JPH04134228A - Distribution type optical fiber sensor and signal treating method - Google Patents

Abstract

PURPOSE:To improve the measuring distance and the temperature resolution of temperature distribution in a low or high temperature region by making the control of AOM used for an optical directional coupler and the sensitivity setting, synchronized with the control, of a detector, and effectively assigning a dynamic range of a device to a to-be-measured object. CONSTITUTION:A device is composed of a semiconductor laser generation part 1, acoustooptic modulator (hereinafter abbreviated to AOM) 2, to-be-measured optical fiber 3, spectroscope 4, detectors 5 and 6 using photoelectric converters, amplifiers 7 and 8, digital signal treatment part 9, computer 10, timing generator 11, and to-be- measured object 12. The timing generator 11 outputs the control signal of the AOM 2 besides a laser pulse trigger signal. The rise of the control signal and a trigger signal have a time difference corresponding to a distance to a boundary point. This guids Raman scattering light after the boundary point to the detectors gain-set conforming to the level. That is, a lower gain is set in a high temperature region and a higher gain is set in the measurement of a low temperature region, and the AOM is controlled so as to avoid the measurement of the boundary point. Thus temperature distribution can be measured over the overall length of an optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a distributed optical fiber temperature sensor and a temperature measurement method.

[Prior Art] A block diagram of a conventional distributed optical fiber temperature sensor is shown in FIG. A laser pulse oscillated from a laser pulser 20 in the light source section is input to an optical fiber 22 to be measured, and Raman scattered light generated in the optical fiber 22 returns to the input end. The Raman scattered light is transmitted to the optical directional coupler 2.
1, the light is guided to the measuring device, and first, the Stokes light and anti-Stokes light in the Raman scattered light are separated and detected by the filter 23, and each is converted into an electric signal proportional to the intensity by the photoelectric conversion unit 24, 24'. The electrical signals are amplified by preamplifiers 25 and 25' respectively, and averaged by 25 and 25'.
In step 6, averaging processing is performed a predetermined number of times. The averaged signal is transmitted to the signal processing section 27, where the ratio of the Stokes light and anti-Stokes light signals is calculated, and processing such as conversion from signal delay time to temperature distribution with respect to distance is performed.

C Problems to be Solved by the Invention C Devices other than optical fibers are placed in a temperature environment within the guaranteed operating temperature range, and one end of the optical fiber is connected here. On the other hand, objects to be measured at extremely high or low temperatures are generally insulated from the room temperature range (and inevitably there will be at least one point in the optical fiber with an extreme temperature gradient. For example, measurement of temperature distribution in an electric furnace, etc.) A temperature gradient of 200 to 500 degrees Celsius occurs over a few cm.The loss at such temperature boundary points is 3 to 10 dB, which significantly wastes the dynamic range and degrades the measurement distance and temperature resolution. .

Furthermore, if the gain is set in accordance with the signal level after the temperature boundary point, there is a problem in that the detector is saturated by the signal before the boundary point.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a light source that enters a laser pulse into an optical fiber to be measured, and a detector that detects the return light from the optical fiber. In a distributed optical fiber sensor comprising an acousto-optic modulator that guides light to the optical fiber, a detector that photoelectrically converts the returned light into an electrical signal, and a signal processing unit that calculates a physical quantity distribution regarding the distance of the optical fiber from the electrical signal. , a distributed optical fiber sensor that measures multiple measurement areas divided by boundary points where steep gradients of physical quantities occur, each with a different gain, and a laser pulse input from a light source into an optical fiber to be measured. However, in a signal processing method that photoelectrically converts the return light from the optical fiber and processes the photoelectrically converted electrical signal in a signal processing unit to calculate a physical quantity distribution regarding the distance of the optical fiber, a steep gradient of the physical quantity occurs. The present invention provides a signal processing method characterized in that a plurality of measurement regions divided by boundary points are measured with different gains.

FIG. 2 shows detection signals in a conventional device, where I3 is a laser pulse trigger signal and 14 is a Raman scattering signal. In No. 14, the dynamic range is insufficient due to local loss. FIG. 1 shows an embodiment, where 1 is a laser generator such as a semiconductor laser, 2 is an acousto-optic modulator (hereinafter referred to as AOM), and 2 is an acousto-optic modulator (hereinafter referred to as AOM).
), 3 is an optical fiber to be measured, 4 is a spectrometer, 5.
6 is a detector using a photoelectric converter such as a photodiode;
7.8 is an amplifier, 9 is a digital signal processing unit, 10 is a computer, 11 is a timing generator, and 12 is an object to be measured. The timing generator 11 outputs a control signal for the AOM 2 in addition to the laser pulse trigger signal. The rise of this control signal and the trigger signal have a time difference corresponding to the distance to the boundary point. As a result, the Raman scattered light after the boundary point is guided to a detector whose gain is set according to this level.

[For use] Figure 3 is a timing chart in the embodiment, and 1
5 is a laser pulse trigger signal, 16 is an AOM control signal, and 17 is a Raman scattering signal. Using the AOM control signal 16, a gain is set for each section between the boundary points so that continuous measurements including boundary points having a steep temperature gradient are not performed, and each section is individually measured. FIG. 3 shows control so that only one section is measured. That is, a lower gain is set in the high temperature region, a higher gain is set for measurement in the low temperature region, and the AOM is controlled so as to avoid measurements at the boundary points.

In the present invention, temperature, pressure, breaking point, etc. are measured as physical quantities to be measured.

[Example] Using an optical fiber to be measured with a measurement distance of 2 km, temperature distribution was measured with the transimpedance set to 4 MΩ. The distance from the pulsed semiconductor laser that is the light source is 60 (1
Rapid temperature drop of 1000° from room temperature 23°C at point m
A loss of approximately 4 dB occurred due to C/m, and a portion of -120° C. was detected, making subsequent measurements impossible due to insufficient dynamic range.

Therefore, for the measurement of the part after the distance of 600 m, the transimpedance was set to IOMΩ, and the
Measurement was avoided for ±30 m (±0.3 μsec) before and after the point m by AOM control, and the measurement was performed again, and it became possible to measure the temperature distribution over the entire length of the optical fiber.

In the present invention, a rapid temperature change of several hundred'C/m
Preferably, the method of the invention is applied when a loss of ~6 dB occurs.

[Effect of the invention] The present invention controls the AOM used in the optical directional coupler and sets the sensitivity of the detector in synchronization with the AOM, and effectively allocates the dynamic range of the device to the measurement target. It has the excellent effect of improving the measurement distance and temperature resolution of the temperature distribution in the area.

[Brief explanation of the drawing]

1 and 3 show embodiments of the present invention, FIG. 1 is a block diagram of a distributed optical fiber temperature sensor, and FIG. 3 is a block diagram of a distributed optical fiber temperature sensor.
This figure is a timing chart, FIG. 2 is a timing chart of a conventional example, and FIG. 4 is a block diagram of a conventional distributed optical fiber temperature sensor. 2...AOM 11...Timing generator Figure 2

Claims (4)

[Claims] (1) a light source that injects a laser pulse into an optical fiber to be measured; an acousto-optic modulator that guides the return light from the optical fiber to a detector; a detector that photoelectrically converts the return light into an electrical signal; In a distributed optical fiber sensor equipped with a signal processing unit that calculates a physical quantity distribution related to the distance of the optical fiber from the electric signal, a plurality of measurement areas divided by boundary points where a steep gradient of the physical quantity has occurred are each measured with a different gain. A distributed optical fiber sensor that measures (2) Injecting a laser pulse from a light source into an optical fiber to be measured, photoelectrically converting the return light from the optical fiber,
In a signal processing method in which a photoelectrically converted electrical signal is processed by a signal processing unit to calculate a physical quantity distribution related to the distance of the optical fiber, a plurality of measurement areas divided by boundary points where a steep gradient of the physical quantity occurs is divided into different measurement areas. A signal processing method characterized by measurement by gain. (3) A light source that injects a laser pulse into the optical fiber to be measured, an acousto-optic modulator that guides the backward Raman scattered light from the optical fiber to the detector, and a Stokes light contained in the backward Raman scattered light. A distributed optical fiber temperature sensor comprising a detector that photoelectrically converts each anti-Stokes light into an electric signal, and a signal processing unit that calculates a temperature distribution with respect to the distance of the optical fiber from the electric signal,
A distributed optical fiber temperature sensor that measures multiple measurement areas, each with a different gain, divided by boundary points where steep temperature gradients occur. (4) Injecting a laser pulse from a light source into an optical fiber to be measured, photoelectrically converting Stokes light and anti-Stokes light contained in the backward Raman scattered light from the optical fiber,
In a signal processing method in which a photoelectrically converted electrical signal is processed by a signal processing unit to calculate the temperature distribution with respect to the distance of the optical fiber, a plurality of measurement regions divided by boundary points where a steep temperature gradient occurs are each processed with different gains. signal processing method to measure.