JPH05172657A - Optical fiber distributed temperature sensor - Google Patents

Abstract

PURPOSE:To obtain an optical fiber distributed temperature sensor which enables measurement of a temperature distribution at a high accuracy without limiting a detection temperature range by shortening processing time of detection light with a simple construction. CONSTITUTION:An optical fiber temperature distributed sensor makes a laser pulse 12 enter an optical fiber 13 and return light from the optical fiber is detected to measure a temperature distribution along the optical fiber. Then, Brillouin scattred light out of the return lights is used to measure temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed optical fiber temperature sensor for injecting a laser pulse into an optical fiber and detecting return light from the optical fiber to measure a temperature distribution along the optical fiber.

[0002]

2. Description of the Related Art In a conventional distributed optical fiber temperature sensor, the Raman scattered light of the return light from the optical fiber is used and the temperature distribution is measured by utilizing the temperature dependence of the intensity of the anti-Stokes light of the Raman scattered light. Was. FIG. 3 shows an example of the configuration of a conventional distributed optical fiber temperature sensor.
Shown in. A laser pulse 3 generated from the laser diode 2 is incident on the optical fiber 4.

A laser pulse 3 is introduced into an optical fiber 5 to be measured, whose temperature distribution is to be measured, through an optical coupler 1 composed of an optical fiber coupler, an acousto-optical element, etc. (solid arrow). FIG. 4 shows the frequency relationship between the incident pulse and the Raman scattered light. 20 indicates an incident pulse, and 21 and 22 indicate Stokes light and anti-Stokes light, respectively. The frequency difference portion indicated by J and K in the figure is several tens GHz.

Of the backscattered light (returned light) reflected in the optical fiber 5, the Raman scattered light travels backward as indicated by the broken line arrow, passes through the optical coupler 1, and is detected by a photoelectric converter, amplifier, AD converter, etc. Guided by 6 and 7. The detection systems 6 and 7 detect the intensities of the Stokes light and the anti-Stokes light of the Raman scattered light separated by a filter or the like (not shown). The detected Stokes light and anti-Stokes light intensity signals are averaged through an adder 8 and converted by a computer 9 into a temperature distribution and the data is displayed.

[0005]

However, the conventional distributed optical fiber temperature sensor utilizing Raman scattered light has the following problems.

First, since the Raman scattered light signal to be detected is extremely weak, it is usually necessary to perform averaging processing tens of thousands of times in order to improve the S / N ratio of the detected signal. Therefore, it takes a long time to process the detection signal, and a time delay occurs between the actual temperature of the optical fiber and the display temperature.

Secondly, when detecting a change in the intensity of the anti-Stokes light, incident light of sufficient intensity is required to obtain high temperature accuracy, but it is difficult to inject high intensity light into the optical fiber. It was Especially in the low temperature region, the strength becomes weak, so that the measurable temperature range is restricted.

The present invention has been made in view of the above-mentioned drawbacks of the prior art. It is a distribution that can shorten the detection light processing time with a simple structure and can measure the temperature distribution with high accuracy without limiting the detection temperature range. Type optical fiber temperature sensor.

[0009]

In order to achieve the above object, the present invention provides a distributed type in which a laser pulse is incident on an optical fiber and the return light from the optical fiber is detected to measure the temperature distribution along the optical fiber. In the optical fiber temperature sensor, the temperature is measured using the Brillouin scattered light of the return light.

In a preferred embodiment, the Brillouin scattered light and the reference light are interfered with each other to detect a frequency change of the Brillouin scattered light, and the temperature is measured based on the frequency change. In a further preferred embodiment, a CW laser is used as the reference light.

[0011]

[Operation] Brillouin scattering
When light of a certain frequency is incident on a substance, it is a phenomenon in which light of a frequency shifted by the amount corresponding to the energy of the acoustic phonon of the substance is scattered, the light interacts with the sound wave in the substance, and the frequency is Scattered with a slight shift, fluctuations in material density are caused by sound waves, and the refractive index changes. Since the frequency of such Brillouin scattered light changes with temperature, the temperature can be detected by measuring the frequency change.

More specifically, first, a laser pulse of frequency ν is incident on the optical fiber to be measured, and Brillouin scattered light is detected from the return light from this optical fiber. The frequency of Brillouin scattered light is ν-Δν depending on the temperature.
Has changed to. Here, the CW laser light that gives a continuous output of a known frequency ν-Δν 'from another laser light source is caused to interfere. From the spectrum of this interference light, the frequency difference Δν−Δν ′ between the Brillouin scattered light and the CW laser light is obtained. If Δν−Δν ′ is obtained, Δν is obtained and the temperature is further obtained.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the structure of a distributed optical fiber temperature sensor according to the present invention. A laser light source 11 composed of a solid-state laser or a semiconductor laser for emitting an incident laser pulse has four terminals NO. 1-NO. No. 4 of the optical coupler 10 having No. Connected to one terminal. The NO. An optical fiber 13 to be measured whose temperature distribution is to be detected is connected to the three terminals. Further, the NO. A photodetector 14 including a photoelectric conversion photodiode or the like is connected to the two terminals, and the photodetector 14 is connected to a computer 18 via a gate circuit 16 and a spectrum analyzer 17.

Further, the NO. A CW laser 15 that outputs laser light in a continuous oscillation state is connected to the four terminals. Further, an optical fiber 19 for the reference temperature portion is provided between the optical coupler 10 and the optical fiber 13, and the reference temperature is detected by a solid-state element such as a thermistor and input to the computer 18.

The operation of the distributed optical fiber temperature sensor having the above structure will be described below. The laser light source 11 sends the laser pulse 12 of the frequency ν to the NO.
It is incident on one terminal as shown by an arrow P. The optical coupler 10 sends the laser pulse 12 to the NO. Guided to the optical fiber 13 to be measured connected to the three terminals, the laser pulse propagates in the optical fiber 13 as indicated by arrow E. From each point in the optical fiber 13, Brillouin scattered light having a frequency different from the incident laser pulse by Δν propagates in the opposite direction as a return light as indicated by a broken arrow F.

The Brillouin scattered light is guided to the photodetector 14 by the optical coupler 10 (arrow G), and the light intensity is detected as described later. On the other hand, from the CW laser 15, continuous wave laser light whose frequency is different from the incident laser pulse by Δν 'is input as reference light as shown by an arrow H, and the optical coupler 10 guides it to the photodetector 14 (arrow I). ). The photodetector 14 simultaneously detects the Brillouin scattered light from the optical fiber 13 for the measured light and the reference laser light from the CW laser 15. Here, in order to obtain information regarding the distance direction, the detection signal is subdivided by the gate circuit 16. 2A and 2A show Brillouin scattered light detection signals, and FIGS. 2A and 2B show CW laser light detection signals.

Subsequently, the spectrum analyzer 17
As shown in FIG. 2 (B), signals extracted by the gate circuit 16 (between broken lines in FIGS. 2 (A) (a) (b)).
And the frequency difference Δν′−Δν is measured. Here, since the reference signal Δν ′ is known, the frequency change Δν of the Brillouin scattered light can be obtained. For the optical fiber to be measured, obtain information on temperature and frequency changes in advance,
Convert the measured frequency change to temperature (Fig. 2
(C)).

Such temperature measurement is repeated by shifting the gate of the gate circuit 16. That is, the same signal processing is performed by shifting the subdivided distance (broken line position) in FIG. In this way, the relationship between the distance obtained at each gate and the temperature is displayed (FIG. 2D). In this case, in order to improve the temperature accuracy, C in the optical fiber 19 for the reference temperature part
It is also possible to measure the frequency difference from the W laser light and use the data for the temperature conversion process. The distance is determined by the delay time from the emission of the pulsed light to the detection of the return light signal.

In the above embodiment, since the frequency shift of the Brillouin scattered light is as small as several tens GHz, the spectral line width of the incident laser pulse light should be as narrow as possible. DPY is a laser that meets these requirements.
A solid-state laser such as (diode, pumping, YAG laser) is preferable (spectrum width of about 1 MHz). If it is a semiconductor laser, a DFB laser is preferable.

The CW laser light used for interference needs to have an interference frequency with the Brillouin scattered light within the measurement range of the spectrum analyzer, so that it is desirable that the frequency is close to the Brillouin scattered light. Further, it is desirable that this laser light also has a narrow spectral line width for the same reason as the above-mentioned incident laser. Since the frequency of the light source used here is used as the reference light, it is configured so that it can be precisely controlled. In this case, if the wavelength can be finely adjusted from the outside, accuracy can be improved.

[0021]

As described above, according to the present invention, the Brillouin scattered light is detected from the return light from the optical fiber to measure the temperature distribution. Therefore, the conventional weak Raman scattered light is used. In addition, the averaging process is not required many times, and the measurement time is shortened and the measured temperature distribution can be displayed almost in real time. Moreover, since the temperature sensitivity is as high as several MHz / ° C. near room temperature, the temperature resolution is improved and highly accurate temperature distribution measurement is achieved. In particular, even in the low temperature region where the conventional measurement has been limited, it is possible to relatively easily measure up to absolute zero.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a distributed optical fiber temperature sensor according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a frequency difference between excitation light and Brillouin scattered light.

FIG. 3 is an explanatory diagram of a detection signal processing procedure by the distributed optical fiber temperature sensor according to the embodiment of the present invention.

FIG. 4 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 10 Optical Coupler 11 Laser Light Source 12 Incident Pulse Laser 13 Optical Fiber 14 Photodetector 15 CW Laser

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Claims (3)

[Claims] 1. A laser pulse is incident on an optical fiber,
A distributed optical fiber temperature sensor for detecting a return light from the optical fiber to measure a temperature distribution along the optical fiber, wherein the temperature is measured by using Brillouin scattered light among the returned light. Temperature sensor. 2. The distributed optical fiber according to claim 1, wherein the Brillouin scattered light and the reference light are caused to interfere with each other to detect a frequency change of the Brillouin scattered light, and the temperature is measured based on the frequency change. Temperature sensor. 3. The distributed optical fiber temperature sensor according to claim 2, wherein a CW laser is used as the reference light.