A sagnac type soil moisture monitoring method based on hollow photonic crystal fiber

Abstract

This invention provides a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber. It utilizes a moisture-sensitive material injected into the hollow-core photonic crystal fiber, employs a Sagnac interference structure, and leverages phase modulation and intrinsic frequency tracking to achieve high-precision measurement and efficient moisture inversion. The method includes the following steps: injecting moisture-sensitive material into the hollow channel of the hollow-core photonic crystal fiber; splitting the light source signal into two beams via a broadband light source through a second fiber coupler; one beam undergoes polarization filtering through a fiber polarizer before entering a first coupler, with the two ends of the first coupler coupled to the two ends of the hollow-core photonic crystal fiber to form a closed fiber loop; the other beam directly enters the fiber loop; a phase modulator is added to one arm of the first coupler, with the modulation frequency matched to the intrinsic frequency of the hollow fiber; the adjustment amount Δf of the phase modulator frequency is measured, and the soil moisture is calculated using Δf.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, specifically to a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber. Background Technology

[0002] In the environmental assessment and safety risk control of power transmission and transformation projects, accurate monitoring of soil erosion is a crucial aspect. Soil moisture, as a key factor affecting soil erosion resistance and stability, requires accurate, real-time, and highly sensitive monitoring for prevention of soil erosion and assessment of the project's long-term environmental impact. However, traditional soil moisture measurement techniques have gradually revealed various limitations in practical engineering applications, especially under special environments such as high voltage and strong electromagnetic interference, making it difficult to meet the increasingly demanding requirements for data accuracy and reliability in modern environmental impact assessments.

[0003] The limitations of traditional humidity measurement methods in terms of sensitivity and resolution are becoming increasingly apparent. Traditional humidity sensors mostly rely on electrochemical principles or gravimetric methods to measure moisture content. While they can provide basic humidity information, the results are often inaccurate due to environmental factors. Furthermore, electrical sensors are often unsuitable for special applications such as high-pressure applications, and electrochemical sensors exhibit poor stability under certain extreme conditions. Long-term use is affected by aging and temperature changes, limiting their reliability and applicability in practical applications.

[0004] Fiber optic sensing technology has gained increasing attention in environmental and engineering monitoring fields due to its resistance to electromagnetic interference, corrosion resistance, and ability to enable long-distance distributed monitoring. Among these technologies, the Sagnac interferometer principle achieves highly sensitive measurement of humidity parameters by detecting the phase difference caused by changes in external physical quantities as light propagates in a ring interferometer. However, traditional solid-core optical fibers in humidity sensing typically rely on coatings with hygroscopic materials or etched gratings, which suffer from slow response, stability significantly affected by coatings, and cross-sensitivity (such as temperature and humidity coupling).

[0005] In summary, in view of the shortcomings of traditional soil moisture monitoring technology in terms of sensitivity, environmental adaptability and long-term reliability, and in combination with the urgent need for high-precision, anti-interference and real-time soil moisture monitoring in the environmental impact assessment of power transmission and transformation projects, a Sagnac interferometric soil moisture sensor based on hollow photonic crystal fiber was developed. Summary of the Invention

[0006] This invention aims to address the technical deficiencies of existing technologies by providing a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber. It utilizes moisture-sensitive materials injected into the hollow-core photonic crystal fiber, employs a Sagnac interference structure, and leverages phase modulation and intrinsic frequency tracking to achieve high-precision measurement and efficient moisture inversion.

[0007] This invention provides the following technical solution: a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber, the method comprising the following steps:

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

[0009] 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.

[0010] 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.

[0011] 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.

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

[0013] (1),

[0014] Where n0 is the initial refractive index, k is the humidity response coefficient, and ξ is the soil relative humidity.

[0015] Furthermore, in step 2, the fiber optic ring is composed of multiple Sagnac rings connected in series, and the multiple Sagnac rings are cascaded in the same path.

[0016] Furthermore, 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 phases of the two optical paths are controlled by phase modulators respectively.

[0017] 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:

[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 optic 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), we get:

[0030] (7).

[0031] Furthermore, the phase modulator is 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-core photonic crystal fiber.

[0032] Furthermore, 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.

[0033] Furthermore, the humidity response coefficient k is calibrated by selecting different humidity environments, measuring the refractive index change of the humidity-sensitive material respectively, and determining the value range of k through equation (1), which is usually from 0.0002 / %RH to 0.0004 / %RH.

[0034] Furthermore, the two ends of the first coupler are coupled to the two ends of the hollow photonic crystal fiber, and a micron-level gap is left at the coupling point, 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 refractive index.

[0035] Furthermore, a protective membrane is provided at the coupling point, the protective membrane being a porous material with a pore size of 1-10 micrometers.

[0036] This invention discloses a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber. This method not only achieves breakthroughs in sensitivity and accuracy by combining hollow-core photonic crystal fiber with the Sagnac interference principle, but also exhibits unique advantages in anti-interference, corrosion resistance, long-term stability and networking capability due to its all-fiber design. It precisely meets the urgent need for soil moisture monitoring in complex environments such as high-voltage power transmission and transformation, and is an innovative monitoring method with important practical value and broad application prospects.

[0037] It boasts ultra-high sensitivity and accuracy, precisely capturing minute changes. Moisture-sensitive materials (such as polyvinyl alcohol hydrogel) are directly injected into the core channel of a hollow photonic crystal fiber, confining the light field within the sensitive material. This significantly increases the interaction path and intensity between light and water molecules, allowing even minute changes in soil moisture to cause significant alterations in the refractive index of the moisture-sensitive material, which are efficiently sensed by the light field. Employing the Sagnac interferometry principle, the refractive index change of the moisture-sensitive material is converted into a non-reciprocal phase difference in the interferometer. Through stepped square wave modulation and linear frequency sweeping technology, the intrinsic frequency offset Δf of the ring interferometer is tracked and locked in real time. Frequency measurement exhibits extremely high accuracy and anti-interference capabilities, converting humidity information into a stable and precise digital signal. This achieves ultra-high sensitivity and measurement accuracy that traditional intensity-type sensors struggle to reach, reliably detecting subtle dynamic changes in soil moisture and accurately reflecting humidity changes through real-time modulation frequency adjustments. This enables soil moisture monitoring in complex environments of power transmission and transformation projects, providing accurate, real-time, and highly reliable humidity data support for soil erosion risk assessment. It serves as a decision-making basis for scientific environmental assessments and early warning of engineering safety risks. Attached Figure Description

[0038] Figure 1 Schematic diagram of the optical path for monitoring soil moisture using a Sagnac type hollow photonic crystal;

[0039] Figure 2 Schematic diagram of injecting hollow fiber into a humidity-sensitive medium;

[0040] Figure 3 This is a schematic diagram of phase modulation principle;

[0041] Figure 4 This is a schematic diagram of the interference signal principle.

[0042] Figure 5 This is a schematic diagram of a multi-ring cascade.

[0043] Figure 6 This is a schematic diagram of a multi-ring cascade based on an optical switch. Detailed Implementation

[0044] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0045] This invention discloses a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber, and constructs a novel soil moisture monitoring system that has both high sensitivity and high accuracy, and can cope with the challenges of complex environments, and has broad application prospects.

[0046] The Sagnac interferometer of the present invention is constructed as follows: Figure 1 As shown, a broadband light source is used as the laser source to shorten the coherence length, thereby avoiding the effects of polarization crosstalk and other related errors caused by excessive coherence of the light source. The broadband light source provides a wide wavelength range, enabling the sensing system to effectively reduce interference signal noise and improve system stability and measurement accuracy in practical monitoring.

[0047] The light source splits into two beams proportionally via a second fiber coupler, thereby achieving signal distribution. One half of the beam undergoes polarization filtering via a fiber polarizer to reduce errors caused by reflections at the fiber-air interface or the inherent characteristics of the fiber itself. The polarization-filtered beam then enters the first coupler, whose two ends are coupled to the two ends of a hollow-core photonic crystal fiber (HC-PCF) to form a closed fiber loop.

[0048] At the coupling point, a micrometer-sized gap is designed to allow water vapor molecules to enter the hollow optical fiber and interact with the internal humidity-sensitive material. To protect the optical fiber and its coupling points, a suitable protective film is used to prevent soil particles and other contaminants from directly contacting the fiber surface, while ensuring that water vapor can enter smoothly.

[0049] Inside the fiber optic loop, clockwise and counterclockwise beams travel a certain distance before returning to the coupler for interference. During interference, the beams pass through a fiber polarizer again to ensure that only light of the same polarization state can effectively interfere, thus enhancing signal stability. The interfered beams then return to the second coupler, where half of the light is received by a detector to obtain the interference signal for subsequent signal processing and humidity inversion.

[0050] Furthermore, a lithium niobate (LiNbO3) phase modulator is incorporated into one arm of the first coupler, employing stepped square wave modulation. In this operation, the lithium niobate phase modulator alters the phase of the beam, facilitating real-time tracking of intrinsic frequency changes caused by humidity variations. This design not only improves the system's response speed but also enhances monitoring accuracy, enabling the Sagnac interferometer to provide efficient and precise humidity monitoring data in dynamic and variable soil environments.

[0051] In this invention, the selection, design, and application of humidity-sensitive materials are crucial for achieving highly sensitive soil moisture monitoring. Therefore, polyvinyl alcohol (PVA) hydrogel was selected as the core humidity-sensitive material. PVA is a polymer with good hydrophilicity, and its excellent hydration properties allow it to exhibit significant refractive index changes under different relative humidity conditions. This characteristic enables PVA hydrogel to respond efficiently 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, to obtain the ideal humidity-sensitive response, it is necessary to optimize the compositional parameters of PVA hydrogel, such as molecular weight, initial concentration, crosslinking agent dosage, and water content. By adjusting the molecular weight, the physical properties and solubility of PVA can be affected, thereby controlling its water absorption rate and hydration capacity. Meanwhile, the selection of the initial concentration is a crucial factor in ensuring the formation of a uniform hydrogel; too low a concentration will not be sufficient to form an effective gel network, while too high a concentration may lead to material inhomogeneity, affecting optical properties. Therefore, a suitable concentration range needs to be found to balance the material's strength and humidity sensitivity.

[0055] On the other hand, the amount of crosslinking agent also significantly affects the performance of the hydrogel. An appropriate amount of crosslinking agent can enhance the mechanical strength and stability of PVA hydrogels, preventing deformation during repeated water absorption and dehydration, thus ensuring the repeatability and stability of its humidity response. Furthermore, adjusting the water content is also crucial for achieving linear and repeatable refractive index changes. While excessively high water content can improve the material's moisture response speed, it may also lead to a decrease in the gel's mechanical properties. Therefore, systematic optimization should be performed to obtain a humidity-sensitive medium material that combines both strength and sensitivity.

[0056] In applications, the prepared PVA hydrogel humidity-sensitive medium is precisely injected into the cavity of a hollow photonic crystal fiber, forming a humidity-sensitive structure inside the fiber, such as... Figure 2As shown, due to the high hydrophilicity of this hydrogel, water vapor molecules can rapidly penetrate into the optical fiber cavity and interact with the humidity-sensitive medium. With changes in ambient humidity, the refractive index of the hydrogel changes accordingly during moisture absorption or loss, directly affecting the intrinsic frequency of the optical fiber. By monitoring this frequency change in real time, accurate inversion of soil moisture can be achieved. Furthermore, the design of the humidity-sensitive material must fully consider its compatibility with the optical fiber material to ensure that, during long-term use, there will be no material aging or optical loss due to interaction, thus affecting the overall performance and stability of the system.

[0057] In the Sagnac-type soil moisture monitoring system of this invention, stepped square wave modulation technology is used to modulate the optical signal to enhance the accuracy and stability of moisture detection, such as... Figure 3 As shown. Specifically, the optical signal in the fiber optic loop is divided into two optical paths, one propagating clockwise and the other counterclockwise, and the phases of these two paths are controlled by modulators.

[0058] When clockwise light reaches the modulator, its modulation phase is α. Counterclockwise light experiences a delay while passing through the fiber optic loop, reaching the modulator with a phase of β. According to the principle of Sagnac interference, the phase difference between clockwise and counterclockwise light is:

[0059] (2)

[0060] When the modulation frequency is matched to the transit time required for light to propagate in the fiber optic loop, the phase difference between the two beams will remain constant, thus keeping the intensity of the interfering light at a constant value. In this case, the interference intensity can be expressed 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 optic loop, the optical path difference will be zero. For example, if the modulation speed is slower than the transit time, the modulation phase of both the forward and reverse beams will be α. In this case, the phase difference is 0, and the interference intensity reaches its maximum value, such as... Figure 4 As shown. When the modulation frequency is f, the transit time τ of light in the optical fiber can be calculated using the fiber length L and the effective refractive index n, as shown in the following formula:

[0064] (4)

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

[0066] (5)

[0067] When humidity changes cause a shift in the refractive index (n) of the optical fiber, the transit time of light within the loop also changes due to the corresponding change in optical path length, thus affecting the required modulation frequency matching. This necessitates real-time monitoring of the modulation frequency adjustment to achieve accurate monitoring of humidity changes.

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

[0069] (6)

[0070] By monitoring and adjusting the modulation frequency in real time, changes in soil humidity can be accurately reflected, achieving highly sensitive humidity monitoring. This process not only improves the accuracy of monitoring but also enhances the system's responsiveness to dynamic humidity changes, providing crucial support for practical applications.

[0071] In the Sagnac-type soil moisture monitoring system of this invention, signal processing and moisture inversion are the core processes for achieving high-precision moisture monitoring. This process begins with the detector acquiring the interference signal. The detector converts the received optical signal into an electrical signal, forming a time-domain interference signal waveform. The intensity of this electrical signal is closely related to the optical path difference and modulation frequency; therefore, a high-speed analog-to-digital converter (ADC) is needed to convert the analog signal into a digital signal for subsequent digital signal processing.

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

[0073] Track and adjust the modulation frequency to make the interference pattern as follows Figure 4 As shown in (a), the adjustment amount Δf is the frequency shift. Substituting equation (1) into equation (6) yields:

[0074] (7)

[0075] The modulation frequency is adjusted proportionally to the relative humidity of the soil. In a laboratory environment, different modulation frequencies are measured under varying humidity levels to calibrate the coefficients, enabling real-time measurement of soil moisture in practical applications.

[0076] Cascade multiple optical paths, such as Figure 5As shown, simultaneous measurements at multiple points can be achieved by burying the sample in different locations in the soil. Optical switches can also be used to switch between multiple optical paths to simplify the optical path, such as... Figure 6 As shown.

[0077] This invention provides a Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber (HC-PCF), which boasts high sensitivity and accuracy, and can adapt to complex soil environments. By injecting polyvinyl alcohol (hydrogel) into the hollow fiber as a moisture-sensitive material and combining it with the Sagnac interferometry principle for real-time monitoring, the system can accurately capture minute changes in soil moisture. Furthermore, the use of stepped square wave modulation technology enhances the stability and response speed of the interference signal, ensuring accurate detection under dynamic humidity conditions. This method effectively optimizes the moisture introduction mechanism, prevents soil pollution, and accurately reflects humidity changes by adjusting the modulation frequency in real time, thus demonstrating broad application prospects in agricultural monitoring and environmental protection, meeting the needs of modern precision 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), specifically including equipment construction, parameter settings, and analysis of actual measurement results.

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

[0080] This invention uses polyvinyl alcohol (PVA) hydrogel as the humidity-sensitive material, but other humidity-sensitive materials can also be used. The following are 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 the PVA hydrogel changes from 1.4900 to 1.5020.

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

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

[0085] (2) Hydroxypropyl methylcellulose (HPMC)

[0086] Refractive index change: As relative humidity changes from 40% to 90%, the 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 a relatively slow response to humidity, but it has strong mechanical strength and chemical stability, making it 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 highly sensitive to changes in humidity, making it particularly suitable for fine monitoring in high humidity environments.

[0093] To construct the Sagnac interferometer ring, the system employed a broadband light source (wavelength range of 400-800 nm). A second fiber optic coupler split the light source signal into two beams. One beam underwent polarization filtering via a fiber polarizer before entering the first coupler, while the other beam entered the fiber ring directly. It is important to note that a micrometer-level gap was incorporated at the coupling point of the first coupler to ensure effective penetration of water vapor molecules into the hollow fiber. Furthermore, a protective film was used on the fiber and its coupling points to prevent interference from soil particles.

[0094] After coupling, the optical signal propagates clockwise and counterclockwise within the fiber optic loop, returning to the coupler after traveling a certain path to form interference. At this point, the optical signal passes through a fiber polarizer to ensure effective interference of light with the same polarization state, and finally, the interference signal is sent to a detector for acquisition. In practice, a lithium niobate phase modulator is used for stepped square wave modulation, and the modulation frequency is adjusted to match the intrinsic frequency of the optical fiber.

[0095] In practical applications, lithium niobate phase modulators (LiNbO3) are used to adjust the frequency of fiber optic interference signals to track changes in soil moisture in real time. The specific calculation steps employed in this method are as follows:

[0096] Step 1: Initialize and set the frequency range of the modulator, which is usually determined based on 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 that the operating frequency range of the hollow fiber is covered.

[0097] Step 2: Linear frequency modulation. The modulator begins to gradually change its frequency in a predetermined linear frequency sweep pattern. During this process, the interference signal will change until the modulation frequency matches the eigenfrequency 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 it matches the intrinsic frequency of the hollow optical fiber. At this time, by measuring the frequency adjustment amount Δf, the refractive index change caused by the humidity change can be determined.

[0099] Step 4: Finally determine the value of Δf. When the interference waveform reaches the DC level, record the modulation frequency value at this time. The frequency difference Δf at this time is the intrinsic frequency offset of the optical fiber, which represents the degree of change in soil moisture.

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

[0101] Calibrating the refractive index n and humidity response coefficient k of the moisture-sensitive material is a crucial step in ensuring the accurate measurement of soil moisture by this monitoring system. The specific steps are as follows:

[0102] Calibrate humidity response coefficient k

[0103] Through laboratory testing, different humidity environments (e.g., RH = 40%, 60%, 80%) were selected.

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

[0105] Calibrated refractive index n

[0106] The refractive index of the humidity-sensitive material is measured using a refractometer under different humidity conditions. Based on the changes in humidity in the soil environment, this step allows us to obtain a precise relationship between the refractive index of the humidity-sensitive material and humidity.

[0107] Real-time humidity monitoring and calibration calculates soil moisture changes by adjusting the modulation frequency based on the calibrated humidity response coefficient k and refractive index change, combined with real-time interference signals.

[0108] The results show that the monitoring system's response time under different humidity conditions does not exceed 2 seconds, and linear changes in humidity are reflected in the detector signal in real time. Experimental results indicate that in low humidity environments (e.g., RH 40%), the interference intensity of the fiber optic ring exhibits a stable DC level, while in high humidity environments (e.g., RH 80%), the interference signal waveform shows a significant step change. By simultaneously monitoring multiple measurement points, the repeatability and stability of the experimental results are significantly ensured, verifying the system's practicality and sensitivity in complex soil environments.

[0109] This embodiment calculates soil moisture data based on changes in modulation frequency:

[0110] The modulation frequency changes by Δf = 0.5 kHz.

[0111] Fiber length L=1m

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

[0113] (Optical fiber) Initial refractive index n = 1.4900

[0114] Calculate the refractive index change Δn:

[0115] The frequency change Δf is proportional to the humidity change ΔRH (the relationship was obtained through experimental calibration). According to equation (6), Δn is approximately 0.000075.

[0116] Based on the humidity response coefficient k, the humidity change ΔRH is derived to be 0.25%.

[0117] In addition, by burying multiple monitoring units in different soil samples, such as Figure 5 or Figure 6 As shown, it can achieve multi-point synchronous measurement. This provides great convenience and scalability for accurate monitoring of soil erosion in environmental assessment and safety risk control of power transmission and transformation projects, meeting diverse monitoring needs. The system's performance has demonstrated good durability, flexibility, and reliability in various practical applications.

[0118] In summary, the Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber has been verified through practical examples to demonstrate its effectiveness in high-sensitivity and high-precision moisture detection, proving the important application value and practical potential of this technology in environmental assessment and safety risk control of power transmission and transformation projects.

[0119] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. These changes involve related technologies well known to those skilled in the art, and all of them fall within the protection scope of the present invention.

[0120] Many other changes and modifications can be made without departing from the concept and scope of this invention. It should be understood that this invention is not limited to the specific embodiments, and the scope of this invention is defined by 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 a humidity-sensitive material into the hollow channel of a hollow photonic crystal fiber. The humidity-sensitive material is any one of polyvinyl alcohol (PVA) hydrogel, hydroxypropyl methylcellulose (HPMC), or 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; 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. Calculate the soil moisture using Δf. The calculation of Δf involves the optical signal in the fiber optic loop being divided into clockwise and counterclockwise optical paths, the phases of which are controlled by phase modulators. 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）。 2. The Sagnac type soil moisture monitoring method based on hollow photonic crystal fiber according to claim 1, 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.

3. The Sagnac type soil moisture monitoring method based on hollow 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.

4. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 1, 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.

5. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 1, 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.

6. 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.

7. The Sagnac-type soil moisture monitoring method based on hollow-core photonic crystal fiber according to claim 6, 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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