JPH04181127A - Distribution-type optical fiber sensor - Google Patents

Abstract

PURPOSE:To largely shorten measurement time by AD-converting electric signals in parallel by respectively different sampling frequencies and respectively adding them to previous addition results. CONSTITUTION:A high resolution measurement region is specified in advance so that measurement length is approximately 2km or less. Then average data of mega-sampling P/S (MSPS) is used to measure a temperature distribution over an entire length of 10km in a measurement system for slow speed sampling, that is, a preamplifier for AD-conversion 8, an AD preamplifier 9 and an AD- converter 11, added 15 to data of an addition memory 17 and sequentially compensated from a temperature at an incident end. On the other hand, in a high speed sampling measurement system, that is, an AD preamplifier 10 and an AD-converter 12, temperature conversion is performed based on data of 100 MSPS and specified start-stop addresses (a distance from the incident end). At this time a temperature at a start point is known from the temperature data of 10km to compensate an attenuation rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a distributed optical fiber sensor that can measure one signal simultaneously and in parallel with different resolutions and can significantly shorten measurement time.

[Prior Art] FIG. 3 shows a block diagram of a distributed optical fiber temperature sensor, which is a type of conventional optical fiber sensor. A laser pulse emitted from a laser pulser 30 such as a semiconductor laser in the light source section is input to an optical fiber 32 to be measured, and backward Raman scattered light generated in the optical fiber 32 returns to the input end. The backward Raman scattered light is guided to the measuring device by a light directional coupler 31, and first passes through a filter 3.
3, the Stokes light and the anti-Stokes light in the Raman scattered light are separately detected, and each is converted into an electric signal proportional to the intensity by the photoelectric conversion section 34, 34'. The electrical signals are each amplified by preamplifiers 35 and 35', and converted into digital signals by AD converters 36 and 36'. The digitized signal is transmitted to the signal processing section 37, where it undergoes processing such as averaging, taking the ratio of the Stokes light and anti-Stokes light signals, and converting it into a temperature distribution.

The AD converter 6 or 6' and the signal processing section 7 shown in FIG. 3 are shown in detail in FIG. 2.

1 is an AD preamplifier, 2 is an AD converter, 3 is an AD memory, 4 is an adder, 5 is an addition memory, and 6 is a control circuit that specifies the start and end addresses of the AD memory 3 using a sampling clock signal. The data that has been added to the addition memory 5 for a predetermined number of measurements is further transmitted to a computer or the like, where it is averaged, and the temperature distribution is calculated by taking the ratio with the averaged data of the anti-Stokes optical signal.

Conventionally, measurement has been performed using a single system (one AD converter, one AD memory, one adder, and one addition memory for each electrical signal). One method is to increase the memory capacity to cope with longer optical fiber distances, to thin out the sampling points using low-speed sampling to measure the overall physical quantity distribution, and to use high-speed sampling to measure local and steep physical quantity distributions using time-sharing methods. Two switching methods have been proposed.

[Problems to be Solved by the Invention] The problems of the former of the above two methods are as follows.

1. Since the amount of data to be averaged increases, the calculation time increases, and the effective (repetition period) becomes longer (and the measurement time becomes longer.The measurement length is 10 km, and the sampling interval is set to 1.
m, data recording requires 10000 x 10 (ns
ec) = 100 u sec, and if the cycle time of the adder is 80 nsec, then 10000 x 80(n
sec) = 800 μsec, and the repetition period is small (900 μsec is required in both cases).

2. Memory usage efficiency is poor.

The problems with the latter method are as follows.

1. It lacks real-time performance because measurements are switched in a time-division manner. That is, when performing high-resolution measurements, other areas are not observed at all, and there is a possibility that sudden temperature changes may be overlooked.

2. In order to perform highly accurate measurements, it is necessary to perform correction based on the attenuation rate of the optical fiber, but this correction requires accurate temperature at the correction starting point. For example, in order to perform high-resolution measurement of 1kl++ between 6km and 7km from the light source out of a total length of 10km by high-speed sampling, accurate temperature at the 6km point is required. Therefore, in this method, correction is performed using the latest temperature data sampled at low speed, but even if the temperature data is refreshed at regular intervals, a sudden temperature change will cause an error.

[One Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems. an optical directional coupler that guides the returned light to a measuring device, a photoelectric conversion section that photoelectrically converts the returned light into an electrical signal, and a signal processing section that calculates a physical quantity distribution regarding the distance of the optical fiber to be measured from the electrical signal. In the distributed optical fiber sensor, the signal processing unit includes a plurality of AD converters that convert the electric signals in parallel and AD convert them at different sampling frequencies, and converts the AD-converted multiple types of digital signals into previous addition results. The present invention provides a distributed optical fiber sensor characterized by comprising a plurality of adders corresponding to the plurality of AD converters that respectively add.

For example, in measuring temperature distribution over a distance of 10 km, it is not necessarily necessary to measure at the highest resolution over the entire area. Rather, the area that requires high-resolution measurement is limited, and depending on the target for which the optical fiber is attached, the area that requires high-resolution measurement may be known in advance or can be estimated to some extent after collecting data for a certain period of time. For example, when used to monitor the temperature of a power transmission cable, the joint part of the cable corresponds to this.

In order to reduce the amount of data and processing time by focusing on the above points, the two methods described above have been proposed, but these methods have the same problems as described above. Therefore, in the present invention, a system that samples and averages the entire area with low resolution and a system that measures a specific area with high resolution are operated in parallel to solve the problem without sacrificing the merits of the technology. are doing.

In addition to temperature distribution, the sensor of the present invention can also be used to measure defects such as optical fiber breakage and damage, pressure, and the like.

[Operation] The system using the present invention operates as follows. First of all,
The high-resolution measurement area is specified in advance so that the measurement length is 2 km or less. However, the number of high-resolution measurement areas may be two or more. Next, AD conversion and averaging are performed at two different sampling frequencies. In one measurement system, 2
0M S P S (Mega Sampling P
er sec, ) using the average data of the total length 1
Measure the temperature distribution at 0km. At this time, temperature correction is performed sequentially starting from the temperature at the entrance end as usual. The other measurement system performs temperature conversion based on 100M5PS averaging data and specified start-end addresses (distance from the incident end). At this time, the temperature at the starting point is known from the temperature data for 10 km, and the attenuation rate is corrected.

Since the temperature distribution over 10 km is based on 5 m sampling, if this data is directly used as the starting point temperature of the high-resolution data, an error may occur. That is, when the starting point has a steep temperature peak. This case can be avoided by correcting the flat part closest to the starting point as a new starting point.

[Example] Figure 1 shows an example, and in order to apply it to a temperature sensor, another similar system for anti-Stokes optical signal is required.
(Convenience 2ch,) Necessary. 19 is an input signal which is a Stokes optical signal, 7 is an input buffer amplifier, 8 is a double filter for low-speed AD converter, 9.10 is an AD preamplifier (odd numbers are for low-speed sampling), 11.1
2 is ADD converter 11 is 20M5PS, 12 is 10
It is 0M5PS. 13.14 is AD memory, 2
It is controlled by the memory control circuit No. 4. The memory control circuit performs write (w) according to each sampling clock (low speed, high speed) and measurement area designation signal 23 (high speed only).
rite pulse, read pulse, and address to the memory. 22 is an output data path.

In this embodiment, the memory capacity was set to 2 kW. If this addition cycle is 80 n5ec, it will take 2000 to record the data.
X 50nsec=1004sec, 2000
x 80nsec=160 u see, total 2
It will be 60 μsec.

[Effects of the Invention] The present invention has the excellent effect of significantly shortening the measurement time by using a general-purpose adder. However, a low-cost system can be constructed without compromising real-time performance or accuracy.

[Brief explanation of the drawing]

FIG. 1 shows a block diagram of an embodiment of the present invention, and FIGS. 2 and 3 are block diagrams of a conventional example. 11, 12...AD converter 13, 14...AD memory 15, 16...Adder 17, 18...Addition memory 19...Input signal 20...Low speed clock 21...High speed clock 22... Output data path 23... Measurement area designation signal 24... Memory control circuit #, -1 1 Parable

Claims (3)

[Claims] (1) A light source that injects a laser pulse into the optical fiber to be measured, an optical directional coupler that guides the return light from the optical fiber to the measurement device, and a photoelectric conversion 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 to be measured from the electrical signal,
The signal processing unit includes a plurality of AD converters that perform AD conversion on the electric signals in parallel and each having a different sampling frequency;
A distributed optical fiber sensor comprising: a plurality of adders corresponding to the plurality of AD converters that respectively add a plurality of types of D-converted digital signals to previous addition results. (2) The plurality of AD converters include a first AD converter that performs AD conversion at a certain sampling frequency in order to measure the entire optical fiber under test; A second AD that performs AD conversion at a higher sampling frequency for measuring resolution
2. The distributed optical fiber sensor according to claim 1, further comprising a control circuit for specifying a measurement area of said second AD converter. (3) The returned light is backward Raman scattered light, has a spectrometer that separates Stokes light and anti-Stokes light in the backward Raman scattered light, and photoelectric conversion converts each of the Stokes light and anti-Stokes light into electricity. and a signal processing section that calculates the temperature distribution with respect to the distance of the optical fiber to be measured from the ratio of the two electrical signals, and the signal processing section parallelizes each of the two electrical signals of the Stokes light and the anti-Stokes light. a plurality of AD converters each performing AD conversion at a different sampling frequency;
2. The distributed optical fiber temperature sensor according to claim 1, further comprising a plurality of adders corresponding to the plurality of AD converters, each adding a plurality of types of converted digital signals to previous addition results.