JP2013253953A - Refractive index sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refractive index sensor which hardly suffers physical damage and is applicable for measuring a refractive index of a liquid, gas and solid.SOLUTION: There is provided a refractive index sensor which comprises: a light source 2; an optical bandpass filter 3 in which an object to be measured is in contact with a dielectric multilayer film 8 formed on one end surface 7A of an optical fiber 7 and light output from the light source 2 is incident via the optical fiber 7; a measurement unit 4 for measuring the spectrum of light reflected by the object to be measured; and a refractive index calculation unit for calculating the refractive index of reflected light on the basis of the center wavelength of the spectrum of the reflected light, wherein the dielectric multilayer film 8 has a reflective multilayer film obtained by laminating a plurality of high refractive index layers and a plurality of low refractive index layers on both sides of a cavity layer disposed approximately on the center. The combined optical thickness of the high refractive index layers and the low refractive index layers is λ0/2 when the center wavelength of the light output from the light source 2 is defined as λ0, and the optical thickness of each of the layers is different from λ0/4, respectively.

Description

  The present invention relates to a refractive index sensor that measures the refractive index of an object to be measured.

  Refractive index sensors are importantly used in various industrial fields such as biochemistry, chemistry, and environmental analysis. Among refractive index sensors, a refractive index sensor using an optical fiber (optical fiber refractive index sensor) is cheaper than other sensors, and can be miniaturized, remotely controlled, possibility of multiplexing, convenience of design, electromagnetic It has the advantage of being less susceptible to the influence of the world, and many studies have been made in recent years.

  Conventional optical fiber refractive index sensors include, for example, fiber Bragg grating (FBG), long-period grating (LPG), interferometers (Mach-Zehnder, Michelson, Fabry-Perot, etc.), taper, etc. It is classified into a technique using processing (for example, see Patent Document 1).

  However, an optical fiber refractive index sensor using a grating such as FBG or LPG has high sensitivity, but high spatial resolution cannot be obtained. In addition, an optical fiber refractive index sensor using an interferometer and a taper process has high sensitivity and high spatial resolution, but the sensing head is likely to be physically damaged. In addition, an optical fiber refractive index sensor using an interferometer and a taper process can be applied only to the measurement of the refractive index of a liquid or gas, and the measurement system is complicated and expensive.

JP 2004-191110 A

  The present invention has been proposed in view of such a situation, the sensing head is not easily damaged physically, can be applied to the measurement of the refractive index of liquid, gas and solid, and the measurement system is simple and inexpensive. An object is to provide a refractive index sensor.

  The present inventors have found that the center wavelength of the spectrum of the reflected light reflected by the object to be measured is incident on the optical band-pass filter is linearly shifted by the refractive index of the object to be measured. It came to do.

  That is, the refractive index sensor according to the present invention includes a light source unit that outputs light, a dielectric multilayer film formed on one end surface of the optical transmission line, and one end in contact with the optical transmission line on the other end side. An optical bandpass filter in which an object to be measured having a predetermined refractive index is in contact with a dielectric multilayer film, and light output from a light source unit is incident through an optical transmission path, and an optical bandpass filter The refractive index calculation that calculates the refractive index of the reflected light based on the center wavelength of the spectrum of the reflected light measured by the measurement unit and the measurement unit that measures the spectrum of the reflected light reflected by the object to be measured The dielectric multilayer film includes a cavity layer disposed substantially at the center, a plurality of high refractive index layers on both sides of the cavity layer, and a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer. Reflective multilayer film with high refractive index layer and low refraction The optical thickness combined with the layers is λ0 / 2 when the central wavelength of the light output from the light source unit is λ0, and the optical thickness of each of the high refractive index layer and the low refractive index layer is Are different from λ0 / 4.

  The refractive index measurement method according to the present invention includes an output step of outputting light, and a dielectric multilayer film is formed on one end surface of the optical transmission line, and one end contacting the optical transmission line is a dielectric on the other end side. An incident step for allowing the light output in the output step to enter the optical band-pass filter in contact with the object to be measured having a predetermined refractive index on the multilayer film, and the optical in the incident step. The refractive index of the reflected light is calculated based on the measurement step of measuring the spectrum of the reflected light reflected by the object to be measured and the center wavelength of the spectrum of the reflected light measured in the measuring step. The dielectric multilayer film includes a cavity layer disposed substantially at the center, and a plurality of high refractive index layers and a plurality of refractive indexes smaller than the high refractive index layers on both sides of the cavity layer. Low refractive index layer The optical thickness of the combined high-refractive index layer and low-refractive index layer is λ0 / 2 when the center wavelength of the light output from the light source unit is λ0. The optical thickness of each of the high refractive index layer and the low refractive index layer is a value different from λ0 / 4.

  Further, in the refractive index sensor according to the present invention, a dielectric multilayer film is formed on one end surface of the optical transmission line, and one end in contact with the optical transmission line is on the dielectric multilayer film on the other end side. An optical bandpass filter in contact with an object to be measured having a predetermined refractive index, a light source unit that outputs light having a wavelength in the vicinity of the center wavelength of the optical bandpass filter and makes it incident through the optical transmission line, and the optical band Indicated by the detection unit that converts the optical power of the reflected light that is reflected by the object to be measured and returned by the measured object into an electric signal and detects the electric signal obtained as a detection output by the detection unit A refractive index calculation unit that calculates a refractive index of the reflected light based on the optical power of the reflected light, and the dielectric multilayer film is formed on a cavity layer disposed substantially in the center and on both sides of the cavity layer. Multiple high refraction And a reflective multilayer film in which a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer are stacked, and the optical thickness of the high refractive index layer and the low refractive index layer combined is Λ0 / 2 when the center wavelength of the light output from the light source unit is λ0, and the optical thicknesses of the high refractive index layer and the low refractive index layer are λ0 / 4, respectively. It is a different value.

  In the refractive index measurement method according to the present invention, a dielectric multilayer film is formed on one end face of an optical transmission line, and the one end in contact with the optical transmission line is on a dielectric multilayer film on the other end side. An incident step in which light having a wavelength near the center wavelength of an optical bandpass filter that is in contact with an object to be measured having a refractive index is incident through the optical transmission path; and the light incident on the optical bandpass filter is the object to be measured A detection step of converting and detecting the optical power of the reflected light reflected and returned by the electric signal, and the optical power of the reflected light indicated by the electric signal obtained as a detection output by the detection step A refractive index calculating step for calculating a refractive index of the reflected light, and the dielectric multilayer film includes a cavity layer disposed substantially at the center, a plurality of high refractive index layers and the high refractive index layers on both sides of the cavity layer. refraction A reflective multilayer film in which a plurality of low refractive index layers having a refractive index smaller than that of the layer is laminated, and the optical thickness of the high refractive index layer and the low refractive index layer combined is output from the light source unit. Λ0 / 2 when the center wavelength of the light is λ0, and the optical thicknesses of the high refractive index layer and the low refractive index layer are different from λ0 / 4.

  In the present invention, the thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured is different depending on the thickness of the other high refractive index layer and the low refractive index layer. Can do.

  In the present invention, the cavity layer and the low refractive index layer are made of SiO 2 and have a thickness of 225 nm. The high refractive index layer is made of TiO 2 and has a thickness of 225 nm. The thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured shall be 1.7 times or more than the thickness of the other high refractive index layer and the low refractive index layer. Can do.

  Furthermore, in the present invention, the total of the cavity layer, the high refractive index layer, and the low refractive index layer in the dielectric multilayer film may be 25 layers or more.

  ADVANTAGE OF THE INVENTION According to this invention, a sensing head is hard to receive a physical damage, can apply to the measurement of the refractive index of a liquid, gas, and solid, and can provide a refractive index sensor with a simple and cheap measurement system.

It is a block diagram which shows the structural example of the refractive index sensor to which this invention is applied. It is a figure which shows typically the structural example of an optical band pass filter. It is sectional drawing which shows the structural example of an optical band pass filter. It is a figure which shows the electron micrograph of an optical band pass filter. It is a figure for demonstrating the measurement principle by the refractive index sensor to which this invention is applied. It is a graph which shows the simulation result of the refractive index dependence of the spectrum of reflected light. It is a graph which shows the simulation result of the refractive index dependence of the center wavelength of the spectrum of reflected light. It is a graph of the experimental result which shows the refractive index dependence of the spectrum of reflected light when a liquid is used for a to-be-measured object. It is a graph which shows the experimental result and simulation result of the refractive index dependence of the center wavelength of the spectrum of reflected light. It is a graph which shows the experimental result of the refractive index dependence of the spectrum of reflected light when using a solid for a to-be-measured object. It is a graph which shows the experimental result of the thickness dependence of the cavity layer with respect to a sensitivity. It is a graph which shows the thickness dependence of the cavity layer with respect to the center wavelength of a reflected light spectrum. It is a graph which shows the experimental result of the thickness dependence of the high refractive index layer and low refractive index layer with respect to a sensitivity. It is a graph which shows the experimental result of the thickness dependence of the high refractive index layer and the low refractive index layer with respect to the center wavelength of a reflected light spectrum. It is a graph which shows the simulation result of the thickness dependence of the uppermost layer of the dielectric multilayer film with respect to a sensitivity. It is a block diagram which shows the other structural example of the refractive index sensor to which this invention is applied. The center wavelength of the spectrum of the reflected light is shifted to the longer wavelength side when water (n = 1.33) is contacted as the object 29 to be measured from the state where the optical bandpass filter is in contact with air (n = 1). It is a graph which shows a mode that it does. When water (n = 1.33) is contacted as the object to be measured 29 from a state where air (n = 1) is in contact with the optical bandpass filter, a wavelength in the vicinity of the center wavelength of the optical bandpass filter It is a graph which shows the result of having converted the reflected light power of this into voltage, and observing on a time-axis. It is a figure which shows the wavelength which can detect the shift of the center wavelength in an optical band pass filter as a linear change of optical power. Depending on the detection unit when liquid (water, ethanol, three kinds of refractive index matching oils) having various refractive indexes (n) is brought into contact as an object to be measured from the state where the optical bandpass filter is in contact with air It is a graph which shows the result of having observed the time change of the voltage of the electric signal obtained as a detection output. It is a graph which shows the result of having plotted the voltage when a refractive index changes and is stabilized with respect to the refractive index. The vertical axis is normalized assuming that the optical reflectance when the fiber end is in contact with air is 100%, and the sensitivity of the optical fiber type refractive index sensor using an optical bandpass filter is expressed as a single mode optical fiber (SMF). It is a graph showing the result of comparing the refractive index dependence of the light reflectance with the sensitivity of a simple refractive index sensor using the intensity change of Fresnel reflection at the fiber end. It is a graph which shows the result of having compared the sensitivity by differentiating with a refractive index and taking an absolute value. It is a block diagram which shows the structural example of the ultrasonic detection apparatus to which the said optical fiber type refractive index sensor is applied. It is a graph which shows the detection result of the ultrasonic wave by the said ultrasonic detection apparatus. It is a graph which shows the result of having plotted distance with respect to time delay about the detection result of the ultrasonic wave by the said ultrasonic detection apparatus. It is a graph which shows the result of having plotted signal strength Vp-p with respect to distance about the detection result of the ultrasonic wave by the said ultrasonic detection apparatus.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this specification, “thickness” refers to a physical thickness, that is, an actual thickness of a layer (film) of interest. In this specification, “optical thickness” refers to a value obtained by multiplying the physical thickness by the refractive index of the film at the wavelength of the light, assuming that light of a certain wavelength passes through the film. .

[First Embodiment]
The present invention is applied to a refractive index sensor 1 configured as shown in the block diagram of FIG. 1, for example. The refractive index sensor 1 includes a light source unit 2, an optical bandpass filter 3, a measurement unit 4, and a refractive index calculation unit 5. The refractive index sensor 1 according to the embodiment of the first invention shown in FIG. 1 has a spectrum of reflected light obtained by reflecting the light incident on the optical bandpass filter 3 by the object to be measured 9 as will be described in detail later. The measurement principle is that the center wavelength of the signal shifts linearly with the refractive index of the object 9 to be measured.

  The light source unit 2 is composed of, for example, a broadband light source that emits broadband light, and outputs light as shown in FIG. The light source unit 2 can be composed of, for example, an ASE (Amplified Spontaneous Emission) light source that emits amplified spontaneous emission light.

  FIG. 2 is a diagram schematically illustrating a configuration example of the optical bandpass filter 3. In the optical bandpass filter 3, for example, as shown in FIGS. 1 and 2, a dielectric multilayer film 8 is formed on one end surface 7A of an optical fiber 7 by vapor deposition. Further, the optical bandpass filter 3 has a measured object made of a liquid, gas or solid having a predetermined refractive index on the dielectric multilayer film 8 on the side of the other end 8B from the one end 8A in contact with the optical fiber 7. 9 touches. The optical fiber 7 has a two-layer structure of a fiber core 7a and a clad 7b that covers the periphery of the fiber core 7a. Light output from the light source unit 2 is incident on the optical bandpass filter 3 through the optical fiber 7. The optical bandpass filter 3 can measure the refractive index of the DUT 9 in contact with the dielectric multilayer film 8 because the center wavelength of the structure shifts substantially linearly with respect to the refractive index. This is due to a change in Fresnel reflection that occurs at the contact surface between the dielectric multilayer film 8 and the DUT 9.

  FIG. 3 is a cross-sectional view showing a configuration example of the optical bandpass filter 3. FIG. 4 is a view showing an electron micrograph of the optical bandpass filter 3. As shown in FIGS. 3 and 4, the dielectric multilayer film 8 includes a cavity layer 10 disposed substantially at the center, and a plurality of high refractive index layers 11 and high refractive indexes on both sides in the thickness direction of the cavity layer 10. A reflective multilayer film in which a plurality of low refractive index layers 12 having a refractive index smaller than that of the layer 11 are alternately laminated. The dielectric multilayer film 8 functions as a band-pass filter that transmits only light of a specific wavelength. The total optical thickness of the high refractive index layer 11 and the low refractive index layer 12 is λ0 / 2 when the center wavelength of the light output from the light source unit 2 is λ0. The optical thicknesses of the high refractive index layer 11 and the low refractive index layer 12 are different from λ0 / 4. By setting the optical thickness of each layer of the high refractive index layer 11 and the low refractive index layer 12 to a value different from λ0 / 4, up to the end of the optical fiber (one end surface 7A) where the optical bandpass filter 3 exists. It can prevent that the quantity of the reflected light reflected by the to-be-measured object 9 reaches significantly decreases. Thereby, it can prevent that a refractive index sensitivity falls.

  The reflective multilayer film is, for example, silicon, amorphous silicon, hydrogenated amorphous silicon, silicon dioxide, tantalum pentoxide, alumina, titanium dioxide, zirconium dioxide, hafnium dioxide, lanthanum dioxide, cerium dioxide, antimony trioxide, indium trioxide, oxidation Oxide such as magnesium and sodium dioxide, silicon oxynitride, silicon nitride, aluminum nitride, zirconium nitride, hafnium nitride, lanthanum nitride, etc., or magnesium fluoride, cerium trifluoride, It is composed of fluorides such as calcium fluoride, lithium monofluoride, and trisodium aluminum hexafluoride.

  In the dielectric multilayer film 8, the high refractive index layer or the low refractive index layer in contact with the object to be measured 9, that is, the thickness of the uppermost layer 13 is such that the high refractive index layer 11 other than the uppermost layer 13 and the low refractive index. It is preferable that the thickness varies depending on the thickness of the layer 12. By making the thickness of the top layer 13 different depending on the thicknesses of the high refractive index layer 11 and the low refractive index layer 12 other than the top layer 13, the refractive index sensitivity can be made positive or negative, and zero. It has a positive maximum and a negative maximum. Therefore, the refractive index sensitivity of the refractive index sensor 1 can be controlled by changing the thickness of the uppermost layer 13.

  For example, when the cavity layer 10 and the low refractive index layer 12 are made of SiO 2 having a thickness of 225 nm and the high refractive index layer 11 is made of TiO 2 having a thickness of 225 nm, the high refractive index in contact with the object 9 to be measured. The thickness of the layer 11 or the low refractive index layer 12 is preferably 1.7 times or more than the thicknesses of the other high refractive index layers 11 and low refractive index layers 12. By setting such a condition, the refractive index sensitivity of the refractive index sensor 1 can be controlled and the refractive index sensitivity can be set to a positive maximum, so that the refractive index sensitivity can be improved. Also, under such conditions, the dielectric multilayer film 8 includes the cavity layer 10, the high refractive index layer 11, and the low refractive index layer 12 from the viewpoint of improving the refractive index sensitivity of the refractive index sensor 1. The total is preferably 25 layers or more.

  The dielectric multilayer film 8 can be manufactured using a thin film forming apparatus by selecting two types of materials having different refractive indexes, that is, a high refractive index layer 11 and a low refractive index layer 12. In order to stack such thin film materials to form a multilayer film, various forming apparatuses and forming methods can be used. Among them, the sputtering method (sputtering method) is preferable because there is no need to use a high-risk gas or a toxic gas, and the surface roughness of the deposited film is relatively good.

  As shown in FIGS. 3 and 4, the dielectric multilayer film 8 is formed by forming a dielectric multilayer film 8 on one end of a FIX (Fiber In Axis) stub 14 (stubbed dielectric multilayer film 8). Is preferably used. By using the dielectric multilayer film 8 thus stubbed, the handling of the sensor head in the optical bandpass filter 3 can be facilitated, and the management of the optical bandpass filter 3 can be facilitated. Can do. When the stub dielectric multilayer film 8 is used, the stub dielectric multilayer film 8 and the optical fiber 7 are connected to each other by butting in the sleeve, and fixing them with an adhesive having a refractive index matching. Good.

  The cavity layer 10 is a layer at a predetermined position in the reflective multilayer film in which the high refractive index layers 11 and the low refractive index layers 12 are alternately stacked. For example, the cavity layer 10 includes the high refractive index layer 11 or the low refractive index layer 12 and Consists of the same material. The optical thickness of the cavity layer 10 is preferably λ0 / 2 or more when the center wavelength of the light output from the light source unit 2 is λ0.

  The light source unit 2, the optical bandpass filter 3, and the measurement unit 4 are connected by an optical fiber 7 through a circulator 6. The circulator 6 causes the light output from the light source unit 2 to enter the dielectric multilayer film 8 of the optical bandpass filter 3. Further, the circulator 6 causes the reflected light, which is the light incident on the optical bandpass filter 3 and reflected by the object 9 to be measured, to enter the measuring unit 4.

  The measurement unit 4 is configured by, for example, an optical spectrum analyzer (OSA), and receives the reflected light reflected by the object 9 to be measured, and measures the spectrum of the reflected light as shown in FIG. . In the measurement unit 4, for example, the measurement object 9 having a first refractive index (n1) and the measurement object having a second refractive index (n2) larger than the first refractive index as the measurement object 9. The reflected light reflected at 9 is measured.

  FIG. 5 is a diagram for explaining the principle of measurement by the refractive index sensor 1. As shown in FIG. 5, the center wavelength λ2 of the spectrum of the reflected light reflected by the measurement object 9 having the second refractive index is equal to the reflected light reflected by the measurement object 9 having the first refractive index. It shifts to the long wavelength side by Δλ from the center wavelength λ1 of the spectrum. Further, the center wavelength of the spectrum of the reflected light reflected by the device under test 9 changes linearly depending on the refractive index of the device under test 9.

  The refractive index calculation unit 5 is composed of, for example, a personal computer. The refractive index calculation unit 5 calculates the refractive index of the reflected light based on the center wavelength of the spectrum of the reflected light measured by the measurement unit 4. As described above, since the center wavelength of the spectrum of the reflected light is linearly shifted with respect to the refractive index, the refractive index calculation unit 5 can measure the refractive index of the object 9 to be measured. For example, the refractive index calculation unit 5 uses a plurality of measured objects 9 having a known refractive index to calculate a graph of the refractive index dependence of the central wavelength of the spectrum of reflected light, and based on this graph The refractive index of the reflected light is calculated from the center wavelength of the spectrum of the reflected light measured by the measuring unit 4.

  As described above, the refractive index sensor 1 includes the optical bandpass filter 3 in which the dielectric multilayer film 8 is formed on the one end surface 7A of the optical fiber 7, and the one end 8A in contact with the optical fiber 7 An object to be measured 9 is in contact with the dielectric multilayer film 8 on the other end 8B side. As described above, the optical bandpass filter 3 has the dielectric multilayer film 8 formed at the tip end portion of the fiber 7, so that the sensing head is hardly subject to physical damage. The refractive index can be measured.

  In the refractive index sensor 1, for example, an optical bandpass filter (BOF: Band-pass filter On Fiber-end) 3 can be formed by vapor-depositing a dielectric multilayer film 8 on one end surface 7 A of the optical fiber 7. it can. Therefore, the same optical bandpass filter 3 can be mass-produced, and the measurement system of the refractive index sensor 1 can be made simple and inexpensive.

  Furthermore, the refractive index sensor 1 can obtain a high spatial resolution because the optical bandpass filter 3 does not have a relatively wide detection region unlike FBG and LPG.

  In the above description, the light source unit 2 is configured by a broadband light source that emits broadband light. However, the present invention is not limited to this example. For example, a wavelength sweep laser light source may be used as the light source unit 2, and the center wavelength of the spectrum of reflected light may be measured in the measurement unit 4. Further, the light source unit 2 may output two wavelengths of light, and the measurement unit 4 may measure the ratio of the two wavelengths in the spectrum of the reflected light.

  In the above description, the optical fiber 7 is used. However, the present invention is not limited to this example, and another optical transmission line into which the laser beam output from the light source unit 2 is incident may be used. For example, a plate-like or sheet-like optical waveguide may be used instead of the optical fiber 7. Such an optical transmission line can be composed of, for example, an inorganic material or an organic material. Examples of inorganic materials include quartz glass and silicon, and examples of organic materials include high-purity polyimide resins, polyamide resins, and polyether resins.

  In the above description, the refractive index sensitivity of the refractive index sensor 1 is controlled by changing the thickness of the uppermost layer 13 in the optical bandpass filter 3, but the present invention is limited to this example. is not. For example, the refractive index sensitivity of the refractive index sensor 1 can be controlled by changing the number of layers of the dielectric multilayer film 8 and the thickness of the cavity layer 10, the high refractive index layer 11, or the low refractive index layer 12. It is.

  In the above description, the dielectric multilayer film 8 has a reflective multilayer film in which a plurality of high refractive index layers 11 and a plurality of low refractive index layers 12 are alternately stacked on both sides in the thickness direction of the cavity layer 10. However, the present invention is not limited to this example. For example, the reflective multilayer film may have a structure in which a plurality of high refractive index layers 11 and a plurality of low refractive index layers 12 are laminated in any order on both sides of the cavity layer 10.

  Hereinafter, specific examples of the present invention will be described. The scope of the present invention is not limited to the following examples. In this example, the operation as a refractive index sensor was confirmed by experiments and simulations.

  A cross-sectional view of a model of the optical bandpass filter 3 used in the simulation is shown in FIG. In this model, a cavity layer 10 (thickness: 925 nm) is disposed substantially at the center of the dielectric multilayer film 8 constituting the optical bandpass filter 3. Further, on both sides of the cavity layer 10, a high refractive index layer (TiO2 layer: refractive index (n) = 2.22, thickness: 215 nm) 11 and a low refractive index layer (SiO2 layer: refractive index (n) = 1. 46, thickness: 215 nm) 12 are alternately arranged. The total optical thickness of the high refractive index layer 11 and the low refractive index layer 12 is λ0 / 2 when the center wavelength of the light output from the light source unit 2 is λ0. The optical thicknesses of the high refractive index layer 11 and the low refractive index layer 12 are different from λ0 / 4. The thickness of the high refractive index layer 11 in contact with the DUT 9, that is, the uppermost layer (TiO 2) 13 is 450 nm.

  FIG. 6 is a graph showing a simulation result of the refractive index dependence of the spectrum of reflected light. FIG. 7 is a graph showing a simulation result of the refractive index dependence of the center wavelength of the spectrum of reflected light. The simulation result shown in FIG. 6 uses the characteristic matrix method. As shown in FIGS. 6 and 7, the refractive index n of the object 9 to be measured is n = 1.0 (a in FIG. 6), n = 1.3 (b in FIG. 6), n = 1.6 (FIG. 6) and n = 1.9 (d in FIG. 6), the center wavelength of the spectrum of the reflected light reflected by the DUT 9 becomes a dependency coefficient +0.6893 nm / RIU (refractive index). (unit) was found to shift to the longer wavelength side.

  In the experimental system, the refractive index sensor 1 shown in FIG. 1 was used. The light source unit 2 was used as an amplified spontaneous emission (ASE) light source as a broadband light source. The measurement unit 4 used an optical spectrum analyzer (OSA). The refractive index calculation unit 5 used a personal computer. As the object 9 to be measured, air (n to 1.00), six types of liquid, and two types of solid were used. Examples of liquids include water (n to 1.33), ethanol (n to 1.36), 30% brine (n to 1.38), matching oil (I) (n to 1.44), and matching oil (II ) (N to 1.60) and matching oil (III) (n to 1.74) were used. As the solid, rubber (n to 1.50) and plastic (n to 1.65) were used.

  FIG. 8 is a graph of experimental results showing the refractive index dependence of the spectrum of reflected light when a liquid is used for the DUT 9. In FIG. 8, (a) is air, (b) is water, (c) is ethanol, (d) is 30% salt water, (e) is matching oil (I), (f) is matching oil (II), (G) is a result of matching oil (III). From the results shown in FIG. 8, it was found that the center wavelength of the spectrum of the reflected light shifts to the longer wavelength side as the refractive index increases.

  FIG. 9 is a graph showing experimental results and simulation results of the refractive index dependence of the center wavelength of the spectrum of reflected light. In FIG. 9, (Exp .: y = 0.6118x + 1542.2) is a result of the experimental system, and (Sim .: y = 0.6893x + 1542) is a simulation result. It was found that the dependence coefficient (+0.6118 nm / RIU) of the experimental value was in good agreement with the dependence coefficient (+0.6893 nm / RIU) obtained by simulation.

  FIG. 10 is a graph showing an experimental result of the refractive index dependence of the spectrum of reflected light when a solid is used for the object 9 to be measured in the optical bandpass filter 3. In FIG. 10, (a) is the result of air, (b) is the result of rubber, and (c) is the result of plastic. When the result shown in FIG. 10 is applied to the result shown in FIG. 9 (Exp .: y = 0.6118x + 1542.2) and the refractive index of rubber and plastic is inversely calculated, the refractive index of rubber is n = 1. .49, the refractive index of the plastic was n = 1.65. This result was found to be in good agreement with the original value.

  FIG. 11 is a graph showing experimental results of the dependency of the cavity layer 10 on the sensitivity. The thickness of the cavity layer 10 was changed in the range of 0.8 to 1.2 μm. As shown in FIG. 11, it was found that the sensitivity of the center wavelength of the spectrum of the reflected light with respect to the refractive index increases in proportion to the thickness of the cavity layer 10 (y = 3.7987x−2.8893).

  FIG. 12 is a graph showing the thickness dependence of the cavity layer 10 with respect to the center wavelength of the reflected light spectrum. The thickness of the cavity layer 10 was changed in the range of 0.8 to 1.2 μm. As shown in FIG. 12, it was found that the center wavelength of the spectrum of the reflected light increases in proportion to the thickness of the cavity layer 10 (y = 681x + 872.8). Moreover, since the upper limit of the bandwidth of amplified spontaneous emission (ASE) used in this example is about 1600 nm, it is preferable that the thickness of the cavity layer 10 is 1.0 μm from the result shown in FIG. I understood.

  FIG. 13 is a graph showing experimental results of the thickness dependence of the high refractive index layer 11 and the low refractive index layer 12 with respect to sensitivity. The thicknesses of the high refractive index layer 11 and the low refractive index layer 12 were changed in the range of 0.20 to 0.24 μm. As shown in FIG. 13, it was found that the sensitivity of the center wavelength of the spectrum of the reflected light with respect to the refractive index increases in proportion to the thickness of the high refractive index layer 11 and the low refractive index layer 12 (y = 15. 489x-2.3987).

  FIG. 14 is a graph showing experimental results of the thickness dependence of the high refractive index layer 11 and the low refractive index layer 12 with respect to the center wavelength of the reflected light spectrum. The thicknesses of the high refractive index layer 11 and the low refractive index layer 12 were both changed in the range of 0.20 to 0.24 μm. As shown in FIG. 14, it was found that the center wavelength of the spectrum of the reflected light increases in proportion to the thicknesses of the high refractive index layer 11 and the low refractive index layer 12. Further, as described above, the upper limit of the bandwidth of amplified spontaneous emission (ASE) used in this example is about 1600 nm. Therefore, from the results shown in FIG. 14, the high refractive index layer 11 and the low refractive index layer 12 are used. It was found that the thickness of the film was preferably 0.225 μm.

  FIG. 15 is a graph showing a simulation result of the thickness dependence of the uppermost layer 13 of the dielectric multilayer film 8 with respect to sensitivity. The influence on the refractive index sensitivity was confirmed while reducing the thickness of the uppermost layer 13 from 0.45 μm to 0.17 μm. It was confirmed that the refractive index sensitivity of the optical bandpass filter 3 vibrates as shown in FIG. 15 with respect to the thickness of the uppermost layer 13. That is, it was found that the period of vibration of the refractive index sensitivity with respect to the thickness of the uppermost layer 13 corresponds to about twice the thickness of the high refractive index layer 11 and the low refractive index layer 12.

  Further, the influence of the refractive index sensitivity accompanying the thickness of the cavity layer 10 was confirmed. By changing the thickness of the cavity layer 10 to 1.0 μm (symbol (♦)) and 0.95 μm (symbol (□)) and changing the thickness of the uppermost layer 13 under the conditions, the influence on the refractive index sensitivity It was confirmed. As a result, as shown in FIG. 15, it was confirmed that the maximum value of the refractive index sensitivity was changed by the change in the thickness of the cavity layer 10. Further, when the thickness of the uppermost layer 13 is 1.7 times (about 0.4 μm) with respect to the thicknesses of the other high refractive index layers 11 and low refractive index layers 12 (0.225 μm), refraction is performed. It was found that the rate sensitivity was a positive maximum. By arranging these simulation results, it was found that the refractive index sensitivity can be improved with the change in the structure of the optical bandpass filter 3 such as the thickness of the uppermost layer 13 and the cavity layer 10. .

  As in the embodiment of the first invention described above, an optical bandpass filter (BOF: Band-pass filter On Fiber-end) 3 constructed by depositing the dielectric multilayer film 8 on the tip of the optical fiber 7 is used. The optical fiber type refractive index sensor 1 is based on the principle that the center wavelength of the spectrum of the reflected light reflected from the object 9 is linearly shifted by the refractive index of the object 9 to be measured. By detecting the center wavelength of the optical spectrum with the measurement unit 4 using an optical spectrum analyzer (OSA), a high spatial resolution of several meters can be obtained, and a robust and simple system can be constructed. It is possible to measure not only gas / liquid but also solid refractive index. The optical fiber type refractive index sensor 1 can also be used as a pressure sensor with extremely low temperature dependence.

  Here, as in the case of the optical fiber type refractive index sensor 1, if the center wavelength of the reflected light spectrum is detected by an optical spectrum analyzer (OSA), high-speed operation is difficult due to the limit of the wavelength sweep speed. However, as in the second embodiment of the present invention described below, the information on the center wavelength is converted into voltage using the slope of the reflected light spectrum, and the operation is speeded up by detecting in the electrical domain. can do.

[Second Embodiment]
That is, the present invention is applied to a refractive index sensor 21 having a configuration as shown in the block diagram of FIG. 16, for example. The refractive index sensor 21 includes a light source unit 22, an optical bandpass filter 23, a measurement unit 24, and a refractive index calculation unit 25.

  In the refractive index sensor 21 according to the embodiment of the second invention shown in FIG. 16, the light incident on the optical bandpass filter 23 is reflected by the object to be measured 29 as in the embodiment of the first invention. The measurement principle is that the center wavelength of the spectrum of the reflected light is linearly shifted by the refractive index of the object to be measured 29. In the measurement unit 24, information on the center wavelength is obtained by using the slope of the reflected light spectrum. The voltage is converted into a voltage by the detection unit 24A, and is observed and detected on the time axis by the observation unit 24B in the electrical region.

  In the second embodiment of the present invention, the light source unit 22 uses a wavelength-tunable laser diode (TLD) and can be set or adjusted to a required wavelength.

  The optical fiber type refractive index sensor 21 in the second embodiment of the invention includes a dielectric multilayer film 28 at the tip of an optical fiber 27 having the same structure as that of the optical fiber type refractive index sensor 1 in the embodiment of the first invention. An optical bandpass filter (BOF: Band-pass filter On Fiber-end) 23 is used.

  In the optical band-pass filter 23 having the same structure as the optical band-pass filter 3 shown in FIG. 3 described above, the center wavelength depends on the refractive index of the substance in contact as shown in FIG. Accordingly, since the center wavelength of the spectrum of the reflected light is shifted to the longer wavelength side, the refractive index of the substance brought into contact with the optical bandpass filter 23 is detected by detecting the reflected light power at a wavelength near the center wavelength. Can be measured.

  For example, when water (n = 1.33) is brought into contact with the optical bandpass filter 23 from the state in which air (n = 1) is in contact with the object 29 to be measured, as shown in FIG. Since the center wavelength of the spectrum shifts to the long wavelength side, as shown in FIG. 18, the refractive index changes by converting the reflected light power of a wavelength near the center wavelength into a voltage and observing it on the time axis. Can be observed as a change in the voltage, and the refractive index of the substance brought into contact with the optical bandpass filter 23 can be measured.

  In this optical fiber type refractive index sensor 21, a laser beam having a wavelength of 1543.5 nm in the vicinity of the center wavelength of the optical bandpass filter 23 is emitted from a light source unit 22 using a tunable laser diode (TLD). Is emitted.

  Here, the wavelength of the laser light emitted from the light source unit 22 is such that air, water, ethanol, 30% salt water, matching oil, and matching oil are brought into contact with the optical bandpass filter 3 in the first embodiment of the present invention. 8 shows the refractive index dependence of the spectrum of the reflected light when the optical band pass filter is used, and the shift of the center wavelength in the optical bandpass filter 23 having the same structure as the optical bandpass filter 3 as shown in FIG. The wavelength that can be detected as a linear change was determined to be 1543.5 nm. The incident light power was −7 dBm.

  The light source unit 22, the optical bandpass filter 23, and the measurement unit 24 are connected by an optical fiber 27 through a circulator 26. Then, the circulator 26 causes the laser light having a wavelength of 1543.5 nm output from the light source unit 22 to enter the dielectric multilayer film 28 of the optical bandpass filter 23. Further, the circulator 26 causes the reflected light obtained by reflecting the laser light having a wavelength of 1543.5 nm incident on the optical bandpass filter 23 and reflected by the measurement object 29 to enter the detection unit 24 </ b> A of the measurement unit 24.

  The measurement unit 24 includes a detection unit 24A and an observation unit 24B.

  The detection unit 24A includes a photodetector that converts the light power of the reflected light into an electric signal indicated by a voltage by photoelectrically converting the light reflected by the measurement object 29 incident through the circulator 26. The electrical signal obtained by the detection unit 24A is input to the observation unit 24B.

  The observation unit 24B includes an oscilloscope for observing a waveform on the time axis of an electric signal indicating the optical power of the reflected light obtained as a detection output by the detection unit 24A as a voltage.

  Furthermore, the refractive index calculation unit 25 is configured by, for example, a personal computer. The refractive index calculation unit 25 is indicated by, for example, the voltage of the electric signal observed by the observation unit 24A with reference to the graph of the refractive index dependence of the center wavelength of the spectrum of the reflected light as shown in FIG. The refractive index of the measurement object 29 is calculated from the optical power of the reflected light.

  In this optical fiber type refractive index sensor 21, a dielectric multilayer film 28 is formed on one end face of an optical fiber 27, and one end in contact with the optical transmission path is on the dielectric multilayer film 28 on the other end side. Then, a laser beam having a wavelength near 1543.5 nm in the vicinity of the center wavelength of the optical bandpass filter 23 in contact with the measured object 29 having a predetermined refractive index is incident from the light source unit 22 through the optical fiber 27, and the optical bandpass The laser light incident on the filter 23 is detected by converting the optical power of the reflected light that is reflected by the measured object 29 and returning to an electrical signal by the detection unit 24 and obtained as a detection output by the detection unit 24. Based on the optical power of the reflected light indicated by the electric signal, the refractive index calculating unit 25 calculates the refractive index of the reflected light.

  That is, in the optical fiber type refractive index sensor 21, the center wavelength of the spectrum of the reflected light obtained by reflecting the laser beam having a wavelength of 1543.5 nm incident on the optical bandpass filter 23 by the measurement object 29 is the measurement object 29. The measurement principle is to shift linearly according to the refractive index of the light, and the information of the center wavelength is converted into voltage by the detector 23 using the slope of the reflected light spectrum, and observed on the time axis by the observation unit 24 in the electrical domain. Thus, the change in the refractive index can be detected at high speed.

  Here, in this optical fiber type refractive index sensor 21, liquids having various refractive indexes (n) (water, ethanol, three types) from the state where the optical bandpass filter 23 is in contact with air as the object 29 to be measured. FIG. 20 shows a result of observing the time change of the voltage of the electric signal obtained as the detection output by the detection unit 24A when the refractive index matching oil) is brought into contact with the observation unit 24B. Further, FIG. 21 shows a result of plotting the voltage when the refractive index is changed and stabilized with respect to the refractive index. Both are in a substantially linear relationship.

  Further, the sensitivity of the optical fiber type refractive index sensor 21 using the optical bandpass filter 23 is the same as that of a simple refractive index sensor using a change in intensity of Fresnel reflection at the end of a single mode optical fiber (SMF). The results compared with the sensitivity are shown in FIG. 22 and FIG. FIG. 22 shows the result of normalizing the vertical axis with the light reflectance when the fiber end is in contact with air being 100% and comparing the refractive index dependence of the light reflectance. Further, FIG. 23 shows the result of comparing the sensitivity by differentiating this by the refractive index and taking the absolute value. While the sensitivity of the Fresnel sensor varies greatly depending on the refractive index of the measurement object, the sensitivity of the optical bandpass filter 23 is constant and can be said to be easy to handle. In particular, in the vicinity of 1.46 which is the refractive index of silica, only this optical bandpass filter 23 has practical sensitivity.

[Application example]
Here, the optical fiber type refractive index sensor 21 can be applied to the measurement of the refractive index of liquid, gas and solid, but it can measure the refractive index at high speed. For example, the ultrasonic wave shown in FIG. Like the detection apparatus 100, it can also be used to detect a burst signal of underwater ultrasonic waves.

  The ultrasonic detection apparatus 100 shown in FIG. 24 detects the burst signal of the underwater ultrasonic wave output from the ultrasonic unit (PZT) 110, and the optical fiber type refractive index sensor in the above-described second embodiment of the invention. 21 is used to submerge the tip of the optical band-pass filter 23 of the optical fiber type refractive index sensor 21 in the water 129 and detect ultrasonic waves in water as a change in the refractive index of the water 129. It is a thing.

  The ultrasonic detection apparatus 100 measurement system uses the optical fiber type refractive index sensor 21 in the above-described second embodiment, and the same components as the optical fiber type refractive index sensor 21 are shown in FIG. Are denoted by the same reference numerals, and detailed description thereof is omitted.

  In this ultrasonic detector 100, the wavelength of the laser beam emitted from the light source unit 22 is set to 1533.8 nm, the optical output power is set to 6.0 dBm, and 1.56 MHz underwater ultrasonic waves are detected. The result was obtained. As a result, the detected signal is delayed in time as the distance between the tip of the optical bandpass filter 23 and the ultrasonic unit (PZT) 110 increases. FIG. 26 shows the result of plotting the distance against this time delay. The slope of this line is equivalent to the general underwater sound velocity, which can be said to be evidence of successful detection of underwater ultrasonic waves. In FIG. 26, the signal intensity is weakened due to attenuation of the ultrasonic waves. The result of plotting the signal intensity Vp-p against the distance is shown in FIG.

  Note that the relationship between sound pressure and refractive index change has been studied for a long time. For underwater ultrasound, for example, the Eykman equation is used (MFJ Eykman, Rec. Tray. Chim. 14, 177-194 (1895) and L Adler and EA Hiedemann, J. Acoust. Soc. Amer. 34, 410 (1962), etc.). When the sound speed is c, the refractive index when there is no sound field is n0, the density is ρ, and the refractive index change Δn when the sound pressure is p is expressed as a function of the sound pressure as shown in the following equation (1).

DESCRIPTION OF SYMBOLS 1,21 Refractive index sensor, 2,22 Light source part, 3,23 Optical band pass filter, 4,24 Measurement part, 5,25 Refractive index calculation part, 6 Circulator, 7 Optical fiber, 8 Dielectric multilayer film, 9 Cover Object to be measured, 10 cavity layer, 11 high refractive index layer, 12 low refractive index layer, 13 top layer, 14 stub, 24A detection unit, 24B observation unit

Claims (10)

A light source unit that outputs light;
A dielectric multilayer film is formed on one end surface of the optical transmission line, and an object to be measured having a predetermined refractive index is in contact with the one end in contact with the optical transmission line on the dielectric multilayer film on the other end side. An optical bandpass filter to which light output from the light source unit is incident via the optical transmission path;
A measuring unit for measuring a spectrum of reflected light reflected by the object to be measured, which is incident on the optical bandpass filter;
A refractive index calculating unit that calculates the refractive index of the reflected light based on the center wavelength of the spectrum of the reflected light measured by the measuring unit;
The dielectric multilayer film is
A cavity layer disposed substantially in the center;
A reflective multilayer film in which a plurality of high refractive index layers and a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer are laminated on both sides of the cavity layer,
The combined optical thickness of the high refractive index layer and the low refractive index layer is λ0 / 2 when the center wavelength of the light output from the light source unit is λ0,
A refractive index sensor in which the optical thickness of each of the high refractive index layer and the low refractive index layer is different from λ0 / 4.   2. The refractive index sensor according to claim 1, wherein a thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured is different depending on thicknesses of the other high refractive index layers and the low refractive index layers. . The cavity layer and the low refractive index layer are made of SiO 2 and have a thickness of 225 nm.
The high refractive index layer is made of TiO 2 and has a thickness of 225 nm.
The thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured is 1.7 times or more than the thickness of the other high refractive index layer and the low refractive index layer. The refractive index sensor according to 2.   4. The refractive index sensor according to claim 3, wherein a total of the cavity layer, the high refractive index layer, and the low refractive index layer in the dielectric multilayer film is 25 or more. An output step for outputting light;
A dielectric multilayer film is formed on one end surface of the optical transmission line, and an object to be measured having a predetermined refractive index is in contact with the one end in contact with the optical transmission line on the dielectric multilayer film on the other end side. An incident step for causing the light output in the output step to enter the optical bandpass filter through the optical transmission path;
A measurement step of measuring the spectrum of the reflected light reflected by the object to be measured by the light incident on the optical bandpass filter in the incident step;
A refractive index calculating step for calculating the refractive index of the reflected light based on the center wavelength of the spectrum of the reflected light measured in the measuring step;
The dielectric multilayer film is
A cavity layer disposed substantially in the center;
A reflective multilayer film in which a plurality of high refractive index layers and a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer are laminated on both sides of the cavity layer,
The combined optical thickness of the high refractive index layer and the low refractive index layer is λ0 / 2 when the center wavelength of the light output from the light source unit is λ0,
The refractive index measurement method, wherein the optical thickness of each of the high refractive index layer and the low refractive index layer is different from λ0 / 4. A dielectric multilayer film is formed on one end surface of the optical transmission line, and an object to be measured having a predetermined refractive index is in contact with the one end in contact with the optical transmission line on the dielectric multilayer film on the other end side. An optical bandpass filter;
A light source unit that outputs light having a wavelength in the vicinity of the center wavelength of the optical bandpass filter and makes the light incident through the optical transmission path;
A detector that detects the light incident on the optical band-pass filter by converting the optical power of the reflected light that is reflected back from the object to be measured into an electrical signal;
A refractive index calculation unit that calculates a refractive index of the reflected light based on the optical power of the reflected light indicated by the electrical signal obtained as a detection output by the detection unit;
The dielectric multilayer film is
A cavity layer disposed substantially in the center;
A reflective multilayer film in which a plurality of high refractive index layers and a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer are laminated on both sides of the cavity layer,
The combined optical thickness of the high refractive index layer and the low refractive index layer is λ0 / 2 when the center wavelength of the light output from the light source unit is λ0,
A refractive index sensor in which the optical thickness of each of the high refractive index layer and the low refractive index layer is different from λ0 / 4.   The refractive index sensor according to claim 6, wherein a thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured is different depending on thicknesses of the other high refractive index layers and the low refractive index layers. . The cavity layer and the low refractive index layer are made of SiO 2 and have a thickness of 225 nm.
The high refractive index layer is made of TiO 2 and has a thickness of 225 nm.
The thickness of the high refractive index layer or the low refractive index layer in contact with the object to be measured is 1.7 times or more than the thickness of the other high refractive index layer and the low refractive index layer. The refractive index sensor according to 7.   The refractive index sensor according to claim 8, wherein a total of the cavity layer, the high refractive index layer, and the low refractive index layer in the dielectric multilayer film is 25 or more. A dielectric multilayer film is formed on one end surface of the optical transmission line, and an object to be measured having a predetermined refractive index is in contact with the one end in contact with the optical transmission line on the dielectric multilayer film on the other end side. An incident step of allowing light having a wavelength near the center wavelength of the optical bandpass filter to be incident through the optical transmission path;
A detection step of detecting the light incident on the optical bandpass filter by converting the optical power of the reflected light reflected by the object to be measured and returning to the electrical signal;
A refractive index calculation step of calculating a refractive index of the reflected light based on the optical power of the reflected light indicated by the electrical signal obtained as a detection output by the detection step;
The dielectric multilayer film is
A cavity layer disposed substantially in the center;
A reflective multilayer film in which a plurality of high refractive index layers and a plurality of low refractive index layers having a refractive index smaller than that of the high refractive index layer are laminated on both sides of the cavity layer,
The combined optical thickness of the high refractive index layer and the low refractive index layer is λ0 / 2 when the center wavelength of the light output from the light source unit is λ0,
The refractive index measurement method, wherein the optical thickness of each of the high refractive index layer and the low refractive index layer is different from λ0 / 4.