JP2015055579A - Displacement and strain distribution measurement device with digital holography - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a displacement and strain distribution measurement device with digital holography, capable of simplifying an optical system for performing phase shift of reference light, and capable of downsizing a measurement head part (a part approaching a measurement object).SOLUTION: Reference light can be made incident to an image element installed around incident light by arranging an optical component (a reflection mirror) reflecting part of incident light into an optical path of incident light to an object. A glass plate projected on a light source side of incident light is used as an optical component. By making incident light as parallel light incident on a convex surface, the reflection angle of incident light is changed with the angle of the convex surface, and thereby incident light as reference light is made incident to an image element.

Description

  The present invention relates to a displacement / strain distribution measuring apparatus using a holographic technique.

  In Patent Document 1, after separating a light source into an object and incident light and reference light, a piezo element is attached to a mirror that reflects the reference light so as to perform phase shift, and an interferometer including the same mirror is provided. A technique that is separately constructed and generates a camera trigger signal based on a change in brightness of interference fringes obtained by the interferometer is disclosed.

  Further, Patent Document 2 discloses an optical that makes it possible to obtain a displacement in three directions and a strain distribution in two directions by using a structure in which a plurality of image pickup devices are arranged around a light beam incident on an object. A system and its calibration technique are disclosed.

JP 2012-181027 A JP 2012-220349 A

In the technique of Patent Document 1, it is necessary to install in the optical path of the reference light, and the design of the optical system is limited. Further, in the technique of Patent Document 2, an optical system for creating reference light is complicated.
SUMMARY OF THE INVENTION In view of the above-described problems of the prior art, an object of the present invention is to simplify an optical system for performing phase shift of reference light, and to reduce the size of a measurement head unit (portion approaching a measurement object). It is possible to provide a displacement / strain distribution measuring apparatus by digital holography that can be used.

The invention according to claim 1 of the present application is an optical system using digital holography that uses a plurality of imaging elements, and includes incident light that irradiates an object with one laser light source, and the plurality of imaging of the incident light. The optical system is characterized in that the means for separating the reference light incident on the element and the plurality of imaging elements are arranged so as to surround the optical path of the incident light.
According to a second aspect of the present invention, the means for separating the incident light into the reference light incident on the plurality of imaging elements is an optical component that reflects a part of the incident light in the optical path of the incident light on the object. The optical system according to claim 1.

  According to the present invention, a displacement / strain distribution measuring device by digital holography capable of simplifying an optical system for performing a phase shift of reference light and miniaturizing a measurement head portion (a portion approaching a measurement object). Can provide.

It is a figure explaining the relationship between a displacement vector and a sensitivity vector. It is a figure explaining the measuring device of the present invention. It is a figure which shows the structure which obtains the reference light by installing a reflective mirror in an irradiation light path (structure in case irradiation light is made into a plane wave). It is a figure which shows the structure which obtains the reference light by installing a reflective mirror in an irradiation optical path (structure which makes a reflective mirror a plane). It is a figure which shows the phase shift mechanism of the reference light by an actuator (structure in case irradiation light is made into a plane wave). It is a figure which shows the phase shift mechanism of the reference light by an actuator (structure in case irradiation light is made into a spherical wave). It is a figure explaining the example of the structure which determines imaging | photography timing from the change of the interference fringe obtained by an interferometer. It is a figure explaining the example of the structure which determines imaging | photography timing from the change of the interference fringe obtained by an interferometer. It is a figure explaining the basic structure of a head separation type measuring device. It is a figure explaining the experimental apparatus of the imaging signal generation by an interferometer. It is a figure explaining the relationship between the input voltage to a piezoelectric element, the intensity | strength of an interference fringe, and a camera trigger signal. It is a figure explaining the application (phase difference distribution) to a displacement measurement experiment. It is a figure explaining the structure of the measurement system which concerns on this invention. It is a figure explaining the structure of a trial image pick-up element part and a circuit.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<Basic configuration for miniaturization>
First, the basic principle of displacement measurement by digital holography will be described. As shown in FIG. 1A, the direction of an angle that is half the angle formed by the incident light from the light source and the light that enters the image sensor is called a sensitivity vector. The phase difference of the complex amplitude of the reproduced image before and after the deformation means the displacement component in the sensitivity vector direction. Note that displacement measurement by digital holography is a known technique.

As shown in FIG. 1B, conventionally, three independent sensitivity vectors e ₁ , e ₂ , and e ₃ can be obtained by using three incident lights. Displacement components in the sensitivity vector direction are obtained from the phase difference before and after deformation, respectively, and displacements in the x, y, and z directions are calculated therefrom (see Patent Document 2). However, in order to create three incident lights and separate them to obtain respective reproduced images, it is necessary to prepare three different lasers, and an optical system such as a lens and a mirror three times as much is required. .

  In the present invention, as shown in FIG. 1C, the incident light is one in the center, and at least three image sensors are used. The idea of using multiple cameras has never been seen in conventional digital holography research.

  Further, as shown in FIG. 2, incident light from a light source (laser light source) is passed through the center, and an image sensor is arranged around the optical path of the incident light, so that it is included in scattered light (object light) from the object surface. The displacement information can be obtained efficiently. As a result, the size can be reduced to a size that can be held with one hand. In FIG. 2, eight image sensors are evenly arranged around the optical path of incident light.

<How to create a reference beam>
(1) Method of arranging an optical component that partially reflects in the optical path of the incident light to the object As shown in FIG. 3, a part of the optical path of the incident light to the object that is the measurement object 2 in the optical system 1 By arranging an optical component that reflects the reference light, it becomes possible to make the reference light incident on the image sensor installed around the incident light. In FIG. 3, a glass plate that is convex toward the light source side of incident light is used. This glass plate acts as a mirror that reflects a portion of the incident light. By making the incident light from the light source incident on the convex surface as parallel light, the reflection angle of the incident light changes depending on the angle of the convex surface, and can be incident on the image sensor as reference light. The light transmitted through the convex surface is irradiated onto the object.
(2) Method for enabling use of planar optical component using spherical wave In FIG. 4, incident light from the light source of FIG. 3 is converted into a spherical wave by a convex lens. And the structure which can be made to inject into the image pick-up element arrange | positioned around incident light as reference light by injecting the spherical wave into a planar glass plate is shown.

This glass plate acts as a mirror that reflects a portion of the incident light. An optical component can be easily created by using a flat glass plate. In addition, suburban design becomes easy. The object to be measured is irradiated with a spherical wave, and the scattered light on the surface of the object passes through the glass plate and enters the image sensor to be photographed as a hologram.
(3) Method for enabling phase shift of reference light by attaching an actuator As shown in FIGS. 5 and 6, an optical component that partially reflects in the optical path of incident light to an object has an out-of-plane direction. An actuator that moves minutely (in other words, the incident light incident direction) is attached. 5 and 6, a piezoelectric element is attached as an actuator. By sending a signal to this actuator and expanding and contracting it, the phase of the reference light incident on the image sensor can be shifted.
(4) Mechanism for determining imaging timing from change in interference fringes obtained by interferometer As shown in FIG. 7, a part of light emitted from a light source is taken out to construct an interferometer. An actuator (piezo element (PZT) in FIG. 7) having a known correspondence relationship with an object attached to the optical component that performs phase shift of the reference light is also attached to the reflective optical component that constitutes the interferometer. By driving with a signal corresponding to the correspondence, the imaging timing is determined from the change in interference fringes obtained by the interferometer. Even when an actuator having a hysteresis in the relationship between the input voltage and the displacement amount, such as a piezo element, photographing can be performed when the predetermined phase shift amount is obtained with good reproducibility.

  In the case of FIG. 7, a piezoelectric element (PZT) is used for the actuator. Further, four piezo elements of the same type as the piezo elements attached to the optical component that performs phase shift of the reference light are bonded in series to form one actuator. At this time, drive signals are input in parallel to the individual piezoelectric elements. As this drive signal, the same signal as that of a piezo element attached to an optical component that performs phase shift of the reference light (corresponding to a reflection mirror that reflects a part of FIGS. 5 and 6) is input.

  By doing so, the amount of movement of the actuator that moves the mirror of the interferometer that determines the imaging timing is exactly four times the amount of displacement of the actuator for phase shifting. That is, when the light and darkness of the interference fringes obtained by the interferometer that determines the photographing timing changes by one cycle, the phase shift amount of the reference light becomes a quarter cycle (phase shift amount π / 2). As a result, it is possible to take an image that is not affected by the hysteresis characteristic of the piezo element and is phase-shifted by a quarter period.

At this time, the angle of incidence on the mirror attached to the actuator of the interferometer that determines the imaging timing according to the reflection direction of the reference light is set as shown in FIG. In the case of the phase shift mechanism of the reference light by the actuator shown in FIG. 6 (configuration in which the irradiation light is a spherical wave), this angle is set to the same value as the inclination of the image sensor, and the phase of the reference light with respect to the amount of movement of the mirror The angle is set so that the phase shift amount of the interference fringes is the same as the amount of movement of the mirror in the interferometer that determines the shift amount and the imaging timing. In this way, when the same type of piezo element is used, it is possible to capture an image that is phase-shifted by ¼ period with high accuracy.
(5) Structure in which the interferometer portion and the digital holography imaging apparatus are separated by connecting with a signal transmission means such as an optical fiber. FIG. 9 shows the basic structure of the head-separated measuring apparatus. An interferometer for detecting the phase shift amount described in (4) above and determining the photographing timing is incorporated in the left part of the figure. The light emitted from the laser light source is sent to the digital holography imaging device via an optical fiber. From the piezo driver, the same signal is sent to the head portion and the interferometer portion through a signal line. A camera trigger signal is generated from the photo sensor of the interferometer, and is input to the driver circuit of the image sensor.
(6) Calibration method The method disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2012-220349) is used for calibration. Thus, in the above (4), even when the phase shift amount is not completely shifted by 1 / N period (N is an integer) due to a shift at the time of installation of the optical system, the obtained phase difference is obtained. And the displacement amount can be obtained for each pixel, so that the displacement distribution and strain distribution can be measured.

The displacements of the object in the x, y, and z directions are d _x , d _y , and d _z , respectively. At this time, when the phase differences obtained by the plurality of image sensors are respectively set to Δφ _i (1 ≦ i ≦ N), the relationship of the formula 1 is established.

Here, S is an N × 3 matrix, which is called a sensitivity matrix. This sensitivity matrix is a matrix that relates to how much the phase difference obtained by each image sensor becomes relative to the displacement of the object, and each component of this matrix is a constant determined if the optical system is determined. Assuming that the pseudo inverse matrix of S is S ⁺ , Equation 2 is obtained.

S ⁺ is a 3 × N matrix. By using this equation, the displacements d _x , d _y , and d _{z of the} object in the x, y, and z directions can be obtained from the phase difference obtained by each image sensor.

Matrix S, using the reference plane used in the calibration are shown in Patent Document 2, it is moved three independent directions (or more) and the phase difference [Delta] [phi _i obtained by moving the reference plane displacement It can be obtained from the quantities d _x , d _y , d _z . For example, when moved in three directions, Equation 3 is obtained. Here, the right subscript j, (Δφ _ij , d _xj , d _yj , d _zj ) represents the number of times of movement.

  Then, Equation 3 can be transformed into Equation 4, from which the matrix S can be obtained.

Further, the value of each component of the matrix S ⁺ can be obtained as a pseudo inverse matrix of the matrix S.
(7) Method for obtaining displacement and distortion from phase differences obtained by a plurality of image sensors Next, a method for obtaining displacement and distortion from phase differences obtained by a plurality of image sensors will be described. The value of each component of the pseudo inverse matrix S ⁺ is expressed by Equation 5.

The displacements in the x, y, and z directions can be obtained from Equation 6 using the phase differences obtained by a plurality of image sensors and the values of each component of the matrix S ⁺ .

The strains ε _x and ε _y and the strain γ _{xy in the} x and y directions can be obtained as shown in Equation 7.

Thus, the phase difference obtained by a plurality of image sensors and the value of each component of the matrix S ⁺ can be obtained as shown in Equation 8.

<Embodiment>
(1) Embodiment of trigger signal generation by interferometer An embodiment of image signal generation by the above-described interferometer will be described. FIG. 10 shows an experimental apparatus for generating an imaging signal by an interferometer. This optical system is a digital holographic optical system that measures the out-of-plane displacement of a measurement target object using one image sensor.

  The light from the light source is divided into two, and one of them is incident on an interferometer for determining the photographing timing of the image sensor. Four piezo stages are attached in series to one mirror of the interferometer, and the voltage output from the piezo driver is applied to each piezo element in parallel. The same voltage is also applied to a piezo element that displaces one of the mirrors of the digital holographic optical system. By doing so, the interferometer for determining the imaging timing is subjected to the four-phase phase shift while the displacement measurement optical system performs the one-phase phase shift. In the interferometer for determining the imaging timing, the light and darkness of the interferometer is converted into a voltage signal by the photosensor, and the trigger signal of the camera is generated by binarizing it with an electric circuit.

  The piezo driver receives a triangular wave signal from a signal generator. Thus, every time the displacement measurement optical system performs a phase shift of ¼ period, a trigger signal of the camera is generated, and a hologram with a phase shift of exactly ¼ period is photographed.

  FIG. 11 shows the input voltage to the piezo element, the interference fringe intensity obtained by this apparatus, and the camera trigger signal. FIG. 12 shows the phase difference distribution obtained with this apparatus. In these cases, the displacement near the sample tip is 10 micrometers, 30 micrometers, and 50 micrometers.

  When the results of the displacement obtained from these were compared with the measurement results when the mirror was positioned with an accuracy of 1 nanometer or less using a piezo stage with feedback, an error of 20 meters in displacement was found. As a result, almost good results were obtained.

(2) Embodiment of trigger signal generation by interferometer FIG. 13 shows an example of a displacement / strain distribution measurement system. A glass element for reflecting the reference light and entering the imaging element is arranged in front of the object. A part of the incident light is reflected to become reference light, and the transmitted light is irradiated to the object from the front.

  Furthermore, the phase shift of the reference light can be performed by moving the glass element in the direction of the incident light using the piezo element. Also, by stacking multiple piezo elements made according to the same standard and applying a voltage in parallel, it is possible to obtain multiple phase shift amounts of the reference light. ), The reference light can be phase-shifted a plurality of times during one period.

  FIG. 14 shows a prototype image sensor unit for a small measuring apparatus. The φ10 hole at the center (the portion surrounded by reference numerals 1, 2, 3, and 4) is a portion through which incident light passes. An image sensor with a side of 2.0 mm is disposed around the hole. The image sensor is attached so as to face the central direction at an angle of 15 degrees. FIG. 14C shows a schematic block diagram of the prototyped circuit.

The present invention can achieve the following effects.
(1) The optical system is simplified, and an inexpensive and small measuring device can be made.
(2) The measurement head portion (portion approaching the measurement object) becomes smaller and lighter and can be used at the inspection site of a large structure.
(3) Even if the number of image sensors increases, the optical system does not become complicated, and a large number of image sensors can be used. If the number of image sensors is increased, noise is reduced by the effect of averaging, and measurement accuracy is improved.
(4) Depending on the inclination of the object, there may be an image sensor in which the reproduced image of the object and its phase difference distribution cannot be obtained with high accuracy, and this may cause a region that cannot be measured. Therefore, the possibility is reduced (a reproduction image of an object and its phase difference distribution need only be obtained with a minimum of three image sensors).

1 Optical system 2 Measurement target

The invention according to claim 1 of the present application is an optical system used in a displacement / strain distribution measuring apparatus using digital holography , and is incident on a plurality of image sensors and incident light that irradiates an object with laser light from a laser light source. Means for separating the reference light, and the separating means functions as a curved mirror convex toward the light source side of the incident light, and when the incident light irradiated on the object is parallel light, A means for generating reference light incident on a plurality of image pickup devices, wherein the plurality of image pickup means are arranged around an optical path of the incident light, and the object light incident on the object and reflected from the object And an optical system characterized in that the reference light is superimposed and received.
According to a second aspect of the present invention, in the displacement / strain distribution measuring apparatus using digital holography, means for separating laser light from a laser light source into incident light that irradiates an object and reference light that is incident on a plurality of image sensors; A head having the plurality of imaging means arranged around the optical path of the incident light, a light source device including an interferometer that determines imaging timing of the laser light source and the plurality of imaging elements, and the light source device An apparatus for measuring a displacement / strain distribution, comprising: an optical fiber that transmits laser light from a laser light source to the head.
According to a third aspect of the present invention, in the displacement / strain distribution measuring apparatus using digital holography, means for separating the laser light from the laser light source into incident light that irradiates an object and reference light that is incident on a plurality of imaging elements; A head having the plurality of imaging means arranged around the optical path of the incident light, a light source device including an interferometer that determines imaging timing of the laser light source and the plurality of imaging elements, and the light source device An optical fiber that transmits laser light from a laser light source to the head, and the means for separating functions as a curved mirror that is convex toward the light source side of incident light and irradiates the object with incident light. Is a means for generating reference light that is incident on the plurality of image pickup devices when the light is parallel light, the plurality of image pickup means are disposed around an optical path of the incident light, and the incident light is It enters the body for receiving overlapping the reference light reflected object light from said object, it is the displacement and strain distribution measuring device and said.
The invention according to claim 4 is characterized in that, when there are four or more image sensors, a displacement in three directions forming a three-dimensional space is obtained from four or more phase differences using a pseudo inverse matrix. Item 4. The displacement / strain distribution measuring apparatus according to Item 2 or 3.
According to a fifth aspect of the present invention, the interferometer is configured so that the phase shift amount of the reference light with respect to the movement amount of the separating unit is the same as the phase shift amount of the interference fringes in the interferometer that determines the photographing timing. The displacement / strain distribution measuring apparatus according to any one of claims 2 to 4, wherein an equipped mirror is set.

Claims (2)

An optical system using digital holography using a plurality of image sensors,
Means for separating one laser light source into incident light that irradiates an object, and reference light that is incident on the plurality of image sensors, and the incident light;
The optical system, wherein the plurality of imaging elements are arranged around an optical path of the incident light.   The means for separating the incident light into reference light incident on the plurality of image sensors is an optical component that reflects a part of the incident light in an optical path of incident light on an object. The optical system described in 1.

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