JPH02201130A - Optical fiber system distribution type temperature sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a temperature sensor, and particularly to 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.
This method measures the temperature distribution along the longitudinal direction of the optical fiber by detecting it using a method called ``main reflectometry''. Two known methods are known, which use Raman scattered light and Rayleigh scattered light, and the former's signal is weaker (approximately 1/1
00 to 1/1000), but the amount of signal change with respect to temperature change is large, and it is considered to be a promising method.

An optical fiber type distributed temperature J, E sensor (hereinafter simply referred to as a Raman type temperature sensor) that uses Raman scattered light has a wavelength λ0, a pulse width ``I''W,
When light with a pulse period Tp ((a) #light in FIG. 5) is incident, anti-Stokes light with a wavelength λa and Stokes light with a wavelength λS, etc., which are two components of Raman scattered light generated in the optical fiber, are reflected. The light (see (1) in Fig. 5) is measured at the sampling time interval Ts, and the pulsed light incident time is set to 1=
0, the light intensity of each reflected light is a function of time 1
a(t).

I 5 (t) and the ratio of these I a (t)
/ l 5(t) is purely a function of temperature, and the time from the light pulse input to the time it returns to the light pulse input @(f & directionally scattered light J1 measuring section) is 2xL/Co ( CO: An apparatus has been proposed that measures linear temperature distribution along an optical fiber by using light in the optical fiber.

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

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

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

Of the reflected light split by the optical splitter 32, the optical fiber 2
3a enters the old OR measuring circuit 30a for anti-Stokes light, which is composed of an optical filter 4a for anti-Stokes light, a light receiver 5a, and an averaging processing circuit 6at'', and from this light intensity, the anti-Stokes light intensity is calculated. The time function I a(t) of 0 is obtained. On the other hand, the optical splitter 32
Of the separated backscattered lights, the one guided to the optical fiber 23s is sent to the Stokes light OT D R, which is composed of a Stokes light optical filter 4s, a light receiver 5s, and an averaging processing circuit 65''C''. The light enters a measuring circuit 30s, and a time function 13(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 synchronizing signal from the trigger circuit 1, and the sampling of reflected light is performed by the averaging processing circuit 6a.

6 seconds at constant time intervals Ts shown in FIG.

Obtained M tm Z numbers I a(t) and I 5(t)
is input to the temperature distribution calculation circuit 7, and I a(t)/I
By performing the calculation of 5(t), the linear temperature distribution along the sensor optical fiber is measured.

[Task II that the invention attempts to solve! JI] A Raman temperature sensor or C9-type temperature sensor is a promising method that can measure linear temperature distribution using the method described above, but since the optical fiber used as the sensor has optical transmission loss,
This placed certain limits on long-distance J1 measurements. That is, Fig. 5 (
As shown in b), the reflected light intensity attenuates toward the far end of the sensor, and when the sensor becomes longer than a certain length, the reflected light intensity at the far end becomes weaker than the sensitivity of the light receiver and becomes unmeasurable. In order to improve this, if the sensitivity of the light receiver is increased, the light receiver becomes saturated with respect to the strong reflected light from the near end, making it impossible to measure the near end.

In addition, the reflected light is measured at the sampling time, and the shorter this time interval '1゛s is, the shorter the temperature measurement distance interval is, and the measurement distance accuracy is improved, but the shorter the sampling time interval Ts, the more the average The processing circuit needs to be high-speed, making it expensive. Therefore, the method of improving the accuracy of the measured distance 11 by shortening the sampling time has a limit due to economic efficiency.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical fiber distributed temperature sensor that improves the drawbacks of the prior art described above, enables long-distance measurement, and improves the accuracy of the measured distance M.

[Means for Solving the Problems] The optical fiber type distributed temperature sensor of the present invention allows a light pulse to be incident on a sensor optical fiber from a light source in a measurement system, and is formed by backscattered light generated by the fiber. The reflected light is guided to the measurement system, and from these reflected lights, light in a wavelength region corresponding to the center wavelength of the Raman scattered light or Rayleigh scattered light is separated and guided to a light receiver, and the intensity of these lights is sampled. In addition to determining the temperature distribution of the optical fiber, the generation value of the reflected light is calculated from the difference between the incident time of the optical pulse and the time when the reflected light reaches the measurement system! In an optical fiber type distributed temperature sensor that requires [A division ax IrtII control means that divides the range into a plurality of sections and sets only the reflected light corresponding to each a-town section to enter the receiver, and Set the sensitivity of the receiver to 11
The system is configured to include means for

As a more preferable form, when one delay is 1 m IJ' is shorter than the sampling time IFfl interval for measuring the reflected light and when these phases are sampled t'::J [11
Set a delay time set of m, and set 1? at each delay time. The measurement point control means sequentially extends the sampling start time by a time corresponding to fl, thereby collectively shortening the interval between temperature measurement points.

When an optical splitter and an optical filter are used to separate light in a wavelength range corresponding to the center wavelength of Raman scattered light or Rayleigh scattered light from the width of reflected light and guide it to a light receiver, the optical switch Any place along the path F6 until the reflected light in the measurement system is guided to the optical receiver by this wavelength separation means, that is, before the optical splitter, between the optical splitter and each optical filter, between each optical filter and the optical receiver. It can be installed somewhere between the container and the container.

The receiver sensitivity adjustment means may be configured such that the receiver itself is configured such that the sensitivity is automatically increased for each measurement section as the intensity of the reflected light it receives becomes smaller, or the sensitivity is adjusted in advance for each measurement section. The sensitivity may be adjusted and set and an adjustment command given to the light receiver, or the intensity of the reflected light may be monitored and the sensitivity of the light receiver may be automatically adjusted.

The division control means and measurement point control means can be a timing adjustment function by a controller.

[Function] The division control means controls the optical switch to divide the temperature measurement distance range of the sensor into a plurality of sections, and set only the reflected light corresponding to each measurement section to enter the light receiver. . The sensitivity of the light receiver is adjusted in accordance with the intensity of the reflected light for each set measurement section. Under this control, the temperature distribution for each measurement section is sequentially measured. The intensity of reflected light attenuates as it gets closer to the far end of the sensor, but since the sensitivity of the receiver is adjusted for each measurement section, the intensity of reflected light at the far end will never be weaker than the sensitivity of the receiver. In addition, the receiver is not saturated even with strong reflected light from the near end.
Measurement is not impossible, so long-distance measurement is possible.

On the other hand, in the second embodiment, the measurement point control means delays the sampling start time for measuring the reflected light by a delay time shorter than the sampling time interval, and repeatedly performs the temperature measurements. Therefore, the interval between the temperature measurement points is shortened overall, the temperature measurement distance resolution is increased, and the accuracy of the optical fiber type distributed temperature sensor is greatly improved.

[Example] Hereinafter, an example of the optical fiber type distributed temperature sensor according to the present invention will be explained with reference to FIG.
This is almost the same as that shown in FIG. 5, and the differences are as follows.

That is, the optical switch 8 was inserted between the optical splitters 31 and 32, and the controller 11 was inserted in place of the recovery circuit 1. The optical switch 8 controls the temperature J of the sensor.
One distance measurement^ is divided into multiple sections, and only the reflected light corresponding to each measurement section is used as the anti-Stokes light 0TDII.
Measurement circuit 30a and Stokes light 01 DI? The timing adjustment is performed by the controller 11 via the line 12.The functions of the controller 11 include the creation of a tri-force signal for the pulsed light 8.21\, which will be described later. The main tasks are to create a delay time signal for the 0TDR measurement circuit 30a, 30s + temperature distribution calculation circuit v87, etc.

Next, the operation of the optical fiber distributed temperature sensor according to the present invention will be explained with reference to FIG.

The light emitted from the pulsed light source 2 in the measuring device 10 is guided to the sensor optical fiber 20 via the optical fiber 21 and the optical splitter 31, and excites backscattered light in the optical fiber, and a part of the light is The reflected light returns to the measuring device 10 side, and is guided to the optical branch '832 via the optical splitter 31, optical fiber 22, and optical switch 8, where it is divided into two. One of the divided lights is transmitted through an optical fiber 23a to an optical filter 4a, which is a wavelength separation means for anti-Stokes light, and a light receiver 5.
0■ old (enters the measurement circuit 30a) for anti-Stokes light composed of a and averaging processing circuit 6a. 1t!One light is passed through an optical fiber 23s to an optical filter serving as a wavelength separation means for 2 Stokes light. 0TD for Stokes light consisting of 4s, light receiving H5s and averaging processing circuit 65"C"
Enter the R measurement circuit 30s.

(In the measurement circuits 30a and 30s, the light from the optical splitter 32 is converted into light in the respective wavelength range by the optical filters 4a and 4s, and then enters the light receivers 5a and 5s, The light intensity information is converted into an electrical analog signal according to the light intensity information.The light intensity electrical signals output from the light receivers 5a, 5s are input to the averaging processing circuits 6a, 6s; The time function 1a(t) of the Andestokes light intensity and the time function r5(t) of the Stokes light intensity are obtained, and the S/N ratio is improved at the same time. The information on the obtained time functions 1 a(t) and I 5(t) is input to the temperature distribution calculation circuit 7, and I a(t
)/I5(t) is performed to obtain a linear temperature distribution along the sensor optical fiber 20. In addition, the pulse light source 2 and the above-mentioned averaging processing circuits 6a, 6s, 0TDRa1
Synchronization between the temperature distribution circuits 30a and 30s and the temperature distribution calculation circuit 7 is performed by a synchronization signal generated from the controller 11.

The optical switch 8 is turned ON (optical shutter open) for a certain time width T SWD every time a predetermined time arrives after the pulsed light is emitted, according to an opening/closing control signal from the controller 11. The temperature measurement distance range is divided into a plurality of sections, and only the reflected light corresponding to each measurement section is received 85a.

It will be incident on 5s. In order to enable long-distance measurement, the gain or sensitivity of these light receivers 35a and 5s is automatically controlled to increase as the intensity of the received reflected light decreases.

By the way, the time width controlled by the controller 11 using the jlN close control signal for the purpose of dividing the measurement distance range is determined by turning on the optical switch 8 after the pulsed light is emitted, as shown in FIG. 2(C). There are two control time widths Tswl and a constant time width TSWp for maintaining this ON state. Averaging processing time #P16a, 6s sampling is performed over a fixed time width T' during which this optical switch 8 is open.
For the purpose of performing within +vp, as shown in Figure 2(d),
Sampling stop time width (Tmi) and execution time width (
Set T[p). That is, the sampling stop time (T
lli) is adjusted in length so that the first sampling point always occurs within the above-mentioned constant time width T 5ivp, and the sampling execution time ('r lρ) is adjusted such that the sampling point 11 & occurs within the time width T swp. length (Fig. 2(d)).

In this way, the controller 11 mutually synchronizes the opening/closing control of the optical switch 8 and the sampling time.
The stop time 'T' rg i is gradually lengthened and the temperature distribution measurement is repeated, that is, first, when sampling is stopped 1j
Measure the temperature distribution with n'rtgi=0, then stop time Tni=N x Tip (N = 1.2.-
),! : The temperature distribution is successively measured, and finally the entire temperature distribution is measured as shown in FIG. 2(e).

At this time, the intensity of the reflected light attenuates as it approaches the far end of the sensor, but as the intensity of the reflected light weakens in each measurement section, the sensitivity of the receivers 5a and 5s increases to an appropriate value. The intensity of reflected light at the near end will never be weaker than the sensitivity of the receiver, and the receiver will not become saturated even with strong reflected light from the near end, so measurements will not become impossible.
The sensitivity adjustment of the light receivers 5a and 5s for each measurement section is as follows:
This may be done in advance, or may be automatically adjusted by monitoring the intensity of the reflected light.

In addition, as the sensitivity adjustment means of the light receivers 5a and 5s, (1)
A method for adjusting the amplification factor of an electrical signal amplifier used after a photoelectric conversion element in a light receiver; (2) When using, for example, an avalanche photodiode (APD) as a photoelectric conversion element; is a method of changing the gain of the APD by changing its bias voltage and temperature,
and (3) a method of adjusting the optical signal by inserting an optical attenuator before the electrical signal light enters the optical-electrical conversion element and changing the attenuation rate.

In the case of method (3) above, the optical attenuator may be inserted just before the optical/electric conversion element used in the optical receiver, or between the optical splitter 31 and the optical receivers 5a and 5b. Where should I insert it?

1. The sampling execution time Tno is determined by the light receiving sensitivity and optical loss of the optical fiber for the sensor, etc. The longer it is, the longer the grill distance M of the sensor can be, and research and development on increasing the distance is progressing. In this embodiment, this can be expanded N times at once. 6 Next, a method for improving the temperature measurement distance resolution will be described with reference to FIG. 3.

This is set as a delay time set such that one delay time is shorter than the sampling time interval GoS for measuring the reflected light, and the sum of these is the sampling time interval 'I's, and the delay time corresponding to each delay time is set. The sampling start time is sequentially extended by an amount of time to repeatedly measure the temperature, thereby shortening the interval between temperature measurement points overall. By improving the temperature measurement distance resolution in this way, the accuracy of the optical fiber type distributed temperature sensor will be significantly improved.

Now, the minimum delay time interval Δt and the sampling stop time T
The relationship with li is 'l'ni=mxΔt (m=o, 1, 2.-, H)
M=Ts/Δt, this is created by the controller 11, and averaging processing time II! Give to 16a and 6s. As a result, Tni=O, T'ni as shown in Fig. 3(a)(b)(c)
= Δt, Ti1 = 2Δt... When temperature measurements are performed sequentially according to the L formula, the final result is fine sampling of the delay time interval Δt, as shown in Figure 3(d) for the case of M=5 Get results.

The sampling time interval at this time, that is, the delay time rrI
The J interval Δt is equivalent to the sampling time interval Ts
The distance between the temperature measurement points is M.
This means that the length can be shortened by a factor of 1.

Therefore, the thermometer a1 distance resolution is significantly improved.

Here, the delay time interval Δt is set to 1/M of the sampling time interval Ts, but any set may be used as long as its size is shorter than Ts and the sum of them is Ts.

If you combine the functions explained in Figures 2 and 3 above,
It is possible to realize an optical fiber type distributed temperature sensor that is capable of long-distance measurement and has extremely high measurement distance accuracy. Furthermore, by changing the delay time as appropriate, the temperature distribution of the sensor can be viewed finely or roughly.

In the embodiments described above, Raman scattered light was mainly explained as backscattered light, but the principles of the present invention can be applied as is even if Raman scattered light is used or a combination thereof is used.

[Effects of the Invention 1 According to the present invention, the following remarkable effects can be achieved.

(1) By dividing the temperature measurement range of the sensor into multiple sections and adopting measurement technology that corresponds to each measurement section 1721, the performance of equipment such as light sources, optical elements, light receivers, and averaging processing devices is improved compared to conventional methods. Even at the same level, long-distance measurement using an optical fiber distributed temperature sensor becomes possible.

(2) By appropriately delaying the sampling time, it is possible to obtain an optical fiber type distributed temperature sensor whose measurement distance accuracy is significantly improved compared to the conventional one.

(3) ] Dual distribution performance can be achieved without sacrificing economic efficiency.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical fiber type distributed temperature sensor according to an embodiment of the present invention, and FIG.
e) is an explanatory diagram of the measurement concept of dividing the temperature measurement distance range into a plurality of sections, FIGS. FIG. 4 is a block diagram of the previously proposed optical fiber type distributed temperature sensor, and FIG. 5 (aHb) is an explanatory diagram of its measurement concept. In the figure, ■ is a trigger circuit, 2 is a pulsed light source, and 4s. 4a is an optical filter, 5s and 5a are light receivers, 6S6a is an averaging processing circuit, 7 is a temperature distribution calculation circuit, 8 is an optical switch, 10 is a measuring device, 11 is a controller, 20 is an optical fiber for sensor, 21.22゜23a and 23s are optical fibers, 31.32 is an optical splitter, 30s is a measurement circuit with an OT connection for Stokes light, and 30a is an OT for anti-Stokes light.
The H 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 reflected light formed by backscattered light generated in the fiber is guided to the measurement system, and these reflected lights are From inside,
Light in the wavelength range corresponding to the center wavelength of Raman scattered light or Rayleigh scattered light is separated and guided to a receiver, and the intensity of these lights is sampled to determine the optical fiber temperature distribution, as well as the incident time of the optical pulse and the reflected light. In an optical fiber type distributed temperature sensor that determines the generation position of reflected light from the difference in the time when the reflected light reaches the measurement system, an optical switch is provided in the measurement system in the middle of the path that guides the reflected light to the receiver, and division control means for controlling a switch to divide the temperature measurement distance range of the sensor optical fiber into a plurality of sections and setting only reflected light corresponding to each measurement section to enter the light receiver; 1. An optical fiber type distributed temperature sensor comprising means for adjusting the sensitivity of a light receiver in accordance with the intensity of reflected light in a section. 2. Set a delay time set in which the first delay time is shorter than the sampling time interval for measuring the reflected light and the sum of the delay times is the sampling time interval, and sequentially extend the sampling start time by the time corresponding to each delay time. 2. The optical fiber type distributed temperature sensor according to claim 1, further comprising measurement point control means for reducing the interval between the temperature measurement points overall.