WO2001055672A1

WO2001055672A1 - Fiber optic cable and method of measuring distortion

Info

Publication number: WO2001055672A1
Application number: PCT/JP2001/000464
Authority: WO
Inventors: Takeshi Genji; Kazuo Imamura
Original assignee: Mitsubishi Cable Industries, Ltd.
Priority date: 2000-01-28
Filing date: 2001-01-24
Publication date: 2001-08-02
Also published as: JP2001208915A

Abstract

A fiber optic cable (100) comprises at least one metering fiber (20) including a plurality of kinds of fiber gratings (21) different in the reflection or transmission characteristic. The metering fiber (20) includes a first fiber grating which reflects or transmits light of a first wavelength, and a second fiber grating which reflects or transmits light of a second wavelength different from the first wavelength. The second fiber grating is distant lengthwise from the first fiber grating. Distortion is monitored in the respective positions of the first and second fiber gratings using the wavelength shift of the first and second fiber gratings.

Description

Description Optical fiber cable and strain measurement method

The present invention relates to an optical fiber cable and a strain measurement method. In particular, the present invention relates to an optical fiber cable suitable for high-resolution strain measurement and a strain measuring method using the optical fiber cable. Background art

If tension is applied to the optical fiber and elongation strain occurs, the optical fiber will deteriorate.To ensure long-term reliability of the optical communication line, it is necessary to manufacture the optical cable, lay it, and use it in the application environment. It is necessary to accurately measure the strain of the optical fiber at each stage. Conventional methods for measuring such distortion include the optical pulse method, in which the change in optical signal delay time due to the occurrence of distortion is directly measured on the time axis, and the method of measuring distortion from the phase change of a sine wave modulation signal. The phase method was used.

According to the above strain measurement method, strain can be measured with high accuracy, but when the strain is unevenly distributed along the longitudinal direction of the optical fiber, the value of the strain averaged along the longitudinal direction is detected. I could only do it. Since the strength degradation of an optical fiber is determined by the maximum strain, even if the average value of the strain is small, if a large maximum strain occurs locally, the degradation of the optical fiber progresses greatly at that part Will be. For this reason, if only the strain value averaged in the longitudinal direction is measured, the reliability of the optical fiber cannot be sufficiently guaranteed.

Therefore, a method of measuring the strain distribution along the longitudinal direction of the optical fiber has been proposed. This method combines a technique for analyzing nonlinear interactions between light waves with sound waves and frequency conversion and a time-domain measurement technique similar to OTDR (optical time domain reflectometry).

However, in this method, the distance resolution (time resolution) is limited to about several meters depending on the light pulse width. Could not be achieved. On the other hand, optical fiber cores housed in optical fiber cables are usually helically twisted at a pitch of about 10 cm. For this reason, the above-mentioned conventional measurement method cannot measure the maximum strain occurring in the optical fiber with high accuracy.

In addition, the above method has problems that the strain measurement resolution is low (about 0.015% or more) and the measurement time is relatively long (5 minutes or more). Further, the above method has a problem that a special and expensive dedicated measuring instrument is required.

The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide an optical fiber cable capable of measuring strain with high distance resolution and a strain measuring method. Invention

An optical fiber cable according to the present invention is an optical fiber cable including a plurality of optical fiber cores, wherein at least one of the plurality of optical fiber cores has a plurality of types having different reflection characteristics or transmission characteristics. Wherein the plurality of types of fiber gratings reflect at least a first fiber grating that reflects or transmits the first wavelength light, and reflects a second wavelength light that is different from the first wavelength. Or a transmitting second fiber grating, wherein the second fiber grating is formed at a position along the axial direction from a position where the first fiber grating is formed, and According to the wavelength shifting of each of the first fiber grating and the second fiber grating, 1 strain of that of an position of the fiber grating and the second fiber grating is monitored. Thereby, to achieve the above object, in one embodiment, the optical fiber core having the plurality of types of fiber gratings, the measurement light including at least the first wavelength light and the second wavelength light, It propagates with communication light that can communicate without being affected by the measurement light.

At least two of the plurality of optical fiber cores are the optical fiber cores having the plurality of types of fiber bug ratings, and each of the optical fiber cores having the plurality of types of fiber gratings is The first fiber group containing The rating and the second fiber grating are preferably arranged symmetrically with respect to a center axis of the optical fiber cable having the plurality of optical fiber cores.

A distortion measuring method according to the present invention includes: a step of injecting light of a light source including the light of the first wavelength and the light of the second wavelength into the optical fiber cable; Detecting a second wavelength shift from the first wavelength shift and the second fiber grating, thereby detecting a first amount of distortion at the first fiber grating location and the second fiber shift; Obtaining a second distortion amount at the position of the grating.

Another distortion measuring method according to the present invention includes: a step of injecting light of a light source including the light of the first wavelength and the light of the second wavelength into the optical fiber cable; and Determining the amount of distortion at the position of the pair of fiber gratings by taking the difference between the wavelength shifts of the pair of fiber gratings symmetrically arranged with respect to the axis.

The optical fiber cable of the present invention has a plurality of types of fiber gratings, and each of the plurality of types of fiber gratings can be arranged at a short distance, so that a strain measurement having a higher distance resolution than the conventional technology can be performed. Possible fiber optic cables and strain measurement methods can be provided.

When the measuring light and the communication light are propagated to the optical fiber, all the optical fibers mounted on the optical fiber can be used for communication. When the measurement light and the communication light propagate through the optical fiber, the reflection wavelength of the grating is set outside the communication wavelength band, so that the measurement light and the communication light are transmitted to the same optical fiber without affecting the communication light. May be transmitted. In this way, the measurement can be performed while the communication using the communication light is in a live state, and all the optical fiber cores mounted on the optical fiber can be used for communication. Furthermore, if the same type of fiber grating is symmetrically arranged with respect to the central axis of the optical fiber cable, the fiber gratings compare the light reflected or transmitted, and the It is possible to detect the shift amount due to the bending of the light with high sensitivity. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a cross-sectional view schematically illustrating an embodiment of an optical fiber cable according to the present invention, and FIG. 1B is a cross-sectional view schematically illustrating a tape core housed in the optical fiber cable. FIG.

FIG. 2 is a diagram schematically showing the measurement core 20 on which the fiber grating 21 is formed.

FIG. 3 is a diagram for explaining a method of manufacturing the measurement core 20.

FIG. 4 is a view for explaining a method of manufacturing the measurement core 20.

FIG. 5 is a diagram for explaining a method of manufacturing the measurement core 20.

Figure 6 (a) shows the measurement core 2 with multiple fiber gratings 21 formed.

FIG. 6 (b) is a graph schematically showing the reflection characteristics of the measuring core 20. FIG.

FIG. 7 is a graph showing the reflection beak wavelength (reflection characteristics) of each grating 21 at the time of making the grating 21 and after releasing the tension.

FIG. 8 is a graph showing the relationship between the reflection wavelength at irradiation and the reflection wavelength after tension release. FIG. 9 is a configuration diagram for explaining an embodiment of the distortion amount measuring method according to the present invention.

FIG. 10 is a configuration diagram in which the optical fiber cable 100 is connected to the optical measurement system 40.

FIG. 11 is a graph showing the relationship between the reflected wavelength shift amount [nm] and the distortion amount [%] for each position [mm] of the optical fiber cable 100.

FIG. 12 (a) is a cross-sectional view schematically showing an SZ type optical fiber cable 110, and FIG. 12 (b) is a schematic view showing a tape core 50 stored in the optical fiber cable 110. FIG. 12 (c) is a diagram for explaining a bent state of the optical fiber cable 110.

FIG. 13 is a graph showing the relationship between the reflection wavelength shift amount [nm] and the distortion amount [%] for each position [mm] of the optical fiber cable 110.

Fig. 14 illustrates the configuration in which the measurement core 20 is used as a communication optical fiber core. FIG.

FIG. 15 (a) is a cross-sectional view schematically showing an optical fiber cable 120 equipped with a pair of measurement cores in which fiber gratings 21 having the same reflection characteristics are arranged point-symmetrically. FIG. 15 (b) is a diagram schematically showing a pair of measurement core wires 20a-20a '.

FIG. 16 is a cross-sectional view schematically showing an optical fiber cable 130 on which two pairs of measurement core wires are mounted.

FIG. 17 (a) is a schematic diagram showing an example in which the optical fiber cable 70 is embedded in a road, and FIG. 17 (b) is a diagram showing an example of embedding when performing two-dimensional measurement. . BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)

Embodiment 1 of the present invention will be described with reference to FIGS. First, refer to FIGS. 1 (a) and 1 (b). FIG. 1A schematically shows a cross section of an optical fiber cable 100 according to the present embodiment, and FIG. 1B shows a tape-shaped optical fiber core included in the optical fiber cable 100. (Tape cord) A cross section of 50 is schematically shown.

The optical fiber cable 100 is an optical fiber cable of a tape slot type, and includes a slot rod 60 having a slot (groove) 55 on its outer periphery and a tape housed in the slot 55 in a superimposed manner. It has a core wire 50 and a sheath (for example, a PE sheath) 64 covering the slot rod 60. At the center of the slot rod 60, a tension member (tensile strength member) 62 for securing the strength of the optical fiber cable is disposed. In this embodiment, an optical fiber cable of a tape slot type is used, but an optical fiber cable of a strand structure or a tube structure may be used.

The tape core 50 accommodated in the optical fiber cable 100 has a plurality of optical fiber cores. In the present embodiment, a 4-core tape core is used as the tape core 50. The tape core 50 is an optical fiber core for strain measurement (hereinafter referred to as “measurement core”). Line ". ) And an optical fiber core 30, and these optical fiber cores are covered with a tape coating 52. In this embodiment, a plurality of types of fiber gratings having different reflection characteristics (or transmission characteristics) are formed on the measurement core 20.

Next, the measuring core 20 will be described in detail with reference to FIG. FIG. 2 schematically shows an optical fiber portion of the measuring core 20. The optical fiber portion is composed of a core 22 on which a fiber grating 21 is written and a periphery of the core 22. And a clad 23 formed in the center.

The grating 21 written (formed) on the core 22 of the measuring core 20 changes the refractive index of the core 2 along the axial direction with a period Λ (for example, 0.3 to 0.6 zm). Having a short-period refractive index modulation structure. Light having a Bragg wavelength defined by the period Λ and the average refractive index of the core 21 is selectively reflected by the grating 21. In the present specification, the Bragg wavelength may be referred to as “reflection peak wavelength”.

Since the Bragg wavelength changes depending on the period に as described above, if distortion occurs in the measuring core wire 20 due to heat or external force, the period 部分 in the portion where the distortion occurs is in an unstrained state. Will change (shift) from the value of. The magnitude of this change (shift amount) can be optically observed as the shift amount at the Bragg wavelength (reflection peak wavelength). In the measurement core 20, a plurality of types of fiber gratings 21 having different reflection characteristics are formed at positions separated from each other along the axial direction. Therefore, by observing the shift amount in each fiber grating 21, it is possible to monitor the amount of distortion at the position of each fiber grating 21. In other words, by detecting the shift amount of the reflection peak wavelength generated due to the change in the physical state of each fiber grating, it becomes possible to sense the stress, etc., occurring at multiple locations on a long fiber. .

Next, the configuration of the measuring core 20 and the method of manufacturing the grating 21 will be described with reference to FIG. As shown in Fig. 3, the measurement core wire 20 covers the core 22 on which the grating 21 is written, the cladding 23 formed around the core 22, and the outer surface of the cladding 23. And a coating layer 24. In this embodiment The core 22 used is doped with Ge having a concentration similar to that of Ge contained in the core of the normal specification optical fiber. Here, the normal specification optical fiber is an optical fiber core connected to the measuring core 20 (for example, a general optical communication core accommodated in an optical fiber cable for communication use). That is. The core of such an optical fiber core is usually doped with Ge so that the relative refractive index difference becomes about 0.9%.

The core 22 of the measurement core wire 20 shown in the figure includes, in addition to Ge, Sn, Sn and Al, or Sn, A in addition to Ge in order to constantly increase the photoinduced refractive index change. It is preferable that the dopants 1 and B are doped in the core 22. For example, in addition to the same amount of Ge as that of the core of the optical fiber of the above-mentioned normal specification (an amount such that the relative refractive index difference is 0.9%), the concentration is not less than 1 ◦ 0,000 ppm, preferably the concentration. It is only necessary to co-dope Sn with a concentration of 1000 to 150 ppm, or Sn with such a concentration and A1 with a concentration of 1000 ppm or less. Such de one-flop may be performed by various known methods, for example, when carried out by immersion, the compounds of the G e and S n (the case of S n, e.g., _{S n C l 2 '2 H2O} ) Can be mixed with methyl alcohol and immersed in the solution.

The coating layer 24 is formed by a single coating method following the drawing process of the optical fiber consisting of the core 22 and the cladding 23 so as to have a film thickness of at least about 30 zm, for example. It is. The material (covering agent) for forming the coating layer 24 and the thickness of the coating layer 12 may be appropriately determined according to the required conditions. For example, the elastic modulus of the optical fiber (Young's modulus E), the coefficient of thermal expansion (linear thermal expansion coefficient a), the temperature coefficient of the refractive index (thermo-optic coefficient), and the modulus of elasticity of the material of the coating layer (Yang's modulus) It is preferable to determine based on the thermal expansion coefficient (linear thermal expansion coefficient) and the like. Table 1 below shows the parameters of the measurement core wire 20 used in the present embodiment.

(Hereinafter the margin) (table 1 )

In the present embodiment, as the material of the coating layer 24, both the property of curing with ultraviolet light of a certain wavelength band (first ultraviolet ray) and the property of transmitting ultraviolet light of another wavelength band (second ultraviolet ray) are used. Is used. In the specification of the present application, such a resin is sometimes referred to as an “ultraviolet transmitting type ultraviolet curable resin”.

The ultraviolet-transmissive ultraviolet-curing resin transmits at least ultraviolet rays in a specific wavelength band (for example, a wavelength band of 240 nm to 270 nm) to be applied to the core for writing the grating 21 (preferably, On the other hand, this ultraviolet ray is transmitted with little absorption. On the other hand, the ultraviolet ray having a wavelength shorter or longer than the specific wavelength band is absorbed to cause a curing reaction. In other words, although the same resin has different UV absorption characteristics depending on the wavelength, a resin that is UV-transmissive in a specific wavelength band, but is UV-curable in a shorter or longer wavelength range than the above-mentioned specific wavelength band. Will be used to form the coating layer 24.

In the present embodiment, the curing reaction is started and accelerated by receiving ultraviolet rays in a wavelength range shorter than 240 nm or a wavelength range longer than 270 nm, for example, for urethane acrylate or epoxy acrylate. A resin containing a suitable photoinitiator (photoinitiator) is used as the “ultraviolet-transmitting ultraviolet-curing resin”.

After coating the outer peripheral surface of the optical fiber with such a resin layer, first, the coating layer is irradiated with first ultraviolet rays to cure the coating layer 24.

For the optical fiber core covered with the cured coating layer 24, the second It is preferable to fill the core 22 with hydrogen before irradiating the ultraviolet rays from the viewpoint of increasing the photoinduced refractive index change. Therefore, in this embodiment, this high-pressure hydrogen filling is performed. Specifically, the optical fiber core wire 1 is placed in a sealed container filled with hydrogen and left at room temperature under a pressure of about 2 OMPa for about 2 weeks.

Next, the grating 21 is written to the core 22 by irradiating the second ultraviolet ray from outside the optical fiber core wire, that is, outside the coating layer 24. The writing of the grating 21 may be performed by using various well-known methods. For example, when the writing is performed by the phase mask method, a grid-like shape is placed immediately before the optical fiber core wire to be the measurement core wire 20. A phase mask 25 is disposed, and a coherent ultraviolet laser beam having a wavelength of 2 66 nm, which is a quadruple wavelength (4 ω) from an Nd-YAG laser source, is applied to the phase mask 25 by a cylindrical lens. Irradiation may be performed in a state where light is collected by the system 26. As a result, the ultraviolet laser light passes through the phase mask 25 and the coating layer 24, and the refractive index of the grating pitch portion corresponding to the lattice pitch of the phase mask 25 with respect to the core 22 is increased, so that the Bragg grating is increased. 2 1 will be written. The wavelength of the second ultraviolet ray is, for example, 150 to 400 nm, and the irradiation energy is 0.1 to: L 0 kJ / cm ² .

In the present embodiment, a plurality of fiber gratings 21 having the structure shown in FIG. 2 or FIG. 3 are formed in one core 22 at intervals along the axial direction. For example, 20 or more short-period fiber gratings with a length of 5 to 20 mm are formed at 50 mm intervals. At this time, the reflectance of each fiber grating is set to about 4 to 5% or more, and the center wavelength of the reflected light is shifted slightly (for example, at 0.5 nm intervals). In order to write such a fiber grating on one fiber core, for example, while using one type of phase mask 25 and ultraviolet light, the tension applied to one fiber core is gradually increased. The writing of the gradation may be performed while changing. Due to the difference in the tension at the time of writing the grating, the grating interval of each grating changes when the tension is released even if the grating interval at the time of writing is constant.

As shown in Fig. 4, while applying tension to the optical fiber core that becomes the measurement core 20, a sweep 21 is applied through the phase mask 25 to perform the grating 21 Was written. Sweep irradiation was performed by moving the reflection mirror 27. In this embodiment, since the grating 21 is written without removing the coating layer 24, a high-strength optical fiber core wire (measurement core wire 20) whose mechanical strength is not reduced is manufactured. can do. The broadband light source 31 and the spectroscope 32 are connected to the optical fiber core (irradiated fiber) 20 on which the grating 21 is to be written via the force bra 33, and the reflected light waveform is observed while observing the reflected light waveform. Grating 21 1 was written. As a broadband light source 31, a superluminescence diode light source (SLD light source) was used. An optical isolator 34 was arranged between the light source 31 and the power bra 33. As the spectroscope 32, for example, a spectroscope in the infrared wavelength region having a resolution of 0.1 nm or less can be used.

As shown in FIG. 5, the tension of the irradiated fiber 20 is applied by using a fixed drum 35 and a rotating drum (tension applying drum) 36, and the amount of applied tension is determined by the rotation angle of the rotating drum 36. Set by controlling. More specifically, the fiber 20 to be irradiated is wound around and fixed to those of the fixed drum 35 and the rotating drum 36, and then a stepping motor (not shown) connected to the rotating drum 36 is driven. Then, the rotating drum 36 was rotated, and the set tension was applied to the irradiated fiber 20. In this embodiment, the tension is set in 20 steps, and gratings 21 having different reflection wavelengths are continuously formed on the same irradiated fiber 20 with the same mask at intervals of 50 mm.

FIG. 6 (a) schematically shows the fabricated measurement core 20 and FIG. 6 (b) shows the reflection characteristics of the measurement core 20. FIG. Twenty gratings 21 having different reflection wavelengths are formed on the measurement core 20 at intervals of about 0.82 nm. A plurality of types of gratings are arranged in one irradiated fiber 20 at a close distance interval of 50 mm between the gratings 21. As can be seen from FIG. 6 (b), the reflection characteristics of each grating 21 are well controlled.

Figure 7 shows the reflection peak wavelength (reflection characteristics) of each grating 21 at the time of making the grating 21 and after releasing the tension. From Fig. 7, it can be seen that the reflection peak wavelength after releasing the tension can be linearly controlled by the amount of rotation of the rotating drum (tension applying drum) 36. That is, depending on the amount of elongation of the irradiated fiber 20, —The reflection wavelength of Ting 2 1 can be controlled linearly. Also, from the change in the reflection wavelength at the time of the preparation of the grating 21, it can be confirmed that the refractive index of the core portion is lowered due to the high elasticity effect accompanying the application of the tension. In the present embodiment, it was possible to shorten the wavelength by about 1 nm by giving the irradiated fiber a maximum of about 1.4%.

FIG. 8 shows the relationship between the reflection wavelength at the time of irradiation and the reflection wavelength after the tension is released, which is an index of the amount of tension applied to the fiber 20 to be irradiated. As can be seen from FIG. 8, extremely good linearity can be confirmed. Therefore, it can be understood that the wavelength can be set with high accuracy by controlling the amount of tension applied to the irradiation target fiber 20.

Next, with reference to FIG. 9, the basic principle of measuring the strain distribution (or stress distribution) using the optical fiber cable 100 including the measuring core 20 will be described. The measuring core wire 20 included in the optical fiber cable 10 ° has n (n is an integer of 2 or more) fiber gratings FBG 1 to FBGn. In this example, the measuring core 20 is coupled to the broadband light source 31 and the spectroscope 32 via the force bra 33. The broadband light source 31, the spectroscope 32, and the power blur 33 constitute an optical measurement system 40. In the present embodiment, for example, a super luminescence diode light source (SLD light source) can be used as the broadband light source 31 as described above, and the spectroscope 32 has, for example, an infrared light having a resolution of 0.1 nm or less. A spectrometer in the wavelength region can be used. The light source is not limited to the broadband light source, and may be any light source including at least the wavelength of the measurement light.

First, the broadband light (measurement light) emitted from the broadband light source 31 enters the measurement core 20 via the force bra 33. This broadband light first enters the fiber grating FBG1. Of the incident broadband light, light having a reflection peak wavelength λ 1 determined by the period Λ 1 of the fiber grating FBG 1 is selectively reflected to the left. The reflected light having the wavelength of 1 enters the spectroscope 32 via the force bra 33. Above the optical fiber core 1, a transmitted light spectrum is schematically shown, and below a measurement core 20, a reflected light spectrum is schematically shown.

Of the broadband light incident on the first fiber grating FBG 1, the band component not reflected by the fiber grating FBG 1 is incident on the next fiber grating FBG 2. Here, similarly, the second fiber grating FBG 2 The light of the reflection peak wavelength 2 determined by the period Λ 2 is selectively reflected to the left, and the reflected light of the wavelength 2 enters the spectroscope 32 via the force bra 3 3.

In the configuration example shown in Fig. 9, the fiber gratings FBG1 to FBGn have fiber reflection gratings such that the reflection peak wavelengths 1 to n increase as the distance from the entrance end of the measurement core 20 increases. 1 to FBG n have been produced. <That is, the relationship of 1 1 2 1 ”“ n (1) 入 n has been established.

In this example, for example, if a stress is applied to the fiber grating FBG 3 having a reflection peak wavelength λ3, the stress causes a strain in the fiber, and as a result, the reflection peak wavelength of the FBG 3 increases from λ3. (Into 3 + Δ people). By measuring this wavelength shift amount Δ person, the amount of strain at the position of FBG 3 can be monitored, and the stress applied to the position of FBG 3 can be measured. <About FBG 3 As described above, the distortion and stress at each position can be measured for FBG1 to FBGη in the same manner. Therefore, to increase the distance resolution, it is sufficient to reduce the distance d between the fiber gratings. To increase the number of measurement points, the number of fiber gratings 21 (the number of set wavelengths) formed on the measurement core wire 20 is increased. do it. Note that the relationship between the wavelength shift amount and the strain (or stress) may be measured and recorded in advance.

FIG. 10 schematically shows a configuration in which the measurement core wire 20 of the optical fiber cable 100 shown in FIG. 1 is connected to the optical measurement system 40, and the optical fiber cable 100 is bent. It is bent at radius R. The outer diameter of the optical fiber cable 100 used in the present embodiment is 13 mm, and the slot cycle is 500 mm. As shown in Fig. 1, the four-core cable 50 including the measuring core 20 is housed in the slot 55 so that it is the outermost layer in the slot 55 of the optical fiber cable 100. are doing. For the optical fiber except the measurement core 20 inside the tape core 50, use the commonly used communication optical fiber core 30 for the core 30, and use the four lower layers of the tape core 50 Similarly, the optical fiber core 30 for communication which is usually used is also used for the optical fiber core included in the tape core 51. The optical fiber cable 100 used was a Z type.

Fig. 11 shows the reflection wavelength shift amount [nm] and distortion (amount) [%] for each position [mm] of the optical fiber cable 100 in the configuration shown in Fig. 10. You. Measurements were performed for cable bending radii R of 38 cm, 25 cm, and 17 cm. Based on the results of an experiment conducted separately, a coefficient of 1.34 [nm / N] (1.83 [nm /%]) between the tension applied to the fiber grating and the amount of change in the reflected wavelength was determined. The coefficient was used to convert the reflected wavelength shift amount into distortion (amount).

As can be seen from Fig. 11, the repetition of the strain in the tensile direction and the strain in the compressive direction was observed at the slot period, and the uneven distribution of the strain generated according to the storage shape of the optical fiber could be observed. By this measurement, the amount of distortion in both directions could be detected at multiple points (distance resolution: 50 mm) with a measurement accuracy of about 0.002%.

Next, the same measurement was performed for the SZ type optical fiber cable shown in Fig. 12. FIG. 12 (a) schematically shows a cross section of the SZ type optical fiber cable 110, and FIG. 12 (b) shows a tape core 50 stored in the optical fiber cable 110. Is schematically shown. The tape core 50 is the same as the four-core tape shown in FIG. 1 and includes a measurement core 20. The slot diameter of the slot rod 61 of the optical fiber cable 110 is 10 mm, and the SZ pitch is 320 mm. The tape core 50 is accommodated in one of the five-groove slots having a groove angle amplitude 6> 270 degrees so that the tape core 50 is located on the outermost layer. At the center of the slot rod 61, a tension member 62 is disposed.

As shown in Fig. 12 (c), the optical fiber cable 110 was bent at a cable bending radius R17.5 cm, and the reflection waveform was measured. The bending was performed in three directions, inward bending, outward bending, and intermediate bending, based on the line 61c. Figure 13 shows the measurement results of the reflected light waveform.

As can be seen from Fig. 13, in the outward bending (bending configuration in which the wire 61c is an outer arc), tensile strain is applied to the entire measurement core wire, and the inward bending (bending in which the wire 61c is an inner arc). In the (curved form), it was observed that a compressive strain was applied in a direction opposite to the outward bending. In the intermediate bending (bending mode in which the outer and inner arcs of the line 61c are uniformly present in the optical fiber cable trajectory), it was confirmed that the tension and the compression were periodically distributed at the slot pitch.

In the above embodiment, the axial distortion is measured from the shift amount of the reflected light peak wavelength. However, the amount of bending of the core wire can also be obtained from the shift amount of the full width at half maximum of the reflected light. (However, in this case, it is necessary to form a fiber grating with high reflectivity. If the relationship between the reflection peak of the measurement core wire 20 and the temperature is measured and recorded, the temperature can be corrected.

In the present embodiment, after the cladding layer is covered with an ultraviolet-transmissive ultraviolet-curing resin, the core is irradiated with ultraviolet rays so as to pass through the resin layer without peeling the resin layer, thereby writing the grating. In. According to such a manufacturing method, the optical fiber core can exhibit high mechanical strength even after writing the grating. As a result, a large strain can be applied to the optical fiber at the time of fabrication, and a large strain applied to the optical fiber can be measured. In addition, since the applied stress at the time of fabricating the fiber grating can be set widely, it is possible to widen the wavelength setting range. Further, by using the fiber grating having the secured mechanical strength, it is possible to easily realize multi-core tape or optical cable in a conventional automatic manufacturing apparatus for optical communication cables. If the method of writing the grating with the resin layer removed is adopted, the mechanical strength of the optical fiber core will be significantly reduced, and the fiber core may be broken by about 1% axial strain. . On the other hand, according to the optical fiber core manufactured by the method of the present embodiment, since the mechanical strength is 5 to 6 times that of the optical fiber, stable measurement can be performed up to 5% strain. is there.

The optical fiber cable according to the present embodiment has a measurement core wire 20 having a plurality of types of fiber gratings 21 having different reflection characteristics. Since a plurality of types of fiber gratings 21 can be arranged at short distance intervals, it is possible to provide an optical fiber cable capable of measuring strain with high distance resolution. Therefore, it is possible to measure the local maximum strain of the optical fiber, which cannot be measured by the conventional technique. In addition, compared with the conventional technology, there is an advantage that the distortion amount can be measured with high accuracy (for example, about 0.002%). _C Furthermore, the distortion amount at each position (or Stress) can be measured in real time by wavelength multiplexing, which is more suitable than the conventional technology that required a relatively long measurement time (5 minutes or more). In addition, the optical fiber cable of the present embodiment is special or expensive. There is also an advantage that distortion measurement can be performed easily without using a dedicated measuring instrument.As shown in Fig. 14, the communication light is propagated through the measuring core 20 and the measuring core 20 is used. Can be used as a communication optical fiber. In this case, if the communication light and the measurement light in a wavelength band different from the communication wavelength band of the communication light are combined into a measurement core 20 by a power bra (eg, a WDM power bra) 33, communication by the communication light is performed. The strain distribution of the optical fiber can be measured in the live state without affecting the optical fiber.

By using the measurement core wire 20 that propagates the measurement light and the communication light in this manner, all the optical fiber cores mounted on the optical fiber cable 100 can be mounted without installing the optical fiber core dedicated to strain measurement. The wires can be used for communication. When propagating communication light that can be communicated without being affected by the measurement light in the measurement core wire 20, for example, the measurement light wavelength is 1.55〃m for the communication wavelength of 1.31〃m band. The fiber grating 21 may be formed so as to be in a band or a measurement light wavelength in a 1.65〃m band for a communication wavelength of 1.55〃m.

As shown in Figs. 15 (a) and 15 (b), a pair of fiber bag gratings 21 having the same reflection characteristic is point-symmetrical (that is, symmetric with respect to the central axis of the optical fiber cable). The measurement core 20a-20a '(or 20b-20b') can be arranged.

By using the optical fiber cable 120 containing the pair of measurement cores 20a-20a 'in this way, the reflection peak of the measurement core 20a and the measurement core 20a' By comparing with the reflection peak (for example, by taking the difference between the two peaks), the cable bending (or the stress due to the cable bending) can be detected with high sensitivity. That is, as shown in Fig. 15 (b), for example, when tensile stress is applied to the 2nd position of the measuring core 20a, compressive stress is applied at the 2nd position of the measuring core 20a ' The measurement cores 20 a and 20 a ′ that are located point-symmetrically in the cable have opposite stress distributions caused by the cable bending, and are thus observed. The peak wavelength change also has the opposite value. As a result, by measuring the gap between the peak wavelengths that match each other in a state where the cable is not bent, the shift is doubled compared to the measurement with a single core (one measuring core wire 20). Because the amount can be detected, the measurement sensitivity can be doubled. Also, for measurement By comparing the obtained tensile stress and compressive stress, it is also possible to detect in which direction the optical fiber cable is bent. Furthermore, in addition to the measurement cores 20a-20a, another pair of measurement cores 20b-20b is mounted, so that the four pairs of measurement cores are orthogonal to each other. If the guard wire is provided, it is possible to detect bending in all directions in the optical fiber cable 120 other than the portion having the spiral housing structure, and it is also possible to detect the direction. By using a pair of measurement cords 20a-20a ', for example, it is possible to cancel the effect of temperature and observe only the effect of stress.

Further, as shown in FIG. 16, an optical fiber cable 130 equipped with two pairs of measurement cores 20a-20a 'and 2Ob-20b' may be used. With the optical fiber cable 130 having such a configuration, similarly to the optical fiber cable 120, the distribution of stress and bending can be detected with high sensitivity. If the outer sheath is provided with markings that indicate the surface direction without spiraling the internal structure, or if the cable is laid out in a non-circular shape, the cable can be bent in such a way that the core wire arrangement position can be identified. And its direction can be observed by reflected light measurement.

(Embodiment 2)

Next, a method for detecting a stress distribution using the optical fiber cable according to the present invention will be described with reference to FIGS. 17 (a) and (b).

In this example, the same optical fiber cable 70 as in the above embodiment is embedded in the road 72. One end of the optical fiber cable 70 is connected to the monitoring device 71. The monitoring device 71 includes a configuration similar to that of the optical measurement system 40 in FIG. 9 inside. The optical fiber cable 70 has a large number of built-in fiber gratings, and the reflection peak wavelength of each fiber grating is slightly different.

As shown in FIG. 17 (a), when automobiles 73 and 74 exist on the road 72, a stress is generated in a corresponding portion of the optical fiber cable 70 due to the influence. Based on the principle described above, the monitoring device 71 can detect the axial stress distribution on the optical fiber cable 70.

Further, as shown in FIG. 17 (b), the optical fiber cable 70 can be buried meandering in a monitoring area having a two-dimensional spread, and the monitoring area shown in FIG. 17 (b) can be buried. It is possible to detect which part of the region and how much stress is generated. Although the description has been given of the road 72, stress in a tunnel or a bridge can be similarly detected. In addition, even when stress or bending occurs in the optical fiber cable due to landslide, etc., it is possible to observe the state of the optical fiber cable remotely without going to the place where the landslide occurred.

According to this example, since a fiber grating is used, stress and strain can be measured with high accuracy, and multipoint measurement can be realized using one fiber. In addition, since fine bag ratings can be formed at intervals of several tens of mm, a high position resolution can be obtained. If the optical fiber cable 70 is configured as shown in FIG. 14, all optical fiber cores mounted on the optical fiber cable 70 can be used for communication. Furthermore, by writing a grating on the communication optical fiber, it is possible to give the communication optical fiber a function of monitoring its own tension. The coating agent described in the first embodiment satisfies the characteristics required for a communication optical fiber, and the grating manufacturing method described in the first embodiment is also applicable to a communication optical fiber.

When the optical fiber cable 70 is configured as shown in FIG. 15 or FIG. 16, the shift amount due to the bending of the optical fiber cable 70 can be detected with high sensitivity. It is possible to detect the amount of bending and the direction of bending. Industrial applicability

According to the present invention, since at least one of the plurality of optical fiber cores included in the optical fiber cable has a plurality of types of fiber gratings, strain measurement with a higher distance resolution than the conventional technology can be performed. An optical fiber cable can be provided. In addition, if the optical fiber cable of the present invention is spatially arranged, a two-dimensional or three-dimensional stress distribution can be detected with high accuracy. Furthermore, if the same type of fiber grating is arranged symmetrically with respect to the center axis of the optical fiber cable, the shift amount due to the bending of the optical fiber cable can be detected with high sensitivity, and the bending amount and the direction of the bending can be detected. it can.

Claims

The scope of the claims

1. An optical fiber cable including a plurality of optical fiber cores, wherein at least one of the plurality of optical fiber cores has a plurality of types of fiber gratings having different reflection characteristics or transmission characteristics. ,

The plurality of types of fiber gratings include at least a first fiber grating that reflects or transmits light of a first wavelength, and a second fiber grating that reflects or transmits light of a second wavelength different from the first wavelength. The second fiber grating is formed at a position along the axial direction from the position where the first fiber grating is formed, and the first fiber grating and the second fiber grating The optical fiber cable monitors the amount of distortion at each position of the first fiber grating and the second fiber grating by the wavelength shift from each of the two fiber gratings.

2. The optical fiber core having the plurality of types of fiber gratings is affected by the measurement light including at least the first wavelength light and the second wavelength light, and the measurement light. 2. The optical fiber cable according to claim 1, wherein the optical fiber cable propagates with communication light that can communicate without communication.

3. At least two of the plurality of optical fiber cores are the optical fiber cores having the plurality of types of fiber gratings,

The first fiber grating and the second fiber grating included in each of the optical fiber cores having the plurality of types of fiber gratings are the center of the optical fiber cable having the plurality of optical fiber cores. 3. The optical fiber cable according to claim 1, wherein the optical fiber cable is symmetrically arranged with respect to the axis.

4. A step of injecting light of a light source including the light of the first wavelength and the light of the second wavelength into the optical fiber cable according to any one of claims 1 to 3, The first wavelength shift from the fiber grating and the second wavelength shift Detecting a second wavelength shift from the bag grating, thereby obtaining a first strain amount at the position of the first fiber grating and a second strain amount at the position of the second fiber grating.

A strain measurement method.

5. The optical fiber cable according to claim 3, wherein the light of the light source including the light of the first wavelength and the light of the second wavelength,

The difference between the wavelength shift of a pair of fiber bag gratings symmetrically arranged with respect to the center axis of the optical fiber cable is obtained to determine the amount of distortion at the position of the pair of fiber bagging. Process and

A strain measurement method.