Optical fiber sensor

An optical fiber sensor includes: a central core disposed at a center of an optical fiber; and an outer peripheral core that spirally surrounds the central core. The effective refractive index ne2 of the outer peripheral core is lower than the effective refractive index ne1 of the central core. A ratio between the effective refractive index ne2 and the effective refractive index ne1 matches a ratio between an optical path length of the central core and an optical path length of the outer peripheral core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage application of International Application No. PCT/JP2017/044173, filed on Dec. 8, 2017, which claims priority from Japanese Patent Application No. 2017-026270, filed on Feb. 15, 2017. The contents of these applications are incorporated herein in their entirety.

TECHNICAL FIELD

The present invention relates to an optical fiber sensor.

BACKGROUND

In the related art, optical fiber sensors for measuring various physical quantities (for example, stress, strain, temperature, or the like) using an optical fiber as a sensor are known. The optical fiber sensor measures the various physical quantities described above, based on the light reception result obtained by causing light to be incident from the first end portion of the optical fiber as a sensor and receiving transmitted light (or scattered light) to be emitted from the second end portion of the optical fiber or reflected light (or scattered light) to be emitted from the first end portion. Representative examples of such an optical fiber sensor include an FBG type optical fiber sensor, a scattered light type optical fiber sensor, or the like.

In the FBG type optical fiber sensor, Fiber Bragg Grating (FBG) is formed in the core of the optical fiber. The FBG type optical fiber sensor is used for measuring the distribution of various physical quantities in the longitudinal direction of the optical fiber by utilizing the characteristic of the FBG that the reflection characteristic of the FBG varies according to the surrounding environment. Incidentally, the FBG type optical fiber sensor is used, for example, in optical frequency domain reflectometry (OFDR).

In the scattered light type optical fiber sensor, an ordinary optical fiber on which FBG or the like is not formed is used as a sensor. The scattered light type optical fiber sensor is used for measuring the distribution of various physical quantities in the longitudinal direction of the optical fiber by utilizing the characteristic that the scattered light (for example, Rayleigh scattered light) generated in the optical fiber varies according to the surrounding environment.

Patent Documents 1 and 2 below disclose methods for measuring strain occurring in, for example, a structure by OFDR using an FBG type optical fiber sensor. Further, Patent Documents 3 to 6 and Non-Patent Documents 1 and 2 below disclose an optical fiber sensor in which an FBG is formed in a multi-core fiber having a plurality of cores. For example, in the following Non-Patent Document 2, the shape of the optical fiber sensor (shape of a structure to which the optical fiber sensor is attached) is measured by OFDR.

Here, for example, the above-described multi-core fiber is an optical fiber having a core (central core) formed at a center of an optical fiber and a plurality of cores (outer peripheral cores) formed so as to spirally surround the central core. For example, three outer peripheral cores are disposed with an interval of 120°. In Patent Documents 3 to 6 and Non-Patent Documents 1 and 2 below, an FBG is formed in each core of such a multi-core fiber.[Patent Document 1] Japanese Patent No. 5232982[Patent Document 2] Japanese Patent No. 5413931[Patent Document 3] U.S. Pat. No. 8,116,601[Patent Document 4] U.S. Pat. No. 8,630,515[Patent Document 5] U.S. Pat. No. 8,773,650[Patent Document 6] U.S. Pat. No. 9,417,057[Non-Patent Document 1] P. S. Westbrook et al., “Integrated optical fiber shape senor modules based on twisted multicore fiber grating arrays”, Proc. SPIE 8938, Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XIV, 89380H (Feb. 20, 2014)[Non-Patent Document 2] “LUNA Fiber Optic Shape Sensing”, Document #: SS00021-D-TS, Luna Innovations Incorporated, Jun. 21, 2013

In the above-described multi-core fiber having the central core and the outer peripheral core, the central core is parallel to the axis of the optical fiber, so the optical path of the central core is linear. On the other hand, since the outer peripheral core is spirally wound, the optical path length of the outer peripheral core is longer than the optical path length of the central core. Therefore, when such a multi-core fiber is used as an optical fiber sensor, a positional deviation occurs between the measurement point of the central core and the measurement point of the outer peripheral core. For example, in the configuration of the multi-core fiber disclosed in the above-described Non-Patent Document 1, when the fiber length is 2 [m], the distance between the cores is 35 [μm], and the number of spirals of the outer peripheral core per unit length is 50 [Turn/m], the optical path length difference between the central core and the outer peripheral core from the first end portion to the second end portion of the optical fiber is about 120 [μm].

Here, in the OFDR, the resolution in the longitudinal direction of the fiber is, for example, about 40 [μm]. Therefore, when a multi-core fiber in which the outer peripheral core is spirally wound around the central core is used as the optical fiber sensor, the positional accuracy in the longitudinal direction is deteriorated by the optical path length difference between the central core and the outer peripheral core. In particular, when the length of the optical fiber sensor is increased, the position error due to the optical path length difference between the center core and the outer peripheral core is accumulated and increased, and it becomes difficult to secure the measurement accuracy over the entire length of the optical fiber sensor.

SUMMARY

One or more embodiments of the present invention provide high measurement accuracy over the entire length of an optical fiber sensor, even when the length of the optical fiber sensor is increased.

An optical fiber sensor according to one or more embodiments of the present invention includes a central core that is formed at a center of an optical fiber; and at least one outer peripheral core that is formed so as to spirally surround the central core, wherein when a distance between the central core and the outer peripheral core is d, and the number of spirals of the outer peripheral core per unit length is fw, an effective refractive index ne1of the central core and an effective refractive index ne2of the outer peripheral core satisfy the following Expression (1).

Here, in the optical fiber sensor, the effective refractive index ne2of the outer peripheral core may be set to be lower than the effective refractive index ne1of the central core so as to match a ratio between the optical path lengths of the central core and the outer peripheral core.

Further, in the optical fiber sensor, a ratio between the effective refractive index ne2of the outer peripheral core and the effective refractive index ne1of the central core may be set so as to satisfy the following Expression (2).

Further, in the optical fiber sensor, a ratio between a molar concentration m1of a dopant to be doped to the central core and a molar concentration m2of an dopant to be doped to the outer peripheral core may be set so as to satisfy the following Expression (3).

Further, in the optical fiber sensor, germanium having the same concentration may be doped as a first dopant to the central core and the outer peripheral core, and a second dopant having an effect of decreasing the refractive index may be doped to the outer peripheral core.

Further, in the optical fiber sensor, an FBG may be formed over an entire length in a longitudinal direction or in a partial region in the longitudinal direction.

According to one or more embodiments of the present invention, the effective refractive index ne1of the central core of the optical fiber sensor and the effective refractive index ne2of the outer peripheral core are set so as to satisfy the above Expression (1). Therefore, the optical path length difference between the central core and the outer peripheral core can be made smaller than the optical path length difference A in the case where the effective refractive indices of the central core and the outer peripheral core are the same. Thus, even when the length of the optical fiber sensor is increased, it is possible to realize high measurement accuracy over the entire length thereof.

DETAILED DESCRIPTION

Hereinafter, embodiments of an optical fiber sensor will be described in detail with reference to the drawings. In the drawings referred to below, in order to facilitate understanding, scales of dimensions of respective members may be appropriately changed and shown as necessary. In the following description, an FBG type optical fiber sensor in which an FBG is formed in the core of an optical fiber will be described as an example. However, the optical fiber sensor is not limited to the FBG type optical fiber sensor, but is also applicable to other optical fiber sensors such as a scattered light type optical fiber sensor.

<Configuration of Optical Fiber Sensor>

FIG. 1is a perspective view showing an optical fiber sensor according to one or more embodiments of the present invention. Further,FIG. 2is a cross-sectional view of the optical fiber sensor according to one or more embodiments of the present invention. As shown inFIGS. 1 and 2, an optical fiber sensor1according to one or more embodiments is a multi-core fiber optical fiber sensor including a central core11, a plurality of outer peripheral cores12(outer peripheral cores12ato12c), and a cladding13. Incidentally, the outer peripheral surface of the cladding13may be covered with a coating (not shown).

The central core11is a core formed at the center of the optical fiber sensor1in parallel with the axis of the optical fiber sensor1. By this central core11, an optical path which is linear with respect to the longitudinal direction of the optical fiber sensor1is formed at the center of the optical fiber sensor1. The central core11is formed of silica glass containing, for example, germanium (Ge) (first dopant). In addition, FBG is formed over the entire length of the central core11. The diameter of the central core11is set to, for example, 5 to 7 [m].

The outer peripheral core12is a core formed to spirally surround the central core11. In one or more embodiments, the three outer peripheral cores12ato12care separated from each other by a predetermined distance d (seeFIG. 2) in the radial direction with respect to the central core11, and are disposed at intervals of an angle θ (for example, 120°) in a cross section orthogonal to the longitudinal direction. These outer peripheral cores12ato12cextend in the longitudinal direction of the optical fiber sensor1so as to spirally surround the central core11while maintaining a distance of an angle θ from each other (seeFIG. 1). These outer peripheral cores12ato12cform three spiral optical path surrounding the central core11in the optical fiber sensor1.

Similar to the central core11, the plurality of outer peripheral cores12ato12care formed of silica glass containing, for example, germanium (Ge) (first dopant). In addition, FBG is formed over the entire length of the outer peripheral cores12ato12c. Here, for example, a ratio between the concentration (molar concentration) of germanium to be doped to the central core11and the concentration (molar concentration) of germanium to be doped to the outer peripheral cores12ato12cis set so as to satisfy a predetermined relationship (details will be described later). The outer peripheral cores12ato12chave the same diameter (or substantially the same diameter) as the central core11, and are set, for example, in the range of 5 to 7 [μm].

This is to adjust the ratio of the effective refractive indices of the central core11and the outer peripheral core12by adjusting the concentration of germanium, which is a dopant having an effect of increasing the refractive index, and eliminate (or reduce) the optical path length difference between the central core11and the outer peripheral core12. Specific adjustment of the effective refractive indices of the central core11and the outer peripheral cores12ato12cwill be described later.

The distance d between the central core11and the outer peripheral cores12ato12cis set in consideration of the crosstalk between the cores, the optical path length difference between the central core11and the outer peripheral cores12ato12c, the strain amount difference between the central core11and the outer peripheral cores12ato12cwhen the optical fiber sensor1is bent, and the like. For example, in a case where the optical fiber sensor1is used for measuring its own shape (shape of a structure to which the optical fiber sensor is attached), it is desirable that the distance between the central core11and the outer peripheral cores12ato12cis, for example, 35 [μm], and the number of spirals of the outer peripheral core per unit length is, for example, about 50 [turn/m].

The cladding13is a common cladding covering the periphery of the central core11and the outer peripheral cores12ato12c. The outer shape of the cladding13is cylindrical. Since the central core11and the outer peripheral cores12ato12care covered with the common cladding13, it can be said that the central core11and the outer peripheral cores12ato12care formed inside the cladding13. The cladding13is formed of silica glass, for example.

<Adjustment of Effective Refractive Index>

Next, the adjustment of the effective refractive indices of the central core11and the outer peripheral core12will be described in detail.FIG. 3is a diagram showing an optical path length difference between the central core and the outer peripheral core in one or more embodiments of the present invention. In the following description, it is assumed that the distance between the central core11of the optical fiber sensor1and the outer peripheral core12(outer peripheral cores12ato12c) is d, and the number of spirals of the outer peripheral core12per unit length of the optical fiber sensor1is fw.

InFIG. 3, the straight line denoted by reference numeral P1indicates the central core11, and the straight line denoted by reference numeral P2indicates the outer peripheral core12. However, inFIG. 3, only the central core11and the outer peripheral core12corresponding to one spiral of the outer peripheral core12are shown. Assuming that the optical path length of the central core11corresponding to one spiral of the outer peripheral core12is L1and the optical path length of the outer peripheral core12corresponding to one spiral of the outer peripheral core12is L2, the relationship therebetween is expressed by the following Expression (4).
L2=√{square root over (L12+(2πd)2)}  (4)

Since the outer peripheral core12is formed so as to spirally surround the central core11, in a case where the effective refractive indices of the central core11and the outer peripheral core12are the same, the optical path length L2of the outer peripheral core12is set to be shorter than the optical path length L1of the central core11. Specifically, when the optical path length difference between the central core11and the outer peripheral core12is denoted by A and the length of the outer peripheral core12is denoted by L1+A, the optical path length difference A between the central core11and the outer peripheral core12is expressed by the following Expression (5).

The length B of the central core11corresponding to the optical path length difference A represented by the above Expression (5) (in other words, the length B in the longitudinal direction of the optical fiber sensor1having the optical path length difference A) is expressed by the following Expression (6).

Here, the effective refractive index of the central core11is denoted by ne1, and the effective refractive index of the outer peripheral core12is denoted by ne2. When the effective refractive indices ne1, ne2satisfy the following Expression (7), the optical path length difference between the central core11and the outer peripheral core12can be made smaller than the optical path length difference A in the case where the effective refractive indices of the central core11and the outer peripheral core12are the same. That is, by setting the effective refractive index ne1of the central core11and the effective refractive index ne2of the outer peripheral core12so as to satisfy the following Expression (7), the optical path length difference can be made smaller as compared with a case where the effective refractive indices of the central core11and the outer peripheral core12are the same. As a result, even when the length is increased, it is possible to realize high measurement accuracy over the entire length thereof.

Next, the FBG formed on the central core11and the plurality of outer peripheral cores12will be discussed. FBGs to be formed on the central core11and the plurality of outer peripheral cores12are formed at the same cycle along the longitudinal direction of the optical fiber sensor1. As described above, since the outer peripheral core12is formed so as to spirally surround the central core11, in a case where the effective refractive indices of the central core11and the outer peripheral core12are the same, the optical path length L2of the outer peripheral core12is set to be shorter than the optical path length L1of the central core11.

Therefore, the period of the FBGs formed in the outer peripheral core12(the period along the outer peripheral core12) is longer than the period of the FBGs formed in the central core11. Assuming that the Bragg wavelength of the FBG formed in the central core11is λ1and the Bragg wavelength of the FBG formed in the outer peripheral core12is λ2, the relationship therebetween is expressed by the following Expression (8).

The Bragg wavelength λBof the FBG is expressed by the following Expression (9) where A is the length of one period of the periodic structure of the refractive index formed in the optical fiber sensor1and neis the effective refractive index.
λB=2neΛ  (9)

If the ratio between the effective refractive index ne1of the central core11and the effective refractive index ne2of the outer peripheral core12is set as shown in the following Expression (10), the ratio between the optical path lengths of the central core11and the outer peripheral core12is canceled. In the Expression (10), since the right side has a value smaller than 1, the value of the effective refractive index ne2is smaller than the value of the effective refractive index ne1. In other words, the effective refractive index ne2of the outer peripheral core12is set to be lower than the effective refractive index ne1of the central core11so as to match a ratio between the optical path lengths of the central core11and the outer peripheral core12. This makes it possible to set (or reduce) the optical path length difference between the central core11and the outer peripheral core12and the Bragg wavelength difference between the central core11and the outer peripheral core12to zero. As a result, even when the length of the optical fiber sensor1is increased, it is possible to realize high measurement accuracy over the entire length thereof.

Incidentally, for example, by adjusting the ratio of germanium to be doped to the central core11and the outer peripheral core12, it is possible to realize the ratio of the effective refractive indices ne1, ne2shown in the above Expression (10). Specifically, a ratio between a molar concentration m1of germanium to be doped to the central core11and a molar concentration m2of germanium to be doped to the outer peripheral core12may be set so as to satisfy the following Expression (11).

<Method of Manufacturing Optical Fiber Sensor>

Next, a method of manufacturing the above-described optical fiber sensor will be described.FIG. 4is a flowchart showing a method of manufacturing the optical fiber sensor according to one or more embodiments of the present invention. In manufacturing an optical fiber sensor, first, a step of forming the core material of the central core11and the core material of the outer peripheral core12having effective refractive indices different from each other and having an effective refractive index difference is performed (step S1). The number of core materials of the outer peripheral core12may be one or more than one.

Specifically, the core material of the central core11to which germanium, a dopant having an effect of increasing the refractive index, is doped at a predetermined concentration, and the core material of the outer peripheral core12to which germanium is doped at a lower concentration than the core material of the central core11are formed. For example, a ratio between a molar concentration m1of germanium to be doped to the core material of the central core11and a molar concentration m2of germanium to be doped to the core material of the outer peripheral core12is adjusted so as to satisfy, for example, the above-described Expression (11). It is desirable to measure the formed core material with a preform analyzer to check whether or not an effective refractive index difference is obtained.

Next, a step of inserting the formed core materials of the central core11and the peripheral core12into a glass tube (capillary) and preparing a base material of the optical fiber sensor1is performed (step S2). Specifically, the base material of the optical fiber is prepared by respectively inserting the core materials of the central core11and the outer peripheral core12formed in the above step S1into the capillary having holes formed at positions where the core materials of the central core11and the outer peripheral core12are to be disposed, and melting and elongating it. The capillary finally becomes the cladding13of the optical fiber sensor1.

Next, a step of drawing the base material while rotating it and forming a covering material having ultraviolet (UV) permeability on the outer side of the cladding is performed (step S3). Specifically, the base material prepared in step S2is set in a drawing machine, and then the base material is drawn while being rotated by the drawing machine. At this time, a covering material having ultraviolet permeability is formed on the outer periphery of the strand obtained by drawing, that is, on the outer periphery of the cladding13. Here, the base material is drawn while being rotated in order to make the outer peripheral core12spiral. The covering material having ultraviolet permeability is formed on the outer periphery of the cladding in order to form the FBG in the central core11and the outer peripheral core12by irradiating the optical fiber with ultraviolet rays while winding up the optical fiber.

Subsequently, a step of irradiating ultraviolet light transmitted through the phase mask from above the covering material to form FBGs in the central core11and the outer peripheral core12is performed (step S4). Since germanium is doped to the central core11and the outer peripheral core12, and the central core11and the outer peripheral core12are irradiated with ultraviolet light through the phase mask, the germanium doped to the central core11and the outer peripheral core12reacts with the ultraviolet light. Thereby, FBG having a structure in which the refractive index periodically changes in the longitudinal direction is formed in the central core11and the outer peripheral core12.

Here, in a case where the diameter of the central core11and the outer peripheral core12is about 5 to 7 [μm], the distance d between the central core11and the outer peripheral core12is 35 [μm], and the number fwof spirals of the outer peripheral core per unit length is 50 [turn/m], from the above-described Expression (8), the difference in Bragg wavelength between the central core11and the outer peripheral core12is about 95 [μm] in the wavelength range of 1.55 μm. The effective refractive index of the core to which germanium is doped is, for example, about 1.48, and the effective refractive index difference of the outer peripheral core12for correcting the difference in the Bragg wavelength (the effective refractive index difference of the outer peripheral core12with respect to the central core11) is about 0.0001.

Further, in a case where the distance d between the central core11and the outer peripheral core12is 50 [μm], and the number fwof spirals of the outer peripheral core per unit length is 100 [turn/m], from the above-described Expression (8), the difference in Bragg wavelength between the central core11and the outer peripheral core12is about 770 [μm], which is large. In order to correct this difference in Bragg wavelength, it is necessary to make the effective refractive index difference of the outer peripheral core12(the effective refractive index difference of the outer peripheral core12with respect to the central core11) about 0.0007.

FIG. 5is a diagram showing an example of characteristics of the optical fiber sensor according to one or more embodiments of the present invention. Specifically,FIG. 5is a diagram showing the relationship between the effective refractive index ratio (ne2/ne1) between the central core11and the outer peripheral core12and the optical path length difference between the central core11and the outer peripheral core12. Further, inFIG. 5, the horizontal axis represents the effective refractive index ratio between the central core11and the outer peripheral core12, and the vertical axis represents the optical path length difference between the central core11and the outer peripheral core12.

The characteristics shown inFIG. 5correspond to the case where the distance d between the central core11and the outer peripheral core12is 35 [μm], the length of one spiral of the outer peripheral core12in the longitudinal direction of the optical fiber sensor1is 20 [mm], and the total length of the optical fiber sensor1is 2 or more [m]. As shown inFIG. 5, the optical path length difference between the central core11and the outer peripheral core12is proportional to the effective refractive index ratio between the central core11and the outer peripheral core12.

Here, as shown inFIG. 5, if the optical path length difference between the central core11and the outer peripheral core12is within the range indicated by A inFIG. 5, the optical path length difference becomes smaller than the optical path length difference therebetween in the case where the effective refractive indices of both are the same. Therefore, in the above case, it is desirable to set the effective refractive index ratio (ne2/ne1) between the central core11and the outer peripheral core12so as to satisfy the following expression.
0.99988<(ne2/ne1)<1

As described above, in one or more embodiments, the effective refractive index ne1of the central core11of the optical fiber sensor1and the effective refractive index ne2of the outer peripheral core are set so as to satisfy the above-described Expression (7). Therefore, the optical path length difference between the central core11and the outer peripheral core12can be made smaller than the optical path length difference A in the case where the effective refractive indices of the central core11and the outer peripheral core12are the same. Thus, even when the length of the optical fiber sensor1is increased, it is possible to realize high measurement accuracy over the entire length thereof.

Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. For example, in the above-described embodiments, an example in which the effective refractive index of the outer peripheral core12is adjusted by adjusting the concentration of germanium, which is a dopant having an effect of increasing the refractive index, has been described. However, the effective refractive indices of the outer peripheral cores12ato12cmay be adjusted, by setting the concentrations of germanium to be doped to the central core11and the plurality of outer peripheral cores12to be the same, and doping a dopant (second dopant) having an effect of decreasing the refractive index, such as boron (B) and fluorine (F), to the plurality of outer peripheral cores12ato12c.

The method of adjusting the effective refractive indices of the central core11and the outer peripheral core12is not limited to a method of changing the concentration and type of dopants. For example, the effective refractive index may be adjusted by changing the diameters of the central core11and the outer peripheral core12. Alternatively, for example, the effective refractive index may be adjusted by individually providing low refractive index layers around the outer peripheral cores12ato12c.

Further, in the above-described embodiments, the example in which the outer peripheral core12formed so as to spirally surround the central core11is configured with the three outer peripheral cores12ato12chas been described. However, the number of the outer peripheral cores12may be arbitrary and may be, for example, only one, or four or more. For example, assuming that the number of the outer peripheral cores12is six, it is desirable because the outer peripheral cores12can be close packed and arranged together with the center core11, when viewed in the cross section of the optical fiber sensor1.

In addition, in the above-described embodiments, in order to facilitate understanding, the example in which FBG is formed over the entire length in the longitudinal direction of the optical fiber sensor1, in the central core11and the outer peripheral cores12ato12cof the optical fiber sensor1has been described. However, the FBGs are not necessarily formed over the entire length in the longitudinal direction of the optical fiber sensor1, and it may be formed only in a partial region in the longitudinal direction.

In addition, the FBGs formed in the central core11and the outer peripheral cores12ato12cmay have a constant period, or the period may be continuously changed (chirped grating). Further, the FBGs need not necessarily be formed on the central core11and the outer peripheral cores12ato12cof the optical fiber sensor1, and the FBG may not be formed.1optical fiber sensor11central core12outer peripheral core12ato12couter peripheral core