JPH02201129A - Optical fiber system distribution type temperature sensor - Google Patents

Abstract

PURPOSE:To obtain the temperature of a sensor with high accuracy by obtaining true Raman scattered light component obtained by subtracting conversion intensity in the object wavelength area of separated Rayleigh scattered light from the light intensity of separated stokes light, etc., and obtaining the temperature of an optical fiber for a sensor from the above-mentioned component. CONSTITUTION:Light emitted from a pulse light source 2 in a measuring device 10 is conducted to the optical fiber 20 for the sensor and excites backscattered light. A part of the backscattered light returns to the device 10 and conducted to OTDR measurement circuits 30a and 30r. In the circuits 30a and 30r, the backscattered light is converted into the light in the wavelength area where the respective central wavelengths are lambdaa and lambdar by filters 4a and 4r and enters photodetectors 5a and 5b. The signals are inputted in averaging processing circuits 6a and 6r and SN ratio is enhanced. The information is inputted in a temperature distribution arithmetic circuit 7 to obtain the temperature distribution of the optical fiber 20 for the sensor. Then, the time function of the light intensity corresponding to the light intensity of antistokes light and the Rayleigh light is inputted in the arithmetic circuit 7 so as to obtain the temperature of the optical fiber for the sensor from the true Raman scattered light component.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Use] The present invention relates to a temperature sensor, particularly an optical fiber type distributed temperature sensor.

[Prior Art] An optical fiber type distributed temperature sensor utilizes the fact that the intensity of scattered light in an optical fiber changes depending on the temperature.
The temperature distribution along the longitudinal direction of the optical fiber is measured by detecting the temperature using the main reflector method. Two methods are known, one using Raman scattered light and the other using Rayleigh scattered light, and the former's signal is weaker (approximately 100 ~
1/1000), but it has a large signal change layer with respect to temperature changes, and is considered to be a promising method.

The optical fiber distributed temperature sensor using Raman scattered light (hereinafter simply referred to as Raman temperature sensor) proposed earlier by the present applicant has a wavelength λ0, a pulse width Tw, and a pulse frequency jtjl T from one end of the optical fiber. P light is input, and anti-Stokes light with a wavelength λa and Stokes light with a wavelength λS, which are two components of Raman scattered light generated within the optical fiber, are generated as a function of time, assuming that the pulse light input time is 1=0. 1 a(t), measured as I 5(t),
These ratios Ia(t)/I5(t) are purely a function of temperature, and after the light pulse enters, it is determined that This device measures linear temperature distribution along an optical fiber by utilizing the fact that the time is 2XL, /Co (CO: the speed of light in an optical fiber).

Backscattered light measurement of anti-Stokes light and Stokes light is performed using the u constant method, which is almost the same as 0TDR, which is used for detecting a break point of an optical fiber.

The above Raman temperature sensor can be used to determine the temperature distribution in the longitudinal direction of a power cable by laying a sensor optical fiber along the salt cable, for example, and can be used to control power transmission capacity, etc. It is possible to detect areas where the temperature is partially high due to deterioration or the like. Also,
When used for fire detection in buildings, tunnels, etc., it is possible to locate the location of a fire outbreak.

FIG. 4 shows a specific example of the configuration of the Raman temperature sensor, which is composed of a measuring device 10 and a sensor optical fiber 20.

Light emitted from the pulse light source 2 in the measuring device 10 is guided to the sensor optical fiber 20 via the optical fiber 21 and the optical branch 2I131, and excites backscattered light within the optical fiber. A part of the excited backscattered light returns to the measuring device 10 side and is guided to the optical splitter 32 via the optical splitter 31 and the optical fiber 22.

Of the backscattered light split into two by the optical splitter 32, the one guided to the optical fiber 23a is an anti-Stokes optical 0TDR consisting of an optical filter 4a with a center wavelength λa, a light receiver 5a, and an averaging circuit 6a. It enters the measurement circuit 30a, and from this light intensity, the time function 1 of the anti-Stokes light intensity is calculated.
a(t) is found. On the other hand, among the backscattered light split into two by the optical branch 81132, the light guided to the optical fiber 23s is passed through an optical filter 4s with a center wavelength λS and a light receiver 5.
The light enters the Stokes light 0TDR measuring circuit 30s, which is formed by an averaging processing circuit 65''c41!, and a time function 15(t) of the Stokes light intensity is determined from this light intensity.

The pulse light source 2 and the averaging processing circuits 6a and 6s are synchronized by a synchronization signal from the trigger circuit 1.

The obtained time functions 1a(t) and I5(t) are input to the temperature distribution calculation circuit 7, and Ia(t)/I5(t
), the linear temperature distribution along the sensor optical fiber is measured.

[Problem to be solved by the invention] The Raman temperature sensor can measure linear temperature distribution using the method described above, and among the linear temperature distribution measurement methods that have been studied so far, it is highly sensitive and practical. This method has the potential to measure temperature distribution over long distances.

By the way, the wavelength λr of Rayleigh scattered light is the wavelength of the incident light ^
0, the wavelength distribution of the Rayleigh scattered light is almost the same as the wavelength distribution of the incident light, as shown in FIG.

On the other hand, the wavelength λa of the antistoic light is shorter than the wavelength λ0 of the incident light by Δλ" (several to several tens r+1),
Conversely, the wavelength λS of Stokes light is Δ from the wavelength λ0 of the incident light.
It becomes longer by λr.

In addition, Raman scattered light (anti-Stokes light and Stokes light) is extremely weak (approximately 1/10
0 to 1/1 (100), and each wavelength is close to each other, so the incident pulsed light is not a complete nine-color light (light with a single wavelength), so as shown in Figure 2, A portion of the Rayleigh scattered light component also enters the wavelength region of Raman scattered light.

For this reason, it is difficult to completely separate only the Raman scattered light component, which imposes a certain limit on temperature measurement accuracy.

SUMMARY OF THE INVENTION An object of the present invention is to improve the above-mentioned drawbacks of the prior art and provide a highly accurate optical fiber type distributed temperature sensor.

Means for Solving Problems] The optical fiber type distributed temperature sensor of the present invention makes a light pulse enter a sensor optical fiber from a light source in a measurement system, and backscattered light generated in the fiber is transmitted to the measurement system. means for separating light in the center wavelength range of Rayleigh scattered light and Stokes light or anti-Stokes light of Raman scattered light from among these backscattered lights, and the separated Stokes light or anti-Stokes light. Fixing the central wavelength range of the light as the ↑ wavelength range, the converted light intensity of the 'NN patriarchal range of the separated Rayleigh scattered light is calculated from the light intensity of the separated Stokes light or antis I-Ras light. The device is equipped with a calculation means for subtracting the true Raman scattered light component and calculating the temperature of the sensor optical fiber from the ratio of the found true Raman scattered light component and the light intensity of the 11η Rayleigh scattered light. It is.

Another type of layer is to input a light pulse from a light source in a measurement system to a sensor optical fiber, guide the backscattered light generated in the fiber to the measurement system, and select Rayleigh scattered light from among these backscattered lights. A means for separating light in a center wavelength range and a center wavelength range of Stokes light and anti-Stokes light of the Raman scattered light, and a means for separating light in a center wavelength range of the Stokes light and anti-Stokes light of the Raman scattered light, and using the separated center wavelength range of the Stokes light and the center wavelength range of the anti-Stokes light as target wavelength ranges, respectively. By subtracting the taX light intensity in the target wavelength range of the separated Rayleigh scattered light from the light intensity of the separated Stokes light and anti-Stokes light at a fixed value, the Stokes light and anti-Stokes light as true Raman scattered light components are obtained. Calculating means for determining the light intensity of the anti-Stokes light and calculating the temperature of the sensor optical fiber from the ratio of the light intensity of the Stokes light as the determined true Raman scattered light component and the light intensity of the anti-Stokes light are provided. It can also be done.

r effect l The light intensity of Raman scattered light (anti-Stokes light, Stokes light) and Rayleigh scattered light is proportional to the incident light intensity ■0, but the temperature dependence of the former is large and the latter is small. Therefore, the Rayleigh scattered light Ignoring the temperature dependence of the former compared to the former, and assuming that the temperature functions of the anti-Stokes light and Stokes light are fa (di) and f5 (T), the light intensity of the anti-Stokes light, Stokes light, and shear scattered light is T a(t), I
5(t), I r(t) is Ia(t)=Baxfa
(T) xlo - (i) I 5(t) = B s
x f s(■)x I o・(2)I r(t)=
Br x Io - (3) However, Ba,
Bs, Br: constant of proportionality, I a(t)/I rft) or I
It can be seen that 5(t)/I r(t) is purely a function of temperature.

? The iAX means calculates the temperature of the sensor optical fiber by utilizing information on Rayleigh scattered light as well as information on one or both of anti-Stokes light and Stokes light caused by Raman scattered light. The calculation procedure is to calculate the pure anti-Stokes light or Stokes light as a true Raman scattered light component by subtracting the converted light intensity in the target wavelength range of Rayleigh scattered light from the light intensity of Stokes light and/or anti-Stokes light. The light intensity is determined and the ratio is taken between this and the light intensity of the Rayleigh scattered light, but the light intensity of the pure Stokes light and the anti-Stokes light are both determined and the ratio of the two is taken. Since only the Raman scattered light component is completely separated, the accuracy of the optical fiber type distributed temperature sensor is greatly improved.

Note that the generation position of the backscattered light can be determined from the difference between the incident time of the optical pulse and the time at which the backscattered light reaches the measurement system.

As a means to separate light in the center wavelength range of Rayleigh scattered light and Raman scattered light, or more precisely, light in the wavelength range corresponding to the center wavelength, optical demultiplexers and optical directional couplings suitable for each wavelength are used. A combination of a filter or a splitter and a filter can be used.

[Example 1] Hereinafter, an example of the optical fiber type distributed temperature sensor according to the present invention will be described with reference to FIG. Basic configuration 1 of the optical fiber type distributed temperature sensor according to this embodiment is almost the same as the sensor example not shown in FIG. 11, and the differences are as follows.

First, in terms of the configuration, as an 0TDR measurement circuit, a Rayleigh scattered light 0TDR measurement circuit 30r is inserted in place of the Stokes scattered light 0TDR measurement circuit 30s. This 0TDR meter for Rayleigh scattered light 1JR30r is composed of an optical filter 4r with a center wavelength λr, a light receiver 5r, and an averaging processing circuit 6r.
-X light Jn OT" DR measurement 4R30a is composed of an optical filter 4a having a center wavelength λa, a light receiver 5a, and an averaging processing circuit 6a.

Next, the operation of the optical bias distribution type temperature sensor shown in FIG. 1 will be explained.

Light (wavelength λO) emitted from the pulse light source 2 in the n1 device 10 is guided to the sensor optical fiber 20 via the optical fiber 21 and the optical splitter 31, and excites backscattered light within the optical fiber. do. A portion of the excited backscattered light returns to the measurement device 10 side and is guided to the two 0TDR measurement circuits 30a and 30r via the optical splitters 31 and 32. That is, what is split into two by the optical branch 3i32 and guided to the optical fiber 23a is a 0TDR meter aμm for anti-Stokes light.
The light that enters the circuit 30a and is guided to the optical fiber 23r enters the Rayleigh scattered light 0 'T'' D R measurement circuit 30r.

In both 0TDR measurement circuits 30a and 3Or, the backscattered light is first converted into light in the wavelength range at the respective center wavelengths λa and λ' by the filters 4a and 4r, and then sent to the light receiver 5.
a. Enter 5r. The signals converted into electrical information from the light receivers 5a and 5r"e are then input to averaging processing circuits 6a and 6r to correct the S/N ratio. The information is input to a temperature distribution calculation circuit 7, The temperature distribution of the sensor optical fiber 20 is determined.

Next, the concept of the calculation processing method in the temperature distribution calculation circuit 7 will be described.

Since the light intensities obtained by the measuring circuits 30a and 30r are apparent values, the time functions of the light intensity corresponding to the light intensities of the anti-Stokes light and the Rayleigh light are respectively expressed as ■"a (t), I'r(t).

Here, the reason for the apparent light intensity is that, as shown in FIG. 2, the Rayleigh scattered light component is also included in the wavelength region 5060 in which Raman scattered light is generated. These determined apparent light intensities 1'a(t), I'rm
is input to the arithmetic circuit 7 to obtain the true scattered light intensity.

First, although the wavelength region 40 in which Rayleigh scattered light occurs as shown in FIG. t) ...(4) However, I r(t) ; Rayleigh scattered light intensity.

In addition, the wavelength region 50 in which anti-Stokes scattered light is generated is
When the wavelength is fixed at , the light intensity ratio of anti-Stokes scattered light and Rayleigh scattered light is constant, so Ia(t) = I'a(t) - CIXI'r(t) -
(5) However, I a (t); anti-Stokes light intensity C3; proportionality constant I a (t) obtained in this way is a true Raman scattered light component, and the above equations (1) and (3) By using , the temperature function fan can be found using the following equation.

f am= (I'a(t)-C,X I'r(
t) ) x 1 / I'r(t)
-(6) Since the temperature function fa(T) can be calibrated in advance for the selected optical fiber, conversely, when the temperature function fa(T) is obtained from the scattered light + ft information, The temperature of the sensor optical fiber is measured with high precision.

Although the above example uses anti-Stokes light as Raman scattered light information, the temperature of the sensor optical fiber can be similarly measured using Stokes light. At this time, the temperature function fa(T) is as follows.

f sm= (1'Nt) -C, x I'r(t)
)X1/I'r(t)...
(7) Also, the distance to the point where Raman scattered light is generated is 1
-1 If the speed of light in the optical fiber is Co, and the time from the input of the optical pulse until it returns is t, then from the following equation (8), the distance 11
Find iL.

L=COX t/2 - (8) In this way, linear temperature distribution measurement along the sensor optical fiber 20 can be performed.

Note that the proportionality constants C+ and Cx are the sensor optical fiber 20.
If it is short, it can be treated as constant, but if it is long, the distance attenuation characteristics differ depending on each wavelength, so it is necessary to perform this correction. This compensation can be easily performed based on the distance attenuation characteristics of the optical fiber for the target wavelength and the wavelength distribution of the incident pulsed light.

In Figure 1, a splitter is used as the wavelength separation means, but a splitter, directional coupler, or a combination of these may also be used.If a splitter is used, Raman scattered light and Rayleigh scattered light can be separated. The branching ratio may be a reciprocal ratio of the respective light intensities (for example, when the light intensity ratio is 100;1, the branching ratio is set to 1;100).

FIG. 3 shows another embodiment of the optical fiber type distributed temperature sensor of the present invention.

In this embodiment, in addition to the OT D Rtf measuring circuit 30a for anti-Stokes light and the 0TDR measuring circuit 30s for Stokes scattered light shown in FIG. 4, the 0TDR measuring circuit further includes an 0TDR measuring circuit 30r for Rayleigh scattered light. ing. Also, the optical branching devices 31 and 32 are changed to optical demultiplexing devices 9,
This makes it easy to separate backscattered light, and along with this, 0
The optical filters 4a, 4s, and 4r are omitted from the 'T'DR measurement circuits 30a, 30s, and 30r.

Next, the operation of the optical fiber type distributed temperature sensor shown in FIG. 3 will be explained.

Light (wavelength λ0) emitted from the pulsed light source 2 in the J1 device 10 is guided to the sensor optical fiber 20 via the optical fiber 21 and the waveform generator 9, and excites backscattered light within the optical fiber. . A part of the excited backscattered light returns to the measurement device 10 side and is transmitted to the optical demultiplexer 9 and the optical fibers 23a and 23s.
, 23r, the 0TDR measurement circuits 30a, 30s,
Guided to 30r.

Of the backscattered light output by the optical demultiplexer into three parts, what is guided to the optical fiber 23a is light in the wavelength range at the center wavelength λa, and the light is sent to the optical receiver 5 which converts the light into an electrical signal.
The anti-Stokes light 0TDR measurement cycle n 30 a, which is composed of the anti-Stokes light intensity and the averaging processing circuit 6a, is entered, and from this light intensity, the time function I' of the apparent anti-Stokes light intensity is calculated.
a(t) is found. Similarly, 0T for Stoke light
From the DR measurement circuit 30s and the Rayleigh scattered light 0TDR measurement device 30r, time functions 1'5(t) and I'r(t) of the corresponding apparent light intensity are obtained.

These obtained apparent light intensity I'a(t)1'
5(t) and I'r(t) are input to the arithmetic circuit 7, and the true scattered light intensity is determined.

First, the Rayleigh scattered light intensity 1 r(t) is given by ``1 (4)
), ignoring the Raman scattered light component, I r(t
)=I'r(t) (9).

Furthermore, if the wavelength is fixed in the wavelength range 5060 where Raman scattered light is generated, the light intensity ratio of Raman scattered light and Rayleigh scattered light is constant, so Ta(t) = I'a(t) - C,
XI'r(t) - (10)I 5(t) = I'
5(t)-C2x I'r(t)...(11)
However, I a(t) ; anti-Stokes light intensity I 5
(t); Stokes light intensity ct, C2; proportionality constant I ff (t) and I 5 (t) obtained in this way
is the true Raman scattered light component, I a(t)/I
Since 5(t) is purely a function of temperature, (1'a(t) C+ x I'r(t) )x
1/ (1'5(t) -C,x I'r(t))
By performing the calculation, a highly accurate thermometer i1! I can do lj.

Further, the distance to the point where Raman scattered light is generated can be determined from the above equation (8).

In this way, also in the case of FIG. 3, linear temperature distribution measurement can be performed on the sensor optical fiber 20.

In addition, in Fig. 3, a splitter is used as a means of wavelength division, but as shown in Fig. 1, it is better to use a combination of a splitter and a filter. Alternatively, the branching ratio of Raman scattered light and Rayleigh scattered light may be set as the reciprocal ratio of the respective light intensities.

[Effects of the Invention] As described above, according to the present invention, the following remarkable effects can be achieved.

(1) Even if the incident pulsed light is not completely monochromatic light (light having a single wavelength), the Raman scattered light can be separated and the temperature of the sensor can be measured with high precision.

(2) Since Rayleigh scattered light is also measured, this in
From the information, it is possible to understand temporal changes in optical fiber loss, and to recognize abnormalities in the sensor optical fiber.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a configuration example of an optical fiber type distributed temperature sensor according to the present invention, FIG. 2 is a conceptual diagram of the wavelength distribution of backscattered light generated within the optical fiber sensor, and FIG. 3 is a diagram showing an example of the configuration of an optical fiber type distributed temperature sensor according to the present invention. A diagram showing an example of the composition of a song for a distribution type temperature sensor,
FIG. 4 is a diagram showing an example of the configuration of an optical fiber type distributed temperature sensor that has already been proposed. In the figure, 1 is a trigger circuit, 2 is a pulse light source, 4s, 4a,
4r is an optical filter, 5s, 5a. 5r is a light receiver, 6s, 6a, 6r are averaging processing circuits, 7
9 is a temperature distribution calculation circuit, 9 is an optical demultiplexer, 10 is a measuring device,
20 is optical fiber for sensor 21, 22, 23s, 23a
, 23r is an optical fiber, 31.32 is an optical splitter, 30s
is the 0TDR measurement circuit for Stokes light, 30a is the 0TDR measurement circuit for anti-Stokes light, and 30r is the 0TDR measurement circuit for Rayleigh light.
A TDR measurement circuit is shown.

Claims (1)

[Claims] 1. A light pulse is input from a light source in a measurement system to a sensor optical fiber, and backscattered light generated in the fiber is guided to the measurement system, and Rayleigh scattering is extracted from the backscattered light. means for separating light in a center wavelength range of light and light in a center wavelength range of Stokes light or anti-Stokes light of Raman scattered light; and fixing the center wavelength range of the separated Stokes light or anti-Stokes light as a target wavelength range. , the true Raman scattered light component is obtained by subtracting the converted light intensity in the target wavelength range of the separated Rayleigh scattered light from the light intensity of the separated Stokes light or anti-Stokes light, and the obtained true Raman scattered light is obtained. 1. An optical fiber type distributed temperature sensor, comprising: arithmetic means for determining the temperature of the sensor optical fiber from the ratio of the light intensity of the Rayleigh scattered light. 2. Inject a light pulse into the sensor optical fiber from the light source in the measurement system, guide the backscattered light generated in the fiber to the measurement system, and select the center wavelength range of the Rayleigh scattered light and the means for separating light in the center wavelength range of the Stokes light and anti-Stokes light of the Raman scattered light, and fixing the separated center wavelength range of the Stokes light and the center wavelength range of the anti-Stokes light as target wavelength ranges, respectively; The converted light intensity in the target wavelength range of the separated Rayleigh scattered light is subtracted from the light intensity of the separated Stokes light and anti-Stokes light, respectively, to obtain the Stokes light and anti-Stokes light as true Raman scattered light components. The present invention is characterized by being provided with a calculation means for determining the light intensity and determining the temperature of the sensor optical fiber from the ratio of the determined light intensity of Stokes light and anti-Stokes light as the determined true Raman scattered light component. Optical fiber type distributed temperature sensor.