Method and system for measuring temperature and stress of multimode optical fiber based on double-beam interference angle

Abstract

The present application relates to the technical field of optical fiber optical measurement, and more particularly to a kind of based on double-beam interference angle multimode fiber temperature, stress measurement method and system, by constructing the double-beam interference system containing measurement arm and reference arm, utilize tail fiber laser, optical fiber beam splitter, multimode fiber, collimator and imaging unit realize signal acquisition, utilize multimode fiber under the action of temperature and stress the difference of exit wavefront characteristics, when measurement arm fiber is disturbed by outside, not only cause interference fringe shift, also can cause the angle of interference fringe to rotate.By imaging unit interference image, the movement of demodulation interference fringe is to obtain the total amount change of temperature and stress, simultaneously calculate the change amount of interference angle, utilize the different sensitivity of angle change to temperature and stress to realize the decoupling and distinction measurement of double parameter.

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 temperature and stress in multimode fiber based on dual-beam interference angle. 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 a significant coupling effect: temperature changes and external forces simultaneously cause changes in parameters such as fiber refractive index and mode coupling strength, resulting in superimposed changes in the characteristic parameters of the output optical signal (such as intensity, wavelength, and phase). Therefore, how to accurately demodulate the independent information of temperature and stress from the coupled sensing signals and effectively distinguish between them is a key technical bottleneck in the practical application of multimode fiber sensing technology, and also a focus and challenge of research in this field.

[0003] It should be noted that existing demodulation differentiation methods generally suffer from the common drawback of failing to simultaneously achieve 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 insufficient 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 temperature and stress in multimode optical fibers based on dual-beam interference angle.

[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 system based on dual-beam interference angle includes:

[0007] A fiber optic laser is used to output a single-frequency laser.

[0008] A 1×2 fiber optic beam splitter, the input end of which is connected to the pigtail laser, is used to split the laser into two equal beams, namely a measurement beam and a reference beam.

[0009] The multimode fiber of the measuring arm and the multimode fiber of the reference arm are respectively connected to the two output ends of the 1×2 fiber beam splitter. The multimode fiber of the measuring arm is used to sense changes in external temperature and stress.

[0010] The fiber collimator includes a first fiber collimator and a second fiber collimator, which are respectively disposed at the output ends of the measuring arm multimode fiber and the reference arm multimode fiber, and are used to convert the transmitted light in the measuring arm multimode fiber and the reference arm multimode fiber into a spatially collimated beam.

[0011] A beam combiner crystal is used to combine two spatially collimated beams for interference.

[0012] The imaging unit is used to record the interference image after beam combining.

[0013] Furthermore, the beam-combining crystal is a beam-splitting prism or a non-polarizing beam-splitting mirror, used to achieve spatial overlap of two beams and generate double-beam interference fringes.

[0014] Furthermore, the imaging unit employs a CCD camera.

[0015] A method for measuring the temperature and stress of multimode fiber based on dual-beam interference angle, applied to the aforementioned multimode fiber temperature and stress measurement system based on dual-beam interference angle, includes:

[0016] S1: Establish a reference state. Under stress-free and constant temperature conditions, acquire reference interference images through the imaging unit and record the position and angle of the initial interference fringes.

[0017] S2: When the multimode fiber of the measuring arm is subjected to temperature change ΔT and / or stress change Δε, the effective refractive index and geometric length of the multimode fiber change, resulting in a change in the phase of the outgoing wavefront.

[0018] S3: Real-time acquisition of the interference image after disturbance through the imaging unit;

[0019] S4: Perform image processing on the acquired interference image to extract the center shift Δx of the interference fringes and the rotation angle Δθ of the interference fringes;

[0020] S5: Establish the sensitivity matrix of temperature and stress, and use the different sensitivity coefficients of the fringe movement Δx and rotation angle Δθ to calculate the temperature change ΔT and stress change Δε.

[0021] Furthermore, the specific method for extracting the interference fringe rotation angle Δθ is as follows:

[0022] Perform Fourier transform or Hough transform on the interference image to calculate the angular deviation of the principal axis of the interference fringes relative to the initial state.

[0023] Furthermore, the imaging unit employs a CCD camera.

[0024] This invention provides a method and system for measuring temperature and stress in multimode fiber based on dual-beam interference angle. By constructing a dual-beam interference system including a measuring arm and a reference arm, signal acquisition is achieved using a pigtail laser, fiber beam splitter, multimode fiber, collimator, and imaging unit. Utilizing the differences in the emitted wavefront characteristics of the multimode fiber under temperature and stress, when the measuring arm fiber is disturbed, not only does the interference fringes shift, but the angle of the interference fringes also rotates. The amount of movement of the interference fringes is demodulated using the interference image from the imaging unit to obtain the total change in temperature and stress. Simultaneously, the change in the interference angle is calculated. The different sensitivities of angle change to temperature and stress are used to achieve decoupling and distinguishable measurement of the two parameters. This invention has a simple structure, low cost, and can achieve high-precision, high-efficiency simultaneous measurement and differentiation of temperature and stress, making it suitable for industrial monitoring and aerospace fields. Attached Figure Description

[0025] 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:

[0026] Figure 1 A schematic diagram of the structure of the multimode fiber temperature and stress measurement system based on dual-beam interference angle described in the embodiment of the present invention;

[0027] Figure 2 A schematic diagram of an interference image recorded by a CCD camera under no stress or temperature in a multimode fiber temperature and stress measurement system based on dual-beam interference angle, as described in an embodiment of the present invention.

[0028] Figure 3 A schematic diagram of the interference image recorded by a CCD camera under stress in the multimode fiber temperature and stress measurement system based on dual-beam interference angle, as described in the embodiment of the present invention.

[0029] Figure 4 This is a schematic diagram of the interference image recorded by a CCD camera under temperature influence in the multimode fiber temperature and stress measurement system based on dual-beam interference angle, as described in an embodiment of the present invention. Detailed Implementation

[0030] 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.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0032] 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.

[0033] 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.

[0034] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0035] like Figure 1 As shown in the embodiment of the present invention, a multimode fiber temperature and stress measurement system based on dual-beam interference angle includes:

[0036] Fiber laser 1, used to output single-frequency laser;

[0037] A 1×2 fiber optic beam splitter 2, the input end of which is connected to the pigtail laser, is used to split the laser into two equal beams, namely a measurement beam and a reference beam;

[0038] The multimode fiber 3 of the measuring arm and the multimode fiber 4 of the reference arm are respectively connected to the two output ends of the 1×2 fiber beam splitter. The multimode fiber 3 of the measuring arm is used to sense changes in external temperature and stress.

[0039] The fiber collimator includes a first fiber collimator 5 and a second fiber collimator 6, which are respectively disposed at the output ends of the measuring arm multimode fiber 3 and the reference arm multimode fiber 4, and are used to convert the transmitted light in the measuring arm multimode fiber 3 and the reference arm multimode fiber 4 into a spatially collimated beam.

[0040] The beam combiner crystal 7 is used to combine two spatially collimated beams for interference.

[0041] Imaging unit 8 is used to record the interference image after beam combining.

[0042] In some embodiments, the imaging unit 8 is a CCD camera, and those skilled in the art can choose flexibly, without limitation.

[0043] like Figure 1 As shown, this embodiment of the invention provides a multimode fiber temperature and stress measurement system based on dual-beam interference angle, comprising 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, a second fiber collimator 6, a bundle combiner crystal 7, and an imaging unit 8. Optical path connection: The single-frequency laser output from the pigtail laser 1 is incident on the 1×2 fiber beam splitter 2 with a splitting ratio of 50:50. The wavelength of the single-frequency laser can be set to 1550nm, which can be selected as needed, splitting the single-frequency laser into two beams: a measurement beam and a reference beam. The measurement beam enters the measurement arm multimode fiber 3, and the reference beam enters the reference arm multimode fiber 4. The other ends of the two measurement arm multimode fibers 3 and the reference arm multimode fiber 4 are respectively connected to the first fiber collimator 5 and the second fiber collimator 6, converting the guided wave mode within the fiber into a spatial Gaussian beam. Two collimated beams are incident on the combining crystal 7 at a certain angle. The two collimated beams are combined and interfere on the prism surface of the combining crystal 7. The resulting interference pattern is received and recorded by the imaging unit 8 located behind the combining crystal.

[0044] In some embodiments, the beam-combining crystal 7 is a beam-splitting prism or a non-polarizing beam-splitting mirror, used to achieve spatial overlap of two beams and generate double-beam interference fringes. Those skilled in the art can choose flexibly, and there is no limitation thereto.

[0045] Implementation principle:

[0046] This embodiment utilizes the different wavefront response characteristics of multimode optical fibers under temperature and stress:

[0047] Optical path difference and fringe shift: When the optical fiber of the measuring arm is subjected to temperature rise or tensile stress, the effective refractive index and physical length of the optical fiber change, resulting in a change in optical path difference, which causes the interference fringes to shift (i.e., shift amount) in the direction perpendicular to the fringe.

[0048] Wavefront tilt and angle variation: When multimode fiber is subjected to stress (especially transverse stress or bending), the internal mode coupling changes, causing the wavefront of the emitted beam to tilt (i.e., a change in the beam pointing angle); while simple temperature changes mainly cause uniform expansion and have little effect on wavefront tilt. This wavefront tilt manifests as a rotation of the entire interference fringes (i.e., an angle variation).

[0049] Demodulation process:

[0050] Initial calibration: Under standard atmospheric pressure and 25°C, start the fiber laser, and the imaging unit records as follows: Figure 2 The initial interference fringe image is shown. The initial fringe angle and center position are calculated using image processing algorithms (such as the gray-scale centroid method).

[0051] Sensitivity matrix calibration:

[0052] Apply a known temperature change (e.g., 10°C) only to the measuring arm, keeping it stress-free, record the fringe movement and angle changes, and calculate the sensitivity coefficient. Apply a known stress change only to the measuring arm, keeping the temperature constant, record the fringe movement and angle changes, and calculate the sensitivity coefficient. Construct the sensitivity matrix.

[0053] Actual measurement:

[0054] The measuring arm is placed in the environment to be measured. The imaging unit acquires interferometric images in real time. For example... Figure 3 As shown, when subjected to stress alone, the interference fringes undergo significant rotation; as Figure 4 As shown, when only temperature is involved, the interference fringes mainly undergo translation, with minimal rotation. A two-dimensional Fourier transform is performed on the real-time image to extract the current fringe shift and rotation angle. Simultaneously, the ambient temperature change ΔT and stress change Δε are obtained.

[0055] Experimental results:

[0056] In this embodiment, a 5-meter-long step-index multimode fiber was tested. When the temperature rose from 20°C to 50°C without stress, the fringes shifted by 15 fringe spacings, with an angle change of less than 0.1°. When a stress of 500 με was applied and the temperature remained constant, the fringes shifted by 5 fringe spacings, but the angle rotated by 2.5°. Using the method of this invention, the superimposed signals were successfully decoupled, with a temperature measurement error of less than ±0.5°C and a stress measurement error of less than ±10 με, demonstrating that this method can effectively distinguish between temperature and stress cross-sensitivity issues.

[0057] Accordingly, this embodiment of the invention also provides a method for measuring the temperature and stress of multimode fiber based on a dual-beam interference angle, applied to the aforementioned multimode fiber temperature and stress measurement system based on a dual-beam interference angle, comprising:

[0058] S1: Establish a reference state. Under stress-free and constant temperature conditions, acquire reference interference images through the imaging unit and record the position and angle of the initial interference fringes.

[0059] S2: When the multimode fiber of the measuring arm is subjected to temperature change ΔT and / or stress change Δε, the effective refractive index and geometric length of the multimode fiber change, resulting in a change in the phase of the outgoing wavefront.

[0060] S3: Real-time acquisition of the interference image after disturbance through the imaging unit;

[0061] S4: Perform image processing on the acquired interference image to extract the center shift Δx of the interference fringes and the rotation angle Δθ of the interference fringes;

[0062] S5: Establish the sensitivity matrix of temperature and stress, and use the different sensitivity coefficients of the fringe movement Δx and rotation angle Δθ to calculate the temperature change ΔT and stress change Δε.

[0063] Furthermore, the specific method for extracting the interference fringe rotation angle Δθ is as follows:

[0064] Perform Fourier transform or Hough transform on the interference image to calculate the angular deviation of the principal axis of the interference fringes relative to the initial state.

[0065] Furthermore, the imaging unit employs a CCD camera.

[0066] This invention provides a method and system for measuring temperature and stress in multimode fiber based on dual-beam interference angle. By constructing a dual-beam interference system including a measuring arm and a reference arm, signal acquisition is achieved using a pigtail laser, fiber beam splitter, multimode fiber, collimator, and imaging unit. Utilizing the differences in the emitted wavefront characteristics of the multimode fiber under temperature and stress, when the measuring arm fiber is disturbed, not only does the interference fringes shift, but the angle of the interference fringes also rotates. The amount of movement of the interference fringes is demodulated using the interference image from the imaging unit to obtain the total change in temperature and stress. Simultaneously, the change in the interference angle is calculated. The different sensitivities of angle change to temperature and stress are used to achieve decoupling and distinguishable measurement of the two parameters. This invention has a simple structure, low cost, and can achieve high-precision, high-efficiency simultaneous measurement and differentiation of temperature and stress, making it suitable for industrial monitoring and aerospace fields.

[0067] 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 method for measuring temperature and stress in multimode fiber based on dual-beam interference angle, applied to a multimode fiber temperature and stress measurement system based on dual-beam interference angle, the measurement system comprising: Fiber optic lasers are used to output single-frequency laser light. A 1×2 fiber optic beam splitter, the input end of which is connected to the pigtail laser, is used to split the laser into two equal beams, namely a measurement beam and a reference beam. The multimode fiber of the measuring arm and the multimode fiber of the reference arm are respectively connected to the two output ends of the 1×2 fiber beam splitter. The multimode fiber of the measuring arm is used to sense changes in external temperature and stress. The fiber collimator includes a first fiber collimator and a second fiber collimator, which are respectively disposed at the output ends of the measuring arm multimode fiber and the reference arm multimode fiber, and are used to convert the transmitted light in the measuring arm multimode fiber and the reference arm multimode fiber into a spatially collimated beam. A beam combiner crystal is used to combine two spatially collimated beams for interference. The imaging unit is used to record the interference image after beam combining; The measurement method is characterized by: S1: Establish a reference state. Under stress-free and constant temperature conditions, acquire reference interference images through the imaging unit and record the position and angle of the initial interference fringes. S2: When the multimode fiber of the measuring arm is subjected to temperature change ΔT and / or stress change Δε, the effective refractive index and geometric length of the multimode fiber change, resulting in a change in the phase of the outgoing wavefront. S3: Real-time acquisition of the interference image after disturbance through the imaging unit; S4: Perform image processing on the acquired interference image to extract the center shift Δx of the interference fringes and the rotation angle Δθ of the interference fringes; S5: Establish the sensitivity matrix of temperature and stress, and use the different sensitivity coefficients of the fringe movement Δx and rotation angle Δθ to calculate the temperature change ΔT and stress change Δε.

2. The method for measuring temperature and stress in multimode fiber based on dual-beam interference angle according to claim 1, characterized in that, The beam-combining crystal is a beam-splitting prism or a non-polarizing beam-splitting mirror, used to achieve spatial overlap of two beams and generate double-beam interference fringes.

3. The method for measuring temperature and stress in multimode fiber based on dual-beam interference angle according to claim 1, characterized in that, The imaging unit uses a CCD camera.

4. The method for measuring temperature and stress in multimode fiber based on dual-beam interference angle according to claim 1, characterized in that, The specific method for extracting the interference fringe rotation angle Δθ is as follows: Perform Fourier transform or Hough transform on the interference image to calculate the angular deviation of the principal axis of the interference fringes relative to the initial state.

5. The method for measuring temperature and stress in multimode fiber based on dual-beam interference angle according to claim 1, characterized in that, The imaging unit uses a CCD camera.

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