JP2005337845A - Optical fiber for strain sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber for a strain sensor capable of preventing increase of a transmission loss and enlarging a strain measuring dynamic range. <P>SOLUTION: In this optical fiber 1 for the strain sensor installed on a measuring object and used for measuring a strain of the measuring object, a coating layer 3 comprising polyimide is formed on the outer periphery of an optical fiber 2 having a plurality of vacancies in a clad provided on the periphery of a core. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is installed on a measurement target object by being attached to a structure or embedded in a structure or underground, and is used for measuring distortion of the measurement target object such as a structure or the ground. The present invention relates to an optical fiber for a strain sensor.

  As a conventional strain sensor optical fiber, there is a strain sensor optical fiber 41 as shown in FIG. 4A (see, for example, Patent Document 1). The strain sensor optical fiber 41 is obtained by coating the outer periphery of the optical fiber 42 with an ultraviolet curable resin 43 and coating the outer periphery of the ultraviolet curable resin 43 with a fiber reinforced plastic 44 for preventing disconnection of the optical fiber 42. . The fiber reinforced plastic 44 has a spiral protrusion 45b as shown in FIG. 4B or a hook-like shape as shown in FIG. Protrusions 45c and mesh-like irregularities 45d as shown in FIG. 4 (d) are provided.

  Further, as a conventional optical fiber for a strain sensor, the outer periphery of the fiber reinforced plastic 44 shown in FIG. 4A is further coated with polyethylene, and grip properties (adhesion) when the polyethylene is embedded in a structure or the ground. Some have been subjected to uneven embossing to improve the thickness (for example, see Patent Document 2).

JP 2000-227368 A Japanese Patent Laid-Open No. 2002-23030

  However, the conventional strain sensor optical fiber 41 generally uses a single mode optical fiber (compliant with ITU-T. Rec. G. 652.B) for optical communication as the optical fiber 42. Normally, a single mode optical fiber has a very large bending loss of 5 dB / m or more when bent with a bending diameter of 20 mm in light having a wavelength of 1.55 μm.

  Accordingly, when the fiber reinforced plastic 44 is coated on the outside of the optical fiber 42, the fiber reinforced plastic 44 is cured and shrunk, and thus the optical fiber 42 receives a side pressure and bends slightly. There is a problem that the transmission loss of 41 increases.

  Further, in the conventional strain sensor optical fiber 41, an ultraviolet curable resin 43 is coated on the outer periphery of the optical fiber 42. Since the ultraviolet curable resin 43 has a low degree of adhesion (low adhesion) with the glass portion (cladding layer) of the optical fiber 42, when the strain sensor optical fiber 41 is subjected to a large strain, the ultraviolet curable resin 43 is light. There is a possibility of peeling from the fiber 42.

  When this peeling occurs, strain is not transmitted to the central portion of the optical fiber 42, but the strain measurement sensitivity of the strain sensor optical fiber 41 is deteriorated. For this reason, the conventional strain sensor optical fiber 41 has a problem that the strain measurement dynamic range cannot be made too large.

  Therefore, an object of the present invention is to provide an optical fiber for a strain sensor that can prevent an increase in transmission loss and expand a strain measurement dynamic range.

  The present invention was devised to achieve the above object, and the invention of claim 1 is for a strain sensor that is installed in a measurement object and used to measure the distortion of the measurement object. An optical fiber for a strain sensor, in which a coating layer made of polyimide is formed on the outer periphery of an optical fiber having a plurality of holes in a clad provided around a core.

  The invention of claim 2 is an optical fiber for a strain sensor that is installed on an object to be measured and is used to measure the strain of the object to be measured, and a plurality of voids are provided in a cladding provided around the core. A first coating layer made of polyimide is formed on the outer periphery of the optical fiber having holes, and a second coating layer made of fiber-reinforced plastic is formed on the outer periphery of the first coating layer, and the outer periphery of the second coating layer A strain sensor optical fiber having a third coating layer made of polyethylene and an embossed outer periphery of the third coating layer.

  According to a third aspect of the present invention, the optical fiber has an inner diameter of 3 to 10 μm in a clad provided around the core so as to be line symmetric with respect to the central axis of the core and at equal intervals. 3. A holey fiber having four or more holes and surrounding an even number of cores, and having a bending loss of 1 dB / m or less at a bending diameter of 20 mm in light having a wavelength of 1.55 μm. It is an optical fiber for distortion sensors given in the description.

  According to the present invention, the following excellent effects are exhibited.

  (1) An increase in transmission loss can be prevented.

  (2) The strain measurement dynamic range can be expanded.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.

  FIG. 1 is a structural diagram of an optical fiber for a strain sensor showing a preferred embodiment of the present invention.

  As shown in FIG. 1, the strain sensor optical fiber 1 according to the present embodiment is a first made of polyimide on the outer periphery of an optical fiber 2 having a plurality of holes in a clad provided around a core. A coating layer 3 is formed, and a second coating layer 4 made of fiber reinforced plastic (for example, glass FRP) for preventing disconnection of the optical fiber 2 is formed on the outer periphery of the first coating layer 3. In order to improve the grip property (adhesiveness) when the third coating layer 5 made of polyethylene is formed on the outer periphery of the coating layer 4 and embedded in the structure or the ground on the outer periphery of the third coating layer 5. The concave-convex shape (concave shape in FIG. 1) is embossed to form a concave portion 6 having a substantially U-shaped cross section.

  Here, “an optical fiber having a plurality of holes in a clad provided around the core” will be described.

  In recent years, a photonic crystal fiber (PCF) has attracted attention as an optical fiber having features that cannot be realized with an optical fiber composed of a conventional core and cladding. PCF is an optical fiber in which a photonic crystal structure is provided in a clad. The photonic crystal structure is a periodic structure having a refractive index. Specifically, by providing a honeycomb-structured space such as a honeycomb in the cladding, a photonic band gap (PBG) that is a forbidden band of light is generated.

  For example, Knight et al., In Science 282, 1476, (1998) report PCF using PBG as a guiding principle, and Cregan et al., In Science, 285, 1537, (1999), describes a PBG structure as a guiding principle. The hollow core PCF is reported.

  Recently, although not an optical fiber having a perfect PBG structure, due to the difference in the glass composition of the core and the cladding, a plurality of voids are present in the cladding in the vicinity of the core of the conventional optical fiber having a relative refractive index difference. A holey fiber having a feature that has not been obtained in the past has been reported by forming holes, lowering the effective refractive index of the clad, and expanding the relative refractive index difference between the core and the clad.

  The feature of this holey fiber is that the refractive index of the hole is about 1, and the effective relative refractive index difference is much larger than that of a normal single mode optical fiber consisting of a core and a clad. This is a highly effective point. For this reason, the holey fiber has a feature that bending loss generated when the holey fiber is bent is extremely small.

  For example, Tatsumi et al. (Merber et al. “A Study on Practical Use of Holy Fiber”, IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers, January 16, 2003, Vol. -50), an effective relative refractive index difference between the core and the clad is obtained by the holey fiber in which 4 to 6 holes are formed in the clad near the core of the optical fiber having the structure of a normal single mode optical fiber. It has been reported that a holey fiber whose bending loss is 1/100 that of a normal single mode optical fiber has been realized.

  FIG. 2 is a cross-sectional view showing a holey fiber used in the present embodiment. As shown in FIG. 2, the holey fiber 20 used as the optical fiber 2 in FIG. 1 includes a center of the core 21 in a clad 22 made of pure quartz provided around a core 21 in which germanium is added to pure quartz. Four holes 23 having an inner diameter d that surrounds the core 21 and extends in the axial direction so as to be line-symmetrical and equidistant with respect to the axis of symmetry are 3 to 10 μm, preferably 5 to 8 μm, more preferably 6 to 7 μm. More than this, preferably 4 to 8, and even numbers (six in FIG. 2) are formed.

  The holey fiber 20 has a bending loss of 1 dB / m or less, preferably 0.1 dB / m or less, more preferably 0.05 dB / m or less at a bending diameter of 20 mm in light having a wavelength of 1.55 μm. It is.

  The reason why the inner diameter d of the hole 23 is 3 to 10 μm is that if the inner diameter d of the hole 23 is less than 3 μm, the bending loss of the holey fiber 20 does not become so small, and the inner diameter d of the hole 23 exceeds 10 μm. This is because the effect of further reducing the bending loss of the holey fiber 20 cannot be obtained.

  The reason why the number of the holes 23 is four or more and the even number is that if the number of the holes 23 is two, the bending loss of the holey fiber 20 is not so small, and if the number of the holes 23 is an odd number, the holey fiber 20 is not. This is because when stress is applied to the film, a portion where stress is concentrated locally may occur.

  In light having a wavelength of 1.55 μm, the bending loss at a bending diameter of φ20 mm is set to 1 dB / m or less. Generally, the wavelength of light transmitted to the holey fiber 20 is 1.55 μm, and the bending loss at a bending diameter of φ20 mm. This is because the transmission loss of the optical fiber 1 for strain sensor after manufacture increases when the value exceeds 1 dB / m.

  FIG. 3 is a diagram illustrating an example of bending characteristics of a holey fiber.

  In FIG. 3, in a holey fiber having four holes, the bending characteristics when light having a wavelength of 1.55 μm is transmitted are plotted with the hole diameter d (μm) of each hole on the horizontal axis and the vertical axis bent. It was shown for the bending loss (dB / m) when the diameter was 20 mm.

  As shown in FIG. 3, a holey fiber having no holes, that is, a single mode optical fiber, has a very large bending loss exceeding 5 dB / m. It can be seen that hole loss of holey fibers having holes decreases as the hole diameter d increases. In particular, a holey fiber having holes with a hole diameter d of 3 μm has a bending loss of 1 dB / m, and a holey fiber having holes with a hole diameter d of 7 μm has a bending loss of 0.3 dB / m. A holey fiber having holes with d of 10 μm has a bending loss of 0.04 dB / m.

  Next, a method for using the strain sensor optical fiber 1 will be described.

  The strain sensor optical fiber 1 is installed on a concrete structure such as a building, a bridge, or a tunnel, or is embedded in the structure or the ground. The strain or the object to be measured such as the ground is distorted. Used to measure.

  Specifically, light is incident on one or both ends of the installed strain sensor optical fiber 1, and the reflected light, scattered light (for example, backscattered light), and light loss are measured, whereby the strain sensor light. Various physical quantities, such as temperature distribution, can be measured over the length of the range in which the fiber 1 is installed, as well as structural and underground strains.

  The operation of the present embodiment will be described.

  The strain sensor optical fiber 1 uses a holey fiber 20 as the optical fiber 2 shown in FIG. 1 that has a very small bending loss and is resistant to bending compared to a normal single mode optical fiber that is generally used as an optical fiber for optical communication. doing. When the second coating layer 4 made of fiber reinforced plastic is formed outside the holey fiber 20, the holey fiber 20 is subjected to a lateral pressure and is bent slightly during manufacture in which the second coating layer 4 is cured and shrunk. Since 20 is resistant to bending, an increase in transmission loss of light transmitted through the strain sensor optical fiber 1 after manufacture can be prevented.

  In the strain sensor optical fiber 1, a first coating layer 3 made of polyimide is formed on the outer periphery of the holey fiber 20. Polyimide has a higher degree of adhesion (high adhesion) to the clad (glass portion) 22 of the holey fiber 20 than an ultraviolet curable resin generally used in optical fibers for optical communication. Even if a large strain is applied to the fiber 1 by an external force, the first coating layer 3 is not peeled off from the holey fiber 20 and is not displaced.

  Therefore, the distortion is always reliably transmitted to the core 21 which is the central portion of the holey fiber 20, and the strain measurement sensitivity of the strain sensor optical fiber 1 is not deteriorated. That is, the strain sensor optical fiber 1 can expand the strain measurement dynamic range as compared to the conventional strain sensor optical fiber 41 described with reference to FIG.

  In the embodiment described above, the optical fiber 2 is described using the holey fiber 20 having the refractive index of the core 21 higher than that of the clad 22. However, instead of the holey fiber 20, a holey having the same refractive index of the core and the clad is used. A fiber may be used. Further, PCF may be used as the optical fiber 2. In these cases, the same effects as described above can be obtained.

1 is a structural diagram showing a preferred embodiment of the present invention. It is a cross-sectional view of a holey fiber. It is a figure which shows an example of the bending characteristic of a holey fiber. FIG. 4A is a structural view of a conventional strain sensor optical fiber, and FIGS. 4B to 4D are side views showing an example of a coating layer thereof.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber for strain sensors 2 Optical fiber having a plurality of holes 3 First coating layer (coating layer)
4 Second coating layer 5 Third coating layer 6 Recess

Claims (3)

  An optical fiber for a strain sensor, which is installed on an object to be measured and used for measuring strain of the object to be measured, and an outer periphery of the optical fiber having a plurality of holes in a clad provided around the core Further, a strain sensor optical fiber, wherein a coating layer made of polyimide is formed.   An optical fiber for a strain sensor, which is installed on an object to be measured and used for measuring strain of the object to be measured, and an outer periphery of the optical fiber having a plurality of holes in a clad provided around the core In addition, a first coating layer made of polyimide is formed, a second coating layer made of fiber reinforced plastic is formed on the outer periphery of the first coating layer, and a third coating made of polyethylene is formed on the outer periphery of the second coating layer. An optical fiber for a strain sensor, wherein a coating layer is formed, and an outer periphery of a third coating layer is embossed. The above optical fiber has an even number of four or more holes having an inner diameter of 3 to 10 μm so that the core provided in the clad provided around the core is line-symmetrical and equidistant with respect to the central axis of the core. 3. An optical fiber for a strain sensor according to claim 1, wherein the optical fiber is a holey fiber having a bending loss of 1 dB / m or less at a bending diameter of 20 mm in light having a wavelength of 1.55 μm and surrounding the core. .

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