JPH06180256A - Temperature measuring method - Google Patents

Abstract

PURPOSE:To realize a highly accurate temperature distribution measurement for reducing the operation processing times and completing the processing in a short time by utilizing inductive Raman scattered light in a temperature distribution measuring method using an optical fiber sensor. CONSTITUTION:While a pulse light of a laser light sources 1 is used as a laser diode light at a level where there generates inductive Raman scattering, an inductive Raman backscattered light generating in an optical fiber sensor 3 is picked up by an optical directional coupler 2, and a secondary anti-stokes which is of a specified degree, is selected by an interference filter 4. The selected light is converted into an electrical signal by a light receiver and the signal is processed on the basis of a specified formula through a signal processing circuit 6 to find out its temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed temperature measuring method using an optical fiber as a temperature sensor.

[0002]

2. Description of the Related Art When an optical pulse is incident from one end of an optical fiber, the optical pulse traveling through the optical fiber produces scattered light such as Rayleigh scattering, Brillian scattering, and Raman scattering in a medium at an arbitrary position of the optical fiber. Looking at the Raman scattered light, the light that is usually generated is natural Raman scattered light, which is a scattered light due to the linear optical effect. On the other hand, it is known that when the incident light intensity is increased, scattered light due to a nonlinear optical effect called stimulated Raman scattered light is generated.

From the Raman scattered light, it is possible to know from which position of the optical fiber the backward Raman scattered light returning backward is measured by measuring the time when the backward Raman scattered light returns to the incident end. Further, although the Raman scattered light is weak, its intensity sensitively depends on temperature, and the temperature at a predetermined position can be measured by measuring the intensity change.

Such a conventional temperature measuring method is described, for example, in the technical report “Distributed temperature sensor using LD pumped solid-state laser”.
(The 5th Workshop on Lightwave Sensing Technology, Japan Society of Applied Physics, 1
(May 990), or in Japanese Patent Application Laid-Open No. 1-212326.

The measuring methods disclosed in the above publications are
Generally, OTDR (Optical Time Domain)
n Reflectometry), and the details thereof are described in detail in the above-mentioned publications and the like, so an outline thereof will be briefly described with reference to FIG.

As shown in the figure, the laser light source 1 including, for example, a laser diode is driven to send out an optical pulse signal having a reference wavelength, which is incident on the incident end of the optical fiber sensor 3.
Backscattered light that scatters at each part in the length direction of the optical fiber and returns to the incident end is extracted by the optical directional coupler 2, and Raman scattered light of the Raman scattered light is extracted from the backscattered light through two filters 4 having different wavelengths. The Stokes light and the anti-Stokes light are extracted and converted into electric signals by the light receiving elements 5 and 5, respectively, and the distribution temperature for each position in the length direction of the optical fiber, which represents the time of the light reception signal, is calculated from the light reception power of the signal to the signal processing circuit 6. Then, it is calculated and measured based on a predetermined arithmetic expression.

[0007]

By the way, in the above-mentioned conventional method for measuring the distributed temperature, the pulsed light of the light source is incident as the light of such intensity that Raman scattering becomes natural Raman scattering.
When the intensity of the pulsed light from the light source is increased, the stimulated Raman scattering state occurs at a certain level or higher.

Natural Raman scattering is, for example, as shown in FIG.
As shown in (b), when the light intensity of the Stokes light and the anti-Stokes light is plotted on the vertical axis on the logarithmic axis and the length distance of the optical fiber sensor is plotted on the horizontal axis, the intensity level is weak but linear with the distance. Changes to a stable change. However, it is assumed that the optical fiber is uniform and the temperature is uniform. And
As shown in (c), when the temperature changes in a part of the length distance, the light intensity changes, and the temperature can be measured from the change in the intensity.

In this case, the value of the ratio of the Stokes light to the anti-Stokes light is generally used with the Stokes light as a reference, but the intensity of the anti-Stokes light may be used for the measurement.

On the other hand, in the stimulated Raman scattering state, a change in intensity can be measured at the incident end, but the change in intensity changes greatly with distance and is unstable. Is said to be difficult to apply. Therefore, in the conventional temperature measurement, a method is adopted in which the intensity of the light pulse is limited to a predetermined level or less so that the stimulated Raman scattering does not occur.

However, since the backscattered light received in the natural Raman scattering state is weak, noise is likely to enter, and the averaging process of the received light signal is repeated tens of thousands times in order to improve the S / N ratio. The measurement accuracy is secured.

Therefore, it is difficult to improve the measurement accuracy, it takes a long time to perform the measurement, and it is not suitable for the purpose of detecting a sudden temperature change. Therefore, the temperature measurement method using the stimulated Raman scattered light positively is carried out. I couldn't.

In the present invention, the pulsed light intensity of the light source incident on the optical fiber sensor is stimulated by taking into account the problem of the conventional method of measuring the distributed temperature by the optical fiber which is measured in the natural Raman state. The scattered light is set to a level at which it will be actively used as backward Raman scattered light, and the temperature distribution is measured using a signal within the range where the stimulated Raman scattered light is stable. It is an object of the present invention to provide a temperature measuring method having a short period.

[0014]

As a means for solving the above-mentioned problems, the present invention provides pulsed light from one end of an optical fiber, and temporally samples and measures Raman backscattered light generated in the optical fiber. In the temperature measurement method that measures the temperature distribution of the optical fiber by performing arithmetic processing on the obtained data, set the intensity of the light source used to a level at which Raman backscattered light generated in the optical fiber becomes stimulated Raman scattered light. Then, the temperature is measured by receiving the Raman scattered light returning from the time from the pulse incident end to the optical fiber to the position where the intensity of the stimulated Raman scattered light reaches the peak.

In this case, it is preferable to use a second-order anti-Stokes light as the stimulated Raman scattered light.

[0016]

In the temperature measuring method according to the present invention described above, the light pulse intensity of the light source is set to a level at which the light scattering in the optical fiber becomes the stimulated Raman scattering state, and the light is incident on the optical fiber sensor. Stimulated Raman scattered light has a higher intensity than natural Raman scattered light and can obtain a large S / N ratio of the received light signal. That is, it can be obtained in a shorter time.

In the stimulated Raman scattering state, conventionally, it is considered that the change in the light intensity returning from each part of the optical fiber is not a stable linear change as under natural Raman scattering, but is unstable. Was there.

However, as a result of various experiments, as shown in FIG. 4, the light intensity changes in a curve shape as the distance from the incident end increases, but the light intensity locally changes at a certain distance apart from the peak value at the incident end. The result is that there is a position Lp that exhibits a specific peak value.

The change of the light intensity is unstable at a position farther than the distance Lp, but is stable from the incident end to the distance Lp, and if the light intensity and the wavelength of the light source are determined, the respective changes will occur. Shows a constant change in state.

In this case, as shown in FIG. 4, when the temperature partially changes in the section of the distance Lv of the optical fiber, the light intensity changes according to the temperature. Therefore, the distribution temperature of a predetermined portion can be measured by obtaining the light intensity change of the portion where the temperature change occurs with respect to the curve serving as the reference temperature.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram of a measuring apparatus used to carry out the temperature measuring method of the present invention.

The laser light source 1 is a semiconductor laser excitation type YLF.
The optical fiber used for the optical fiber sensor 3 using a pulse laser is a multimode GI fiber. The optical directional coupler 2 uses a 50:50 coupler. An interference filter 4 selects Raman scattered light of a predetermined order. The light selected by the interference filter 4 is received as continuous light by the light receiver 5 made of Ge-APD,
It is converted into an electric signal. A signal processing circuit 6 includes a program for calculating a predetermined temperature, a circuit for controlling the pulsed light of the light source, and the like. Reference numeral 7 is a personal computer for displaying data.

In this embodiment, the light receiver 5 is not provided with two sets for Stokes light and anti-Stokes light as in the conventional device, but actually only one set for anti-Stokes light is used. There is. As shown in FIG. 3, the natural Raman scattered light of (a) can be obtained even if the first-order Stokes light and anti-Stokes light are weak, whereas the stimulated Raman scattered light of (b) is obtained. Stokes light is the primary, secondary ...
This is because the ghost part of each order has an effect, and in this embodiment, the secondary light of the anti-Stokes light, which has little effect, is used.

FIG. 2 shows an example of a result obtained by measuring the secondary light of the anti-Stokes light according to the measuring principle shown in FIG.
This measurement result shows that the OTD of the second anti-Stokes light when the temperature between the distance 1870 m and 1970 m from the fiber end is changed from 20 ° C. at room temperature to 80 ° C.
The R waveform was observed with the pump light power of 300 (W), and the ratio of scattered light was taken and recorded. The number of addition operations is 5000, and the rate of change in the temperature dependence of scattered light is 0.02% / ° C.

Of course, the above measurement results are obtained at the measurement position before the peak position under stimulated Raman scattering. It will be understood from the above results that the temperature can be measured in the range up to the peak position of the light intensity.

The temperature change rate of natural Raman scattered light is, for example, 0.1 to 0.8% / ° C. according to the above-mentioned technical report, whereas the temperature change rate of stimulated Raman scattered light is However, since the scattered light power is large, the number of repeated addition operations can be reduced, which has the advantage of shortening the measurement time. However, since the rate of temperature change is small, it is suitable for measuring a state in which the temperature is greatly different, such as 20 → 80 ° C. as in the embodiment.

[0028]

As described above in detail, in the temperature measuring method of the present invention, the light intensity of the light source is set to a level at which stimulated Raman scattering occurs or more, and under the condition, the backward Raman scattered light attenuated from the optical fiber entrance end peaks again. Since the temperature change is measured from the change of the received signal in the range up to, the temperature measurement with the same or higher accuracy in a short time compared with the conventional temperature measurement method using optical fiber using only Raman. In addition, various advantages such as application to abnormality detection such as fire position detection where the temperature change is relatively large can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of a distributed temperature measuring device according to an embodiment.

[Fig. 2] Example of temperature measurement result by optical fiber sensor

FIG. 3 is an explanatory diagram of natural Raman scattering and stimulated Raman scattering.

FIG. 4 is an explanatory diagram of a measurement principle using stimulated Raman scattered light.

FIG. 5 is a schematic block diagram of a conventional distributed temperature measuring device.

FIG. 6 is an explanatory diagram of the same measurement method as above.

[Explanation of symbols]

 1 Laser Light Source 2 Optical Directional Coupler 3 Optical Fiber Sensor 4 Interference Filter 5 Light Receiver 6 Signal Processing Circuit 7 Personal Computer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jiro Morita 1 of 105 Nishikawara, Kakogawa-cho, Kakogawa-shi, Hyogo Prefecture (72) Inventor Hideaki Nijima 1-3-3 Shimaya, Konohana-ku, Osaka Sumitomo Electric Industries Inside the Osaka Works Co., Ltd. (72) Yoshikazu Murata, 1-3 1-3 Shimaya, Konohana-ku, Osaka Sumitomo Electric Industries Co., Ltd. Osaka Works

Claims (2)

[Claims] 1. A temperature of an optical fiber is obtained by inputting pulsed light from one end of the optical fiber, temporally sampling and measuring Raman backscattered light generated in the optical fiber, and applying arithmetic processing to the obtained data. In the temperature measurement method for measuring the distribution, the intensity of the light source used is set to a level at which the Raman backscattered light generated in the optical fiber becomes stimulated Raman scattered light, and the intensity of the stimulated Raman scattered light from the pulse incident end to the optical fiber is set. A temperature measuring method characterized by receiving Raman scattered light returning from a position where the intensity reaches a peak and measuring the temperature. 2. The temperature measuring method according to claim 1, wherein second-order anti-Stokes light is used as the stimulated Raman scattered light.