Fiber deformation sensor based on sagnac interference and deformation measurement method

Abstract

This invention discloses a fiber optic deformation sensor and deformation measurement method based on Sagnac interferometry. The sensor includes a Sagnac interferometer optical path and a photoelectric conversion module. The Sagnac interferometer optical path includes a sensing fiber loop, which includes at least one segment of polarization-maintaining fiber. The polarization-maintaining fiber has an axially stretchable elastic helical structure. The interference light intensity output by the Sagnac interferometer optical path changes monotonically with the axial stretch of the polarization-maintaining fiber. The photoelectric conversion module receives the interference light intensity and converts it into an electrical signal. This invention achieves wide-range deformation measurement while maintaining high sensitivity by setting the polarization-maintaining fiber as an axially stretchable elastic helical structure. It uses an intensity modulation method to directly convert the signal into an electrical signal through a photodetector, eliminating the need for a spectrometer, significantly reducing system cost. Furthermore, the structure is simple and easy to fabricate, making it widely applicable to various high-precision, large-deformation real-time monitoring scenarios.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a fiber optic deformation sensor based on Sagnac interferometry and a deformation measurement method. Background Technology

[0002] Deformation sensing has important applications in structural health monitoring, smart wearable devices, and soft robot control. The performance of deformation sensors directly determines the accuracy and reliability of the system's perception of deformation states. Traditional resistive and capacitive deformation sensors are susceptible to electromagnetic interference and lack stability in complex environments. Fiber optic sensors, with their advantages of electromagnetic interference resistance, small size, and flexible wearability, have become an ideal choice.

[0003] Fiber Bragg grating (FBG)-based sensors are the mainstream technology for fiber optic deformation sensing, measuring deformation magnitude through Bragg wavelength shift. However, their traditional forms are limited by the intrinsic rigidity of silicon-based optical fibers, with a deformation measurement range typically less than 1%, making it difficult to meet the needs of large deformation scenarios such as human joint movement and soft robot motion. Interferometric sensors stand out due to their ultra-high sensitivity, but are limited by the free spectral range, resulting in a narrow measurement range and difficulty in meeting the needs of large deformation monitoring. Sagnac interferometric sensors have advantages such as high sensitivity and simple structure, but they are constrained by the inherent rigidity of optical fibers, making it difficult to achieve effective response to millimeter-level deformations, thus failing to meet the needs of large deformation scenarios. At the same time, traditional Sagnac sensors mostly use wavelength demodulation methods, requiring expensive equipment such as spectrometers, which limits their engineering application.

[0004] Therefore, further consideration is needed on how to develop an optical fiber deformation sensor that can measure large deformations, maintain high sensitivity, and be cost-effective. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a fiber optic deformation sensor based on Sagnac interferometry that can measure large deformation, maintain high sensitivity and low cost.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A fiber optic deformation sensor based on Sagnac interference includes a Sagnac interference optical path and a photoelectric conversion module. The Sagnac interference optical path includes a sensing fiber loop, which includes at least one section of polarization-maintaining fiber. The polarization-maintaining fiber has an elastic helical structure that can be stretched axially. The interference light intensity output by the Sagnac interference optical path changes monotonically with the axial stretch of the polarization-maintaining fiber. The photoelectric conversion module is used to receive the interference light intensity and convert it into an electrical signal.

[0008] As an optimization, the sensing fiber loop also includes an elastic frame, and the polarization-maintaining fiber is wound axially on the elastic frame.

[0009] As an optimization, the elastic frame is a cylindrical helical spring, and the polarization-maintaining optical fiber is wound along the helical direction of the cylindrical helical spring and fixed on the outer surface of the cylindrical helical spring.

[0010] As an optimization, the polarization-maintaining optical fiber and the cylindrical helical spring are bonded together with an adhesive. The adhesive can be a UV-curable adhesive or an epoxy resin adhesive.

[0011] As an optimization, the photoelectric conversion module includes a photodetector, and the output end of the Sagnac interference optical path is electrically connected to the input end of the photodetector.

[0012] As an optimization, the photoelectric conversion module is also used to convert the electrical signal into a deformation and output it according to a preset mapping relationship between the deformation and the interference light intensity. To achieve this function, the sensor may also include a storage unit for storing the mapping relationship. The mapping relationship between the deformation and the interference light intensity is obtained through calibration experiments, and this mapping relationship is pre-stored in the storage unit. Based on the preset mapping relationship in the storage unit, the corresponding deformation value is calculated and output. At this time, the user can directly obtain the deformation without self-calibration or external calculation.

[0013] The present invention also discloses a fiber optic deformation measurement method based on Sagnac interferometry. Using the sensor described above, the sensor is calibrated to determine the mapping relationship between the deformation and the interference light intensity. The elastic spiral structure is fixed on the object to be measured, and the interference light intensity after the object to be measured undergoes deformation is obtained. The current deformation is obtained according to the mapping relationship.

[0014] As an optimization, the calibration method of the sensor is as follows: apply multiple known deformations to the elastic helical structure, record the corresponding interference light intensities, and fit the mapping relationship.

[0015] Compared with existing technologies, this invention achieves wide-range deformation measurement by setting the polarization-maintaining fiber as an axially stretchable elastic helical structure. At the same time, it retains high sensitivity based on the Sagnac interference principle and adopts an intensity modulation method to directly convert the light into an electrical signal through a photodetector, eliminating the need for a spectrometer, which greatly reduces the system cost. Moreover, the structure is simple and easy to manufacture, and it can be widely used in high-precision real-time monitoring scenarios of large deformations in complex working conditions such as structural health monitoring, soft robots, and smart wearables. Attached Figure Description

[0016] Figure 1 This is a diagram illustrating the working mechanism of the sensor in this invention;

[0017] Figure 2The graph shows the response characteristics of the deformation sensor in the 1570~1605 nm wide-band spectrum under different deformation magnitudes in this invention.

[0018] Figure 3 for Figure 2 A magnified view of a single interference peak at 1592–1598 nm in the middle region;

[0019] Figure 4 This is a fitted curve of the sensor loading response in this invention;

[0020] Figure 5 This is the fitted curve of the sensor unloading response in this invention;

[0021] Figure 6 This is a graph showing the stability performance of the sensor in this invention under 0% and 20% deformation.

[0022] Figure 7 This is a diagram showing the error characteristics of the sensor in this invention;

[0023] Figure 8 This is a repeatability test diagram of the sensor in this invention;

[0024] Figure 9 This is a test graph showing the response of the sensor in this invention under different temperature conditions. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] The fiber optic deformation sensor based on Sagnac interference in this specific embodiment includes a Sagnac interference optical path and a photoelectric conversion module. The Sagnac interference optical path includes a sensing fiber loop, which includes at least one section of polarization-maintaining fiber. The polarization-maintaining fiber has an elastic helical structure that can be stretched axially. The interference light intensity output by the Sagnac interference optical path changes monotonically with the axial stretch of the polarization-maintaining fiber. The photoelectric conversion module is used to receive the interference light intensity and convert it into an electrical signal.

[0027] The sensing fiber loop also includes an elastic frame, and the polarization-maintaining fiber is wound axially on the elastic frame.

[0028] The elastic frame is a cylindrical helical spring, and the polarization-maintaining optical fiber is wound along the helical direction of the cylindrical helical spring and fixed on the outer surface of the cylindrical helical spring.

[0029] The polarization-maintaining optical fiber and the cylindrical helical spring are bonded together with adhesive.

[0030] The photoelectric conversion module includes a photodetector, and the output end of the Sagnac interference optical path is electrically connected to the input end of the photodetector.

[0031] The photoelectric conversion module is also used to convert the electrical signal into a deformation and output it according to a preset mapping relationship between the deformation and the interference light intensity.

[0032] A fiber optic deformation measurement method based on Sagnac interferometry is provided. The sensor described above is calibrated to determine the mapping relationship between the deformation and the interference light intensity. The elastic spiral structure is fixed on the object to be measured, and the interference light intensity after the object to be measured undergoes deformation is obtained. The current deformation is obtained according to the mapping relationship.

[0033] The calibration method for the sensor is as follows: apply multiple known deformations to the elastic helical structure, record the corresponding interference light intensities, and fit the mapping relationship.

[0034] Principle explanation:

[0035] The Sagnac interferometer splits a laser beam into two beams, one clockwise (CW) and the other counterclockwise (CCW), which then propagate in opposite directions along the same closed loop before converging and interfering.

[0036] Its interference light intensity is:

[0037]

[0038] in, , These represent the light intensities of the two beams of light. This represents the optical path difference between the two beams of light.

[0039] Based on Stokes' theorem, the propagation time of CW and CCW transmission light can be further deduced:

[0040]

[0041] Where S is the area enclosed by the closed optical path. Let be the rotational angular velocity. From this, the Sagnac phase difference between the two beams can be obtained:

[0042]

[0043] For an N-turn closed loop, the total phase difference is:

[0044]

[0045] When a suitable fiber length L is selected to satisfy the phase difference When m is the interference order (rounded to the nearest integer), interference valleys will appear in the transmission spectrum. From this, the wavelength positions corresponding to the interference valleys can be derived, and the free spectral range (FSR) is defined as the wavelength interval between adjacent interference valleys:

[0046]

[0047] This relationship indicates that the FSR is inversely proportional to the length L of the PMF; that is, the longer the fiber, the narrower the FSR, and the more interference valleys there are within the same wavelength range. When external deformation is applied to the fiber, the birefringence B and length L of the PMF change, thereby causing a change in the phase difference.

[0048]

[0049] This change originates from the photoelastic effect, specifically the deformation-induced change in the refractive index of the optical fiber, expressed as:

[0050]

[0051] Where P e s,f This is the effective photoelastic coefficient of the PMF fast and slow axes. Substituting this formula into the expression for phase difference change, we can obtain the phase difference change directly caused by deformation:

[0052]

[0053] In the formula This is a constant describing the deformation-induced birefringence change. Changes in phase difference directly affect light intensity. The change in shape modulates the light intensity, thus achieving modulation of the deformation. The sensitivity of this sensor is:

[0054]

[0055] Besides using sensitivity to measure the sensing performance of a sensor, resolution (R) is also an important parameter for evaluating its sensing performance, which can be expressed as:

[0056]

[0057] Figure 1The working mechanism of the fiber optic sensing structure is illustrated. The polarization-maintaining fiber is made in a spring shape. Under external load, the sensing fiber undergoes axial tensile deformation, causing a change in the effective enclosed area of ​​the fiber loop. Simultaneously, under the modulation of the elasto-optic effect, the refractive index of the fiber itself and the geometric optical path of the transmitted light change synchronously. The combined effect of these two factors causes an additional absolute optical path difference between the two coherent beams propagating in opposite directions within the loop, thereby inducing a Sagnac interference phase difference. The corresponding shift occurs, which is ultimately reflected in the regular response of the output light intensity of the interference system.

[0058] A deformation sensor testing system was constructed, including an ASE light source (C+L band), a spectrum analyzer (AQ6370D), a cleaver (S326), a fusion splicer (S178C), a PMF (partition matrix), a 2*2 fiber optic coupler (split ratio 50:50), a photodetector (JY-PR-200M), and a multimeter. The deformation generating device was a large extensometer (LD-5000 electronic tensile testing machine). The deformation sensing unit was made of polarization-maintaining fiber wound on a spring, with its two ends connected to the two ends of a fiber optic beam splitter. A broadband laser, through the beam splitter, transmitted light in 50% increments to the polarization-maintaining fiber and output it to the spectrometer at the other end.

[0059] A spring with an adhesive PMF (partially bonded polymer film) was vertically fixed to a large deformation extensometer. The length of this spring was 10 cm. Interference spectra under different deformations were recorded using a spectrometer. The test environment conditions were 50% humidity and 20℃.

[0060] from Figure 2 As can be seen, the interference intensity of the sensor decreases with increasing deformation. However, once the deformation exceeds 40%, the interference signal shifts from being dominated by pure photoelastic phase modulation to being dominated by grating wavelength-selective modulation. That is, the PMF evolves from uniform waveguide deformation to periodic refractive index modulation, limiting the reduction in light intensity. Therefore, no significant change in light intensity can be observed when the Sagnac loop area increases. Figure 3 Specifically, it demonstrates the change in light intensity with the magnitude of deformation. With smaller deformations, the light intensity decreases only slightly, approximately 1 dB. When the deformation reaches 40%, the maximum spectral change is 4.036 dB. This change in spectral intensity can be converted into a change in electrical signal using a photodetector, thus eliminating the need for a spectrometer to directly measure the electrical signal and reducing sensor costs.

[0061] Figure 4 and Figure 5 The hysteresis response characteristics of the sensor are shown, revealing a cubic relationship between the hysteresis response and the change in light intensity. Figure 4The test results of the loading response are shown. The average sensitivity of the sensor is 0.3876 dB / mm and the goodness of fit is 0.998, which can be obtained from formula (9). Figure 5 The test results for the unloading response are presented, with an average sensitivity of 0.3858 dB / mm and a goodness of fit of 0.999. The low hysteresis response indicates that it can be measured continuously.

[0062] Figure 6 The sensor's stability was demonstrated at 0% and 20% deformation, indicating its good stability and ability to accurately monitor deformation. Furthermore, multiple tests were conducted at deformations of 10%, 20%, 30%, and 40%. Generally, full-scale error (FSE) is used to evaluate system accuracy, using the formula:

[0063]

[0064] here, This represents the maximum absolute error of the deformation magnitude. See [link to deformation sensor error characteristics] for details. Figure 7 As shown. Within its operating range, the sensor exhibits a full-scale error of 1.57%, demonstrating robust stability and repeatability, thus meeting the stringent requirements for deformation monitoring.

[0065] The output voltage was measured using a photodetector. Figure 8 The repeatability test of the sensor output through the photodetector was demonstrated, showing that the sensor can directly output an effective detection signal using the photodetector, exhibiting good output adaptability, significantly reducing the complexity of instrument operation, and improving the convenience of the detection process. To investigate the temperature response characteristics of the sensor, the fabricated Sagnac ring sensor structure was placed on a constant-temperature heating platform for testing. Figure 9 Voltage response data obtained by the photodetector from the sensor under different temperature conditions are presented. In the experiment, the heating platform was gradually heated from 30°C to 60°C at a rate of 5°C every 15 minutes. The test results show that the output voltage did not fluctuate significantly with the increase of ambient temperature, indicating that the sensor has good temperature insensitivity and weak temperature cross-sensitivity effect.

[0066] To highlight the comprehensive performance advantages of the Sagnac interferometric fiber optic deformation sensor of this invention, Table 1 compares it with fluorescent and micro / nano fiber optic deformation sensing solutions in terms of core dimensions such as measurement range, modulation method, sensitivity, demodulation complexity (M), and fabrication cost. While the fluorescent sensor can achieve large-scale deformation measurement comparable to this solution and has a simple demodulation process, it suffers from inaccurate quantification of sensitivity and insufficient long-term stability. The micro / nano fiber optic sensor, although exhibiting excellent weak strain detection performance, has an extremely narrow measurement range, making it unsuitable for large deformation scenarios, and its fabrication and packaging require extremely high process precision, hindering its engineering application. In contrast, the sensor of this invention, based on the principle of light intensity modulation, achieves ultra-wide deformation measurement from 0% to 40% while maintaining a sensitivity of 0.3876 dB / mm. It also features simple demodulation, low fabrication cost, excellent stability, and engineering adaptability, making it significantly advantageous in high-precision monitoring scenarios of large deformation.

[0067]

[0068] Table 1

[0069] Table 2 compares and analyzes the sensor of this invention with traditional Sagnac interferometric, FPI interferometric, and distributed FBG fiber optic deformation sensors. Compared to these traditional micro-deformation sensing solutions, the sensor of this invention overcomes the core bottleneck of limited measurement range in micro-deformation structures. While retaining the advantages of low demodulation complexity and low fabrication cost, it achieves ultra-wide deformation measurement capabilities that are difficult for traditional solutions to cover. This solution does not require high-precision micro-nano fabrication processes, avoiding the technical shortcomings of high demodulation costs and system complexity. It has unique advantages in large deformation monitoring scenarios, complementing micro-deformation sensors and providing a cost-effective and highly reliable new solution for large deformation monitoring.

[0070]

[0071] Table 2

[0072] Sensors of different specifications were fabricated, and after each sensor was calibrated, the deformation of the object to be measured was detected.

[0073] A first sensor was fabricated, using a polarization-maintaining fiber from a certain brand, 1m in length, with a center wavelength of 1550nm, a typical insertion loss of dB 0.30, and a minimum return loss of 60dB. The sensor was calibrated with a strain range of 0-40%, and the light intensity change was related to the cube of the strain magnitude. R0 2 =0.999, low hysteresis response, temperature response between 30°C and 60°C, and almost no change in light intensity. When testing object I, after deformation, the measurement result showed a 2.1 dB decrease in light intensity, which corresponds to the sensor's 20% deformation calibration and is consistent with actual 20% deformation.

[0074] Simultaneously, another existing sensor was used to test the object: a fluorescent sensor with a light source of 654 ~ 660 nm, a strain range of 0-40%, and a linear relationship between light intensity change and strain response. R 2 =0.993, temperature response is between 30-60 degrees Celsius, light intensity hardly changes. Measurement results show a 1.1dB decrease in fiber optic output light intensity, which corresponds to a 20% deformation calibration of the sensor, consistent with actual 20% deformation. Comparing the two, although both can perform accurate measurements, the fluorescence sensor's fiber optic brightness can only quantitatively analyze the deformation magnitude, and it is more difficult to manufacture. Moreover, quantitative analysis requires the use of lasers in other wavelength bands, which results in higher costs. In contrast, the sensor in this work can perform rapid and accurate measurements at a lower cost.

[0075] A second sensor was fabricated, using a polarization-maintaining fiber from a specific brand, 1m in length, with a center wavelength of 1550nm, attenuation ≤0.5dB / km, crosstalk ≤-40dB / 4m, insertion loss of 0.2dB, and a minimum return loss of 50dB. The sensor was calibrated with a strain range of 0-40%, and the change in light intensity was correlated with the square root of the strain. R0 2 =0.999, low hysteresis response, temperature response between 30°C and 60°C, and almost no change in light intensity. Measurements were taken of object II, and the result showed a 0.46 dB decrease in light intensity. This change in light intensity corresponds to a 10% deformation calibration of the sensor, which is consistent with an actual 10% deformation.

[0076] Simultaneously, another existing sensor was used to test the object: a micro-nano fiber optic sensor with a 1550nm light source and a strain range of 0-0.45%. The light intensity change and strain response showed a linear relationship. R 2 =0.999, temperature has a significant impact on the test. The test results show that the light intensity at output port 1 decreased by 4dB, while the light intensity at output port 2 increased by 2dB. This change in light intensity corresponds to a 0.034% deformation calibration of the sensor, which is consistent with the actual 0.034% deformation calibration. Compared to the two, although both can perform the test correctly, the micro-nano fiber optic sensor is more difficult to manufacture and has a smaller strain measurement range. The sensor in this work can measure a wider range.

[0077] A third sensor was fabricated, using a polarization-maintaining fiber from a specific brand, 1m in length, with a center wavelength of 1550nm, a maximum insertion loss of 0.5dB, and a return loss of 20dB. The sensor was calibrated; its strain range is 0-400,000. The change in light intensity is related to the square root of the strain, R 2=0.999, low hysteresis response, temperature response between 30°C and 60°C, and almost no change in light intensity. When testing object III, after deformation, the measurement result is a 3dB decrease in light intensity, corresponding to the sensor's 30% deformation calibration, which is consistent with the actual 30% deformation.

[0078] Simultaneously, the object was tested using another existing sensor: a conventional Sagnac interferometer sensor with a 1550nm light source and a strain range of 0-5187 nm. The wavelength change is linearly related to the strain, R 2 =0.999, temperature has a significant impact on the test. The measurement result shows a wavelength redshift of 15.1nm, which corresponds to a sensor value of 1000. The calibration conforms to the actual 1000 The deformation is measured. In contrast, traditional Sagnac interferometric sensors can only measure minute strains, suffer from wavelength drift, are difficult to demodulate, and are expensive. The sensor in this work can measure a wider range, is intensity-modulated, and is less expensive.

[0079] This invention utilizes a photoelectric element (PMF) as the core sensing element, constructing the sensing unit through spring winding. It leverages the deformation-induced photoelastic effect and optical path difference variation to achieve linear modulation of the deformation signal into interference light intensity, enabling high-fidelity sensing of deformation information. Experimental results show that the proposed sensor can stably measure a wide deformation range of 0-40%. The sensor output light intensity exhibits a good cubic correlation with the deformation, with a goodness of fit of 0.999, a full-scale error as low as 1.57%, and an average sensitivity of 0.3876 dB / mm, with low hysteresis during measurement. Furthermore, the sensor demonstrates excellent stability and cyclic repeatability, is insensitive to environmental temperature changes, and effectively suppresses the interference of temperature cross-sensitivity on the measurement results. This intensity-modulated sensor can directly convert photoelectric signals through a photodetector, simplifying the demodulation process. The core component is inexpensive to manufacture, and the structure is simple and easy to integrate, providing a reliable technical solution and experimental support for high-precision real-time monitoring of large deformations in complex scenarios across multiple fields.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A fiber optic deformation sensor based on Sagnac interferometry, characterized in that: It includes a Sagnac interference optical path and a photoelectric conversion module. The Sagnac interference optical path includes a sensing fiber loop, which includes at least one section of polarization-maintaining fiber. The polarization-maintaining fiber has an elastic helical structure that can be stretched axially. The intensity of the interference light output by the Sagnac interference optical path changes monotonically with the amount of axial stretching of the polarization-maintaining fiber. The photoelectric conversion module is used to receive the interference light intensity and convert it into an electrical signal.

2. The fiber optic deformation sensor based on Sagnac interferometry according to claim 1, characterized in that: The sensing fiber loop also includes an elastic frame, and the polarization-maintaining fiber is wound axially on the elastic frame.

3. The fiber optic deformation sensor based on Sagnac interferometry according to claim 2, characterized in that: The elastic frame is a cylindrical helical spring, and the polarization-maintaining optical fiber is wound along the helical direction of the cylindrical helical spring and fixed on the outer surface of the cylindrical helical spring.

4. The fiber optic deformation sensor based on Sagnac interferometry according to claim 3, characterized in that: The polarization-maintaining optical fiber and the cylindrical helical spring are bonded together with adhesive.

5. The fiber optic deformation sensor based on Sagnac interferometry according to claim 1, characterized in that: The photoelectric conversion module includes a photodetector, and the output end of the Sagnac interference optical path is electrically connected to the input end of the photodetector.

6. The fiber optic deformation sensor based on Sagnac interferometry according to claim 1, characterized in that: The photoelectric conversion module is also used to convert the electrical signal into a deformation and output it according to a preset mapping relationship between the deformation and the interference light intensity.

7. A method for measuring fiber optic deformation based on Sagnac interferometry, characterized in that: Using the sensor as described in claim 1, the sensor is calibrated to determine the mapping relationship between the deformation and the interference light intensity. The elastic spiral structure is fixed on the object to be measured, and the interference light intensity after the object to be measured undergoes deformation is obtained. The current deformation is obtained according to the mapping relationship.

8. The fiber optic deformation measurement method based on Sagnac interferometry according to claim 7, characterized in that: The calibration method for the sensor is as follows: apply multiple known deformations to the elastic helical structure, record the corresponding interference light intensities, and fit the mapping relationship.

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