JPS63236938A - Method for detecting water immersion of optical fiber cable connection part - Google Patents

Abstract

PURPOSE:To quickly detect the water immersion of a connection box by providing a branch part to an optical fiber for monitoring and inserting it into the connection box for a transmission line, and entering a light pulse from its end part and measuring the light intensity of a reflected pulse from the branch part. CONSTITUTION:The optical fiber 22 for monitoring is made to penetrate the optical fiber cable transmission line 23 and an optical fiber 25 for water immersion detection which is branched from the optical fiber 22 for monitoring is positioned in the connection box 24. Then the light pulse is entered into one end of the transmission line 23 to detect difference in reflected light intensity between the glass of the optical fiber for water immersion detection and air, and the glass and water. Namely, Fresnel reflection 31 is caused in the absence of water, but Fresnel reflection 34 is caused in case of water immersion while the reflected light intensity decreases, so the water immersion is easily detected to take a proper countermeasure to prevention against the breaking of the optical fiber due to deterioration in its strength.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting water intrusion in an optical fiber cable connection section, and more specifically, to detect water intrusion in an optical fiber cable connection section, etc., and to detect deterioration in the strength of an optical fiber due to water intrusion and to prevent fiber damage due to water intrusion. This invention relates to technology for preventing the occurrence of increased transmission loss in optical fibers due to hydrogen generation due to breakage and flooding of optical fibers.

<Technical Background of the Invention> In an optical fiber cable transmission line, an optical fiber cable
If water enters the optical fiber under stress such as bending or tension on the fiber or its connection surface, even if there is a minute scratch on the surface of the optical fiber, the moisture will accelerate it and damage the fiber's mechanical properties. The fiber strength may deteriorate and the fiber may break. Furthermore, when metal inside the cable connection surface corrodes due to water intrusion, hydrogen gas is generated and the inside of the cable is filled with hydrogen gas. This hydrogen gas not only invades the inside of the fiber and causes an increase in fiber loss, but also forms hydroxyl groups in the fiber, causing an irrecoverable increase in loss. Therefore, if water has entered the cable, it is important to detect it as soon as possible in order to prevent breakage and increased loss. Conventionally, as a countermeasure to this problem, the cable was made airtight and nitrogen gas was injected from one end to increase the internal pressure of the cable to prevent water from entering from the outside. If the cable breaks, this is detected by a drop in the cable's internal pressure, and the location of the break is estimated from the cable's internal pressure distribution. When the relationship between the breakage position and the cable's internal pressure distribution when the cable is damaged, a characteristic diagram as shown in FIG. 7 is obtained.

The position indicated by the arrow 11 in FIG. 7 is the cable breakage position, from which gas is leaking. A solid line 12 indicates the cable internal pressure distribution.

It can be seen that the cable internal pressure generally decreases linearly toward the cable breakage position. Therefore, by installing a pressure sensor on each cable tangential surface and measuring the pressure within each connection surface, the cable breakage position can be estimated.

<Problems to be Solved by the Invention> However, this method detects damage to the cable, and does not directly detect water intrusion within the cable or the connection surface of the cable.

Furthermore, when the diameter of the cable is made smaller or the length of the optical fiber cable transmission line is made longer, the pressure of the injected gas spreads over the entire cable, but the beam remains the same for a long time. Furthermore, there is a problem in that the time from the occurrence of damage until the pressure drop is detected is long, resulting in a delay in detection.

The present invention eliminates the drawbacks of such cable breakage detection methods, and monitors the tangential surfaces within a long optical fiber cable transmission line for the presence or absence of water intrusion. The purpose of the present invention is to provide a method for detecting water intrusion in optical fiber cable connections to prevent breakage of fiber optic cables.

Means for Solving the Problems> In order to achieve the above object, the method for detecting water intrusion in an optical fiber cable connection section according to the present invention is to penetrate a monitoring optical fiber through an optical fiber cable transmission line and At least one optical fiber for water immersion detection branched from the monitoring optical fiber is provided in at least a part of the connection part housing provided in the optical fiber cable transmission line, and the end face of the optical fiber for water immersion detection is The connecting portion is arranged to be located inside the housing, and a light pulse is made to enter the monitoring optical fiber from one end or both ends of the optical fiber cable transmission path, and is reflected by the end face of the water immersion detection optical fiber to cause the monitoring measuring the light intensity of the reflected light pulse returning to the input end of the optical fiber for use in water detection, and detecting the presence or absence of water intrusion inside the connection part housing in which the optical fiber for water intrusion detection is installed from the light intensity of the obtained reflected light pulse. It is characterized by:

Function> As explained above, the method for detecting water intrusion in an optical fiber cable connection part of the present invention detects water intrusion branched into a monitoring optical fiber that is passed through an optical fiber cable transmission path in an optical fiber cable connection part box. When an optical fiber for monitoring is provided and a light pulse is incident on the monitoring optical fiber from one end (or both ends may be sufficient) of the optical fiber cable transmission line, the interface between glass and air and glass and water at the end face of the optical fiber for monitoring. By detecting the difference in the intensity of reflected light at the connection point, it is possible to detect the ingress of water or oil into the connection part, and to prevent the deterioration of the mechanical strength of the optical fiber due to water intrusion, the resulting breakage of the optical fiber, and the generation of hydrogen due to water intrusion. It is possible to prevent an increase in optical fiber transmission loss due to

Embodiments Next, typical embodiments of the present invention will be described with reference to the drawings.

(Example 1) Figure 1 shows an optical fiber for monitoring, an optical fiber for detecting water intrusion, and an optical pulse tester arranged in an optical fiber cable transmission line in order to carry out the method of detecting water intrusion of an optical fiber cable connection part of the present invention. FIG.

In the figure, 21 is a light pulse test word, 22 is a monitoring optical fiber, 24 is a cable tangential surface, and 25 is an optical fiber for detecting water intrusion that is energized from the monitoring optical fiber. 26 is a schematic diagram showing the arrangement of the monitoring optical fiber 22 and the water immersion detection optical fiber 25 in the monitoring optical fiber 22, and 26 in FIG. 2 shows an optical branch for guiding to an optical fiber 25. In addition, the end face of the optical fiber 25 for water immersion detection is exposed so as to be located as low as possible in the cable connection surface 24, and the end face is cut perpendicular to the central axis of the optical fiber 25 for water immersion detection.

When carrying out the method of detecting water immersion in the optical fiber cable connection part of this embodiment, the monitoring optical fiber 22 is passed through the optical fiber cable transmission path 23 according to the configuration shown in FIG. 1 and FIG. After connecting the above-described optical fiber 25 for water immersion detection to the branch 26 of the monitoring optical fiber cable 22 located in the connection surface 24, the end of the cable is connected to the optical pulse test wire 21, and a normal optical pulse test is performed. , the intensity of reflected light from the end faces of the monitoring optical fiber 22 and the water immersion detection optical fiber 25 may be measured.

For reference, the results of a light pulse test with the above-mentioned configuration (FIG. 1) in the absence of water intrusion are shown, and the relationship between time and reflected light intensity has a characteristic curve as shown in FIG. 3. However, the third
The characteristic curve diagram in the figure is a characteristic curve diagram converted into an optical fiber length (from the end of the optical pulse tester 21) versus optical signal level (8).

The optical signal peak 31 appearing in the characteristic curve of FIG. 3 is caused by Fresnel reflection at the end face of the water immersion detection fiber 25, and one Fresnel reflection peak is generated for each water immersion detection fiber. , and the beak 32 is a beak caused by Fresnel reflection at the end of the monitoring fiber 22. In addition, the optical signal component 33 that attenuates with a constant slope in the section between each beak is behind the Rayleigh that occurs due to non-uniformity of the material in the fiber when the optical pulse passes through the monitoring fiber 22. This is a scattered light signal.

- When light traveling through a medium with a refractive index of n and enters a medium with a refractive index of n2, reflection of the light will occur at the interface. At this time, if the direction of light incidence is perpendicular to the interface, the reflectance R is expressed as R= ((n, -n2)/(n, -n2)+2. Wavelength 1.3- Alternatively, the refractive index of air, water, and quartz glass for light in the 1.54 band is 1.00 each.
, 1.33, 1.47. Therefore, the wavelength at the interface between silica glass and air is 1.3 Am~1.5-
The reflectance of light in the band is 3.6%, and the reflectance of light in the 1.3 to 1.5 wavelength band at the interface between the quartz glass and water is 0.25%. In other words, when the interface of quartz glass changes from air to water, the peak 3 of the reflected light at that interface
1 is 11.5dB? l! decreases.

Consider a case where one or more of the connections '@z4 in FIG. 1 is flooded with water, and the tip of the water immersion detection fiber 25 is submerged in the water. In this case, when a pulse test is performed on the monitoring fiber 22, the optical signal waveform as shown in FIG.
1! be measured. In the waveforms in FIG. 4, 31 indicates Fresnel reflection from the detection fiber 25 on the connection surface without water intrusion, and 34
shows Fresnel reflection from the detection fiber 25 at the connection surface where water has occurred. That is, when the end face of the detection fiber 25 is submerged in water, the Fresnel reflected light intensity at this end face decreases for the above-mentioned reason, and this is observed as a decrease in the Fresnel reflected light intensity of the optical signal waveform in the pulse test. Therefore, by monitoring a significant decrease in the Fresnel reflection pulses 31, water ingress within the connection plane can be detected.

In this embodiment, an optical branch 26 is inserted into each connection surface,
A part of the light guided through the monitoring fiber 22 is transmitted to the detection fiber 2.
5, this leakage amount becomes a loss in the monitoring fiber 22. The magnitude of this loss cr, (dB) is crX=10 jog, where a is the proportion of light branched from the fiber 22 to the water immersion detection fiber 25. It is expressed as (1/(1a)). If a is sufficiently small (a<0.1), α can be approximately expressed as α=4.343Xa. Therefore, for example, if a is 0.05, α8 will be approximately 0.22 dB. On the other hand, assuming that the monitoring fiber is a graded index fiber with a core diameter of 50-1 and a relative refractive index difference of 1%, and the width of the optical pulse incident on the monitoring fiber during a pulse test is 1ρS, the above branching When the ratio a is 0.05, the intensity level of the Fresnel reflected pulsed light from the water immersion detection fiber 25 is approximately 13 dB larger than the Rayleigh backscattered light signal of the monitoring fiber 22 in the absence of water immersion. Therefore, this Fresnel reflection component has a sufficiently large intensity relative to the Rayleigh backscattered light and can be detected.

In the above embodiments, it is assumed that monitoring is performed only at one end of the optical fiber cable transmission line, but if the optical fiber cable transmission line is long, it may not be possible to monitor the entire transmission line from one end.

In such a case, the following Embodiment 2 will be used.

(', t! Example 2) That is, as shown in FIG.
The light emitted by the optical pulse tester 21 on the side closer to the water immersion detection optical fiber 25 branched from the monitoring optical fiber 22 in the tangential surface 24 connected to the optical fiber cable transmission #I 23 The connection is such that the Fresnel reflected pulse of the pulse can be detected.

According to this Example 2, each optical pulse test u21
By performing detection up to the midpoint of the optical fiber cable transmission line 23 using the above, it becomes possible to detect the entire transmission line. Therefore, the applicable optical fiber cable transmission path length is doubled compared to the first embodiment. moreover,
FIG. 6 shows a third embodiment in which monitoring is performed from both ends. However, FIG. 6 shows the optical fiber cable transmission line from one end to its midpoint, and it is assumed that the other end has a similar arrangement.

In this embodiment, two optical fibers 25 for water immersion detection having different connection directions are connected in part or all of the connection plane in the optical fiber cable transmission path. According to this third embodiment, by inputting an optical signal from the water immersion detection fiber 25 in the connection plane and receiving it at another connection plane position, it is possible to make a call between the positions of the connection box. There are advantages in this point. This advantage is particularly useful in cable maintenance work and the like.

Note that since the present invention is characterized by detecting water intrusion based on the difference in refractive index between air and water, the refractive index of liquid other than water that enters the connection surface is approximately 1.25. If this is the case, it should be possible to detect an intrusion. Therefore, according to the present invention, it is also possible to detect oil or the like that enters the connection surface.

<Effects of the Invention> As is clear from the above description, according to the method for detecting water intrusion at the connection portion of an optical fiber cable according to the present invention, water intrusion at the connection portion of a long optical fiber cable transmission line can be detected directly from the end of the transmission line. Since it is possible to monitor water intrusion, it is possible to detect water intrusion quickly and take appropriate measures before the optical fiber breaks due to strength deterioration or the optical fiber transmission loss increases due to hydrogen generation. .

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the method for detecting water intrusion in an optical fiber cable connection part of the present invention, and FIG. 2 is a westward monitoring light for cable connection in the embodiment shown in FIG. A schematic diagram showing the arrangement of fibers and optical fibers for water immersion detection. Figure 3 shows the state of the reflection peak of optical pulses from the ends of the monitoring optical fiber and the optical fiber for water immersion detection when there is no water seepage in the cable connection section. FIG. 4 is a characteristic curve diagram showing the state of the reflection peak of optical pulses from the ends of the monitoring optical fiber and the water intrusion detection optical fiber when there is water intrusion in the cable connection part, and FIG. 5 is the characteristic curve diagram of the present invention. An explanatory diagram showing an embodiment of the second embodiment of the method for detecting water intrusion in an optical fiber cable connection part, and FIG. 6 shows an embodiment of the third embodiment of the method for detecting water intrusion in an optical fiber cable connection part of the present invention. The explanatory diagram, FIG. 7, is a characteristic curve diagram showing a change in the cable internal pressure distribution when an optical fiber cable is broken, with conventional measures taken to prevent water from entering inside the optical fiber cable. In the figure, 21... Optical pulse tester, 22... Optical fiber for monitoring, 23... Optical fiber cable transmission line, 24... Cable tangential surface, 25... Optical fiber for water immersion detection, 26 ...Light branching, 31...Fresnel reflection at the end face of the optical fiber for water immersion detection, 32...Fresnel reflection at the end face of the monitoring optical fiber. Patent applicant: Agent of Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] A monitoring optical fiber is passed through an optical fiber cable transmission line, and at least one branched from the monitoring optical fiber is flooded into at least a part of the connection part housing provided in the optical fiber cable transmission line. A detection optical fiber is provided,
The end face of the optical fiber for detecting water intrusion is arranged so as to be located inside the connection part housing, and a light pulse is made to enter the monitoring optical fiber from one end or both ends of the optical fiber cable transmission path, and at this time, water intrusion is detected. A connection section provided with a water immersion detection optical fiber, which measures the light intensity of a reflected light pulse that is reflected at the end face of the detection optical fiber and returns to the input end of the monitoring optical fiber, and determines the light intensity of the obtained reflected light pulse. A method for detecting water intrusion in an optical fiber cable connection part, characterized by detecting the presence or absence of water intrusion inside a housing.