JPH0533344B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a moisture detection method using an optical fiber.

[Background technology and problems]

Conventionally, moisture detection methods using optical fibers use optical fibers as light guide lines, and the light is interrupted or transmitted in response to the presence or absence of water along the guide line. Such methods are known.

4A and 4B show a moisture detection element using a right triangular prism. In the figure, 1 is a right triangular prism in cross section, 2 and 3 are optical fibers, and a rod lens 4 is attached to a right angle corresponding surface 5. , these rod lenses 4 are connected to optical fibers 2,
3 is connected. When the right-angle corresponding surface 5 is fixed in a horizontal position and light is made to enter the optical fiber 2 from the direction of the arrow, if the surfaces that sandwich the right angle of the prism 1 are in the air, the incident light will be reflected as shown in Figure A. The light is reflected by the surfaces sandwiching the right angle and is emitted to the optical fiber 3. On the other hand, if the surfaces sandwiching the right angle of the prism 1 are flooded with water, the optical fiber 2
The light is emitted directly into the water.

In this way, the light is emitted or cut off from the optical fiber 3 depending on whether or not there is water at the reflective interface of the prism 1, so if the prism 1 is placed at the water detection position, the presence or absence of water at that position can be detected. can be detected. Prepare a large number of such moisture detection elements,
By attaching it to the required location, extending the optical fiber, and connecting a light source, for example a light emitting diode, to one end of the optical fiber and a photoelectric diode to the other end, the presence or absence of water at the location where the moisture detection element is installed can be monitored from a remote location. be able to.

However, the situation of water intrusion in areas away from the mounting position of this moisture detection element is unknown.

Recently published Journal of the Institute of Electronics and Communication Engineers 2/'85,
According to Vol. 68, No. 2, pages 157 et seq., under the title "Increased optical loss in the long wavelength band in silica-based optical fibers", when the transmitted light wavelength exceeds 1.0 μm, It has been reported that silica-based optical fiber contains a large amount of P ₂ O ₅ dopant, and that transmission loss increases when this optical fiber is immersed in water for a long time, and countermeasures have been taken. Based on this phenomenon, the degree of water immersion in the optical fiber is measured by passing white light containing a wavelength of 1.0 μm or more through a P ₂ O ₅ doped silica optical fiber and measuring the transmission attenuation. A moisture detection method was proposed in Japanese Patent Application No. 78175/1983.

In Figure 2, 6 is a white light source and 7 is a white light source.
It is a silica-based optical fiber doped with P ₂ O ₅ . This optical fiber 7 is manufactured, for example, by the well-known MCVD method. 8 is a demultiplexer, which separates light into short wavelength light of 1.0 μm or less and long wavelength light of 1.0 μm or more. 9
10 is a photodetector for the long wavelength light, and 10 is a photodetector for the short wavelength light, both of which are composed of photodiodes and the like, and convert optical input into electrical signals.
11 is a divider that performs calculations based on the electrically converted output P _A of long wavelength light and the electrically converted output P _B of short wavelength light. Note that a tungsten lamp or the like is used as the white light source.

When a silica-based optical fiber doped with P ₂ O ₅ is immersed in water, the loss for each wavelength at the initial stage and after 8 months is shown in Figure 3 (cited in the above-mentioned publication).

According to Figure 3, there is no change in transmission loss for light with a wavelength of 1.0 μm or less even if the optical fiber is immersed in water for a long time, and for light with a wavelength of 1.0 μm or more, as shown in the figure, there is no change in transmission loss. It is recognized that the transmission loss increases significantly.

Therefore, if the light from the white light source 6 is transmitted through the P ₂ O ₅ doped silica optical fiber 7 and split into light with a wavelength of 1.0 μm or more and light with a wavelength of 1.0 μm or less by the demultiplexer 8, the light will not be transmitted. Light of 1.0 μm or more is attenuated due to increased transmission loss due to water immersion in the optical fiber 7, but optical output of 1.0 μm or less is not affected by water immersion and the loss is unlikely to increase. , assuming that the light wavelength distribution from the white light source 6 does not change each time the measurement is made, the light output P _B for wavelengths of 1.0 μm or less is the optical output P _A for wavelengths of 1.0 μm or more.
By dividing by the divider 11, the degree of water intrusion can be determined by the long wavelength over time.

Since the division is performed as described above, the increase in loss due to bending of the optical fiber, etc. can be compensated for.

The above is separated into light with a wavelength of 1.0 μm or more by a demultiplexer.
The purpose is to detect the degree of flooding by splitting light into wavelengths of 1.0 μm or less and calculating both outputs, but in FIG. The optical fiber 7 transmits the transmitted light, the splitter 8 extracts the transmitted light with a wavelength of 1.0 μm or more, the light receiver 9 converts it electrically and outputs it, and if this is repeated over time, the wavelength 1.0 The degree of flooding can be determined by the amount of light attenuation of μm or more.

However, being able to determine the degree of flooding by extracting only wavelengths of 1.0 μm or more requires that the white light source does not fluctuate, and if the light source fluctuates, it will be inaccurate.

[Means to solve the problem]

As explained above, even if a P ₂ O ₅ doped silica optical fiber is immersed in water for a long period of time, the loss of light in the wavelength range of 1.0 μm or less, which is included in a white light source, will hardly change due to moisture. Therefore, it can be used as a stable reference light, and the present invention applies feedback to the white light source so that the output of the receiver for short wavelength light is constant, and then uses the output of the receiver for long wavelength light to It is constructed to be able to detect moisture.

〔Example〕

The present invention will be explained below with reference to an embodiment shown in FIG. The same parts as in FIG. 2 are indicated by the same reference numerals.

A white light source 6, for example, a tungsten lamp, is disposed at one end of the P ₂ O ₅ doped silica optical fiber 7, and light containing light with a wavelength of 1.0 μm or more is made to enter the optical fiber 7.

The other end of the optical fiber 7 is output to a demultiplexer 8 that separates the white light into long wavelength light of 1.0 μm or more and short wavelength light of 1.0 μm or less, and the demultiplexed wavelength is
The light of 1.0 μm or more and the light of 1.0 μm or less are incident on a long wavelength light receiver 9 and a short wavelength light receiver 10, respectively, and are electrically converted.

The light receiver 10 is a photodiode, and the light receiver 1
0 is input to the control system 12 and compared with the standard setting value S for short wavelength light. If the output signal from the light receiver 10 is lower than the standard setting value S, the white light source 6 is driven. The output of the power source 13 is increased, and if it is higher than the set value S, the output of the drive power source 13 is lowered, and the output of the white light source 6 is adjusted so that the output of the light receiver for short wavelength light is always kept constant. .

In other words, the white light source 6 emits light while being fed back by the output of the light receiver 10.

If such feedback is applied, for example, for some reason, it becomes a moisture detection element.
Even if the P ₂ O ₅ doped silica optical fiber is bent, the white light source 6 is automatically adjusted, for example, in a direction where the output is increased, and the light output at the light receiver 10 for short wavelengths of 1.0 μm or less is Without changing, the output of the white light source 6 itself is emitted in a corrected state, and with respect to the constant output of the light receiver 10, the long wavelength attenuated output of the light receiver 9 naturally shows a reliable value. Therefore, the presence or absence of moisture and changes in the state of flooding can be determined simply by the electrically converted output each time from the optical receiver 10 of this long wavelength light.

The present invention makes it possible to know the state of flooding by embedding the above-mentioned optical fiber inside the corrosion-resistant layer of a power cable, particularly a cross-linked polyethylene insulated power cable, and also by wiring the optical fiber inside a duct or tunnel. It can also detect flooding.

〔effect〕

In the present invention, the influence of moisture is small.
Using light in the short wavelength region of 1.0 μm or less as a reference light, the output of the wavelength light of 1.0 μm or less after passing through the P ₂ O ₅ doped silica optical fiber serving as the moisture detection element is always constant. Since it measures the long wavelength light output of μm or more, the value shown as the long wavelength light output accurately indicates the presence or absence of water intrusion and its condition, and even if the optical fiber shows bending over time, Since the loss due to bending is corrected by adjusting the output of the white light source, the value is extremely accurate in this respect as well.

Furthermore, by keeping the amount of white light constant, there is no need to provide a divider as disclosed in the above-mentioned Japanese Patent Application No. 78175/1982, and the light output with a wavelength of 1.0 μm or less is fed back to the white light source. This is technically easy, and the moisture detection method of the present invention can be realized at low cost without using special parts.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the invention. Figure 2 shows the previously proposed moisture detection method. FIG. 3 is a graph showing transmission loss due to water immersion in a P ₂ O ₅ doped silica optical fiber. Figure 4 shows conventional optical fibers in A and B.
FIG. 2 is an explanatory diagram of a moisture detection element using a prism. 6... White light source, 7... P ₂ O ₅ doped silica optical fiber, 8... Demultiplexer, 9, 10... Light receiver, 11... Divider, 12... Control system, 13... Drive power supply.

Claims (1)

[Claims] 1. A white light source is placed at one end of a P ₂ O ₅ doped silica optical fiber placed along the moisture detection object, and a light source with a wavelength of 1.0 μm or more and a light with a wavelength of 1.0 μm or less are placed at the other end of the optical fiber. Arrange a demultiplexer that separates light into light, and input the optical output from the demultiplexer into a receiver with a wavelength of 1.0 μm or less, respectively, so that the output of the receiver with a wavelength of 1.0 μm or less is constant. , a moisture detection method characterized by applying feedback to a white light source and checking the presence or absence of moisture in the object to be detected and its condition based on the output of a photoreceiver with a wavelength of 1.0 μm or more.