JP2650417B2 - Distributed optical fiber temperature sensor and temperature measuring method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed optical fiber temperature sensor and a temperature measuring method, and more particularly to a distributed optical fiber temperature sensor and a temperature measuring method with improved temperature resolution and S / N ratio. It is about.

[Prior Art] FIG. 4 shows a block diagram of a conventional distributed optical fiber temperature sensor. The laser pulse oscillated from the laser pulser 10 of the light source unit is incident on the optical fiber 12 to be measured, and the Raman scattered light generated in the optical fiber 12 returns to the incident end. The Raman scattered light is transmitted to the optical directional coupler 11.
The Stokes light and the anti-Stokes light in the Raman scattered light are separated and detected by the filter 13 and converted into electric signals proportional to the intensity by the photoelectric converters 14 and 14 ', respectively. The electrical signals are
The signal is amplified by 5 and 15 ', and is averaged by the averager 16 a predetermined number of times. The signal subjected to the averaging process is transmitted to the signal processing unit 17, 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 into a temperature distribution is performed. Here, there is a close relationship between the temperature resolution and the S / N ratio, and it is known that an improvement in the S / N ratio is essential to increase the temperature resolution. Usually, in the averager,
2 ¹⁰ by taking the arithmetic mean of ~ 2 ¹⁶ measurements results S / N
The ratio has improved. The upper limit of the number of additions and the measurement time, restrictions on the circuit, and has a 2 ¹⁶ times the versatility of the components.

[SUMMARY OF THE INVENTION] Conventionally, there is a limit to the improvement of the S / N ratio by average count (2 ¹⁶ times). Therefore, in order to realize higher resolution, it is necessary to use a special high-sensitivity fiber or to increase the incident power of the laser pulse. The former is costly and loses the advantages of a distributed fiber optic temperature sensor that allows the use of versatile communication optical fibers. In the latter case, there is a problem that the pulse width becomes large, so that the distance resolution proportional to the pulse width is sacrificed.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and a light source unit for injecting a laser pulse into an optical fiber to be measured and a Raman scattered light from the optical fiber to be measured are measured. An optical directional coupler that guides light to the device, a filter that separates Stokes light and anti-Stokes light in Raman scattered light, and a photoelectric conversion unit that photoelectrically converts the Stokes light and anti-Stokes light into electric signals proportional to their respective intensities An averaging unit for averaging the electric signal from the photoelectric conversion unit for a predetermined number of times, a memory for storing n (n is an integer of 2 or more) data subjected to the averaging processing for a predetermined number of times, A distributed optical fiber temperature sensor characterized by comprising a signal processing unit that performs weighted moving averaging using the data of The Raman scattered light from the optical fiber to be measured is guided to the measuring device by the optical directional splicer, and the Stokes light and the anti-Stokes light included in the Raman scattered light are separated by a filter. And the anti-Stokes light are each converted into an electric signal proportional to the intensity by a photoelectric conversion unit, and the electric signal is averaged a predetermined number of times by an averaging processing unit, and the averaged data is n (n is 2 or more) The temperature control method is characterized in that the data is stored in a memory and a weighted moving average process is performed by a signal processing unit using the n data.

[Operation] First, it is assumed that the system continuously repeats the averaging cycle. Now, assuming that the heat capacity of the optical fiber is C _F and the heat capacity of the object to be measured is C _M , the condition for establishing a highly sensitive distributed optical fiber temperature sensor is C _F << C _M. The thermal resistance from the optical fiber to be measured and R _FM, when assuming that there is a stepwise change in temperature in the series, the transient thermal response is defined by a time constant τ = R _FM · C _F occurs. However, it is assumed that the temperature difference between the optical fiber cable and the core can always be ignored. Therefore, the system is not required to have real-time performance beyond the transient thermal response. Now, the temperature variation of the shaped steps is T, the predetermined time or time constant to reach 63% of the variation amount T as tau _{1 =} RC, 1 cycle averaging process, for example, the time required for 2 ¹⁶ times T ₀ and when, T ₀ <
If <CR, the S / N ratio can be improved using a weighted moving average of past average data. That is,
Weighting coefficients _{K 1, ····, K n (} 0 <K i ≦ 1), average data A _1, · · ·, When A _n, moving average weighted, And the improvement ratio of the S / N ratio is At the maximum, SNI = 3 × Log ₂ n (dB).

In addition, in order to perform the normal weighted moving average processing,
A memory for storing _n data A ₁ to An is required,
The following operation can also reduce the capacity of the memory.
From the above equation and the condition that K ₁ = K ₂ =... = K _n , And the next weighted moving average by the latest average data A is , And the time weight factor K ₁ ~K _n equals, no rapid temperature change as a prerequisite, can the A _MA ≒ A _1, thus A _'MA
Is Becomes That is, if the current result A _MA is multiplied by (n-1) / n, the latest data A is multiplied by 1 / n and added, and the result A ′ _MA is newly rewritten as A _MA , and Only one memory is needed.

Embodiment FIG. 1 shows an embodiment of the present invention, in which 1 is a laser pulsar of a light source unit, 2 is an optical directional coupler such as an acousto-optic element, 3 is an optical fiber to be measured, and 4 is an optical fiber. It is a filter for separating Stokes light from Stokes light, and is usually formed of a transparent medium or a diffraction grating provided with a dielectric multilayer film. 5, 5 'are photoelectric conversion units such as photodiodes, 6, 6' are preamplifiers, 7 is an averager, 8 is a memory for storing average data, and 9 is a signal processing unit. Reference numeral 8 may be an external auxiliary storage device such as a hard disk, but is preferably SRAM or DRAM in terms of access time. The past n average data are stored in the n memories, and the oldest data is rewritten with the latest data.

In this case, the capacity of the memory 8 can be reduced by the following operation. For example, A _MA = K (A ₁ + A ₂ + ... +
A _10), when the K = 0.1, weighted moving average with the latest average data _{A, A 'MA = (0.1A} 1 + 0.1A 2 + ··· + 0.1A 10) -0.1A 1 + 0.1A , and the time weight factor K ₁ ~K _n equals, no rapid temperature change as a prerequisite, can the a _MA ≒ a _1, thus a _'MA
Is the _{A 'MA = A MA -0.1A 1} + 0.1A = 0.9A MA + 0.1A. That is, if the current result A _MA is multiplied by 0.9 and the latest data A is multiplied by 0.1 and added, and the result A ′ _MA is newly rewritten as A _MA , the memory can be reduced to one memory.

Hereinafter, description will be made with reference to the output result of one embodiment of the present invention. Raise optical fiber at room temperature (25 ℃), 1100m
The temperature output of the distributed optical fiber temperature sensor system was measured in a state where a portion around 1140 m from the above was heated to 70 ° C.
Figure 2 is a temperature distribution output is not performed weighted moving average processing after 2 ¹⁶ times averaging process.

FIG. 3 shows a temperature output result of the same data as in FIG. 2 using the method of the present invention, where the number of age data is 10, the weighting factors are all 0.1, and one memory is used. The S / N ratio is improved by about 10 dB in the theoretical area as compared with FIG.

Note that the number of average data and the weight coefficient set as described above can be freely adjusted according to needs such as a measurement environment, required accuracy, and response speed, and are not limited to the above.

[Effects of the Invention] The present invention has an excellent feature that the S / N ratio can be improved more than the S / N ratio improvement rate of the averager. The number of points (corresponding to n described above) can be programmed, and a flexible system can be constructed without imposing a load on the optical fiber material, the laser pulser, and the memory capacity.

[Brief description of the drawings]

1 and 3 show an embodiment of the present invention. FIG. 1 is a block diagram of a distributed optical fiber temperature sensor of the present invention, and FIG. 3 is a diagram of temperature distribution data processed by the measuring method of the present invention. FIGS. 2 and 4 show conventional examples, FIG. 2 is a graph of temperature distribution data processed by a conventional method, and FIG. 4 is a block diagram of a conventional apparatus. 1 laser pulsar 3 optical fiber 7 averager 8 memory 9 signal processing unit

Claims (2)

(57) [Claims] 1. A light source unit for injecting a laser pulse into an optical fiber to be measured, a light directional coupler for guiding Raman scattered light from the optical fiber to be measured to a measuring device, Stokes light and anti-Stokes in the Raman scattered light A filter that separates light, a photoelectric conversion unit that photoelectrically converts Stokes light and anti-Stokes light into electric signals that are each proportional to the intensity, and an averaging processing unit that averages the electric signal from the photoelectric conversion unit a predetermined number of times. A memory for storing n pieces of data (n is an integer of 2 or more) subjected to a predetermined number of times of averaging processing, and a signal processing unit for performing weighted moving averaging processing using the n pieces of data. A distributed optical fiber temperature sensor. 2. A laser pulse oscillated from a light source section is incident on an optical fiber to be measured, and Raman scattered light from the optical fiber to be measured is guided to a measuring device by an optical directional coupler.
The Stokes light and the anti-Stokes light included in the Raman scattered light are separated by a filter, the Stokes light and the anti-Stokes light are respectively converted into electric signals proportional to the intensity by a photoelectric conversion unit, and the electric signals are averaged. A predetermined number of averaging processes are performed by
A temperature measurement method, wherein the temperature is stored in a memory and a weighted moving average process is performed by a signal processing unit using the n pieces of data.