JP2007271513A - Optical fiber cable and optical fiber physical quantity variation detecting sensor using the same, and method for detecting physical quantity - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a physical quantity variation detecting sensor having high reliability, over a long period of time. <P>SOLUTION: This sensor is provided with an optical fiber cable, having a structure where an optical coated fiber is inserted into a metal tube, low strength parts with smaller mechanical strength than its periphery being provided to a plurality of places in the longitudinal direction, and the optical coated fiber that is not fixed to the metal tube, a light source for emitting light into the optical coated fiber at one end of the optical fiber cable, and a measurement means for detecting the backscattered light of the light. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical fiber cable used for detecting a change in physical quantity generated in the environment using an optical fiber, an optical fiber physical quantity change detection sensor using the same, and a physical quantity change detection method.

  In addition to being used as an optical medium for optical communication, optical fibers are also used as various sensors that detect changes in physical quantities. Such a sensor detects a change in propagation characteristics of the optical fiber in accordance with a change in the environment in which the optical fiber is installed. As this sensor, there is an optical fiber strain sensor. The optical fiber strain sensor detects an optical fiber cable strain generated when an optical fiber cable serving as a detection unit is fixed to a land, a building, or the like and is generated by an earthquake or a landslide in the environment. In particular, since an optical fiber cable can be laid over 100 km or more, it has an advantage that such a change in physical quantity can be immediately detected in a wide range.

  In order to detect the strain in this optical fiber strain sensor, an optical pulse is incident from one end of the optical fiber in the installed optical fiber cable, and the optical loss of the backscattered light returned by scattering in the optical fiber is reduced. Examine fluctuations. In particular, when this optical fiber is distorted by the BOTDR (Brillouin Optical Time Domain Reflectometry) method for detecting the time change of this backscattered light, the location and the size of the distortion are determined. Can be detected. An example of such an optical fiber strain sensor is described in Patent Document 1, for example. The backscattered light detected here is due to Brillouin scattering in the optical fiber. By analyzing the spectrum of this backscattered light by the BOTDR method, the weak distortion applied to the optical fiber is immediately detected in a wide range. We were able to.

  The characteristics of an optical fiber strain sensor are determined by the structure of an optical fiber cable that serves as a detection unit. In general, since the optical fiber itself is flexible, it is easily distorted and bent. For this reason, in such an optical fiber cable, the optical fiber core wire that is protected by coating the surface of the optical fiber with plastic or the like is protected with a metal tube or resin, and when an external force is applied to the optical fiber, no stress is applied to the optical fiber It has a form like this. On the other hand, there is one that is fixed to a cable in a state where tension is intentionally applied to the optical fiber in order to detect fine distortion. For example, as described in FIG. 1 of Patent Document 1, an optical fiber is inserted by inserting an optical fiber core into a metal tube, holding the optical fiber core in a tensioned state, and caulking the metal tube. The core wire was fixed to the metal tube. When this optical fiber cable was laid, when the metal tube was distorted by an external force due to an earthquake or the like, this distortion could be detected from one end of the optical fiber by the BOTDR method. As a result, changes in physical quantities of buildings and land caused by earthquakes etc. could be detected immediately over a wide range.

JP-A-2005-274200

  However, when manufacturing such an optical fiber cable, it has been very difficult and time-consuming to caulk and fix it to a metal tube without applying stress to the optical fiber core wire. Furthermore, since this optical fiber is in a state where tension is applied, a mechanical load is always applied to the optical fiber even after an external force such as an earthquake does not act after the optical fiber cable is manufactured. . For this reason, the strength of this optical fiber may deteriorate over time due to this long-term load. These optical fiber physical quantity fluctuation sensors are mainly intended for the detection of natural disasters such as earthquakes, so their long-term reliability is important, but for this reason, the long-term reliability of this optical fiber strain sensor is low. It was.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an invention that solves the above problems when detecting a change in physical quantity.

In order to solve the above problems, the present invention has the following configuration.
The gist of the invention described in claim 1 is that, in an optical fiber cable having a structure in which an optical fiber core wire is inserted into a metal tube, the mechanical strength is lower than the surroundings at a plurality of locations in the longitudinal direction. The optical fiber cable is provided with a portion, and the optical fiber core wire is not fixed to the metal tube.
The gist of the invention described in claim 2 resides in the optical fiber cable according to claim 1, wherein the low-strength portion is formed in the metal tube.
The gist of the invention described in claim 3 is characterized in that the low-strength portion is formed by applying any one of caulking, pressing, heat application, and bending to the metal tube. 2 in the optical fiber cable.
The gist of the invention described in claim 4 resides in the optical fiber cable according to claim 2, wherein the low-strength portion is formed by partially reducing the thickness of the metal tube.
The subject matter of claim 5 is characterized in that a coating layer is formed on the surface of the metal tube, and the low-strength portion is formed by partially reducing the thickness of the coating layer. It exists in the optical fiber cable of Claim 1.
The gist of the invention described in claim 6 is that the optical fiber cable according to any one of claims 1 to 5 laid at a location where a change in physical quantity is measured, and the optical fiber at one end of the optical fiber cable. The present invention resides in an optical fiber physical quantity fluctuation detection sensor comprising a light source that makes light incident on a core wire and a measuring unit that detects backscattered light of the light.
The gist of the invention described in claim 7 resides in the optical fiber physical quantity variation detection sensor according to claim 6, wherein a weight is connected to the low strength portion.
The gist of the invention described in claim 8 is that an OTDR measuring device having the light source and measuring means for detecting the backscattered light is connected to one end of the optical fiber cable. 7 is an optical fiber physical quantity variation detection sensor according to the seventh aspect.
According to a ninth aspect of the present invention, there is provided an optical fiber cable according to any one of the first to fifth aspects, wherein the optical fiber cable is laid at a position where a change in physical quantity is measured, and the optical fiber core is provided at one end of the optical fiber cable. The present invention resides in a physical quantity variation detection method characterized in that light is incident on a line and a backscattered light is measured to identify a location of the physical quantity variation generated in the optical fiber cable.

  Since this invention is comprised as mentioned above, the optical fiber physical-quantity fluctuation | variation detection sensor excellent in long-term reliability can be obtained.

  Hereinafter, the best mode for carrying out the present invention will be described.

(First embodiment)
The optical fiber cable according to the first embodiment of the present invention is used by being laid at a place where a physical quantity change is measured, particularly as a physical quantity change detection unit in an optical fiber physical quantity change detection sensor. FIG. 1 is a cross-sectional view (a) in a direction perpendicular to the light traveling direction of the optical fiber cable according to the first embodiment of the present invention, and a cross-sectional view in the longitudinal direction in the II direction (b). . The optical fiber cable 1 includes a metal tube 2 and an optical fiber core wire 3 inserted through the metal tube 2. The optical fiber core wire 3 is not fixed to the metal tube 2. That is, the outer diameter of the optical fiber core wire 3 is smaller than the inner diameter of the metal tube 2, and the optical fiber core wire 3 can freely move inside the metal tube 2. Therefore, no tension is applied to the optical fiber core wire 3 and no mechanical load is applied. Further, the low strength portion 4 having a small mechanical strength is formed at a plurality of locations in the longitudinal direction of the optical fiber cable 1.

  The metal tube 2 is made of, for example, stainless steel or copper, and an outer diameter of 1.5 to 2.0 mm and an inner diameter of about 1.0 to 1.5 mm can be used. The metal tube 2 has a mechanical strength that is not deformed unless a particularly large external force is applied, with the optical fiber core wire 3 inside. Further, a coating layer can be provided on the surface of the metal tube 2 to protect the surface. For example, polyethylene is used as the coating layer.

  The optical fiber core 3 is obtained by coating the surface of a normal optical fiber composed of a core and a clad with a plastic or the like, and allows light to enter from one end and propagate through the optical fiber. As the optical fiber, for example, a pure silica optical fiber is used if light having a wavelength of 1.55 μm is propagated to the optical fiber. Since the optical fiber core wire 3 is inserted through the inside of the metal tube 2, the outer diameter thereof is sufficiently smaller than the inner diameter of the metal tube 2, for example, about 0.25 mm. Its length is about the same as that of the metal tube 2. Since the optical fiber core wire 3 is flexible, its shape is determined by the shape of the metal tube 2 on the outside. That is, the shape of the optical fiber cable 1 is determined by the shape of the metal tube 2.

  The low-strength portions 4 are provided at a plurality of locations over the longitudinal direction of the optical fiber cable 1. For example, the low-strength portions 4 having a length of about 3 to 5 mm can be provided at intervals of 1 m. The low-strength portion 4 is a portion of the optical fiber cable 1 that has a lower mechanical strength than its surroundings, and when an external force is applied to the optical fiber cable 1, this portion is easily selectively deformed. Therefore, when an external force is applied due to an earthquake or the like, the optical fiber cable 1 is easily deformed with this portion as a vertex. In response to this, bending also occurs locally in the optical fiber core wire 3 inside the metal tube 2. At this time, the optical fiber core wire 3 is freely deformed in accordance with the deformation of the metal tube 2, but the optical fiber core wire 3 is not fixed to the metal tube 2 and is not tensioned. On the other hand, a large load is not applied.

  The low-strength portion 4 can be formed on the metal tube 2 by various methods, for example. For example, it is formed by either caulking the metal tube 2 (caulking), applying heat partially (heat application), or partially bending mechanically (bending). it can. At this time, it is necessary that the optical fiber core wire 3 is not fixed to the metal tube 2 by caulking, heating, bending, or the like introduced into the metal tube 2. In particular, the caulking is easy because it is not necessary to fix the inner optical fiber core. The low strength portion 4 can also be formed by partially reducing the thickness of the metal tube 2. For this purpose, for example, the metal tube 2 having a uniform wall thickness is deformed by being partially pressed from the outside, partially mechanically polished from the outside, or chemically. Etching may be performed. In this case, the low-strength portion 4 can be formed with almost no influence on the optical fiber core wire 3. FIG. 2 shows an optical fiber cable 11 in which a low-strength portion 4 is formed in the metal tube 2 by caulking, and FIG. 3 shows a longitudinal direction of the optical fiber cable 12 in which the low-strength portion 4 is formed by reducing the thickness of the metal tube 2. Each of the cross sections is shown. The reference numerals in these figures are the same as those in FIG. In addition to the above method, a method that can partially reduce the mechanical strength of the metal tube 2 can be used in the same manner.

  Further, in the structure in which the coating layer is provided on the surface of the metal tube 2, the low-strength portion 4 can be formed by reducing the thickness of the coating layer at regular intervals. 4A and 4B are a cross-sectional view (a) in a direction perpendicular to the light traveling direction of the optical fiber cable 13 and a cross-sectional view (b) in the longitudinal direction in the II direction. In this optical fiber cable 13, the surface of the metal tube 2 is covered with the coating layer 5, and the low-strength portion 4 is formed in the coating layer 5. The covering layer 5 is polyethylene, and the thickness can be 0.5 mm in the low-strength portion 4 and 1 mm in other portions. In forming the low-strength portion 4 in this case, a method such as pressure application, heat application, polishing, and etching can be used as in the case of forming the low-strength portion 4 in the metal tube 2. Since the coating layer 5 and the optical fiber core 3 are not in direct contact with each other, in this case, the influence of the optical fiber core 3 due to the formation of the low strength portion can be particularly reduced.

  In the optical fiber cables 1, 11, 12, and 13 described above, no mechanical load is applied to the optical fiber core wire 3 and the optical fiber therein when no external force such as an earthquake is applied. Therefore, it has high reliability over a long period of time. Therefore, if this optical fiber cable 1 is used, an optical fiber physical quantity variation detection sensor having high long-term reliability can be obtained.

  Also, when manufacturing a conventional optical fiber cable in which the optical fiber core is fixed by caulking a metal tube, the amount of crimping is such that the optical fiber core is fixed and the optical fiber in the optical fiber is not damaged. There was a need to adjust. On the other hand, when the optical fiber cable 1 is manufactured, the process of forming the low strength portion 4 can be easily performed by the above-described method, and thus the possibility that the optical fiber is broken is small. Therefore, since this optical fiber cable 1 can be easily manufactured, the cost is low.

(Second Embodiment)
The optical fiber physical quantity variation detection sensor according to the second embodiment of the present invention has a large external force generated by an earthquake, a landslide, etc. by the optical fiber cable 1 laid at a location where a physical quantity variation such as strain is measured. Detects the occurrence of FIGS. 5A and 5B are configuration diagrams of the optical fiber physical quantity variation detection sensor according to the second embodiment, in which FIG. 5A shows a state where no external force is applied, and FIG. 5B shows an external force indicated by an arrow. This represents a partially added state. In this optical fiber physical quantity variation detection sensor, an OTDR measuring device 6 is connected to one end of the optical fiber cable 1.

  Since the optical fiber cable 1 is the same as that of FIG. 1, the description about the structure is abbreviate | omitted. Here, the optical fiber cable 1 is fixedly installed at a place where a change in physical quantity is to be detected, and when a large bend occurs at this place, as described above, the optical fiber cable 1 has a shape with the low-strength portion 4 as a vertex. Deform. For this reason, the low-strength portions 4 are formed in the metal tube 2 at intervals of 1 m, for example. In addition, when the place which measures the fluctuation | variation of a physical quantity is a part instead of the full length of this optical fiber cable 1, the low intensity | strength part 4 is not provided uniformly over the full length of the optical fiber cable 1, but the fluctuation | variation of a physical quantity is detected. You may form only in the power place. In this case, since the mechanical strength of the entire optical fiber cable 1 is increased, the handling becomes easy at the time of laying, and the metal tube 2 is easily deformed only in a place where a change in physical quantity is to be detected.

  The OTDR (Optical Time Domain Reflectometry) measuring instrument 6 functions as a light source for inputting pulsed light to one end of the optical fiber in the optical fiber core wire 3, and this pulsed light is transmitted in the optical fiber. It has a function as a measuring means for detecting backscattered light that has been scattered and returned to one end thereof. For this reason, it has the semiconductor laser which oscillates pulsed light, and the photodetector which detects backscattered light. This pulsed light is, for example, light having a wavelength of 1.55 μm, and propagates in the optical fiber toward the other end. Since the backscattered light detected at this time is due to Rayleigh scattering, the light detected by the photodetector is light having the same wavelength as the incident pulsed light.

  In this optical fiber physical quantity variation detection sensor, when a partial external force indicated by an arrow is applied as shown in (b) from a state where no external force is applied as shown in FIG. The optical fiber cable 1 (metal tube 2) is deformed in a shape with the apex as the apex. In response to this, the optical fiber core wire 3 inside thereof is also deformed into the same shape, and a bend (a change in physical quantity) partially occurs. The OTDR measuring device 6 detects a change in backscattered light caused by this bending.

  In this optical fiber physical quantity variation detection sensor and physical quantity variation detection method, light is incident on the optical fiber in the optical fiber core wire 3 from one end (incident point) of the optical fiber cable 1 according to the first embodiment. This light propagates in the optical fiber toward the other end. At this time, in each part in the optical fiber, backscattering of light occurs toward one end where the light is incident. This backscattering is mainly generated by two types of mechanisms, Rayleigh scattering and Brillouin scattering. The former is characterized by no change in the wavelength of light, and the latter is characterized by a change in wavelength, and either scattered light may be detected. However, scattered light by Rayleigh scattering generally has a higher intensity, and when detecting this, only light having the same wavelength as the incident light needs to be detected, and therefore a method using this is simpler. For this reason, the OTDR measuring device 6 was connected here. In this case, the OTDR measuring device 6 detects this backscattered light and analyzes it in time series.

  When a large bend occurs in the optical fiber, a large light loss occurs at that point. Further, as shown in FIG. 6, the time until the back scattered light generated by the pulsed light incident from one end returns to this one end is proportional to the distance from this one end to the location where this back scattering occurs. Therefore, it is possible to specify the position where the optical fiber is bent.

  In this optical fiber physical quantity variation detection sensor, the optical fiber core wire 3 is not fixed to the metal tube 2 in the optical fiber cable 1. Therefore, even if a large external force such as an earthquake is not applied, the optical fiber core wire 3 may be slightly deformed inside the metal tube 2 even by a weak vibration and bend. Since this optical fiber physical quantity variation detection sensor aims to detect only a large external force generated by an earthquake or the like, it is preferable to ignore the minute variation of backscattered light due to such a small bend. In particular, since it is not necessary to detect such minute fluctuations, the sensitivity of the photodetector for detecting the backscattered light in the OTDR measuring device 6 may be low.

  If a BOTDR (Brillouin Optical Time Domain Reflectometry) measuring device is used in place of the OTDR measuring device 6, the degree of bending is similarly determined at which location by backscattering due to Brillouin scattering. Whether it has occurred can be detected. In this case, unlike the case of the Rayleigh scattering described above, since the change in the wavelength of the backscattered light is also detected, it is necessary to detect light in a wide range of wavelengths.

  As described above, if the optical fiber cable 1 is fixed and installed in the section for measuring the fluctuation of the physical quantity, the metal tube 2 can be applied when a large external force is partially applied due to an earthquake or a collapse of earth and sand. Accordingly, the optical fiber core wire 3 is bent, and the size and location of the optical fiber core wire 3 can be detected immediately.

  Since this optical fiber physical quantity variation detection sensor does not require a highly sensitive photodetector, an inexpensive OTDR measuring device 6 can be used. As described above, the optical fiber cable 1 has high long-term reliability and low cost. Therefore, this optical fiber physical quantity variation detection sensor and physical quantity variation detection method also have high long-term reliability and low cost.

(Third embodiment)
FIGS. 7A and 7B are configuration diagrams of the optical fiber physical quantity variation detection sensor according to the third embodiment, in which FIG. 7A shows a state where no external force is applied, and FIG. 7B shows an external force indicated by an arrow. This represents a partially added state. The function of this optical fiber physical quantity variation detection sensor is the same as that of the optical fiber physical quantity variation detection sensor according to the second embodiment, except that a weight 7 is connected to the low-strength portion 4.

  The weight 7 is connected to the low strength portion 4 of the optical fiber cable 1 in a suspended form. This weight is a weight that does not deform the low-strength portion 4 in a state where a large external force due to an earthquake or the like is not applied. Accordingly, the weight is appropriately determined depending on the material and strength of the metal tube 2 and the strength and interval of the low strength portion 4.

  In this optical fiber physical quantity variation detection sensor, when an earthquake or the like occurs, in addition to the external force due to this, a load due to a large movement of the weight 7 is applied to the low strength portion 4. For this reason, the optical fiber cable 1 is particularly easily deformed by the weight 7. As described above, in this optical fiber physical quantity fluctuation detection sensor, it is preferable to ignore the slight bending of the optical fiber, but the weight 7 increases the deformation of the optical fiber cable 1 even when the same external force is applied. Can do. Therefore, the optical fiber can be bent greatly with a small external force, and the optical fiber physical quantity variation detection sensor is more sensitive to an external force such as an earthquake.

  The weight 7 may be connected to all the low-strength portions 4 in the optical fiber cable 1 or may be connected only to a portion where an external force is particularly sensitively detected. In the latter case, the manufacturing cost can be reduced particularly compared to the former.

  A load is always applied to the optical fiber cable 1 by the weight 7, but this load is applied to the low-strength portion 4 and the metal tube 2. Since the internal optical fiber core wire 3 is not fixed to these, no load is applied. Therefore, this does not impair long-term reliability. Therefore, a high-sensitivity, long-term reliability and low-cost optical fiber physical quantity variation detection sensor can be obtained.

  In the optical fiber physical quantity fluctuation detection sensor described above, the fluctuation of the physical quantity generated in the optical fiber is detected by measuring the optical loss by the OTDR method or the BOTDR method. Any means that can measure optical loss can be used in the same manner.

It is sectional drawing which shows the structure of the optical fiber cable used as the 1st Embodiment of this invention. It is sectional drawing which shows the structure of the optical fiber cable used as the 1st modification of the 1st Embodiment of this invention. It is sectional drawing which shows the structure of the optical fiber cable used as the 2nd modification of the 1st Embodiment of this invention. It is sectional drawing which shows the structure of the optical fiber cable used as the 3rd modification of the 1st Embodiment of this invention. It is a figure which shows the structure of the optical fiber physical-quantity fluctuation | variation detection sensor used as the 2nd Embodiment of this invention. It is a figure which shows the relationship between the intensity | strength of backscattered light and the distance from an incident point measured in the optical fiber physical-quantity fluctuation | variation detection sensor used as the 2nd Embodiment of this invention. It is a figure which shows the structure of the optical fiber physical-quantity fluctuation | variation detection sensor used as the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11, 12, 13 Optical fiber cable 2 Metal pipe 3 Optical fiber core wire 4 Low intensity | strength part 5 Coating layer 6 OTDR measuring instrument 7 Weight

Claims (9)

In an optical fiber cable having a structure in which an optical fiber core is inserted into a metal tube,
A plurality of locations extending in the longitudinal direction are provided with low-strength portions whose mechanical strength is smaller than the surroundings,
The optical fiber cable is characterized in that the optical fiber core wire is not fixed to the metal tube.   The optical fiber cable according to claim 1, wherein the low-strength portion is formed in the metal pipe.   The optical fiber cable according to claim 2, wherein the low-strength portion is formed by partially crimping, pressing, applying heat, or bending the metal pipe.   The optical fiber cable according to claim 2, wherein the low-strength portion is formed by partially reducing the thickness of the metal tube.   The optical fiber cable according to claim 1, wherein a coating layer is formed on a surface of the metal tube, and the low-strength portion is formed by partially reducing a thickness of the coating layer. The optical fiber cable according to any one of claims 1 to 5, wherein the optical fiber cable is laid at a location where a change in physical quantity is measured;
At one end of the optical fiber cable, a light source that makes light incident on the optical fiber core wire;
An optical fiber physical quantity variation detection sensor, comprising: a measuring unit that detects backscattered light of the light.   The optical fiber physical quantity variation detection sensor according to claim 6, wherein a weight is connected to the low strength portion.   8. The optical fiber physical quantity fluctuation detection sensor according to claim 6, wherein an OTDR measuring device having the light source and a measuring means for detecting the backscattered light is connected to one end of the optical fiber cable. .   The optical fiber cable according to any one of claims 1 to 5, wherein the optical fiber cable is laid at a location where a change in physical quantity is measured, light is incident on the optical fiber core at one end of the optical fiber cable, and backscattering is performed. A physical quantity variation detection method characterized by identifying a location of a physical quantity variation occurring in the optical fiber cable by measuring light.

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