JP6314678B2 - Optical fiber temperature distribution measuring device - Google Patents

Description

  The present invention relates to an optical fiber temperature distribution device configured to measure a temperature distribution along an optical fiber using Raman backscattered light using an optical fiber on which an optical pulse is incident as a sensor.

  One type of distributed measurement device using an optical fiber as a sensor is an optical fiber temperature distribution measurement device configured to measure a temperature distribution along the optical fiber. This apparatus uses backscattered light generated in an optical fiber.

  Backscattered light includes Rayleigh scattered light, Brillouin scattered light, and Raman scattered light, but temperature-dependent back Raman scattered light is used for temperature measurement. Measure. The back Raman scattered light includes anti-Stokes light generated on the short wavelength side with respect to the wavelength of incident light and Stokes light generated on the long wavelength side.

  The optical fiber temperature distribution measuring device measures the intensity Ias of the anti-Stokes light and the intensity Ist of the stosk light, calculates the temperature from the intensity ratio, and displays the temperature distribution along the optical fiber. It is used in fields such as facility temperature management, disaster prevention research and research, and air conditioning in power plants and large buildings.

  Japanese Patent Application Laid-Open No. 2012-27001 discloses that a true temperature is measured using a reference thermometer arranged in the vicinity of an optical fiber, a calculated value at a position where the reference thermometer is arranged, and a true temperature T A technique is disclosed in which temperature calibration is performed at two temperatures by making two combinations with the above. Such temperature calibration can effectively suppress errors in the measured temperature.

JP 2012-27001 A

  The above two-point temperature calibration method is technically superior in that it reduces the measurement temperature error. However, in actual operation, there is a problem that a facility for calibration is required at the site where the optical fiber is laid, and a power source necessary for this facility may not be secured.

That is, in order to perform two-point temperature calibration, it is necessary to measure the temperature at two locations T ₁ and T ₂ , so even if one T ₁ is set as the fiber ambient temperature, the other T ₂ is It is necessary to create a predetermined temperature environment by heating with a heater or cooling with a cooler. In particular, if the two temperatures used for calibration are not close to the temperature of the object to be measured by the sensor optical fiber 3, the error of the measured value will not be reduced. For example, if the measurement target is around 200 ° C., it is desirable to calibrate with _one T ₁ at room temperature and the other T ₂ at around 200 ° C.

  However, it is not uncommon for the optical fiber laying site to be a desert or a remote polar area, and it is impossible to secure a power source required for a heater or a cooler. Alternatively, even if there is a power source at the installation location of the apparatus main body, it is not uncommon that the optical fiber serving as the sensor is located away from the apparatus main body and the power supply cannot be secured. For this reason, there are cases where the method using the two-point temperature calibration cannot be effectively used in actual operation.

  An object of the present invention is to provide an optical fiber temperature distribution measuring device capable of performing practical and effective temperature calibration.

The optical fiber temperature distribution device of the present invention uses an optical fiber on which an optical pulse is incident as a sensor, and measures the temperature distribution along the optical fiber using Raman backscattered light. In the distribution device, the reference temperature measuring means provided in the vicinity of the optical fiber, the anti-Stokes light and the Stokes light gain ratio fluctuation value at the connection part for making the optical pulse incident on the optical fiber, and the reference temperature measuring means Temperature calibration means for calibrating the measured temperature using a correction formula using the obtained temperature measurement data as a parameter.
According to this optical fiber temperature distribution device, the measured temperature is calibrated using the correction equation using the gain ratio fluctuation value of the anti-Stokes light and the Stokes light at the connection as a parameter. Temperature calibration can be performed.

  Loss determining means may be provided for determining the gain ratio variation value from measurement data of anti-Stokes light and Stokes light before and after the connecting portion.

  The temperature calibration unit may use a value different from the gain ratio fluctuation value of the anti-Stokes light and the Stokes light at the actual connection as a parameter.

  As the parameter, the Raman shift frequency of the optical fiber obtained from the gain ratio variation value, the Raman shift frequency of the temperature reference portion provided in front of the connection portion, and the true temperature value measured by the reference temperature measuring means May be included.

  The temperature calibration unit may calibrate the measurement temperature in real time using the temperature measurement data obtained from the reference temperature measurement unit in real time.

  According to the optical fiber temperature distribution device of the present invention, the measurement temperature is calibrated using the correction equation using the gain ratio fluctuation value of the anti-Stokes light and the Stokes light at the connection portion as a parameter, so that two-point temperature calibration is not used. Effective temperature calibration can be performed.

The block diagram which shows the structure of an optical fiber temperature distribution measuring apparatus. The figure which shows the relationship between the distance x, anti-Stokes light intensity Ias, and Stokes light intensity Ist. The figure which shows the example of an effect of the temperature calibration in the optical fiber temperature distribution measuring apparatus of this invention. It shows the measured temperature error after calibration with .DELTA.G _c = -0.002 and ΔG _c = + 0.062.

  Embodiments of an optical fiber temperature distribution measuring apparatus according to the present invention will be described below.

  FIG. 1 is a block diagram showing a configuration of an optical fiber temperature distribution measuring apparatus according to this embodiment.

  As shown in FIG. 1, the optical fiber temperature distribution measuring apparatus according to the present embodiment is configured by using, for example, a laser diode, and a light source 1 that emits pulsed light, and wavelength of backward Raman scattered light into Stokes light and anti-Stokes light. The optical demultiplexer 2 to be separated, the optical fiber 3 laid on the site, the O / E converter 4 st that receives the Stokes light emitted from the optical demultiplexer 2, and the anti-Stokes light emitted from the optical demultiplexer 2 Receiving the O / E converter 4as, the amplifier 5st for amplifying the output signal of the O / E converter 4st, the amplifier 5as for amplifying the output signal of the O / E converter 4as, and the output signal of the amplifier 5st as a digital signal From the A / D converter 6st for converting the output signal of the amplifier 5as to a digital signal, the A / D converter 6st and the A / D converter 6as An arithmetic control unit 7 that executes arithmetic operations based on digital signals and controls each part of the apparatus, a pulse generator 8 that is connected to the light source 1 and is controlled by the arithmetic control unit 7, and windings of several tens to several hundreds of meters A temperature reference unit 9 comprising a measured optical fiber 9A and a thermometer 9B made of, for example, a platinum resistance thermometer and measuring the actual temperature of the optical fiber 9A, and a platinum resistance thermometer provided near the optical fiber 3, for example. A reference thermometer 11 that measures the actual temperature in the vicinity of the optical fiber 3, a loss determination unit 12 connected to the calculation control unit 7, and a temperature correction unit 13 that performs temperature calibration. The O / E conversion unit 4st and the O / E conversion unit 4as are configured using, for example, a photodiode.

  As shown in FIG. 1, one end of the optical fiber 9A is connected to the input / output end of the optical demultiplexer 2, and the other end of the optical fiber 9A is connected to the optical fiber 3 via the connection portion CN. The connection portion CN that connects the optical fiber 9A and the optical fiber 3 of the temperature reference portion 9 can be configured using an optical connector or by fusion splicing.

  As shown in FIG. 1, the output signals of the thermometer 9 </ b> B and the reference thermometer 11 are respectively input to the calculation control unit 7. In addition, a timing signal that defines the timing of the optical pulse is output from the arithmetic control unit 7 toward the pulse generation unit 8.

  Next, the operation of the optical fiber temperature distribution measuring apparatus of this embodiment will be described.

  A pulse signal corresponding to the timing signal from the arithmetic control unit 7 is output from the pulse generation unit 8 and applied to the light source 1. From the light source 1, pulsed light corresponding to the pulse signal is emitted. The pulsed light is incident on the incident end of the optical demultiplexer 2, and the pulsed light emitted from the input / output end of the optical demultiplexer 2 is emitted to the optical fiber 9A of the temperature reference portion 9, and further via the connection portion CN. Then, the light is emitted to the optical fiber 3.

  On the other hand, the backward Raman scattered light generated in the optical fiber 3 is incident from the incident / exit end of the optical demultiplexer 2 via the connection portion CN and the optical fiber 9A of the temperature reference portion 9, and in the optical demultiplexer 2. The wavelength is separated into Stokes light and anti-Stokes light.

  Stokes light emitted from one emission end of the optical demultiplexer 2 is incident on the O / E converter 4st, and emitted from the other emission end of the optical demultiplexer 2 to the O / E converter 4as. Anti-Stokes light is incident, and the O / E converter 4st and the O / E converter 4as each output an electrical signal corresponding to the incident light.

  The amplifier 5st and the amplifier 5as amplify the electric signals output from the O / E converter 4st and the O / E converter 4as, respectively. A / D converters 6st and 6as convert the signals output from amplifiers 5st and 5as into digital signals, respectively.

  Based on the digital signals output from the A / D converters 6st and 6as, the arithmetic control unit 7 calculates the temperature from the two components of the backscattered light, that is, the intensity ratio of the Stokes light and the anti-Stokes light. Based on this, the temperature distribution along the optical fiber 3 is displayed on the display means (not shown). The arithmetic control unit 7 stores in advance the relationship between the intensity ratio and the temperature in the form of a table or an expression.

  The loss determination unit 12 is a characteristic component of the optical fiber temperature distribution measuring device of the present invention, and calculates a gain ratio variation value (loss variation) of the connection unit CN. The operation of the loss determination unit 12 will be described later.

  The temperature correction unit 13 calibrates the temperature value calculated by the calculation control unit 7 using the temperature measurement data of the reference thermometer 11 provided in the vicinity of the optical fiber 3 used as the temperature sensor and the calibration formula, and has high accuracy. The temperature measurement result of is output. The calibration formula can be stored in the arithmetic control unit 7 or the temperature correction unit 13, for example. Details of the calibration formula will be described later.

  Next, the operation of the optical fiber temperature distribution measuring apparatus of the present invention will be described while comparing with the principle of the conventional two-point temperature calibration.

  When the signal strengths of the Stokes light and the anti-Stokes light are expressed as a function of time based on the light emission timing in the light source 1, the speed of light in the optical fiber 3 is known. It can be replaced with a function of distance. That is, it can be regarded as a distance distribution of light intensity in which the horizontal axis is the distance and the vertical axis is the intensity of Stokes light and anti-Stokes light generated at each distance point of the optical fiber.

  The anti-Stokes light intensity Ias and the Stokes light intensity Ist both depend on the temperature of the optical fiber 3, and the intensity ratio Ias / Ist of both lights also depends on the temperature of the optical fiber 3. Therefore, if the intensity ratio Ias / Ist is known, Raman The temperature at the location where the scattered light is generated can be known. Here, the intensity ratio Ias / Ist is a function Ias (x) / Ist (x) of the distance x, and the temperature distribution T along the optical fiber 3 from the distance distribution of the intensity ratio Ias (x) / Ist (x). (X) can be obtained.

In the configuration shown in FIG. 1, when the temperature of the temperature reference portion 9 is T ₀ (K), the intensity ratio R _r (T ₀ ) between the anti-Stokes light (AS) and the Stokes light (ST) is _{expressed by the following} equation (1). Is required.

Here, L _r is defined as follows.

  When the temperature of the optical fiber 3 is T (K), the intensity ratio R (T) between the anti-Stokes light (AS) and the Stokes light (ST) is obtained by the equation (3).

Here, L ₁ is defined as follows.

At the stage where the Raman shift frequency omega ₁ of the optical fiber 3 is not known, the calculated values T 'is the fiber temperature obtained by calculation using the known Raman shift frequency omega _r, the T and ω1 to omega _r in (3) T It is obtained by the equation (5) solved by replacing with '.

When the equation (5) is solved for the true value T of the fiber temperature, the equation (6) is obtained, so that the true temperature T can be obtained from the calculated value T ′ obtained by the calculation. This equation (6) includes the loss variation ΔG _{c at the} connection portion CN and the Raman shift frequency ω ₁ of the optical fiber 3 as unknowns.

In the two-point temperature calibration disclosed in Japanese Patent Application Laid-Open No. 2012-27001, the true temperature is measured using the reference thermometer 11 arranged in the vicinity of the optical fiber 3, and the position where the reference thermometer 11 is arranged Two combinations of the calculated value T ′ and the true temperature T in FIG. This example calculates values T ₁ 'and a true value T _1, calcd T _2' and the true value T _2. By substituting these combinations into the equation (6), the loss fluctuation ΔG _c can be canceled and the Raman shift frequency ω ₁ can be obtained by the equation (7).

  As a result of the two-point temperature calibration, an accurate temperature can be calculated by the equation (8).

Thus, in the conventional two-point temperature calibration, the equation (6) includes the gain ratio fluctuation value (loss fluctuation ΔG _c ) at the connection portion CN and the Raman shift frequency ω ₁ of the optical fiber 3 as unknowns. On the other hand, the loss determination unit 12 in the optical fiber temperature distribution measuring device of the present invention obtains the loss fluctuation ΔG _c in the connection part CN by calculating from the measurement data. This reduces the number of unknowns and requires only one point for temperature calibration instead of two.

Next, a method for determining the gain ratio variation value (loss variation ΔG _c ) of the connection unit CN and the temperature calibration method performed by the temperature correction unit 13 performed by the loss determination unit 12 will be described.

  FIG. 2 is a graph in which the horizontal axis is the distance x from the light source 1 in the fiber direction, and the vertical axis is a logarithm display of the anti-Stokes light intensity Ias and the Stokes light intensity Ist adjusted for the fiber loss.

  First, as in the prior art, the anti-Stokes light intensity Ias and the Stokes light intensity Ist are measured. Anti-Stokes light and Stokes light have different fiber losses because of their different optical wavelengths, but, as before, for example, by multiplying the fiber loss of anti-Stokes light by an appropriate factor, the fiber loss of anti-Stokes light and Stokes light will be the same Adjust as follows.

  In FIG. 2, the position of the connecting portion CN is at a distance from L2 to L3. Since the connection part CN connects the apparatus main body including the temperature reference part 9 and the optical fiber 3 outside the apparatus, the distance from the light source 1 to the connection part CN is known to the apparatus designer. In the measurement data, since a signal whose band is limited electrically and optically appears, a position having a slight width from L2 to L3 is shown.

The ratio Ias (x) / Ist (x) between the anti-Stokes light and the Stokes light in the range from L1 to L2 on the apparatus main body side with respect to the position of the connection part CN is the gain ratio G1, and the sensor is located more than the position of the connection part CN When the ratio Ias (x) / Ist (x) of anti-Stokes light to Stokes light in the range from L3 to L4 on the optical fiber 3 side is the gain ratio G2, the gain ratio fluctuation value at the connection portion CN, that is, the loss fluctuation ΔG _c is G2-G1.

  Although the gain ratio variation value of the connection portion CN is very small, the connection portion CN is not far from the light source 1 and has a sufficient signal amplitude, so the influence of noise is small. In order to further reduce the influence of the noise, the loss determination unit 12 can obtain G1 after moving average the signals of Ias (x) and Ist (x) between L1 and L2, and L3 G2 can be obtained after moving average the signals of Ias (x) and Ist (x) between L4 and L4.

(6) is the true value T ₁ and the calculated value T ₁ 'performs one-point temperature calibration, is if it is known, is rewritten into the following equation.

The left side of equation (9) is a quaternary function related to ω ₁ , but as an actual value, the Raman shift frequency ω ₁ is two orders of magnitude smaller than the frequency ω ₀ of the light source, so the appearance is a linear monotonically increasing function. It has become. On the other hand, the right side of the equation (9) is an exponential function related to ω _1, and is a monotonically increasing or decreasing function. Therefore, obtaining the intersection of the graph on the left side and the right side can be obtained by performing a relatively simple calculation process using a computer or the like. For example, there is a method in which the initial value of the variable ω ₁ is ω ₁ = ω _r and the value of the variable ω ₁ is slightly increased or decreased sequentially while comparing the left side and the right side. Alternatively, there is a method in which the value of the variable ω ₁ is temporarily selected at two locations, and the next point is selected in a direction in which the difference between the left side and the right side at the two points becomes smaller. As described above, ω ₁ can be obtained by a very general numerical analysis method.

As described above, the fluctuation value (loss fluctuation ΔG _c ) of the gain ratio Ias (x) / Ist (x) is obtained from the measurement data before and after the position of the connection portion CN, and obtained from the one-point temperature calibration based on that. When ω ₁ is obtained by solving the equation (9) obtained by simple numerical analysis, the temperature calibration of the equation (8) used in the conventional two-point temperature calibration can be performed.

  FIG. 3 shows an example of the effect of temperature calibration in the optical fiber temperature distribution measuring apparatus of the present invention, where the horizontal axis shows the actual temperature of the measurement target site, and the vertical axis shows the error in the measured temperature.

The parameters used in the example of FIG. 3 are as follows.
Raman shift frequency ω _r = 74.1 × 10 ¹² (rad / Hz) used for temperature calculation before calibration
Actual temperature T ₁ = 352.12 (K) (78.97 (° C.))
Calculated temperature value T1 ′ before calibration = 356.91 (K) (83.76 (° C.))

  As shown in FIG. 3, the temperature measurement error before calibration is as large as about 2 ° C. at the actual temperature of −140 ° C. and about 6 ° C. at the actual temperature of + 160 ° C. This is because of the error due to the connection loss between the apparatus main body and the optical fiber 3 used as the temperature sensor, the Raman shift frequency of the optical fiber used in the temperature reference section 9 inside the apparatus main body, and the Raman shift frequency of the optical fiber 3 used as the temperature sensor. This is considered to be due to the difference. On the other hand, the temperature measurement error after calibration using the calibration according to the present invention is as small as about 0 ° C. at the actual temperature of −140 ° C. and about 2 ° C. at the actual temperature of + 160 ° C., which is greatly improved. Yes.

  In the example of FIG. 3, the error is 0 ° C. at + 80 ° C. after one-point temperature calibration, and the error is also 0 ° C. at −140 ° C. This is the two-point temperature at −140 ° C. and + 80 ° C. The same effect as the calibration is obtained.

In the first place, the two-point temperature calibration method is only forcibly determining the Raman shift frequency ω ₁ so that the error is 0 ° C. at the two points of the calculated value T ₁ ′ and the calculated value T ₂ ′. Thus, the Raman shift frequency is not the true Raman shift frequency of the sensor optical fiber. If the same idea is taken, the gain ratio fluctuation value (loss) of the anti-Stokes light and the Stokes light at the connection portion CN is set so that the error becomes 0 ° C. at two points of the calculated value T ₁ ′ and the calculated value T ₂ ′. It is also possible to forcibly determine the fluctuation ΔG _c ) to an arbitrary value in the loss determination unit 12.

Figure 4 is obtained by drawing a measured temperature error after calibration with two .DELTA.G _c of .DELTA.G _c = -0.002 and ΔG _c = + 0.062. In FIG. 4, as in FIG. 3, the horizontal axis represents the actual temperature of the measurement target site, and the vertical axis represents the error in the measured temperature.

As shown in FIG. 4, ΔG _c = −0.002 corresponds to the result of calculating loss fluctuation ΔG _c based on the measurement data shown in FIG. 3, and two points at −140 ° C. and + 80 ° C. It has the same effect as temperature calibration. On the other hand, when the loss fluctuation ΔG _c = + 0.062 regardless of the loss of the connection CN in the measurement data, the effect is the same as that of performing two-point temperature calibration at + 80 ° C. and + 176 ° C.

Thus, the between the measured temperature error and the value of loss variation .DELTA.G _c, the temperature measurement error of the low temperature side when the value of .DELTA.G _c is reduced decreases, the temperature measurement error of the high temperature side when the value of .DELTA.G _c becomes larger There is a tendency to become smaller. There are various types of optical fiber, but it is generally a GI type multimode fiber (MMF) that is installed as a sensor optical fiber, and its parameters such as refractive index and loss factor are different. However, it can be expected to some extent before measurement. Therefore, it is possible to estimate the value of the loss fluctuation ΔG _c and the measurement temperature error to some extent for a typical optical fiber. For example, when it is desired to accurately measure around + 300 ° C., the value of the loss fluctuation ΔG _c . You can give a rough idea of how much should be set.

If one-point temperature calibration is performed with the value of loss fluctuation ΔG _c based on such a guideline, it is not as accurate as two-point temperature calibration, but it is more two-point temperature calibration than simple one-point temperature calibration. It is possible to realize a small measured temperature error close to. In FIG. 4, even if ΔG _c = + 0.062 is tentatively determined and one-point temperature calibration is performed as one guideline in a high temperature application, the effect of suppressing the error is large in a temperature range of + 200 ° C. or higher. Is clear.

  In the above-described embodiment, the reference thermometer 11 is arranged in the vicinity of the optical fiber 3 used as the temperature sensor and the temperature measurement data is obtained offline. However, the data of the reference thermometer 11 is captured in real time. The temperature calibration parameters may be changed in real time.

  As described above, according to the present invention, the gain ratio fluctuation value is calculated from the measurement data of the anti-Stokes light and the Stokes light before and after the connecting portion, and the gain ratio fluctuation value and the one-point temperature calibration value are used for the sensor. Since the Raman shift frequency of the optical fiber is obtained, the calibration work is simplified as compared with the two-point temperature calibration, and it is possible to eliminate the restrictions on the calibration equipment at the calibration site.

  In addition, the gain ratio fluctuation value is not obtained from the measurement data of the anti-Stokes light and the Stokes light before and after the connection part, but the error is reduced in the desired temperature range if the measurement temperature error is desired to be reduced. By using the gain ratio fluctuation value estimated in advance, it is possible to significantly improve the temperature error even if it is not as much as two-point temperature calibration in the temperature range.

  The scope of application of the present invention is not limited to the above embodiment. The present invention is widely applied to an optical fiber temperature distribution device configured to measure a temperature distribution along the optical fiber using Raman backscattered light using an optical fiber on which an optical pulse is incident as a sensor. Can be applied.

3 Optical fiber 11 Reference thermometer (reference temperature measurement means)
12 Loss determination unit (temperature calibration means)
CN connection

Claims (5)

In an optical fiber temperature distribution device configured to measure a temperature distribution along the optical fiber using Raman backscattered light using an optical fiber on which an optical pulse is incident as a sensor,
A reference temperature measuring means provided in the vicinity of the optical fiber;
Calibrates the measurement temperature using a correction equation using as parameters the gain ratio fluctuation value of anti-Stokes light and Stokes light at the connection where the optical pulse is incident on the optical fiber and the temperature measurement data obtained from the reference temperature measurement means. Temperature calibration means to
An optical fiber temperature distribution device comprising: The optical fiber temperature distribution device according to claim 1, further comprising a loss determination unit that determines the gain ratio variation value from measurement data of anti-Stokes light and Stokes light before and after the connecting portion. 3. The optical fiber temperature distribution device according to claim 1, wherein the temperature calibration unit uses a value different from the gain ratio fluctuation value of the anti-Stokes light and the Stokes light in the actual connection portion as a parameter. . As the parameter, the Raman shift frequency of the optical fiber obtained from the gain ratio variation value, the Raman shift frequency of the temperature reference portion provided in front of the connection portion, and the true temperature value measured by the reference temperature measuring means The optical fiber temperature distribution device according to any one of claims 1 to 3, wherein: 5. The light according to claim 1, wherein the temperature calibration unit calibrates the measurement temperature in real time using the temperature measurement data obtained from the reference temperature measurement unit in real time. Fiber temperature distribution device.