Patent Publication Number: US-2023161102-A1

Title: Sensing cable and sensing system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of priority from Japanese Patent Application No. 2021-172829 filed on Oct. 22, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a sensing cable and a sensing system. 
     2. Related Art 
     Sensing systems that are equipped with a sensing cable including a multi-core optical fiber having a first core, a second core, and a cladding, and that detects a physical quantity, such as pressure, based on crosstalk between the first core and the second core have conventionally been known (JP-A-4-307328). 
     In the sensing cable of JP-A-4-307328, when light coupling to the second core from the first core is to be induced, it is necessary to reduce the optical confinement effect in the first core and, consequently, light leaking from the first core to the cladding propagates across the surrounding of the first core. Accordingly, a rate of an optical power coupled in the second core to an optical power leaked from the first core becomes low and, for example, problems such as lowered detection sensitivity or lowered energy efficiency have occurred in some cases. 
     Therefore, an object of the present disclosure is to obtain improved novel sensing cable and sensing system that are capable of improving, for example, the detection sensitivity and the energy efficiency. 
     SUMMARY 
     A sensing cable according to one aspect of the present disclosure includes a first optical fiber; a second optical fiber that extends along the first optical fiber and that is spaced from the first optical fiber; and a transmitting material including an intervention portion present between the first optical fiber and the second optical fiber, the transmitting material being configured to transmit light from the first optical fiber to the second optical fiber through the intervention portion. On a cross-section intersecting a longitudinal direction of the first optical fiber at a position at which the intervention portion is arranged, an outer periphery of the first optical fiber includes a first section and a second section, the first section being in contact with the intervention portion and being optically connected to the intervention portion, the second section being separated from the intervention portion. 
     A sensing cable according to one aspect of the present disclosure includes a first optical fiber; a second optical fiber that extends along the first optical fiber and that is spaced from the first optical fiber; a transmitting material including an intervention portion present between the first optical fiber and the second optical fiber, the transmitting material being configured to transmit light from the first optical fiber to the second optical fiber through the intervention portion; and a suppression material configured to suppress travel of light from the first optical fiber to a direction deviated from the intervention portion on an outer periphery of the first optical fiber. 
     A sensing system according to one aspect of the present disclosure includes: a light source; the sensing cable; and a measurement device configured to measure at least one of an external force acting on the sensing cable and a state change of the sensing cable based on inspection light, the inspection light being input to the first optical fiber from the light source and being output from the second optical fiber through the transmitting material. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an exemplary schematic diagram illustrating a sensing system of a first embodiment; 
         FIG.  2    is an exemplary and schematic cross-section of the sensing system of the first embodiment; 
         FIG.  3    is a graph showing an example of pressure detection values of the sensing system of the first embodiment and a conventional pressure sensor changing over time; 
         FIG.  4    is an exemplary and schematic cross-section of a sensing cable of a second embodiment; 
         FIG.  5    is an exemplary schematic configuration diagram of a sensing cable of a third embodiment; 
         FIG.  6    is an exemplary schematic configuration diagram of a sensing cable of a fourth embodiment; 
         FIG.  7    is an exemplary and schematic cross-section of a sensing cable of a fifth embodiment; 
         FIG.  8    is an exemplary schematic configuration diagram of a sensing cable of the fifth embodiment; 
         FIG.  9    is an exemplary and schematic cross-section of a sensing cable of a sixth embodiment; 
         FIG.  10    is an exemplary and schematic cross-section of a sensing cable of a seventh embodiment; and 
         FIG.  11    is an exemplary schematic configuration diagram of a sensing cable of an eighth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present disclosure will be disclosed. Configurations, actions and effects (results) provided by the configurations of the embodiments described in the following are one example. The present disclosure can be implemented by other means than the configurations disclosed in the following embodiments. Moreover, according to the present disclosure, it is possible to obtain at least one of various effects (including secondary effects) that can be obtained by the configuration of the embodiments. 
     The embodiments described in the following include similar configurations. Therefore, according to the configurations of the respective embodiments, similar actions and effects based on the similar configurations can be obtained. Furthermore, in the following, like reference symbols are assigned to those similar configurations, and duplicated explanation can be omitted. 
     Moreover, in the present specification, ordinal numbers are used for convenience of distinguishing parts, members, portions, and the like, and are not to indicate priority or order. 
     First Embodiment 
       FIG.  1    is a schematic configuration diagram of a sensing system  100  according to a first embodiment. The sensing system  100  includes a sensing cable  10 A, an optical fiber  20 , and a measurement device  30 . In  FIG.  1   , as for a measuring unit  10   a  of the sensing cable  10 A, a cross-section taken along a longitudinal direction is illustrated. 
     The sensing system  100  can measure physical quantities, such as pressure and force acting on the measuring unit  10   a  of the sensing cable  10 A, state changes, such as bend, kink, and compression, of the sensing cable  10 A based on an external force acting on the measuring unit  10   a,  and the like. 
     The measurement device  30  includes a light source  31 , a light receiving unit  32 , and a processing unit  33 . 
     The light source  31  has, for example, a laser diode, and outputs light (inspection light) having, for example, a wavelength of 400 nm to 550 nm. The light source  31  may output continuous light, or may output optical pulses of predetermined time interval as the inspection light. 
     The light receiving unit  32  has, for example, a photodiode, and detects intensity of light input from a delivery optical fiber  22 . The light receiving unit  32  can be referred to as detecting unit also. 
     The processing unit  33  acquires reception light intensity in the light receiving unit  32 , and calculates a physical quantity, such as force and pressure according to the reception light intensity, and a state change amount, such as a curvature radius of bend of the sensing cable  10 A, a kink angle, a diameter change amount. Moreover, the processing unit  33  switches between emission and stop of emission of the inspection light in the light source  31 , and changes an output condition of the inspection light. The processing unit  33  can be referred to as control unit or arithmetic unit also. 
     In the present embodiment, the light source  31  and the light receiving unit  32  are optically connected to different pieces of the optical fibers  20 , respectively. 
     The optical fiber  20  optically connected to the light source  31  is partially housed inside the measuring unit  10   a  of the sensing cable  10 A at a position apart from the light source  31 . The measuring unit  10   a  is a portion on which an external force acts in the sensing cable  10 A. A section housed inside the measuring unit  10   a  out of the optical fiber  20  connected to the light source  31  is a first optical fiber  21 - 1 . Moreover, out of the optical fiber  20 , a section between the first optical fiber  21 - 1  and the light source  31  is the delivery optical fiber  22 . The first optical fiber  21 - 1  and the delivery optical fiber  22  may be a single optical fiber formed in one piece from the beginning, or may be separate optical fibers connected to each other by fusion or the like. 
     The optical fiber  20  optically connected to the light receiving unit  32  is partially housed inside the measuring unit  10   a  at a position apart from the light receiving unit  32 . A section housed inside the measuring unit  10   a  out of the optical fiber  20  is a second optical fiber  21 - 2 . Moreover, out of the optical fiber  20 , a section between the second optical fiber  21 - 2  and the light receiving unit  32  is the delivery optical fiber  22 . The second optical fiber  21 - 2  and the delivery optical fiber  22  may be a single optical fiber formed in one piece from the beginning, or may be separate optical fibers connected to each other by fusion or the like. 
       FIG.  2    is a cross-section intersecting with a longitudinal direction of the measuring unit  10   a  of the sensing cable  10 A. As illustrated in  FIGS.  1 ,  2   , the first optical fiber  21 - 1  and the second optical fiber  21 - 2  both extend substantially parallel in an X direction in the sensing cable  10 A and are spaced apart from each other. The X direction is a longitudinal direction of the sensing cable  10 A. In the present embodiment, the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are separated from each other. 
     The sensing cable  10 A includes a transmitting material  11 A, a cover  12 , and a plug  13  in addition to the first optical fiber  21 - 1  and the second optical fiber  21 - 2 . 
     The transmitting material  11 A can transmit the inspection light, and is made, for example, from a synthetic resin material. The transmitting material  11 A is arranged so as to surround around the first optical fiber  21 - 1  and the second optical fiber  21 - 2 , and extends in the X direction. Moreover, the transmitting material  11 A includes an intervention portion  11   a  that is positioned between the first optical fiber  21 - 1  and the second optical fiber  21 - 2 . The intervention portion  11   a  also extends in the X direction. The transmitting material  11 A is only required to be capable of propagating the inspection light, and it may be, for example, air, water, other liquids, or the like also. 
     The transmitting material  11 A has a substantially circular cross-sectional shape. Moreover, the cover  12  has substantially uniform thickness, and has a tubular shape or a cylindrical shape, and surrounds an outer periphery of the transmitting material  11 A. The cover  12  can improve protection of the sensing cable  10 A. 
     Each of the first optical fiber  21 - 1  and the second optical fiber  21 - 2  includes a core  20   a  and a cladding  20   b.  The core  20   a  can transmit the inspection light, and is made from, for example, quartz glass in the present embodiment. Furthermore, the cladding  20   b  is made from, for example, a synthetic resin material. The cladding  20   b  may be capable of transmitting the inspection light, or may be incapable of transmitting the inspection light. The core  20   a  may be made from a synthetic resin material transparent to the inspection light, such as methacrylic resin and fluororesin. In this case, both the first optical fiber  21 - 1  and the second optical fiber  21 - 2  may be a plastic optical fiber made from a synthetic resin material. 
     In the present embodiment, the cladding  20   b  is removed in a portion  20   c  facing each other, in other words, the portion  20   c  facing the intervention portion  11   a,  from the outer periphery of the first optical fiber  21 - 1  and the second optical fiber  21 - 2 . Thus, the core  20   a  of the first optical fiber  21 - 1  and the intervention portion  11   a  of the transmitting material  11 A face or come into contact with each other, and the core  20   a  of the second optical fiber  21 - 2  and the intervention portion  11   a  of the transmitting material  11 A face or come into contact with each other, and the core  20   a  of the first optical fiber  21 - 1 , the intervention portion  11   a,  and the core  20   a  of the second optical fiber  21 - 2  are thus optically connected to one another. The portion  20   c  can be formed, for example, by mechanical treatment, such as grinding, polishing, and shot blasting, or by chemical treatment, such as etching. Moreover, in the portion  20   c,  it is not necessary to remove the cladding  20   b  completely, and the cladding  20   b  may remain in very thin thickness, or the cladding  20   b  may remain in a small amount at intervals. The portion  20   c  can be referred to as removal portion also. Furthermore, the intervention portion  11   a  can be referred to as light guide portion of inspection light also. The intervention portion is only required to be a medium that propagates a signal light from the first optical fiber  21 - 1  to the second optical fiber  21 - 2 , and may be, for example, resin, air, water, or the like. 
     Moreover, the first optical fiber  21 - 1  and the second optical fiber  21 - 2  have a structure in which the inspection light is relatively easy to diffuse. As one example, the first optical fiber  21 - 1  and the second optical fiber  21 - 2  may include plural nanostructures in the core  20   a,  or near the interface between the core  20   a  and the cladding  20   b.  The nanostructure is, for example, a filler (for example, particles, such as minute particles or cylindrical tubes) or a void (for example, tubes or minute air space), and may include plural kinds of fillers or plural kinds of voids, or may include both a filler and a void. A cross-sectional diameter perpendicular to the longitudinal direction of the sensing cable  10 A of the nanostructure is, for example, 100 nm or less. The nanostructure can be referred to as diffusion factor also. Moreover, to facilitate diffusion of the inspection light, in at least one of the first optical fiber  21 - 1  and the second optical fiber  21 - 2 , a fiber external layer may be roughened after removing the cladding  20   b,  or the fiber external layer may be roughened without removing the cladding  20   b.  Moreover, the structure in which the inspection light is easy to diffuse described above may be arranged in at least one of the first optical fiber  21 - 1  and the second optical fiber  21 - 2  only at a portion facing the other. This can promote light coupling between the first optical fiber  21 - 1  and the second optical fiber  21 - 2  efficiently. 
     Furthermore, the refractive index of the transmitting material  11 A is equal to or higher than the refractive index of the core  20   a  of the first optical fiber  21 - 1 , and is equal to or lower than the refractive index of the core  20   a  of the second optical fiber  21 - 2 . The refractive index of the transmitting material  11 A may be higher than the refractive index of the core  20   a  of the first optical fiber  21 - 1  and less than the refractive index of the core  20   a  of the second optical fiber  21 - 2 , or may be substantially the same as the refractive index of the core  20   a  of the first optical fiber  21 - 1  and the refractive index of the core  20   a  of the second optical fiber  21 - 2 . 
     In the configuration as described, when deformation, such as bend, compression, and kink, occurs in the first optical fiber  21 - 1 , or the first optical fiber  21 - 1  and the second optical fiber  21 - 2  come closer to each other due to an external force or the like acting on the measuring unit  10   a  of the sensing cable  10 A, light confinement capability of the first optical fiber  21 - 1  is reduced, or crosstalk between the first optical fiber  21 - 1  and the second optical fiber  21 - 2  increases. Consequently, an amount of the inspection light coupled in the second optical fiber  21 - 2  from the first optical fiber  21 - 1  through the intervention portion  11   a  of the transmitting material  11 A increases. Thus, the reception light intensity increases in the light receiving unit  32 . 
     Furthermore, when the first optical fiber  21 - 1  includes a diffusion factor as described above, the amount of light coupled in the second optical fiber  21 - 2  from the first optical fiber  21 - 1  through the intervention portion  11   a  is more likely to increase, and is more sensitively affected by an external force, deformation, and the like. 
     Therefore, according to the present embodiment, an external force acting on the sensing cable  10 A, or a degree of state change of the sensing cable  10 A can be measured based on the reception light intensity of the inspection light in the light receiving unit  32 . In this process, the processing unit  33  can calculate an external force or a degree of state change corresponding to the reception light intensity in the light receiving unit  32  based on a correlation between a reception light intensity in the light receiving unit  32  and an external force and a degree of state change that has been experimentally acquired in advance. 
     In the present embodiment, the refractive index of the cladding  20   b  is set to be lower than the refractive index of the core  20   a  and the refractive index of the transmitting material  11 A. Therefore, the inspection light leaked from the first optical fiber  21 - 1  is difficult to be transmitted to the transmitting material  11 A through the cladding  20   b,  and is to be mainly coupled in the intervention portion  11   a  through the portion  20   c.  That is, the cladding  20   b  of the first optical fiber  21 - 1  suppresses leakage of light from the first optical fiber  21 - 1  in a direction deviating from the intervention portion  11   a  and the second optical fiber  21 - 2  on the outer periphery of the first optical fiber  21 - 1 . By the cladding  20   b  as described, the inspection light leaked from the first optical fiber  21 - 1  is to be coupled in the intervention portion  11   a  and the second optical fiber  21 - 2  more efficiently. The cladding  20   b  is one example of suppression material. 
     Moreover, in the configuration described above, it has been found that the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are preferable to be a multimode optical fiber that transmits the inspection light at an inspection wavelength in a multimode from the viewpoint of likeliness of leakage of the inspection light to the transmitting material  11 A, and eventually, from the viewpoint of improving the inspection sensitivity. Furthermore, from the same viewpoint, it has been found that the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are preferable to be configured to have the transmission loss with respect to the inspection light of 0.3 dB/m or more. 
       FIG.  3    is a graph showing an example of pressure detection values of the sensing system  100  of the first embodiment and pressure detection values of a conventional pressure sensor changing over time. This graph shows a result of pressure measurement of a test fluid in an experimental facility. From  FIG.  3   , it is found that detection values of pressure of the test fluid are preferably in agreement between those of the sensing system  100  of the present embodiment indicated by a broken line and those of the conventional pressure sensor indicated by a solid line. It has experimentally been confirmed that according to the sensing system  100  of the present embodiment, measurement performance equivalent to or higher than the conventional pressure sensor can be obtained as in this example. 
     As explained above, in the present embodiment, the cladding  20   b  (suppression material) of the first optical fiber  21 - 1  suppresses travel of light toward a direction deviating from the intervention portion  11   a  of the transmitting material  11 A from the first optical fiber  21 - 1 . 
     According to the configuration as described, the inspection light leaked from the first optical fiber  21 - 1  can be transmitted to the second optical fiber  21 - 2  efficiently through the intervention portion  11   a  of the transmitting material  11 A, and the detection sensitivity and the energy efficiency can be improved in measurement of an external force and a state change by the sensing system  100 . 
     Second Embodiment 
       FIG.  4    is a cross-section intersecting the longitudinal direction of the measuring unit  10   a  of a sensing cable  10 B of a second embodiment. As illustrated in  FIG.  4   , in the present embodiment, a cladding  20   b  of the first optical fiber  21 - 1  and the second optical fiber  21 - 2  is removed in than a wider range than the first embodiment, and the cladding  20   b  is formed thinner in an entire area, and is arranged in fragments on an opposite side to the intervention portion  11   a.  In such a configuration also, the cladding  20   b  can suppress leakage of light from the first optical fiber  21 - 1  to the transmitting material  11 A, in other words, travel of light from the first optical fiber  21 - 1  to a direction deviated from the intervention portion  11   a  in a portion in which the cladding  20   b  is arranged. Therefore, according to the present embodiment also, the inspection light leaked from the first optical fiber  21 - 1  can be more efficiently transmitted to the second optical fiber  21 - 2  through the intervention portion  11   a  compared to a configuration without the cladding  20   b,  and the detection sensitivity and the energy efficiency can be improved in measurement of an external force and a state change by the sensing system  100 . 
     Third Embodiment 
       FIG.  5    is a cross-section taken along a longitudinal direction of a sensing cable  10 C of a third embodiment. As illustrated in  FIG.  5   , in the present embodiment, the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are constituted of a single optical fiber, and are connected through a turning portion  23 . The optical fiber  20  is, for example, plastic optical fiber. In the configuration as described also, actions and effects similar to those of the first embodiment can be obtained. 
     Fourth Embodiment 
       FIG.  6    is a cross-section taken along a longitudinal direction of a sensing cable  10 D of a fourth embodiment. As illustrated in  FIG.  6   , in the present embodiment also, the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are constituted of a single optical fiber, and are connected through the turning portion  23 . However, in the present embodiment, the turning portion  23  is arranged at a position deviated from the measuring unit  10   a.  In the configuration as described also, actions and effects similar to those of the embodiments described above can be obtained. Moreover, according to the present embodiment, because an external force does not act on the turning portion  23 , it is possible to prevent the leakage state of light from changing at the turning portion  23  as affected by an action of the external force, and prevent the change from affecting the reception light intensity at the light receiving unit  32 . Therefore, according to the present embodiment, the measurement accuracy can be further improved. 
     Fifth Embodiment 
       FIG.  7    is a cross-section intersecting a longitudinal direction of the measuring unit  10   a  of a sensing cable  10 E of a fifth embodiment, and  FIG.  8    is a cross-section taken along a longitudinal direction of the sensing cable  10 E. 
     As illustrated in  FIG.  7   , in the present embodiment, a transmitting material  11 E is present only between the first optical fiber  21 - 1  and the second optical fiber  21 - 2 . That is, the transmitting material  11 E only includes the intervention portion  11   a.  Furthermore, inside the cover  12 , that is, between the first optical fiber  21 - 1 , the second optical fiber  21 - 2 , and the transmitting material  11 E, and the cover  12 , a housing chamber S is formed. In the housing chamber S, for example, an inactive gas such as nitrogen, or a gas such as air is put. In this case, the refractive index of the gas put in the housing chamber S is lower than the refractive index of the core  20   a  of the first optical fiber  21 - 1 , the refractive index of the core  20   a  of the second optical fiber  21 - 2 , and the refractive index of the transmitting material  11 E. 
     The outer periphery of the first optical fiber  21 - 1  and the second optical fiber  21 - 2  have the portion  20   c  and a portion  20   d.  In the portion  20   c,  the cladding  20   b  is removed, and core  20   a  faces and is in contact with the intervention portion  11   a.  Moreover, in the portion  20   d,  the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are separated from the intervention portion  11   a,  that is, the transmitting material  11 E. In at least a part of the portion  20   d,  the cladding  20   b  is arranged. In the configuration as described, the inspection light leaked from the first optical fiber  21 - 1  is not to leak to the housing chamber S through the portion  20   d  or the cladding  20   b,  but is coupled with the intervention portion  11   a  through the portion  20   c,  and eventually with the second optical fiber  21 - 2 . Therefore, according to the present embodiment also, the inspection light leaked from the first optical fiber  21 - 1  can be efficiently transferred to the second optical fiber  21 - 2  through the intervention portion  11   a,  and the detection sensitivity and the energy efficiency can be improved in measurement of an external force and a state change by the sensing system  100 . The portion  20   c  is one example of a first section, and the portion  20   d  is one example of a second section. 
     Moreover, as illustrated in  FIG.  8   , in the present embodiment, the transmitting material  11 E extends in the longitudinal direction of the measuring unit  10   a.  Furthermore, the transmitting material  11 E is arranged not across the entire section, but in a part of the measuring unit  10   a  in the longitudinal direction, and is distributed so as to be separated from one another at a plurality of positions in the longitudinal direction. In this case, for example, by outputting the inspection light as optical pulse, and by adopting a technique of optical time-domain reflectometry (OTDR), it becomes possible to identify a position at which an external force acts in the measuring unit  10   a,  and a position at which the transmitting material  11 E affected by an external force is arranged, based on a time waveform of the reception light intensity in the light receiving unit  32 . 
     Sixth Embodiment 
       FIG.  9    is a cross-section intersecting a longitudinal direction of the measuring unit  10   a  of a sensing cable  10 F of a sixth embodiment. As illustrated in  FIG.  9   , in the present embodiment, along with the first optical fiber  21 - 1  and the second optical fiber  21 - 2 , at least in the measuring unit  10   a,  the cladding  20   b  is not provided. As described above, the refractive index of the gas in the housing chamber S is lower than the refractive index of the first optical fiber  21 - 1 , the second optical fiber  21 - 2 , and the transmitting material  11 E. Moreover, the refractive index of the cover  12  is also lower than the refractive index of the first optical fiber  21 - 1 , the second optical fiber  21 - 2 , and the transmitting material  11 E. Accordingly, also in the configuration without the cladding  20   b  as the present embodiment, the inspection light leaked from the first optical fiber  21 - 1  does not leak to the housing chamber S or the cover  12  from the portion  20   d,  and is coupled with the intervention portion  11   a  through the portion  20   c,  and eventually with the second optical fiber  21 - 2 . Therefore, according to the present embodiment also, the inspection light leaked from the first optical fiber  21 - 1  can be transferred more efficiently to the second optical fiber  21 - 2  through the intervention portion  11   a,  and the detection sensitivity and the energy efficiency can be improved in measurement of an external force and a state change by the sensing system  100 . 
     Seventh Embodiment 
       FIG.  10    is a cross-section intersecting a longitudinal direction of the measuring unit  10   a  of a sensing cable  10 G of a seventh embodiment. As illustrated in  FIG.  10   , in the present embodiment, the sensing cable  10 G includes plural sets S 1 , S 2  that respectively include the first optical fiber  21 - 1  and the second optical fiber  21 - 2  positioned so as to sandwich the intervention portion  11   a  of a transmitting material  11 G, similarly to the fifth embodiment. By thus providing a plurality of pieces of the sets S 1 , S 2 , benefits, for example, that of a double measurement system can be established, or that an error or variation of measurement can be suppressed by calculating an average value based on respective measurement values of the two sets S 1 , S 2  can be produced. 
     As illustrated in  FIG.  10   , in the present embodiment, the transmitting material  11 G is shared by the plural sets S 1 , S 2 . The configuration as described is beneficial in that time and effort in manufacturing and cost can be reduced by reducing the number of parts, and in that the measuring unit  10   a  can be formed in a compact size, compared to a configuration in which the plural sets S 1 , S 2  respectively have independent pieces of the transmitting material  11 G. 
     Moreover, as illustrated in  FIG.  10   , in the present embodiment, directions in which the first optical fiber  21 - 1  and the second optical fiber  21 - 2  are aligned in the plural sets S 1 , S 2  intersect with each other. According to the configuration as described, benefits, for example, that a direction in which an external force acts, and a direction of a state change caused in the sensing cable  10 G can be measured based on respective measurement values of the plural sets S 1 , S 2  can be produced. 
     Eighth Embodiment 
       FIG.  11    is a schematic configuration diagram of the sensing system  100  including a sensing cable  10 H of an eighth embodiment. In  FIG.  11   , as for the sensing cable  10 H, a cross-section taken along a longitudinal direction is illustrated. 
     As illustrated in  FIG.  11   , in the present embodiment, the light source  31  and the light receiving unit  32  are optically connected to a coupler  24  through the delivery optical fiber  22 . Moreover, the coupler  24  optically connects two pieces of the delivery optical fibers and the first optical fiber  21 - 1 . In this configuration, the inspection light output from the light source  31  is input to the first optical fiber  21 - 1  through the delivery optical fiber  22  and the coupler  24 , is reflected on an end portion on the opposite side to the light source  31  in the first optical fiber  21 - 1 , and is input to the light receiving unit  32  through the coupler  24  and the other delivery optical fiber  22 . When an external force acts on the measuring unit  10   a,  the inspection light leaked from the first optical fiber  21 - 1  is transmitted to the second optical fiber  21 - 2  through the transmitting material  11 A, and the light reception intensity in the light receiving unit  32  decreases for the corresponding amount. Therefore, according to the configuration of the present embodiment also, an external force acting on the sensing cable  10 H, or an amount of state change of the sensing cable  10 A can be measured based on the reception light intensity in the light receiving unit  32 . In the first to the seventh embodiments described above, an external force and a state change measured become larger as the reception light intensity increases, but in the present embodiment, an external force and a state change measured become larger as the reception light intensity decreases. 
     As above, the embodiments of the present disclosure have been exemplified, the above embodiments are one example, and are not intended to limit the scope of the present disclosure. The above embodiments can be implemented in various other forms, and various omission, replacement, combination, and change are possible in a range not departing from the gist of the present disclosure. Moreover, respective configurations, specifications such as shapes (structure, type, direction, model, size, length, width, thickness, height, quantity, arrangement, position, material, and the like) can be appropriately changed to be implemented. 
     For example, when refractive indexes of the first optical fiber, the transmitting material (intervention portion), and the second optical fiber are set to be substantially the same and a configuration in which the first optical fiber, the transmitting material, and the second optical fiber are substantially symmetrical with respect to the transmitting material is provided, by optically connecting the light source and the second optical fiber, and by optically connecting the light receiving unit and the first optical fiber, measurement can be performed by switching the first optical fiber and the second optical fiber. 
     According to the present disclosure, for example, improved and novel sensing cable and sensing system can be obtained. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.