Multimode fiber temperature and stress measurement and differential modulation methods and systems

Abstract

This invention relates to the field of fiber optic measurement technology, and particularly to a method and system for measuring and modulating temperature and stress in multimode fiber optics. The method utilizes a single-frequency laser emitted from a pigtail laser, which is then split by a beam splitter and fed into the multimode fiber of the measurement and reference arms. The outgoing light is collimated, polarized, and then combined by a polarization combining crystal. After being modulated by a quarter-wave plate, interference fringes are finally recorded by a polarization mask camera. A four-step phase-shifting algorithm is used to reconstruct the wavefront phase image, extracting two characteristic parameters: the change in the tilt angle of the interference fringes and the change in the wavefront phase distribution. This invention leverages the differences in wavefront response mechanisms of multimode fibers under temperature and stress to achieve two-parameter decoupling. It offers advantages such as high real-time performance, resistance to strong light interference, and a compact structure, making it suitable for integrated precision temperature and stress measurement in industrial monitoring and aerospace fields.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic measurement technology, and particularly relates to a method and system for measuring and differentiating temperature and stress in multimode optical fibers. Background Technology

[0002] In critical fields such as industrial monitoring, aerospace, and large-scale civil engineering, accurate and simultaneous detection of temperature and stress is essential for ensuring the safe operation of equipment and the stability of engineering structures. Multimode fiber, with its significant advantages including low cost, simple fabrication process, strong resistance to electromagnetic interference, wide transmission bandwidth, and ease of distributed sensing, has become an important application medium in temperature and stress sensing. However, the sensing characteristics of multimode fiber exhibit significant coupling effects; temperature changes and external forces simultaneously alter parameters such as fiber refractive index and mode coupling strength, leading to superimposed changes in the characteristic parameters of the output optical signal (such as intensity, wavelength, and phase). Therefore, accurately demodulating the independent information of temperature and stress from the coupled sensing signals and effectively distinguishing between them is a key technical bottleneck in the practical application of multimode fiber sensing technology, and also a focus and challenge in this field of research.

[0003] It should be noted that existing demodulation differentiation methods generally suffer from the common drawback of failing to balance demodulation accuracy, detection efficiency, and anti-interference performance. High-precision solutions typically rely on complex detection systems and lengthy demodulation processes, resulting in low detection efficiency. While simpler solutions offer higher detection efficiency, they suffer from deficiencies in demodulation accuracy and anti-interference performance. Furthermore, with the application of wavefront phase detection technology in fiber optic sensing, its high sensitivity and fast response speed provide a new technical direction for solving the aforementioned coupling demodulation challenges. However, currently, there is no effective technical solution that combines wavefront phase detection technology with multimode fiber temperature-stress demodulation differentiation, making it difficult to fully leverage the precise sensing advantages of wavefront phase detection technology for changes in fiber micro-parameters. Summary of the Invention

[0004] In view of this, the present invention aims to provide a method and system for measuring and differentiating temperature and stress in multimode optical fibers, which solves the problems of high cost and inability to perform temperature and stress differentiation measurement functions in the prior art.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows:

[0006] A multimode fiber temperature and stress measurement and differential modulation system, comprising:

[0007] The light source module is used to output single-frequency laser light.

[0008] The beam splitting coupling module is used to split the single-frequency laser beam into a measurement beam and a reference beam;

[0009] The sensing and reference module includes a measurement arm multimode fiber and a reference arm multimode fiber, wherein the measurement arm multimode fiber is used to sense ambient temperature and stress.

[0010] The polarization modulation module includes a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combining crystal, and a quarter-wave plate, which are used to combine two beams and convert them into orthogonally circularly polarized light.

[0011] The detection and processing module is used to record the interference fringe formation image. For the interference fringe image, four interference images with different phases are acquired in real time, and the wavefront image at the current wavelength is obtained through four-step phase shift solution.

[0012] The light source module outputs the single-frequency laser, which is then uniformly split into the measurement light and the reference light by the beam splitting and coupling module. The measurement light passes through the multimode fiber of the measurement arm, the first fiber collimator, and the first polarizer before entering the polarization combining crystal. The reference light passes through the multimode fiber of the reference arm, the second fiber collimator, and the second polarizer before entering the polarization combining crystal. The polarization combining crystal combines the two beams and converts them into orthogonally circularly polarized light by the quarter-wave plate, forming interference fringes. The detection and processing module calculates the movement of the interference fringes and the change in the interference angle based on the change in the wavefront phase distribution of the wavefront image. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of the multimode fiber, a two-parameter response equation is established for the change in wavefront phase and the changes in temperature and stress. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

[0013] Furthermore, the light source module employs a fiber optic laser.

[0014] Furthermore, the beam splitting coupling module employs a 1×2 fiber beam splitter.

[0015] Furthermore, the detection and processing module employs a polarization mask camera and a processing unit. The polarization mask camera records and acquires four interference images with different phases in real time, and the processing unit is used to perform a four-step phase shift solution on the interference images to obtain the wavefront image at the current wavelength.

[0016] Furthermore, the polarization mask camera employs a pixel-level polarizer array to simultaneously acquire interference images in four polarization directions—0°, 45°, 90°, and 135°—in a single exposure.

[0017] A method for measuring and modulating the temperature and stress of multimode optical fibers, comprising:

[0018] S1. Construct an interferometric optical path system, which includes a pigtail laser, a 1×2 fiber beam splitter, a measurement arm multimode fiber, a reference arm multimode fiber, a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combiner crystal, a quarter-wave plate, and a polarization mask camera.

[0019] S2. The single-frequency laser output from the pigtail laser is split into two beams by the 1×2 fiber beam splitter and coupled into the multimode fiber of the measurement arm and the multimode fiber of the reference arm, respectively.

[0020] S3. After the two beams are collimated by the fiber collimator and polarized by the polarizer, they are incident on the polarization combining crystal for beam combining. The combined beam is converted into orthogonally circularly polarized light by a quarter-wave plate.

[0021] S4. Use a polarization mask camera to record interference images containing orthogonal polarization state information in real time;

[0022] S5. The processing unit performs a four-step phase-shifting process on the acquired interference image to obtain the wavefront image of the wavefront phase distribution of the optical field at the current wavelength.

[0023] S6. Calculate the amount of movement of the interference fringes and the amount of change of the interference angle based on the change in the wavefront phase distribution of the wavefront image.

[0024] S7. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of multimode optical fiber, a two-parameter response equation for the wavefront phase change, temperature change, and stress change is established. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

[0025] Furthermore, the four-step phase shift process includes:

[0026] The polarization mask camera acquires four interference images with a phase difference of 90° within one sampling period. The processing unit uses a four-step phase shift algorithm to calculate the wrapped phase of the interference images and obtains a continuous wavefront phase distribution through a phase unwrapping algorithm.

[0027] Furthermore, based on the difference in phase modulation characteristics of the outgoing wavefront under temperature and stress fields of multimode fiber, a two-parameter response equation is established for the wavefront phase change, temperature change, and stress change. This equation is then decoupled through matrix operations and simultaneously outputs the measured values ​​of temperature and stress, including:

[0028] By fitting the wavefront phase distribution using Zernike polynomials, Zernike coefficients of a specific order are extracted. Utilizing the different sensitivity coefficients of stress and temperature to each Zernike coefficient, a sensitivity matrix is ​​constructed. By inverting the matrix, the differentiation and demodulation of temperature and stress are achieved.

[0029] This invention provides a method and system for measuring and modulating temperature and stress in multimode optical fibers. A single-frequency laser emitted from a pigtail laser is split and fed into the multimode optical fibers of the measurement and reference arms. The outgoing light is collimated, polarized, and then combined by a polarization combining crystal. It is then modulated by a quarter-wave plate, and finally, interference fringes are recorded by a polarization mask camera. A four-step phase-shifting algorithm is used to reconstruct the wavefront phase image, extracting two characteristic parameters: the change in the tilt angle of the interference fringes and the change in the wavefront phase distribution. This invention utilizes the differences in wavefront response mechanisms of multimode optical fibers under temperature and stress to achieve two-parameter decoupling. It has advantages such as high real-time performance, resistance to strong light interference, and compact structure, making it suitable for integrated precision measurement of temperature and stress in industrial monitoring and aerospace fields. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings,

[0031] Figure 1 A schematic diagram of the structure of the multimode fiber temperature and stress measurement and differentiation modulation system described in the embodiment of the present invention;

[0032] Figure 2 A schematic diagram of an interference image recorded by a polarization mask camera in a multimode fiber temperature and stress measurement and differentiation demodulation system described in an embodiment of the present invention, without stress or temperature.

[0033] Figure 3 A schematic diagram of an interference image recorded by a polarization mask camera under stress in the multimode fiber temperature and stress measurement and differentiation demodulation system described in the embodiment of the present invention;

[0034] Figure 4 This is a schematic diagram of the interference image recorded by the polarization mask camera under temperature influence in the multimode fiber temperature, stress measurement and differentiation demodulation system described in the embodiment of the present invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0036] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0037] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0038] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0039] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0040] The multimode fiber temperature and stress measurement and differentiation modulation system described in this invention includes:

[0041] The light source module is used to output single-frequency laser light.

[0042] The beam splitting coupling module is used to split the single-frequency laser beam into a measurement beam and a reference beam;

[0043] The sensing and reference module includes a measurement arm multimode fiber and a reference arm multimode fiber, wherein the measurement arm multimode fiber is used to sense ambient temperature and stress.

[0044] The polarization modulation module includes a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combining crystal, and a quarter-wave plate, which are used to combine two beams and convert them into orthogonally circularly polarized light.

[0045] The detection and processing module is used to record the interference fringe formation image. For the interference fringe image, four interference images with different phases are acquired in real time, and the wavefront image at the current wavelength is obtained through four-step phase shift solution.

[0046] The light source module outputs the single-frequency laser, which is uniformly split into the measurement light and the reference light by the beam splitting and coupling module. The measurement light passes through the multimode fiber of the measurement arm, the first fiber collimator, and the first polarizer before entering the polarization combining crystal. The reference light passes through the multimode fiber of the reference arm, the second fiber collimator, and the second polarizer before entering the polarization combining crystal. The polarization combining crystal combines the two beams and converts them into orthogonally circularly polarized light by the quarter-wave plate, forming interference fringes. The detection and processing module calculates the shift of the interference fringes and the change in the interference angle based on the change in the wavefront phase distribution of the wavefront image. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of the multimode fiber, a two-parameter response equation is established for the change in wavefront phase and the changes in temperature and stress. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

[0047] In some embodiments, the light source module uses a fiber optic laser, and those skilled in the art can choose flexibly, without limitation.

[0048] In some embodiments, the beam splitting coupling module uses a 1×2 fiber beam splitter. Those skilled in the art can choose flexibly, and there is no limitation on this.

[0049] In some embodiments, the detection and processing module employs a polarization mask camera and a processing unit. The polarization mask camera records and acquires four interference images of different phases in real time, and the processing unit is used to obtain the wavefront image at the current wavelength through a four-step phase shift solution.

[0050] In some embodiments, the polarization mask camera employs a pixel-level polarizer array to simultaneously acquire interference images in four polarization directions (0°, 45°, 90°, and 135°) in a single exposure.

[0051] like Figure 1 As shown, this embodiment provides a multimode fiber temperature and stress measurement and differentiation modulation system, including: a pigtail laser 1, a 1×2 fiber beam splitter 2, a measurement arm multimode fiber 3, a reference arm multimode fiber 4, a first fiber collimator 5 and a second fiber collimator 6, a first polarizer 7 and a second polarizer 8, a polarization combining crystal 9, a quarter-wave plate 10, a polarization mask camera 11, and a processing unit (not shown in the figure).

[0052] Optical path connection and working principle:

[0053] The single-frequency laser output from the pigtail laser 1 is incident on a 1×2 fiber beam splitter 2 and split into two beams: a measurement beam and a reference beam. These beams enter the measurement arm multimode fiber 3 and the reference arm multimode fiber 4, respectively. The measurement arm multimode fiber 3 is placed in the test environment, while the reference arm multimode fiber 4 is placed in a constant-temperature, stress-free environment. The two beams exit from the ends of the fibers. The measurement beam is collimated into parallel light by the first fiber collimator 5, and the reference beam is similarly collimated into parallel light by the second fiber collimator 6. The measurement beam then passes through the first polarizer 7, and the reference beam passes through the second polarizer 8, where they are adjusted to be orthogonal P-beams and S-beams (linearly polarized light). The two linearly polarized beams are then combined by a polarization combining crystal 9, and the combined beam passes through a quarter-wave plate 10. The fast axis of the quarter-wave plate 10 forms a 45-degree angle with the linearly polarized light, converting the combined light into left-handed and right-handed circularly polarized light. The two light beams overlap in space and interfere with each other. The polarization mask camera 11 records and acquires four interference images with different phases in real time. The processing unit performs a four-step phase shift solution on the interference images to obtain the wavefront image at the current wavelength. This system can simultaneously measure temperature and stress changes. When temperature and stress are applied to the multimode fiber of the measuring arm, the effective refractive index will change. Finally, the temperature and stress can be effectively measured by measuring the shift of the interference fringes. At the same time, since the multimode fiber is affected by the phase change of the emitted wavefront under stress and temperature changes, temperature and stress can be distinguished by measuring the change of the two-beam interference angle.

[0054] like Figure 2 As shown, the interference image recorded by the polarization mask camera under no stress or temperature shows that the interference fringes are regularly distributed horizontally or vertically in the absence of external interference (reference state).

[0055] like Figure 3 As shown, the interference image recorded by the polarization mask camera under stress is illustrated. When axial stress is applied to the multimode fiber of the measuring arm, the outgoing wavefront tilts due to the slight geometric deformation and elasto-optic effect of the fiber, causing the interference fringes to rotate as a whole. The processing unit characterizes the stress magnitude by calculating the tilt angle θ of the fringes.

[0056] like Figure 4 The image shown illustrates the interference pattern recorded by a polarization mask camera under temperature influence. When the temperature of the multimode fiber in the measuring arm is changed, the effective refractive index and length of the fiber change due to thermo-optic effects and thermal expansion, primarily causing changes in the density of the interference fringes (i.e., phase shift Δ). ), while the change in stripe angle is minimal.

[0057] Accordingly, this embodiment of the invention also provides a method for measuring and modulating the temperature and stress of multimode optical fibers, including:

[0058] S1. Construct an interferometric optical path system, which includes a pigtail laser, a 1×2 fiber beam splitter, a measurement arm multimode fiber, a reference arm multimode fiber, a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combiner crystal, a quarter-wave plate, and a polarization mask camera.

[0059] S2. The single-frequency laser output from the pigtail laser is split into two beams by the 1×2 fiber beam splitter and coupled into the multimode fiber of the measurement arm and the multimode fiber of the reference arm, respectively.

[0060] S3. After the two beams are collimated by the fiber collimator and polarized by the polarizer, they are incident on the polarization combining crystal for beam combining. The combined beam is converted into orthogonally circularly polarized light by a quarter-wave plate.

[0061] S4. Use a polarization mask camera to record interference images containing orthogonal polarization state information in real time;

[0062] S5. The processing unit performs a four-step phase-shifting process on the acquired interference image to obtain the wavefront phase distribution of the optical field at the current wavelength.

[0063] S6. Calculate the shift of the interference fringes and the change of the interference angle based on the change in the wavefront phase distribution.

[0064] S7. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of multimode optical fiber, a two-parameter response equation for the wavefront phase change, temperature change, and stress change is established. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

[0065] In some embodiments, the four-step phase shift process includes:

[0066] The polarization mask camera acquires four interference images with a phase difference of 90° within one sampling period. The processing unit uses a four-step phase shift algorithm to calculate the wrapped phase of the interference images and obtains a continuous wavefront phase distribution through a phase unwrapping algorithm.

[0067] In some embodiments, based on the differences in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of multimode optical fiber, a two-parameter response equation is established for the wavefront phase change and the temperature and stress changes. The measured values ​​of temperature and stress are then decoupled and simultaneously output through matrix operations, including:

[0068] By fitting the wavefront phase distribution using Zernike polynomials, Zernike coefficients of a specific order are extracted. Utilizing the different sensitivity coefficients of stress and temperature to each Zernike coefficient, a sensitivity matrix is ​​constructed. By inverting the matrix, the differentiation and demodulation of temperature and stress are achieved.

[0069] Wavefront detection and demodulation process:

[0070] The polarization mask camera 11 is located behind the quarter-wave plate 10 and is used to record the interference pattern. The polarization mask camera integrates a micro-polarization array, which can simultaneously acquire interference patterns in four polarization directions (0°, 90°, 45°, and 135°) in a single exposure, i.e., four images required for four-step phase shifting.

[0071] The processing unit receives these four images and uses a four-step phase-shifting algorithm:

[0072] I(x,y)=A(x,y)+B(x,y)cos[ (x,y)+δ]

[0073] Where I is light intensity, A is background light, and B is modulation amplitude. Let δ be the phase to be determined, and δ be the phase shift (0, π / 2, π, 3π / 2). The wavefront phase distribution at the fiber optic output end can be reconstructed through calculation. (x,y).

[0074] Experimental verification and decoupling:

[0075] In the experiment, the system was subjected to temperature scanning (20℃-100℃) and stress loading (0-2000με). The results showed that stress mainly caused a linear change in the interference fringe angle, while temperature mainly caused a linear shift in the wavefront phase. Although there was a weak cross-sensitivity between the two, by constructing a sensitivity matrix and then inverting the matrix, the temperature change ΔT and the stress change ΔT could be calculated simultaneously. The system in this embodiment achieves real-time demodulation of video frame rate (30fps), effectively overcoming the shortcomings of traditional spectrometers, such as slow scanning speed and large influence from light source power fluctuations.

[0076] This invention provides a method and system for measuring and modulating the temperature and stress of multimode optical fibers. A single-frequency laser emitted from a pigtail laser is split and fed into the multimode optical fibers of the measurement and reference arms. The emitted light is collimated, polarized, and then combined by a polarization combining crystal. It is then modulated by a quarter-wave plate, and finally, interference fringes are recorded by a polarization mask camera. A four-step phase-shifting algorithm is used to reconstruct the wavefront phase image, extracting two characteristic parameters: the change in the tilt angle of the interference fringes and the change in the wavefront phase distribution. This invention utilizes the differences in the wavefront response mechanisms of multimode optical fibers under temperature and stress to achieve two-parameter decoupling. It has advantages such as high real-time performance, resistance to strong light interference, and compact structure, making it suitable for integrated precision measurement of temperature and stress in industrial monitoring and aerospace fields.

[0077] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A multimode fiber optic temperature and stress measurement and differential modulation system, characterized in that, include: The light source module is used to output single-frequency laser light. The beam splitting coupling module is used to split the single-frequency laser beam into a measurement beam and a reference beam; The sensing and reference module includes a measurement arm multimode fiber and a reference arm multimode fiber, wherein the measurement arm multimode fiber is used to sense ambient temperature and stress. The polarization modulation module includes a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combining crystal, and a quarter-wave plate, which are used to combine two beams and convert them into orthogonally circularly polarized light. The detection and processing module is used to record the interference fringe formation image. For the interference fringe image, four interference images with different phases are acquired in real time. The wavefront image of the wavefront phase distribution of the optical field at the current wavelength is obtained through four-step phase shift solution. The light source module outputs the single-frequency laser, which is then uniformly split into the measurement light and the reference light by the beam splitting and coupling module. The measurement light passes through the multimode fiber of the measurement arm, the first fiber collimator, and the first polarizer before entering the polarization combining crystal. The reference light passes through the multimode fiber of the reference arm, the second fiber collimator, and the second polarizer before entering the polarization combining crystal. The polarization combining crystal combines the two beams and converts them into orthogonally circularly polarized light by the quarter-wave plate, forming interference fringes. The detection and processing module calculates the movement of the interference fringes and the change in the interference angle based on the change in the wavefront phase distribution of the wavefront image. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of the multimode fiber, a two-parameter response equation is established for the change in wavefront phase and the changes in temperature and stress. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

2. The multimode fiber temperature and stress measurement and differentiation modulation system according to claim 1, characterized in that, The light source module uses a fiber laser.

3. The multimode fiber temperature and stress measurement and differentiation modulation system according to claim 1, characterized in that, The beam splitting coupling module uses a 1×2 fiber beam splitter.

4. The multimode fiber temperature and stress measurement and differentiation modulation system according to claim 1, characterized in that, The detection and processing module employs a polarization mask camera and a processing unit. The polarization mask camera records and acquires four interference images with different phases in real time. The processing unit is used to perform a four-step phase shift solution on the interference images to obtain the wavefront image at the current wavelength.

5. The multimode fiber temperature and stress measurement and differentiation modulation system according to claim 4, characterized in that, The polarization mask camera uses a pixel-level polarizer array to simultaneously acquire interference images in four polarization directions (0°, 45°, 90°, and 135°) in a single exposure.

6. A method for measuring and modulating the temperature and stress of multimode optical fibers, characterized in that, include: S1. Construct an interferometric optical path system, which includes a pigtail laser, a 1×2 fiber beam splitter, a measurement arm multimode fiber, a reference arm multimode fiber, a first fiber collimator, a second fiber collimator, a first polarizer, a second polarizer, a polarization combiner crystal, a quarter-wave plate, and a polarization mask camera. S2. The single-frequency laser output from the pigtail laser is split into two beams by the 1×2 fiber beam splitter and coupled into the multimode fiber of the measurement arm and the multimode fiber of the reference arm, respectively. S3. After the two beams are collimated by the fiber collimator and polarized by the polarizer, they are incident on the polarization combining crystal for beam combining. The combined beam is converted into orthogonally circularly polarized light by a quarter-wave plate. S4. Use a polarization mask camera to record interference images containing orthogonal polarization state information in real time; S5. The processing unit performs a four-step phase-shifting process on the acquired interference image to obtain the wavefront image of the wavefront phase distribution of the optical field at the current wavelength. S6. Calculate the amount of movement of the interference fringes and the amount of change of the interference angle based on the change in the wavefront phase distribution of the wavefront image. S7. Based on the difference in the phase modulation characteristics of the outgoing wavefront under temperature and stress fields of multimode optical fiber, a two-parameter response equation for the wavefront phase change, temperature change, and stress change is established. The measured values ​​of temperature and stress are decoupled and output simultaneously through matrix operations.

7. The method for measuring and modulating the temperature and stress of multimode optical fiber according to claim 6, characterized in that, The four-step phase shift process includes: The polarization mask camera acquires four interference images with a phase difference of 90° within one sampling period. The processing unit uses a four-step phase shift algorithm to calculate the wrapped phase of the interference images and obtains a continuous wavefront phase distribution through a phase unwrapping algorithm.

8. The method for measuring and modulating the temperature and stress of multimode optical fiber according to claim 6, characterized in that, Based on the difference in the phase modulation characteristics of the emitted wavefront under temperature and stress fields of multimode optical fiber, a two-parameter response equation is established for the wavefront phase change, temperature change, and stress change. This equation is then decoupled through matrix operations and simultaneously outputs the measured values ​​of temperature and stress, including: By fitting the wavefront phase distribution using Zernike polynomials, Zernike coefficients of a specific order are extracted. Utilizing the different sensitivity coefficients of stress and temperature to each Zernike coefficient, a sensitivity matrix is ​​constructed. By inverting the matrix, the differentiation and demodulation of temperature and stress are achieved.

Images