JP2900081B2 - Distributed optical fiber sensor, distributed optical fiber temperature sensor, and signal processing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a distributed optical fiber sensor, a distributed optical fiber temperature sensor, and a signal processing method.

[Prior Art] FIG. 4 shows a block diagram of a conventional distributed optical fiber temperature sensor. The laser pulse oscillated from the laser pulser 20 of the light source unit is incident on the optical fiber 22 to be measured, and the Raman scattered light generated in the optical fiber 22 returns to the incident end. The Raman scattered light is guided to the measurement device by the optical directional coupler 21, and first, Stokes light and anti-Stokes light in the Raman scattered light are separated and detected by the filter 23, and the intensity thereof is detected by the photoelectric conversion units 24 and 24 ′. Is converted to an electric signal proportional to. The electrical signals are preamplifiers 25 and 2 respectively.
The signal is amplified by 5 ', and the averager 26 performs an averaging process for a predetermined number of times. The signal subjected to the averaging process is transmitted to the signal processing unit 27, where the ratio of the signal of the Stokes light to the signal of the anti-Stokes light is calculated, and a process such as conversion from the delay time of the signal to a temperature distribution related to the distance is performed.

[Problems to be Solved by the Invention] Devices other than the optical fiber are placed in a temperature environment within an operation guarantee temperature range, and one end of the optical fiber is connected here. On the other hand, the measurement object having extremely high or low temperature is generally insulated from the normal temperature region, and inevitably at least one point having an extreme temperature gradient occurs in the optical fiber. For example, in the measurement of the temperature distribution in an electric furnace, a temperature gradient of 200 to 500 ° C. occurs at several cm. The loss at such a temperature boundary point is 3 to 10 dB, remarkably wasting the dynamic range and degrading the measurement distance and the temperature resolution. Further, when the gain is set in accordance with the signal level after the temperature boundary point, there is a problem 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 a light source for injecting a laser pulse into an optical fiber to be measured, and a detector for detecting a return light from the optical fiber. A distributed optical fiber sensor comprising an acousto-optic modulator that guides light, a detector that photoelectrically converts the return light into an electric signal, and a signal processing unit that calculates a physical quantity distribution related to the distance of the optical fiber from the electric signal. A distributed optical fiber sensor characterized in that a plurality of measurement areas divided by a boundary point where a steep physical quantity gradient occurs are measured with different gains, and a laser pulse is injected from a light source to an optical fiber to be measured. The photoelectric conversion of the return light from the optical fiber, and processing of the photoelectrically converted electrical signal by a signal processing unit to determine the distance of the optical fiber. In a signal processing method for calculating a physical quantity distribution, there is provided a signal processing method characterized in that a plurality of measurement areas divided by a boundary point where a steep gradient of a physical quantity occurs are measured with different gains.

FIG. 2 shows a detection signal in the conventional apparatus, 13 is a laser pulse trigger signal, and 14 is a Raman scattering signal.
14 has a shortage of dynamic range due to local loss. FIG. 1 shows an embodiment, in which 1 is a laser generator such as a semiconductor laser, 2 is an acousto-optic modulator (hereinafter abbreviated as AOM), 3 is an optical fiber to be measured, 4 is a spectroscope, and 5 and 6 are fort diodes and the like. Detectors using photoelectric converters, 7, 8
Is an amplifier, 9 is a digital signal processing unit, 10 is a computer, 11 is a timing generator, and 12 is a measurement target. The timing generator 11 outputs a control signal of the AOM 2 in addition to the laser pulse trigger signal. The rise of the control signal and the trigger signal have a time difference corresponding to the distance to the boundary point. The scattered light is guided to a detector whose gain is set according to this level.

[Operation] FIG. 3 is a timing chart in the embodiment.
15 is the laser pulse trigger signal, 16 is the AOM control signal, 17
Is the Raman scattering signal. The AOM control signal 16 sets a gain for each section between the boundary points so as to prevent continuous measurement including a boundary point having a steep temperature gradient, and measures each individually. FIG. 3 shows a control in which only one section is measured. That is, a lower gain is set in the high temperature region, a higher gain is set in the low temperature region, and the AOM is controlled so as to avoid the measurement of the boundary point.

In the present invention, the physical quantities to be measured include temperature, pressure, break point, and the like.

Example A temperature distribution was measured using an optical fiber 3 to be measured having a measurement distance of 2 km and a transimpedance of 4 MΩ. At a distance of 600m from the pulsed semiconductor laser, which is the light source, a sudden temperature drop from room temperature 23 ° C to 1000 ° C / m
A loss of 4 dB occurred and the part at -120 ° C was detected, and subsequent measurements became impossible due to insufficient dynamic range.

Therefore, the transimpedance was set to 10 MΩ for the measurement after the point at a distance of 600 m, and the measurement was performed again by avoiding the measurement by AOM control for ± 30 m (± 0.3 μsec) before and after including the 600 m point. The temperature distribution can be measured over the entire length of the optical fiber.

In the present invention, 3 to 6 d at a rapid temperature change of several hundred degrees Celsius / m.
It is preferable to apply the method of the present invention when B loss occurs.

[Effects of the Invention] The present invention controls the AOM used for 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 object to be measured, so that the temperature can be lowered or lowered. It has an excellent effect of improving the measurement distance and the temperature resolution of the temperature distribution in the region.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 3 show an embodiment of the present invention, and FIG. 1 is a block diagram of a distributed optical fiber temperature sensor;
FIG. 2 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

Claims (4)

(57) [Claims] 1. A light source for injecting a laser pulse into an optical fiber to be measured, an acousto-optic modulator for guiding return light from the optical fiber to a detector, and a detector for photoelectrically converting the return light to an electric signal And a signal processing unit that calculates a physical quantity distribution related to the distance of the optical fiber from the electric signal, wherein a plurality of measurement areas divided by a boundary point where a steep physical quantity gradient occurs are different from each other. A distributed optical fiber sensor characterized by measuring by gain. 2. A physical quantity relating to a distance of the optical fiber by applying a laser pulse from a light source to the optical fiber to be measured, photoelectrically converting return light from the optical fiber, processing the photoelectrically converted electric signal by a signal processing unit. A signal processing method for calculating a distribution, wherein a plurality of measurement areas divided by a boundary point where a steep physical quantity gradient occurs are measured with different gains. 3. A light source for injecting a laser pulse into an optical fiber to be measured, an acousto-optic modulator for guiding backward Raman scattered light from the optical fiber to a detector, and a Stokes contained in the backward Raman scattered light. In a distributed optical fiber temperature sensor including a detector that photoelectrically converts light and anti-Stokes light into an electric signal, and a signal processing unit that calculates a temperature distribution related to the distance of the optical fiber from the electric signal, a steep temperature gradient is obtained. A distributed optical fiber temperature sensor, wherein a plurality of measurement regions divided by generated boundary points are measured with different gains. 4. A laser pulse is incident on an optical fiber to be measured from a light source, Stokes light and anti-Stokes light contained in backward Raman scattered light from the optical fiber are photoelectrically converted, and the photoelectrically converted electric signal is converted into a signal. A signal processing method for calculating a temperature distribution with respect to the distance of the optical fiber by processing in a processing unit, wherein a plurality of measurement areas divided by a boundary point where a steep temperature gradient occurs are measured with different gains. Signal processing method.