JP2012251963A - Refractive index detection method and optical fiber sensor system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber sensor system having small energy loss, and a refractive index detection method.SOLUTION: A waveguide body which guides diffracted light generated by diffracting incident laser light, while totally reflecting the diffracted light is disposed in an analyte, and a refractive index is detected on the basis of a phase change occurring in the diffracted light due to total reflection. The optical fiber sensor system comprises: a projector which emits laser light; a first optical fiber which has one end connected to the projector and guides the laser light; a long-sized waveguide body to which the other end of the first optical fiber is connected; a second optical fiber which faces the first optical fiber and has one end connected to the waveguide body; a light receiver to which the other end of the second optical fiber is connected; and an analyzer which analyzes a signal outputted from the light receiver.

Description

  The present invention relates to a detection method and an optical fiber sensor system for detecting a refractive index of a subject using diffracted light of laser light guided by an optical fiber.

  Conventionally, various sensor systems using optical fibers have been proposed. In the conventional sensor system, a sensor part that leaks the light in the core is provided in the middle part of the optical fiber, and it is possible to detect fluctuations in distortion, temperature, etc. by detecting transmission loss due to scattering of the leaked light, etc. (For example, refer to Patent Document 1 and Patent Document 2.)

  In the sensor part of the sensor system, the optical fiber composed of the core and the clad is partially removed to expose the core, or the core diameter of the sensor part other than the sensor part is the core of the optical fiber. The light guided by the optical fiber is effectively leaked by making it smaller or larger than the diameter, or by using only the clad without the core in some cases.

  In particular, in a sensor system used for measuring the concentration of a surfactant, a coloring indicator is added to the surfactant solution of the analyte, and the energy of the evanescent wave generated in the sensor part that leaks light is transferred to the coloring indicator. Thus, it has been proposed to measure the concentration by detecting light transmission loss depending on the concentration of the surfactant solution (see, for example, Patent Document 3).

Japanese Patent No. 3180959 JP 2007-040738 A JP 2009-063538 A

  However, in the conventional optical fiber sensor system, the light leaked in the sensor unit is only energy loss, and the sensor system has a lot of energy loss.

  In particular, in order to increase the sensor sensitivity in the optical fiber sensor system, it is better to leak as much light as possible, and there is a problem that energy loss increases accordingly.

  In view of such a current situation, the present inventors have conducted research and development to configure an optical fiber sensor system with less energy loss by utilizing a phase change known as a Goose Henschen shift. It has come to make.

  According to the refractive index detection method of the present invention, in the refractive index detection method for detecting the refractive index of an object, a waveguide for guiding the diffracted light generated by diffracting incident laser light while totally reflecting it is provided. It is disposed in the subject and detects the refractive index based on the phase change that occurs in the diffracted light due to total reflection.

  In the optical fiber sensor system of the present invention, a projector that emits laser light, a first optical fiber that guides the laser light by connecting one end to the projector, and a second optical fiber sensor that connects the other end of the first optical fiber are connected. A laser beam incident from one optical fiber is diffracted to generate diffracted light and guided while being totally reflected, and one end is connected to the waveguide facing the first optical fiber. An optical fiber sensor system comprising: a second optical fiber; a photoreceiver connected to the other end of the second optical fiber; and an analyzer for analyzing a signal output from the photoreceiver. As the sensor unit, the refractive index of the subject in contact with the waveguide is detected based on the phase change generated in the diffracted light by total reflection at the waveguide.

Furthermore, the optical fiber sensor system of the present invention is also characterized by the following points.
(1) The waveguide is made of the same material as the cores of the first and second optical fibers, and the radial dimension of the waveguide is the same as that of the cores of the first optical fiber and the second optical fiber. It must be larger than the dimensions.
(2) The size of the core of the second optical fiber is larger than the size of the core of the first optical fiber.
(3) The waveguide has a tapered shape in which the radial dimension is gradually enlarged or gradually reduced from the first optical fiber side to the second optical fiber side.
(4) The waveguide has the outer peripheral shape of the end portion on the first optical fiber side matched with the outer peripheral shape of the end portion of the first optical fiber, and the outer periphery of the end portion on the second optical fiber side The shape is matched with the outer peripheral shape of the end of the second optical fiber.
(5) In the waveguide, the radial dimension at the end portion on the first optical fiber side is larger or smaller than the radial dimension of the first optical fiber.
(6) In the waveguide, the radial dimension at the end portion on the second optical fiber side is larger or smaller than the radial dimension of the second optical fiber.
(7) The waveguide has a cross-sectional shape parallel to the radial direction and a shape having symmetry with the center of the cross-section as the rotational symmetry axis.
(8) The waveguide has an elliptical cross-sectional shape parallel to the radial direction.

  According to the refractive index detection method of the present invention, a waveguide that guides the diffracted light generated by diffracting the incident laser light while totally reflecting it is disposed in the subject, and the total reflection is caused. Thus, by detecting the refractive index based on the phase change of the Goose-Henschen shift that occurs in the diffracted light, it is not necessary to leak light, and a detection method with low energy loss can be obtained.

  In addition, according to the optical fiber sensor system of the present invention, the longitudinal waveguide that guides the laser light incident from the first optical fiber while diffracting the light and generating the diffracted light as a total reflection is used as the sensor unit. By detecting the refractive index of the subject in contact with the waveguide based on the phase change of the Goose-Henschen shift caused in the diffracted light due to total reflection at the waveguide, there is no need to leak light and energy loss Of the optical fiber sensor system.

It is explanatory drawing of a Goose Henschen shift. It is a schematic explanatory drawing of the optical fiber sensor system which concerns on this invention. It is explanatory drawing of other embodiment of the waveguide body which is a sensor part. It is explanatory drawing of other embodiment of the waveguide body which is a sensor part. It is explanatory drawing of other embodiment of the waveguide body which is a sensor part. It is explanatory drawing of other embodiment of the waveguide body which is a sensor part. It is explanatory drawing of other embodiment of the waveguide body which is a sensor part. It is the graph which showed the result of having measured the transmitted light quantity with respect to the incident light wavelength in the air in the 1st Example, and underwater. It is the graph which showed the result of having measured the transmitted light quantity with respect to the incident light wavelength in the air in the 2nd Example, and underwater. It is a graph which shows the relationship between the mixing ratio of the mixed solution of ethanol and water, and a refractive index. It is a graph which shows the change of the wavelength used as the largest transmitted light quantity in an optical fiber sensor system with respect to the mixing ratio of the mixed solution of ethanol and water. It is the graph which showed the result of having measured the transmitted light quantity with respect to the incident light wavelength in the air in 4th Example. It is explanatory drawing of the interference phenomenon which this invention utilizes. It is the graph which compared the theoretical value regarding the relationship between the length of a waveguide, and the wavelength of the light which injects into a waveguide, and a measured value.

  The refractive index detection method of the present invention and the optical fiber sensor system using this refractive index detection method do not use transmission loss as in the conventional optical fiber sensor system, but goose generated during total reflection of light. The refractive index of the subject is detected using the phase change of the Henschen shift.

The Goos Shen shift, as shown in FIG. 1, the total reflection at the boundary surface P of the light L in the material A in the refractive index n ₁ is a substance A having a refractive index n ₁ and the substance B having a refractive index n ₂ In this case, a phase delay occurs, and reflection occurs after a slight delay as if it is reflected by the virtual reflection surface P ′ on the side of the substance B having the refractive index n ₂ from the boundary surface. Here, what oozes out toward the substance B having the refractive index n ₂ is an electromagnetic field accompanying the propagation of the light L, and an evanescent wave is generated by this electromagnetic field.

Since the magnitude of the phase change due to the Goose Henschen shift has a correlation with the refractive index n ₂ , in the present invention, the influence of the phase change caused by the Goose Henschen shift is detected using light interference, and the refractive index n ₂ can be specified.

  In particular, in order to effectively cause the Goose Henschen shift, in the present invention, the diffracted light is guided while being totally reflected in the waveguide serving as the sensor unit, and the laser is emitted from the optical fiber connected to the waveguide. When light enters the waveguide, diffraction is generated to obtain diffracted light.

  By arranging the waveguide in the subject, the waveguide and the subject are brought into contact with each other, and a phase change due to the Goose Henschen shift is caused in the diffracted light guided through the waveguide. Here, the subject is generally a liquid or a gas, but may be solid as long as it can be in close contact with the waveguide, or it may be solid only when measuring the refractive index. You may do.

  Hereinafter, an optical fiber sensor system of the present invention will be described in detail with reference to the drawings. FIG. 2 is a schematic diagram of the optical fiber sensor system of the present invention.

  The optical fiber sensor system includes a projector 11 that emits laser light, a first optical fiber 12 that connects one end to the projector 11 to guide the laser light, and a longitudinal waveguide that connects the other end of the first optical fiber 12. Output from the optical receiver 13, the second optical fiber 14 opposite to the first optical fiber 12 and having one end connected to the waveguide 13, the optical receiver 15 connected to the other end of the second optical fiber 14, and the optical receiver 15 And an analyzer 16 for analyzing the received signal.

  The projector 11 is a laser light source that can emit laser light having a predetermined wavelength. The projector 11 may be a laser light source that can emit laser light having a single wavelength, or may be a laser light source that can emit laser light having a wavelength in a predetermined band. When a laser light source that can emit laser light having a predetermined band wavelength is used, a control signal can be input from the analyzer 16 so that the laser light is emitted based on the control of the analyzer 16. It is desirable.

  The first optical fiber 12 is an optical fiber that can guide the laser light emitted from the projector 11, and includes a core 12a, a clad 12b, and an abdominal membrane (not shown). As the core 12a of the first optical fiber 12, it is desirable to use one having a diameter as small as possible so that strong diffraction can be generated in the light incident on the waveguide 13.

  The waveguide 13 is made of a light-transmitting material made of the same material as the core 12a of the first optical fiber 12, and diffracts the laser light incident from the first optical fiber 12 to generate diffracted light and totally reflects the light. It is going to guide while letting. By using the same material for the waveguide 13 and the core 12a of the first optical fiber 12, the laser light incident on the waveguide 13 from the first optical fiber 12 is incident on the waveguide 13 without leaking. be able to. The waveguide 13 is not necessarily made of the same material as the core 12a of the first optical fiber 12, and the laser light incident on the waveguide 13 from the first optical fiber 12 is guided with as little leakage as possible. What is necessary is just to be able to guide into the body 13.

  The waveguide 13 may be a cylindrical body in the simplest case, but is not limited to the cylindrical body, and the shape of the cross section parallel to the radial direction is a shape having symmetry with the center of the cross section as the rotational symmetry axis. Also good. Specifically, a rectangular column having a rectangular or square cross section, a hexagonal column having a hexagonal cross section, an elliptical column having an elliptical cross section, etc. It may be.

  The waveguide 13 is not necessarily limited to a rod having a uniform cross section in the longitudinal direction. As shown in FIG. 3, the waveguide 13 extends from the first optical fiber 12 side to the second optical fiber 14 side. The taper may have a taper shape in which the radial dimension is gradually enlarged, or conversely, as shown in FIG. 4, the taper is obtained by gradually reducing the radial dimension from the first optical fiber 12 side to the second optical fiber 14 side. It is good also as a shape.

  The waveguide 13 is formed separately from the first optical fiber 12 by using the same material as the core 12a of the first optical fiber 12, and the first waveguide 13 is formed by using an existing fusion splicer using discharge. One optical fiber 12 is connected.

  Therefore, in the waveguide 13, as shown in FIGS. 3 and 4, the outer peripheral shape of the end of the waveguide 13 on the first optical fiber 12 side is made to coincide with the outer peripheral shape of the end of the first optical fiber 12. However, in some cases, as shown in FIG. 5, at the end portion of the waveguide 13 on the first optical fiber 12 side, it is possible to perform the connection work with the fusion splicer. The radial dimension may be larger than the radial dimension of the first optical fiber 12, and conversely, as shown in FIG. 6, the radial direction at the end of the waveguide 13 on the first optical fiber 12 side. May be smaller than the diameter of the first optical fiber 12.

  The second optical fiber 14 is an optical fiber that has one end connected to the waveguide 13 so as to face the first optical fiber 12, and guides the diffracted light in the waveguide 13 to the light receiver 15 in the subsequent stage, and the core 14a. And a clad 14b and an abdominal membrane (not shown).

  The core 14a of the second optical fiber 14 is formed of the same transparent material as the waveguide 13 and the core 12a of the first optical fiber 12, and the core 14a of the waveguide 13 and the second optical fiber 14 The diffracted light in the waveguide 13 can be guided to the core 14a of the second optical fiber 14 without causing reflection at the interface. As in the case of the first optical fiber 12, the waveguide 13 and the core 14a of the second optical fiber 14 are not necessarily made of the same material, and light in the waveguide 13 is lost as much as possible. It suffices as long as it can be incident on the core 14a of the second optical fiber 14.

  Here, the light guided from the waveguide 13 to the core 14a of the second optical fiber 14 is interference light. That is, the light guided by the second optical fiber 14 travels straightly through the waveguide 13 from the core 12a of the first optical fiber 12 to the core 14a of the second optical fiber 14, and the waveguide 13 The light is interfered with the diffracted light that has undergone the Goose Henschen shift phase change due to total reflection in the light, and this interference light is guided to the light receiver 15.

  Therefore, if the diameter of the core 14a of the second optical fiber 14 is reduced, the diffracted light that interferes with the straight light is restricted from entering the second optical fiber 14, so that the diffracted light having a small phase shift from the straight light. Only the incident light enters the second optical fiber 14, and a diffracted wave obtained by amplifying a straight wave by superposition of the waves is guided to the light receiver 15.

  On the other hand, when the diameter of the core 14a of the second optical fiber 14 is increased, diffracted light having a large phase shift from the straight light also enters the second optical fiber 14, and the phase shift is large in the wave superposition. Interference light that has undergone relaxation due to the component of the diffracted light is guided to the light receiver 15.

  The influence of the diffracted light having a large phase shift from the straight light can be adjusted by the dimension of the core 14a of the second optical fiber 14, and can be appropriately determined according to the sensitivity required for the sensor unit.

  When the dimension of the core 14a of the second optical fiber 14 is made larger than the dimension of the core 12a of the first optical fiber 12, the dimension of the second optical fiber 14 is made larger than the dimension of the first optical fiber 12. Alternatively, the dimensions of the second optical fiber 14 may be matched with the dimensions of the first optical fiber 12 by adjusting the clad 14 b of the second optical fiber 14.

  The second optical fiber 14 is connected to the waveguide 13 using a fusion splicer, and as shown in FIGS. 3 and 4, the outer periphery of the end of the waveguide 13 on the second optical fiber 14 side. If the shape is made consistent with the outer peripheral shape of the end portion of the second optical fiber 14, the connection work by the fusion splicer can be performed more reliably.

  However, in some cases, as shown in FIG. 5, the radial dimension at the end of the waveguide 13 on the second optical fiber 14 side may be larger than the radial dimension of the second optical fiber 14. Conversely, as shown in FIG. 6, the radial dimension at the end of the waveguide 13 on the second optical fiber 14 side may be smaller than the radial dimension of the second optical fiber 14.

  If necessary, for example, as shown in FIG. 7, the waveguide 13 has a radial dimension at the end on the first optical fiber 12 side larger than the radial dimension of the first optical fiber 12. The radial dimension at the end of the two optical fibers 14 may be smaller than the radial dimension of the second optical fiber 14.

  The light receiver 15 includes a photoelectric conversion element such as a photosensor, and receives light guided by the second optical fiber 14 and outputs a signal corresponding to the intensity of light, that is, the amount of transmitted light.

  The analyzer 16 identifies the amount of transmitted light from the signal output from the light receiver 15, and identifies the refractive index from the amount of transmitted light.

  Here, when the projector 11 is a laser light source that emits laser light having a single wavelength, the refractive index is specified based on the transmitted light amount and refractive index correlation table registered in the analyzer 16 in advance. It is said.

  Further, when the projector 11 is a laser light source that emits laser light having a wavelength in a predetermined band, the transmitted light amount is detected while changing the emitted wavelength, and the wavelength at which the transmitted light amount is maximized is identified and specified. The refractive index is specified based on the correlation table of wavelength and refractive index.

  Further, when there is a physical property correlation table having a correlation with the refractive index, such as sugar content, the physical property value can be specified using the correlation table. That is, a change in the sugar content of the subject can also be detected.

  In particular, the analyzer 16 is composed of a personal computer, and based on the operation program of the optical fiber sensor system, the analyzer 16 functions as a control unit, and the transmitted light amount / refractive index correlation table or the wavelength / refractive index correlation table. Furthermore, by storing a correlation table of physical property values such as refractive index and sugar content in a hard disk in advance, an optical fiber sensor system capable of detecting not only the refractive index but also a predetermined physical property value can be obtained.

  Here, the waveguide 13 serving as the sensor unit is disposed in a gaseous or liquid subject or a solid subject so that the subject is brought into contact with the waveguide 13 so that the waveguide 13 The refractive index or predetermined physical property value of the subject in contact with 13 is detected.

  As a first embodiment, a first optical fiber having a typical core diameter of 8.2 μm and a cladding diameter of 125 μm, and a second optical fiber having the same core diameter as the first optical fiber of a typical value of 8.2 μm and a cladding diameter of 125 μm The sensor part was created by connecting each end of a cylindrical waveguide having a diameter of 125 μm and a length of 100 mm using a fusion splicer.

  FIG. 8 shows the result of measuring the amount of transmitted light with respect to the incident light wavelength in air and water using this sensor unit.

  As shown in FIG. 8, superposition of linear light and diffracted light at a specific wavelength (in FIG. 8, about 921 nm in air (solid line in FIG. 8) and about 925 nm in water (dashed line in FIG. 8)). It can be seen that there is a peak wavelength shift due to the difference in refractive index around the sensor part between air and water.

  As a second embodiment, a first optical fiber having a core diameter of 8.2 μm typical and a cladding diameter of 125 μm and a second optical fiber having a core diameter of about 50 μm and a cladding diameter of 125 μm are used. A sensor part was created by connecting each end of a cylindrical waveguide having a length of 100 mm using a fusion splicer.

  FIG. 9 shows the result of measuring the amount of transmitted light with respect to the incident light wavelength in air and water using this sensor unit.

  As shown in FIG. 9, linear light and diffracted light are superimposed at a specific wavelength (in FIG. 9, about 909 nm in the air (solid line in FIG. 9) and about 913 nm in water (dashed line in FIG. 9)). It can be seen that there is a peak wavelength shift due to the difference in refractive index around the sensor part between air and water.

  Further, as can be seen from the comparison between FIG. 8 and FIG. 9, by increasing the core diameter of the second optical fiber, the rate of change of the light intensity before and after the peak of the transmitted light amount is reduced. It can be seen that the sensitivity of the sensor part can be adjusted by the fiber core diameter.

  The position of the peak in FIGS. 8 and 9 varies depending on the diameter and length of the waveguide, and the diameter and length of the waveguide so that a peak occurs at a predetermined wavelength portion. The shape may be adjusted.

  Further, when attention is paid to the one-dot chain line parallel to the vertical axis displayed near about 915 nm shown in FIG. 9, the light intensity in water is higher than the light intensity in air for light having a wavelength of about 915 nm. Since it is increased by about 7 dB, an optical fiber sensor system using this fluctuation in light intensity can be obtained.

  As a third example, detection of the mixing ratio of a mixed solution of ethanol and water was performed using the sensor unit of the first example.

  As shown in FIG. 10, it is known that the refractive index of the mixed solution of ethanol and water varies depending on the mixing ratio. Using this, the practicality of the optical fiber sensor system of the present invention was verified. .

  As a result of immersing the sensor part in a mixed solution of ethanol and water with each mixing ratio and specifying the wavelength that maximizes the amount of transmitted light while varying the incident light wavelength, as shown in FIG. It was confirmed that there was a correlation between the mixing ratio of the mixed solution and the wavelength at which the amount of transmitted light was maximum. From this, it can be understood that the optical fiber sensor system of the present invention can be used as a sensor of a mixing ratio of at least a mixed solution of ethanol and water.

  As a fourth embodiment, measurement was performed in a wider band than the first and second embodiments described above.

  Here, the sensor unit uses a first optical fiber having a core diameter of about 50 μm and a cladding diameter of 125 μm, and a second optical fiber having the same core diameter as the first optical fiber of about 50 μm and a cladding diameter of 125 μm. It was created by connecting each end of a cylindrical waveguide having a diameter of 125 μm and a length of 98 mm using a fusion splicer.

  The subject was air, the sensor unit was placed in the air, light with a wavelength of 700 to 1700 nm was incident, and the amount of transmitted light with respect to the incident light wavelength was measured.

  FIG. 12 is a graph showing the measurement results, and it can be seen that there are a plurality of peaks.

  The appearance of these peaks will be specifically described with reference to FIG. Here, the diameter dimension of the waveguide is d, the length is L, the incident point of light in the waveguide is S, and the incident point of virtual light to be described later is S ′. In the waveguide, light is incident on the center of the waveguide by the first optical fiber. For convenience of explanation, the center of the waveguide is the incident point S.

  As shown in FIG. 13 (a), the interference between the straight traveling light linearly traveling in the waveguide and the diffracted light that is totally reflected once in the waveguide is the same as the light irradiated from the incident point S and the virtual light. It can be considered as interference of light irradiated from the incident point S ′. Here, the position of the incident point S ′ is a position away from the incident point S by d which is the diameter dimension of the waveguide.

  Similarly, the interference between the straight traveling light linearly traveling in the waveguide and the diffracted light totally reflected twice in the waveguide is the light irradiated from the incident point S as shown in FIG. And interference of light emitted from the virtual incident point S ′. Here, the position of the incident point S ′ is a position away from the incident point S by twice the diameter d of the waveguide.

  Similarly, the interference between the straight traveling light linearly traveling in the waveguide and the diffracted light totally reflected three times in the waveguide is the light irradiated from the incident point S as shown in FIG. And interference of light emitted from the virtual incident point S ′. Here, the position of the incident point S ′ is a position separated from the incident point S by 3 times d which is the diameter dimension of the waveguide.

  As described above, since the diffracted light is totally reflected in the waveguide, it can be considered that there are a plurality of incident points.

  The plurality of peaks described above is a virtual incident point S ′ where light that causes interference that reinforces the linearly traveling light linearly traveling in the waveguide is incident by changing the wavelength of the light incident on the waveguide. This corresponds to the change of the position.

  That is, each peak appears corresponding to each number of times of total reflection in the waveguide, which causes interference that reinforces the linearly traveling light linearly traveling in the waveguide.

When this is applied to Young's experiment, the incident point S and the incident point S ′ correspond to the slit in Young's experiment, and the light is strengthened when light of wavelength λ is incident on the waveguide. The condition that results in a bright line is that the wavelength of light in the refractive index n is λ / n = λ ′.
x = m'Lλ '/ d'
It becomes.

  Here, m ′ is an arbitrary integer, d ′ is an interval between the incident point S and the incident point S ′, and x is a virtual image where an interference fringe is projected from an intermediate point between the incident point S and the incident point S ′. It is the distance from the position of the foot of the perpendicular when the perpendicular is lowered to the screen.

In this case, if the bright line interval is Δx,
Δx = (m ′ + 1) Lλ ′ / d′−m′Lλ ′ / d ′ = Lλ ′ / d ′
It is.

  On the other hand, in a waveguide that causes total reflection of diffracted light, the peripheral surface that causes total reflection is a mirror surface, and as described above, there are a plurality of incident points. The exit point that is the connection portion with the second fiber is also affected by the mirror surface, and can be regarded as having a plurality of exit points.

  Moreover, the distance between each of the plurality of emission points is d which is the diameter of the waveguide due to the mirror surface condition, and if Δx is exactly d, the bright lines of the interference fringes Will be output. Here, for convenience of explanation, let D be the period of the interference fringes. Therefore, D = d.

From this, the bright line interval Δx is the period of the interference fringes, and Δx = D.
D = Lλ ′ / d ′ = L (λ / n) / d ′
It becomes.

Furthermore, since d ′ is an integral multiple of d, which is the radial dimension of the waveguide, it can be expressed as d ′ = md, where m is an arbitrary integer.
λ = (Dmd / L) × n
The following relational expression is obtained.

here,
λ: wavelength D: period of interference fringes m: arbitrary integer corresponding to the order of the mirror image (1, 2, 3,...)
d: Diameter dimension of the waveguide L: Length dimension of the waveguide n: Refractive index of the waveguide.

  From the above equation, it can be seen that when the shape of the waveguide is specified, the wavelength of each peak appearing in FIG. 12 can be specified. In FIG. 12, the symbol “◯ to 4” indicates that the peak corresponds to m = 4, and the symbol “◯ to 5” indicates that the peak corresponds to m = 5. The sign “6” indicates that the peak corresponds to m = 6, and the sign “◯ = 7” indicates that the peak corresponds to m = 7.

  Accordingly, when the length of the waveguide was varied and the wavelength at which the peak of the transmitted light amount occurred with respect to the incident light wavelength was measured, as shown in FIG. 14, the peak was observed at the wavelength corresponding to the above equation. It was.

  The solid line on the lower right in FIG. 14 is a theoretical curve with λ = (Dmd / L) × n at each value of m. The white circles indicate the measured peaks corresponding to m = 1, and the white squares indicate Actual measurement peak corresponding to the case of m = 2, white diamond mark is the actual measurement peak corresponding to the case of m = 3, cross mark is the actual measurement peak corresponding to the case of m = 4, plus mark is m = 5 An actual measurement peak corresponding to the case, a white triangle mark indicates an actual measurement peak corresponding to m = 6, and a black circle mark indicates an actual measurement peak corresponding to m = 7.

  As shown in FIG. 12, the peaks appearing corresponding to each value of m do not have the same shape before and after the peak, and have various shapes.

  When a waveguide is used as a sensor body, it is desirable to have high sensitivity, and therefore, it is considered that a sensor having a large change rate before and after the peak is preferable as the sensor body.

  Therefore, in the case of FIG. 12, a highly sensitive sensor body can be provided by using a waveguide having a shape of m = 4. In this case, the wavelength of the laser beam to be used is also specified.

  When m is large, the number of total reflections increases and the number of external interactions increases, which is advantageous for high sensitivity. However, if m is too large, interference may be blurred due to the bending of the sensor fiber or slight asymmetry, and m = 4 is optimal. In this embodiment, since the waveguide is cylindrical, m = 4 seems to be suitable. However, if the waveguide has a shape other than the cylinder, the case of m = 4 is always preferable. However, it may be a different value of m.

  The refractive index detection method of the present invention and the optical fiber sensor system using this refractive index detection method can detect the refractive index of a subject in real time, for example, change in sugar content of a material solution during the process of producing a soft drink Can be monitored in real time.

  Moreover, it can be used as a sensor system that detects not only the sugar content but also a physical property value having a correlation with a refractive index, such as a mixing ratio in a mixed solution of ethanol and water.

11 Floodlight
12 First optical fiber
12a core
12b cladding
13 Waveguide
14 Second optical fiber
14a core
14b cladding
15 Receiver
16 analyzer

Claims (10)

In the refractive index detection method for detecting the refractive index of the subject,
A waveguide that guides the diffracted light generated by diffracting the incident laser light while totally reflecting it is arranged in the subject, and the refractive index is based on the phase change that occurs in the diffracted light due to total reflection. Refractive index detection method. A projector that emits laser light;
A first optical fiber for guiding the laser beam by connecting one end to the projector;
A longitudinal waveguide for connecting the other end of the first optical fiber to diffract the laser light incident from the first optical fiber to generate diffracted light and to guide it while totally reflecting;
A second optical fiber having one end connected to the waveguide facing the first optical fiber;
A receiver connected to the other end of the second optical fiber;
An optical fiber sensor system comprising an analyzer for analyzing a signal output from the light receiver,
An optical fiber sensor system that uses the waveguide as a sensor unit and detects a refractive index of a subject in contact with the waveguide based on a phase change generated in diffracted light by total reflection at the waveguide.   The waveguide is made of the same material as the cores of the first and second optical fibers, and the radial dimension of the waveguide is the same as that of the cores of the first optical fiber and the second optical fiber. The optical fiber sensor system according to claim 2, wherein the optical fiber sensor system is larger than a dimension of the core.   The optical fiber sensor system according to claim 2 or 3, wherein a dimension of the core of the second optical fiber is larger than a dimension of the core of the first optical fiber.   5. The waveguide according to claim 2, wherein the waveguide has a tapered shape in which a radial dimension is gradually enlarged or gradually reduced from the first optical fiber side to the second optical fiber side. 6. Fiber optic sensor system.   In the waveguide, the outer peripheral shape of the end portion on the first optical fiber side is matched with the outer peripheral shape of the end portion of the first optical fiber, and the end portion on the second optical fiber side is matched. The optical fiber sensor system according to any one of claims 2 to 5, wherein an outer peripheral shape is matched with an outer peripheral shape of an end portion of the second optical fiber.   6. The waveguide according to claim 2, wherein a radial dimension at an end portion on the first optical fiber side of the waveguide is larger or smaller than a radial dimension of the first optical fiber. An optical fiber sensor system according to claim 1.   6. The waveguide according to any one of claims 2 to 5, wherein a radial dimension at an end portion on the second optical fiber side is larger or smaller than a radial dimension of the second optical fiber. An optical fiber sensor system according to claim 1.   9. The optical fiber sensor system according to claim 2, wherein the waveguide has a shape having a cross section parallel to the radial direction and a shape having a rotational symmetry axis at the center of the cross section.   The optical fiber sensor system according to claim 2, wherein the waveguide has an elliptical cross-sectional shape parallel to the radial direction.

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