CN211978164U - Distributed optical fiber temperature measurement system based on double-end demodulation - Google Patents

Abstract

The utility model discloses a distributed optical fiber temperature measurement system based on bi-polar demodulation, including optical structure, processing module and optic fibre utilize bi-polar demodulation to overcome the influence of RDTS wavelength correlation loss in practical application, utilize optic fibre subregion correlation noise reduction technique effectively to have solved the both ends of the temperature curve SNR that bi-polar demodulation obtained than central part much lower problem simultaneously, have further promoted RDTS's temperature measurement precision. The utility model discloses a method helps improving the wholeness ability of distributed optical fiber temperature measurement system in special environment such as fire detection, power cable detection and nuclear power station detection.

Description

Distributed optical fiber temperature measurement system based on double-end demodulation

Technical Field

The utility model relates to a distributing type optical fiber sensing system, concretely relates to distributing type optical fiber temperature measurement system based on bi-polar demodulation.

Background

Raman Distributed Temperature Sensors (RDTS) have been studied for many years and, due to their well-known advantages over electrically equivalent devices, have been successfully used in many different fields, such as fire detection, power cable monitoring and leakage detection. Most RDTS systems are based on the Optical Time Domain Reflectometry (OTDR) principle for backscattered Raman scattered anti-Stokes (AS) and Stokes (S) optical signals. Where single-ended RDTS represents the most common solution in long distances, but it is inherently affected by AS and S Wavelength Dependent Loss (WDL); the slow variation of the WDL over time actually causes slow and undetectable distortion in the demodulation temperature profile. This is typically due to fiber aging, but is strongly enhanced in certain harsh application environments, such as nuclear power plant monitoring, where the presence of ionizing radiation can significantly increase fiber loss over time and result in significant WDL. Studies have also shown that degradation of the fibers in high temperature and humid environments can greatly increase fiber loss. Furthermore, in geothermal well applications, the WDL of the optical fiber typically varies over time due to the effects of high temperature and hydrogen concentration.

For use in a wider field, it is preferable to improve the measurement accuracy by post-processing without expensive light sources, filters, circuits, and the like. A band pass filter that cuts off unnecessary signal bands or an adaptive filter that extracts an effective signal band based on a designed noise model may be used as post-processing for noise reduction. But the frequency band of the noise overlaps with the frequency band of the signal component, that is, in any filtering process, the signal component is attenuated with the suppression of the noise, and even if the filtering process is performed in order to suppress the reduction of the measurement accuracy, the signal component itself is attenuated, and the noise of the anti-stokes optical signal becomes particularly large at the far end. In practical RDTS systems both WDL and local losses must be effectively accounted for and eliminated, which can be achieved by using an alternate double-ended demodulation scheme (also known as loop demodulation). In this scheme, the AS and S optical signals are obtained in the forward and backward directions, and then averaged appropriately. But double-ended demodulation also has one such problem: the signal to noise ratio at the two ends of the demodulation temperature curve is much lower than in the central part.

SUMMERY OF THE UTILITY MODEL

The utility model discloses the problem that will solve is: how to overcome the influence of wavelength-dependent loss of RDTS in practical application, solve the problem that two ends of a temperature curve signal-to-noise ratio obtained by double-end demodulation are much lower than a central part, and further improve the temperature measurement precision of RDTS so as to meet the application requirement of RDTS in more complex environments.

The utility model provides a problem is solved like this: a double-ended demodulation scheme based on fiber partition correlation noise reduction is provided.

The specific technical scheme is as follows:

the utility model provides a distributed optical fiber temperature measurement system based on bi-polar demodulation, includes optical structure, processing module and optic fibre, its characterized in that: the optical structure is configured to detect a first stokes light signal and a first anti-stokes light signal from backscattered light generated when light is input to the first end of the optical fiber, and to detect a second stokes light signal and a second anti-stokes light signal from backscattered light generated when light is input to the second end of the optical fiber; the processing module is configured to calculate, within a region including the first end of the optical fiber, a first region length based on a first correlation between the second stokes light signal and at least one of the first stokes light signal and the first anti-stokes light signal, calculate a second region length based on a second correlation between the second anti-stokes light signal and at least one of the first stokes light signal and the first anti-stokes light signal, smooth the second stokes light signal in the first region, smooth the second anti-stokes light signal in the second region, calculate a temperature of the sampling point using the smoothed second stokes light signal, the smoothed second anti-stokes light signal, the first stokes light signal, and the first anti-stokes light signal.

Further, pulsed light is alternately input to the first end and the second end of the optical fiber through the optical switch.

Further, the first region is elongated as the first correlation becomes smaller, and the second region is elongated as the second correlation becomes smaller.

Further, the length of the first region and the second region sets an upper limit.

Further, when the first correlation is equal to or greater than the threshold, the second stokes light signal is not smooth; when the second correlation is equal to or greater than another threshold, the second anti-stokes light signal is not smooth.

Further, a pearson product-moment correlation coefficient is applied to the first correlation and the second correlation.

Further, a spearman rank correlation coefficient is applied to the first correlation and the second correlation.

Further, the fiber has a constant temperature in the region around the sampling point; the area around the sampling point is larger than a zeroth-order width of a temperature distribution obtained when another constant temperature different from the constant temperature is given to a minimum heating length portion centered on the sampling point, and is smaller than a principal component width of the temperature distribution.

The beneficial technical effects of the utility model are that, utilize bi-polar demodulation to overcome the influence of RDTS wavelength correlation loss in practical application, utilize the technique of making an uproar of optic fibre subregion relativity to fall simultaneously and effectively solved the both ends of the temperature curve SNR that bi-polar demodulation obtained than central part much lower problem, further promoted RDTS's temperature measurement precision. The utility model discloses a method helps improving the wholeness ability of distributed optical fiber temperature measurement system in special environment such as fire detection, power cable detection and nuclear power station detection.

Drawings

Fig. 1 shows the general structure of a distributed optical fiber temperature measurement system 1 based on double-end demodulation;

fig. 2 shows a case where a portion of the optical fiber 3 is immersed in hot water of 55 c when the room temperature is 24 c;

FIG. 3 shows the results obtained from FIG. 2 and equation (3);

FIG. 4 shows a typical example of a calculated impulse response;

fig. 5 to 7 show a comparison between an output waveform estimated from an impulse response with respect to each immersion length in hot water and an actually obtained output waveform;

fig. 8 shows an output waveform in the case where a center portion to which no high temperature is applied is provided between two high temperature application portions of 0.2m, and the width of the center portion is gradually changed;

fig. 9 shows a flow chart executed when the distributed optical fiber thermometry system 1 based on double-ended demodulation measures temperature;

FIG. 10 shows a comparison between Pearson product-moment correlation coefficients and spearman rank correlation coefficients for a set of experimental data;

fig. 11 shows another example of a flowchart executed when the temperature measured by the temperature detector 42 is corrected by the corrector 43;

FIG. 12 shows another flowchart executed when the temperature measured by the temperature detector 42 is corrected by the corrector 43;

fig. 13 shows another example of a flowchart executed when the distributed optical fiber thermometry system 1 measures temperature.

Detailed Description

Fig. 1 shows the overall structure of a distributed fiber optic thermometry system 1 based on double-ended demodulation, comprising an optical structure 2, an optical fiber 3 and a processing module 4. The optical structure 2 comprises a laser 21, a beam splitter 22, an optical switch 23, a filter 24, a detector 25a and a detector 25 b. The processing module 4 comprises an instructor 41, a temperature detector 42 and a corrector 43. The laser 21 emits laser pulses of a predetermined wavelength range at predetermined time intervals in accordance with the instruction from the commander 41. The optical splitter 22 inputs the pulsed light emitted from the laser 21 into the optical switch 23, and the optical switch 23 alternately inputs the pulsed light into the first end and the second end of the optical fiber 3 in a predetermined period as instructed by the commander 41. In this embodiment, the length of the optical fiber 3 is L meters, the position of the first end is 0 meters, and the position of the second end is L meters.

When the pulsed light propagates through the optical fiber 3, raman scattering occurs, and forward scattered light advancing in the propagation direction and backward scattered light advancing in the return direction are generated, and the backward scattered light is input again to the optical splitter 22 through the optical switch 23. The backscattered light input into the spectroscope 22 is emitted to the filter 24, and long-wavelength light (stokes light) and short-wavelength light (anti-stokes light) are extracted from the backscattered light. The detectors 25a and 25b convert the anti-stokes light and the stokes light into electrical signals, respectively, and send the electrical signals to the temperature detector 42 and the corrector 43. The corrector 43 corrects the anti-stokes light signal and the temperature measurer 42 measures the temperature using the stokes light signal and the anti-stokes light signal.

The temperature detector 42 measures the temperature of each position of the optical fiber 3 according to the following formula (1), and when the ratio of the two components is used, the difference between the two weak components is enhanced and comes to practical value. The gain and compensation depend on the design of the optical fiber 3 and are therefore pre-calibrated.

Gain/[ offset-2 x ln (anti-stokes/stokes) light (1)

When the incident position of the optical switch 23 to the optical fiber 3 is fixed at one of the first end or the second end, the temperature measurement can be achieved using the above formula (1). When the incident position is alternately switched to the first end and the second end with a constant period, the anti-stokes optical signal and the stokes optical signal are averaged (average value is calculated) with respect to the position of the optical fiber 3, and thus this method is called double-ended demodulation. The double-ended demodulation changes the above equation (1) to the following equation (2), i.e., averages the anti-stokes optical signal and the stokes optical signal at each position of the optical fiber 3 using the above equation (1).

Gain/[ offset-2 x ln (average anti-stokes light intensity/average stokes light intensity) ] (2)

If 01S denotes a stokes light signal in the case where pulsed light is input to the first end (0 to L meters), 01A denotes an anti-stokes light signal in the case where pulsed light is input to the first end (0 to L meters), 02S 'denotes a stokes light signal in the case where pulsed light is input to the second end (L to 0 meters), 02A' denotes an anti-stokes light signal in the case where pulsed light is input to the second end (L to 0 meters), and 02S 'and 02A' are inverted with respect to the elapsed time, 02S and 02A are obtained. The positions can be unified through inversion, and the influence of the optical fiber loss on the measured temperature can be eliminated by using double-end demodulation.

Next, the relationship between the length of the temperature measuring optical fiber and the demodulation temperature is explained, and FIG. 2 shows a case where a part of the optical fiber 3 is immersed in hot water of 55 ℃ when the room temperature is 24 ℃. When the length of immersion in hot water is extended from 0.5m to 10.5m, the maximum temperature becomes 55 ℃, which is the same as the temperature when the length of immersion in hot water is 2m or more. Therefore, in order to accurately measure the temperature, it is preferable to extend the length of the temperature measuring fiber. The sensitivity of the thermometric system at this time can be expressed by the following equation (3).

Sensitivity (peak temperature of hot water soak site-room temperature measured with fiber before and after soak site)/applied temperature 100%

(3)

Fig. 3 shows the results obtained from fig. 2 and equation (3), where the curves show a slight overshoot, because the impulse response of the system is not gaussian, but the waveform of the impulse response has a negative component close to the sinc function and a high order peak. The minimum length for which the sensitivity is 100% or considered 100% is called the minimum heating length.

As can be seen from fig. 2, the temperature profile of the fiber immersed in the hot water section can be seen as a single square wave into which the impulse response is convoluted, and fig. 4 shows a typical example of the calculated impulse response. When using backward raman scattered light for temperature measurement, the impulse response can be viewed as a wave form in which a window function is applied to the sinc function in order to smoothly attenuate the optical signal away from the center. When the impulse response is convolved into a temperature distribution along the fiber 3, an approximately accurate output prediction can be achieved.

Fig. 5 to 7 show that the output waveform can be predicted approximately accurately as found from the comparison between the estimated output waveform and the actually obtained output waveform with respect to the impulse response for each immersion length in the hot water. When the immersion length in hot water is 3.25m, the peaks are smoothed due to interference of convolutions of the impulse response.

Fig. 8 shows an output waveform in the case where a center portion to which no high temperature is applied is provided between two high temperature application portions of 0.2m, and the width of the center portion is gradually changed. The peak temperature is normalized to 1 and the reference temperature is normalized to 0. When the length of the central portion is 1.2 m to 1.6 m, two high temperature application portions can be considered. This is because of the interference caused by the amplification of the impulse response waveform. When the distance between the two high temperature application portions is the half width of the impulse response of fig. 8 or more, it can be considered that there are two high temperature application portions. Preferably, the distance is equal to half the value of the zeroth order width at which the gradient reverses or is greater to ensure that the two portions are significantly spaced from each other. As can be seen from fig. 8, when the lowest temperature of the central non-heating portion is equal to the reference temperature, the distance between the two high temperature-applied portions is greater than the main peak width and approximately equal to the main component width, i.e., the disturbance of the impulse response waveform can be ignored in fig. 8.

In order to accurately measure the temperature, it is preferable to focus on the temperature variation in a range in which the center position is the current processing position and the width is equal to or greater than the zero-order width and equal to or less than the width of the principal component. The pulsed light propagates while being gradually broadened and gradually attenuated due to influences such as broadening of wavelength, incident angle, scattering, and the like, and therefore, it is preferable to average values of the near end, the center, and the far end.

For measuring the temperature with higher accuracy, it is preferable that the plurality of portions are determined so that the range of difference between the convolution of the impulse response, which is calculated and stored at the center position of each region, and the output data shown in fig. 5 to 7 is not a problem, and is considered to be the same. The form of the impulse response wave slightly changes over time due to the degradation of the laser light or the like. Therefore, in a constant cycle, the impulse response is preferably calibrated at the same position as the initially obtained position for a more accurate measurement of the temperature.

For use in a wider field, it is preferable to improve the measurement accuracy by post-processing without expensive light sources, filters, circuits, and the like. A band pass filter that cuts off unnecessary signal bands or an adaptive filter that extracts an effective signal band based on a designed noise model may be used as post-processing for noise reduction. But the frequency band of the noise overlaps with the frequency band of the signal component, that is, in any filtering process, the signal component is attenuated with the suppression of the noise, and even if the filtering process is performed in order to suppress the reduction of the measurement accuracy, the signal component itself is attenuated, and the noise of the anti-stokes optical signal becomes particularly large at the far end. That is, it is preferable to focus on a method of reducing the noise of the far-end anti-stokes optical signal in order to reduce the noise. Therefore, the present embodiment uses double-ended demodulation and the relationship between the fluctuation of the stokes light signal and the fluctuation of the anti-stokes light signal.

When the temperature changes, the stokes light signal and the anti-stokes light signal also synchronously change along with the change of the temperature. That is, when the synchronization range is specified, it is considered that the temperatures of the other ranges do not change with respect to the next position of the optical fiber on the light source side, or only the temperature gradient changes smoothly. Thus, the minimum heating length described with reference to fig. 2 to 4 can be focused on. It is believed that the minimum heating length response is approximately the same within a given fiber segment as measured by temperature measurements taken by detecting backscattered raman light. When a portion of the optical fiber of the minimum heating length is heated beyond the region where the temperature remains constant, a waveform substantially identical to the impulse response of fig. 2 is obtained. As described above, as the range having an influence on the surroundings, it is preferable to focus on a range having a width equal to or larger than the zeroth-order width and equal to or smaller than the width of the principal component having an amplitude that is approximately attenuated to zero.

Fig. 9 shows a flow chart executed when the distributed optical fiber temperature measurement system 1 based on double-ended demodulation measures temperature. According to the measurement accuracy distribution of the optical fiber 3 in the double-ended demodulation method, the optical fiber 3 is equally divided into a first region, a central region, and a third region. In the first region, the measurement accuracy is low near 0 meter (first end); the second region is a central portion; in the third region, the measurement accuracy near L meters (second end) is low. The corrector 43 executes the flowchart of fig. 7 for three regions.

First, the corrector 43 determines whether the currently processed region of the optical fiber 3 is the first region of 1/3 equal to or smaller than the total length of the optical fiber 3 (step S1). When the determination in step S1 is yes, the corrector 43 calculates, for each sampling point, a large value of the correlation in a predetermined range having a width equal to or larger than the zeroth-order width of the response waveform of the minimum heating length and equal to or smaller than the principal component width of the response waveform of the minimum heating length.

Next, the corrector 43 targets 02S and 02A as objects to be processed, and 01S and 01A as objects, and calculates four correlated large values. For example, the corrector 43 selects the smaller one of the correlation between 02S and 01S and the correlation between 02S and 01A as the large value α _02S (step S2). Next, the corrector 43 selects the smaller one of the correlation between 02A and 01S and the correlation between 02A and 01A as the large value α _02A (step S3).

Next, the corrector 43 sets the average region such that the larger each correlation is, the smaller the average range of the center position of its current processing region is. For example, the corrector 43 rounds 1/α _02A and 1/α _02S to the nearest integers, and determines each integer of samples as one side of each average range (step S4). Next, the corrector 43 averages 02S and 02A within the average range determined for each sampling point. Thereafter, the corrector 43 averages 01S and 02S at each sampling point, uses the average value as a stokes component, averages 02A and 01A at each sampling point, uses the average value as an anti-stokes component, and calculates the temperature (step S5). 01S and 01A are targets. Therefore, 01S and 01A do not switch.

When the determination in step S1 is "no", the corrector 43 determines whether the currently processed region of the optical fiber is the third region equal to or larger than 2/3 of the total length of the optical fiber 3 (step S6). In the third region, 02S corresponds to 01S, and 02A corresponds to 01A. Therefore, when it is determined as "yes" in step S6, the corrector 43 replaces 02S with 01S, 01S with 02S, 02A with 01A, and 01A with 02A. The corrector 43 performs the same process as steps S2 to S4 (step S7). Therefore, the corrector 43 selects the smaller of the two calculated correlations relating to 01S as the large value α _01S, and selects the smaller of the two calculated correlations relating to 01A as the large value α _ 01A. The corrector 43 determines each integer of the samples as one side of each average range by rounding 1/α _01A and 1/α _01S to the nearest integer. Next, the corrector 43 averages 01S and 01A within each averaging range determined for each sampling point. Thereafter, corrector 43 averages 01S and 02S at each sampling point, uses the average value as a stokes component, averages 02A and 01A at each sampling point, uses the average value as an anti-stokes component, and calculates the temperature. 02S and 02A are targets. Therefore, 02S and 02A do not switch. When the determination in step S6 is no, the corrector 43 averages 01S and 02S at each sampling point, uses the average value as the stokes component, averages 02A and 01A, uses the average value at the anti-stokes component, and calculates the temperature (step S8). That is, in the regions 1/3 through 2/3, conventional double-ended demodulation is used as the temperature calculation method without performing averaging.

The method carries out relative weighting on the values with small noise and high reliability, and eliminates the noise. Although one of the smaller values in step S2 and step S3 is used, a larger value may be used. Alternatively, an average of the values of step S2 and step S3 may be used. When the smaller one is used, the data other than the portion regarded as the temperature change is equalized. When a larger one is used, even data that is hidden in a slight change in noise is not deleted as much as possible.

There are many ways to determine the correlation. For example, Pearson product-moment correlation coefficients may be used. The pearson product-moment correlation coefficients 02A and 01S are represented by the following formula (4).

Correlation coefficient α ═ (covariance of 02A, the range of which is the same as the specified range of 01S covariance)/(standard deviation of 01S of the same range)/(standard deviation of 02A of the same range) (4)

The correlation coefficient of Pearson's product-moment centered at the sampling point k of the optical fiber 3 is α [ k ]. An array of 01S is 01S [ k ]. An array of 02A is 02A [ k ]. The number of samples in the specified range is n, and the average 01S [ k ] of the specified range is 01 Save. The average value 02A [ k ] for the specified range is 02 Aave. The above formula (4) can be represented by formula (5).

As another example, when the modified spearman rank correlation coefficient is used, the n numbers of 01S and 02A (n in the above equation (5)) in the specified range are sorted, and the pearson product-moment correlation coefficient is used for sorting. When there are two or more of the same ranks, a compensation formula is used. However, for the stokes component and the anti-stokes component, there are generally few cases where two or more have the same rank. Thus, the one that appears previously may be considered a higher level.

For example, in the portion shown in the impulse response of fig. 2, 3.6m is set as a range satisfying the above condition. Fig. 10 shows a comparison between pearson product-moment correlation coefficients and spearman rank correlation coefficients for a set of experimental data. In general, a Pearson product-moment correlation coefficient of 1 or-1 indicates complete correlation. The absolute value of the pearson product-moment correlation coefficient is 0.4 or more than 0.4 and less than 0.7, indicating that the correlation is high. The absolute value of the pearson product-moment correlation coefficient is 0.2 or more and less than 0.4, indicating that the correlation is low. A pearson product-moment correlation coefficient with an absolute value less than 0.2 indicates no correlation. However, although the gradient of spearman is much greater than that of pearson, an approximate ratio of 1:1 is achieved in a range less than 0.2 indicating no correlation and in a range 0.3 or more indicating low correlation.

The inverse of the large value of the correlation coefficient in fig. 9 is an index of the average range, however, the inverse is not always required to be used. The larger the correlation coefficient, the narrower the average range is. And the smaller the correlation coefficient, the wider the average range is.

Fig. 11 shows another example of the flowchart executed when the temperature measured by the temperature detector 42 is corrected by the corrector 43. As shown in fig. 11, the corrector 43 performs steps S11 to S13 identical to steps S1 to S3 of fig. 9. Next, the corrector 43 executes steps S14 to S18 for the correlation large value α _02S and the correlation large value α _ 02A. Detailed descriptions of steps S14 to S18 will be given. The correlation large value α _02s and the correlation large value α _02a are shortened to "α _".

The corrector 43 determines whether the correlation large value α _ is equal to or smaller than the first threshold value (0.2 or less) (step S14). When the determination in step S14 is yes, the temperature detector 42 expands the average range of the currently processed sample points to the upper limit value (step S15). For example, the upper limit value may be 6 samples as one side or 11 samples as the total. When the determination in step S14 is no, the corrector 43 determines whether the correlation coefficient α _ is equal to or smaller than a second threshold value (e.g., 0.55) which is larger than the first threshold value of step S14 (step S16). When the determination in step S16 is yes, the corrector 43 determines the average range as "1" (step S17). When the determination in step S16 is "NO", the temperature detector 42 calculates an integer by rounding to 1/α _ n. The nearest integer is taken and the integer of the sample is determined as one side of the average range (step S18). After step S15, step S17, or step S18 is performed, step S19 which is the same as step S5 of fig. 9 is performed. In this process, when the correlation coefficient is equal to or greater than the second threshold value, averaging is not performed.

When the determination in step S11 is no, steps S20 to S22, which are the same as steps S6 to S8 of fig. 9, are performed. That is, when it is determined to be yes in step S20, the average range with respect to the correlation large value α _01S and the correlation large value α _01A is determined using the first threshold value and the second threshold value, and the temperature is calculated. When the determination in step S20 is no, the temperature is calculated using a conventional double-ended method.

Fig. 12 shows another flowchart executed when the temperature measured by the temperature detector 42 is corrected by the corrector 43. Differences between fig. 12 and fig. 11 will be described. First, the optical fiber 3 is divided into a first portion near 0 m (first end) where the measurement accuracy is low, a central third portion where the measurement accuracy is low, a fifth portion near L m (second end), a second portion between the first portion and the third portion, and a fourth portion between the third portion and the fifth portion, on the average, based on the measurement accuracy distribution in the longitudinal direction of the optical fiber 3. The corrector 43 executes the flowchart of fig. 12 for five sections.

The corrector 43 determines whether the currently processed portion of the optical fiber 3 is the first portion of 1/5 equal to or less than the total length of the optical fiber 30 (step S31). When the determination in step S31 is yes, the corrector 43 performs steps S32 to S39, which are the same as steps S12 to S19.

When the determination in step S31 is "no", the corrector 43 determines whether the currently processed portion of the optical fiber 3 is equal to or greater than 4/5 of the total length of the optical fiber 3 (step S40). When it is determined as "yes" in step S40, after replacing 02S with 01S, 01S with 02S, 02A with 01A, and 01A with 02A, the same processing as in steps S32 to S39 is performed (step S41). However, as for the upper limit values of the correlation coefficient and the average range, other values may be used as the threshold values.

When the determination in step S40 is "no", the corrector 43 determines whether the currently processed section is equal to or smaller than 2/5 of the total length of the optical fiber 3 or equal to or larger than 3/5 of the total length of the optical fiber 3 (step S42). When it is determined as "yes" in step S42 and the currently processed section is equal to or smaller than 2/5 of the total length of the optical fiber 3, the same processing as steps S32 to S39 is performed. However, the threshold values of the correlation coefficient and the upper limit value with respect to the average range are different from the threshold values of step S32 to step S39. When it is determined as "yes" in step S42 and the currently focused section is equal to or larger than 3/5 of the total length of the optical fiber 3, the same processing as step S41 is performed. However, the threshold value and the upper limit of the average range regarding the correlation coefficient are different from the threshold value of step S41. Therefore, the minimum heating length according to the sectional change can be corrected. When the determination in step S42 is no, the same step S44 as step S22 is performed.

In fig. 9, 11 and 12, the average range is equal to or less than the principal component width of the minimum heating length. This is because when the averaging range exceeds the principal component width of the minimum heating length, the probability that crosstalk adjacent to another signal affects the averaged signal becomes higher. For example, when the obtained correlation coefficient is-1, it is determined that complete correlation has occurred. However, in the present embodiment, the correlation is regarded as noise. The reason is as follows: the Stokes and anti-Stokes curves are convex upward as the temperature increases. The Stokes and anti-Stokes curves are convex downward as the temperature decreases. In this case, the anti-stokes curve never has an upside-down shape of the stokes curve. The anti-stokes curve may have a shape in which the stokes curve is inverted upside down only when noise occurs or a connector with poor connection or poor fusion, or optical fibers having a large difference in refractive index are fused to each other.

In fig. 9, 11 and 12, the optical fiber is divided into three or five parts. However, the optical fiber may be divided into two portions at one center, or four portions obtained by dividing the two portions, or eight portions obtained by dividing the four portions. In these cases, only the central portion where the temperature is calculated without processing 01S, 01A, 02S, and 02A is deleted.

In an embodiment, double ended demodulation is used. When pulsed light is input to the second end, the 02S is averaged in accordance with at least one of the correlation coefficients of 02S and 01S and the correlation coefficients of 02S and 01A within a predetermined range of sampling points including a partial range of the first end side. In this case, the noise of 02S is reduced. Further, the 02A is averaged within an averaging range according to at least one of the correlation coefficients of 02A and 01S and the correlation coefficients of 02A and 01A, in which case the noise of 02A is reduced. When the average 02S and the average 02A are used, the measured temperature can be corrected. When pulsed light is input to the first end, 01S is averaged within an average range within a predetermined range including sampling points of a partial range of the second end side according to at least one of correlation coefficients of 01S and 02S and correlation coefficients of 01S and 02A. In this case, the noise of 01S is reduced. Further, 01A is averaged within an averaging range according to at least one of the correlation coefficients of 01A and 02S and the correlation coefficients of 01A and 02A. In this case, the noise of 01A is reduced. When the average 01S and the average 01A are used, the measured temperature can be corrected. When averaging 02A, it is preferred that 01S and 01A are the components just before 02A, relative to the switching of the optical switch 23. When averaging 01A, it is preferable that 02S and 02A are the components before 01A in terms of switching of the optical switch 23.

In the present embodiment, the average value of the Stokes optical signal and the anti-Stokes optical signal is calculated within the average range. However, when the variability of the data within the average range is suppressed, it is not necessary to calculate the average value. Thus, another average may be used, for example an arithmetic average, a geometric average or a harmonic average taking into account the w-weights. When calculating the average values of 01S and 02S and the average values of 01A and 02A, an arithmetic average value considering the weights may be used.

The averaging may be performed after smoothing the Stokes light signal and the anti-Stokes light signal within a smoothing range according to a correlation large value of the Stokes light signal and the anti-Stokes light signal within a predetermined range including predetermined sampling points. In this case, the measured temperature can be corrected. For example, when the correlation is small, the noise becomes large, and therefore it is preferable to smooth the two optical signals. In this case, noise can be reduced. Preferably, the smaller the correlation, the longer the smoothing range. In this case, the noise is reduced more. Preferably, the upper limit of the smoothing range is determined. In this case, redundancy of the smoothing range is suppressed, suppressing a decrease in temperature measurement accuracy. When the correlation is small, the temperature variation around the sampling point is small, and therefore, both the smoothing processing and the reduction in the accuracy of measuring the temperature are suppressed. On the other hand, when the correlation is large, the smoothing range is shortened, or no correction is performed. When the correlation is large, the temperature around the sampling point changes greatly, so the influence of noise is small, and the accuracy of measuring the temperature can be maintained. However, in the processes of fig. 9, 11, and 12, when the correlation coefficient between anti-stokes optical signals is high, it is preferable not to perform smoothing. This is because the anti-stokes optical signal is small and detection of the anti-stokes component becomes difficult due to the smoothing process. This process is explained based on fig. 13.

Fig. 13 shows another example of a flowchart executed when the distributed optical fiber thermometry system 1 measures temperature. In step S3 of fig. 9, the corrector 43 determines whether the correlation coefficient between 01A and 02A is equal to or smaller than a threshold (S51). When the determination in step S51 is "no", the correlation between 01A and 02A is high. Thus, the flowchart terminates, the smoothing of the anti-stokes light signal is not performed and the accuracy of fig. 9, 11 and 12 is maintained.

When the determination in step S51 is yes, the corrector 43 calculates a correlation large value α between the Stokes optical signal and the anti-Stokes optical signal within a predetermined range including the sampling points, the width of which is equal to or larger than the zeroth-order width of the minimum heating length response waveform and equal to or smaller than the principal component width of the minimum heating length response waveform with respect to each sampling point (step S52). The sampling points are all target points for measuring the temperature in the longitudinal direction of the optical fiber 3.

Next, the corrector 43 determines whether the correlation coefficient α is equal to or smaller than a threshold value (e.g., 0.2 or less) (step S53). When the determination in step S53 is yes, the corrector 43 expands the smooth range of the current processing sample point to the upper limit (step S54). The upper limit may be 11 samples, where one side may be 6 samples with respect to the sample point. When determined as "no" in step S53, the corrector 43 calculates an integer by rounding 1/α to the nearest integer, and determines the number of samples on the side of the smoothing range to the calculated integer (step S55). After performing step S54 or step S55, the corrector 43 smoothes the Stokes optical signal and the anti-Stokes optical signal, respectively, within the determined smoothing range (step S56). Specifically, both 01A and 01S are smoothed within a smoothing range determined according to the large correlation coefficient values of 01A and 01S, both 02A and 02S are smoothed within a smoothing range determined according to the large correlation coefficient values of 02A and 02S, and when the correlation coefficient α is 1 or close to 1, the smoothing range is 1 and no smoothing is performed. When the correlation coefficients of the anti-stokes light signals of the other regions of fig. 9, 11, and 12 are equal to or less than the threshold value, the averaging process of fig. 13 is not necessarily performed.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any person skilled in the art can make any simple modification, equivalent change and modification to the above embodiments according to the technical spirit of the present invention without departing from the scope of the present invention.

Claims (8)

1. The utility model provides a distributed optical fiber temperature measurement system based on bi-polar demodulation, includes optical structure, processing module and optic fibre, its characterized in that: the optical structure is configured to detect a first stokes light signal and a first anti-stokes light signal from backscattered light generated when light is input to the first end of the optical fiber, and to detect a second stokes light signal and a second anti-stokes light signal from backscattered light generated when light is input to the second end of the optical fiber; the processing module is configured to calculate, within a region including the first end of the optical fiber, a first region length based on a first correlation between the second stokes light signal and at least one of the first stokes light signal and the first anti-stokes light signal, calculate a second region length based on a second correlation between the second anti-stokes light signal and at least one of the first stokes light signal and the first anti-stokes light signal, smooth the second stokes light signal in the first region, smooth the second anti-stokes light signal in the second region, calculate a temperature of the sampling point using the smoothed second stokes light signal, the smoothed second anti-stokes light signal, the first stokes light signal, and the first anti-stokes light signal.

2. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: pulsed light is alternately input to the first and second ends of the optical fiber through the optical switch.

3. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: the first region is elongated as the first correlation becomes smaller, and the second region is elongated as the second correlation becomes smaller.

4. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 3, wherein: the length of the first region and the second region sets an upper limit.

5. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: when the first correlation is equal to or greater than the threshold, the second stokes light signal is not smooth; when the second correlation is equal to or greater than another threshold, the second anti-stokes light signal is not smooth.

6. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: the pearson product-moment correlation coefficient is applied to the first correlation and the second correlation.

7. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: a spearman rank correlation coefficient is applied to the first correlation and the second correlation.

8. The distributed optical fiber temperature measurement system based on double-end demodulation as claimed in claim 1, wherein: the fiber has a constant temperature in a region around the sampling point; the area around the sampling point is larger than a zeroth-order width of a temperature distribution obtained when another constant temperature different from the constant temperature is given to a minimum heating length portion centered on the sampling point, and is smaller than a principal component width of the temperature distribution.

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