Water level measuring device based on distributed temperature sensor and high-precision measuring system

Abstract

The invention provides a water level measuring device based on a distributed temperature sensor and a high-precision measuring system.The water level measuring device comprises an optical fiber rod, a base, a polygonal rod and a shell, the polygonal rod and the shell are fixed to the base, and a vertical polygonal through hole matched with the polygonal rod in shape is formed in the optical fiber rod; the polygonal rod is inserted into the optical fiber rod to enable the optical fiber rod to be fixed on the base, the bottom of the shell is of a mesh structure, the optical fiber rod is located in the shell, sensing optical fibers of the distributed temperature sensors are wound on the optical fiber rod, and according to the spatial resolution requirement of the water level of accumulated water and the spatial resolution of the sensing optical fibers corresponding to the distributed temperature sensors, the sensing optical fibers are distributed. The winding length of the sensing optical fiber in the corresponding height in the vertical direction of the optical fiber rod is adjusted, and the temperature measurement precision of the distributed temperature sensor is smaller than or equal to the minimum difference value of the air temperature and the accumulated water temperature. The device is passive equipment, is low in energy consumption, has universality and is high in measurement accuracy.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing, specifically relating to a water level measuring device and a high-precision measuring system based on a distributed temperature sensor. Background Technology

[0002] Traditional water accumulation monitoring methods largely rely on active devices such as electrode sensors, ultrasonic sensors, and image recognition. These devices are susceptible to environmental interference and have high maintenance costs. With technological advancements, a distributed temperature sensing (DTS) method for tunnel water accumulation has been proposed. By encapsulating the sensing fiber in a two-dimensional planar spiral configuration, the spatial resolution of the sensor is improved. Active heating significantly enhances the temperature difference at the water-air interface, enabling the identification of water accumulation events and their extent. However, this method requires active heating, which is more energy-intensive and easily affected by external environmental conditions, leading to increased installation, usage, and maintenance workload. Furthermore, while heating can determine the water-air interface when measuring the water level in a fixed container and heating within that container, it becomes impossible to determine the heating duration and required temperature difference to accurately reflect the water-air interface if the water extent and level are unknown. Therefore, this heating method lacks universality and has low measurement accuracy. Summary of the Invention

[0003] This invention provides a water level measuring device and a high-precision measuring system based on a distributed temperature sensor, to solve the problems that current water level measuring devices based on distributed temperature sensors rely on active equipment, have high energy consumption, lack universality, and have low measurement accuracy.

[0004] According to a first aspect of the present invention, a water level measuring device based on a distributed temperature sensor is provided, comprising an optical fiber rod, a base, a polygonal rod fixed on the base, and a housing. The optical fiber rod has a vertical polygonal through hole matching the shape of the polygonal rod. After the polygonal rod is inserted into the optical fiber rod, the optical fiber rod is fixed on the base. The bottom of the housing has a mesh structure. The optical fiber rod is located inside the housing and a sensing optical fiber of the distributed temperature sensor is wound around it. According to the spatial resolution requirements of the accumulated water level and the spatial resolution of the corresponding sensing optical fiber of the distributed temperature sensor, the winding length of the sensing optical fiber within the corresponding height in the vertical direction of the optical fiber rod is adjusted. The temperature measurement accuracy of the distributed temperature sensor is less than or equal to the minimum difference between the air temperature and the accumulated water temperature.

[0005] During measurement, water enters the housing through the mesh structure at the bottom of the housing, ensuring that the water level inside and outside the housing remains consistent. The sensing fiber inside the housing measures the temperature. Based on the temperature difference on the sensing fiber, the length of the sensing fiber located in the water is determined. Based on the length of the sensing fiber located in the water and the winding length of the sensing fiber within the corresponding height in the vertical direction of the fiber rod, the water level is determined, which is used for temperature measurement over a longer distance.

[0006] Optionally, when the spatial resolution requirement of the accumulated water level is k1, it means that the minimum value required for the water level change to be distinguished by temperature difference in the vertical direction of the optical fiber rod is k1. When the spatial resolution of the sensing fiber corresponding to the distributed temperature sensor is k2, it means that the length of the sensing fiber in the temperature change region must be at least k2 to distinguish the temperature change. In this case, the length L of the sensing fiber that needs to be wound within each k1 height in the vertical direction of the optical fiber rod is greater than or equal to k0 * k2, where k1 and k2 are any values ​​greater than 0, and k0 is any value greater than 1 set by the user to ensure measurement accuracy.

[0007] Optionally, the device can be applied to various underground facilities in cities, including tunnels, underpasses, underground parking garages, urban drainage networks and subway tracks. The lower temperature section of the sensing fiber below the boundary line is used as the sensing fiber segment that measures the temperature of the water. There is a significant temperature difference between the submerged and unsubmerged sections.

[0008] According to a second aspect of the present invention, a high-precision measurement system including the above-described water level measuring device is provided, further comprising a laser generator, a wavelength division multiplexer, a data acquisition card, and a processor. The output end of the laser generator is connected to the sensing optical fiber in the water level measuring device through the wavelength division multiplexer, and the wavelength division multiplexer is also connected to the processor through the data acquisition card.

[0009] The laser generator provides the laser signal to the sensing fiber via the wavelength division multiplexer (WDM). Upon receiving the laser signal, the sensing fiber transmits the scattered signal back to the WDM via the WDM. The WDM provides the Stokes and anti-Stokes signals from the scattered signal to the data acquisition card. The data acquisition card acquires the Stokes and anti-Stokes signals and provides them to the processor. The processor performs cumulative averaging on the acquired Stokes and anti-Stokes signals and caches them in binary format. It compensates for the time delay errors of the digital Stokes and anti-Stokes signals, ensuring that they correspond to each other at the same vertical position on the fiber rod in the water level measuring device. The compensated Stokes and anti-Stokes signals are then denoised to obtain a ratio sequence of the denoised Stokes and anti-Stokes signals. The observed temperature sequence is calculated based on this ratio sequence. Wiener deconvolution is used to process the observed temperature sequence to obtain the true temperature sequence. Based on the positional relationship between the true temperature sequence and the axial direction of the fiber rod, the water-air boundary is obtained, thus determining the accumulated water level.

[0010] Optionally, it also includes a pulse modulator and an erbium-doped fiber amplifier. The output of the laser generator is connected to the sensing fiber in sequence through the pulse modulator, the erbium-doped fiber amplifier and the wavelength division multiplexer. The pulse modulator modulates the laser signal into a pulsed laser. The erbium-doped fiber amplifier provides an average power margin while ensuring eye safety and nonlinear threshold. The wavelength division multiplexer uses a low-noise photodiode and a transimpedance amplifier to form a linear detection channel for outputting Stokes and anti-Stokes signals, avoiding excessive noise in the APD Geiger mode and balancing the electrical bandwidth of the two channels to reduce differential group delay.

[0011] The data acquisition card operates at a 1GHz sampling rate and external trigger mode, with the recording length matched to the round-trip time, and is set with appropriate pre-trigger to capture the baseline.

[0012] Optionally, the compensation for the time delay error of the Stokes and anti-Stokes signals in the digital domain includes: performing baseline alignment correction on the Stokes and anti-Stokes signals in the digital domain; and performing alignment compensation on the time delay error of the two signals based on the cross-correlation peak values ​​of the Stokes and anti-Stokes signals, so that the two signals correspond to each other at the same position in the vertical direction of the fiber optic rod in the water level measuring device.

[0013] Optionally, the denoising of the compensated Stokes and anti-Stokes signals includes: performing zero-point and drift correction on the compensated Stokes and anti-Stokes signals respectively, using the static unheated section as a reference, and eliminating the slowly varying level bias; and using a multi-scale chain strategy of "moving average + discrete wavelet decomposition" to suppress random noise.

[0014] Optionally, the moving average reduces high-frequency white noise without sacrificing transition edges; the discrete wavelet decomposition selects an approximately orthogonal wavelet basis, based on noise estimation. A soft threshold λ is set, and scale-adaptive shrinkage is implemented on the detail coefficients to preserve interface mutations.

[0015] Optionally, the denoising of the compensated Stokes and anti-Stokes signals further includes: denoising the slow baseline drift caused by differential attenuation and dispersion using a polynomial. The polynomial is modeled and removed from the logarithmic or ratio domain of the compensated Stokes and anti-Stokes signals. If there is a small group delay difference in the spectral paths of the two channels, a fractional delay filter is used in the frequency domain for subsampling point-level phase compensation to keep the ratio flat throughout the entire range.

[0016] Optionally, processing the observed temperature sequence using Wiener deconvolution includes applying the following formula to the observed temperature sequence. Processing:

[0017]

[0018] in This represents the spectral information after the Fourier transform of the actual temperature sequence. The Fourier transform of the system's point spread function (PSF) is given. for The conjugate complex number of , k is the noise ratio coefficient of the Wiener filter, which reflects the ratio of the signal power spectrum to the noise power spectrum.

[0019] The beneficial effects of this invention are:

[0020] 1. This invention includes a housing surrounding the fiber optic rod wound with sensing fibers. The bottom of the housing is designed with a mesh structure to filter out foreign objects in the accumulated water. This not only protects the sensing fibers from external foreign objects entering the housing and directly affecting the fibers, thus preventing interference with temperature measurement, but also ensures consistent water levels inside and outside the housing, preventing significant fluctuations in the water level within the housing. This further guarantees the accuracy of temperature measurement by the sensing fibers. Furthermore, this invention includes a polygonal rod inside the housing. After inserting the polygonal rod, which is fixed to the base, into the fiber optic rod, the fiber optic rod is secured. On the base, this design facilitates the removal and installation of the fiber optic rod, making routine maintenance of the sensing fiber optic rod easier. Furthermore, the polygonal shape prevents the fiber optic rod from rotating under water accumulation. Since the fiber optic rod and base are not tightly connected, water can enter the space between the fiber optic rod and the polygonal shape through the gap between them. This water can then transfer its temperature to the sensing fiber optic rod, allowing the sensing fiber optic rod to perceive the water temperature from all angles. This increases the difference between the measured water temperature and the air temperature, further ensuring the accuracy of the sensing fiber optic temperature measurement.

[0021] Based on the above structure, this invention proposes a new measurement method. It selects a distributed temperature sensor whose temperature measurement accuracy is less than or equal to the minimum difference between the air temperature and the water temperature. According to the spatial resolution requirements of the water level and the spatial resolution of the corresponding sensing fiber of the distributed temperature sensor, the winding length of the sensing fiber in the vertical direction of the fiber rod is adjusted. Since the above structure can ensure that the water and air temperatures have significant differences, the measurement method of this invention can achieve accurate measurement of the water level even if the water range and water level are unknown. The winding of the sensing fiber on the fiber rod makes up for the insufficient resolution of the sensing fiber when measuring the water level. Furthermore, the long temperature measurement distance and passive characteristics allow the device to be placed in remote and complex environments, greatly expanding the monitoring range and making the measurement device universal.

[0022] 2. This invention designs the fiber optic rod and housing as cylindrical, allowing for rapid equilibrium even in the event of small fluctuations within the housing. The bottom of the fiber optic rod is designed with a mesh structure, and transverse guide holes are distributed along the vertical direction of the fiber optic rod. The space between the water-inflowing fiber optic rod and the polygonal rod is also polygonal, stabilizing fluctuations and further reducing them, thus ensuring the accuracy of temperature measurement in the sensing fiber. Furthermore, the transverse guide holes along the vertical direction of the fiber optic rod allow the water to further influence the temperature measured by the sensing fiber, increasing the temperature difference between the measured water and air, thereby further ensuring the accuracy of temperature measurement in the sensing fiber.

[0023] 3. In calculating the observed temperature sequence, this invention uses Stokes and anti-Stokes signals, and performs time delay error compensation and noise reduction on both. The ratio of the obtained Stokes and anti-Stokes signals can accurately represent the observed temperature sequence, improving the measurement accuracy of the observed temperature sequence. Based on this, the observed temperature sequence is processed to obtain the true temperature sequence, and then the water-air boundary is obtained based on the true temperature sequence. Because the measurement accuracy of the observed temperature sequence is high, the accuracy of determining the water-air boundary is improved, and the accuracy of water level measurement is also improved. This invention first uses Wiener deconvolution to process the observed temperature sequence to obtain the true temperature sequence, and then obtains the water-air boundary based on the true temperature sequence. This can improve the spatial resolution of determining the water-air boundary, thereby improving the spatial resolution of water level measurement. The temperature measurement accuracy of this invention can reach within 1℃, and the spatial resolution of water level measurement can reach the cm level. Attached Figure Description

[0024] Figure 1 This is an exploded view of components of an embodiment of the water level measurement device based on a distributed temperature sensor of the present invention;

[0025] Figure 2 This is a schematic diagram of the assembly structure of an embodiment of the water level measuring device based on a distributed temperature sensor of the present invention;

[0026] Figure 3 This is a schematic diagram of an optical fiber rod with sensing optical fibers wound around it.

[0027] Figure 4 This is a schematic diagram of the measurement results;

[0028] Figure 5 This is a schematic diagram of an embodiment of the high-precision water level measurement system of the present invention. Detailed Implementation

[0029] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention, and to make the above-mentioned objectives, features and advantages of the embodiments of the present invention more apparent and understandable, the technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0030] In the description of this invention, unless otherwise specified and limited, it should be noted that the term "connection" should be interpreted broadly. For example, it can be a mechanical connection or an electrical connection, or it can be a connection between two internal components. It can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above term according to the specific circumstances.

[0031] See Figure 1This is an exploded view of components from an embodiment of the water level measurement device based on a distributed temperature sensor according to the present invention. (Combined with...) Figure 2 and Figure 3 As shown, the water level measuring device may include an optical fiber rod 1, a base 2, a polygonal rod 3 fixed on the base 2, and a housing 4. The optical fiber rod 1 has a vertical polygonal through hole that matches the shape of the polygonal rod 3. After the polygonal rod 3 is inserted into the optical fiber rod 1, the optical fiber rod 1 is fixed on the base 2. The bottom of the housing 4 has a mesh structure 41. The optical fiber rod 1 is located inside the housing 4 and a sensing optical fiber 5 of a distributed temperature sensor is wound around it. According to the spatial resolution requirements of the accumulated water level and the spatial resolution of the sensing optical fiber 5 corresponding to the distributed temperature sensor, the winding length of the sensing optical fiber 5 within the corresponding height in the vertical direction of the optical fiber rod 1 is adjusted. The temperature measurement accuracy of the distributed temperature sensor is less than or equal to the minimum difference between the air temperature and the accumulated water temperature.

[0032] In this embodiment, the polygonal rod 3 can be a square column, and the polygonal through hole provided in the corresponding optical fiber rod 1 can be a square through hole. The sensing optical fiber can be wound around the optical fiber rod in a spiral manner. The length of the sensing optical fiber can reach tens of kilometers, and one device can be connected to multiple temperature measuring optical fibers. Therefore, this monitoring device can be installed in various remote tunnels or urban drainage networks and other environments where existing equipment is difficult to install for water level monitoring, and a modulation and demodulation device can be used to realize real-time measurement of water levels in multiple scenarios. When the spatial resolution requirement of the accumulated water level is k1, it means that the minimum value of the water level change that the optical fiber rod needs to distinguish through temperature difference in the vertical direction is k1. When the spatial resolution of the sensing optical fiber corresponding to the distributed temperature sensor is k2, it means that the length of the sensing optical fiber in the temperature change area must be at least k2 to distinguish the temperature change. At this time, the length L of the sensing optical fiber that needs to be wound within each k1 height in the vertical direction of the optical fiber rod is greater than or equal to k0 * k2, where k1 and k2 are any values ​​greater than 0, and k0 is any value greater than 1 set by human intervention to ensure measurement accuracy, which can generally be 1.5 or 2.

[0033] In one example, urban flooding typically occurs during the hot and rainy summer months. The temperature of the floodwater is lower than the air temperature, with a temperature difference usually greater than 1°C. In this case, the temperature measurement accuracy of the distributed temperature sensor can be less than or equal to 1°C. When the spatial resolution of the sensing fiber corresponding to the distributed temperature sensor is 2 m, meaning the length of the sensing fiber in the temperature change zone is at least 2 m to distinguish temperature changes, if the spatial resolution of the floodwater level is required to be at the cm level, then the length of the sensing fiber that needs to be wound within each 1 cm height of the fiber rod is L ⩾ k0 * 2 m. If higher accuracy is desired, k0 can be a value greater than 1. When k0 = 1.5, the length is L ⩾ 1.5 * 2 m.

[0034] When the fiber optic rod is cylindrical, the length L of the sensing fiber that needs to be wound within each k1 height in the vertical direction of the fiber optic rod is L = n * 2πr, where n is the number of windings, r is the radius of the fiber optic rod, n is an integer greater than 0, and r is any value greater than 0. According to the formula n * r ⩾ k0 * k2 / 2π, the number of windings n of the sensing fiber and the radius r of the fiber optic rod within each k1 height in the vertical direction of the fiber optic rod can be designed to meet the required spatial resolution of the water level.

[0035] During measurement, water enters the housing through the mesh structure at the bottom, maintaining a consistent water level inside and outside. The sensing fiber inside the housing measures the temperature. Based on the temperature difference along the sensing fiber, the length of the fiber submerged in the water is determined. The water level is then determined based on the length of the fiber submerged and the length of its winding along the vertical direction of the fiber rod. After measuring the corresponding temperature, the temperatures of each fiber segment are compared to determine the temperature difference boundary. The segment with the lower temperature below this boundary is considered the fiber submerged in the water. In the experiment, the immersion depth was 10cm, and the measured length of the submerged fiber was 30 meters. The calculated immersion depth was 30 ÷ 3.14 = 9.55cm. The error is within 1cm, meeting the requirements. Furthermore, as... Figure 4 As shown, there is a significant temperature difference between the submerged and unsubmerged parts.

[0036] As can be seen from the above embodiments, the present invention provides a housing outside the optical fiber rod wound with sensing optical fiber, and designs the bottom of the housing as a mesh structure to filter foreign objects in the accumulated water. This not only protects the sensing optical fiber and prevents external foreign objects from entering the housing and directly affecting the sensing optical fiber and its temperature measurement, but also ensures that the water level inside and outside the housing is consistent and that the water level inside the housing does not fluctuate significantly, thereby further ensuring the accuracy of temperature measurement of the sensing optical fiber. The invention also includes a polygonal rod inside the housing. After the polygonal rod, which is fixed on the base, is inserted into the optical fiber rod, the optical fiber rod is fixed on the base. This facilitates the installation and removal of the optical fiber rod, making it easier to perform routine maintenance on the sensing optical fiber. Furthermore, the polygonal rod prevents the optical fiber rod from rotating under the influence of water accumulation. Since the optical fiber rod and the base are not tightly connected, water can enter the space between the optical fiber rod and the polygonal rod through the gap between the optical fiber rod and the base. The water in this space can also transfer its temperature to the sensing optical fiber through the optical fiber rod, allowing the sensing optical fiber to sense the water temperature from all angles. This increases the difference between the measured water temperature and the air temperature, thereby further ensuring the accuracy of the temperature measurement by the sensing optical fiber.

[0037] Furthermore, while the sensing fiber in this invention measures the temperature of the water inside the casing, the water inside and outside the casing are connected, and the extent and level of the water are unknown. Therefore, it is still impossible to measure the water level by increasing the temperature difference at the water-air interface through heating. To address this, this invention proposes a new measurement method based on the aforementioned structure. It employs a distributed temperature sensor whose temperature measurement accuracy is less than or equal to the minimum difference between the air temperature and the water temperature. Based on the spatial resolution requirements of the water level and the spatial resolution of the corresponding sensing fiber in the distributed temperature sensor, the winding length of the sensing fiber within the corresponding height in the vertical direction of the fiber rod is adjusted. Since the above structure ensures a significant temperature difference between the water and air, this method can accurately measure the water level even when the extent and level are unknown. Winding the sensing fiber onto the fiber rod compensates for the insufficient resolution of the sensing fiber when measuring water level. The longer measurement distance and passive nature of the device allow it to be installed in remote and complex environments, greatly expanding the monitoring range and making the measurement device universally applicable.

[0038] The measuring device of this invention is a passive device. The system mainly uses optical fiber for measurement. The passive nature of optical fiber means that it does not require an external power supply, reducing the system's energy consumption and maintenance costs. This invention has strong anti-interference capabilities. The main component of optical fiber is silicon dioxide, which has excellent stability and is resistant to electromagnetic interference. This allows it to be widely used in various underground facilities in cities, such as tunnels, underpasses, underground parking garages, urban drainage networks, and subway tracks, greatly expanding the field and scope of urban waterlogging detection. Especially in remote tunnels and urban drainage networks, it can provide real-time monitoring capabilities that are difficult to achieve with other equipment.

[0039] In addition, in the above embodiment, the top end of the polygonal rod 3 may be provided with a threaded post 6. After the threaded post 6 passes through the polygonal through hole in the optical fiber rod 1, the optical fiber rod 1 is fixed to the base 2 by a nut 7. The optical fiber rod 1 and the housing 4 may both be cylindrical. The upper end of the housing 4 is provided with a protective cover 8 and a through hole 12 for the sensing optical fiber to pass through. The protective cover 8 can be fixed to the housing 4 by a protective cover bolt 9 and a protective cover nut 10. The base 2, the polygonal rod 3 and the housing 4 may be integrally formed. The base 2 may be provided with a screw hole 11. The base 2 is fixed by passing a screw through the screw hole 11. In another embodiment, the bottom of the optical fiber rod 1 may also be a mesh structure, and the optical fiber rod 1 may have transverse guide holes distributed in the vertical direction. This invention designs the fiber optic rod and housing as cylindrical shapes, allowing for rapid equilibrium even in the event of small fluctuations within the housing. The bottom of the fiber optic rod is designed with a mesh structure, and transverse guide holes are distributed along the vertical direction of the fiber optic rod. The space between the flowing water and the polygonal rod is also polygonal, stabilizing fluctuations and further reducing their intensity, thus ensuring the accuracy of temperature measurement in the sensing fiber. Furthermore, the transverse guide holes along the vertical direction of the fiber optic rod allow the water to influence the temperature measured by the sensing fiber from another angle, increasing the temperature difference between the measured water and air, thereby further ensuring the accuracy of temperature measurement in the sensing fiber.

[0040] from Figure 4 It can be seen that there is a significant temperature difference between the submerged and unsubmerged parts. However, when the above-mentioned water level measuring device is applied to long-distance temperature measurement to achieve long-distance water level measurement, the temperature difference becomes less obvious due to factors such as noise transmitted along the way and time delay differences, and the measurement accuracy and spatial resolution also decrease. Therefore, this invention proposes... Figure 5The high-precision water level measurement system shown includes a laser generator (DFB), a wavelength division multiplexer (WDM), a data acquisition card (DAQ), and a processor (PC). The output of the DFB is connected to the sensing fiber in the water level measurement device via the WDM, and the WDM is also connected to the PC via the DAQ. The laser generator provides a laser signal to the sensing fiber via the WDM. After receiving the laser signal, the sensing fiber transmits the scattered signal back to the WDM via the WDM. The WDM then provides the Stokes and anti-Stokes signals from the scattered signal to the data acquisition card, which performs processing on the Stokes and anti-Stokes signals. The data is collected and provided to the processor; the processor performs cumulative averaging on the collected Stokes and anti-Stokes signals and caches them in binary format; it compensates for the time delay error of the Stokes and anti-Stokes signals in the digital domain, so that the two signals correspond to each other at the same position in the vertical direction of the fiber optic rod in the water level measuring device; it denoises the compensated Stokes and anti-Stokes signals respectively to obtain the ratio sequence of the denoised Stokes and anti-Stokes signals; it calculates the observed temperature sequence based on the ratio sequence; it processes the observed temperature sequence using Wiener deconvolution to obtain the true temperature sequence; and it obtains the water-air boundary line based on the positional relationship between the true temperature sequence and the axial direction of the fiber optic rod, thereby obtaining the water level.

[0041] In this embodiment, the wavelength division multiplexer can use a low-noise photodiode and a transimpedance amplifier to form a linear detection channel for outputting Stokes and anti-Stokes signals, avoiding excessive noise in the APD Geiger mode and balancing the electrical bandwidth of the two channels to reduce differential group delay. The compensation for delay errors of the digital domain Stokes and anti-Stokes signals includes: baseline alignment correction of the digital domain Stokes and anti-Stokes signals; alignment compensation of the delay errors of the two signals based on the cross-correlation peak values ​​of the Stokes and anti-Stokes signals, ensuring that they correspond to each other at the same position in the vertical direction of the fiber optic rod in the water level measuring device. The denoising of the compensated Stokes and anti-Stokes signals can include: zero-point and drift correction of the compensated Stokes and anti-Stokes signals respectively, using the static unheated section as a reference to eliminate slowly varying level bias; and using a multi-scale chain strategy of "moving average + discrete wavelet decomposition" to suppress random noise. The moving average weakens high-frequency white noise without sacrificing transition edges; the discrete wavelet decomposition can select approximately orthogonal wavelet bases, based on noise estimation... A soft threshold λ is set, and scale-adaptive shrinkage is implemented on the detail coefficients to preserve interface mutations.

[0042] The denoising of the compensated Stokes and anti-Stokes signals may further include: denoising the slow baseline drift caused by differential attenuation and dispersion using a polynomial... The model is created and the polynomial is removed from the logarithmic or ratio domain of the compensated Stokes and anti-Stokes signals. If there is a small group delay difference in the spectral paths of the two channels, a fractional delay filter is used in the frequency domain for subsampling-point-level phase compensation to keep the ratio flat throughout the measurement range. In this invention, both the observed temperature sequence and the actual temperature sequence correspond to the position of the fiber rod along its axial direction (the fiber rod's axis can be perpendicular to the water surface). When calculating the observed temperature sequence, this invention is based on the Stokes and anti-Stokes signals, and performs delay error compensation and noise reduction on both. The ratio of the obtained Stokes and anti-Stokes signals can accurately represent the observed temperature sequence, improving the measurement accuracy of the observed temperature sequence. The observed temperature sequence is calculated based on the ratio sequence of the Stokes and anti-Stokes signals using conventional techniques in the prior art, which will not be elaborated here.

[0043] In distributed fiber optic temperature sensing systems, the spatial response characteristics of the system are determined by the finite time width of the laser pulse and the finite bandwidth of the system components such as amplifiers and acquisition cards. Specifically, the system's response to the real temperature distribution can be described by the point spread function (PSF). The PSF actually reflects the spatial response of the system to an ideal unit pulse input. Physically, the shape of the PSF is influenced by the combined effects of the laser pulse envelope, the rise and fall edge characteristics, and the bandwidth of the detection system; in some cases, it can also be obtained experimentally through inversion by measuring the ideal temperature level response. Since the actually observed temperature distribution is a spatial convolution of the real temperature field and the PSF, the temperature transition region, which originally had sharp boundaries such as the water-air interface, will be widened, the slope will be softened, and it may cause a systematic drift of the interface position, thus affecting the accuracy of water level positioning. This convolution process can be described as follows:

[0044]

[0045] in, For the observed temperature information, Let h(x) represent the true temperature distribution, h(x) denote the point spread function, and n(x) represent the superimposed noise. To improve spatial resolution and eliminate boundary ambiguity caused by convolution, the Wiener deconvolution algorithm is adopted. This algorithm can effectively deconvolve the observed signal at a certain signal-to-noise ratio while fully considering system noise, effectively improving the spatial resolution of the system and thus restoring a temperature distribution closer to the truth, ensuring "sub-resolution interface localization capability at a nominal resolution of 2 m". The entire process can be described by the following formula:

[0046]

[0047] in and These are the spectral information after Fourier transform of the observed temperature sequence and the actual temperature sequence, respectively. The Fourier transform of the system's point spread function (PSF) is given. for The conjugate complex number of is given by , where k is the noise ratio coefficient of the Wiener filter, reflecting the ratio of the signal power spectrum to the noise power spectrum. When the noise is small, K→0, the Wiener deconvolution approaches the ideal inverse filter; when the noise is large, it can avoid the excessive amplification of high-frequency noise, achieving a trade-off between filtering and deconvolution.

[0048] Output of Wiener deconvolution By performing an inverse Fourier transform, the sharpened temperature distribution in the spatial domain can be obtained. Compared with the original observed signal, this distribution can better reproduce the actual temperature field, especially the temperature abrupt changes near the interface, effectively eliminating the boundary widening and positional shift caused by the system response, and improving the accuracy and quantitative resolution of interface positioning.

[0049] This high-precision water level measurement system may also include a pulse modulator (AOM) and an erbium-doped fiber amplifier (EDFA). The output of the laser generator (DFB) is connected to the sensing fiber via the AOM, EDFA, and WDM. The pulse modulator modulates the laser signal into a pulsed laser, and the EDFA provides an average power margin while ensuring eye safety and a nonlinear threshold. The WDM uses a low-noise photodiode and a transimpedance amplifier to form a linear detection channel for outputting Stokes and anti-Stokes signals, avoiding excessive noise in the APD Geiger mode, and the electrical bandwidth of the two channels is balanced to reduce differential group delay. The data acquisition card operates at a 1 GHz sampling rate and external trigger mode, with the recording length matched to the round-trip time, and an appropriate pre-trigger is set to capture the baseline. Considering the complexity of the urban underground environment and electromagnetic interference, the system clock and trigger are allocated from the same source, and the optical path and circuit modules are thermally designed and mechanically protected to reduce phase noise introduced by temperature drift and micro-vibration. The processor PC can send a modulation signal to the pulse modulator AOM, so that the pulse modulator AOM modulates the laser signal provided by the laser generator DFB according to the modulation signal.

[0050] As can be seen from the above embodiments, when calculating the observed temperature sequence, the present invention is based on Stokes and anti-Stokes signals, and performs time delay error compensation and noise reduction on both. The ratio of the obtained Stokes and anti-Stokes signals can accurately represent the observed temperature sequence, improving the measurement accuracy of the observed temperature sequence. On this basis, the observed temperature sequence is processed to obtain the true temperature sequence, and then the water-air boundary is obtained based on the true temperature sequence. Since the measurement accuracy of the observed temperature sequence is high, the determination accuracy of the water-air boundary is improved, and the measurement accuracy of the accumulated water level is also improved. The present invention first uses Wiener deconvolution to process the observed temperature sequence to obtain the true temperature sequence, and then obtains the water-air boundary based on the true temperature sequence. This can improve the spatial resolution of the determination of the water-air boundary, thereby improving the spatial resolution of the accumulated water level measurement. The temperature measurement accuracy of the present invention can reach within 1°C, and the spatial resolution of the water level height measurement can reach the cm level.

[0051] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0052] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is defined solely by the appended claims.

Claims

1. A water level measuring device based on a distributed temperature sensor, characterized in that, The device includes an optical fiber rod, a base, a polygonal rod fixed on the base, and a housing. The optical fiber rod has a vertical polygonal through-hole that matches the shape of the polygonal rod. After the polygonal rod is inserted into the optical fiber rod, the optical fiber rod is fixed on the base. The bottom of the housing has a mesh structure. The optical fiber rod is located inside the housing and a sensing optical fiber of a distributed temperature sensor is wound around it. According to the spatial resolution requirements of the water level and the spatial resolution of the sensing optical fiber corresponding to the distributed temperature sensor, the winding length of the sensing optical fiber within the corresponding height in the vertical direction of the optical fiber rod is adjusted. The temperature measurement accuracy of the distributed temperature sensor is less than or equal to the minimum difference between the air temperature and the water temperature. During measurement, water enters the housing through the mesh structure at the bottom of the housing, ensuring that the water level inside and outside the housing remains consistent. The sensing fiber inside the housing measures the temperature. Based on the temperature difference on the sensing fiber, the length of the sensing fiber located in the water is determined. Based on the length of the sensing fiber located in the water and the winding length of the sensing fiber within the corresponding height in the vertical direction of the fiber rod, the water level is determined, which is used for temperature measurement over a longer distance.

2. The water level measuring device based on a distributed temperature sensor according to claim 1, characterized in that, When the spatial resolution requirement of the accumulated water level is k1, it means that the minimum value required for the water level change to be distinguished by temperature difference in the vertical direction of the optical fiber rod is k1. When the spatial resolution of the sensing fiber corresponding to the distributed temperature sensor is k2, it means that the length of the sensing fiber in the temperature change region must be at least k2 to distinguish the temperature change. At this time, the length L of the sensing fiber that needs to be wound within each k1 height in the vertical direction of the optical fiber rod is greater than or equal to k0 * k2, where k1 and k2 are any values ​​greater than 0, and k0 is any value greater than 1 set by the user to ensure measurement accuracy.

3. The water level measuring device based on a distributed temperature sensor according to claim 1 or 2, characterized in that, This device is applied to various underground facilities in cities, including tunnels, underpasses, underground parking garages, urban drainage networks, and subway tracks. The lower temperature section of the sensing fiber below the dividing line is used to measure the temperature of the accumulated water. The sensing fiber segment located in the accumulated water shows a significant temperature difference between the submerged and unsubmerged parts.

4. A high-precision measurement system comprising the water level measuring device according to any one of claims 1 to 3, characterized in that, It also includes a laser generator, a wavelength division multiplexer, a data acquisition card, and a processor. The output of the laser generator is connected to the sensing fiber in the water level measuring device through the wavelength division multiplexer, and the wavelength division multiplexer is also connected to the processor through the data acquisition card. The laser generator provides the laser signal to the sensing fiber via the wavelength division multiplexer (WDM). Upon receiving the laser signal, the sensing fiber transmits the scattered signal back to the WDM via the WDM. The WDM provides the Stokes and anti-Stokes signals from the scattered signal to the data acquisition card. The data acquisition card acquires the Stokes and anti-Stokes signals and provides them to the processor. The processor performs cumulative averaging on the acquired Stokes and anti-Stokes signals and caches them in binary format. It compensates for the time delay errors of the digital Stokes and anti-Stokes signals, ensuring that they correspond to each other at the same vertical position on the fiber rod in the water level measuring device. The compensated Stokes and anti-Stokes signals are then denoised to obtain a ratio sequence of the denoised Stokes and anti-Stokes signals. The observed temperature sequence is calculated based on this ratio sequence. Wiener deconvolution is used to process the observed temperature sequence to obtain the true temperature sequence. Based on the positional relationship between the true temperature sequence and the axial direction of the fiber rod, the water-air boundary is obtained, thus determining the accumulated water level.

5. The high-precision measurement system according to claim 4, characterized in that, It also includes a pulse modulator and an erbium-doped fiber amplifier. The output of the laser generator is connected to the sensing fiber in sequence through the pulse modulator, the erbium-doped fiber amplifier, and the wavelength division multiplexer. The pulse modulator modulates the laser signal into a pulsed laser. The erbium-doped fiber amplifier provides an average power margin while ensuring eye safety and nonlinear threshold. The wavelength division multiplexer uses a low-noise photodiode and a transimpedance amplifier to form a linear detection channel for outputting Stokes and anti-Stokes signals, avoiding excessive noise in the APD Geiger mode and balancing the electrical bandwidth of the two channels to reduce differential group delay. The data acquisition card operates at a 1 GHz sampling rate and external trigger mode, with the recording length matched to the round-trip time, and an appropriate pre-trigger is set to capture the baseline.

6. The high-precision measurement system according to claim 4, characterized in that, The compensation for the time delay error of the Stokes and anti-Stokes signals in the digital domain includes: performing baseline alignment correction on the Stokes and anti-Stokes signals in the digital domain; and aligning the time delay error of the two signals according to the cross-correlation peak value of the Stokes and anti-Stokes signals, so that the two signals correspond to each other at the same position in the vertical direction of the fiber optic rod in the water level measuring device.

7. The high-precision measurement system according to claim 4, characterized in that, The denoising of the compensated Stokes and anti-Stokes signals includes: performing zero-point and drift correction on the compensated Stokes and anti-Stokes signals respectively, using the static unheated section as a reference, and eliminating the slowly varying level bias; and using a multi-scale chain strategy of "moving average + discrete wavelet decomposition" to suppress random noise.

8. The high-precision measurement system according to claim 7, characterized in that, This moving average reduces high-frequency white noise without sacrificing transition edges; the discrete wavelet decomposition selects an approximately orthogonal wavelet basis, based on noise estimation. A soft threshold λ is set, and scale-adaptive shrinkage is implemented on the detail coefficients to preserve interface mutations.

9. The high-precision measurement system according to claim 4, characterized in that, The denoising of the compensated Stokes and anti-Stokes signals also includes: denoising the slow baseline drift caused by differential attenuation and dispersion using a polynomial... The polynomial is modeled and removed from the logarithmic or ratio domain of the compensated Stokes and anti-Stokes signals. If there is a small group delay difference in the spectral paths of the two channels, a fractional delay filter is used in the frequency domain for subsampling point-level phase compensation to keep the ratio flat throughout the entire range.

10. The high-precision measurement system according to claim 4, characterized in that, The processing of the observed temperature sequence using Wiener deconvolution includes applying the following formula to the observed temperature sequence. Processing: ， in This represents the spectral information after the Fourier transform of the actual temperature sequence. The Fourier transform of the system's point spread function (PSF) is given. for The conjugate complex number of , k is the noise ratio coefficient of the Wiener filter, which reflects the ratio of the signal power spectrum to the noise power spectrum.

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