Sagnac type soil humidity monitoring method based on hollow-core photonic crystal fiber

Abstract

The invention provides a Sagnac type soil humidity monitoring method based on a hollow-core photonic crystal fiber. According to the Sagnac type soil humidity monitoring method based on the hollow-core photonic crystal fiber, a humidity-sensitive material is injected into the hollow-core photonic crystal fiber, a Sagnac interference structure is adopted, and high-precision measurement and efficient humidity inversion are achieved through phase modulation and eigen frequency tracking; the method comprises the following steps: injecting a humidity-sensitive material into a hollow channel of the hollow-core photonic crystal fiber; a broadband light source equally divides a light source signal into two beams of light signals through a second optical fiber coupler, one beam of light signals enters a first coupler after being subjected to polarization filtering processing through an optical fiber polarizer, the two ends of the first coupler are coupled with the two ends of a hollow-core photonic crystal fiber to form a closed optical fiber ring, and the other beam of light signals directly enters the optical fiber ring; a phase modulator is added to one arm of the first coupler, and the modulation frequency is matched with the eigenfrequency of the hollow optical fiber; and measuring the adjustment amount delta f of the frequency of the phase modulator, and calculating the soil humidity through the delta f.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of optical fiber sensing, in particular to a Sagnac type soil humidity monitoring method based on a hollow photonic crystal fiber. BACKGROUND

[0002] In the environmental assessment and safety risk control of power transmission and transformation projects, accurate monitoring of soil erosion is one of the important links. As a key factor affecting the soil erosion resistance and stability, accurate, real-time and high-sensitivity monitoring of soil humidity is of great significance for preventing water and soil erosion and evaluating the long-term impact of the project on the environment. However, in the practical application of traditional soil humidity measurement technology, especially in special environments such as high voltage and strong electromagnetic interference, the limitations of the technology gradually appear, and it is difficult to meet the increasing requirements of modern engineering environmental assessment for data accuracy and reliability.

[0003] The limitations of traditional humidity measurement methods in sensitivity and resolution gradually appear. Traditional humidity sensors rely on electrochemical principles or gravimetric methods to measure moisture content, which can provide basic humidity information, but often result in inaccurate measurement results due to environmental factors. In addition, in special application fields such as high voltage, electrical sensors often cannot be applied, and electrochemical sensors have poor stability under extreme conditions, and are affected by aging and temperature changes during long-term use, limiting their reliability and application range in practical applications.

[0004] Optical fiber sensing technology gradually attracts attention in the field of environmental and engineering monitoring due to its characteristics such as anti-electromagnetic interference, corrosion resistance and realization of long-distance distributed monitoring. Among them, the Sagnac interference principle realizes high-sensitivity measurement of humidity parameters by detecting the phase difference caused by the change of external physical quantity when light propagates in a ring interferometer, however, traditional solid core optical fiber in humidity sensing usually relies on coating humidity absorbing materials or writing gratings, which has the problems of slow response, large influence of coating on stability, cross-sensitivity (such as temperature and humidity coupling) and the like.

[0005] In summary, in view of the deficiencies of traditional soil humidity monitoring technology in sensitivity, environmental adaptability and long-term reliability, combined with the urgent need for high-precision, anti-interference and real-time soil humidity monitoring in the environmental assessment of power transmission and transformation projects, a Sagnac interference type soil humidity sensor based on a hollow photonic crystal fiber is developed. SUMMARY

[0006] The application aims to overcome the technical defects of the prior art and provide a Sagnac type soil humidity monitoring method based on a hollow photonic crystal fiber. The method injects humidity-sensitive materials into the hollow photonic crystal fiber, adopts a Sagnac interference structure, uses phase modulation and intrinsic frequency tracking to realize high-precision measurement, and realizes efficient humidity inversion.

[0007] The application provides the following technical scheme: a Sagnac type soil humidity monitoring method based on a hollow core photonic crystal fiber, the method comprising the following steps:

[0008] Step 1: injecting a humidity-sensitive material into a hollow channel of the hollow core photonic crystal fiber;

[0009] Step 2: a wide-spectrum light source divides light source signals into two light signals through a second optical fiber coupler, one of the two light signals is subjected to polarization filtering processing through an optical fiber polarizer and then enters a first coupler, two ends of the first coupler are coupled with two ends of the hollow core photonic crystal fiber to form a closed optical fiber ring, and the other light signal directly enters the optical fiber ring;

[0010] Step 3: a phase modulator is added in one arm of the first coupler to perform stepwise square wave modulation, the modulation frequency is adjusted in real time through linear sweep, and the modulation frequency is matched with the intrinsic frequency of the hollow fiber until the modulation frequency is matched with the intrinsic frequency of the hollow fiber;

[0011] Step 4: the adjustment amount Δf of the phase modulator frequency is measured through monitoring of an interference waveform, the adjustment amount Δf of the phase modulator frequency is an intrinsic frequency offset amount of the hollow fiber, and the soil humidity is calculated through Δf.

[0012] Further, the humidity-sensitive material in step 1 is any one of polyvinyl alcohol (PVA) hydrogel, hydroxypropyl methyl cellulose (HPMC) and polyurethane (PU) hydrogel, and the refractive index expression of the humidity-sensitive material is:

[0013] (1),

[0014] wherein n0 is an initial refractive index, k is a humidity response coefficient, and ξ is relative soil humidity.

[0015] Further, the optical fiber ring in step 2 is composed of a plurality of series-connected Sagnac rings, and the plurality of Sagnac rings are cascaded in the same path.

[0016] Further, the calculation of Δf in step 4: the light signals in the optical fiber ring are divided into clockwise light paths and counterclockwise light paths, and the phases of the two light paths are controlled by the phase modulator;

[0017] The phase of the clockwise light path when reaching the phase modulator is α, the phase of the counterclockwise light path when reaching the phase modulator is β, and the phase difference is:

[0018] (2),

[0019] The interference intensity of the phase modulator is:

[0020] (3),

[0021] I0 is the maximum intensity of the interference signal;

[0022] The transit time τ of the optical signal in the fiber loop is:

[0023] (4),

[0024] L is the length of the optical fiber, n is the refractive index of the humidity-sensitive material, and c is the speed of light;

[0025] The modulation frequency f is:

[0026] (5),

[0027] The adjustment amount Δf of the modulation frequency is:

[0028] (6),

[0029] Substituting equation (1) into equation (6) gives:

[0030] (7).

[0031] Further, the phase modulator adopts a lithium niobate phase modulator, and the frequency range of the linear sweep of the phase modulator is determined according to the intrinsic frequency range of the hollow photonic crystal fiber.

[0032] Further, the calibration of the refractive index n of the humidity-sensitive material uses a refractometer to measure the refractive index of the humidity-sensitive material under different humidity conditions.

[0033] Further, the calibration of the humidity response coefficient k selects different humidity environments and measures the refractive index change of the humidity-sensitive material, respectively, and determines the value range of k by equation (1), which is usually 0.0002 / % RH to 0.0004 / % RH.

[0034] Further, the two ends of the first coupler are coupled with the two ends of the hollow photonic crystal fiber, and a micron-level gap is left at the coupling position, so that water vapor molecules can enter the hollow fiber through the gap and interact with the humidity-sensitive medium, thereby causing a change in the refractive index.

[0035] Further, a protective film is provided at the coupling position, and the protective film is a porous material with a pore size of 1-10 microns.

[0036] The application discloses a Sagnac type soil humidity monitoring method based on a hollow photonic crystal fiber.

[0037] The method has super-high sensitivity and precision, can accurately capture slight changes, and directly injects a humidity-sensitive material (such as a polyvinyl alcohol hydrogel) into a core channel of the hollow photonic crystal fiber, so that a light field is constrained to propagate in the sensitive material, the action path and intensity of light and water molecules are greatly increased, slight changes in soil water vapor can cause significant changes in the refractive index of the humidity-sensitive material, and the light field can efficiently perceive the changes; the Sagnac interference principle is used to convert the refractive index changes of the humidity-sensitive material into non-reciprocal phase differences of an interferometer. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 A hollow photonic crystal Sagnac type soil humidity monitoring optical path principle diagram;

[0039] Figure 2 A principle diagram for injecting a humidity-sensitive medium into a hollow fiber;

[0040] Figure 3 A phase modulation principle diagram;

[0041] Figure 4 An interference signal principle diagram;

[0042] Figure 5 A multi-loop cascade principle diagram;

[0043] Figure 6 A multi-loop cascade principle diagram based on an optical switch. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of the present application.

[0045] The application discloses a Sagnac type soil humidity monitoring method based on a hollow photonic crystal fiber, and constructs a novel soil humidity monitoring system, which has high sensitivity and high precision, can cope with the challenge of complex environment, and has wide application prospect.

[0046] The Sagnac interferometer of the present application is constructed as shown in Figure 1 The wide-spectrum light source is used as a laser light source to shorten the coherence length, so as to avoid the influence of polarization crosstalk and other related errors caused by the strong coherence of the light source. The wide-spectrum light source provides a wide wavelength range, so that the sensing system can effectively reduce the noise of the interference signal in actual monitoring, and improve the stability and measurement accuracy of the system.

[0047] The light source couples the light into two beams through the second optical fiber coupler, and then realizes the distribution of the signal. Half of the light beams are subjected to polarization filtering through the optical fiber polarizer to reduce the errors caused by the interface reflection of the optical fiber and air or the characteristics of the optical fiber. The light beams subjected to polarization filtering then enter the first coupler, and the two ends of the first coupler are coupled with the two ends of the hollow photonic crystal fiber (HC-PCF) to form a closed optical fiber ring.

[0048] At the coupling position, a micron-level gap is designed to allow water vapor molecules to enter the hollow optical fiber through the gap and interact with the internal humidity-sensitive material. In order to protect the optical fiber and its coupling part, a suitable protective film is used to prevent soil particles and other pollutants from directly contacting the surface of the optical fiber, while ensuring that water vapor can enter smoothly.

[0049] Inside the optical fiber ring, the clockwise and counterclockwise propagating light beams return to the coupler after a certain propagation distance in the optical fiber ring to interfere. In the interference process, the light beams pass through the optical fiber polarizer again to ensure that only light beams of the same polarization state can effectively interfere and enhance the stability of the signal. Then, the light beams subjected to interference return to the second coupler, and half of the light beams are received by the detector to obtain the interference signal and perform subsequent signal processing and humidity inversion.

[0050] In addition, a lithium niobate (LiNbO3) phase modulator is added in one arm of the first coupler, and a stepped square wave modulation method is adopted. In this operation, the lithium niobate phase modulator can change the phase of the light beam, facilitating real-time tracking of the intrinsic frequency change caused by the change in humidity. This design not only improves the response speed of the system, but also improves the monitoring accuracy, so that the Sagnac interferometer can provide efficient and accurate humidity monitoring data in a dynamic and variable soil environment.

[0051] In the present application, the selection, design and application of the humidity-sensitive material are key links to achieve high-sensitivity soil humidity monitoring. Therefore, polyvinyl alcohol (PVA) hydrogel is selected as the core humidity-sensitive material. PVA is a high-molecular-weight material with good hydrophilicity, and its excellent hydration characteristics can exhibit significant refractive index changes under different relative humidity conditions. This characteristic enables PVA hydrogel to efficiently respond to changes in soil moisture, thereby providing accurate humidity monitoring results.

[0052] (1)

[0053] where n0 is the initial refractive index, k is the humidity response coefficient, and ξ is the relative humidity.

[0054] In the specific design process, in order to obtain an ideal humidity response, the molecular weight, initial concentration, crosslinker amount and water content of the PVA hydrogel and other composition parameters need to be optimized. By adjusting the molecular weight, the physical properties and solubility of PVA can be affected, thereby controlling the water absorption rate and hydration capacity. At the same time, the selection of the initial concentration is an important factor to ensure the formation of uniform hydrogel. If the concentration is too low, it is not enough to form an effective gel network, and if the concentration is too high, it may cause the material to be uneven, affecting the optical properties. Therefore, a moderate concentration range needs to be found to balance the strength and humidity sensitivity of the material.

[0055] On the other hand, the amount of crosslinking agent also has a significant impact on the performance of the hydrogel. An appropriate amount of crosslinking agent can enhance the mechanical strength and stability of the PVA hydrogel, preventing deformation during repeated water absorption and dehydration, thereby ensuring the repeatability and stability of its humidity response. In addition, the adjustment of water content is also a key to achieving linear and repeatable refractive index changes. While a high water content can increase the moisture response speed of the material, it may also lead to a decrease in the mechanical properties of the gel. Therefore, systematic optimization should be performed to obtain a humidity-sensitive medium material that has both strength and sensitivity.

[0056] In application, the prepared PVA hydrogel humidity-sensitive medium is precisely injected into the inner cavity of the hollow photonic crystal fiber, forming a humidity-sensitive structure inside the fiber, as shown in Figure 2As shown in FIG. 4, due to the high hydrophilicity of the hydrogel, water vapor molecules can quickly penetrate into the fiber cavity and interact with the humidity-sensitive medium. With the change of the environmental humidity, the refractive index of the hydrogel will change accordingly during the process of water absorption or dehydration, which directly affects the intrinsic frequency of the optical fiber. By monitoring the change of the frequency in real time, the soil humidity can be accurately inverted. In addition, the compatibility of the humidity-sensitive material with the optical fiber material should also be fully considered to ensure that there is no material aging or optical loss due to interaction during long-term use, which will affect the overall performance and stability of the system.

[0057] In the Sagnac-type soil humidity monitoring system of the present application, a stepped square wave modulation technique is used to modulate the optical signal to enhance the precision and stability of humidity detection, as shown in FIG. 5. Figure 3 Specifically, the optical signal in the fiber ring is divided into two optical paths of clockwise propagation and counterclockwise propagation, and the phases of the two lights are controlled by the modulator.

[0058] When the clockwise propagating light reaches the modulator, its modulation phase is α, and the counterclockwise propagating light has a delay during its passage through the fiber ring, and its phase is β when it reaches the modulator. According to the principle of Sagnac interference, the phase difference between the clockwise light and the counterclockwise light is:

[0059] (2)

[0060] When the modulation frequency matches the transit time of the light in the fiber ring, the phase difference of the two beams will remain constant, so that the intensity of the interference light remains at a constant value. At this time, the interference intensity can be represented as:

[0061] (3)

[0062] Where I0 is the maximum intensity of the interference signal. When Δϕ is constant, the interference intensity I will also be a corresponding constant value.

[0063] However, if the modulation frequency does not match the transit time of the light in the fiber ring, the optical path difference will be zero. For example, if the modulation speed is slower than the transit time, the modulation phases of the clockwise and counterclockwise lights are both α. In this case, the phase difference is 0, and the interference intensity reaches the maximum value, as shown in FIG. 6. Figure 4 When the modulation frequency is f, the transit time τ of the light in the fiber can be calculated by the length L of the fiber and the effective refractive index n of the light, as follows:

[0064] (4)

[0065] Where c is the speed of light. According to the relationship between the transit time and the modulation frequency:

[0066] (5)

[0067] When the refractive index n of the optical fiber changes due to humidity changes, the transit time of light in the loop will also change due to the corresponding change in optical path, thereby affecting the required modulation frequency matching. It is necessary to monitor the adjustment amount of the modulation frequency in real time to achieve accurate monitoring of humidity changes.

[0068] The adjustment amount Δf of the modulation frequency is positively correlated with the change Δn of the refractive index. When the refractive index changes slightly, the transit time of the optical fiber will increase or decrease accordingly, so the modulation frequency must be adjusted accordingly to maintain the matching with the modified transit time.

[0069] (6)

[0070] Through real-time monitoring and adjustment of the modulation frequency, the humidity changes in the soil environment can be accurately reflected, and high-sensitivity humidity monitoring can be achieved. This process not only improves the accuracy of monitoring, but also enhances the response capability of the system to dynamic humidity changes, providing an important guarantee for practical application.

[0071] In the Sagnac-type soil humidity monitoring system of the present application, the process of signal processing and humidity inversion is the core of realizing high-precision humidity monitoring. The process starts with the acquisition of the interference signal by the detector, which converts the received optical signal into an electrical signal to form the time-domain interference signal waveform. The intensity of this electrical signal is closely related to the optical path difference and the modulation frequency, so a high-speed analog-to-digital converter (ADC) is needed to convert the analog signal to a digital signal for subsequent digital signal processing.

[0072] In the digital signal processing stage, to improve the signal-to-noise ratio, digital filtering, averaging processing and Fourier transform methods can be used to remove high-frequency noise and obtain clear interference signals.

[0073] Tracking and adjusting the modulation frequency so that the interference pattern is as shown in Figure 4 (a), and the adjustment amount Δf is the frequency shift amount. Substituting equation (1) into equation (6) gives:

[0074] (7)

[0075] The adjustment of the modulation frequency is proportional to the relative humidity of the soil. In a laboratory environment, different modulation frequencies are measured at different humidities to calibrate the coefficient, so that the soil humidity can be measured in real time in practical applications.

[0076] Cascade multiple optical paths, such as Figure 5As shown, by burying at different positions in the soil, multi-point synchronous measurement can be achieved. Light switches can also be used to switch between multiple optical paths to simplify the optical path, such as Figure 6 As shown.

[0077] The present application provides a Sagnac type soil moisture monitoring method based on hollow core photonic crystal fiber (HC-PCF), which has the advantages of high sensitivity and high precision, and can adapt to complex soil environment. By injecting polyvinyl alcohol (hydrogel) as a humidity-sensitive material in the hollow fiber, and combining with the Sagnac interference principle for real-time monitoring, the system can accurately capture the small water vapor changes in the soil. In addition, the use of step square wave modulation technology promotes the stability and response speed of the interference signal, ensuring accurate detection under dynamic humidity conditions. The design of this method effectively optimizes the water vapor introduction mechanism, prevents soil pollution, and accurately reflects humidity changes by adjusting the modulation frequency in real time, thus showing a wide application prospect in the fields of agricultural monitoring and environmental protection, and meeting the needs of modern fine management.

[0078] This embodiment demonstrates the practical application of a Sagnac type soil moisture monitoring method based on hollow core photonic crystal fiber (HC-PCF), which specifically includes device construction, parameter setting and analysis of actual measurement results.

[0079] First, when constructing the monitoring system, a suitable hollow core photonic crystal fiber is selected, with a core size of 200 microns and an outer diameter of about 1 millimeter. To meet the sensitivity requirements of the system, polyvinyl alcohol (PVA) hydrogel is injected as a humidity-sensitive material, with an initial molecular weight of PVA set to 130,000, a crosslinking agent dosage of 5%, and a water content of 80%. By optimizing these parameters, the humidity-sensitive layer is ensured to have significant refractive index changes under different relative humidity (RH) conditions. Within the experimental humidity range, the refractive index of the humidity-sensitive layer changes from 1.4900 (at 40% RH) to 1.5020 (at 90% RH), and the corresponding humidity response coefficient k is experimentally determined to be 0.0003 / %.

[0080] The present application uses polyvinyl alcohol (PVA) hydrogel as a humidity-sensitive material, but other humidity-sensitive materials can also be used. Here are some experimental data for several common humidity-sensitive materials:

[0081] (1) Polyvinyl Alcohol (PVA) Hydrogel

[0082] Refractive index change: As the relative humidity (RH) increases from 40% to 90%, the refractive index of PVA hydrogel changes from 1.4900 to 1.5020.

[0083] Humidity response coefficient (k): The experimentally determined humidity response coefficient is 0.0003 / % RH.

[0084] Performance characteristics: PVA hydrogel has good hydrophilicity, high stability and response speed, suitable for detection in different humidity ranges.

[0085] (2) Hydroxypropyl methylcellulose (HPMC)

[0086] Refractive index change: Relative humidity changes from 40% to 90%, refractive index changes from 1.4800 to 1.4950.

[0087] Humidity response coefficient (k): The humidity response coefficient is 0.0002 / % RH.

[0088] Performance characteristics: HPMC has relatively slow response to humidity, but has strong mechanical strength and chemical stability, suitable for long-term use.

[0089] (3) Polyurethane (PU) hydrogel

[0090] Refractive index change: Relative humidity changes from 30% to 85%, refractive index changes from 1.4950 to 1.5050.

[0091] Humidity response coefficient (k): The humidity response coefficient is 0.0004 / % RH.

[0092] Performance characteristics: PU hydrogel is very sensitive to humidity changes, especially suitable for fine monitoring in high humidity environments.

[0093] To construct the Sagnac interference ring, a broadband light source (wavelength range 400-800 nm) is used, which is divided into two beams by a second fiber coupler. One beam is polarized after passing through the fiber polarizer and then enters the first coupler, and the other beam directly enters the fiber ring. It should be noted that a micron gap is set at the coupling of the first coupler to ensure that water vapor molecules can effectively penetrate the hollow fiber. On the other hand, the fiber and its coupling parts use a protective film to prevent soil particles from interfering with the fiber.

[0094] After coupling, the light signal propagates in the fiber ring in clockwise and counterclockwise directions, and after a certain path, it returns to the coupler to form interference. At this time, the light signal passes through the fiber polarizer to ensure that the light of the same polarization state can effectively interfere, and finally the interference signal is sent to the detector for collection. In actual operation, a lithium niobate phase modulator is used for step square wave modulation, and the modulation frequency is adjusted to adapt to the intrinsic frequency of the fiber.

[0095] In practical application, a lithium niobate phase modulator (LiNbO3) is used to adjust the frequency of the fiber interference signal to track the changes of soil humidity in real time. The specific calculation steps of this method are as follows:

[0096] Step 1: Initialize the frequency range of the modulator, usually determined by the intrinsic frequency range of the hollow-core photonic crystal fiber. For example, set the frequency range from 10 Hz to 1 kHz to ensure coverage of the working frequency range of the hollow fiber.

[0097] Step 2: Linear frequency modulation, the modulator starts to change its frequency in a predetermined linear sweep manner. During this process, the interference signal will change until the modulation frequency matches the intrinsic frequency of the hollow fiber.

[0098] Step 3: Detect the interference waveform, by monitoring the changes in the interference waveform in real time, the system can identify the DC level state when the intrinsic frequency of the hollow fiber is matched. At this time, by measuring the adjustment amount △f of the frequency, the refractive index change caused by the humidity change can be determined.

[0099] Step 4: Finally determine the value of △f, when the interference waveform reaches the DC level, record the modulation frequency value at this time, the frequency difference △f at this time is the intrinsic frequency offset of the fiber, which represents the degree of soil humidity change.

[0100] In the experiment, in order to measure the response under different humidity conditions, the relative humidity is set from 40% to 90%. Through the real-time feedback of the modulator, the adjustment amount of the frequency is monitored, and the humidity is measured in real time according to formula (7). The frequency monitoring and adjustment of each time is recorded in real time by ADC, and the change of the interference signal is shown in the image of the change of the modulation frequency, which is processed into electronic signal for further analysis.

[0101] Calibration of the refractive index n and humidity response coefficient k of the humidity-sensitive material is a key step to ensure the accuracy of the monitoring system in measuring soil humidity. The specific steps are as follows:

[0102] Calibration of the humidity response coefficient k

[0103] Through laboratory tests, different humidity environments (such as RH = 40%, 60%, 80%, and

[0104] 90%) are selected to measure the refractive index change of the humidity-sensitive material, and the value of k for different humidity-sensitive materials is calculated according to formula (1). The value of k is usually in the range of 0.0002 / % to 0.0004 / %.

[0105] Calibration of the refractive index n

[0106] The refractometer is used to measure the refractive index value of the humidity-sensitive material under different humidity conditions. According to the change of humidity in the soil environment, the accurate relationship between the refractive index of the humidity-sensitive material and the humidity can be obtained through this step.

[0107] Real-time humidity monitoring and calibration is based on the calibrated humidity response coefficient k and the refractive index change, combined with real-time interference signal to adjust the modulation frequency, and calculate the soil humidity change.

[0108] The results show that the response time of the monitoring system under different humidity conditions is not more than 2 seconds, and the linear change of humidity can be reflected in the detector signal in real time. The experimental results show that in a low humidity environment (such as RH 40%), the interference intensity of the optical fiber ring presents a stable direct current level, while in a high humidity environment (such as RH 80%), the interference signal waveform shows obvious step changes. Through synchronous monitoring of multiple measurement points, the repeatability and stability of the experimental results are significantly guaranteed, verifying the practicability and sensitivity of the system in complex soil environments.

[0109] This example is based on the change of modulation frequency to calculate the soil humidity data:

[0110] Modulation frequency change Δf = 0.5 kHz

[0111] Optical fiber length L = 1 m

[0112] Humidity response coefficient k = 0.0003 / %RH

[0113] Initial refractive index of optical fiber n = 1.4900

[0114] Calculate the refractive index change Δn:

[0115] The frequency change Δf is proportional to the humidity change ΔRH (obtained by experimental calibration), and Δn is obtained according to formula (6), which is about 0.000075.

[0116] According to the humidity response coefficient k, the humidity change ΔRH is derived as 0.25%.

[0117] In addition, by burying multiple monitoring units in different soil samples, as shown in Figure 5 or Figure 6 , multi-point synchronous measurement can be realized. For the environmental assessment and safety risk control of power transmission and transformation projects, the precise monitoring of soil loss provides great convenience and expandability, and can meet the diversified monitoring needs, and the performance of the system has good durability, flexibility and reliability in various practical applications.

[0118] In summary, the Sagnac type soil humidity monitoring method based on hollow photonic crystal fiber is verified by actual examples to be effective in high sensitivity and high precision humidity detection, and proves the important application value and practical potential of the technology in the environmental assessment and safety risk control of power transmission and transformation projects.

[0119] The preferred embodiments of the present application have been described in detail above with reference to the accompanying drawings, but the present application is not limited to the above-described embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the spirit and scope of the present application, which involve related technologies well known to those skilled in the art, and these all fall within the scope of the patent of the present application.

[0120] Many other changes and modifications can be made to the application without departing from the spirit and scope of the application. It is to be understood that the application is not limited to the specific embodiments described herein, but only to the scope of the appended claims.

Claims

1. A Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber, characterized in that, The method includes the following steps: Step 1: Inject humidity-sensitive material into the hollow channel of the hollow-core photonic crystal fiber; Step 2: The broadband light source splits the light source signal into two beams through the second fiber coupler. One beam is polarized and filtered by the fiber polarizer and then enters the first coupler. The two ends of the first coupler are coupled to the two ends of the hollow-core photonic crystal fiber to form a closed fiber loop. The other beam enters the fiber loop directly. Step 3: Add a phase modulator to one arm of the first coupler to perform stepped square wave modulation. Adjust the modulation frequency in real time by linear frequency sweep until the modulation frequency matches the intrinsic frequency of the hollow fiber. Step 4: By monitoring the interference waveform, measure the adjustment amount Δf of the phase modulator frequency. The adjustment amount Δf of the phase modulator frequency is the intrinsic frequency offset of the hollow optical fiber. Soil moisture is calculated through Δf.

2. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 1, characterized in that, In step 1, the humidity-sensitive material is any one of polyvinyl alcohol (PVA) hydrogel, hydroxypropyl methylcellulose (HPMC), and polyurethane (PU) hydrogel. The refractive index expression of the humidity-sensitive material is: （1）， Where n0 is the initial refractive index, k is the humidity response coefficient, and ξ is the soil relative humidity.

3. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 2, characterized in that, In step 2, the fiber optic ring consists of multiple Sagnac rings connected in series, and the multiple Sagnac rings are cascaded in the same path.

4. A Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to any one of claims 1-3, characterized in that, In step 4, Δf is calculated as follows: the optical signal in the fiber optic loop is divided into clockwise optical path and counterclockwise optical path, and the phase of the two optical paths is controlled by the phase modulator respectively. The phase of the clockwise optical path reaching the phase modulator is α, and the phase of the counterclockwise optical path reaching the phase modulator is β. The phase difference is: （2）， The interference intensity of the phase modulator is: （3）， I0 is the maximum intensity of the interference signal; The transit time τ of the optical signal in the fiber optic loop is: （4）， L is the length of the optical fiber, n is the refractive index of the humidity-sensitive material, and c is the speed of light; The modulation frequency f is: （5）， The adjustment amount Δf of the modulation frequency is: （6）， Substituting equation (1) into equation (6), we get: （7）。 5. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 1, characterized in that, The phase modulator is a lithium niobate phase modulator, and the frequency range of the linear sweep frequency of the phase modulator is determined according to the intrinsic frequency range of the hollow-core photonic crystal fiber.

6. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 2, characterized in that, The refractive index n of the humidity-sensitive material is calibrated by measuring the refractive index of the humidity-sensitive material under different humidity conditions using a refractometer.

7. A Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 2 or 4, characterized in that, The humidity response coefficient k is calibrated by selecting different humidity environments, measuring the refractive index change of the humidity-sensitive material, and determining the value range of k through equation (1). It is usually from 0.0002 / %RH to 0.0004 / %RH.

8. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 1, characterized in that, The first coupler is coupled to both ends of a hollow photonic crystal fiber. A micron-level gap is left at the coupling point, allowing water vapor molecules to enter the hollow fiber through the gap and interact with the humidity-sensitive medium, thereby causing a change in refractive index.

9. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 8, characterized in that, A protective membrane is provided at the coupling point. The protective membrane is a porous material with a pore size of 1-10 micrometers.

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