JPH0531099B2 - - 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.

3A and 3B 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, such as a light emitting diode, to one end of the optical fiber and a photodiode 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. I can do it.

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

[Means to solve the problem]

Recently published Journal of the Institute of Electronics and Communication Engineers 2/'85,
According to Vol. 68, No. 2, pp. 157 et seq., under the title "Increased optical loss in the long wavelength range 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 to counter this phenomenon. .

The present invention is based on the above-mentioned article that if a silica optical fiber doped with P ₂ O ₅ is in a flooded area, the wavelength
The fact that the transmission loss gradually increases in the wavelength range of 1.0 μm or more, that is, the transmitted light is attenuated, and even if the optical fiber is in a flooded area,
Focusing on the fact that no attenuation occurs in the wavelength range of 1.0 μm or less, we aim to provide a moisture detection method using such a P ₂ O ₅ doped silica optical fiber. In particular, if the present invention is applied to cables such as cross-linked polyethylene insulated power cables, where water trees may occur due to the presence of moisture, ultimately leading to damage to the cable itself, the conventional discrete points can be eliminated. This allows for a more linear monitoring system configuration than that used for monitoring.

〔Example〕

The present invention will be explained below with reference to embodiments shown in the drawings.

In Figure 1, 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 2 (cited in the above-mentioned publication).

According to Figure 2, 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 separated by the demultiplexer 8 into light with a wavelength of 1.0 μm or more and light with a wavelength of 1.0 μm or less, the light is 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, so 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 optical output P _B for wavelengths of 1.0 μm or less is the optical output P _A for wavelengths of 1 μm or more.
By dividing by the divider 11, the degree of flooding can be determined economically based on the long wavelength.

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.

As mentioned above, a P ₂ O ₅ doped silica optical fiber is embedded along the length of a cross-linked polyethylene insulated power cable, for example inside a corrosion protection layer, with a white light source at one end and a white light source at the other end. By connecting a demultiplexer and other devices, a linear (continuous) monitoring system can be created in place of the conventional discrete point monitoring system.

The present invention can be widely applied not only as a moisture detection method for detecting flooding of power cables, but also as a moisture detection method for monitoring the condition of tunnels, ducts, etc.

〔effect〕

The present invention skillfully utilizes the fact that transmission loss increases in the long wavelength region of 1 μm or more due to flooding of the silica-based optical fiber doped with P ₂ O ₅ , whereas there is no transmission loss in the short wavelength region of 1 μm or less. The optical fiber monitors the presence or absence of water intrusion and its degree, and the long-term moisture history of the optical fiber can be determined. In addition, the effects of optical losses such as bending of optical fibers and coupling losses of connectors occur both in the short wavelength region and in the long wavelength region, but since division is performed, the effects are extremely small.

According to the present invention, it is possible to provide a moisture detection method that allows monitoring at linear or continuous positions rather than monitoring at discrete point positions as in the conventional method.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of the invention. Figure 2 is
2 is a graph showing transmission loss due to water immersion in a P ₂ O ₅ doped silica optical fiber. FIG. 3 is an explanatory diagram of a conventional water immersion detection element using an optical fiber and a prism. 6... White light source, 7... P ₂ O ₅ doped silica optical fiber, 8... Demultiplexer, 9, 10... Light receiver,
11...Divider.

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 demultiplexes the light, input the optical output from the demultiplexer with a wavelength of 1.0 μm or more and the optical output with a wavelength of 1.0 μm or less into a photoreceiver, and divide the outputs of both the photoreceivers. wavelength
A moisture detection method characterized by detecting the degree of water immersion of an object to be detected by determining the ratio of output with a wavelength of 1.0 μm or more to output with a wavelength of 1.0 μm or less.