A long-distance optical fiber Raman scattering temperature measurement monitoring system

Abstract

The application discloses a long-distance optical fiber Raman scattering temperature measurement and monitoring system, and relates to the technical field of optical fiber sensing temperature measurement, which comprises injecting pulsed laser into a sensing optical fiber through a laser emission module to excite back Raman scattering light; a receiving and separating module separates the scattering light into Stokes light and anti-Stokes light signals; a processing and positioning module converts signal intensity into data distributed along the optical fiber position; a temperature demodulation module establishes a calibration equation based on the intensity ratio of the two and calculates initial temperature data along the optical fiber; and a compensation and correction module constructs a correction model fusing inherent attenuation of the optical fiber and external disturbance characteristics, compensates and corrects the initial data, and finally generates accurate temperature field data distributed along the optical fiber. The system improves the spatial consistency, absolute accuracy and long-term environmental stability of long-distance distributed temperature measurement through accurate calculation of the scattering light intensity ratio and systematic error modeling compensation.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic temperature sensing technology, specifically a long-distance fiber optic Raman scattering temperature monitoring system. Background Technology

[0002] Existing distributed fiber optic Raman temperature measurement systems typically rely on detecting the intensity of the backscattered and antiscattered light generated in the sensing fiber and demodulating the temperature based on the physical relationship between their intensity ratio and temperature. While this method works under ideal laboratory conditions over short distances, it suffers from significant drawbacks in long-distance engineering applications. The system's temperature demodulation primarily depends on direct or simplified processing of the two scattered light signals, failing to adequately consider the inherent wavelength-dependent attenuation characteristics of the sensing fiber during long-distance transmission, which causes asymmetric effects on the dual-channel signals. This error, introduced by the fiber's inherent properties, accumulates with distance, leading to systematic distortion of the measured temperature distribution along the fiber's spatial distribution and a decrease in accuracy at long distances.

[0003] Current technologies lack effective mechanisms for distinguishing and compensating for non-temperature-dependent disturbances introduced by the external environment. During installation and use, sensing optical fibers inevitably experience changes such as micro-bending and stress. These changes affect the transmission loss of both Stokes and anti-Stokes light, but their impact is not entirely equivalent to the effect of temperature changes. Conventional demodulation methods struggle to separate such disturbance effects from the actual temperature change signal, leading to errors in temperature demodulation results. Consequently, the reliability and stability of measurement data are difficult to guarantee in complex field environments. Summary of the Invention

[0004] This invention aims to solve at least one of the technical problems existing in the prior art;

[0005] Therefore, this invention proposes a long-distance fiber optic Raman scattering-based temperature monitoring system, comprising:

[0006] The laser emitting module continuously injects pulsed laser light of a specific wavelength into the sensing fiber deployed in the environment under test, thereby exciting the sensing fiber to generate backscattered Raman light containing Stokes light and anti-Stokes light.

[0007] The receiving and separating module receives and separates the Stokes light signal and the anti-Stokes light signal in the backscattered Raman light to obtain the Stokes light intensity sequence and the anti-Stokes light intensity sequence.

[0008] The processing and positioning module performs time-domain analysis on the Stokes light intensity sequence and the anti-Stokes light intensity sequence respectively, converts the time information into spatial position information along the length direction of the sensing fiber, and obtains the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position.

[0009] The temperature demodulation module establishes a calibration equation for the ratio of Stokes light intensity to anti-Stokes light intensity. Based on the calibration equation, it calculates and processes the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position to obtain the initial temperature data distributed along the fiber position.

[0010] The compensation and correction module constructs a temperature demodulation correction model based on the inherent attenuation characteristics and external disturbance features of the sensing fiber, and uses the temperature demodulation correction model to compensate and correct the initial temperature data to generate the final temperature field data distributed along the length of the sensing fiber.

[0011] Furthermore, the continuous injection of pulsed laser light of a specific wavelength into the sensing fiber deployed in the environment under test to excite the sensing fiber to generate backscattered Raman light containing Stokes light and anti-Stokes light includes:

[0012] Generate a laser pulse signal with constant peak power and preset pulse width;

[0013] The laser pulse signal is coupled into the incident end of the sensing optical fiber through a wavelength division multiplexing device;

[0014] The laser pulse signal is controlled to propagate forward in the sensing optical fiber. The photons of the laser pulse signal undergo inelastic collisions with molecules in the optical fiber material, resulting in a frequency shift.

[0015] Collect the Raman scattered light generated by the inelastic collision and propagating in the reverse direction along the optical fiber, wherein the backscattered Raman light includes Stokes light with a wavelength longer than the incident light and anti-Stokes light with a wavelength shorter than the incident light.

[0016] Further, the step of receiving and separating the Stokes light signal and the anti-Stokes light signal in the backscattered Raman light to obtain the Stokes light intensity sequence and the anti-Stokes light intensity sequence includes:

[0017] The collected back Raman scattered light is guided to the optical path separation device;

[0018] By utilizing the filtering characteristics of the optical path separation device for Stokes and anti-Stokes wavelengths, the mixed scattered light is separated into independent Stokes and anti-Stokes optical paths.

[0019] Stokes optical detectors and anti-Stokes optical detectors are used respectively to convert the optical signals in the Stokes optical path and the anti-Stokes optical path into corresponding electrical signals, which are analog voltage sequences that vary with time.

[0020] The analog voltage sequence corresponding to the Stokes optical path and the analog voltage sequence corresponding to the anti-Stokes optical path are synchronously sampled and converted from analog to digital to obtain the digitized Stokes light intensity sequence and anti-Stokes light intensity sequence.

[0021] Further, the step of performing time-domain analysis on the Stokes light intensity sequence and the anti-Stokes light intensity sequence respectively, converting the time information into spatial position information along the length direction of the sensing fiber, and obtaining the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position, includes:

[0022] Determine the transmission speed of the pulsed laser in the sensing optical fiber;

[0023] The time coordinates of each sampling point in the Stokes light intensity sequence and the anti-Stokes light intensity sequence are mapped to the corresponding spatial position coordinates on the sensing fiber according to the calculation relationship of multiplying the transmission speed by time and then dividing by two, where time refers to the time interval from pulse emission to the return of scattered light.

[0024] Establish a table of correspondence between spatial location coordinates and light intensity values, and form a set of data on the distribution of Stokes light intensity along the fiber position and a set of data on the distribution of anti-Stokes light intensity along the fiber position.

[0025] The distribution data sets of Stokes light intensity along the fiber position and the distribution data sets of anti-Stokes light intensity along the fiber position are smoothed and filtered to suppress local intensity fluctuations caused by random noise.

[0026] Further, the calibration equation for establishing the ratio of Stokes light to anti-Stokes light intensity is used to calculate and process the distribution data of Stokes light intensity and anti-Stokes light intensity along the fiber position based on the calibration equation, to obtain the initial temperature data distributed along the fiber position, including:

[0027] Based on Raman scattering theory, a temperature function is constructed with the ratio of Stokes light intensity to anti-Stokes light intensity as the variable. The temperature function includes constant terms related to the properties of the optical fiber material.

[0028] Under a known constant reference temperature, the reference Stokes light intensity value and the reference anti-Stokes light intensity value are measured, and the constant term in the temperature function is solved using the reference Stokes light intensity value and the reference anti-Stokes light intensity value.

[0029] The temperature function obtained after solving for the constant term is defined as the calibration equation;

[0030] For each spatial location point on the optical fiber, the intensity value of that spatial location point in the Stokes light intensity distribution data along the optical fiber position and the intensity value of the same location point in the anti-Stokes light intensity distribution data along the optical fiber position are substituted into the calibration equation for calculation.

[0031] Output the temperature calculation results for each spatial location point. All calculation results are arranged in order of location to form the initial temperature data distributed along the fiber optic location.

[0032] Further, the step of constructing a temperature demodulation correction model based on the inherent attenuation characteristics and external disturbance features of the sensing fiber, and using the temperature demodulation correction model to compensate and correct the initial temperature data to generate final temperature field data distributed along the length of the sensing fiber, includes:

[0033] The relationship between the attenuation coefficient of the sensing fiber and the wavelength, as well as the loss parameters of the fiber at different temperatures, are obtained to establish a fiber attenuation-temperature correlation database.

[0034] The overall attenuation trend of the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position are analyzed. Combined with the fiber attenuation-temperature correlation database, the light intensity change component caused by the inherent loss of the fiber is separated.

[0035] Monitor the temporal stability of the backscattered Raman light signal and identify abnormal fluctuation patterns in the intensity of scattered light caused by external disturbances such as microbending and stress.

[0036] Based on the light intensity variation component caused by the inherent loss of the optical fiber and the abnormal fluctuation mode of the scattered light intensity, the temperature demodulation correction model is constructed. The input of the temperature demodulation correction model is the initial temperature data and the corresponding spatial location, and the output is the temperature correction amount.

[0037] The initial temperature data distributed along the fiber position is input into the temperature demodulation correction model to calculate the temperature correction amount at each position point. The temperature correction amount is then added to the corresponding initial temperature data to generate the final temperature field data distributed along the length of the sensing fiber.

[0038] Furthermore, the monitoring of the temporal stability of the backscattered Raman light signal, and the identification of abnormal fluctuation patterns in scattered light intensity caused by external disturbances such as microbending and stress, include:

[0039] Multiple sets of Stokes light intensity sequences and anti-Stokes light intensity sequences were continuously acquired at fixed time intervals;

[0040] For the same spatial location point, compare and analyze the light intensity values ​​of the spatial location point in multiple sets of sequences, and calculate the variance or standard deviation of its change over time;

[0041] Spatial locations where the variance or standard deviation exceeds a preset stability threshold are marked as abnormal fluctuation points;

[0042] Analyze the distribution characteristics of the abnormal fluctuation points on the optical fiber. If the abnormal fluctuation points show a local clustering or continuous distribution pattern, it is determined that the region showing the local clustering or continuous distribution pattern has an abnormal fluctuation pattern of the scattered light intensity caused by micro-bending or stress.

[0043] Record the fiber start position, end position, and fluctuation intensity level corresponding to the abnormal fluctuation mode of the scattered light intensity.

[0044] Furthermore, the workflow of the temperature demodulation correction model also includes a specific compensation step for fiber optic connector loss:

[0045] The spatial locations of all fiber optic splices or connectors are marked in the fiber optic cable laying information.

[0046] In the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position, locate the position point corresponding to the fiber fusion splice or connector, and identify the abrupt change value of the light intensity at the position point.

[0047] Based on the wavelength division multiplexing transmission principle of Raman scattered light, the theoretical loss coefficients of Stokes light and anti-Stokes light at the fiber optic splice or connector are calculated respectively.

[0048] The actual insertion loss value at the fiber optic splice or connector is calculated by comparing the abrupt change in light intensity with the theoretical loss coefficient for the corresponding wavelength.

[0049] In the temperature demodulation correction model, for each fiber optic splice or connector location, a local compensation function based on its actual insertion loss value is introduced to correct the temperature demodulation error caused by insertion loss near the location.

[0050] Furthermore, the method also includes a spatial resolution enhancement step for the temperature data:

[0051] The relationship between the pulse width of the pulsed laser and the spatial resolution of the final temperature field data is analyzed.

[0052] Pulse code modulation technology is used to modulate the pulsed laser of a specific wavelength into a pulse train with a specific coding sequence;

[0053] At the receiving end, the backscattered Raman light signal is decoded to match the encoding sequence of the transmitting end, and the scattered light components with different time delays are separated.

[0054] The decoding process improves the ability to discriminate the timing of scattered light signals, thereby achieving spatial positioning accuracy that exceeds the limitations of the original pulse width.

[0055] The higher-precision scattered light position information obtained through the decoding process is used to update the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position. Then, through the calibration equation and the temperature demodulation correction model, the final temperature field data with enhanced spatial resolution is generated.

[0056] Furthermore, the method also includes system self-calibration and fault diagnosis steps:

[0057] A constant temperature reference unit is set at the end of the sensing fiber or at a specific calibration point, and the constant temperature reference unit provides a known and stable temperature value.

[0058] The temperature measurement value at the corresponding location of the isothermal reference unit is periodically read from the final temperature field data;

[0059] The measured temperature value is compared with the known temperature value of the constant temperature reference unit to calculate the system drift error under long-term monitoring.

[0060] Based on the system drift error, the parameters in the temperature demodulation correction model are adjusted in reverse, or the constant terms in the calibration equation are updated.

[0061] Simultaneously, by analyzing the continuous abnormal temperature segments or signal loss segments in the final temperature field data that are inconsistent with the known environment, and combining the principle of optical time domain reflectance analysis, the breakage, excessive bending, or aging and deterioration points of the sensing optical fiber are diagnosed and located.

[0062] Compared with the prior art, the beneficial effects of the present invention are:

[0063] A calibration equation based on the intensity ratio of Stokes light to anti-Stokes light was established, and point-by-point calculations were performed on the two light intensity data distributed along the optical fiber according to this equation. This scheme converts the absolute light intensity information of the two channels into a ratio that is temperature-sensitive and can cancel out common noise such as light source fluctuations for calculation. This enables direct and highly sensitive extraction of temperature information, improves the anti-interference foundation of temperature demodulation, and the output initial temperature data already has a high relative resolution capability.

[0064] A temperature demodulation correction model is constructed that simultaneously quantifies the inherent attenuation characteristics of the sensing fiber and the features of external disturbances. This model is then used to compensate for and correct the initial temperature data calculated based on the light intensity ratio. This scheme mathematically models and estimates the systematic losses and random disturbances during fiber transmission. This effectively compensates for measurement deviations caused by long-distance transmission and non-temperature factors, improving the spatial consistency, absolute accuracy, and long-term measurement stability of temperature data along the entire length of the sensing fiber in complex environments. Attached Figure Description

[0065] Figure 1 This is a timing diagram of the long-distance fiber optic Raman scattering temperature monitoring system described in this invention.

[0066] Figure 2 This is a flowchart of optical signal reception and separation. Detailed Implementation

[0067] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0068] See Figure 1 The laser emitting module continuously injects pulsed laser light of a specific wavelength into the sensing fiber deployed in the environment under test. This pulsed laser excites the sensing fiber to generate backscattered Raman light containing Stokes and anti-Stokes beams. The receiving and separating module receives and separates the Stokes and anti-Stokes beam signals from the backscattered Raman light, thereby obtaining the Stokes and anti-Stokes beam intensity sequences. The processing and positioning module performs time-domain analysis on the Stokes and anti-Stokes beam intensity sequences, converting the time information into spatial position information along the length of the sensing fiber, obtaining the distribution data of the Stokes and anti-Stokes beam intensities along the fiber. The temperature demodulation module establishes a calibration equation for the ratio of Stokes and anti-Stokes beam intensities. Based on this calibration equation, it calculates and processes the distribution data of the Stokes and anti-Stokes beam intensities along the fiber to obtain the initial temperature data distributed along the fiber. The compensation and correction module constructs a temperature demodulation correction model based on the inherent attenuation characteristics of the sensing fiber and the features of external disturbances. This temperature demodulation correction model is used to compensate and correct the initial temperature data, generating the final temperature field data distributed along the length of the sensing fiber.

[0069] See Figure 2In one embodiment of the present invention, a laser emitting module generates a laser pulse signal with a constant peak power and a preset pulse width. The laser pulse signal is coupled into the incident end of the sensing fiber through a wavelength division multiplexing device. The laser pulse signal is controlled to propagate forward in the sensing fiber. Photons of the laser pulse signal undergo inelastic collisions with molecules in the fiber material, resulting in a frequency shift. Raman scattered light generated by the inelastic collisions and propagating backward along the fiber is collected. The backscattered Raman light includes Stokes light with a wavelength longer than the incident light and anti-Stokes light with a wavelength shorter than the incident light. In some embodiments, the laser emitting module uses a pulsed laser to generate a laser pulse signal with a wavelength of 1550 nanometers, a pulse width of 10 nanoseconds, and a peak power of 1 watt. The wavelength division multiplexing device efficiently injects the laser pulse signal into the sensing fiber while allowing the backscattered Raman light to be output from the same port. Optionally, the repetition frequency of the laser pulse signal is adjustable to adapt to the monitoring requirements of sensing fibers of different lengths, and the pulse width of the laser pulse signal can be adjusted within the range of 5 nanoseconds to 20 nanoseconds to match spatial resolution requirements.

[0070] In specific implementations, the receiving and separating module guides the collected backscattered Raman light to an optical path separator. Utilizing the filtering characteristics of the optical path separator for Stokes and anti-Stokes wavelengths, the mixed scattered light is separated into independent Stokes and anti-Stokes optical paths. Stokes and anti-Stokes optical detection components are then used to convert the optical signals in the Stokes and anti-Stokes optical paths into corresponding electrical signals, which are time-varying analog voltage sequences. The analog voltage sequences corresponding to the Stokes and anti-Stokes optical paths are synchronously sampled and converted from analog to digital to obtain digitized Stokes and anti-Stokes light intensity sequences. In some embodiments, the optical path separator is a dichroic mirror combination, the Stokes optical detection component is an InGaAs photodiode, and the anti-Stokes optical detection component is a silicon photomultiplier tube. Synchronous sampling is achieved by a high-speed data acquisition card, with the sampling rate matched to the repetition frequency of the laser pulse signal. Optionally, the analog-to-digital converter can be set to 16 bits to improve the dynamic range, and the sampling interval of the analog voltage sequence can be set to 1 nanosecond to correspond to spatial positioning accuracy. It can be understood that the generation of the Stokes light intensity sequence and the anti-Stokes light intensity sequence depends on the linearity of the photoelectric conversion and the timing accuracy of the synchronous sampling.

[0071] In practical implementation, using an example scenario in oil pipeline temperature monitoring, a laser emitting module injects laser pulse signals into sensing optical fibers laid along the pipeline. By comparing the intensity of backscattered Raman light under different pulse widths, a pulse width of 10 nanoseconds results in higher scattered light signal intensity, while a pulse width of 5 nanoseconds results in lower scattered light signal intensity but finer temporal resolution. The data comparison shows that the choice of pulse width needs to balance signal strength and temporal resolution, thus affecting spatial positioning accuracy. The formula describes the relationship between the energy of the laser pulse signal, peak power, and pulse width:

[0072]

[0073] in: The energy of a laser pulse signal is expressed in joules. This represents the peak power of the laser pulse signal, measured in watts. This represents the pulse width of the laser pulse signal, measured in nanoseconds, which can be converted to seconds for calculation. It can be understood that the energy of the laser pulse signal affects the intensity of the backscattered Raman light, and changes in the pulse width directly lead to differences in the amplitude of the scattered light signal.

[0074] In one embodiment of the present invention, the processing and positioning module determines the transmission speed of the pulsed laser in the sensing fiber. The time coordinates of each sampling point in the Stokes intensity sequence and the anti-Stokes intensity sequence are mapped to corresponding spatial coordinates on the sensing fiber based on the calculation relationship of transmission speed multiplied by time and then divided by two. Here, time refers to the time interval from pulse emission to the return of scattered light. A correspondence table between spatial coordinates and light intensity values ​​is established to form a distribution data set of Stokes intensity along the fiber position and an anti-Stokes intensity along the fiber position. The distribution data sets of Stokes intensity along the fiber position and the anti-Stokes intensity along the fiber position are then subjected to smoothing filtering to suppress local intensity fluctuations caused by random noise. In some embodiments, the transmission speed of the pulsed laser in the sensing fiber is calculated based on the effective refractive index of the fiber. The smoothing filtering uses a five-point moving average algorithm, and the data format of the distribution data sets of Stokes intensity along the fiber position and the anti-Stokes intensity along the fiber position is a two-dimensional array. Optionally, the division factor used in the mapping calculation is two because the path of the scattered light returning is half the round-trip path of the pulsed laser. The window length for smoothing filtering can be adjusted according to the noise level of the measured signal.

[0075] In practical implementation, the temperature demodulation module constructs a temperature function based on Raman scattering theory, using the ratio of Stokes light intensity to anti-Stokes light intensity as the variable. This temperature function includes constant terms related to the fiber material properties. Under a known constant reference temperature, the reference Stokes light intensity and reference anti-Stokes light intensity values ​​are measured. The constant terms in the temperature function are then solved using these values. The temperature function after solving for the constant terms is defined as a calibration equation. For each spatial location point on the fiber, the intensity values ​​of the Stokes light intensity distribution along the fiber and the intensity values ​​of the anti-Stokes light intensity distribution along the fiber are substituted into the calibration equation for calculation. The temperature calculation result for each spatial location point is output. All calculation results are arranged in positional order to form the initial temperature data distributed along the fiber. In some embodiments, the constant reference temperature condition is achieved by immersing a section of sensing fiber in a constant-temperature oil bath. The calibration equation is used to map the intensity ratio at each location point to a temperature value. Optionally, the temperature function is constructed based on the Boltzmann distribution law, and the constant terms are solved during initial system installation or periodic calibration. It is understandable that the accuracy of the initial temperature data depends on the measurement accuracy of the reference Stokes light intensity value and the reference anti-Stokes light intensity value, as well as the applicability of the calibration equation.

[0076] In practical implementation, using an example scenario in cable tunnel temperature monitoring, the processing and positioning module converts time information into spatial location information. By comparing the distribution data of Stokes light intensity along the fiber optic position at different spatial sampling intervals, the data curve is smoother when the sampling interval is 1 meter, while the curve shows more detail but more noticeable noise fluctuations when the sampling interval is 0.1 meters. The data comparison shows that the choice of spatial sampling interval affects the smoothness and detail of the temperature distribution curve. The formula describes the calibration relationship used in the temperature demodulation module:

[0077]

[0078] in: This represents the temperature at position z of the optical fiber, in degrees Celsius. This represents the ratio of the anti-Stokes light intensity to the Stokes light intensity at position z in the optical fiber; it is dimensionless. B is a dimensionless constant related to the Raman scattering cross section of the optical fiber material, and B is a dimensionless constant related to the molecular vibrational properties of the optical fiber material. This is another constant related to the Raman scattering cross-section and frequency, measured in degrees Celsius, where z is the spatial coordinate of the optical fiber, measured in kilometers. This can be understood as a constant... , and The value is determined by measurement under reference temperature conditions; the ratio It is calculated from the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position.

[0079] In one embodiment of the present invention, the compensation and correction module acquires the relationship between the attenuation coefficient of the sensing fiber and the wavelength, as well as the loss variation parameters of the fiber at different temperatures, establishes a fiber attenuation-temperature correlation database, analyzes the overall attenuation trend of the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position, and separates the light intensity variation component caused by the inherent loss of the fiber by combining the fiber attenuation-temperature correlation database. In some embodiments, the fiber attenuation-temperature correlation database is obtained through laboratory calibration and includes the attenuation coefficients of the sensing fiber at multiple discrete temperature points for Stokes light wavelength and anti-Stokes light wavelength. The overall attenuation trend is obtained by fitting the exponential curves of the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position as a function of distance. Optionally, the separation of the light intensity variation component caused by inherent loss is achieved by using a database-based difference method, subtracting the theoretical inherent attenuation curve from the measured attenuation curve.

[0080] In specific implementation, the compensation and correction module monitors the temporal stability of the backscattered Raman light signal to identify abnormal fluctuation patterns in scattered light intensity caused by external disturbances such as microbending and stress. It integrates the light intensity variation component caused by inherent fiber loss with the abnormal fluctuation patterns in scattered light intensity to construct a temperature demodulation correction model. The input to the temperature demodulation correction model is the initial temperature data and its corresponding spatial location, and the output is the temperature correction amount. The initial temperature data distributed along the fiber position is input into the temperature demodulation correction model to calculate the temperature correction amount at each location point. The temperature correction amount is then added to the corresponding initial temperature data to generate the final temperature field data distributed along the length of the sensing fiber. In some embodiments, the temperature demodulation correction model is implemented in the form of a polynomial function, and its coefficients are jointly determined by the fiber attenuation-temperature correlation database and the identified abnormal fluctuation patterns. Optionally, the addition of the temperature correction amount to the initial temperature data is a point-by-point algebraic addition. It can be understood that the accuracy of the final temperature field data depends on the compensation effect of the temperature demodulation correction model on inherent attenuation and external disturbances.

[0081] In practical implementation, using an example scenario in high-voltage transmission line temperature monitoring, the distribution data of Stokes light intensity along the fiber optic cable of a 10-kilometer sensing fiber were analyzed. The data showed a significant additional attenuation trend in the 5-7 kilometer section. Compared with the theoretical values ​​in the fiber attenuation-temperature correlation database, the measured attenuation slope in this section was higher than the theoretical inherent attenuation slope. This data comparison indicates that there is additional loss in this section due to fiber aging or stress. The temperature demodulation correction model outputs a series of negative temperature correction values ​​for this section to compensate for the positive temperature demodulation bias caused by the additional light intensity attenuation. A simplified mathematical expression of the temperature demodulation correction model is described by the following formula:

[0082]

[0083] in: This represents the temperature correction calculated at fiber position z, in degrees Celsius. This represents the additional attenuation coefficient separated at fiber position z, relative to standard conditions, expressed in decibels per kilometer. This represents the intensity level of the anomalous fluctuation mode determined by monitoring time-domain stability at fiber location z, and is dimensionless. and These are the weighting coefficients obtained by fitting the model using a fiber attenuation-temperature correlation database and historical data. The units are degrees Celsius per kilometer per decibel and degrees Celsius per class, respectively. z represents the spatial coordinates of the fiber, in kilometers. This can be understood as an additional attenuation coefficient. The intensity level of the anomalous fluctuation mode is determined by comparing the measured attenuation trend with the theoretical inherent attenuation trend in the fiber attenuation-temperature correlation database. It is a dimensionless quantitative indicator. The intensity level of abnormal fluctuation patterns. Used to characterize fiber position The degree of intensity fluctuation of scattered light caused by external disturbances such as microbending and stress is determined by the following steps: Calculate the fiber position. The relative fluctuation coefficient of scattered light intensity over time :

[0084]

[0085] in, Dimensionless This represents the standard deviation of the light intensity values ​​collected at this location multiple times, in millivolts. The value is the arithmetic mean of multiple light intensity measurements taken at this location, in millivolts; z is the spatial coordinate of the optical fiber, in kilometers. The relative fluctuation coefficient of the system's undisturbed baseline is set. , The relative fluctuation coefficient of fiber optic intensity, measured under standard experimental conditions without external disturbances, is a reference value obtained by those skilled in the art through routine calibration experiments. The intensity level is determined by the ratio of the relative fluctuation coefficients. ,Will and The ratio is divided into 3 levels, when hour, (Slight fluctuations); when hour, (Moderate fluctuation); when hour, (Severe fluctuations); if It was determined that there was no external disturbance. The above. The grading threshold and calculation method can be routinely adjusted by those skilled in the art based on the laying environment of the sensing optical fiber, such as industrial pipelines, cable tunnels, and power transmission lines.

[0086] In one embodiment of the present invention, the temporal stability of the backscattered Raman light signal is monitored, and multiple sets of Stokes light intensity sequences and anti-Stokes light intensity sequences are continuously acquired at fixed time intervals. The light intensity values ​​at the same spatial location are compared and analyzed across the multiple sets of sequences, and their variance or standard deviation over time is calculated. In some embodiments, the fixed time interval is set to 1 minute, and 100 sets of Stokes light intensity sequences and anti-Stokes light intensity sequences are continuously acquired. Variance calculation is performed on 100 light intensity values ​​at each spatial location. Optionally, a preset stability threshold is determined based on the long-term baseline noise level of the system under conditions of no external disturbance. It can be understood that the magnitude of the variance or standard deviation directly reflects the temporal stability of the scattered light signal at the corresponding spatial location.

[0087] In specific implementations, spatial locations with variances or standard deviations exceeding a preset stability threshold are marked as anomalous fluctuation points. The distribution characteristics of these anomalous fluctuation points on the optical fiber are analyzed. If the anomalous fluctuation points exhibit local clustering or continuous distribution, the region exhibiting such a pattern is determined to have an anomalous fluctuation mode in scattered light intensity caused by microbending or stress. The fiber start and end positions and fluctuation intensity levels corresponding to the anomalous fluctuation mode are recorded. In some embodiments, the distribution characteristics of anomalous fluctuation points are determined by statistically analyzing the distance between adjacent anomalous fluctuation points. When the distance between multiple anomalous fluctuation points is less than a set threshold, it is determined to be a local cluster. The fluctuation intensity level is classified according to the multiple by which the variance or standard deviation exceeds the preset stability threshold. Optionally, information on anomalous fluctuation modes in scattered light intensity is recorded in a dedicated log file for use by the temperature demodulation correction model.

[0088] In practical implementation, the workflow of the temperature demodulation correction model includes a specific compensation step for fiber optic splice loss. The spatial locations of all fiber optic splices or connectors are marked in the fiber optic laying information. The corresponding locations of the splices or connectors are located using the distribution data of Stokes light intensity along the fiber optic position and the distribution data of anti-Stokes light intensity along the fiber optic position, and the abrupt changes in light intensity at these locations are identified. The theoretical loss coefficients of Stokes light and anti-Stokes light at the fiber optic splice or connector are calculated based on the wavelength division multiplexing transmission principle of Raman scattered light. Optionally, the theoretical loss coefficients are preset based on the type of fiber optic connector and the splice quality. It can be understood that the abrupt change in light intensity refers to the step drop in the distribution data curve at the location of the fiber optic splice or connector.

[0089] In practical implementation, the abrupt change in light intensity is converted into a loss value (dB) for the corresponding wavelength using the optical power-loss conversion relationship. This converted loss value is then compared with the theoretical loss coefficient (dB / km) for the corresponding wavelength, combined with the theoretical loss value calculated based on the physical length of the fiber optic connector. The actual insertion loss value at the fiber optic splice or connector is calculated. In the temperature demodulation correction model, a local compensation function based on the actual insertion loss value is introduced for each fiber optic splice or connector location to correct temperature demodulation errors caused by insertion loss near the location. The optical power-loss conversion relationship is a standard conversion method in fiber optic sensing, converting the voltage abrupt change in light intensity (mV) into an optical power abrupt change (dBm), and then converting it to a loss value (dB) at the fiber optic connector. The physical length of the fiber optic connector is the inherent length of the splice / connection area (typically in the centimeter range). The theoretical loss coefficient (dB / km) is converted to a centimeter-level theoretical loss value (dB), and then compared with the loss value (dB) converted from the abrupt change in light intensity. In some embodiments, the local compensation function is a Gaussian attenuation function centered at the fiber fusion splice or connector location. The amplitude of the function is proportional to the actual insertion loss value, and the width is related to the spatial resolution of the pulsed laser. Optionally, the calculation of the actual insertion loss value takes into account the different attenuation characteristics of the fiber fusion splice or connector for Stokes and anti-Stokes light.

[0090] In practical implementation, using an example scenario, a 5-kilometer section of sensing fiber optic cable containing three fiber optic fusion splices was monitored in long-distance oil and gas pipeline monitoring. The distribution of recorded abnormal fluctuation points and the locations of the fiber optic fusion splices were analyzed and compared. See Table 1 for data comparison.

[0091] Table 1: Data on Abnormal Fluctuations and Joint Losses in Pipeline Monitoring

[0092]

[0093] The table data shows that at the fiber optic splice locations of 1.23 km, 2.50 km, and 4.01 km, the abrupt changes in optical intensity were all greater than the theoretical loss coefficient. The calculated actual insertion loss values ​​were used for local compensation. An abnormal fluctuation pattern caused by micro-bending was identified in the 3.15-3.20 km section, with a fluctuation variance significantly higher than the normal background fluctuation at the splice location. Based on the comparison of the above abrupt changes in optical intensity with the theoretical loss coefficient in the same unit, the actual insertion loss value was calculated using the conventional formula for fiber optic splice insertion loss. The formula is described as follows: ,in This represents the calculated actual insertion loss value, expressed in decibels (dB). This represents the average optical intensity up to the fiber optic splice or connector location, extracted from Stokes intensity distribution data or anti-Stokes intensity distribution data along the fiber optic line, in millivolts. This represents the average light intensity after the fiber splice or connector location, extracted from Stokes intensity distribution data or anti-Stokes intensity distribution data along the fiber location, in millivolts. It can be understood as the actual insertion loss value. The calculations are performed separately for Stokes light and anti-Stokes light, and the local compensation functions are constructed based on these two values ​​respectively.

[0094] In one embodiment of the present invention, the system performs a spatial resolution enhancement processing step for temperature data, analyzes the constraint relationship between the pulse width of the pulsed laser and the spatial resolution of the final temperature field data, and uses pulse code modulation (PCM) technology to modulate a pulsed laser of a specific wavelength into a pulse train with a specific coding sequence. In some embodiments, the constraint relationship is reflected in the fact that the spatial resolution is proportional to the product of the pulse width and the speed of light. The PCM technology uses a Gray complement sequence, and the length of the specific coding sequence is 127 bits. Optionally, the width of each sub-pulse in the pulse train is smaller than the original pulsed laser pulse width, but the duration of the entire pulse train is longer. It can be understood that the PCM technology achieves higher temporal discrimination capability by increasing the total injection time of optical energy.

[0095] In specific implementations, the backscattered Raman light signal at the receiving end undergoes decoding processing to match the encoded sequence at the transmitting end, separating the scattered light components with different time delays. This decoding process improves the ability to discriminate the timing of the scattered light signal, thereby achieving spatial positioning accuracy beyond the limitations of the original pulse width. The higher-precision scattered light position information obtained after decoding is used to update the distribution data of Stokes light intensity and anti-Stokes light intensity along the fiber optic position. Finally, through calibration equations and a temperature demodulation correction model, final temperature field data with enhanced spatial resolution is generated. In some embodiments, the decoding process is a correlation operation, performing cross-correlation calculations between the received backscattered Raman light signal sequence and a copy of the transmitting end's encoded sequence. The position of the correlation peak corresponds to the precise time delay of the scattering point. Optionally, the improvement factor in spatial positioning accuracy is related to the length of the encoded sequence and its autocorrelation characteristics.

[0096] In practical implementation, using an example scenario of monitoring localized overheating at power cable joints, the original pulse width of the pulsed laser corresponds to a spatial resolution of 1 meter. Spatial resolution enhancement processing is performed using encoded sequences of different lengths. Comparing the final temperature field data reconstructed using 63-bit and 127-bit encoded sequences, near the cable joint location, the temperature distribution curve obtained using the 127-bit encoded sequence shows sharper edges in temperature changes. Data comparison demonstrates that longer encoded sequences provide finer spatial detail resolution. The formula describes the processing gain brought by pulse code modulation technology:

[0097]

[0098] in: This indicates the processing gain of pulse code modulation, expressed in decibels. This indicates the length of the specific coded sequence used, and is dimensionless. It can be understood that this relates to processing gain. The improvement helps to increase the signal-to-noise ratio while maintaining the same average output optical power, thereby supporting finer time delay discrimination.

[0099] In practical implementation, the system performs system self-calibration and fault diagnosis steps. A constant-temperature reference unit is set at the end of the sensing fiber or a specific calibration point. The constant-temperature reference unit provides a known and stable temperature value. Temperature measurements at the corresponding location of the constant-temperature reference unit are periodically read from the final temperature field data. The measured temperature values ​​are compared with the known temperature value of the constant-temperature reference unit to calculate the long-term monitored system drift error. In some embodiments, the constant-temperature reference unit is a precision temperature control module encapsulated within the sensing fiber terminal box. The known and stable temperature value is set to 25.00 degrees Celsius, and the system drift error is the difference between the measured temperature value and 25.00 degrees Celsius. Optionally, the periodic reading operation cycle is set to 24 hours. It can be understood that the system drift error reflects the slow change in the system baseline caused by factors such as photodetector response and light source fluctuations.

[0100] In practical implementation, the parameters in the temperature demodulation correction model are adjusted in reverse based on the system drift error, or the constant terms in the calibration equation are updated. Simultaneously, persistent abnormal temperature segments or signal loss segments that do not conform to the known environment are analyzed in the final temperature field data. Using optical time-domain reflectometry (OTDR) principles, the breaks, excessive bending, or aging degradation points in the sensing fiber are diagnosed and located. In some embodiments, adjusting the parameters in the temperature demodulation correction model involves adding an offset proportional to the system drift error to the constant terms of the model. Using ODR principles, the fiber break point is determined by analyzing the endpoint where the signal is completely lost in the final temperature field data. Optionally, the diagnosis of aging degradation points is based on abnormal attenuation segments in the final temperature field data that exhibit slow, monotonically changing attenuation, with their demodulated temperature values ​​continuously deviating from the environmental reference values.

[0101] The above embodiments are only used to illustrate the technical methods of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical methods of the present invention without departing from the spirit and scope of the technical methods of the present invention.

Claims

1. A long-distance fiber optic Raman scattering temperature monitoring system, characterized in that, include: The laser emitting module continuously injects pulsed laser light of a specific wavelength into the sensing fiber deployed in the environment under test, thereby exciting the sensing fiber to generate backscattered Raman light containing Stokes light and anti-Stokes light. The receiving and separating module receives and separates the Stokes light signal and the anti-Stokes light signal in the backscattered Raman light to obtain the Stokes light intensity sequence and the anti-Stokes light intensity sequence. The processing and positioning module performs time-domain analysis on the Stokes light intensity sequence and the anti-Stokes light intensity sequence respectively, converts the time information into spatial position information along the length direction of the sensing fiber, and obtains the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position. The temperature demodulation module establishes a calibration equation for the ratio of Stokes light intensity to anti-Stokes light intensity. Based on the calibration equation, it calculates and processes the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position to obtain the initial temperature data distributed along the fiber position. The compensation and correction module constructs a temperature demodulation correction model based on the inherent attenuation characteristics and external disturbance features of the sensing fiber. It then uses this model to compensate and correct the initial temperature data, generating final temperature field data distributed along the length of the sensing fiber, including: The relationship between the attenuation coefficient of the sensing fiber and the wavelength, as well as the loss parameters of the fiber at different temperatures, are obtained to establish a fiber attenuation-temperature correlation database. The overall attenuation trend of the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position are analyzed. Combined with the fiber attenuation-temperature correlation database, the light intensity change component caused by the inherent loss of the fiber is separated. Monitor the temporal stability of the backscattered Raman light signal and identify abnormal fluctuation patterns in the intensity of scattered light caused by external disturbances such as microbending and stress. Based on the light intensity variation component caused by the inherent loss of the optical fiber and the abnormal fluctuation mode of the scattered light intensity, the temperature demodulation correction model is constructed. The input of the temperature demodulation correction model is the initial temperature data and the corresponding spatial location, and the output is the temperature correction amount. The initial temperature data distributed along the fiber position is input into the temperature demodulation correction model to calculate the temperature correction amount at each position point. The temperature correction amount is then added to the corresponding initial temperature data to generate the final temperature field data distributed along the length of the sensing fiber.

2. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 1, characterized in that, The method of continuously injecting pulsed laser light of a specific wavelength into a sensing fiber deployed in the environment under test to excite the sensing fiber to generate backscattered Raman light containing Stokes light and anti-Stokes light includes: Generate a laser pulse signal with constant peak power and preset pulse width; The laser pulse signal is coupled into the incident end of the sensing optical fiber through a wavelength division multiplexing device; The laser pulse signal is controlled to propagate forward in the sensing optical fiber. The photons of the laser pulse signal undergo inelastic collisions with molecules in the optical fiber material, resulting in a frequency shift. Collect the Raman scattered light generated by the inelastic collision and propagating in the reverse direction along the optical fiber, wherein the backscattered Raman light includes Stokes light with a wavelength longer than the incident light and anti-Stokes light with a wavelength shorter than the incident light.

3. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 2, characterized in that, The process of receiving and separating the Stokes light signal and the anti-Stokes light signal in the backscattered Raman light to obtain the Stokes light intensity sequence and the anti-Stokes light intensity sequence includes: The collected back Raman scattered light is guided to the optical path separation device; By utilizing the filtering characteristics of the optical path separation device for Stokes and anti-Stokes wavelengths, the mixed scattered light is separated into independent Stokes and anti-Stokes optical paths. Stokes optical detectors and anti-Stokes optical detectors are used respectively to convert the optical signals in the Stokes optical path and the anti-Stokes optical path into corresponding electrical signals, which are analog voltage sequences that vary with time. The analog voltage sequence corresponding to the Stokes optical path and the analog voltage sequence corresponding to the anti-Stokes optical path are synchronously sampled and converted from analog to digital to obtain the digitized Stokes light intensity sequence and anti-Stokes light intensity sequence.

4. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 3, characterized in that, Time-domain analysis is performed on the Stokes light intensity sequence and the anti-Stokes light intensity sequence respectively, converting the time information into spatial position information along the length of the sensing fiber, and obtaining the distribution data of Stokes light intensity and anti-Stokes light intensity along the fiber position, including: Determine the transmission speed of the pulsed laser in the sensing optical fiber; The time coordinates of each sampling point in the Stokes light intensity sequence and the anti-Stokes light intensity sequence are mapped to the corresponding spatial position coordinates on the sensing fiber according to the calculation relationship of multiplying the transmission speed by time and then dividing by two, where time refers to the time interval from pulse emission to the return of scattered light. Establish a table of correspondence between spatial location coordinates and light intensity values, and form a set of data on the distribution of Stokes light intensity along the fiber position and a set of data on the distribution of anti-Stokes light intensity along the fiber position. The distribution data sets of Stokes light intensity along the fiber position and the distribution data sets of anti-Stokes light intensity along the fiber position are smoothed and filtered to suppress local intensity fluctuations caused by random noise.

5. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 4, characterized in that, The calibration equation for establishing the intensity ratio of Stokes light to anti-Stokes light is used to calculate and process the distribution data of Stokes light intensity and anti-Stokes light intensity along the fiber position based on the calibration equation, thereby obtaining the initial temperature data distributed along the fiber position, including: Based on Raman scattering theory, a temperature function is constructed with the ratio of Stokes light intensity to anti-Stokes light intensity as the variable. The temperature function includes constant terms related to the properties of the optical fiber material. Under a known constant reference temperature, the reference Stokes light intensity value and the reference anti-Stokes light intensity value are measured, and the constant term in the temperature function is solved using the reference Stokes light intensity value and the reference anti-Stokes light intensity value. The temperature function obtained after solving for the constant term is defined as the calibration equation; For each spatial location point on the optical fiber, the intensity value of that spatial location point in the Stokes light intensity distribution data along the optical fiber position and the intensity value of the same location point in the anti-Stokes light intensity distribution data along the optical fiber position are substituted into the calibration equation for calculation. Output the temperature calculation results for each spatial location point. All calculation results are arranged in order of location to form the initial temperature data distributed along the fiber optic location.

6. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 5, characterized in that, Monitoring the temporal stability of the backscattered Raman light signal and identifying abnormal fluctuation modes in scattered light intensity caused by external disturbances such as microbending and stress, including: Multiple sets of Stokes light intensity sequences and anti-Stokes light intensity sequences were continuously acquired at fixed time intervals; For the same spatial location point, compare and analyze the light intensity values ​​of the spatial location point in multiple sets of sequences, and calculate the variance or standard deviation of its change over time; Spatial locations where the variance or standard deviation exceeds a preset stability threshold are marked as abnormal fluctuation points; Analyze the distribution characteristics of the abnormal fluctuation points on the optical fiber. If the abnormal fluctuation points show a local clustering or continuous distribution pattern, it is determined that the region showing the local clustering or continuous distribution pattern has an abnormal fluctuation pattern of the scattered light intensity caused by micro-bending or stress. Record the fiber start position, end position, and fluctuation intensity level corresponding to the abnormal fluctuation mode of the scattered light intensity.

7. A long-distance fiber optic Raman scattering temperature monitoring system according to claim 6, characterized in that, The workflow of the temperature demodulation correction model also includes a specific compensation step for fiber optic connector loss: The spatial locations of all fiber optic splices or connectors are marked in the fiber optic cable laying information. In the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position, locate the position point corresponding to the fiber fusion splice or connector, and identify the abrupt change value of the light intensity at the position point. Based on the wavelength division multiplexing transmission principle of Raman scattered light, the theoretical loss coefficients of Stokes light and anti-Stokes light at the fiber optic splice or connector are calculated respectively. The actual insertion loss value at the fiber optic splice or connector is calculated by comparing the abrupt change in light intensity with the theoretical loss coefficient for the corresponding wavelength. In the temperature demodulation correction model, for each fiber optic splice or connector location, a local compensation function based on its actual insertion loss value is introduced to correct the temperature demodulation error caused by insertion loss near the location.

8. The long-distance fiber optic Raman scattering temperature monitoring system according to claim 7, characterized in that, It also includes a spatial resolution enhancement process for temperature data: The relationship between the pulse width of the pulsed laser and the spatial resolution of the final temperature field data is analyzed. Pulse code modulation technology is used to modulate the pulsed laser of a specific wavelength into a pulse train with a specific coding sequence; At the receiving end, the backscattered Raman light signal is decoded to match the encoding sequence of the transmitting end, and the scattered light components with different time delays are separated. The decoding process improves the ability to discriminate the timing of scattered light signals, thereby achieving spatial positioning accuracy that exceeds the limitations of the original pulse width. The higher-precision scattered light position information obtained through the decoding process is used to update the distribution data of Stokes light intensity along the fiber position and the distribution data of anti-Stokes light intensity along the fiber position. Then, through the calibration equation and the temperature demodulation correction model, the final temperature field data with enhanced spatial resolution is generated.

9. A long-distance fiber optic Raman scattering temperature monitoring system according to claim 1, characterized in that, It also includes system self-calibration and fault diagnosis steps: A constant temperature reference unit is set at the end of the sensing fiber or at a specific calibration point, and the constant temperature reference unit provides a known and stable temperature value. The temperature measurement value at the corresponding location of the isothermal reference unit is periodically read from the final temperature field data; The measured temperature value is compared with the known temperature value of the constant temperature reference unit to calculate the system drift error under long-term monitoring. Based on the system drift error, the parameters in the temperature demodulation correction model are adjusted in reverse, or the constant terms in the calibration equation are updated. Simultaneously, by analyzing the continuous abnormal temperature segments or signal loss segments in the final temperature field data that are inconsistent with the known environment, and combining the principle of optical time domain reflectance analysis, the breakage, excessive bending, or aging and deterioration points of the sensing optical fiber are diagnosed and located.

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