JP2006071532A - Optical fiber temperature distribution sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber temperature distribution sensor for accurately measuring temperature distribution even when a temperature difference in the entire length of an optical fiber (especially, a temperature difference between a measuring instrument installation environment and a measuring object position) is great. <P>SOLUTION: This optical fiber temperature distribution sensor is used for measuring temperature distribution along the optical fiber 1 based on an intensity ratio Ia/Is between stokes light and antistokes light generated in the optical fiber 1. Two reference temperatures are measured with thermometers 11 and 12 different from the relevant sensor. A reading value of temperature based on the intensity ratio Ia/Is is corrected so that the temperatures found from the intensity ratio Ia/Is agree with the two reference temperatures, respectively. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber temperature distribution sensor using Raman backscattered light, and can accurately measure the temperature distribution even when the temperature difference in the entire length of the optical fiber (especially the temperature difference between the measurement device installation environment and the measurement target location) is large. The present invention relates to an optical fiber temperature distribution sensor.

  An optical fiber temperature distribution sensor that uses an optical fiber and measures a temperature distribution along the optical fiber is described in Patent Document 1.

  As shown in FIG. 3, the optical fiber temperature distribution sensor is formed by connecting one end of a sensor optical fiber (hereinafter simply referred to as an optical fiber including a relay optical fiber) 1 to a measuring device 2. The other end (not shown) of the optical fiber 1 is routed to the location to be measured. The measuring apparatus 2 includes a light source 4 that emits light modulated by a predetermined method, an optical demultiplexer 5 that guides the light to the optical fiber 1 and separates backscattered light returned from the optical fiber 1, Of the separated backscattered light, an O / E converter 6s that receives Stokes light, an O / E converter 6a that receives anti-Stokes light, and the average of the received light signals in synchronization with the modulation in the light source 4 Averaging circuits 7s and 7a, a synchronizing circuit 8 for synchronizing them, and a temperature distribution calculating unit 13 for calculating the temperature distribution based on the respective average outputs.

  The light of wavelength λ0 emitted from the light source 4 enters the optical fiber 1 via the relay optical fiber 81 and the optical demultiplexer 5. Raman scattered light (Stokes light of wavelength λs and anti-Stokes light of wavelength λa) is generated by incident light having a wavelength λ0 at various locations in the optical fiber 1. Of the generated Raman scattered light, the back scattered light returning to the incident side is demultiplexed into Stokes light of wavelength λs and anti-Stokes light of wavelength λa by the optical demultiplexer 5, and the respective lights are relayed optical fiber 82. , 83 are led to O / E converters 6s, 6a. The electric signals converted and amplified by the respective O / E converters 6s and 6a have time waveforms as shown in FIGS.

  4 and 5, the horizontal axis is a time axis based on the light emission timing of the light source, and the vertical axis is the output intensity of the O / E converters 6 s and 6 a, that is, the signal intensity of Stokes light and anti-Stokes light. It is. Actually, the signal strengths of the Stokes light and the anti-Stokes light are very weak, so that a waveform having a good S / N as shown in FIGS. 4 and 5 cannot be obtained in one measurement, and the averaging circuit 7s in the subsequent stage is not obtained. 7a, the waveforms shown in FIGS. 4 and 5 are obtained by performing the averaging process in synchronization with a plurality of measurements.

  4 and 5 represent the signal intensities of the Stokes light and the anti-Stokes light received by the O / E converters 6s and 6a as a function of time with reference to the light emission timing of the light source 4. Since the speed of light in the optical fiber 1 is known, it can be replaced with a function of the distance along the optical fiber 1 with respect to the light source 4. Therefore, in FIGS. 4 and 5, the horizontal axis is distance, and the intensity of Stokes light and anti-Stokes light generated at each distance point of the optical fiber, that is, distance distribution can be obtained.

  On the other hand, the Stokes light intensity Ia and the anti-Stokes light intensity Is both depend on the temperature of the optical fiber 1, and the intensity ratio Ia / Is of both lights also depends on the temperature of the optical fiber 1 as shown in the equation (1). To do.

  The coefficients K0, n0, and nk in Equation (1) are determined by the properties of the optical fiber and measurement device components, such as the composition of the optical fiber, the center wavelength of the light source, the wavelength distribution, and the conversion efficiency of the O / E converter. Therefore, if the intensity ratio Ia / Is is known, the temperature at the location where the Raman scattered light is generated can be known.

  Although the expression (1) excludes the distance x, since the intensity ratio Ia / Is is a function of distance Ia (x) / Is (x), light from this intensity ratio Ia (x) / Is (x) A temperature distribution T (x) along the fiber 1 can be determined.

  FIG. 6 shows a temperature distribution T (x) corresponding to the waveforms of FIGS. As shown, high and low peaks appear in the temperature distribution corresponding to the high and low peaks found in the intensity distributions of the Stokes light and the anti-Stokes light. Actually, since the loss when Stokes light and anti-Stokes light propagate through the optical fiber is slightly different, the loss is corrected.

Japanese Patent No. 2136071 (Japanese Patent Publication No. 8-23512) Japanese Patent No. 2977373

  The above-described optical fiber temperature distribution sensor also has characteristics that change with time in the environment in which the optical fiber and the measuring device are placed and the components, so it is necessary to correct temperature measurement errors due to this change.

  In the example of FIG. 3, the reference temperature unit 10 is provided in the measuring device 2, and the temperature of the reference temperature unit 10 is measured by a thermometer 11 provided separately by a method different from the optical fiber every time the temperature distribution is measured. The coefficient in the equation (1) is determined so that the temperature of the reference temperature unit 10 obtained from the ratio Ia / Is matches the temperature (referred to as the reference temperature) measured by the thermometer 11 provided separately, and other coefficients in the optical fiber 1 are determined. The temperature distribution is obtained by applying the coefficient to the temperature calculation at the location.

  Specifically, as shown in FIG. 7, the intensity ratio Ia / Is is a function of temperature, and the slope of the curve representing the function varies depending on the coefficients K0, n0, and nk in Equation (1). It is. These coefficients are determined by the properties of the optical fiber and measurement device components such as the composition of the optical fiber, the center wavelength of the light source, the wavelength distribution, and the conversion efficiency of the O / E converter. If the characteristics of 2 are determined, they are uniquely determined. Since some of these coefficients change depending on the ambient temperature, the reference temperature unit 10 is provided in the measuring apparatus 2 as described above so that correction can be performed.

  However, even when the coefficient in the equation (1) is determined by the above-described method, the coefficient has some error with respect to the true value. It has been found that the accuracy of the temperature obtained by the equation (1) is reduced by such a coefficient setting error. In FIG. 7, the characteristic of the intensity ratio Ia / Is obtained by substituting the temperature into the equation (1) when the coefficient is a true value is indicated by a solid line, and when the coefficient has a setting error, the equation (1) Two characteristics of the intensity ratio Ia / Is obtained by substituting the temperature for are indicated by broken lines. As can be seen from the figure, when there is a setting error in the coefficient, the intensity ratio Ia / Is is close to that of the true value in any characteristic in the vicinity of the reference temperature. However, at a temperature away from the reference temperature, the intensity ratio Ia / Is is far away from that of the true value.

  Conversely, even when the temperature is obtained by substituting the intensity ratio Ia / Is obtained by light measurement into the equation (1), there is an error based on the coefficient setting error with respect to the actual temperature. is there. In other words, even if the temperature in the vicinity of the reference temperature is close to the reference temperature, measurement at a high or low temperature far from the reference temperature is accompanied by a large error.

  Applications of this type of optical fiber temperature distribution sensor include measuring the temperature distribution of a manufacturing facility that is at a high temperature, and measuring the temperature distribution of a road in order to investigate freezing of the road surface. However, in general, the installation environment of the measuring device 2 (one end of the optical fiber 1 is also included here) is about room temperature, whereas in the above-described application, the temperature of the measurement target location is high or low. There is. That is, the conventional method in which the reference temperature is given in the installation environment of the measuring apparatus has a drawback that the temperature at the important measurement target portion becomes inaccurate.

  Accordingly, an object of the present invention is to solve the above-mentioned problems, and an optical fiber temperature distribution sensor capable of measuring a temperature distribution with high accuracy even when a temperature difference in the entire length of the optical fiber (especially a temperature difference between a measuring device installation environment and a measurement target location) is large. Is to provide.

  To achieve the above object, the present invention provides an optical fiber temperature distribution sensor for measuring a temperature distribution along an optical fiber based on an intensity ratio Ia / Is of Stokes light and anti-Stokes light generated in the optical fiber. Two reference temperatures are measured by a thermometer different from the sensor, and the temperature readings based on the intensity ratio Ia / Is are corrected so that the temperatures obtained from the intensity ratios Ia / Is match with each other. It is what I did.

  The two reference temperatures may be measured by separate thermometers.

  A first temperature at which a first reference temperature is measured in the vicinity of the end of the optical fiber connected to the measuring device for measuring the intensity of both lights or in an optical fiber in the measuring device connected in series with the optical fiber. A second thermometer that measures the second reference temperature may be installed on the extension of the optical fiber.

  The at least one reference temperature may be within a temperature range to be measured.

  At least one thermometer may be installed at a measurement target location.

  Regarding the relational expression between the intensity ratio Ia / Is and the temperature, a coefficient is determined so that the calculated value at the location where the first thermometer is installed matches the first reference temperature Tr1 measured by the first thermometer. The second reference temperature T0r2 and the first reference temperature T0r2 measured by the second thermometer with respect to the difference (Tr2-Tr1) between the temperature Tr2 and the first reference temperature Tr1 at the location where the second thermometer is calculated The ratio of the difference (T0r2-Tr1) from the reference temperature Tr1 is applied to the difference between the temperature T1 and the first reference temperature Tr1 at an arbitrary position calculated by the above relational expression, and the first reference temperature Tr1 is added thereto. Thus, the corrected temperature T1 ′ may be obtained.

  The present invention exhibits the following excellent effects.

  (1) The temperature distribution can be accurately measured even when the temperature difference in the entire length of the optical fiber is large.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  First, the configuration of the optical fiber temperature distribution sensor will be described in detail.

  As shown in FIG. 2, the optical fiber temperature distribution sensor according to the present invention includes one end of an optical fiber 1 connected to a measuring device 2 and a temperature reference unit 3 provided in the middle of the optical fiber 1. The measuring apparatus 2 includes a light source 4 that emits light modulated by a predetermined method, an optical demultiplexer 5 that guides the light to the optical fiber 1 and separates backscattered light returned from the optical fiber 1, Of the separated backscattered light, an O / E converter 6s that receives Stokes light, an O / E converter 6a that receives anti-Stokes light, and the average of the received light signals in synchronization with the modulation in the light source 4 Averaging circuits 7s and 7a, a synchronizing circuit 8 for synchronizing the averaging circuits, a temperature distribution calculating unit 9 for calculating the temperature distribution based on the respective average outputs, and the optical fiber 1 in the vicinity of the measuring device 2 And a reference temperature unit 10 for measuring temperature.

  As can be seen from comparison with FIG. 3, the feature of the present invention is that a temperature reference unit 3 is provided. The temperature reference unit 3 has substantially the same structure as the reference temperature unit 10, and measures the temperature of the environment (temperature of the optical fiber) with a thermometer provided separately from the optical fiber 1. The thermometer of the reference temperature unit 10 is a first thermometer 11 that measures a first reference temperature, and the thermometer of the temperature reference unit 3 is a second thermometer 12 that measures a second reference temperature.

  Here, the temperature reference unit 3 will be described in detail. A known temperature sensor such as a platinum resistor, thermistor, thermocouple, or the like is used as the temperature sensor serving as the thermometer 12 in accordance with the desired measurement accuracy and usage environment. Can do. The temperature reference unit 3 accommodates a temperature sensor in a closed space such as a container or a box (not shown), and the optical fiber 1 is passed through the temperature reference unit 3. The length of the optical fiber 1 inserted into the temperature reference unit 3 is longer than the distance responsiveness, preferably 5 having a distance responsiveness, in consideration of the distance responsiveness (distance resolution) of temperature measurement by Raman backscattered light. Double it. At this time, it is necessary to reduce the size of the temperature reference portion 3 so that a temperature distribution (in this case, meaning a temperature difference) does not easily occur in the temperature reference portion 3. For this purpose, the optical fiber 1 is preferably housed in a loop.

  Further, the temperature sensor is placed in the immediate vicinity of the optical fiber 1 so that the temperature of the optical fiber 1 and the temperature of the temperature sensor become the same, but since there is a thermal resistance between them, the change in the ambient temperature of the temperature reference unit 3 If a temperature change occurs inside the temperature reference unit 3 due to the above, a temperature difference appears between the optical fiber 1 and the temperature sensor due to the thermal resistance. Therefore, it is preferable that the temperature reference unit 3 has a structure in which the inside is thermally insulated from the outside.

  The other members in FIG. 2 are the same as those shown in FIG. However, unlike the temperature distribution calculation unit 13 in FIG. 3, the temperature distribution calculation unit 9 obtains a temperature T1 ′ corrected by applying the formula (2) to the reading value T1 obtained by the formula (1). It is like that.

  Next, the temperature distribution calculation procedure will be described with reference to FIG.

  FIG. 1 shows the characteristic of the measurement result with respect to the actual temperature by taking the actual temperature on the horizontal axis and the temperature (measurement result) obtained from the intensity ratio Ia / Is on the vertical axis. A broken line is a reading value before correction, that is, a characteristic before the present invention is applied, and a solid line is a true value (measured value that matches the actual temperature), that is, a characteristic obtained by applying the present invention. In the drawing, the first reference temperature Tr1 is the same as the reference temperature measured by a thermometer different from the sensor in the reference temperature section in the measuring apparatus described so far as the background art. Since the coefficient is set so that the reading with respect to the first reference temperature Tr1 is Tr1, the same Tr1 is shown on the vertical axis.

  The prior art (state before the reading is corrected in the present invention) will be described using the characteristics of the broken line in this figure. The temperature obtained from the intensity ratio Ia / Is is a true value near the first reference temperature Tr1. Close to. However, the error from the true value increases as the distance from the first reference temperature Tr1 increases to the high temperature side or the low temperature side.

  Therefore, in the present invention, the second reference temperature Tr2 is used in addition to the first reference temperature Tr1. The temperature readings based on the intensity ratio Ia / Is are corrected so that the temperatures obtained from the intensity ratios Ia / Is match with respect to these two reference temperatures.

  As described above, the present invention is characterized in that the two reference temperatures are measured by means other than the optical fiber. According to this principle, it is only necessary to know two reference temperatures, and it does not matter where these reference temperatures are measured. However, in practice, a thermometer is installed somewhere to measure the reference temperature. The two reference temperatures can be measured at the same location at different times, but cannot be measured simultaneously at the same location. On the other hand, two reference temperatures can be measured simultaneously by installing thermometers at different locations. Therefore, as shown in FIG. 2, the reference temperatures may be measured by thermometers installed at different locations.

  The specific correction is performed as follows. First, as in the prior art, the first reference temperature at which the intensity ratio Ia / Is at the location where the first thermometer is installed is measured with the first thermometer in relation to the relational expression (1) between the intensity ratio Ia / Is and the temperature. A coefficient is determined so as to coincide with Tr1. Next, the difference (Tr2-Tr1) between the temperature Tr2 and the first reference temperature Tr1 at the installation location of the second thermometer calculated by the relational expression (1) is obtained. At the same time, the difference (T0r2-Tr1) between the second reference temperature T0r2 measured by the second thermometer and the first reference temperature Tr1 is obtained. The ratio of these two differences (with the former as the denominator) is multiplied by the difference between the temperature T1 and the first reference temperature Tr1 at an arbitrary location x calculated by the relational expression (1), and this is added to the first reference temperature Tr1. Is added to obtain a corrected temperature T1 ′. This procedure is expressed by the following equation (2).

  In this way, the measured value at the point x where the actual temperature is TA is corrected from the first reading value T1 to the correction value T1 ', and this measured value T1' is as close as possible to the actual temperature TA. As can be seen from FIG. 1, the error in the prior art is eliminated.

  The temperature Tr2 in the temperature reference unit 3 is a reading obtained by calculating the temperature of the optical fiber 1 in the temperature reference unit 3 from the intensity ratio Ia / Is. At this time, if the measurement range based on the distance resolution includes the outside of the temperature reference unit 3, the influence of the ambient temperature comes out, so the temperature near the center of the optical fiber length in the temperature reference unit 3 is read. Further, at that time, it is possible to read and use the temperature at only one place near the center. However, averaging the readings at a plurality of places near the center can avoid the measurement error and increase the measurement accuracy.

  The calculation formula (3) including the distance x is as follows.

  In order to obtain the temperature distribution, first, readings calculated from the intensity ratio Ia / Is are obtained for a plurality of locations of distance x. This is the temperature distribution before correction. If equation (3) is applied to this temperature distribution, a corrected temperature distribution is obtained.

  The first and second reference temperatures can be arbitrarily measured as long as they are different from each other. However, as described above, the first reference temperature is the temperature of the installation environment of the measuring device 2 (in many cases). It is preferable that the second reference temperature is the temperature (high temperature or low temperature) of the measurement target location. At that time, the second reference temperature may be within the temperature range to be measured. That is, referring to FIG. 1, when trying to measure a temperature range below the actual temperature TA, the second reference temperature T0r2 is lower than the actual temperature TA.

  Here, the temperature dependence of the intensity ratio Ia / Is is as shown in FIG. 8 in the range of the absolute temperature from 0 ° K to several hundred ° K. That is, the linearity is poor in a low temperature region and a high temperature region in which the optical fiber temperature distribution sensor is mainly applied in the present invention. Therefore, it is advantageous to provide the second reference temperature within the temperature range to be measured in order to perform the correction with the equation (2) with higher accuracy. However, when there are other constraints, the second reference temperature does not necessarily have to be within the measured temperature range.

  In addition, when the second reference temperature is provided within the measured temperature range, the temperature actually measured from the measurement target location can be set as the second reference temperature, so the second reference temperature outside the measured temperature range can be There is no need to prepare a thermostat or the like for providing separately.

  Furthermore, when the second reference temperature is provided within the measured temperature range, the measured temperature range changes with time (for example, the initial temperature is 100 to 200 ° C., and the subsequent temperature is 200 to 300 ° C.). This is convenient because the reference temperature automatically falls within the temperature range to be measured.

  The procedure for obtaining the temperature distribution may be to calculate the temperature distribution before correction by calculating all the temperatures at a plurality of locations, and then correct the temperature distribution before correction using the second reference temperature, or calculate the temperature at one location. Then, as soon as the pre-correction temperature is obtained, the temperature may be corrected using the second reference temperature, and the temperature distribution may be obtained by performing this at a plurality of locations.

  The present invention can employ a method of outputting and measuring one pulsed light from a light source during one measurement period, as in the technique of the patent document cited as the background art. Furthermore, the present invention may employ a method different from the above method, for example, a method in which a light source is continuously emitted in a continuous waveform for measurement. The present invention can be applied to all types of optical fiber temperature distribution sensors using optical fibers.

  As the sensor optical fiber, either a multimode optical fiber or a single mode optical fiber can be used.

  The present invention is particularly effective in the case where the temperature difference in the entire length of the optical fiber is large and the temperature of the measurement target portion is higher or lower than that in the installation environment of the measurement apparatus. As an example, a description will be given of road temperature distribution measurement for detecting freezing of a road surface. In road temperature distribution measurement, an optical fiber is buried in the ground near the road surface over a wide area of the road, and the temperature distribution of the road surface is measured over a wide range. At that time, it is important to measure with high accuracy a temperature around 0 ° C. at which freezing is likely to occur. Therefore, the temperature reference unit 3 is installed at a location where a specific temperature (0 ° C.) is expected to be measured on the road to be measured. As a result, the second reference temperature measured by the temperature reference unit 3 is about 0 ° C. In addition, in the season when the temperature is high, the second reference temperature measured by the temperature reference unit 3 is also higher than 0 ° C. However, there is no possibility of freezing of the road surface at this time, so there is no inconvenience. .

FIG. 3 is a characteristic diagram of actual temperature versus temperature measurement value representing the principle of the present invention. It is an apparatus block diagram of the optical fiber temperature distribution sensor which shows one Embodiment of this invention. It is an apparatus block diagram of the optical fiber temperature distribution sensor of background art. It is a time and distance characteristic view of Stokes light intensity. It is a time and distance characteristic view of anti-Stokes light intensity. FIG. 6 is a temperature distribution diagram obtained from the data of FIGS. 4 and 5. It is a characteristic view of temperature to intensity ratio Ia / Is in the optical fiber temperature distribution sensor of the background art. It is a characteristic view with respect to absolute temperature of temperature to intensity ratio Ia / Is.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Measuring apparatus 3 Temperature reference part 10 Standard temperature part 11 Another thermometer (1st thermometer)
12 Another thermometer (second thermometer)

Claims (6)

  An optical fiber temperature distribution sensor that measures a temperature distribution along an optical fiber based on an intensity ratio Ia / Is of Stokes light and anti-Stokes light generated in the optical fiber. An optical fiber temperature characterized in that a reference temperature is measured and a temperature reading based on the intensity ratio Ia / Is is corrected so that the temperatures obtained from the intensity ratios Ia / Is match each other for these two reference temperatures. Distribution sensor.   The optical fiber temperature distribution sensor according to claim 1, wherein the two reference temperatures are measured by different thermometers.   A first temperature at which a first reference temperature is measured in the vicinity of the end of the optical fiber connected to the measuring device for measuring the intensity of both lights or in an optical fiber in the measuring device connected in series with the optical fiber. The optical fiber temperature distribution sensor according to claim 1 or 2, wherein a second thermometer for measuring a second reference temperature is installed on an extension of the optical fiber.   4. The optical fiber temperature distribution sensor according to claim 1, wherein at least one reference temperature is within a temperature range to be measured.   The optical fiber temperature distribution sensor according to any one of claims 1 to 4, wherein at least one thermometer is installed at a location to be measured. Regarding the relational expression between the intensity ratio Ia / Is and the temperature, a coefficient is determined so that the calculated value at the location where the first thermometer is installed matches the first reference temperature Tr1 measured by the first thermometer. The second reference temperature T0r2 and the first reference temperature T0r2 measured by the second thermometer with respect to the difference (Tr2-Tr1) between the temperature Tr2 and the first reference temperature Tr1 at the location where the second thermometer is calculated The ratio of the difference (T0r2-Tr1) with respect to the reference temperature Tr1 is applied to the difference between the temperature T1 and the first reference temperature Tr1 at an arbitrary position calculated by the above relational expression, and the first reference temperature Tr1 is added thereto. The corrected temperature T1 ′ is obtained by the optical fiber temperature distribution sensor according to claim 1.

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