JPH11223751A - Optical fiber cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber cable with high reliability by providing the optical fiber cable with a tension load sensing function thereby preventing the break accident of the optical fiber cable. SOLUTION: An optical fiber 3 for monitoring tension is provided in addition to an optical fiber 5 for communication longitudinally in an optical fiber cable 1 into which a high tensile strength body 2 whose service life up to break is decided based on the history with respect to the tension to be loaded is inserted. The tension load history of the optical fiber cable 1 is managed, so that the break accident of the optical fiber cable 1 can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber cable in which the life of a high tensile strength member is determined by the history of tensile load, and in which the history of tensile load can be managed.

[0002]

2. Description of the Related Art A conventional general optical fiber cable has a built-in tension member (tension member) which is durable against tension in order to protect the optical fiber core wire from destruction when tension is applied. However, when the fiber optic cable is subjected to severe tension loading conditions for an extended period of time, the tension member can collapse rapidly even at tensions within endurance limits. This is generally considered to be caused by creep (tensile load fatigue), and is a phenomenon in which the life is shortened when the fatigue is accumulated.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to prevent an optical fiber cable from being broken due to such a cause, and to provide an optical fiber cable with a function of detecting a tension load. And a highly reliable optical fiber cable.

In order to prevent an optical fiber cable from being broken due to tension load fatigue, it is necessary to always grasp the fatigue characteristics of the optical fiber cable material itself and manage the tension load history. This fatigue property can be checked in advance by a property evaluation test. On the other hand, by measuring the tension load at each point of the fiber optic cable and recording and collecting the tension load history, the remaining life of the fiber optic cable material can be predicted, which can lead to breakage of the fiber optic cable. Can be taken beforehand.

The present invention has been made in view of the fact that a conventional optical fiber cable did not have a function of sensing a tension load, and an object of the present invention is to provide an optical fiber cable having a tension load sensing function. In other words, information on the tension load status at each part of the optical fiber cable can be obtained in real time, and the tension load history based on this information can be easily grasped. The remaining life of the optical fiber cable can be calculated. In this way, an attempt is made to prevent an optical fiber cable from being broken due to fatigue caused by a tensile load.

As a method of imparting a tension sensing ability to an optical fiber itself, there is a method of forming a Bragg grating (diffraction grating) using a photorefractive effect. This is G
When ultraviolet light is applied to the optical fiber core doped with e,
By utilizing the fact that the refractive index slightly increases, a diffraction grating is directly written on an optical fiber by a method such as an interference method or a mask method. When such a diffraction grating is formed, light is selectively reflected at a wavelength corresponding to the grating interval. Also, if we look at the transmitted light that has passed through the optical fiber, the light at that wavelength will be lost, so the wavelength analysis of the reflected or transmitted light will cause the lattice spacing to change due to changes in temperature and mechanical tension. If appropriate temperature compensation is performed, an optical fiber cable having a tension load sensing function can be realized by incorporating an optical fiber having such a diffraction grating in the optical fiber cable.

Conventional optical fiber cables have been laid underground or suspended by telephone poles. But,
Recently, the use of optical fiber cables has been rapidly expanded, and they have been used even under severe conditions such as space and ocean. Along with this, since the management of the tension load history is not performed in the conventional optical fiber cable, the lack of reliability that it is not known when the breakage accident occurs due to fatigue is concerned.

In the optical fiber cable having the tension load sensing function according to the present invention, the load can be measured in real time under the use condition of the optical fiber, and the remaining life of the optical fiber cable can be predicted based on the information. These problems have been overcome, opening the way to the use of highly reliable optical fiber cables even under severe conditions.

[0009]

SUMMARY OF THE INVENTION According to the present invention, there is provided an optical fiber cable in which a high tensile strength member whose life until fracture is determined by the history of applied tension is inserted.
Optical fiber for monitoring the tension load of the optical fiber cable by longitudinally aligning the optical fiber for tension monitoring in addition to the optical fiber for communication, so that the breakage of the optical fiber cable can be prevented beforehand. It is a fiber cable. Further, the tension monitoring optical fiber has an optical fiber diffraction grating (Bragg grating) formed in a part or a plurality of portions of the optical fiber, and the reflected light wavelength or the transmission wavelength is changed by the change of the grating interval depending on the magnitude of the tension load. An optical fiber cable characterized in that it is possible to measure a tension load history on an optical fiber cable by utilizing a change in a wavelength lacking in an optical spectrum. Further, in the above-mentioned optical fiber cable, a tension sensitive portion using a diffraction grating is formed in the optical fiber cable for communication so that the optical fiber cable also serves as an optical fiber for tension monitoring.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are a side sectional view and a transverse sectional view showing a configuration of an optical fiber cable according to an embodiment. That is, the optical fiber cable is configured such that a tensile member (tension member) 2 is inserted into the coating 1, and further, a communication optical fiber 5 and a tension monitoring optical fiber 3 are longitudinally attached and integrated. A diffraction grating 4 is written on the tension monitoring optical fiber 3. When a plurality of the grids are provided, the grids are slightly different from each other so that they can be identified.

The tension monitoring optical fiber 3 is an embodiment in which a plurality of Bragg gratings (diffraction gratings) 4 are formed with a certain space. This is because, when ultraviolet light is applied to a Ge-doped optical fiber core, a diffraction grating is directly applied to the optical fiber by a method such as an interference method or a mask method using a photorefractive effect in which the refractive index slightly increases. What to write. When such a diffraction grating 4 is formed, light having a wavelength corresponding to the grating interval is selectively reflected. In addition, when viewed from light transmitted through an optical fiber, light of that wavelength is lacking. When the optical fiber is subjected to a tension, the optical fiber elongates and the lattice interval also changes. Therefore, the tensile load at each position of the optical fiber can be determined by analyzing the wavelength of the reflected light or the transmitted light. Diffraction grating 4
Since the lattice spacing changes due to fluctuations in temperature and mechanical tension, appropriate temperature compensation is performed. Thus, an optical fiber cable having a tension load sensing function is realized by incorporating an optical fiber having a diffraction grating in the optical fiber cable.

FIG. 2 is a block diagram when measuring the tension with the optical fiber cable having the tension load sensing function. That is, the optical fiber cable 10 having the tension load sensing function is connected to the optical fiber coupler 12 of the tension load history monitoring device 20. The tension load history monitoring device 20 includes a light source 11, an optical fiber coupler 12, a reflected light wavelength analyzer 13,
It comprises a signal converter 14, a tension history recorder 15, a remaining life indicator 16, and the like. Light from the light source 11 is made incident from one end of the monitoring optical fiber 3 of the optical fiber cable 10 and reflected from each diffraction grating 4. Light to optical fiber coupler 12
And then enter the reflected light wavelength analyzer 13 and the signal converter 14. Then, the magnitude of the tension load is obtained from this output, the load history of the tension is totaled by the tension history recorder 15, and the remaining life is obtained and displayed by the remaining life indicator 16. Since the emission wavelength of the light source 11 needs to cover the entire wavelength region of the reflection wavelength of the diffraction grating 4, a broadband laser diode or LED or A
An SE light source is used.

In this embodiment, a plurality of diffraction gratings are formed. However, when the tension is uniform, this is not essential, and one of them is used or a single diffraction grating is used. It just needs to be formed. However, when a tension load is distributed, it is necessary to form a plurality of diffraction gratings. In this case, if the lattice spacing is slightly changed, the reflected light wavelengths are different to separate them, and it is possible to know where the cleave fatigue accumulates. Predict if there is.

As a method of identifying the position, the time required for the reflected light to return is different even if the grating interval is the same in any diffraction grating. Therefore, the time-resolved measurement method known in the reflection measurement method (OTDR) is used. Identify locations by law. In this case, a light pulse is transmitted from the light source.

The reflected light wavelength analyzer 13 converts the wavelength of the reflected light separated by the optical fiber coupler 12 into measurable information. Inside the diffraction angle θ depends on the wavelength λ (sin θ). = Λ / Λ, Λ denotes a built-in flat plate diffraction grating having a function of converting the change in wavelength into spatial information called a diffraction angle. In such a wavelength analysis, a reflected light wavelength is output as a bright line at a specific wavelength. Further, the signal converter 14 has a built-in photodiode array element, and the position x of each photodiode included in the array is determined by the equation x = L sin θ (L is the distance between the diffraction grating plate and the photodiode array). The output signal from each photodiode is signalized in a scanning time series, and the time position of the signal indicates the position of the photodiode that has captured the bright line of the reflected light. Changes in position correspond to changes in wavelength. This change in wavelength is caused by a change in the amount of elongation of the optical cable due to a change in tension.
5 and a change in the tension from the wavelength change is calculated and obtained by a microcomputer incorporated therein, and the history of the tension load is accumulated every moment.

FIG. 3 shows an embodiment in which the tension load history monitoring device 20 is installed separately at both ends of the monitored optical fiber cable 10. In this case, the monitored optical fiber cable 10 is used.
The light source 11 is placed on one side of the optical fiber cable 1 to be monitored.
0, the transmitted light lacking wavelength analyzer 17 and the signal converter 1
4, the tension history recorder 15 and the remaining life indicator 16 are connected in series, the transmitted light wavelength missing by reflection is determined by the transmitted light lacking wavelength analyzer 17, and the tension is measured in the same manner as in the case of the reflected light. Measure the load. The transmitted light lacking wavelength analyzer 17 is structurally the same as the reflected light wavelength analyzer 13 except that the output is not a bright line but a dark line.

The wavelength information is applied to a signal converter 14 and converted into tension data T (t). Further, the tension history recorder 15 stores and stores the tension information. The remaining life estimated from this result is indicated by the remaining life indicator 16.
Is calculated and displayed. In this embodiment, a plurality of diffraction gratings are provided in the tension monitoring optical fiber, so that the tension can be independently measured at each point of the optical fiber cable and the remaining life can be predicted. Therefore,
It is possible to grasp which part of the optical fiber cable is in a state where the tension fatigue accumulates and is easily broken.

Now, assuming that N diffraction gratings are mounted, the local remaining life τ at the position where the j-th diffraction grating 4 is mounted is predicted using the following Larson-Miller equation. It is. ΔH _j / R = T _j · (C + log τ) (1 ≦ j ≦ N) where ΔH _j is the energy of tension, R is Boltzmann's constant, T is temperature (° K), and C is a material constant. In this equation, necessary parameters are obtained by experiments, and τ
Is calculated. By the way, consider a case where a material having a constant temperature and a life of 10 years is subjected to a double tension, for example, C =
If the above equation is applied assuming that the material is No. 2, the lifetime is τ
= 1.1 is obtained.

According to this equation, a margin can be calculated for safety. For example, if a margin for increasing the amount of tensile strength material that satisfies the predicted life is expected to be doubled, the predicted life is increased by almost one digit, and a highly reliable optical fiber cable that does not cause unexpected breakage can be realized. Can be.

In the above embodiment, the monitoring optical fiber is provided for exclusive use, but it is needless to say that the communication optical fiber may be provided with a diffraction grating so that the optical fiber can be shared.

[0021]

As described above, according to the optical fiber cable of the present invention, a tension monitoring optical fiber or a communication optical fiber which is also used as a tension monitoring optical fiber is laid inside the optical fiber cable. By managing the tension load history of the optical fiber cable, the life of the optical fiber cable can be known, and a reliable optical fiber cable can be obtained with a simple configuration.

[Brief description of the drawings]

FIGS. 1A and 1B are a side sectional view and a transverse sectional view showing a configuration of an optical fiber cable according to an embodiment;

FIG. 2 is a block diagram of a tension load history monitoring device for measuring tension with an optical fiber cable,

FIG. 3 is a block diagram of another example of a tension load history monitoring device for measuring tension.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 coating 2 tension member to be monitored 3 tension monitoring optical fiber 4 diffraction grating written on tension monitoring optical fiber 5 communication optical fiber 10 optical fiber cable 11 light source 12 optical fiber coupler 13 reflected light wavelength analyzer 14 signal Transducer 15 Tension history recorder 16 Remaining life indicator 17 Transmitted light lack wavelength analyzer 20 Tension load monitoring device

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kenichi Muta 4-1-1 Minamihashimoto, Sagamihara City, Kanagawa Prefecture Showa Electric Wire & Cable Co., Ltd. (72) Inventor Yuichi Morishita 4-1-1 Minamihashimoto, Sagamihara City, Kanagawa Prefecture Showa Electric Wire and Cable Co., Ltd.

Claims (3)

[Claims] 1. An optical fiber for monitoring tension in addition to an optical fiber for communication is longitudinally arranged in an optical fiber cable into which a high tensile strength member whose life until breaking is determined by a history of applied tension is inserted. An optical fiber cable characterized in that a breakage of the optical fiber cable can be prevented by managing the tension load history of the optical fiber cable. 2. The tension monitoring optical fiber according to claim 1, wherein an optical fiber diffraction grating (Bragg grating) is formed in a part or a plurality of portions of the optical fiber, and the reflected light is changed by a change in the grating interval with respect to the magnitude of the tension load. 2. The optical fiber cable according to claim 1, wherein the tension load history on the optical fiber cable can be measured by utilizing the change in the wavelength or the lack of the transmitted light spectrum. 3. The optical fiber cable according to claim 2, wherein a tension sensitive portion formed by a diffraction grating is formed in the optical fiber cable for communication so as to serve also as an optical fiber for tension monitoring. Fiber optic cable.