JPH06331456A - Optical fiber system apparatus for measuring temperature distribution - Google Patents

Abstract

PURPOSE:To improve temperature measurement precision at a distant from a predetermined point. CONSTITUTION:A light pulse emitted from a high output pulse light source 1 falls into a measuring fiber 3. A spectral device 2 separates and outputs stakes light and anti-stokes light caused by Raman scattering out of back scattering light generated in the fiber 3. Each light signal is amplified by amplifiers 6A and 6C and digitized by a digital averager 8 After the transmission of a trigger signal of pulse light emission, a control circuit 8c switches terminal connection states of optical switches 4A, 4B and electric switches 7A, 7B to the reverse side in 70musec so that the signal may pass through amplifiers 6B and 6D of higher amplifications. Therefore, a signal level where temperature measurement of high precision is possible is maintained over all the regions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type temperature distribution measuring device for measuring temperature distribution in the longitudinal direction of a fiber by utilizing Raman scattered light generated in an optical fiber.

[0002]

2. Description of the Related Art Conventionally, OTDR (Optical Time-Dom)
There is known a device for measuring a temperature distribution over a long distance by using an optical fiber by using ain reflectometry) technology. An example of the configuration of such an optical fiber type temperature distribution measuring device is shown in FIG.
Shown in. In the figure, 1 is a high output pulse light source, 2 is a spectroscopic device, 13 is a built-in reference fiber, 3 is a measuring fiber, 10A and 10B are light receiving elements, 11A and 11B are amplifiers, 8 is a digital averager, and 9 is a computer. is there.

The high-power optical pulse generated from the high-power pulse light source 1 is generated by the spectroscopic device 2 and the built-in reference fiber 1.
It is incident on the measurement fiber 3 coupled to the measurement fiber 3 via the light source 3. The incident pulsed light is scattered little by little as it propagates through each point in the measurement fiber 3, and part of it is returned as backscattered light to the incident end via the core portion of the fiber. The backscattered light generated at each point returns after a time corresponding to the distance to the incident end has elapsed.

The backscattered light, though very small,
Raman scattered light whose wavelength is shifted by Raman scattering is included. The Raman scattered light includes Stokes light whose wavelength is shifted to the long wavelength side and anti-Stokes light whose wavelength is shifted to the short wavelength side with respect to the incident light. The Raman scattered light is generated by the interaction with the thermal vibration of the molecules forming the optical fiber, and its intensity changes depending on the temperature of each part of the fiber. That is, the intensity of the Stokes light and the anti-Stokes light depends on the absolute temperature. Therefore, it is possible to detect the temperature distribution of each part of the fiber by observing the temporal intensity change of the Raman scattered light and combining the relational expressions between each light intensity and the absolute temperature.

From the backscattered light, the spectroscopic device 2
Stokes light and anti-Stokes light are extracted and output. The intensity of each extracted light is extremely weak,
The following amplification means are provided. First, the Stokes light is photoelectrically converted by the light receiving element 10A, and the signal strength thereof is amplified by the amplifier 11A. Similarly, the anti-Stokes light is photoelectrically converted by the light receiving element 10B, and the signal strength is amplified by the amplifier 11B.

The output of each amplifier is the digital averager 8
A / D (analog / digital) converter 8a
Is converted into a digital signal by. The control circuit 8c sends a pulse signal generation trigger signal to the high-output pulse light source 1 at predetermined intervals and controls each part of the digital averager. The averaging circuit 8b calculates the averaging of the detection signals for each incident pulsed light. This reduces noise. The calculated data is sent to the computer 9. Then, the time and the light intensity are converted into temperature, and the temperature distribution at each position in the longitudinal direction of the measuring fiber 3 is obtained and displayed.

In practice, the format described below is often used for reasons such as the simplicity of data processing. First, a built-in reference fiber 13 as shown in the figure is provided, the platinum temperature measuring element 15 is fixed to this, and the temperature measurement result is sent to the computer 9. This is monitored as a reference absolute temperature, and calibration is performed by the relative temperature change amount obtained from the change in intensity of Raman scattered light described above. As a result, the absolute temperature distribution of each part is obtained.

[0008]

By the way, due to the optical loss in the fiber, the signal intensity is strong in the vicinity of the incident end, and the intensity decreases as the distance increases. Therefore, the amplifiers 11A and 11A are arranged so that the signal level in the vicinity of the incident end falls within the input dynamic range of the A / D converter 8a.
The amplification factor of B is adjusted. Since the once-adjusted amplification factor is fixed, a sufficient signal level cannot be ensured in a region farther than a certain region, so that there is a problem that the temperature measurement accuracy is reduced.

The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide an optical fiber type temperature distribution measuring device which improves the accuracy of temperature measurement far from a predetermined point.

[0010]

In order to solve the above problems, according to the present invention, an optical pulse is made incident from an incident end of an optical fiber arranged in a temperature measurement region and is emitted from the incident end. In the optical fiber type temperature distribution measuring device which measures the intensity signal of Raman scattered light and detects the temperature distribution in the longitudinal direction of the optical fiber from the waveform of the intensity signal, the Raman scattered light is amplified. And a control means for controlling the amplification factor of the amplification means to be higher than before after a lapse of a predetermined time after the optical pulse is incident on the optical fiber. To do.

[0011]

The optical pulse is incident from the incident end of the optical fiber arranged in the temperature measurement region, and after a lapse of a predetermined time, the control means controls the amplification factor of the amplification means to be higher than before. Therefore, the signal level of Raman scattered light generated at a distance from the entrance end is increased, and the measurement accuracy at a distance is improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical fiber type temperature distribution measuring apparatus according to the present embodiment. The same parts as those in FIG. 3 are designated by the same reference numerals and the description thereof will be omitted. In the figure, 4A and 4B are optical switches, and 5A to 5D.
Is a light receiving element, 6A to 6D are amplifiers, and 7A and 7B are electric switches.

The high-power pulse light source 1 has a wavelength of 1.32.
μm, and the measuring fiber 3 has a cladding diameter of 1
A graded index type of 25 μm and a core diameter of 50 μm is used. The transmission loss rate in the longitudinal direction of the fiber is about 1 dB per km.

In the figure, Stokes light and anti-Stokes light output from the spectroscopic device 2 are supplied to optical switches 4A and 4B, respectively. The optical switch 4A receives the input Stokes light according to the control signal transmitted from the control circuit 8c of the digital averager 8 and receives the Stokes light.
Supply to A or 5B. Similarly, the optical switch 4B is
The input anti-Stokes light is supplied to the light receiving element 5C or 5D.

Each of the light receiving elements 5A to 5D photoelectrically converts the input light and supplies it to the amplifiers 6A to 6D. Amplifier 6A, 6B
Amplify the strength of the input signal and supply it to the two input terminals of the electric switch 7A. Similarly, amplifier 6
C and 6D respectively supply the amplified signals to the two input terminals of the electric switch 7B.

The electric switches 7A and 7B connect one of the two input terminals to the output terminal according to a control signal transmitted from the control circuit 8c of the digital averager 8. Then, the signals input from the connected input ends are supplied to the A / D converter 8a of the digital averager 8, respectively. Then, in the averaging circuit 8b, the averaging process is performed in consideration of each amplifier amplification factor before and after the switch change.

Next, the operation of this embodiment will be described. First, in the initial setting, the connection state of each switch is set so that the signal supplied from the spectroscopic device 2 passes through the amplifiers 6A and 6C. Also, the amplifier 6
The amplification factors of B and 6D are set higher than the amplification factors of amplifiers 6A and 6C. Then, when a trigger signal is sent from the control circuit 8c to the high output pulse light source 1, the spectroscopic device 2
An optical pulse is incident on the measurement fiber 3 via the.

The backscattered light generated in the measuring fiber 3 is incident on the spectroscopic device 2, and the Stokes light and the anti-Stokes light are separated and output from the backscattered light. Each optical signal is photoelectrically converted by the light receiving elements 5A and 5C, amplified by the amplifiers 6A and 6C, and converted into digital data by the digital averager 8. Then, the calculated data is processed by the computer 9 and the temperature distribution at each position in the longitudinal direction of the fiber 3 is displayed.

Further, the control circuit 8c switches the terminal connection state of each optical switch and electric switch to the opposite side after, for example, 70 μsec has elapsed after the transmission of the trigger signal. That is, the signals thereafter are supplied to the digital averager 8 via the amplifiers 6B and 6D.

FIG. 2A shows the waveforms of the signals output from the amplifiers 6A and 6C, and FIG. 2B shows the amplifiers 6B and 6C.
6 illustrates an example of a waveform of a signal output from 6D.
The horizontal axis (time) corresponds to the longitudinal distance from the fiber entrance end. In addition, in the figure, time t _c indicates the switch change time (at the time when 70 μsec has elapsed from the trigger).

As is clear from each graph, before the time t _c , the outputs of the amplifiers 6A and 6C are gradually attenuated from the maximum value, whereas the outputs of the amplifiers 6B and 6D are "0". . The amplifiers 6A and 6A are separated at time t _c.
The output of C sharply decreases to "0" level. on the other hand,
The outputs of the amplifiers 6B and 6D rise sharply from "0", reach a peak, and then gradually decrease.

Assuming that this time t _c corresponds to a distance of about 7 km converted from the optical propagation velocity in the fiber, the above-mentioned transmission loss rate (about 1 dB / km) causes a signal loss of about 7 dB at that point. Is occurring. Therefore, if each light and electric switch is not switched, a loss of 7 dB or more occurs at a distance farther than this (dotted line part in FIG. 2A). If the loss is large, the signal level required for highly accurate temperature measurement cannot be secured.

Here, this 7 km point is considered to be the limit at which highly accurate temperature detection can be performed, and the amplifiers 6B and 6D having a higher amplification degree are switched. As a result, the distance considered to enable highly accurate temperature measurement was expanded to 10 km or more. Further, microscopically, it is impossible to measure the temperature in the transitional part immediately after the switch is switched, but by means such as slightly shifting the switch switching timing for each trigger, it is possible to achieve high accuracy over the entire area. Can measure temperature.

When only the amplifiers 6B and 6D are used, the level of the signal near the incident end is A / D converter 8.
There is a problem that the input dynamic range of a is exceeded and saturation occurs. There is also a method of switching again by using another amplifier with a higher amplification factor, but if the amplification factor of the amplifier is increased, the amplifier itself and the noise component generated by the light receiving element are also amplified, so conversely the accuracy is increased. It may decrease. By taking into consideration the characteristics of the amplifier, the attenuation characteristics of the fiber, and the like, by appropriately switching the amplifiers with an appropriate number of stages, it is possible to measure temperature in a wide range and with high accuracy.

Further, as in the conventional example shown in FIG. 3, the reference fiber and the temperature measuring body may be provided in front of the temperature measuring fiber to measure the reference absolute temperature.

It should be noted that instead of providing a plurality of photoelectric conversion units and switching them as in this embodiment, a variable amplifier whose amplification factor can be set by a control signal from the outside is provided in a single photoelectric conversion unit. The amplification factor may be changed at a predetermined timing. Similarly, by providing an optical attenuator that attenuates the intensity of transmitted light by a predetermined amount by an external control signal, attenuating the optical signal from the spectroscopic device for a predetermined period, and then canceling the attenuation, the same effect can be obtained. Is obtained.

[0027]

As described above, according to the present invention, when the predetermined time elapses after the optical pulse is incident on the optical fiber, the control means makes the amplification factor of the amplification means higher than before. Controlled. Therefore, it is possible to improve the accuracy of temperature measurement far from the predetermined point.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical fiber type temperature distribution measuring device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an output waveform of each amplifier in the embodiment.

FIG. 3 is a configuration diagram of a conventional optical fiber type temperature distribution measuring device.

[Explanation of symbols]

3 ... Optical fiber for measurement (optical fiber), 6A to 6D ... Amplifier (amplifying means), 4A, 4B ... Optical switch (control means), 7A, 7B ... Electric switch (control means), 8c ...
Control circuit (control means)

Claims (1)

[Claims] 1. An intensity signal of Raman scattered light of backscattered light emitted from an incident end of an optical fiber arranged in a temperature measurement region and emitted from the incident end is measured to measure the intensity. In an optical fiber type temperature distribution measuring device for detecting a temperature distribution in the longitudinal direction of the optical fiber from the waveform of a signal, an amplifying means for amplifying the Raman scattered light, and a predetermined time after the optical pulse enters the optical fiber. An optical fiber type temperature distribution measuring device, comprising: a control unit that controls the amplification factor of the amplification unit to be higher than that after that.