, it improves space utilization. Compared to polarization methods involving millimeter-scale curved waveguides, this invention reduces manufacturing costs and offers a relatively lightweight polarization method. Attached Figure Description

[0014] Figure 1 Here are schematic diagrams of the waveguide directional coupler used for polarization beam splitting according to the present invention; (a) planar schematic diagram of the directional coupler; (b) schematic diagram of the rectangular waveguide directional coupler.

[0015] Figure 2 This is a schematic diagram of the gyroscope resonator structure of the coupled polarization beam splitter of the present invention;

[0016] Figure 3 The following are output spectrum diagrams of the ring resonator according to an embodiment of the present invention: (a) is the output spectrum diagram of the ring resonator without a polarization beam splitter, and (b) is the output spectrum diagram of the ring resonator with a polarization beam splitter applied. In the diagrams, IP represents the output spectrum corresponding to the primary polarization mode, and IS represents the output spectrum corresponding to the secondary polarization mode. Detailed Implementation

[0017] The structure of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that these embodiments are descriptive and not limiting.

[0018] This invention addresses the polarization noise problem in resonant integrated gyroscopes by proposing a coupled polarization beamsplitter structure for the integrated gyroscope resonant cavity. Based on the principle of waveguide directional couplers, it utilizes the difference in phase matching conditions between TE and TM modes to exchange energy between the guided modes within the two waveguides, achieving polarization mode beam splitting. Specifically:

[0019] Using the principle of directional couplers, TE and TM modes can be purposefully separated within a waveguide resonant cavity. In regions with small coupling intervals between two waveguides, the spatial distribution of the waveguide refractive index changes, and the field distribution within the corresponding waveguide is affected by the optical field of the adjacent waveguide. The guided modes of the two waveguides will couple to each other, and their coupling characteristics are determined by the waveguide spacing, coupling length, and guided mode distribution. By designing a suitable waveguide structure, under the condition of phase matching, the energy of one waveguide can be effectively coupled into the other. Directional couplers can achieve mode and energy exchange between parallel adjacent waveguides. Furthermore, for the same rectangular waveguide structure, the effective refractive indices of the TE and TM modes are different. Therefore, when the TM modes of the two waveguides satisfy the phase matching condition while the TE modes are phase mismatched, such as... Figure 1 As shown, this allows for effective separation of the TE and TM mode optical fields, achieving a polarization beam splitting effect. Coupled with the waveguide resonant cavity structure inside the integrated gyroscope, the optical field, during its internal cyclic propagation, induces dissipation of the TM mode optical field, ensuring that the resonant cavity output is the dominant polarization mode optical field, thereby reducing crosstalk between polarization modes. See the following equation:

[0020] N TE1 ≠N TE2

[0021] NTM1 =N TM2 (1)

[0022] Based on the above design concept, the integrated gyroscope resonant cavity structure of the coupled polarization beamsplitter of this invention consists of a straight waveguide and a ring waveguide. A polarization beamsplitter is constructed inside the ring resonant cavity to selectively suppress optical fields of different polarization modes within the resonant cavity, thereby reducing polarization noise. The specific scheme is as follows:

[0023] (1) First, based on the fundamental theories of optical waveguide guided mode transmission and waveguide coupling, a suitable waveguide directional coupler size is designed. For example... Figure 1 The diagram shows a general waveguide directional coupler structure. The refractive indices of the core layers of waveguides a and b are na and nb, respectively, and the refractive index of the cladding medium is n0. The optical field propagates along the z-axis. When the spacing between waveguides a and b is less than the lateral penetration depth of the waveguides, the field distribution inside the waveguides will be affected by the field distribution of the adjacent waveguides. Under the condition that the guided modes of the two waveguides approximately satisfy the same propagation constant, mode coupling and energy exchange will occur. By designing the cross-sectional dimensions of the two waveguides, high-efficiency coupling of TM mode energy can be achieved at a specific coupling length, while the TE mode, due to mode mismatch, still propagates in the original waveguide. When an incident light field is applied to the input end of waveguide a, the beam splitting of the optical field polarization mode is achieved through the directional coupler. Finally, the output end of waveguide a is the TE mode, and the output end of coupled waveguide b is the TM mode. This structure can achieve a polarization extinction ratio of over 10 dB.

[0024] The specific parameters of the waveguide directional coupler used in this invention are as follows:

[0025] The waveguide core is made of silicon nitride with a refractive index of 1.98, while the cladding is made of silicon dioxide with a refractive index of 1.4455. Based on the theoretical analysis of the guided modes of the three-dimensional optical waveguide, the mode field distribution of the rectangular waveguide core and cladding is approximately solved using the Macartiri approximation method and the boundary continuity condition of the electromagnetic field components. TE-like modes and TM-like modes exist in the three-dimensional waveguide.

[0026] When two waveguides are coupled at close range, they can be regarded as a unified system. The waveguide field distribution in the coupling region can be represented as the superposition of two separate mode field distributions, where the coefficients Aa(z) and Ab(z) are the superposition coefficients of the two waveguide optical fields, which vary with the propagation direction z, and βa and βb are the propagation constants of the respective guided modes of the two waveguides.

[0027]

[0028] Using Maxwell's fundamental equations and the mode coupling equations, the coupling relationship between the two mode fields can be obtained:

[0029]

[0030] Ka and Kb represent the self-coupling coefficients, Kab and Kba are the coupling coefficients, and the transmission power of each waveguide is Pa(z) = |A|. a | 2 Pb(z)=|A b | 2 For a dual-waveguide coupled system, the optical energy within any one waveguide will be perturbed by the optical mode of the other waveguide. When the two waveguides approximately satisfy the phase-matching condition, i.e., 2δ=βa-βb+Ka-Kb≈0, the energy conversion between the two waveguides can be maximized. Due to the different guided mode difference between the TE and TM modes between the two waveguides, when waveguide a has a width of approximately 1.7μm and a height of 73.6nm, and waveguide b has a width of approximately 1.6μm and a height of 97.9nm, the TM modes of the two waveguides approximately satisfy the matching condition, while the TE modes are phase-mismatched. Calculations show that the device's polarization effect is approximately 12dB.

[0031] (2) Next, the designed waveguide directional coupler is applied to the gyroscope ring resonator to polarize the optical field mode. For example... Figure 2 As shown, this is a commonly used reflective resonant cavity structure for integrated gyroscopes. The resonant cavity structure consists of a straight waveguide and a waveguide ring. Clockwise and counterclockwise incident light fields are input from one end of the straight waveguide, respectively, and then coupled into the ring waveguide for propagation. The light is then coupled out again and output to the straight waveguide, where the gyroscope's output spectrum is obtained through a detector. To reduce the polarization noise in the resonant cavity, in... Figure 2 The lower region of the ring waveguide is coupled with the designed waveguide directional coupler to create a polarization beamsplitter structure and improve the polarization extinction ratio of the system. As a result, the TM mode component inside the waveguide resonant cavity will be effectively coupled at the intracavity polarization beamsplitter location, causing energy leakage to the outside of the resonant cavity. The TM mode optical field energy is significantly attenuated, while the TE mode inside the ring resonant cavity continues to propagate approximately in its original state, with its energy remaining essentially unchanged, thus maintaining the energy proportion of the TE mode (the dominant polarization mode).

[0032] Figure 3The parameters of the fixed device and environmental factors such as temperature are considered. After the incident linearly polarized light is transmitted through the ring resonant cavity, the output spectrum of the photodetector is the interference superposition of the output spectra of two optical field modes (IP main polarization mode optical field and IS secondary polarization mode optical field). For each polarization mode, its output is the interference field of the waveguide through optical field and the coupled output optical field of the ring resonant cavity, which is represented by the Lorentz output spectrum. Comparing the output spectra of Figure (a) and Figure (b), without the addition of a waveguide polarization beam splitter, i.e., without a coupled polarization beam splitter, the optical field output is the superposition of the main polarization mode resonant spectrum and the secondary polarization mode resonant spectrum. The resonant integrated gyroscope uses the frequency shift of the spectrum corresponding to the main polarization mode (IP) to sense the inertial space angular velocity information. The presence of the secondary polarization mode (IS) will destroy the original main polarization mode spectrum, and the intensity of the resonant spectrum corresponding to the secondary polarization mode is close to 0.12. After applying the polarization beam splitter structure, the optical field power corresponding to the secondary polarization mode in the output spectrum is significantly reduced, and the corresponding resonant spectrum line is deformed and its intensity is attenuated to 6.7% of its original value. This resonant cavity structure can effectively reduce the polarization noise of the gyroscope signal. Finally, the ring resonant cavity coupled with the polarization beam splitter structure is combined with the modulation and demodulation signal processing module in the gyroscope to reduce the gyroscope angular velocity detection error, thereby improving the gyroscope angular velocity detection accuracy.

[0033] Although embodiments and drawings of the present invention have been disclosed for illustrative purposes, those skilled in the art will understand that various substitutions, variations and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. An integrated gyroscope resonant cavity structure for a coupled polarization beam splitter, characterized in that: The gyroscope resonant cavity structure adopts a reflective resonant cavity composed of a straight waveguide and a ring waveguide. The straight waveguide and the ring waveguide complete the input and output of the optical field through the coupling region at the upper end of the reflective resonant cavity. A polarization beam splitter structure is coupled in the lower region of the ring waveguide. The polarization beam splitter structure adopts a waveguide directional coupler. By matching the phase matching condition of the TM mode, the phase mismatch of the TE mode is caused, inducing the TM mode optical field to leak to the external waveguide, while retaining the TE mode to propagate in the ring waveguide. The clockwise and counterclockwise incident light fields are input from both ends of the straight waveguide, respectively. They are coupled into the inner loop of the ring waveguide and transmitted through the waveguide. They are then coupled out again to the straight waveguide, and the gyroscope output spectrum is obtained through the detector.

2. The integrated gyroscope resonator structure of the coupled polarization beam splitter according to claim 1, characterized in that: The waveguide directional coupler uses silicon nitride as the core layer with a refractive index of 1.98; the cladding is silicon dioxide with a refractive index of 1.45; the ring resonator has an input center wavelength of 1550 nm, and the ratio of cavity length to light source wavelength is 7.

7. 10 4 .

3. The integrated gyroscope resonator structure of the coupled polarization beam splitter according to claim 1, characterized in that: The waveguide directional coupler includes waveguide a and waveguide b, with waveguide a having a width of 1.

7. The height is 73.6 nm, and the width of waveguide b is 1.6 nm. The height is 97.9 nm; the polarization beam splitter structure has a polarization effect of 12 dB on the optical field.

4. The integrated gyroscope resonator structure of the coupled polarization beam splitter according to claim 1, characterized in that: The straight waveguide, ring waveguide, and waveguide directional coupler are all fabricated using a deposition and etching process.

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