JP2008224424A - Method of detecting minute displacement and minute displacement gauge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of highly precisely detecting minute displacement and a minute displacement gauge which is small, low-cost, and with high sensitivity, even when the measurement object is a low reflecting or light-absorbing material. <P>SOLUTION: The optical coupler constituted such that the transparent dielectric substance 2 is in close contact with the metal membrane 1 so that the measurement incident light generates the metal intensifying evanescent light on the interface of the metal membrane 1 is arranged to the measurement object 5 via the slit layer 6. The reflection detection light from the optical coupler depending on the minute distance between the metallic membrane 1 and the measurement object 5 generated by the optical mutual reaction between the metal intensifying evanescent light generated on the interface of the gap layer 6 side and the approaching measurement object 5 is measured by the optical detector 4, and thereby the minute displacement of the measurement object 5 is detected. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for detecting a minute displacement of a measurement object with high sensitivity, and a small and inexpensive high-sensitivity minute displacement meter that detects the minute displacement of the measurement object using the method.

  In order to optically detect the displacement of the measurement object in a non-contact manner, as is well known, based on the principles of optical interference such as Doppler shift and light heterodyne interference, the measurement object is irradiated with light from the measurement object. The interference effect of reflected light is utilized. (For example, Patent Documents 1 and 2 etc.)

  However, in the case of these displacement detection methods, the measurement light is directly applied to the measurement object and the reflected light is detected. Therefore, if the measurement object has a high light reflectance such as metal, the displacement detection is performed. However, for low-reflectivity materials and light-absorbing materials such as glass, ceramics, plastics, polymer polymers, and pigments, the above method is used as a target for displacement detection. There is a problem that it is unsuitable.

  In addition, since the interference effect of the reflected light from the measurement object is used to detect the displacement, there are factors that greatly affect the reflectance, that is, the surface shape and surface condition in addition to the material of the measurement object. There is also a problem that the sensitivity of the displacement detection remarkably depends.

  When the measurement object is a transparent object, the measurement light passes through the transparent object, and the light reflected from the opposite interface is also detected simultaneously with the reflected light to be detected from the measurement object surface. Therefore, depending on the conditions of incident light, accurate displacement detection may be difficult with the above method.

  Furthermore, since the reference light is made to interfere with the reflected light from the measurement object, and the displacement of the measurement object is detected from the intensity change and phase change of the interference fringes, the optical system and the measurement device become complicated, and the above In the case of the method, it is difficult to reduce the size and cost of the micro displacement meter.

On the other hand, in an optical coupler made of a transparent dielectric with a thin metal thin film formed in close contact, when light that has passed through the transparent dielectric is incident on the metal thin film at an appropriate incident angle in the internal total reflection region, the transparent dielectric A phenomenon was observed in which metal-enhanced evanescent light with an electric field increased exponentially with respect to the distance from the metal thin film interface on the opposite side of the body with an increased electric field than normal propagating light was generated.
JP 2000-186912 A JP 2006-3290 A

  Therefore, an optical coupler made of a transparent dielectric with a metal thin film formed in close contact, and an object to be measured placed on the metal thin film via an appropriate gap layer such as an air layer, and metal enhancement at the gap layer side interface of the metal thin film. In an incident light condition that generates evanescent light, light is incident on the metal thin film from the transparent dielectric, and the metal-enhanced evanescent light optically interacts with the measurement object close to the metal thin film, thereby causing the light from the optical coupler. The reflected light changes depending on the minute distance between the metal thin film and the measurement object, and this may possibly realize a minute displacement meter that can detect the minute displacement of the measurement object with high sensitivity.

  Here, when the gap layer medium is air and the distance between the metal thin film and the measurement object is about five times or more as large as the incident light wavelength, the metal enhanced evanescent generated at the gap layer side interface of the metal thin film. The optical interaction between the light and the measurement object is remarkably reduced, so that it becomes difficult to obtain reflected light depending on the position of the measurement object, that is, the distance between the metal thin film and the measurement object.

  In view of such circumstances, the present invention is capable of optically interacting with an object to be measured in close proximity to metal-enhanced evanescent light generated at the interface on the gap layer side of the metal thin film by an optical coupler in which the metal thin film is closely formed. In addition, an optical coupler is arranged at a position close to the measurement object, and due to the optical interaction, the reflected light from the optical coupler changes depending on the minute distance between the metal thin film and the measurement object, and the measurement object An object of the present invention is to provide a method for detecting a minute displacement of a measurement object with high sensitivity by measuring reflected light depending on the position of the sensor, and a small and inexpensive highly sensitive minute displacement meter using the method.

  The present inventors, in an optical coupler made of a transparent dielectric having a metal thin film formed in close contact, reflect light from the optical coupler under incident light conditions that generate metal enhanced evanescent light at the air gap layer side interface of the metal thin film. However, the present invention has been completed by discovering that it varies depending on the minute distance between the metal thin film and the measurement object.

  In other words, the invention described in claim 1 is an optical coupler made of a transparent dielectric formed in close contact with a metal thin film, and an object to be measured is disposed on the metal thin film via an appropriate gap layer, and the gap between the metal thin films is obtained. Under incident light conditions that generate metal-enhanced evanescent light at the layer-side interface, incident measurement light is incident on a metal thin film from a transparent dielectric, and this metal-enhanced evanescent light optically interacts with a nearby measurement object. The reflected detection light from the optical coupler depends on the minute distance from the gap layer side interface of the metal thin film to the object to be measured. By measuring the reflected detection light with the photodetector, the minute displacement of the object to be measured can be reduced. It is intended to be detected.

  According to a second aspect of the present invention, there is provided an optical coupler in which a metal thin film is formed in close contact with a transparent dielectric, wherein a measurement object is disposed on the metal thin film through a gap layer, and the transparent dielectric Has a function of guiding incident measurement light so that metal-enhanced evanescent light is generated at the gap layer side interface of the metal thin film, and this metal-enhanced evanescent light optically interacts with the object to be measured in close proximity, and the gap between the metal thin films It is configured to detect the minute displacement of the measurement object by measuring the reflected detection light from the optical coupler depending on the minute distance from the layer side interface to the measurement object with the photodetector. .

  Further, the invention described in claim 3 is that the polarization of the incident measurement light for generating the metal enhanced evanescent light is TM (p) polarized light, and thereby the metal enhanced evanescent light is applied to the gap layer side interface of the metal thin film. It can be generated efficiently, and the minute displacement of the measurement object is detected with high sensitivity.

  In the invention according to claim 4, the minute displacement of the measurement object is a minute position change in the vertical direction with respect to the metal thin film, and the reflected detection light from the optical coupler is remarkably changed by the perpendicular minute displacement. Therefore, the minute displacement of the measurement object can be detected with high sensitivity.

  Further, in the invention according to claim 5, the gap layer medium is air, and the minute distance between the metal thin film and the measurement object is set to about 5 times or less of the incident measurement light wavelength. The metal-enhanced evanescent light generated at the air gap layer side interface can be optically interacted with the measurement object, so that reflected detection light depending on the minute displacement of the measurement object can be obtained. It is a thing.

  Further, in the invention described in claim 6, the metal thin film is, for example, a silver thin film, the transparent dielectric is, for example, a glass prism, the incident measurement light is, for example, laser light, and the photodetector is, for example, a photodiode. In this configuration, a minute displacement of the measurement object close to the metal thin film is detected by measuring the reflected detection light from the optical coupler.

  According to the present invention, the measurement object is low because it uses the optical interaction between the metal-enhanced evanescent light localized at the interface and the measurement object, which is greatly different from the displacement detection method using the normal light propagating. Even a reflective material or a light absorbing material can detect the minute displacement with high sensitivity, and a small and inexpensive highly sensitive minute displacement meter can be realized using this.

  The method for detecting minute displacement and the minute displacement meter according to the present invention include an optical coupler in which a metal thin film is formed in close contact with a transparent dielectric having a refractive index larger than that of the gap layer medium. Metal-enhanced evanescent light is generated at the gap layer side interface of the metal thin film at an appropriate incident angle equal to or greater than the internal total reflection critical angle.

The atmosphere medium in which the measurement object is slightly displaced forms the gap layer. There is no other limitation as long as the refractive index is smaller than that of the transparent dielectric and the viscosity is less than that of the non-absorbing medium. Can occur.
Preferable atmospheric media include air, vacuum, or gases such as nitrogen, argon, and helium. Further, if the liquid is in a transparent liquid having a relatively low refractive index such as water or alcohol, it is possible to detect a minute displacement of the measurement object even in the atmosphere medium.

  The transparent dielectric constituting the optical coupler is not particularly limited as long as it is a non-absorbing substance having a refractive index somewhat higher than that of the gap layer. For example, BK7, glass materials such as SF11 and LaSF9, and transparent plastic materials. Or a transparent polymer can be used, and is selected according to the properties of the target optical material. A preferable transparent dielectric of an optical coupler having a relatively large refractive index and being inexpensive and easy to process includes glass, particularly BK7 glass.

  The shape of the transparent dielectric material that guides the incident measurement light to the metal thin film is not particularly limited. Any shape can be used as long as incident light can be introduced into the metal thin film at an angle, and the shape of the transparent dielectric is selected in accordance with the optical axes of incident measurement light and reflected detection light.

As the metal thin film, a metal material having a high free electron density that easily generates metal-enhanced evanescent light, such as gold, silver, copper, aluminum, indium, magnesium, or an alloy thereof can be used. Silver and gold are particularly preferable metal materials that are likely to generate metal-enhanced evanescent light.
Silver is the most suitable metal material for generating metal-enhanced evanescent light in the wavelength range from near infrared to ultraviolet including visible light than other metals, and has a wide range from near infrared to ultraviolet. Applicable to incident measurement light in the wavelength range.
Gold is a metal material that can stably generate metal-enhanced evanescent light because there is no surface modification such as an oxide film, and has a wavelength range from ultraviolet to blue visible light where a strong absorption band unique to gold material exists. Otherwise, it applies to the present invention.
On the other hand, copper and aluminum are metal thin film materials that can be used in the present invention. Like gold, copper can be used in a wavelength range excluding from ultraviolet rays exhibiting strong light absorption to blue visible light, and aluminum can be used in a wavelength range other than near-infrared rays having strong absorption.

  In order for incident measurement light to generate metal-enhanced evanescent light at the gap layer side interface of the metal thin film in the optical coupler, the metal thin film needs to have an appropriate thickness. For example, when the wavelength of the incident measurement light is in the visible light region, a film thickness of about 50 nm is optimal for silver and gold, and about 15 nm for aluminum with strong light absorption.

  The incident measurement light used in the micro displacement meter of the present invention is preferably monochromatic light with high directivity such as laser light. Even white light can be used as incident measurement light, but when light is incident on the optical coupler at a fixed incident angle, light with a wavelength that does not meet the conditions for generating metal-enhanced evanescent light is displaced by the object being measured. Because it may be reflected as it is without being influenced by the incident light and incident on the photodetector together with the reflected detection light depending on the minute displacement of the measurement object, it may cause noise in the detection light. This is not desirable.

  In addition, in order to measure the minute displacement of the measurement object by measuring the reflected detection light from the optical coupler, a device with stable output of the emitted light is used as the light source of the incident measurement light. As a preferable light source, a light source whose output of emitted light is controlled, for example, a semiconductor laser with stable driving, can be cited.

  Furthermore, although the incident measurement light can be applied to the micro displacement detection method and micro displacement meter of the present invention even if it is non-polarized light, the incident light of TM (p) polarization efficiently generates metal enhanced evanescent light. It is appropriate that the incident measurement light used in is TM (p) polarized light.

  On the other hand, when the incident measurement light is TE (s) polarized light, it is difficult to generate metal enhanced evanescent light on the smooth interface of the metal thin film. It is not suitable as measurement light used in the invention.

The electric field of the metal-enhanced evanescent light generated by the optical coupler at the gap layer side interface of the metal thin film has an intensity that decreases exponentially from the metal thin film interface, and the leaching length is transparent dielectric, metal thin film And the refractive index of the gap layer medium, the thickness of the metal thin film, and the wavelength and incident angle of incident light.
For example, when the gap layer is an air layer, the electric field oozes out to the air side region of about 5 times or less of the incident measurement light wavelength. Therefore, in order to detect a minute displacement of the measurement object, it is appropriate that the distance between the metal thin film and the measurement object is about 5 times or less of the incident measurement light wavelength.

  On the other hand, when the distance between the metal thin film and the measurement object is about 5 times or more of the incident measurement light wavelength, the electric field strength of the metal-enhanced evanescent light is significantly reduced. The action becomes very small, so that the reflected light from the optical coupler is hardly affected by the measurement object. Therefore, in this case, it is difficult to detect a minute displacement of the measurement object.

  As the photodetector for measuring the reflected detection light depending on the minute displacement of the measurement object, one having excellent sensitivity and responsiveness is preferable. In particular, a semiconductor optical sensor such as a pn junction type or a photoconductive type is desirable. In consideration of the wavelength and output of the incident measurement light, the response speed of the displacement to be detected, etc., a photodetector suitable for measuring the reflected detection light is selected. For example, when incident measurement light having a certain output such as laser light is used, a pn junction photodiode is suitable as a photodetector.

  In addition, in the case of the air gap layer, the leaching length of the enhanced electric field of the metal-enhanced evanescent light is about 5 times the incident measurement light wavelength. From the air gap layer interface to about 5 times the incident measurement light wavelength. Therefore, in order to expand the range of minute displacement that can be detected, incident measurement light having a long wavelength is used.

  The minute displacement that can be detected by the present invention is not limited to the direction perpendicular to the metal thin film as long as it is within the region where the electric field of the metal enhanced evanescent light generated on the metal thin film of the optical coupler exists. Even if it is a displacement, it is possible to detect the minute displacement.

  However, since the intensity of the metal-enhanced electric field changes most greatly with respect to the distance in the vertical direction from the metal thin film, the reflected measurement light of the incident measurement light that generates the metal-enhanced evanescent light from the optical coupler is The change due to the minute displacement in the vertical direction is the largest with respect to the metal thin film, and therefore the detection sensitivity for the minute displacement in the vertical direction is the highest.

The method for detecting minute displacement and the minute displacement meter according to the present invention use a transparent dielectric having a high refractive index to generate metal-enhanced evanescent light at the gap layer side interface of a metal thin film, and leached into the gap layer. The optical enhancement electric field interacts optically with the measurement object, and the reflected light from the optical coupler, which depends on the position of the measurement object, is measured to obtain a minute displacement of the measurement object. And
Therefore, the measurement object does not need to be a highly reflective material, such as a metal, in order to function as an object that optically interacts with the metal-enhanced electric field and disturbs the enhanced electric field. Any substance can be used as long as it is different from the layer, and the minute displacement can be detected without depending on the material of the measurement object.

  The material of the measurement object may be, of course, metal, ceramics, glass, plastic, polymer polymer, pigment, paper material, wood, etc., or a composite material thereof. It is applicable to a wide range of measuring objects such as light absorbing materials, transparent materials, etc.

  Further, the shape of the measurement object is not particularly limited, and the displacement detection surface of the measurement object facing the irradiation surface of the incident measurement light does not have to be particularly flat, and has a curved surface shape having a curvature, for example, a hemispherical shape. , Square or needle-like.

  Furthermore, the displacement detection surface of the measurement object may have surface roughness, dirt, etc., and is a target for displacement detection in any state.

  EXAMPLES Next, although an Example demonstrates this invention further based on drawing, the following specific examples do not limit this invention.

  FIG. 1 is a cross-sectional view of an embodiment of a micro displacement meter according to the present invention. Reference numeral 1 is a metal thin film, for example, a silver thin film having a thickness of about 50 nm, 2 is a transparent dielectric, for example, BK7 glass has a 45-degree right triangle prism with a side of 15 mm, and 3 is a light source of incident measurement light, for example, a beam with a wavelength of 0.532 μm Laser diode-pumped solid-state laser having a diameter of about 1 mm, 4 is a photodetector for detecting reflected light from the optical coupler, for example, a silicon pn junction photodiode, and 5 is a measurement object, for example, a light-absorbing dye film A stainless steel rod having a diameter of about 1.8 mm and whose surface is covered and which is finely displaced by driving a piezoelectric element. Reference numeral 6 denotes an air layer which is a gap layer between the metal thin film 1 and the measurement object 5, for example.

  The 45-degree right triangle prism 2 of BK7 glass functions as a transparent dielectric that guides the incident measurement light from the light source 3 to the silver thin film 1 at a required incident angle, and the incident measurement light is on the air gap layer 6 side of the silver thin film 1. It plays the role of generating metal enhanced evanescent light on the interface. That is, the transparent dielectric prism 2 with the silver thin film 1 formed in close contact constitutes an optical coupler. BK7 glass has a refractive index of about 1.52 in the visible light region, and the critical angle of total internal reflection with air is about 41.2 degrees. Above this angle, an appropriate incident angle at which metal-enhanced evanescent light is generated Then, incident measurement light is made incident on the silver thin film 1.

  Even if the thickness of the silver thin film 1 is about half thinner than the thickness of the previous item, or about twice as thick, it is difficult to generate metal-enhanced evanescent light.

  The stainless steel rod is slightly displaced by driving a piezoelectric element attached thereto, and serves as a measurement object.

  The flat surface portion at the tip of the rod, which is the displacement detection surface of the measurement object 5, is coated with a light-absorbing dye, for example, copper phthalocyanine dye with a thickness of about 1.5 μm by a vacuum deposition method. When visible light, particularly green to red light is irradiated, the light is absorbed by the dye layer, and light reflection at the rod tip is extremely small. Therefore, this rod functions as a measurement object of the light absorbing medium.

  An optical coupler is disposed at a position where the enhanced electric field of the metal-enhanced evanescent light generated on the interface on the gap layer 6 side of the metal thin film 1 can optically interact with the measurement object 5, Since the minute distance between the metal thin film 1 and the measurement object 5 changes due to the minute displacement of the measurement object 5 in the vertical direction, the reflected light from the optical coupler of the incident measurement light that generates the metal enhanced evanescent light is reflected. Change. Therefore, this reflected detection light responds sensitively to a minute displacement of the measurement object.

  For example, TM (p) polarized laser light as incident measurement light is incident on the interface of the silver thin film 1 on the air gap layer 6 side through an optical coupler at an incident angle at which metal-enhanced evanescent light is generated, and the piezoelectric element is driven. Then, the reflected detection light from the optical coupler when the measurement object 5 is made slightly close to the metal thin film 1 in a vertical direction from a position sufficiently away from the interface of the metal thin film 1 on the air gap layer 6 side is the photodetector 4. FIG. 2 shows an example of the result of measurement in (1).

FIG. 2 shows the intensity of the reflected detection light when the measuring object 5 is made to approach the metal thin film 1 in the vertical direction from a position at a proximity distance of about 3.6 μm from the interface on the air gap layer side of the metal thin film 1. It shows a change.
As the measurement object 5 approaches the metal thin film, the reflected detection light increases almost linearly. Particularly in this case, the intensity change of the reflected detection light is extremely linear at a distance of about 5 times the wavelength of incident measurement light of 0.532 μm, that is, in a range of about 2.7 μm or less. The intensity change of the reflected detection light is displayed as a ratio with respect to the intensity of the reflected detection light at a position where the above distance is about 4.0 μm.

  In this case, when the measurement object 5 is in contact with the metal thin film 1, the intensity change of the reflected detection light is not observed. On the other hand, the intensity of the reflected detection light was constant even when the measurement object 5 was installed at a position sufficiently separated from the metal thin film 1 by about 4.0 μm or more. Thus, as shown in FIG. 2 of the present embodiment, the minute displacement of the measurement object 5 is within a range where the proximity distance between the metal thin film 1 and the measurement object 5 is about 5 times or less of the incident measurement light wavelength. The reflected detection light depending on the was obtained.

  Here, when the polarization of the incident measurement light is TE (s) polarized light, the reflected detection light that changes according to the minute displacement of the measurement object 5 as shown in FIG. 2 is not observed, and the TE (s) polarized light. Is difficult to generate metal-enhanced evanescent light at the interface of the metal thin film 1.

  The measuring object 5 is slightly displaced in the direction perpendicular to the metal thin film 1 at a position where the distance from the air side interface of the metal thin film 1 is about 5 times or less of the incident measurement light wavelength. If the distance between the measurement object 5 and the measurement object 5 becomes larger than this, the enhancement electric field of the metal-enhanced evanescent light becomes remarkably small, and it becomes difficult to obtain reflected light depending on the minute displacement of the measurement object 5.

Therefore, when the gap layer is an air layer, in order to detect a minute displacement of the measurement object, it is preferable to set the initial position of the measurement object with respect to the metal thin film to about 5 times or less of the incident measurement light wavelength. .
The maximum range of minute displacement that can be detected depends on the setting position of the optical coupler with respect to the measurement object, the range in which the measurement object does not directly collide with the metal thin film due to the minute displacement, and about 5 times the wavelength of the incident measurement light. An optical coupler is arranged with respect to the object to be measured within a range below the extent.

  In FIG. 2 of the embodiment, the position of the measuring object 5 is slightly changed statically and the minute displacement is detected. However, even if the minute displacement is dynamic, that is, minute vibration, the measurement object 5 is periodic and irregular. Regardless of the minute vibration, the minute vibration can be detected as a change in the reflected light from the optical coupler by using a photodetector capable of temporal response to the minute vibration of the measurement object. For detecting a minute displacement in a high frequency band of 100 kHz or higher, for example, a PIN photodiode or an avalanche photodiode (APD) capable of high-speed response is used.

  Here, a small semiconductor solid-state laser is used as the light source for incident measurement light, but a gas laser or a light emitting diode (LED) having directivity by using an optical lens can also be used.

  Further, in the case of this embodiment, the metal-enhanced evanescent light is applied on the silver thin film at an incident angle θ shown in FIG. 1 of about 43 degrees to about 45 degrees, which is not less than the critical total internal reflection angle of about 41.2 degrees. The laser beam is incident in this incident angle range.

  The present invention provides a method for detecting a minute displacement of a displacement body with high sensitivity in a high-speed rotating body such as an ultra-precision motor, a high-speed displacement body and a displacement element, and a minute displacement measurement such as a MEMS (microelectromechanical system). It can be used as a high-sensitivity micro displacement meter that is small and requires low cost.

It is the schematic which shows the structure of one Example of this invention. In this embodiment, the measurement object 5 is minute in the direction perpendicular to the metal thin film 1 under the incident light conditions in which the incident measurement light generates metal enhanced evanescent light at the gap layer 6 side interface of the metal thin film 1 of the optical coupler. It is a figure which shows the intensity | strength change of the reflected detection light measured with the photodetector 4 when approaching.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Metal thin film 2 Transparent dielectric material 3 Light source of incident measurement light 4 Photodetector 5 Measurement object 6 Gap layer

Claims (6)

  An optical coupler made of a transparent dielectric with a metal thin film formed in close contact with the object to be measured via an appropriate gap layer on the metal thin film, and metal enhanced evanescent light generated at the gap layer side interface of the metal thin film In the incident light condition, the incident measurement light is incident on the metal thin film from the transparent dielectric, and depends on a minute distance from the gap layer side interface of the metal thin film to the measurement object. A minute displacement detection method, wherein the minute displacement of the measurement object is detected by measuring the reflected detection light of the object with a photodetector.   An optical coupler made of a transparent dielectric with a metal thin film formed in close contact with the object to be measured via an appropriate gap layer on the metal thin film, and metal-enhanced evanescent light at the gap layer side interface of the metal thin film The incident light is incident on the metal thin film from the transparent dielectric, and the optical coupling depends on a minute distance from the gap layer side interface of the metal thin film to the measurement object. A micro displacement meter configured to detect a micro displacement of the measurement object by measuring reflected detection light from the detector with a photo detector.   The minute displacement detection method or minute displacement meter according to claim 1, wherein the incident measurement light is TM (p) polarized light.   4. The minute displacement of the measurement object is a displacement in a direction perpendicular to the metal thin film, and the minute displacement of the measurement object in the vertical direction is detected. A method for detecting a minute displacement or a minute displacement meter according to claim 1.   The gap layer is an air layer, the minute distance between the metal thin film and the measurement object is set to about 5 times or less of the incident measurement light wavelength, and the reflection depending on the minute displacement of the measurement object. The micro displacement detection method or micro displacement meter according to any one of claims 1 to 4, wherein the optical coupler is arranged with respect to the measurement object so that detection light can be obtained.   The metal thin film is a silver thin film, the transparent dielectric for forming the metal thin film is a glass prism, the incident measurement light is laser light, and the photodetector for measuring the reflected detection light is a photodiode. The micro displacement meter according to claim 5, wherein:

Images