JPH0743286B2 - Optical fiber distributed temperature sensor - Google Patents

Description

The present invention relates 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 with temperature. By detecting this change using a known OTDR method, the length of the optical fiber It is known to obtain temperature information along the direction, and to use Rayleigh scattered light or Raman scattered light. Among these, the method using Raman scattered light is considered to be a promising method because the signal change amount is large with respect to temperature change although the signal is weak.

An optical fiber type distributed temperature sensor utilizing Raman scattered light (hereinafter, simply referred to as Raman optical fiber type distributed temperature sensor) has a wavelength λ _o , a pulse width T _W , from one end of the optical fiber.
The backscattered light of the Stokes light of the wavelength λ _s , which is the two components of the Raman scattered light generated in the optical fiber when the light of the pulse period T _P is incident, and the anti-Stokes light of the wavelength λ _as ,
Assuming that the pulsed light incident time is t = 0, each is a function of time.
Measured as fs (t) and fas (t), the ratio of these is fas
The fact that (t) / fs (t) is purely a function of temperature and the time for the backscattered light returning from the distance l in the optical fiber to reach the end of the optical fiber (end of the optical pulse) is Assuming that the speed of light inside is C ₀ , 2l / C
It is a device that measures the linear temperature distribution along the optical fiber by utilizing the fact that it is _zero . The backscattered light of Stokes light and anti-Stokes light is measured by almost the same measurement method as the OTDR used for detecting the breaking point of the optical fiber.

This Raman optical fiber distributed temperature sensor can be used to control the transmission capacity, etc. by knowing the temperature distribution in the longitudinal direction of the power cable by laying an optical fiber cable for the sensor along the power cable, for example. It is also possible to detect a part where the temperature is high due to deterioration of the cable or the like. Further, when used for controlling the temperature of production lines and equipment of various plants and for detecting fires in buildings, tunnels, etc., it is possible to locate the fire occurrence position.

FIG. 4 shows a configuration example of a conventional Raman optical fiber type distributed temperature sensor. This temperature sensor comprises a sensor optical fiber 6 (usually having a core diameter of 50 μm or more) and a sensor body 22. The light emitted from the sensor pulse light source 4 of the sensor body 22 is guided to the sensor optical fiber 6 through the optical fiber 19a and the optical branching device 5a, and the backscattered light generated in the sensor optical fiber 6 is branched. It is guided to the optical branching device 5b through the device 5a and the optical fiber 19d. Of the backscattered light divided into two here, the one guided to the optical fiber 19b is the central wavelength λ.
OTDR measurement of anti-Stokes light composed of _as optical filter 8a, photoreceiver 3a, and averaging circuit 2a
Used to determine (t). What is guided to the optical fiber 19c is an optical filter 8b having a central wavelength λ _s , a light receiver 3
b, used for obtaining the function fs (t) by the OTDR measurement of the Stokes light in the Stokes light measurement system 30b including the averaging processing circuit 2b. Then, in the temperature distribution calculation circuit 1b, fas
By calculating (t) / fs (t), the linear temperature distribution along the sensor optical fiber 6 is measured.

[Problems to be Solved by the Invention] The Raman optical fiber type distributed temperature sensor can measure temperature by the method described above, and the Raman optical fiber type distributed temperature sensor has been studied so far. Among them, since it has a high sensitivity and is a practical method, it may be possible to measure a long temperature distribution reaching up to about 20 km.

However, in such a long distance measurement, it becomes difficult to secure a satisfactory temperature resolution.
Here, the temperature resolution of the optical fiber type distributed temperature sensor is given by the equation (1).

R _T (X) = 10 × Log ₁₀ {1 + 1 / (n × 1
0 ^{−2 × α / 10 × x} )} / β (1) R _T (X): Temperature resolution at distance x 1 / n: Resolution of sensor body α: Transmission loss of optical fiber x: Distance β: Temperature temperature Assuming that the resolution of the sensor body 1 / n and the transmission loss α of the optical fiber are practical values of 1/2 ¹⁶ (corresponding to 16-bit arithmetic inside the sensor) and 0.5 dB / km, Since the obtained temperature sensitivity is 0.01 dB / ° C, 20k
The temperature resolution at the m point is 0.66 ℃. The temperature resolution considered here does not correspond to the temperature measurement accuracy itself, but corresponds to the temperature difference that can be read, but in the case of general temperature sensors, this temperature resolution is up to about 0.1 ° C, Then, the resolution is 1/2 ¹⁶ ≈1.5 × 10 ^{-5, which} is very high. Nevertheless, the optical fiber type distributed temperature sensor has a problem that the temperature becomes low as described above and the temperature resolution is insufficient.

It is an object of the present invention to provide an optical fiber type distributed temperature sensor capable of measuring the temperature distribution over a long distance by overcoming the above-mentioned drawbacks of the prior art.

[Means for Solving the Problem] An optical fiber type distributed temperature sensor of the present invention is provided in a measurement temperature region, an optical fiber for a sensor having a core diameter of 20 μm or less, and a sensor driven by pseudo random pulses. A light source with a small peak power that causes pulsed light to enter the optical fiber for use and a backscattered light emitted from the optical fiber for a sensor are guided, and the intensity of the Stokes light and anti-Stokes light due to Raman scattering among the backscattered light is The measuring system is provided in synchronism with the pseudo random pulse, and the intensity ratio of the two is calculated to obtain the distribution temperature of the optical fiber.

[Operation] The relationship between the core diameter of the optical fiber used as the sensor optical fiber and the temperature sensitivity is shown in FIG. When the core diameter is 30 μm or more, the temperature sensitivity is about 0.01 dB / ° C, which is almost constant, but the temperature sensitivity increases in a region smaller than 30 μm, and it is almost constant when the core diameter is 20 μm or less. . From this, it can be seen that if a fiber having a core diameter of 20 μm or less is used for a sensor, a sensor having a sensitivity about twice as high as that of a conventional sensor using an optical fiber having a core diameter of 50 μm or more can be obtained.

However, the purpose of increasing the sensitivity using an optical fiber with a core diameter of 20 μm or less was to achieve high resolution in long-distance measurement.
In order to obtain the measurement temperature accuracy commensurate with the resolution, light with a peak power of 100 W or more must be incident on the optical fiber as the measurement light source. A pulsed YAG laser is known as such a high-output light source, and this can be used as a measurement light source. Therefore, when a measurement was attempted using a pulsed YAG laser, the OTDR measurement result of the anti-Stokes light was different from the expected measurement waveform, as shown in Fig. 3, and the saturation level of the photodetector was measured at a short distance. It was found that the excess signal was incident and the photoreceiver was saturated. As a result of examining the cause, it was found that the strong signal in the short distance portion was due to stimulated Raman scattering generated in the optical fiber. Stimulated Raman scattering is a non-linear effect that occurs when the optical fiber incident light level is high, and is a phenomenon in which a laser oscillation phenomenon occurs in an optical fiber. When stimulated Raman scattering occurs, temperature measurement cannot be performed because Raman scattering with an intensity proportional to temperature does not occur. Up to now, we have performed measurements under conditions that do not require consideration of that phenomenon, but with a core diameter of 20μ, which can obtain high-resolution characteristics over long distances.
As a result of using a high-power YAG laser in addition to an optical fiber of m or less, stimulated Raman scattering occurs and temperature distribution measurement cannot be performed. The threshold of incident light that causes stimulated Raman scattering of the optical fiber is several W when the core diameter is 20 μm. Therefore, the light source used to measure the temperature distribution needs to have a peak power of several W or less.
It turns out that measurement cannot be performed.

Therefore, as a result of various studies as to whether or not high-resolution measurement cannot be performed over a long distance using an optical fiber having a core diameter of 20 μm or less, a pseudo used in the communication field when a communication line with a poor S / N is used. We came to think that the random pulse method could be used.

The pseudo-random pulse system is, for example, a signal that is sent as an M-sequence signal, and the receiving side receives an S-N signal with poor S / N correlation by demodulating it with the M-sequence signal used at the time of transmission to demodulate the S-signal. This is a method to improve / N. This method is the measuring means of the optical fiber type distributed temperature sensor.
By applying it to the OTDR and improving the S / N, even if a laser with a small optical fiber incident light level with a single pulse is used, the S / N of the same level or more is secured compared to the conventional sensor. The high-sensitivity optical fiber having a small core diameter according to the invention is used as an optical fiber for a sensor to enable high-resolution temperature distribution measurement over a long distance.

[Embodiment] An embodiment of the optical fiber type distributed temperature sensor of the present invention will be described below with reference to FIG. The basic configuration of the distributed temperature sensor of this embodiment is almost the same as that of the conventional example shown in FIG.
The different points are as follows.

As the light source 4a, a 1,300 nmcw oscillation semiconductor laser with a pigtail (optical output 4 mw) is used, and the drive signal of this laser is generated by the pseudo random signal generation circuit 7. The basic pulse width of the pseudo-random pulse is 10 nsec, and by using the 15th-order M-sequence signal, one cycle is about 330μ.
s. From the light source 4a through the optical fiber 19a in the sensor main body 20 constituting the measurement system, the optical branching device 5a, the core diameter 10μ
The light incident on the sensor optical fiber 6 of m generates Raman scattered light having an intensity according to the temperature in the optical fiber 6 for sensor. In the anti-Stokes light receiving system 30a and the Stokes light receiving system 30b that receive this backscattered light, the synchronization signal 8 from the pseudo random signal generation circuit 7 is received by the averaging processing circuits 2a and 2b, and the received signal is synchronized with this. The averaging process is performed, for example, 2 ¹⁷ times. The received signals of the two systems for which the averaging process has been completed are guided to the temperature signal calculation circuit 1a with a correlation detection function, where the correlation process is performed for each system, and normal OTDR waveforms for anti-Stokes light and Stokes light are obtained. After the restoration, the same processing as that of the conventional sensor such as obtaining the intensity ratio of the anti-Stokes light and the Stokes light is performed, and the temperature distribution signal along the optical fiber can be obtained. According to this, one cycle of the M-sequence signal is about 330 μsec.
It is possible to measure the temperature distribution along an optical fiber with a length of more than km.

In addition, although the above embodiment has described the case where the present invention is applied to the temperature distribution measurement over a long distance, even when the distance is short, there are many connection points of the optical fiber and there is a light transmission loss or the like at the connection point. Also, the present invention is effective. Optical fiber connection
Usually, connector connection or fusion splicing is used, but each of these connection points causes an optical transmission loss of about 0.2 dB. For example, if there is a connection point for every 100 m on an optical fiber with a total length of 50 km, the increase in loss due to the connection will be 5 dB per way.
Which is the case for an optical fiber with a transmission loss of 0.5 dB / km,
It is considered that the loss is equivalent to the addition of an optical fiber with a length of 10 km, which corresponds to an optical fiber with no connection point of 15 km. Therefore, the present invention can be applied to such a case where there are many connection points, and the effect is great.

For example, when using an optical fiber for a sensor with a built-in power cable or along the outside in order to know the temperature distribution after laying the power cable, the number of connection points may increase even if the total length is not long due to the construction work. As expected, it is very effective in such a case to use the optical fiber having a core diameter of 20 μm or less, which is highly sensitive to temperature.

[Effect of the Invention] As described above, according to the present invention, the core of the optical fiber for sensor is 20 μm.
The temperature sensitivity is increased to m or less, and a light source with a small peak power that does not cause stimulated Raman scattering is used as the incident light source.
Furthermore, we have improved the S / N characteristics even with a light source with a small peak power by using a pseudo-random pulse method for the light source and measurement system, so not only long lines but also lines with many connection points, It is possible to perform temperature distribution measurement with a resolution as high as that of a general spot (single point) temperature sensor.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical fiber type distributed temperature sensor showing an embodiment of the present invention, FIG. 2 is a characteristic diagram showing a relationship between a core diameter of an optical fiber for sensor and temperature sensitivity, and FIG. 3 is a pulse oscillation.
FIG. 4 is a light intensity characteristic diagram of an anti-Stokes light OTDR waveform when a YAG laser is used as a light source, and FIG. 4 is a configuration diagram showing an example of a conventional optical fiber type distributed temperature sensor. In the figure, 4a is a light source having a small peak power, 6 is an optical fiber for sensor, 7 is a pseudo random signal generating circuit, and 20 is a sensor main body which constitutes a measuring system.

Claims (1)

[Claims] 1. The core diameter disposed in the measurement temperature region is 20 μm.
The following sensor optical fiber, a light source with a small peak power driven by a pseudo-random signal generated by a pseudo-random signal generation circuit, and a pulse light from the light source are incident on the sensor optical fiber The backscattered light emitted from the optical fiber is guided, and the intensities of the Stokes light and the anti-Stokes light due to the Raman scattered light in the backscattered light are obtained in synchronization with the pseudo random pulse, and the intensity ratio of both is calculated. An optical fiber type distributed temperature sensor, comprising an arithmetic measuring device for obtaining a distributed temperature of the optical fiber for a sensor.