A few-mode fiber stress sensing method based on frequency domain measurement

Abstract

The present application relates to a kind of stress sensing methods of few-mode fiber based on frequency domain measurement.The network analyzer outputs a radio frequency signal with linearly changing frequency over time, which is loaded onto the MZM modulator, modulates the common continuous light source, and then enters photon lantern, few-mode fiber, photon lantern in turn to realize modulation, transmission, demodulation, and finally photoelectric conversion through photodetector.The network analyzer processes electrical signal internally to obtain its frequency domain response.Then fast Fourier transform is performed to obtain time-domain impulse response.The time difference of time-domain impulse response reflects the pressure position, and the amplitude reflects the pressure size to realize stress sensing.The measurement accuracy of the method depends on the bandwidth of network analyzer, and compared with the time-domain measurement method based on optical pulse, the measurement accuracy is higher;The present application finds the mapping relationship between stress and impulse response based on frequency domain measurement technology, realizes stress sensing.Simplify the system complexity, and can solve the contradiction between cost and measurement accuracy in time-domain measurement.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and more specifically, to a few-mode fiber stress sensing method based on frequency domain measurement. Background Technology

[0002] Optical fiber communication plays a crucial role in modern communications. Early communication systems based on single-mode fiber are now approaching the Shannon limit, struggling to meet the ever-increasing demands for transmission rates and capacity. Few-mode fiber offers a highly attractive expansion solution for communication systems. The limited number of orthogonal modes in few-mode fiber can function as independent channels for information transmission, significantly increasing the transmission capacity of communication systems. The development of optical fiber communication has led to widespread attention and development of optical fiber sensors. In the field of sensing, few-mode fiber also offers several advantages over single-mode fiber: the spatial mode diversity of few-mode fiber enables multi-parameter sensing and helps combat the effects of signal fading on sensing systems.

[0003] Distributed sensing is currently the mainstream of fiber optic sensing. It integrates transmission and sensing, continuously sensing physical parameters such as stress, temperature, and vibration at every point along the fiber optic transmission path, obtaining multi-dimensional spatiotemporal distribution information. It plays a significant role in industries such as power, environment, and key security. In recent years, fiber optic stress sensors have seen considerable research and rapid development. Currently, common stress sensing technology uses backscattered light as the information carrier. Backscattered Rayleigh light is commonly detected by an optical time domain reflectometer (OTDR). Traditional OTDRs detect its intensity signal and are often used to locate large energy losses, such as fiber optic line breaks, but are not suitable for stresses with relatively small changes. Later, in 1993, scholars proposed phase-sensitive... -OTDR technology. This technology improves detection sensitivity and enables sensing of even weaker disturbances by modulating the optical phase, which is more sensitive to stress. Subsequently, more and more technologies based on... OTDR technology has been studied for its sensors. However, this technology needs to consider the following issues: 1. Phase noise caused by the light source. Therefore, narrow-linewidth light sources or short probe pulses are required to suppress phase noise; 2. Low power of backscattered Rayleigh light requires appropriate demodulation methods to further improve the signal-to-noise ratio. Currently, heterodyne coherent detection is commonly used for phase demodulation. Backscattered stress sensing technology can achieve long-distance distributed sensing, but methods to improve the signal-to-noise ratio will increase system complexity and cost.

[0004] Using forward-transmitting light as the information carrier is more conducive to achieving integrated communication and sensing. Furthermore, the power of forward-transmitting light is higher than that of backscattered light, resulting in a higher signal-to-noise ratio under similar experimental configurations. Currently, distributed stress sensors based on forward-transmitting light commonly employ polarization coupling and speckle effects. Both of these sensors can achieve distributed sensing, but they cannot be applied in long-distance scenarios.

[0005] Recently, researchers have demonstrated the feasibility of distributed stress sensing based on the mode coupling effect of few-mode fibers. This scheme, due to weak inter-mode coupling, treats multiple linearly polarized modes as independent spatial channels, sensing the magnitude and location of stress through mode coupling under stress. This scheme has been experimentally verified over several kilometers of fiber and has the potential for sensing over even longer distances. Previous research employed time-domain positioning techniques, utilizing the delay time and magnitude of different mode impulse responses to determine the location and magnitude of stress. However, such time-domain measurement methods require short-pulse light sources, and the pulse width directly determines the positioning accuracy. On the other hand, ultrashort pulse light sources are expensive, thus creating a trade-off between cost and positioning accuracy. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a few-mode fiber stress sensing method based on frequency domain measurement, which improves positioning accuracy and reduces costs.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A few-mode fiber stress sensing method based on frequency domain measurement includes the following steps:

[0009] S1. System Setup: The system includes a laser, polarization controller, MZM modulator, optical amplifier, photonic lantern, few-mode fiber, network analyzer, and pressure device. The network analyzer loads the radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator before being output. The modulated light is amplified by the optical amplifier and then input to the photonic lantern for mode selection excitation. The mode-selected excitation light is injected into the few-mode fiber for transmission. The light then passes through the photonic lantern again to demodulate the signal, and then is sent to the network analyzer via the photodetector (PD) on the single-mode pigtail. The pulse response of the light is displayed on the network analyzer's screen. Different pulse response curves are obtained by applying different pressures to the few-mode fiber using the pressure device.

[0010] S2. Data Acquisition and Processing: The frequency domain curve of the signal is obtained by using a network analyzer, and then the time domain curve is obtained by performing IFFT on the frequency domain curve;

[0011] S3. Establish the mapping relationship between pressure and relative power:

[0012] P related (dB)=P crosstalk -P excited (1)

[0013] In the formula, P crosstalk P represents the power of the crosstalk modes detected by the photodetector. excited P represents the power of the excitation mode detected by the photodetector.related This represents the power of the crosstalk mode relative to the excitation mode.

[0014] Based on the above technical means, the network analyzer outputs a radio frequency signal whose frequency varies linearly with time. This signal is loaded onto an MZM modulator to modulate a continuous wave laser source, which then sequentially enters a photonic lantern, a few-mode fiber, and another photonic lantern to achieve modulation, transmission, and demodulation. Finally, it undergoes photoelectric conversion through a photodetector. The network analyzer internally processes the electrical signal to obtain its frequency domain response. A fast Fourier transform is then performed to obtain the time domain impulse response. The time difference of the time domain impulse response reflects the pressure location, and the amplitude reflects the pressure magnitude, thus achieving stress sensing. The measurement accuracy of this method depends on the bandwidth of the network analyzer, and it offers higher accuracy compared to time-domain measurement methods based on optical pulses. This invention uses frequency domain measurement technology to find the mapping relationship between stress and impulse response, achieving stress sensing. This scheme simplifies system complexity and resolves the contradiction between cost and measurement accuracy in time-domain measurements.

[0015] Furthermore, step S3 includes the following steps:

[0016] S31. Excite the LP01 mode using a photon lantern and collect the pulse response curves of the optical fiber under pressures of 0g, 3kg, 6kg, and 9kg, respectively, where 0g represents no external pressure applied to the optical fiber. Obtain the power P of the LP11 mode. LP11 Power P in LP01 mode LP01 The power of LP11 relative to LP01 is calculated using formula (1).

[0017] S32. Excite the LP11 mode with a photon lantern, repeat step S31, and use formula (1) to obtain the power of LP01 relative to LP11.

[0018] S33. Establish The mapping relationship with pressure; with pressure as the independent variable, Use a coordinate system for the dependent variable to plot the relationship curve.

[0019] Furthermore, in this invention, the light source emitted by the laser is a DC light source rather than a short-pulse light source.

[0020] Furthermore, the network analyzer's output port transmits a radio frequency signal whose frequency varies linearly with time. This radio frequency signal is loaded onto an MZM modulator to modulate the frequency of the continuous light source. Then, it undergoes mode-selective excitation via a photonic lantern to obtain the optical field expressions for different transmission modes, as shown in the following formula:

[0021]

[0022] In the formula, n represents the mode n that can be transmitted in a few-mode fiber, P is the power spectrum of the continuous light source, m is the modulation coefficient, f is the light source frequency, and f0 is the frequency of the light source. m t is the frequency of the radio frequency signal, and t is the time.

[0023] Furthermore, after mode n is transmitted through the few-mode fiber, the expression for the output optical field is obtained as shown in formula (3):

[0024]

[0025] Equation (3) shows that the output light field can be expressed as the frequency f integral in the complex domain, where represents the delay time of mode n at frequency f.

[0026] Furthermore, since few-mode fibers are weakly coupled, and inter-mode coupling is ignored, the expression for the signal detected by the photodetector is shown in equation (4):

[0027]

[0028] Where R is the response coefficient of the photodetector.

[0029] Furthermore, substituting formula (3) into formula (4) yields the frequency response of the few-mode fiber, as shown in formula (5):

[0030]

[0031] Where Δτ2 is the LP under the set light source. 11 With LP 01 The time delay difference, C0 and C1 are constants related to the excitation mode power.

[0032] Furthermore, IFFT is performed on the frequency domain curve to obtain the time domain curve. The highest and second highest values ​​in the curve reflect the power of the excitation mode and the crosstalk mode, respectively.

[0033] This invention also provides a few-mode fiber stress sensing system based on frequency domain measurement, including a laser, a polarization controller, an MZM modulator, an optical amplifier, a photonic lantern, a few-mode fiber, a network analyzer, and a pressure device. The network analyzer loads a radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator and then output. The modulated light is amplified by the optical amplifier and then input to the photonic lantern for mode selection excitation. The mode-selectively excited light is injected into the few-mode fiber for transmission. The light is then demodulated again by the photonic lantern and then sent to the network analyzer via a photodetector (PD) on a single-mode pigtail. The pulse response of the light is displayed on the network analyzer's screen. Different pulse response curves are obtained by applying different pressures to the few-mode fiber using the pressure device.

[0034] Furthermore, the pressure device includes a bracket, a turntable, a lead screw, a first pressure plate, a second pressure plate, a base, a pressure display, and a pressure sensor. The bracket is mounted on the base, the second pressure plate is located on top of the base, and the pressure sensor is located on the second pressure plate. The lead screw passes vertically through the bracket and is threadedly connected to it. One end of the lead screw is connected to the first pressure plate, and the other end is connected to the turntable. Rotating the turntable can move the lead screw up and down. The second pressure plate is located directly below the first pressure plate. The pressure sensor is communicatively connected to the pressure display. The main body of the pressure device is a longitudinal displacement stage. By driving the lead screw through the top turntable, the pressure exerted by the first pressure plate on the optical fiber can be controlled to vary. Directly below the first pressure plate is a second pressure plate with an embedded 10kg pressure sensor. The optical fiber is laid flat and fixed on the second pressure plate to receive external pressure.

[0035] Furthermore, both the first and second pressure plates are made of resin material. Using resin material can isolate the optical platform from magnetic interference with the sensor.

[0036] Compared with existing technologies, the advantages of this invention are: The few-mode fiber stress sensing method based on frequency domain measurement provided by this invention has a measurement accuracy that depends on the bandwidth of the network analyzer, and it offers higher accuracy compared to time-domain measurement methods based on optical pulses; this invention uses frequency domain measurement technology to find the mapping relationship between stress and impulse response, thus realizing stress sensing. This scheme simplifies system complexity and resolves the contradiction between cost and measurement accuracy in time-domain measurements. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the system structure of the present invention.

[0038] Figure 2 This is a schematic diagram of the pressure device structure of the present invention.

[0039] Figure descriptions: 1. Bracket; 2. Turntable; 3. Lead screw; 4. First pressure plate; 5. Second pressure plate; 6. Base; 7. Pressure indicator; 8. Fiber optic cable. Detailed Implementation

[0040] 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 a part of the embodiments of the present invention, and not all of the embodiments. The present invention will be described in one embodiment below with reference to specific embodiments. The accompanying drawings are for illustrative purposes only and represent schematic diagrams, not actual pictures, and should not be construed as limiting the present patent. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged, or reduced, and do not represent the actual product size. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0041] In the description of this invention, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing the 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. Therefore, the terms describing positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances. In addition, if the embodiments of this invention involve descriptions of "first," "second," etc., such descriptions are only for descriptive purposes and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of those features. Furthermore, the meaning of "and / or" throughout the text is to include three parallel solutions. Taking "A and / or B" as an example, it includes solution A, or solution B, or a solution that simultaneously satisfies A and B.

[0042] Example 1:

[0043] A few-mode fiber stress sensing method based on frequency domain measurement includes the following steps:

[0044] Step 1. System Setup: The system includes a laser, polarization controller, MZM modulator, optical amplifier, photonic lantern, few-mode fiber, network analyzer, and pressure device. The network analyzer loads the radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator and then output. The modulated light is amplified by the optical amplifier and then input to the photonic lantern for mode selection excitation. The mode-selected excitation light is injected into the few-mode fiber for transmission. The light is then demodulated again by the photonic lantern and then sent to the network analyzer via the photodetector (PD) on the single-mode pigtail. The pulse response of the light is displayed on the network analyzer screen. Different pulse response curves are obtained by applying different pressures to the few-mode fiber using the pressure device. In this embodiment, the light source emitted by the laser is a DC light source rather than a short-pulse light source.

[0045] In this embodiment, the step-type ultra-low attenuation two-mode fiber FMSI-2-ULL from Yangtze Optical Fibre and Cable Joint Stock Limited Company is selected.

[0046] Step 2. Data Acquisition and Processing: The network analyzer can measure the frequency domain curve of the signal, and performing IFFT on it yields the time domain curve. The experiment will save the corresponding time-domain impulse response curves for subsequent data processing. The experiment will save the impulse response data points corresponding to different stress magnitudes. The highest and second highest values ​​reflect the power of the excitation mode and crosstalk mode, respectively.

[0047] Step 3. Establish the mapping relationship between pressure and relative power through calculation formulas:

[0048] P related (dB)=P crosstalk -P excited (1)

[0049] In the formula, P crosstalk P represents the power of the crosstalk modes detected by the photodetector. excited P represents the power of the excitation mode detected by the photodetector. related This represents the power of the crosstalk mode relative to the excitation mode.

[0050] Based on the above technical means, the network analyzer outputs a radio frequency signal whose frequency varies linearly with time. This signal is loaded onto an MZM modulator to modulate a continuous wave laser source, which then sequentially enters a photonic lantern, a few-mode fiber, and another photonic lantern to achieve modulation, transmission, and demodulation. Finally, it undergoes photoelectric conversion through a photodetector. The network analyzer internally processes the electrical signal to obtain its frequency domain response. A fast Fourier transform is then performed to obtain the time domain impulse response. The time difference of the time domain impulse response reflects the pressure location, and the amplitude reflects the pressure magnitude, thus achieving stress sensing. The measurement accuracy of this method depends on the bandwidth of the network analyzer, and it offers higher accuracy compared to time-domain measurement methods based on optical pulses. This invention uses frequency domain measurement technology to find the mapping relationship between stress and impulse response, achieving stress sensing. This scheme simplifies system complexity and resolves the contradiction between cost and measurement accuracy in time-domain measurements.

[0051] Step S3 includes the following steps:

[0052] S31. Excite the LP01 mode using a photon lantern and collect the pulse response curves of the optical fiber under pressures of 0g, 3kg, 6kg, and 9kg, respectively, where 0g represents no external pressure applied to the optical fiber. Obtain the power P of the LP11 mode. LP11 Power P in LP01 mode LP01 The power of LP11 relative to LP01 is calculated using formula (1).

[0053] S32. Excite the LP11 mode with a photon lantern, repeat step S31, and use formula (1) to obtain the power of LP01 relative to LP11.

[0054] S33. Establish The mapping relationship with pressure; with pressure as the independent variable, Use a coordinate system for the dependent variable to plot the relationship curve.

[0055] Furthermore, the network analyzer's output port transmits a radio frequency signal whose frequency varies linearly with time. This radio frequency signal is loaded onto an MZM modulator to modulate the frequency of the continuous light source. Then, it undergoes mode-selective excitation via a photonic lantern to obtain the optical field expressions for different transmission modes, as shown in the following formula:

[0056]

[0057] In the formula, n represents the mode n that can be transmitted in a few-mode fiber, P is the power spectrum of the continuous light source, m is the modulation coefficient, f is the light source frequency, and f0 is the frequency of the light source. m t is the frequency of the radio frequency signal, and t is the time.

[0058] After mode n is transmitted through a few-mode fiber, the expression for the output optical field is as shown in formula (3):

[0059]

[0060] Equation (3) shows that the output light field can be expressed as the frequency f integral in the complex domain, where represents the delay time of mode n at frequency f.

[0061] Since few-mode fibers are weakly coupled, and inter-mode coupling is ignored, the expression for the signal detected by the photodetector is shown in equation (4):

[0062]

[0063] Where R is the response coefficient of the photodetector.

[0064] If we disregard the dispersion of the mode and the linewidth of the light source, and assume that the few-mode fiber used in the experiment only supports two modules, substituting equation (3) into equation (4) yields the frequency response of the few-mode fiber, as shown in equation (5):

[0065] |H(fm)|=C0+C1 cos(2πf m Δτ2) (5)

[0066] Where Δτ2 is the LP under the set light source. 11 With LP 01 The time delay difference, C0 and C1 are constants related to the excitation mode power.

[0067] The time-domain impulse response was obtained using Fast Fourier Transform (FFT). For the experimental FMF, its time-domain impulse response consisted of two peaks. The time difference on the horizontal axis reflected the location of the stress acting on the fiber, while the amplitude change on the vertical axis reflected the magnitude of the stress. An IFFT was then performed on the frequency-domain curve to obtain the time-domain curve. The highest and second-highest values ​​on the curve reflected the power of the excitation mode and crosstalk mode, respectively.

[0068] Example 2

[0069] This embodiment provides a few-mode fiber stress sensing system based on frequency domain measurement, including a laser, a polarization controller, an MZM modulator, an optical amplifier, a photonic lantern, a few-mode fiber, a network analyzer, and a pressure device. The network analyzer loads a radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator and then output. The modulated light is amplified by the optical amplifier and then input to the photonic lantern for mode selection excitation. The mode-selectively excited light is injected into the few-mode fiber for transmission. The light is then demodulated again by the photonic lantern and then sent to the network analyzer via a photodetector (PD) on a single-mode pigtail. The pulse response of the light is displayed on the network analyzer's screen. Different pulse response curves are obtained by applying different pressures to the few-mode fiber using the pressure device.

[0070] Based on the above technical means, the network analyzer outputs a radio frequency signal whose frequency varies linearly with time. This signal is loaded onto an MZM modulator to modulate a continuous wave laser source, which then sequentially enters a photonic lantern, a few-mode fiber, and another photonic lantern to achieve modulation, transmission, and demodulation. Finally, it undergoes photoelectric conversion through a photodetector. The network analyzer internally processes the electrical signal to obtain its frequency domain response. A fast Fourier transform is then performed to obtain the time domain impulse response. The time difference of the time domain impulse response reflects the pressure location, and the amplitude reflects the pressure magnitude, thus achieving stress sensing. The measurement accuracy of this method depends on the bandwidth of the network analyzer, and it offers higher accuracy compared to time-domain measurement methods based on optical pulses. This invention uses frequency domain measurement technology to find the mapping relationship between stress and impulse response, achieving stress sensing. This scheme simplifies system complexity and resolves the contradiction between cost and measurement accuracy in time-domain measurements.

[0071] Example 3

[0072] This embodiment provides a pressure device, which includes a bracket 1, a turntable 2, a lead screw 3, a first pressure plate 4, a second pressure plate 5, a base 6, a pressure display 7, and a pressure sensor. The bracket 1 is mounted on the base 6, the second pressure plate 5 is located on top of the base 6, and the pressure sensor is located on the second pressure plate 5. The lead screw 3 is vertically inserted through the bracket 1 and threadedly connected to the bracket 1. One end of the lead screw 3 is connected to the first pressure plate 4, and the other end is connected to the turntable 2. Rotating the turntable 2 can drive the lead screw 3 to move up and down. The second pressure plate 5 is located directly below the first pressure plate 4. The pressure sensor is communicatively connected to the pressure display 7. The main body of this pressure device is a longitudinal displacement stage. By driving the lead screw 3 through the top turntable 2, the pressure exerted by the first pressure plate 4 on the optical fiber 8 can be controlled to vary. Directly below the first pressure plate 4, there is a second pressure plate 5 with an embedded pressure sensor with a range of 10 kg. The optical fiber 8 is laid flat and fixed on the second pressure plate 5 to receive external pressure.

[0073] Both the first pressure plate 4 and the second pressure plate 5 are made of resin material. Using resin material helps to isolate the optical platform from magnetic interference with the sensor.

[0074] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A few-mode fiber stress sensing method based on frequency domain measurement, characterized in that, Includes the following steps: S1. System Setup: The system includes a laser, polarization controller, MZM modulator, optical amplifier, photonic lantern, few-mode fiber, network analyzer, and pressure device; the network analyzer loads the radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator before being output; The modulated light is amplified by an optical amplifier and then input to a photonic lantern for mode selection excitation. The mode-selectively excited light is injected into a few-mode fiber for transmission. The light is then demodulated again by the photonic lantern and sent to a network analyzer via a photodetector (PD) on a single-mode pigtail. The pulse response of the light is displayed on the network analyzer screen. Different pulse response curves are obtained by applying different pressures to the few-mode fiber using a pressure device. S2. Data Acquisition and Processing: The frequency domain curve of the signal is measured using a network analyzer, and then the time domain curve is obtained by performing IFFT on the frequency domain curve; S3. Establish the mapping relationship between pressure and relative power: （1） In the formula, This represents the power of the crosstalk modes detected by the photodetector. This represents the power of the excitation mode detected by the photodetector. This represents the power of the crosstalk mode relative to the excitation mode; Step S3 includes the following steps: S31. Excite the LP01 mode with a photon lantern and collect the pulse response curves of the optical fiber under pressures of 0g, 3kg, 6kg, and 9kg, respectively, where 0g represents no external pressure applied to the optical fiber. Obtain the power P of the LP11 mode. LP11 Power P in LP01 mode LP01 The power of LP11 relative to LP01 is calculated using formula (1). ; S32. Excite the LP11 mode with a photon lantern, repeat step S31, and use formula (1) to obtain the power of LP01 relative to LP11. ; S33. Establish , The mapping relationship with pressure; with pressure as the independent variable, , Use a coordinate system for the dependent variable to plot the relationship curve.

2. The few-mode fiber stress sensing method based on frequency domain measurement according to claim 1, characterized in that, The laser emits a DC light source.

3. The few-mode fiber stress sensing method based on frequency domain measurement according to claim 1, characterized in that, The network analyzer's output port transmits a radio frequency signal whose frequency varies linearly with time. This radio frequency signal is loaded onto an MZM modulator to modulate the frequency of a continuous light source. Then, it undergoes mode-selective excitation via a photonic lantern to obtain the optical field expressions for different transmission modes, as shown in the following formulas: （2） In the formula, n represents the mode n that can be transmitted in a few-mode fiber, P is the power spectrum of the continuous light source, m is the modulation coefficient, f is the light source frequency, and f0 is the frequency of the light source. m t is the frequency of the radio frequency signal, and t is the time.

4. The few-mode fiber stress sensing method based on frequency domain measurement according to claim 3, characterized in that, After mode n is transmitted through a few-mode fiber, the expression for the output optical field is as shown in formula (3): Equation (3) shows that the output light field is represented as the frequency integral of the complex domain, where represents the delay time of mode n at frequency f.

5. The few-mode fiber stress sensing method based on frequency domain measurement according to claim 4, characterized in that, Since few-mode fibers are weakly coupled, and inter-mode coupling is ignored, the expression for the signal detected by the photodetector is shown in equation (4): （4） Where R is the response coefficient of the photodetector; Substituting formula (3) into formula (4) yields the frequency response of the few-mode fiber, as shown in formula (5): （5） in Under the set light source, LP 11 With LP 01 The time delay difference, C0 and C1 are constants related to the excitation mode power.

6. The few-mode fiber stress sensing method based on frequency domain measurement according to any one of claims 1 to 5, characterized in that, The time-domain curve is obtained by performing IFFT on the frequency domain curve. The highest and second highest values ​​in the curve reflect the power of the excitation mode and the crosstalk mode, respectively.

7. A few-mode fiber stress sensing system based on frequency domain measurement, characterized in that, The method for stress sensing of few-mode fiber based on frequency domain measurement as described in any one of claims 1 to 6 includes a laser, a polarization controller, an MZM modulator, an optical amplifier, a photonic lantern, a few-mode fiber, a network analyzer, and a pressure device; the network analyzer loads a radio frequency signal onto the MZM modulator, and the light emitted by the laser is modulated by the MZM modulator and then output; The modulated light is amplified by an optical amplifier and then input into a photonic lantern for mode selection excitation. The mode-selected excited light is injected into a few-mode fiber for transmission. The light is then demodulated again by the photonic lantern and then sent to a network analyzer via a photodetector (PD) on a single-mode pigtail. The network analyzer displays the pulse response of the light. Different pulse response curves were obtained by applying different pressures to the few-mode optical fiber using a pressure device.

8. The few-mode fiber stress sensing system based on frequency domain measurement according to claim 7, characterized in that, The pressure device includes a bracket (1), a turntable (2), a lead screw (3), a first pressure plate (4), a second pressure plate (5), a base (6), a pressure display (7), and a pressure sensor; the bracket (1) is mounted on the base (6), the second pressure plate (5) is located on the top of the base (6), and the pressure sensor is located on the second pressure plate (5); the lead screw (3) is vertically inserted through the bracket (1) and threadedly connected to the bracket (1); one end of the lead screw (3) is connected to the first pressure plate (4), and the other end is connected to the turntable (2). Rotating the turntable (2) can drive the lead screw (3) to move up and down; the second pressure plate (5) is located directly below the first pressure plate (4); the pressure sensor is communicatively connected to the pressure display (7).

9. The few-mode fiber stress sensing system based on frequency domain measurement according to claim 8, characterized in that, Both the first pressure plate (4) and the second pressure plate (5) are made of resin material.

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