Optical pressure sensor

The present invention aims to provide an optical pressure sensor capable of detecting a more minute pressure change. A base film is formed with a through hole passing first and second surfaces, an optical fiber is fixed to the base film at a region other than the FBG portion such that the FBG portion is positioned on the through hole in plan view. The optical pressure sensor according to the present invention is attached to an object body such that the second surface of the base film is closely attached to a surface of the object body directly or indirectly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pressure sensor equipped with an optical fiber including at least one FBG (Fiber Bragg Grating) portion.

2. Related Art

In the fields of design and analysis of automobiles, ships, airplanes and the like, various optimum designing has been carried out in terms of safety and higher efficiency. In particular, it is important to grasp the fluid physical quantity of air pressure or water pressure that acts on the object surface in the above fields. That is, optimum designing is carried out by feeding back the result obtained from the measurement of air pressure (wind pressure) by using a wind tunnel device that employs a reduced model and/or the measurement of water pressure by using a water tank.

A piezoelectric or semiconductor pressure sensor, and a strain gauge pressure sensor are known for measuring air pressure or water pressure.

However, such configuration has the following problems. In the piezoelectric and semiconductor pressure sensors, it is needed to bore a hole in the reduced model, insert a tube through the hole and electrically connect an opening end of the hole and a pressure sensor main body. The configuration has a technical limitation that the number of measurement points that could be arranged in the reduced model is up to tens to several tens. This is because the arrangement of a large number of measurement points is difficult since the tube has a diameter of several mm. With the strain gauge pressure sensor, an amplifier must be provided at each measurement point, and thus is practically difficult to carry out measurement at a large number of points. Furthermore, it is needed to bore a hole in an object body such as the reduced model in both the piezoelectric or semiconductor type and the strain gauge type. Therefore, the reduced model may be damaged due to water immersion from the hole when the reduced model is used for water pressure measurement. Furthermore, the strain gauge pressure sensor is electric, and therefore it may short-circuit due to water immersion.

An optical pressure sensor having an optical fiber with an FBG portion as a pressure sensitive portion is known as a pressure sensor that measures the pressure of the surface of the object body without boring any holes in the object body such as the reduced model (e.g., see Japanese unexamined publication No. 2002-71323, hereinafter referred to as prior document 1). The FBG portion is a portion where a grating (a diffraction grating) for reflecting only a predetermined wavelength of the incident light with respect to an advancing direction of the incident light is formed. The FBG portion is attached to a base film to form the sensor. When the pressure applying on the base film changes (i.e. when the base film strains due to the pressure change), the pressure change is also occurred on the FBG portion of the optical fiber attached to the base film. The pressure change on the FBG portion causes the diffraction pitch of the grating to change so that a peak of the wavelength of the light to be reflected is shifted. That is, the optical pressure sensor is configured so as to detect the amount of the pressure applying on the FBG portion (the strain amount of the FBG portion) by detecting the shift amount of the peak of the wavelength to be reflected at the time when the pressure applying on the FBG portion changes.

However, it is needed to detect a more minute pressure change in the optical pressure sensor as disclosed in the prior document 1 in order to carry out a safer and more efficient optimum designing. Further, there has been no optical pressure sensor capable of detecting both a positive pressure and a negative pressure (i.e., a differential pressure from a reference pressure).

SUMMARY OF THE INVENTION

In consideration of the above prior art, it is a first object of the present invention to provide an optical pressure sensor capable of detecting a more minute pressure change.

A second object of the present invention is to provide an optical pressure sensor capable of measuring the minute pressure change as a differential pressure with respect to a reference pressure.

One aspect of the present invention provides, in order to achieve the first object, an optical pressure sensor having a base film that includes first and second surfaces, the second surface being closely attached to a surface of an object body directly or indirectly; and an optical fiber that has at least one FBG portion and that is fixed to the base film, wherein the base film is formed with a through hole passing through the first and second surfaces, and the optical fiber is fixed to the base film at a region other than the FBG portion such that the FBG portion is positioned on the through hole in plan view.

The thus configured optical pressure sensor detects the pressure change on a side of the first surface of the base film in a state where the second surface of the base film is closely contacted to the surface of the object body directly or indirectly.

According to the configuration, it is possible to directly detect the pressure change at the FBG portion functioning as a sensitive portion when the pressure applying on the first surface is changed.

In one configuration, the FBG portion is wholly positioned on the through hole of the base film. According to the configuration, it is possible to effectively prevent the base film from hindering or attenuating the strain action of the FBG portion caused by the pressure change. Therefore, it is possible to detect a more minute pressure change in comparison with the conventional optical pressure sensor in which the pressure change is detected based on the bending of the base film.

Alternatively, the FBG portion may be partially positioned on the through hole. According to the configuration, it is possible to lower the sensitivity of the FBG portion in comparison with the one embodiment, thereby detecting the pressure at a portion of the object body subjected to a more high pressure.

The optical fiber preferably includes a plurality of the FBG portions that has different reflection characteristics one another and that are arranged along a longitudinal direction of the optical fiber.

It is possible to detect each of the shifts of the different Bragg wavelengths by having the reflection characteristics (the Bragg wavelengths) of the plurality of the FBG portions different one another. Therefore, it is possible to easily detect the pressures at a plurality of points without using tubes needed in the conventional piezoelectric or semiconductor pressure sensor, thereby detecting a more specific pressure change at each of the plurality of points over a wide area.

In one embodiment, the base film is formed with a plurality of the through holes. In the one embodiment, the optical fiber is arranged such that each of the plurality of FBG portions is arranged on the corresponding through hole of the plurality of through holes in plan view.

In the one embodiment, a flexible cover film is preferably arranged on the first surface of the base film so as to cover any of the plurality of through holes.

According to the configuration, it is possible to have the sensitivity of one FBG portion positioned in the through hole that is covered by the flexible cover film different from the sensitivity of the other FBG portion positioned in the through hole21that is not covered by the cover film. That is, the arrangement of the flexible cover film makes it possible to differ the sensitivity of the FBG portion with respect to each of the measuring points.

Therefore, it is possible to simultaneously detect the pressures at a plurality of measuring points subjected to extremely different pressures.

For instance, it is possible to cover one FBG portion, which is positioned at one area (e.g., a front end portion of an automobile) where the measurement pressure is assumed to be large, by the flexible cover film, and not to cover the other FBG portion, which is positioned at the other area (e.g., a side portion of the automobile) where the measurement pressure is assumed to be small, so that the one area on which the large pressure applies and the other area on which the small pressure applies could be simultaneously detected.

Furthermore, it is possible to easily change the shift amount (sensitivity) of the corresponding FBG portion by replacing one cover film with the other cover film having thickness and/or material different from that of the one cover film.

In another embodiment, the base film is formed with a single through hole.

In the embodiment, the optical fiber is arranged such that the plurality of FBG portions are arranged on the single through hole in plan view.

In the embodiment, a flexible cover film is preferably arranged on the first surface of the base film for covering a predetermined area of the single through hole so as to cover any of the plurality of FBG portions.

According to the configuration, it is possible to have the sensitivity of one FBG portion covered by the flexible cover film different from that of the other FBG portion not covered by the cover film. That is, the arrangement of the flexible cover film makes it possible to differ the sensitivity of the FBG portion with respect to each of the measuring points.

Therefore, it is possible to simultaneously detect the pressures at a plurality of measuring points subjected to extremely different pressures.

For instance, it is possible to cover one FBG portion, which is positioned at one area (e.g., a front end portion of an automobile) where the measurement pressure is assumed to be large, by the flexible cover film, and not to cover the other FBG portion, which is positioned at the other area (e.g., a side portion of the automobile) where the measurement pressure is assumed to be small, so that the one area on which the large pressure applies and the other area on which the small pressure applies could be simultaneously detected.

Furthermore, it is possible to easily change the shift amount (sensitivity) of the corresponding FBG portion by replacing one cover film with the other cover film having thickness and/or material different from that of the one cover film.

In the above various configurations, the optical fiber is preferably fixed to the base film at both sides of the FBG portion.

According to the configuration, it is possible to effectively prevent a positional shift of the FBG portion from the through hole.

The optical fiber may be fixed to the base film with, for example, a fixing film.

In each of the above various configurations, the optical pressure sensor may preferably include a rigid cover plate fixedly attached to the first surface of the base film directly or indirectly, the cover plate being formed with a passing hole at a position corresponding to the FBG portion.

According to the configuration, even if the optical fiber is arranged on the first surface of the base film, it is possible to prevent a bump due to the optical fiber, thereby smoothening the measuring surface. Accordingly, it is possible to detect a minute pressure change on the measuring surface without depending on the surface profile of the base film.

Furthermore, the configuration effectively prevents a positional shift of the FBG portion. In particular, in a case where pressures at a plurality of points on the smooth plane are respectively detected by the plurality of FBG portions, the arrangement of the rigid cover plate makes it possible to reliably hold the plurality of FBG portions at the respective installing positions.

A spacer film is preferably interposed between the base film and the cover plate.

The spacer film is formed with an aperture fluidly connecting the through hole and the corresponding passing hole.

According to the configuration, even if the optical fiber having a relatively large diameter is arranged on the first surface of the base film, the flexible spacer film provides a good contact between the optical fiber and the cover plate, thereby enhancing the smoothening effect by the cover plate.

In each of the above various configurations, the optical pressure sensor preferably further includes an adhesive film fixedly attached to the second surface of the base film.

The adhesive film includes, on a surface opposite the base film, an adhesive layer capable of adhering to the object body.

According to the configuration, it is possible to adhere the base film to the surface of the object body by the adhesive layer of the adhesive film, thereby easily attaching the optical pressure sensor to the object body without damaging the object body.

The FBG portion elongates and compresses in response to the temperature change, so that the Bragg wavelength is shifted. If the external temperature changes during the measurement of the external pressure, the Bragg wavelength of the pressure detecting FBG portion is shifted in response to the external temperature change in addition to the external pressure change.

In order to prevent the disadvantage, in each of the above various configurations, the optical fiber further includes a temperature compensating FBG portion, the temperature compensating FBG portion being inserted into a rigid hollow member fixed to the base film.

According to the configuration, the temperature compensating FBG portion could detect only the temperature change without being affected by the external pressure change. Therefore, it is possible to detect a net pressure change (irrespective of the temperature change) by offsetting the measurement result of the temperature compensating FBG portion from the measurement result of the pressure detecting FBG portion.

In each of the above various configurations, the base film is preferably formed with a groove at the first surface, and the optical fiber is arranged in the groove.

According to the configuration, it is possible to prevent or reduce a bump due to the optical fiber from occurring on the first surface of the base film.

Another aspect of the present invention provides, in order to achieve the second object, an optical pressure sensor including a base member fixedly attached to an object body, and an optical fiber that has at least one FBG portion and that is fixed to the base member, wherein the base member includes a sealed space in which a surface facing an external pressure to be measured is covered with a flexible first cover film, the optical fiber is fixed to the base member at a region other than the FBG portion such that the FBG portion is positioned on the sealed space with the FBG portion contacted to an inner surface of the first cover film, and the change of reflection characteristic of the FBG portion is detected in a state where the sealed space is maintained at a reference pressure.

In the thus configured optical pressure sensor, the change of reflection characteristic of the FBG portion (the shift amount of Bragg wavelength by distortion of the FBG portion) of the optical fiber contacting the inner surface of the first cover film could be detected in a state where the sealed space is maintained at the reference pressure (in a state where the sealed space is preset to the reference pressure).

That is, according to the configuration, it is possible to detect the external pressure change as a differential pressure with reference to the reference pressure.

For example, the FBG portion may be wholly positioned on the sealed space (i.e., the FBG portion is positioned such that the whole FBG portion is not contacted to the base member). According to the configuration, it is possible to effectively prevent the base member from hindering or attenuating the strain action of the FBG portion caused by the pressure change. Therefore, it is possible to detect a more minute pressure change in comparison with the conventional optical pressure sensor in which the bending of the base film is detected by the FBG portion.

Alternatively, the FBG portion may be partially positioned on the sealed space. According to the configuration, it is possible to lower the sensitivity of the FBG portion in comparison with the one embodiment, thereby detecting the pressure at a portion of the object body subjected to a more high pressure.

Preferably, the FBG portion is fixedly attached to the inner surface of the first cover film such that a bending of the FBG portion follows a bending of the first cover film caused by a change of the external pressure with respect to the reference pressure.

According to the configuration, when the first cover film bends due to the external pressure change with respect to the reference pressure in the sealed space, the FBG follows the bending of the first cover film to bend. Accordingly, it is possible to accurately detect a more minute pressure change.

Further, according to the configuration, it is possible to effectively detect the external pressure change, even if the external pressure changes so as to be lower than the reference pressure (i.e. the external pressure changes in a negative direction).

Preferably, the base member has a flexible base film that includes first and second surfaces and a through hole passing through the first and second surfaces, the second surface being closely attached to a surface of the object body directly or indirectly; the first cover film fixedly attached to the first surface of the base film so as to cover one end of the through hole; and a flexible second cover film fixedly attached to the second surface of the base film so as to cover the other end of the through hole for forming the sealed space in cooperation with the first cover film. The optical fiber is interposed between the first surface of the base film and the first cover film such that the FBG portion is positioned on the through hole in plan view.

According to the configuration, it is possible to detect the pressure changes on surfaces of the object body having various shapes as the differential pressure.

The optical fiber preferably includes a plurality of the FBG portions that have different reflection characteristics one another and that are arranged along a longitudinal direction of the optical fiber.

In one embodiment, the base film is formed with a plurality of the through holes and a slit fluidly connecting the plurality of through holes from one another.

The optical fiber is arranged such that each of the plurality of FBG portions is arranged on the corresponding through hole of the plurality of the through holes in plan view.

The first and second cover films are respectively fixed to the first and second surfaces so as to cover the plurality of through holes and the slit.

The base film is further formed with an external communicating slit having a first end fluidly connected to the sealed space that is defined by the plurality of through holes and the slit and a second end opened to the outside.

According to the one embodiment, it is possible to easily detect the differential pressures at a plurality of (a large number of) points without using tubes needed in the conventional piezoelectric or semiconductor pressure sensor by measuring the shift amount of each of the preset Bragg wavelengths. Further, it is possible to easily maintain the pressure in the sealed space at a constant pressure through the external communicating slit since the sealed space is opened to the outside through the external communicating slit.

In another embodiment, the base film is formed with a single through hole.

The optical fiber is arranged such that the plurality of FBG portions are arranged on the single through hole in plan view.

The base film is further formed with an external communicating slit having a first end fluidly connected to the single through hole and a second end opened to the outside.

According to another embodiment, it is also possible to easily detect the differential pressures at a plurality of (a large number of) points without using tubes needed in the conventional piezoelectric or semiconductor pressure sensor by measuring the shift amount of each of the preset Bragg wavelengths. Further, it is possible to easily maintain the pressure in the sealed space at a constant pressure through the external communicating slit since the sealed space is opened to the outside through the external communicating slit.

The base film is preferably formed with a groove at the first surface, and the optical fiber is arranged in the groove.

According to the configuration, it is possible to prevent or reduce a bump due to the optical fiber from occurring on a surface of the optical pressure sensor positioned on a side at which the external pressure is detected.

Alternatively, the first cover film may be formed with a groove at the inner surface, and the optical fiber is arranged in the groove.

According to the configuration, it is also possible to prevent or reduce a bump due to the optical fiber from occurring on a surface of the optical pressure sensor positioned on a side the external pressure is detected.

In each of the above various configurations, the optical fiber preferably further includes a temperature compensating FBG portion, the temperature compensating FBG portion being inserted into a rigid hollow member fixed to the rigid base member.

According to the configuration, it is possible to detect a net pressure change (irrespective of the temperature change) by offsetting the measurement result of the temperature compensating FBG portion from the measurement result of the pressure detecting FBG portion.

Still another aspect of the present invention provides, in order to achieve the second object, an optical pressure sensor including a base member fixedly attached to an object body, and an optical fiber that has at least one FBG portion and that is fixed to the base member, wherein the base member includes a sealed space in which a surface facing an external pressure to be measured is covered with a flexible first cover film, the optical fiber is fixed to the base member at a region other than the FBG portion such that the FBG portion is positioned on the sealed space with the FBG portion contacted to an outer surface of the first cover film, and the change of reflection characteristic of the FBG portion is detected in a state where the sealed space is maintained at a reference pressure.

In the thus configured optical pressure sensor, the change of reflection characteristic of the FBG portion (the shift amount of Bragg wavelength by distortion of the FBG portion) of the optical fiber contacting the outer surface of the first cover film could be detected in a state where the sealed space is maintained at the reference pressure (in a state where the sealed space is preset to the reference pressure).

That is, according to the configuration, it is possible to detect the external pressure change as a differential pressure with reference to the reference pressure.

For example, the FBG portion may be wholly positioned on the sealed space (i.e., the FBG portion is positioned such that the whole FBG portion is not contacted to the base member). According to the configuration, it is possible to effectively prevent the base member from hindering or attenuating the strain action of the FBG portion caused by the pressure change. Therefore, it is possible to detect a more minute pressure change in comparison with the conventional optical pressure sensor in which the bending of the base film is detected by the FBG portion.

Alternatively, the FBG portion may be partially positioned on the sealed space. According to the configuration, it is possible to lower the sensitivity of the FBG portion in comparison with the one embodiment, thereby detecting the pressure at a portion of the object body subjected to a more high pressure.

Preferably, the base member has a flexible base film that includes a first surface on which an external pressure applies and a second surface closely attached to a surface of the object body directly or indirectly, the base film including a through hole passing through the first and second surfaces; the first cover film fixedly attached to the first surface of the base film so as to cover one end of the through hole; and a flexible second cover film fixedly attached to the second surface of the base film so as to cover the other end of the through hole for forming the sealed space in cooperation with the first cover film. The optical fiber is fixed to the outer surface of the first cover film such that the FBG portion is positioned on the through hole in plan view.

In the preferred configuration, the base member has the flexible base film that includes the first and second surfaces and the through hole passing through the first and second surfaces. The first and second cover films are fixedly attached to the base film so as to respectively cover one end and the other end of the through hole to form the sealed space. The optical fiber is fixed to the outer surface of the first cover film such that the FBG portion is positioned on the through hole in plan view. The change of reflection characteristic of the FBG portion (the shift amount of Bragg wavelength by distortion of the FBG portion) of the optical fiber contacting the outer surface of the first cover film in order to detect the external pressure change on the outer surface of the first cover film is detected in a state where the second surface of the base film is closely attached to the surface of the object body directly or indirectly and the sealed space is maintained at the reference pressure (or the sealed space is preset to the reference pressure).

According to the configuration, it is possible to detect the pressure changes on surfaces of the object body having various shapes as the differential pressure.

The first cover film is preferably formed with a groove at the outer surface, and the optical fiber is arranged in the groove.

According to the configuration, it is possible to prevent or reduce a bump due to the optical fiber from occurring on the outer surface of the first cover film forming an outer surface of the optical pressure sensor positioned on a side at which the external pressure is detected.

In each of the above various configurations, a hollow reinforcing member is preferably arranged in the through hole, the reinforcing member has rigidity higher than the base film, and the optical fiber is arranged such that the FBG portion is positioned on a hollow portion of the reinforcing member in plan view.

According to the configuration, it is possible to enhance stability of the FBG portion and the base film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A preferred embodiment of an optical pressure sensor according to the present invention will now be described with reference to the accompanying drawings.

FIG. 1is a schematic plan view of an optical pressure sensor according to a first embodiment of the present invention.FIG. 2(a) is a partially enlarged view of a region II(a) ofFIG. 1, andFIG. 2(b) is a cross sectional view taken along a line II(b)-II(b) ofFIG. 2(a).

As shown inFIGS. 1 and 2, the optical pressure sensor100A according to the present embodiment includes a base film2having a first surface2aand a second surface2b(only the first surface2ais shown inFIG. 1), the second surface2bbeing closely attached to a surface of an object body directly or indirectly, and an optical fiber3fixed to the base film2.

In the present embodiment, the optical fiber3is fixed to the first surface2a, which is on a side opposite the object body, of the base film2.

In place of the configuration, the optical fiber3may be embedded within the base film2.

The optical fiber3includes an FBG portion31. The FBG portion31has a diffraction grating configuration in which the refractive index of a core part of the optical fiber is periodically changed, and is configured to reflect only components having a predetermined wavelength (a Bragg wavelength) out of an incident light entering the optical fiber3. A probe4is attached to a first end of the optical fiber3, and a light source and a photodetector (both are not shown) are attached by way of the probe4. Out of the incident light entering the optical fiber from the light source though the probe4, a light having the wavelength is reflected by the FBG portion31and returned so that the photodetector detects the wavelength of the light.

Specifically, when the FBG portion31strains due to an external pressure change, the grating pitch of the diffraction grating of the FBG portion31expands and contracts according to the strain of the FBG portion31so that the Bragg wavelength to be reflected is shifted. Therefore, it is possible to detect a strain amount of the FBG portion31, that is, an amount of the pressure applying on the FBG portion31by detecting the shift amount of the Bragg wavelength.

It is possible to attach another probe to a second end of the optical fiber3opposite the first end to which the probe4is attached, and to respectively attach the light source to the first end and the photodetector to the second end through the respective proves. In such a configuration, the photodetector detects the wavelength of the incident light that is not passed through the FBG portion31so that the pressure is detected.

In the present embodiment, the optical fiber3includes a plurality of (five in the illustrated embodiment) of FBG portions31arranged along a longitudinal direction of the optical fiber3, the plurality of FBG portion31having different reflection characteristics one another.

The base film2is formed with through holes21as many as the FBG portion31, the through holes21passing through the first surface2aand the second surface2b.

The optical fiber3is arranged such that each of the plurality of FBG portions31is positioned on the corresponding through hole21in plan view.

It is obviously possible that the optical fiber3includes only one FBG portion31or more than five or less than five FBG portions31.

In a case where the plurality of FBG portions31are arranged in the optical fiber3as in the present embodiment, the plurality of FBG portions31are configured so as to have different reflection characteristics (Bragg wavelengths) from one another.

For example, one FBG portion31may be set to have the Bragg wavelength of 1520 nm in a state (an initial state) where no pressure change occurs, and the remaining FBG portions31may be set to have the Bragg wavelengths (1521 nm, 1522 nm, . . . ) that are sequentially increased by 1 nm from one another in the initial state. In this case, the incident light from the light source is set to have a wavelength including all the Bragg wavelengths of the plurality of FBG portions31. That is, it is configured that the light having the wavelength of 1520 nm to 1570 nm enters the optical fiber3from the light source.

The measurement of the shift amounts of the Bragg wavelengths of the respective plural FBG portions31with the configuration makes it possible to easily detect the pressures at a plurality of (a large number of) points without using tubes needed in the conventional piezoelectric or semiconductor pressure sensor, thereby detecting more specific pressure changes at a plurality of points over a wide area.

In the present embodiment, the optical fiber3is provided with a temperature compensating FBG portion32, as shown inFIG. 1.FIG. 3shows a partially enlarged view of a region III inFIG. 1.

The temperature compensating FBG portion32is inserted into a rigid hollow member5fixed to the base film2. In the present embodiment, the optical fiber3is fixed to the first surface2aof the base film2, as described above. Therefore, the hollow member5is also fixed to the first surface2aof the base film2.

The hollow member5has such rigidity that prevents the hollow member5from being deformed by the external pressure and has such an inner diameter that allows the optical fiber3to slidably move within the hollow member5. The hollow member5may be an aluminum pipe.

It is possible to distinguish a error component which is caused by an external temperature change from the measurement value of the FBG portion31by arranging the FBG portion32in the rigid hollow member5in a slidably movable manner as described above, thereby detecting an accurate pressure value.

In other words, the FBG portion31strains in response to both a pressure change and a temperature change so that the Bragg wavelength is shifted. On the other hand, the temperature compensating FBG portion32strains in response to only the temperature change without being subjected to the pressure change since it is arranged within the hollow member5.

Therefore, it is possible to detect a net pressure change (irrespective of the temperature change) by offsetting the measurement result of the temperature compensating FBG portion32from the measurement result of the pressure measurement FBG portion31.

As shown inFIG. 1, only one temperature compensating FBG portion32is arranged on the one base film2in the present embodiment, but obviously, a plurality of the FBG portions32may be arranged. The installing position of the temperature compensating FBG portion32is not particularly limited, but the FBG portion32is preferably arranged close to the FBG portion31to an extent such that the FBG portion32is subjected to the same temperature condition as the pressure measurement FBG portion31.

The hollow member5preferably has an inner diameter close as much as possible to the outer diameter of the optical fiber3within a range in which the optical fiber3can slidably move within the hollow member5. Such a configuration could effectively prevent the FBG portion32from straining in the radial direction while allowing the FBG portion32to elongate or compress in the longitudinal direction in response to the temperature change.

FIG. 4shows an exploded perspective view of the optical pressure sensor100A.

The base film2may be a flexible film having a film thickness of about 30 μm to 500 μm, for example. Examples of such flexible films include polyester films and flexible impact absorbing materials.

According to such a configuration, the optical pressure sensor100A can be closely attached to the object body even when the surface of the object body is not planar, and thus the pressure measurement can be carried out with high accuracy.

Furthermore, in a case where the impact absorbing material is used as the base film2, the base film2can absorb the impact from the outside, thereby preventing the optical fiber3from being damaged due to the impact. Moreover, the configuration could alleviate stress concentration on the optical fiber3even when the base film2is attached to the object body with being forcibly deformed.

In the optical pressure sensor100A according to the present embodiment, it is possible to thicken the base film2to an extent (e.g., about 0.5 to 1.0 mm) such that the attachment of the base film2to the object body is not affected, since the measurement value of the FBG portion31is not directly influenced by the thickness and the hardness of the base film2, as will be described later, thereby enhancing the durability of the optical pressure sensor100A.

The base film2is formed with through holes21passing through the first surface2aand the second surface2b. The shape of the through hole21is not particularly limited to, but is preferably a shape having a diameter substantially the same as or slightly larger than the axial length of the FBG portion31of the optical fiber3to be attached.

The optical fiber3is fixed to the base film2(at the first surface2ain the present embodiment) at a region other than the FBG portion31so that the FBG portion31is positioned on the corresponding through hole21in plan view (FIG. 1).

In the present embodiment, the optical fiber3is firmly fixed to the first surface2aof the base film2at both sides of the FBG portion31by fixing films6.

The fixing film6may be a double-faced tape interposed between the first surface2aand the optical fiber3to firmly fix the first surface2aand the optical fiber3, or may be an adhesive tape attached from above after arranging the optical fiber3on the first surface2a.

An aluminum film may be used as the fixing film6. In this case, the optical fiber3may be arranged on the aluminum film to prevent the optical fiber3from sliding.

In place of using the fixing film6, the optical fiber3may be fixed to the base film2with an adhesive.

The temperature compensating FBG portion32is also fixed to the base film2by using the same configuration as that for the FBG portion31.

Specifically, as shown inFIGS. 1 and 2, the optical fiber3is fixed to the base film2at both sides of the through hole21, on which the FBG portion31is positioned, in the longitudinal direction of the optical fiber3.

The positional shift of the FBG portion31with respect to the through hole21can be effectively prevented by fixing the optical fiber3to the base film2at both sides of the FBG portion31.

More preferably, the optical fiber3is fixed to the base film2at both sides of the FBG portion31with tensile force in the longitudinal direction applied to the FBG portion31, so that the Bragg wavelength in the initial state (where no pressure change occurs) can be stabilized.

The strain amount of the FBG portion31and the shift amount of the Bragg wavelength reflected by the FBG portion31are in a proportional relationship. Therefore, the FBG portion31can be prevented from being unintentionally strained in the initial state where no pressure change occurs by fixing both sides of the FBG portion31with a predetermined magnitude of tensile force applied to the FBG portion31, whereby the Bragg wavelength in the initial state can be stabilized.

The portion of the optical fiber3other than the both sides of the FBG portion31does not necessarily have to be fixed.

In the configuration in which the plurality of FBG portions31are respectively positioned on the corresponding through holes21as in the present embodiment, it is possible to arrange a region of the optical fiber3between the adjacent FBG portions31in a curve shape so as to be positioned in the same plane as the adjacent FBG portions31, and to arrange a region of the optical fiber3positioned on an outer side in the longitudinal direction than one FBG portion31, which is positioned at an end in the longitudinal direction of the optical fiber3, on a region of the base film2different from the through hole21so as to be positioned in the same plane as the one FBG portion31, as shown with a broken line22inFIG. 4. With the configuration, it is possible to prevent the bumps due to the optical fiber3from increasing on the measuring surface.

It is of course possible to firmly fix the portion of the optical fiber3other than the both sides of the FBG portion31to the base film2with an adhesive or the like.

A groove in which the optical fiber3is at least partially positioned may be arranged along the broken line22ofFIG. 4so that the bumps on the measuring surface due to the outer diameter of the optical fiber3are reduced as much as possible to enhance the measurement accuracy.

FIG. 23shows a cross sectional view of the optical pressure sensor in which the groove22ais formed in the first surface2aof the base film2, and the portion of the optical fiber3located other than on the through hole21in plan view is arranged in the groove22a.FIGS. 23(a) and23(b) are cross sectional views taken along a line XXIII(a)-XXIII(a) and a line XXIII(b)-XXIII(b), respectively, inFIG. 1.

As shown inFIG. 23, the arrangement of the optical fiber3in the groove22amakes it possible to prevent or reduce the optical fiber3from projecting outward from the outer surface of the base film2, whereby the bumps on the measuring surface can be prevented or reduced.

In the present embodiment, the first surface2aof the base film2is covered by a flexible cover film7with the optical fiber3arranged on the base film2, as shown inFIG. 4. An example of the cover film7is a polyester film.

The arrangement of the cover film7makes it possible to smoothen the measuring surface, thereby reducing the measurement error to enhance measurement accuracy. Please note that the cover film7doses not affect the following property of the optical pressure sensor100A with respect to the object body since the cover film7is a flexible film.

InFIG. 4, the cover film7is configured to cover the entire surface of the base film2.

In such a configuration, since the bending of the FBG portion31of the optical fiber3follows the bending of the cover film7by an external pressure, the sensitivity of the FBG portion31slightly lowers compared to the configuration in which the cover film7is not arranged. However, the FBG portion31is positioned within the through hole21, as described above. That is, there is a space, which is formed by the through hole21, between the portion of the cover film7contacting the FBG portion31and the object body. Therefore, it is possible to enhance the sensitivity of the FBG portion31and effectively detect a more minute pressure change in comparison with the conventional configuration in which the pressure is detected based on the deformation of the FBG portion involved in the deformation of the base film itself.

The cover film7preferably has a thickness (e.g., 0.05 mm) less than that of the base film2.

InFIG. 4, the cover film7is a single film that covers the entire surface of the base film2, but in place thereof, a plurality of films that covers the through hole21individually may be used as the cover film7.

Furthermore, in place of the configuration shown inFIG. 4, the cover film7may have an opening at a region corresponding to the whole or part of the FBG portion31, so that the whole or part of the FBG portion31is not covered by the cover film7.

According to such a configuration, the measurement sensitivity in the FBG portion31can be prevented from being lowered by the cover film7while enhancing the smoothness of the measuring surface by the cover film7.

In a case where the cover film7covers the entire surface of the base film2as in the present embodiment, the cover film7may be formed with a groove in which the optical fiber3is at least partially arranged.

Specifically, the cover film7may be formed with a groove71aat a region of the inner surface (a surface facing the base film2) of the cover film7corresponding to the optical fiber3, and the optical fiber3may be arranged within the groove71a, as shown inFIG. 24(a). Alternatively, the cover film7may be formed with a groove72a, in which the optical fiber3is at least partially arranged, on the outer surface of the cover film7, as shown inFIG. 24(b).

As shown inFIGS. 2(b) and4, the optical pressure sensor100A according to the present embodiment further includes an adhesive film8firmly fixed to the second surface2bof the base film2.

The adhesive film8has an adhesive layer that can adhere to the object body and that is on an opposite surface of the adhesive film8as the base film2.

The adhesive film8is attached to the second surface2bof the base film2on a side close to the object body with, e.g., an adhesive.

The arrangement of the adhesive film8makes it possible to directly adhere the optical pressure sensor100A to the surface to be measured of the object body. Therefore, the optical pressure sensor100A can be easily attached without damaging the object body (without processing the object body).

In the present embodiment, the cover film7and the adhesive film8that are attached on both surfaces of the base film2close the through hole21formed in the base film2to form a sealed space. The configuration makes it possible to enhance the sensitivity of the FBG portion31since the FBG portion also bends in response to the difference between the internal pressure of the sealed space and the external pressure.

The adhesive layer of the adhesive film8is preferably configured to be strippable with respect to the object body (have low viscosity). Thus, the optical pressure sensor100A can be easily detached from the object body without damaging the object body after the measurement, and both the object body and the optical pressure sensor100A can be reused.

The adhesive film8is also preferably a flexible film (e.g., a polyester film) so that following property of the optical pressure sensor100A with respect to the object body is not affected due to the adhesive film8.

An optical pressure sensor100B according to a variation of the present embodiment will now be described.

FIG. 5shows an exploded perspective view of the optical pressure sensor100B.

As shown inFIG. 5, in the optical pressure sensor100B, the cover film7and the base film2are integrally formed to each other.

That is, the optical pressure sensor100B includes a single film forming the cover film7and the base film2. The single film is folded with the optical fiber3including the FBG portions31,32sandwiched between one part forming the cover film7and the other part forming the base film2.

As described above, in the optical pressure sensors100A,100B, the base film2is formed with the through hole21that passes through the first surface2aand the second surface2b, and the optical fiber3is fixed to the base film2by the fixing film6at the region other than the FBG portion31with the FBG portion31functioning as a pressure sensitive part positioned on the through hole21in plan view.

When measuring the pressure acting on the object body, the optical pressure sensors100A,100B with such a configuration are attached to the object body in a state that the second surface2bof the base film2directly or indirectly face the surface of the object body.

Therefore, the FBG portion31functioning as the pressure sensitive part can directly detect the pressure change on the surface of the first surface2athat is the measuring surface.

Furthermore, since the FBG portion31is arranged on the through hole21of the base film2, the distortion of the FBG portion31will not be hindered or attenuated by the base film2. That is, the FBG portion32bends irrespective of the base film2and the object body. Therefore, according to the optical pressure sensors100A,100B, a more minute pressure change can be detected compared to the conventional optical pressure sensor detecting the pressure change based on the bending of the base film2.

A plurality of the optical pressure sensors100A,100B may be connected, for example in series, to carry out pressure measurement over a wide area.

FIG. 6shows a usage example where a plurality of the optical pressure sensors100A,100B are connected in series.

In the usage example shown inFIG. 6, each of the optical pressure sensors100A,100B includes the probe4and a probe4′ connectable to the probe4respectively arranged at first and second ends of the optical fiber3.

The probe4′ in one optical pressure sensor100A,100B is connected to the probe4in another optical pressure sensor100A,100B, so that all of the FBG portions31of the plurality of optical pressure sensors100A,100B are connected in series.

The pressures of a large number of measuring points can be simultaneously measured with one channel by connecting the plurality of unitized optical pressure sensors100A,100B as described above.

Please note that the measurement can be performed without deteriorating measurement accuracy even if such a connection is made, since the incident light is less attenuated in the optical fiber3.

The configuration in which the FBG portion31is positioned on the through hole21in plan view includes a configuration in which a portion of the FBG portion31is positioned on the through hole21, as shown inFIG. 25, in addition to the configuration in which the entire region of the FBG portion31is positioned on the through hole21, as shown inFIGS. 1 and 2.

Positioning only a portion of the FBG portion31on the through hole21, as shown inFIG. 25, can lower the sensitivity of the FBG portion31.

That is, the sensitivity of the FBG portion31can be adjusted by adjusting the length of the portion of the FBG portion31positioned on the through hole21.

Therefore, the measurable pressure range can be substantially widened by appropriately changing the length of the portion of the FBG portion positioned on the through hole21in accordance with the magnitude of the expected pressure at the measurement point.

Furthermore, the above configuration has an effect of reducing the generation of extra noise due to overreaction of the FBG portion31.

The present embodiment has been described taking the configuration in which the base film2, the cover film7and the adhesive film8have flexibility as an example, but the present invention is not limited thereto.

For example, in a case where the measuring surface is a plane, a rigid base member made of resin material or metal material may be used in place of the base film2.

Furthermore, a rigid cover plate9firmly fixed to the first surface2aof the base film (or the base member) in a direct or indirect manner may be arranged in place of the cover film7.

FIG. 7shows an exploded perspective view of an optical pressure sensor100C according to another variation of the present embodiment.

As shown inFIG. 7, the optical pressure sensor100C includes the rigid cover plate9in place of the cover film7.

Examples of the cover plate9include an aluminum plate.

The cover plate9is formed with passing holes91at positions corresponding to the FBG portions31.

In the optical pressure sensor100C, the cover plate9is securely fixed in an indirect manner to the first surface2aof the base film2by way of a flexible spacer film10.

The spacer film10interposed between the base film2and the cover plate9is preferably formed with holes101for fluidically connecting the passing holes91and the corresponding through holes21.

The passing hole91in the rigid cover plate9may have a diameter substantially the same as or smaller than that of the through hole21of the base film2.

According to the optical pressure sensor100C, the rigid cover plate9eliminates the bumps on the surface due to the diameter of the optical fiber3arranged on the first surface2aof the base film2and the like, thereby smoothening the measuring surface.

Further, the flexible spacer film10E can enhance the contact between the cover plate9and the base film2even when the diameter of the optical fiber3attached to the first surface2aof the base film2is relatively large, thereby increase the effectiveness of the smoothening function by the cover plate9. Accordingly, it is possible to detect a minute pressure change without depending on the surface profile of the base film2.

In a case where pressures at a plurality of points on the smooth plane are respectively detected by the plurality of FBG portions31, the arrangement of the rigid cover plate9makes it possible to reliably hold the plurality of FBG portions31at the respective installing positions.

In the case where the spacer film10is interposed, a single film, which is folded to form the spacer film10and the base film2, may be used, similarly to the configuration shown inFIG. 5.

Although the through holes21of the same number as the FBG portions31of the optical fiber3are formed in the present embodiment, the present invention is not limited to such configuration.

FIG. 8shows a plan view of an optical pressure sensor100D according to still another variation of the present embodiment.

As shown inFIG. 8, the optical pressure sensor100D is configured so that the base film2ais formed with a single through hole21′, and the optical fiber3is arranged such that the plurality of FBG portions31are positioned on the single through hole21′ in plan view.

Alternatively, the base film2may be formed with a plurality of through holes, each of the through holes corresponding to some FBG portions31.

The arrangement of the optical fiber3is not particularly limited, and the optical fiber3may be arranged so as to form a spiral as shown inFIG. 9.

A flexible cover film7′,7″ for covering any of the plurality of through holes21may be arranged on the first surface2aof the base film2.

FIGS. 10(a) and10(b) show plan views of optical pressure sensors100E,100E′ including the cover films7′,7″ for covering any of the plurality of through holes21in place of the cover film7in the optical pressure sensor100A.

In the optical pressure sensors100E,100E′ shown inFIGS. 10(a) and10(b), some of the plurality of through holes21are covered by the flexible cover films7′,7″. The cover films7′,7″ may be formed to cover each of the plurality of through holes21individually (seeFIG. 10(a)) or may be formed to cover two or more through holes21(seeFIG. 10(b)).

In the optical pressure sensor100E shown inFIG. 10(a), only one through hole21at the middle out of the five through holes21is covered by the flexible cover film7′.

In the optical pressure sensor100E′ shown inFIG. 10(b), on the other hand, the entire surface of the first surface2aof the base film2excluding the region near the two through holes21is covered by the flexible cover film7″.

The sensitivity of one FBG portion31positioned in the through hole21that is covered by the flexible cover film7′,7″ becomes lower in comparison with the other FBG portion31positioned in the through hole21that is not covered by the cover film7′,7″, because the strain of the one FBG portion31follows the strain of the cover film7′,7″. That is, the arrangement of the flexible cover film7′,7″ makes it possible to locally lower the sensitivity of the FBG portion31at a desired area.

Therefore, even when detecting pressures at a plurality of measuring points in which the rates of pressure changes are extremely different one another, it is possible to simultaneously detect the pressures at the plurality of measuring points by using the plurality of FBG portion31having the same configurations one another.

For instance, it is possible to cover one FBG portion31, which is positioned at one area (e.g., a front end portion of an automobile) where the measurement pressure is assumed to be large, by the flexible cover film7′,7″, and not to cover the other FBG portion31, which is positioned at the other area (e.g., a side portion of the automobile) where the measurement pressure is assumed to be small, so that the one area on which the large pressure applies and the other area on which the small pressure applies could be simultaneously detected by the FBG portions31having the same configuration.

Furthermore, it is possible to change the shift amount of the FBG portion31by using the cover film7′,7″ of various thickness, thereby finely performing sensitivity adjustment.

Furthermore, it is also possible to cover only part of the FBG portion31with the cover film7,7′,7″, so that the sensitivity of the corresponding FBG portion31is lowered.

The present embodiment has been described taking, as an example, the configuration in which the plurality of FBG portions31are fixed on the one base film2(and the one cover film7and the one adhesive film8are provided for the plurality of FBG portions31), but the present invention is not limited thereto.

FIG. 11shows a plan view of an optical pressure sensor100F having a configuration in which one base film2′ is arranged for one (may be plural) FBG portion31(in other words, a configuration in which the base film2is divided for every plural FBG portions31).

The configuration could deal with various shapes of an object body to be measured, and easily have the sensor pitch (the space between the plurality of FBG portions31) sparse or dense so that the degree of freedom of measurement mode can be heightened.

Second Embodiment

Another embodiment of an optical pressure sensor according to the present invention will now be described with reference to the accompanying drawings.

FIG. 12is a schematic plan view of an optical pressure sensor200A according to the present embodiment.FIG. 13(a) is a partially enlarged view of a region XIII(a) ofFIG. 12, andFIGS. 13(b) and13(c) are cross sectional views respectively taken along a line XIII(b)-XIII(b) and a line XIII(c)-XIII(c) ofFIG. 13(a).

In the figures, the same members as those of the first embodiment have been given the same reference characters.

As shown inFIGS. 12 and 13, the optical pressure sensor200A according to the present embodiment includes a base member20fixed to the object body, and the optical fiber3that has at least the one FBG portion31and that is fixed to the base member20.

The FBG portion31has a diffraction grating configuration in which the refractive index of a core part of the optical fiber is periodically changed, and is configured to reflect only components having a predetermined wavelength (a Bragg wavelength) out of an incident light entering the optical fiber3. The probe4is attached to the first end of the optical fiber3, and the light source and the photodetector (both are not shown) are attached by way of the probe4. Out of the incident light entering the optical fiber from the light source though the probe4, a light having the wavelength is reflected by the FBG portion31and returned so that the photodetector detects the wavelength of the light.

Specifically, when the FBG portion31strains due to an external pressure change, the grating pitch of the diffraction grating of the FBG portion31expands and contracts according to the strain of the FBG portion31so that the Bragg wavelength to be reflected is shifted. Therefore, it is possible to detect a strain amount of the FBG portion31, that is, an amount of the pressure applying on the FBG portion31by detecting the shift amount of the Bragg wavelength.

It is possible to attach another probe to the second end of the optical fiber3opposite the first end to which the probe4is attached, and to respectively attach the light source to the first end and the photodetector to the second end through the respective proves. In such a configuration, the photodetector detects the wavelength of the incident light that is not passed through the FBG portion31so that the pressure is detected.

In the present embodiment, the optical fiber3includes a plurality of (five in the illustrated embodiment) of FBG portions31arranged along a longitudinal direction of the optical fiber3, the plurality of FBG portion31having different reflection characteristics one another.

The base member20is configured to include a sealed space having a surface, on which the external pressure to be measured applies, covered by a flexible first cover film70.

Specifically, the base member20includes the flexible base film2, a flexible first cover film70and a flexible second cover film80.

The base film2includes the first surface2ato which the optical fiber3is fixed; the second surface2bclosely attached to the surface of the object body directly or indirectly; the through holes21passing through the first and second surfaces2a,2b, the through holes21being as many as the FBG portion31; and a slit24fluidly connecting the plurality of through holes21one another.

The first cover film70is fixedly attached to the first surface2aof the base film2so as to cover first ends on one sides of the through holes21and the slit21.

The second cover film80is fixedly attached to the second surface2bof the base film2so as to cover second ends on the other sides of the through holes21and the slit21for forming the sealed space in cooperation with the first cover film70.

The base film2is further provided with an external communicating slit23having a first end fluidly connected to the sealed space that is defined by the plurality of through holes21and a slit24and a second end opened to the outside.

The slit23is connected to an introducing pipe231for introducing air into the sealed space.

The optical pressure sensor200A is configured to detect the change of reflection characteristics (the shift amount of Bragg wavelength by distortion of the FBG portion31) of the FBG portion31of the optical fiber3contacting the inner surface of the first cover film70in a state where the sealed space is maintained at a reference pressure through the slit23opened to the outside.

It is possible to detect the pressure change (the differential pressure) from the reference pressure by the configuration where one surface of the sealed space maintained at the predetermined reference pressure is covered with the first cover film70having flexibility, and the change of the pressure acting on the first cover film70is detected using the change in reflection characteristics of the FBG portion31of the optical fiber3contacting the inner surface of the first cover film70.

The optical fiber3is arranged such that the plurality of FBG portions31are respectively positioned on the corresponding through holes21in plan view.

It is obviously possible that the optical fiber3includes only one FBG portion31, or more or less than five FBG portions31.

In a case where the plurality of FBG portions31are arranged in the optical fiber3as in the present embodiment, the plurality of FBG portions31are configured so as to have different reflection characteristics (Bragg wavelengths) from one another.

For example, one FBG portion31may be set to have the Bragg wavelength of 1520 nm in a state (an initial state) where no pressure change occurs, and the remaining FBG portions31may be set to have the Bragg wavelengths (1521 nm, 1522 nm, . . . ) that are sequentially increased by 1 nm from one another in the initial state. In this case, the incident light from the light source is set to have a wavelength including all the Bragg wavelengths of the plurality of FBG portions31. That is, it is configured that the light having the wavelength of 1520 nm to 1570 nm enters the optical fiber3from the light source.

The measurement of the shift amounts of the Bragg wavelengths of the respective plural FBG portions31with the configuration makes it possible to easily detect the pressures at a plurality of (a large number of) points without using tubes needed in the conventional piezoelectric or semiconductor pressure sensor, thereby detecting more specific pressure changes at a plurality of points over a wide area.

In the present embodiment, the optical fiber3is provided with the temperature compensating FBG portion32, as shown inFIG. 12.FIG. 14shows a partially enlarged view of a region XIV inFIG. 12.

The temperature compensating FBG portion32is inserted into the rigid hollow member5fixed to the base film2. In the present embodiment, the optical fiber3is fixed to the first surface2aof the base film2. Therefore, the hollow member5is also fixed to the first surface2aof the base film2.

The hollow member5has such rigidity that prevents the hollow member5from being deformed by the external pressure and has such an inner diameter that allows the optical fiber3to slidably move within the hollow member5. The hollow member5may be an aluminum pipe.

It is possible to distinguish a error component which is caused by an external temperature change from the measurement value of the FBG portion31by arranging the FBG portion32in the rigid hollow member5in a slidably movable manner as described above, thereby detecting an accurate pressure value.

In other words, the FBG portion31strains in response to both a pressure change and a temperature change so that the Bragg wavelength is shifted. On the other hand, the temperature compensating FBG portion32strains in response to only the temperature change without being subjected to the pressure change since it is arranged within the hollow member5.

Therefore, it is possible to detect a net pressure change (irrespective of the temperature change) by offsetting the measurement result of the temperature compensating FBG portion32from the measurement result of the pressure measurement FBG portion31.

As shown inFIG. 12, only one temperature compensating FBG portion32is arranged on the one base film2in the present embodiment, but obviously, a plurality of the FBG portions32may be arranged. The installing position of the temperature compensating FBG portion32is not particularly limited, but the FBG portion32is preferably arranged close to the FBG portion31to an extent such that the FBG portion32is subjected to the same temperature condition as the pressure measurement FBG portion31.

FIG. 15shows an exploded perspective view of the optical pressure sensor200A.

The base film2may be a flexible film having a film thickness of about 30 μm to 500 μm, for example. Examples of such flexible films include polyester films and flexible impact absorbing materials.

According to such a configuration, the optical pressure sensor200A can be closely attached to the object body even when the surface of the object body is not planar, and thus the pressure measurement can be carried out with high accuracy.

Furthermore, in a case where the impact absorbing material is used as the base film2, the base film2can absorb the impact from the outside, thereby preventing the optical fiber3from being damaged due to the impact. Moreover, the configuration could alleviate stress concentration on the optical fiber3even when the base film2is attached to the object body with being forcibly deformed.

In the optical pressure sensor200A according to the present embodiment, it is possible to thicken the base film2to such an extent (e.g., about 0.5 to 1.0 mm) that the attachment of the base film2to the object body is not affected, since the measurement value of the FBG portion31is not directly influenced by the thickness and the hardness of the base film2, thereby enhancing the durability of the optical pressure sensor200A.

The base film2is formed with the through holes21passing through the first surface2aand the second surface2b. The shape of the through hole21is not particularly limited to, but is preferably a shape having a diameter substantially the same as or slightly larger than the axial length of the FBG portion31of the optical fiber3to be attached.

The optical fiber3is fixed to the base film2(at the first surface2ain the present embodiment) at a region other than the FBG portion31so that the FBG portion31is positioned on the corresponding through hole21in plan view (FIG. 12).

In the present embodiment, the optical fiber3is firmly fixed to the first surface2aof the base film2at both sides of the FBG portion31by the fixing films6.

The fixing film6may be a double-faced tape interposed between the first surface2aand the optical fiber3to firmly fix the first surface2aand the optical fiber3, or may be an adhesive tape attached from above after arranging the optical fiber3on the first surface2a.

In place of using the fixing film6, the optical fiber3may be fixed to the base film2with an adhesive.

The temperature compensating FBG portion32is also fixed to the base film2by using the same configuration as that for the FBG portion31.

Specifically, as shown inFIGS. 12 and 13, the optical fiber3is fixed to the base film2at both sides of the through hole21, on which the FBG portion31is positioned, in the longitudinal direction of the optical fiber3.

The positional shift of the FBG portion31with respect to the through hole21can be effectively prevented by fixing the optical fiber3to the base film2at both sides of the FBG portion31.

The portion of the optical fiber3other than the both sides of the FBG portion31does not necessarily have to be fixed.

In the configuration in which the plurality of FBG portions31are respectively positioned on the corresponding through holes21as in the present embodiment, it is possible to arrange a region of the optical fiber3between the adjacent FBG portions31in a curve shape so as to be positioned in the same plane as the adjacent FBG portions31, and to arrange a region of the optical fiber3positioned on an outer side in the longitudinal direction than one FBG portion31, which is positioned at an end in the longitudinal direction of the optical fiber3, on a region of the base film2different from the through hole21so as to be positioned in the same plane as the one FBG portion31, as shown with a broken line22inFIG. 15. With the configuration, it is possible to prevent the bumps due to the optical fiber3from increasing on the measuring surface.

It is of course possible to firmly fix the portion of the optical fiber3other than the both sides of the FBG portion31to the base film2with an adhesive or the like.

A groove in which the optical fiber3is at least partially positioned may be arranged along the broken line22ofFIG. 15so that the bumps on the measuring surface due to the outer diameter of the optical fiber3are reduced as much as possible to enhance the measurement accuracy.

That is, it is possible to form the groove on the first surface2aof the base film2and to arrange the portion of the optical fiber3other than the portion positioned on the through hole21in plan view within the groove, as inFIG. 23in the first embodiment.

The first cover film70is attached so as to cover the first surface2aof the base film2in a state where the optical fiber3is arranged on the base film2.

The optical fiber3is interposed between the first surface2aof the base film2and the first cover film70in a state where the FBG portion31is positioned on the through hole21in plan view.

The arrangement of the first cover film70makes it possible to smoothen the measuring surface, thereby reducing the measurement error to enhance measurement accuracy.

The first cover film70is preferably made of a flexible film such as a polyester film.

In the present embodiment, although the bending of FBG portion31follows the bending of the first cover film70by the external pressure, the sensitivity of the FBG portion31is satisfactorily maintained since the FBG portion31is positioned on the through hole21and the flexible film is used as the first cover film70.

That is, there is a space, which is formed by the through holes21, between the portion of the first cover film70contacting the FBG portion31and the object body. Therefore, the sensitivity of the FBG portion31can be enhanced so that a more minute pressure change can be effectively detected in comparison with the conventional configuration where the pressure is detected based on the deformation of the FBG portion involved in the deformation of the base film itself.

The first cover film70preferably has a thickness (e.g., 0.05 mm) less than that of the base film2, thereby preventing the measurement sensitivity from lowering as much as possible.

In the present embodiment, the FBG portion31is firmly fixed to the inner surface of the first cover film70so as to follow the bending of the first cover film70involved in the external pressure change with respect to the reference pressure.

Specifically, the FBG portion31is fixed to the inner surface of the first cover film70with an adhesive or the like, so that the bending of the FBG portion31follows the bending of the first cover film70that is occurred in response to the external pressure change with respect to the reference pressure.

The configuration makes it possible to more accurately detect a more minute pressure change.

The configuration also makes it possible to more accurately detect the pressure change, even when the external pressure is lower than the reference pressure (i.e. the external pressure is a negative pressure), that is, even when the first cover film70bends outward.

In a case where the cover film70for covering the entire surface of the base film2is provided as in the present embodiment, it may be possible to form a groove in which the optical fiber3could be at least partially arranged in the first cover film70.

Specifically, the first cover film70may be formed with a groove at a region of the inner surface (a surface facing the base film2) of the first cover film70corresponding to the optical fiber3so that the optical fiber3could be arranged within the groove, as in the configuration shown inFIG. 24(a) of the first embodiment.

In the present embodiment, the second cover film80is firmly fixed to the second surface2bof the base film2.

That is, the sealed space is formed by the through holes21and the slits24formed in the base film2, the first cover film70, and the second cover film80.

The second cover film80is fixed to the second surface2bof the base film2on a side close to the object body by, for example, an adhesive.

The second cover film80may preferably have an adhesive layer capable of adhering to the object body, on an opposite surface of the second cover film80as the base film2.

The arrangement of the adhesive layer in the second cover film80makes it possible to directly adhere the optical pressure sensor200A to the surface of the object body to be measured. Therefore, the optical pressure sensor200A can be easily attached without damaging the object body (without processing the object body).

The adhesive layer of the second cover film80is preferably configured to be strippable with respect to the object body (have low viscosity). Thus, the optical pressure sensor200A can be easily detached from the object body without damaging the object body after the measurement, and both the object body and the optical pressure sensor200A can be reused.

The second cover film80is also preferably a flexible film (e.g., a polyester film) so that following property of the optical pressure sensor200A with respect to the object body is not affected due to the second cover film80.

An optical pressure sensor200B according to a variation of the present embodiment will now be described.

FIG. 16shows an exploded perspective view of the optical pressure sensor200B.

As shown inFIG. 16, in the optical pressure sensor200B, the first cover film70and the base film2are integrally formed to each other.

That is, the optical pressure sensor200B includes a single film forming the first cover film70and the base film2. The single film is folded with the optical fiber3including the FBG portions31,32sandwiched between one part forming the cover film7and the other part forming the base film2.

Alternatively, it is possible to integrally form the first cover film70and the second cover film80, or to integrally form the second cover film80and the base film2.

As described above, in the optical pressure sensors200A,200B, the base member20has the sealed space covered by the first cover film70having flexibility and the second cover film80, and the FBG portion31of the optical fiber3is fixed to the base member20while contacting the inner surface of the first cover film70facing the sealed space. The optical pressure sensors200A,200B are configured to detect the change of reflection characteristics of the FBG portion31(the shift of Bragg wavelength due to strain of the FBG portion31) of the optical fiber3contacting the inner surface of the first cover film70in a state where the sealed space is maintained at the reference pressure (set at the predetermined reference pressure).

According to the thus configured optical pressure sensors200A,200B, it is possible to detect the external pressure change as the differential pressure with respect to the reference pressure.

That is, the optical sensors200A,200B could effectively detect not only the pressure (a positive pressure) at which the capacity of the sealed space decreases but also the pressure (a negative pressure) at which the capacity of the sealed space increases.

Furthermore, since the FBG portion31is positioned in the through hole21in plan view (arranged at the area not contacting the base film2), the strain of the FBG portion31following the bending of the first cover film70will neither be hindered nor attenuated by the base film2. Thus, it is possible to detect a more minute pressure change in comparison with the conventional optical pressure sensor in which the pressure is detected based on the bending of the base film2.

However the FBG portion31of the optical fiber3is fixed to the inner surface of the first cover film70in the present embodiment, it is also possible to fix the optical fiber3to the outer surface of the first cover film70with the FBG portion31positioned on the through hole21in plan view.

FIG. 17shows an exploded perspective view of an optical pressure sensor200C according to a variation of the present embodiment, the optical pressure sensor200B being configured so that the optical fiber3is fixed to the outer surface of the first cover film70.

In the optical pressure sensor200C, the optical fiber3is fixed to the outer surface of the first cover film70in a state where the FBG portion31is positioned on the through hole21in plan view.

Specifically, the optical fiber3is firmly fixed to the outer surface of the first cover film70at both sides of the FBG portion31by fixing films6.

The optical pressure sensor200C is configured to detect the change of reflection characteristics of the FBG portion31(the shift of Bragg wavelength due to strain of the FBG portion31) in a state where the sealed space is maintained at the reference pressure, similar to the optical pressure sensors200A,200B.

The optical pressure sensor200C further includes a third cover film90(a flexible film similar to the first cover film70) having substantially the same size as the base film2, the third cover film90being attached to the outer surface of the first cover film70with the optical fiber3arranged on the outer surface of the first cover film70, thereby more effectively preventing the positional shift of the optical fiber3(a portion of the optical fiber3other than FBG portion31).

The first cover film70need not be the same size (may be smaller) as the base film2as long as it has a size necessary for forming the sealed space (i.e. a size enough to cover all the through holes21and the slits24).

The third cover film70is preferably formed with openings (apertures101) at portions corresponding to the through holes21, thereby enhancing the measurement sensitivity of the FBG portions31.

In a case where the optical fiber3is arranged on the outer surface of the first cover film70as shown inFIG. 17, the first cover film70may be formed with a groove, in which the optical fiber3could be arranged, on the outer surface, as in the configuration shown inFIG. 24(b) in the first embodiment, thereby preventing or reducing the bumps on the measuring surface.

In the present embodiment, the optical fiber3preferably is fixed to the base film2or the first cover film70at both sides of the FBG portion with the FBG portion31bent in an axial direction of the through hole21, so as to create a state in which the Bragg wavelength is shifted by a predetermined wavelength in the initial state.

In a case where the FBG portion31is fixed so as to be linear in the initial state, the shift direction of the Bragg wavelength when the pressure to be detected is a positive pressure (the positive pressure mean the pressure causing the FBG portion31to strain in a positive direction which is a direction from the measuring surface towards the inner side of the sealed space) is same as the shift direction of the Bragg wavelength when the pressure to be detected is a negative pressure. That is, the FBG portion31shifts in the same direction by the same amount irrespective of the strain direction of the FBG portion31if the shift amount of the FBG portion when a given positive pressure applies and that when a given negative pressure applies are same, resulting in impossibility (or difficulty) in detecting whether the pressure is a positive pressure or a negative pressure.

On the other hand, the pre-shift of the Bragg wavelength in any one of pressure applying directions in the initial state could have a shift direction of the Bragg wavelength shifts when a positive pressure is applied different from a shift direction of the Bragg wavelength shifts when a negative pressure is applied.

More specific description will be made hereinafter.

FIG. 18shows a cross sectional view of a configuration where both sides of the FBG portion31are fixed with the FBG portion31pre-bent in one of the pressure applying directions in the optical pressure sensor shown inFIG. 17.

FIG. 18(a) shows the initial state when an external pressure is not applied,FIG. 18(b) shows a state where a positive external pressure is applied, andFIG. 18(c) shows a state where a negative external pressure is applied.

In the configuration shown inFIG. 18, the FBG portion31is bent so that the Bragg wavelength λBin the initial state (hereinafter referred to as initial Bragg wavelength) is pre-shifted in the positive direction, as shown inFIG. 18(a), with respect to the Bragg wavelength in a state where the FBG portion is linear.

Specifically, in the configuration shown inFIG. 18, a spacer71is arranged on the outer surface of the first cover film70so as to be interposed between the outer surface of the first cover film70and the FBG portion31, so that the FBG portion31is pre-bent in one of the axial directions of the through hole21(i.e., one of the pressure applying directions) by the spacer71.

In the configuration shown inFIG. 18, the spacer71is arranged substantially at a center in the axial direction of the FBG portion31, so that the region of the FBG portion31substantially at the center in the axial direction is most away from the first cover film70.

That is, when no external pressure is applied (i.e., when the external pressure is substantially equal to the reference pressure in the sealed space), the region of the FBG portion31substantially at the center in the axial direction is bent so as to be convex upward.

When a positive external pressure applies on the FBG portion31in the initial state as shown inFIG. 18(a) (when an external pressure applies in a direction towards the inner side of the sealed space from the measuring surface, that is, when the external pressure higher that the reference pressure), the FBG portion31bends in such a direction that the bending amount becomes smaller so that the Bragg wavelength shifts in a direction opposite the direction in which the initial Bragg wavelength λBis shifted with the Bragg wavelength in the case where the FBG portion31is substantially linear as a reference (seeFIG. 18(b)). This means that the Bragg wavelength shifts in the negative direction when the positive external pressure applies, assuming that the initial Bragg wavelength λBis 0.

On the other hand, when a negative external pressure applies on the FBG portion31in the initial state as shown inFIG. 18(a) (when the external pressure is lower than the pressure in the sealed space, that is, when the external pressure is lower than the reference pressure), the FBG portion31bends in such a direction that the bending amount becomes larger so that the Bragg wavelength shifts in the same direction as the direction in which the initial Bragg wavelength λBis shifted (seeFIG. 18(c)). This means that the Bragg wavelength shifts in the positive direction when the negative pressure applies, assuming that the initial Bragg wavelength λBis 0.

Therefore, it is possible to definitely detect whether the pressure applying on the FBG portion31is a positive pressure or a negative pressure thus can be more definitely determined by fixing both sides of the FBG portion31to the base film2or the first cover film70with the FBG portion31bent to a certain extent in any one of opposite sides in the axial direction of the through hole21in the initial state so that the Bragg wavelength λBin the initial state is pre-shifted with respect to the Bragg wavelength in the state where the FBG portion31is substantially linear.

When the external pressure applies in such a direction that the bending amount of the FBG portion31becomes smaller from the initial state (when the external positive pressure applies on the configuration shown inFIG. 18), the shift amount of the Bragg wavelength with respect to the initial Bragg wavelength λBbecomes larger as the pressure increases until the FBG portion31becomes linear without bending, and thereafter, when the pressure further increases and the FBG portion31starts to bend towards the side opposite the bent direction in the initial state, the shift amount of the Bragg wavelength with respect to the initial Bragg wavelength λBbecomes smaller as the pressure increases, and the shift amount of the Bragg wavelength with respect to the initial Bragg wavelength λBbecomes zero at the point where the FBG portion31is bent in the opposite direction by the same amount as the bending amount in the initial state.

In other words, when the pressure is applied in such a direction that the bending amount becomes smaller from the initial state, the shift direction of the Bragg wavelength with respect to the initial Bragg wavelength λBswitches with the state in which the FBG portion31becomes substantially linear as a reference.

Therefore, it is preferable that the FBG portion31is bent so as to be convex upward (so as to have a convex shape in a direction away from the sealed space) in the initial state (seeFIG. 18) when it is assumed that the negative pressure is likely to occur and the FBG portion31is bent so as to be convex downward (so as to have a convex shape in a direction approaching the sealed space) in the initial state (not shown) when it is assumed that a positive pressure is likely to occur, based on the measurement position and the type (water pressure, air pressure, and the like) of the external pressure.

Such a preferred configuration makes it possible to prevent the shift direction of the Bragg wavelength with respect to the initial Bragg wavelength λBfrom being switched with the state in which the FBG portion31becomes substantially linear as a reference.

It is possible to detect pressure over a wide area by connecting a plurality of the optical pressure sensors200A-200C, for example in series.

FIG. 19shows a usage example where a plurality of the optical pressure sensors200A-200C are connected in series.

In the usage example shown inFIG. 19, each of the optical pressure sensors200A-200C include the probe4and a probe4′ connectable to the probe4respectively arranged at first and second ends of the optical fiber3.

The probe4′ in one optical pressure sensor200A-200C is connected to the probe4in another optical pressure sensor200A-200C, so that all of the FBG portions31of the plurality of optical pressure sensors200A-200C are connected in series.

The pressures of a large number of measuring points can be simultaneously measured with one channel by connecting the plurality of unitized optical pressure sensors200A-200C as described above.

Please note that the measurement can be performed without deteriorating measurement accuracy even if such a connection is made, since the incident light is less attenuated in the optical fiber3.

The configuration in which the FBG portion31is positioned on the through hole21in plan view includes a configuration in which a portion of the FBG portion31is positioned on the through hole21, as shown inFIG. 25, in addition to the configuration in which the entire region of the FBG portion31is positioned on the through hole21, as shown inFIGS. 12,13and18.

Positioning only a portion of the FBG portion31on the through hole21, as shown inFIG. 25, can lower the sensitivity of the FBG portion31.

That is, the sensitivity of the FBG portion31can be adjusted by adjusting the length of the portion of the FBG portion31positioned on the through hole21.

Therefore, the measurable pressure range can be substantially widened by appropriately changing the length of the portion of the FBG portion positioned on the through hole21in accordance with the magnitude of the expected pressure at the measurement point.

Furthermore, the above configuration has an effect of reducing the generation of extra noise due to overreaction of the FBG portion31.

The present embodiment has been described taking, as an example, the configuration in which the optical pressure sensor200A-200C has flexibility as a whole, but the present invention is not limited thereto.

FIG. 20shows an exploded perspective view of an optical pressure sensor200C according to still another variation of the present embodiment.

As shown inFIG. 20, the optical pressure sensor200C further includes the rigid cover plate9arranged on the measuring surface of the first cover film70on which the external pressure is applied.

The cover plate9is formed with passing holes91at positions corresponding to the FBG portions31.

The passing hole91may have a diameter substantially the same as or smaller than that of the through hole21of the base film2.

The rigid cover plate9eliminates the bumps on the surface due to the diameter of the optical fiber3and the like, thereby smoothening the measuring surface. Accordingly, it is possible to detect a minute pressure change without depending on the surface profile of the base member20.

In a case where pressures at a plurality of points on the smooth plane are respectively detected by the plurality of FBG portions31, the arrangement of the rigid cover plate9makes it possible to reliably hold the plurality of FBG portions31at the respective installing positions.

Although the through holes21of the same number as the FBG portions31are formed in the present embodiment, the present invention is not limited to such configuration.

FIG. 21shows a plan view of an optical pressure sensor200E according to still another variation of the present embodiment.

As shown inFIG. 21, the optical pressure sensor200E is configured so that the base member20is formed with a single through hole21′, and the optical fiber3is arranged such that the plurality of FBG portions31are positioned on the single through hole21′ in plan view.

The base member20is further formed with the external communicating slit23having a first end fluidly connected to the single through hole21′ and a second end opened to the outside.

Alternatively, it is possible to form a through hole corresponding to some of the plurality of FBG portions31.

The arrangement of the optical fiber3is not particularly limited, and the optical fiber3may be arranged so as to form a spiral as shown inFIG. 22.

In each of the embodiments, it is possible to arrange a hollow reinforcing ring210within the through hole21in the base film2, the reinforcing ring210being made of a material having rigidity higher than that of base film2, and position at lease a part of the FBG portion31on the hollow portion of the reinforcing member210in plan view.

The arrangement of the reinforcing member210makes it possible to more enhance stability of the FBG portion31and the cover film2.

FIG. 26(a) shows an example of a configuration where the reinforcing member210is provided.

FIG. 26(b) shows a cross sectional view taking along a line XXVI(b)-XXVI(b) inFIG. 26(a).

The reinforcing member210is preferably configured so that its outer circumferential surface contacts the inner circumferential surface of the through hole21.

The reinforcing member may be made of a metal or a resign having higher rigidity than the base film.