JP2010091716A - Photo elasticity modulator and photo elasticity measuring device including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small-sized photo elasticity modulator which can be easily set to a photo elasticity measuring device and to provide the photo elasticity measuring device including the same. <P>SOLUTION: A piezoelectric element 47 is brought into contact with an end of a peripheral side surface of a disk like optical element 46 and a side opposite to the piezoelectric element 47 of the optical element is brought into contact with a frame 48 to support both ends of the optical element 46. In this construction, the resonance frequency of the photo elasticity modulator 45 is set to be a frequency lower than the resonance frequency of the optical element 46 to vibrate the optical element from the piezoelectric element 47. Since only stress due to inertia of the piezoelectric element 47 is applied to the optical element 46 after vibration applied to the optical element 46 is attenuated, an amount of birefringence is uniformly generated in the optical element 46 and, in this state, light is made to be transmitted by the optical element 46 to apply modulation to the transmitted light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a structure of a photoelastic modulator that changes the polarization angle of light transmitted through an optical element, and includes a measurement object having the photoelastic modulator and having transparency such as a liquid crystal panel or a plasma display panel. The present invention relates to a photoelasticity measurement method including the photoelasticity modulator that measures stress, strain, and the like acting on the light and an apparatus thereof.

  For example, the following are known as conventional photoelastic modulators. That is, as shown in FIG. 12, the modulator 61 is fixed to one end in the longitudinal direction of the optical element 60 made of a rectangular parallelepiped, and the integrated optical assembly is moved through the spring 62 in the width direction orthogonal to the vibration propagation direction. It is configured to be suspended in a box-shaped enclosure 63. That is, the optical assembly can freely vibrate within the enclosure 63 (see Patent Document 1).

Special table 2001-518632

  However, the conventional methods have the following problems.

  The above-mentioned photoelastic modulator has a free end at the propagation direction of the vibration of the optical element in the enclosure. Therefore, when the vibration is applied from the modulator, the vibration mode M as indicated by a chain line in FIG. become. That is, the stress increases at the node NP generated near the center of the vibration propagation direction of the optical element 60, and the amount of birefringence increases. And since a stress becomes small as a vibration goes to a free end, there exists a problem that it does not become a uniform birefringence amount in the whole optical element.

  In addition, the resonant frequency of the photoelastic modulator produced by this method is almost the same as the resonant frequency of a single optical element such as glass, and therefore becomes relatively large. For example, when the resonance frequency is 50 kHz, the size of the optical element is about 10 cm. Therefore, there is a problem that the optical assembly becomes large.

  The present invention has been made in view of such circumstances, and includes a photoelastic modulator having a uniform amount of birefringence throughout the optical element, and capable of being easily installed and miniaturized, and the same. The main object is to provide a photoelasticity measuring device.

In order to achieve such an object, the present invention has the following configuration.
That is, the first invention is a photoelastic modulator that changes the phase difference of two orthogonal components of the polarization of transmitted light,
An optical element that transmits light;
Vibration generating means for applying vibration from a direction perpendicular to the light incident surface while in contact with the optical element;
A frame that contacts and supports a surface facing the contact surface with the vibration generating means in the optical means;
It is provided with.

  (Operation / Effect) According to the photoelastic modulator according to the first aspect of the present invention, the optical element and the vibration generating means vibrate integrally with the frame as a fixed end, so that the frequency is lower than the vibration of the optical element alone. The entire photoelastic modulator can be vibrated.

  That is, the vibration applied to the optical element at the start of fluctuation of the vibration generating means is uniformly dispersed in the process of reciprocating propagation in the optical element, so that the stress is uniformly distributed.

  In addition, it is preferable that the vibration generating unit applies vibration having a frequency lower than a resonance frequency of the optical element alone to the optical element.

  According to this configuration, since the vibration frequency of the vibration generating means is lower than the resonance frequency of the optical element alone, the vibration generated continuously applied to the optical element after the vibration applied to the optical element is attenuated in the optical element. Only a small stress based on the inertial mass of the means acts uniformly in the optical element. Therefore, uniform birefringence is generated in the optical element. As a result, the light transmission position can be arbitrarily set in the optical element, the optical element itself can be reduced, and the photoelastic modulator can be downsized.

  In addition, in the said structure, it is preferable to comprise so that a weight may be given to the said vibration generation means and inertial mass may be increased (Claim 3).

  According to this configuration, the frequency applied from the vibration generating unit can be arbitrarily adjusted in a frequency range lower than the resonance frequency of the optical element alone.

4th invention is the holding means to hold | maintain the measuring object which has the permeability | transmittance which consists of several layers,
Irradiating means for irradiating light toward the measurement object;
A first optical means for transmitting the light and converting it into polarized light composed of polarization components in two orthogonal directions;
A lens that transmits the polarized light and focuses on the interface of the predetermined layer of the measurement object;
Of the polarized light reflected from the focal plane of the measurement object and transmitted through the photoelastic modulator, the first polarized light returning the initial optical path and the second polarized light orthogonal to the first polarized light are separated into separate optical paths. Separating means for outputting,
A member formed with a pinhole that allows only the polarized light reflected from the focal plane to pass back among the separated second polarized light; and
First detection means for detecting the light intensity of polarized light that has passed through the pinhole;
Moving means for relatively moving the lens and the holding means back and forth along the optical axis direction of polarized light;
Rotating means for relatively rotating the holding means and the first optical means around an optical axis;
The polarized light irradiated to the measurement object by the rotating means and the measurement object are rotated relative to each other around the optical axis, and the light intensity of the polarized light detected by the first detection means at least every three predetermined positions. Is obtained based on the position information and the birefringence amount (c) of the optical member including the lens that transmits the polarized light, and [(φ / 2) sin2 (α−θ) + c] Calculating means for approximating the model ² and calculating the difference (φ / 2) and direction (α) of principal stress acting on each layer of the measurement object from the model When,
The polarized light that is disposed between the lens and the separating unit, modulates the polarized light that passes through the separating unit and travels toward the lens, and the polarized light that is reflected by the measurement object and transmitted through the lens and returns to the separating unit. The photoelastic modulator according to claim 3,
It is provided with.

  (Operation / Effect) According to this configuration, the light irradiated from the irradiation unit toward the measurement object held by the holding unit is linearly polarized by the optical unit. The holding means and the measurement object are moved back and forth along the optical axis direction by the moving mechanism while irradiating the polarized light, thereby focusing on a predetermined interface. The polarized light toward the focal plane, or polarized light reflected and returned from the focal plane and other interfaces is transmitted through the photoelastic modulator. At this time, the polarized light that is affected by the amount of change in birefringence is modulated. That is, even when the value of the amount of change in birefringence acting on the predetermined layer is small, the reference threshold level is increased. When the reflected light that has passed through the photoelastic modulator is returned to the first optical means, polarized light that is not affected by the amount of change due to birefringence is returned to the initial optical path as the first polarized light. The reflected light generated by the action of the amount of change due to birefringence is output as a second polarized light to the optical path different from the first polarized light by the optical means.

  When the second polarized light outputted from the separating means passes through the pinhole, only the polarized light reflected and returned from the predetermined focused interface is extracted, and this polarized light is detected by the first detecting means. This process is a one-cycle process, the linearly polarized light and the measurement object are rotated relative to each other, and the one-cycle process is performed for every arbitrary at least three rotation angles, and the light intensity of the second polarized light during each cycle process is determined. Get the amount of change.

The calculation means acquires a plurality of light intensity change amounts and rotation angle position information acquired for each cycle process, and birefringence of an optical member such as a lens through which the position information and polarized light in the apparatus configuration are transmitted. The principal stress acting on each layer of the object to be measured is obtained by approximating the model of [(φ / 2) sin2 (α-θ) + c] ² based on the quantity from the least square method. Difference (φ / 2) and its direction (α) can be obtained.

  In addition, since the optical element which comprises the photoelastic modulator with which this apparatus invention is equipped can generate | occur | produce uniform birefringence, it can permeate | transmit light to the arbitrary positions in an optical element. Therefore, setting is facilitated and the apparatus configuration can be reduced in size.

According to this device invention, a member having a pinhole that allows only polarized light reflected from the focal plane to pass back among the first polarized light separated by the separating means, and
Second detection means for detecting the light intensity of the polarized light that has passed through the pinhole,
Preferably, the calculation means is configured to divide the light intensity of the polarized light detected by the first detection means by the light intensity of the polarization detected by the second detection means (Claim 5).

  According to this configuration, the second polarization component can be corrected as long as the first polarization component is not zero due to the interference reflectance of the first and second polarizations. Further, the amount of change in the light intensity of the reflected light from the first surface of the first layer can be corrected by the amount of light reflected from the first surface of the measurement object that does not include the amount of change in birefringence, and the measurement accuracy can be increased.

  As the rotating means of this apparatus invention, for example, an optical element that transmits polarized light from the first optical means toward the measurement object and rotates it around the optical axis may be used. (Claim 6).

According to this configuration, the model of [(φ / 2) sin2 (α-θ) + c] ² can be approximated by the least square method from the light intensity value of polarized light obtained at each rotation angle, and the direction of the stress difference ( φ / 2) and its orientation (α) are calculated. The model can be estimated from the polarization intensity with at least three different angles. That is, the fourth method invention can be suitably realized.

  According to the photoelastic modulator of the present invention, the vibration frequency applied to the optical element from the vibration generating means is made lower than the resonance frequency of the optical element alone, so that it can be stably duplicated at any location in the optical element. Since the amount of refraction can be contained, light can be transmitted at an arbitrary position. As a result, the photoelastic modulator can be downsized.

  Moreover, according to the photoelasticity measuring apparatus, the apparatus configuration can be reduced in size by including the photoelastic modulator. Furthermore, by using the photoelastic modulator, it is possible to distinguish between compression and tension of the elastic stress acting on the workpiece by one measurement, and to determine its size and direction.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a perspective view showing a schematic configuration of a photoelastic modulator of the present invention, FIG. 2 is a plan view thereof, and FIG. 3 is a cross-sectional view taken along the line XX of FIG.

  As shown in FIG. 1C, the photoelastic modulator 45 opposes the disk-shaped optical element 46, the piezoelectric element 47 that contacts at one end of the optical element 46, and the piezoelectric element 47 across the optical element 46. The frame 48 is configured to come into contact with the optical element 46 in position. The piezoelectric element 47 is connected to a signal generator 49 that generates a voltage application signal at a period corresponding to the resonance frequency of the photoelastic modulator 45.

  Examples of the material of the optical element 46 include synthetic quartz and glass. The birefringence amount of the optical element itself is preferably small. For example, it is 0.001 rad or less.

  The length of the propagation direction of vibration of the optical element 46 is such that the propagation of vibration reciprocating between the contact surface (vibration applying surface) with the piezoelectric element 47 and the contact surface with the frame 48 is reciprocated in the optical element. The time period of the specified vibration corresponding to the optical element 46 intermittently applied from the piezoelectric element 47 is set to be longer than the time. That is, the propagation speed V of the vibration obtained from the physical properties of the material of the optical element 46, the length 2L of the optical element 46 when the vibration reciprocates in the optical element, and the period of time of vibration applied from the piezoelectric element 47 The optical element 46 is set to such a length that the relationship with the (interval) T satisfies the relationship 2L / V <T.

  Note that the optical element 46 and the piezoelectric element 47 are connected to each other with an adhesive 50 such as an epoxy resin in a state where there is no foreign matter on the contact surfaces of both elements.

  The frame 48 has a concave shape, and the optical element 46 can be stored in the concave portion. The material of the frame 48 is preferably capable of reflecting a signal propagated from the optical element 46, and for example, aluminum, duralumin, which is a kind of aluminum alloy, is more preferable.

  Note that the optical element 46 and the frame 48 are connected with an adhesive 50 such as an epoxy resin in a gap between the frame 48 and the optical element 46 formed around the contact surface in a state where there is no foreign matter on the contact surface. It is filled and adhered and fixed. The photoelastic modulator 45 is installed by rotating 45 degrees from the horizontal direction with respect to the optical axis.

  Next, the operation of the photoelastic modulator of the above embodiment will be described.

  The resonance frequency of the photoelastic modulator 45 including the optical element 46, the piezoelectric element 47, and the frame 48 is set in the signal generator 49. That is, since the photoelectric elastic modulator 45 composed of a plurality of members is larger than the mass of the optical element 46 alone, the resonance frequency thereof is lower than the resonance frequency of the optical element 46 alone. When a predetermined sine wave signal set in the signal generator 49 is applied to the piezoelectric element 47, the piezoelectric element 47 is activated. In conjunction with this operation, the same vibration as the resonance frequency of the photoelastic modulator 45 is applied from the piezoelectric element 47 to the optical element 46.

  The vibration applied to the optical element 46 propagates back and forth between the piezoelectric element 47 and the frame 48. At this time, since the vibration frequency of the piezoelectric element 47 is lower than the resonance frequency of the optical element 46 alone, after being attenuated in a short time while being dispersed radially in the process of reciprocating propagation in the optical element 46, the inertia of the piezoelectric element 47 is obtained. Only the stress due to is applied to the optical element 46.

  That is, a uniform stress is applied in the optical element 46. In other words, birefringence acts uniformly in the optical element 46.

  As shown in FIG. 1 and FIG. 2, when light is transmitted from the circular surface side in plan view to the optical element 46 in the state where birefringence is generated, the light transmitted through the optical element 46 is transmitted. The polarization state changes. That is, the polarization of light transmitted through the optical element 46 is modulated by the vibration frequency.

  The degree of modulation is proportional to the magnitude of birefringence generated in the optical element 46, and the magnitude of birefringence is proportional to the magnitude of distortion generated in the optical element 46.

Next, a specific example of the photoelastic modulator of the above embodiment will be described.
In this specific example, the optical element 46 is made of synthetic quartz having a diameter of 15 mm and a thickness of 3 mm. At this time, since the resonance frequency of the photoelastic modulator 45 was 50 kHz, the signal generator 49 was set to generate a 50 kHz sine wave signal.

  When the above setting was completed, light was transmitted through the optical element 46 while operating the photoelastic modulator 45. That is, as a result of conducting an experiment of transmitting light to a plurality of positions of the optical element, the measurement result was 0.1 μm or less at all positions, so the distortion generated in the optical element 46 alone was 0.1 μm or less. I knew there was. Here, since the frequency of the piezoelectric element 47 is low, it is determined that minute distortion is uniformly distributed throughout the optical element.

  When the vibration frequency generated in the optical element 46 is around 50 kHz, the vibration generated in the optical element 46 is in the vibration mode M of the fixed end supported by the frame 48 on one side, as shown by the chain line in FIG. Become. Therefore, the vibration node NP generated in the optical element 46 exists only at the contact point between the optical element 46 and the frame 48.

  The velocity (V) of vibration propagating through the optical element 46 is determined as approximately 6000 m / s at room temperature from the physical properties of the material, and the time for reciprocating through the optical element 46 is 5 μs. That is, the resonance frequency of the optical element 46 is 200 kHz, which is higher than the frequency 50 kHz of the piezoelectric element 47. Therefore, as long as vibration is continuously applied, a slight distortion is always generated uniformly in the optical element 46.

  From the above, the polarization modulation of the light becomes the same no matter which position in the optical element 46 is transmitted.

  Next, a photoelasticity measuring apparatus provided with the photoelastic modulator 45 will be described.

  FIG. 5 is a diagram showing a schematic configuration of the photoelasticity measuring apparatus.

  In this example apparatus, a measurement object W in which two transparent glass substrates W1 and W2 are overlapped at a minute interval as in a liquid crystal panel or a plasma display, and an opening H in the center are formed. A light source 1 that irradiates light toward the measuring object W held flat on the mounting table 8 is provided. On the optical path from the light source 1 to the measurement object W, the first polarization detector 3, the polarizing plate 40, the linear polarizing plate 41, the λ / 2 wavelength plate 42, the third polarizing beam splitter 43, the λ / 4 wavelength plate 44, A movable base 7 that includes an optical system unit 5 and an objective lens 6 and moves back and forth is provided. A lens 9a, a pinhole 16, and a second ftdiode 9b are arranged on the optical path of the light branched by the third polarizing beam splitter 43 toward the optical path different from the initial optical path. Each configuration will be specifically described below.

  The light source 1 generates light having a predetermined center circumference. For example, in the case of the present embodiment, a semiconductor laser having a center circumference of 635 nm is used. The light source 1 is installed so that the output laser light is composed of a horizontal polarization component, and this light is directed to the first polarization beam splitter 10 that constitutes the first polarization detection unit 3. The light source 1 corresponds to the irradiation means of the present invention.

  The first polarizing beam splitter 10 has a polarity that transmits only the horizontal component. In the case of the present embodiment, all the light from the light source 1 is transmitted in the straight direction and directed to the polarizing plate 40.

  The polarizing plate 40 is a Faraday Rotator, and rotates the horizontally polarized light from the first polarizing beam splitter 10 by 45 °. This polarized light is directed to the linearly polarizing plate 41. Further, the reflected light reflected and returned from the measurement object W is further rotated by 45 °, and in this optical system, the component of the reflected light is converted into only the vertical polarization component. As a result, the total amount of reflected light does not pass straight through the first polarizing beam splitter 10 but goes to the first photodiode 21 orthogonal to the initial light.

  The linearly polarizing plate 41 converts the initial light and reflected light that travels back and forth into linearly polarized light.

  In the process of transmitting 45 ° linearly polarized light incident from the linearly polarizing plate 41, the λ / 2 wavelength plate 42 returns 45 ° further to return to the linearly polarized light that is the original horizontal component and is directed to the third polarizing beam splitter 43. Dodge. The reflected light is also returned by 45 °, and the polarization direction is rotated by 90 ° in cooperation with the λ / 2 wavelength plate 42 and the Faraday Rotator 40 on the downstream side, so that the reflected light to the first polarizing beam splitter 10 is only the vertical component. To do. That is, the total amount of reflected light is used.

  The third polarization beam splitter 43 is disposed across the initial optical path and the two optical paths of linearly polarized light that are separated and output by the BDP 12 on the downstream side. Further, the third polarization beam splitter 43 has a polarity, and linearly polarized light of a horizontal component goes straight, and a vertical component is directed in an orthogonal direction. That is, all the linearly polarized light from the λ / 2 wavelength plate 42 is transmitted to the λ / 4 wavelength plate 44 on the downstream side, and is generated under the influence of stress in the process of transmitting the measurement target W in the reflected light. The horizontal component is directed in a direction orthogonal to the initial optical path. That is, it is directed to the second photodiode 9b.

  The λ / 4 wavelength plate 44 is disposed across the initial optical path and two linearly polarized optical paths separated and output by the downstream BDP 12. That is, the linearly polarized light reaching from the third polarizing beam splitter 43 is transmitted and further inclined by 45 ° to be circularly polarized. Further, both the first and second vertical and horizontal polarizations A and B reflected from the measurement object W, separated by BDP, and transmitted through the upstream λ / 4 wave plate 13 are further 45 Rotate to return circularly polarized light to horizontal and vertical polarization, respectively. In other words, when stress is applied to the glass substrate W1 or W2 of the measurement object W, a different polarization component is generated from the incident polarization component by photoelasticity, and the resulting elliptically polarized light is allowed to pass through. Convert to linearly polarized light including the amount of change. The λ / 4 wavelength plate 44 corresponds to the first optical means of the present invention.

  The optical system unit 5 includes a λ / 4 wavelength plate 13, a BDP 12, and a photoelastic modulator 45 in order from the light source 1 side.

  The λ / 4 wavelength plate 13 is disposed across the initial optical path and two linearly polarized light paths separated and output by the downstream BDP 12. That is, circularly polarized light from the upstream side is transmitted, and further rotated by 45 ° to return to linearly polarized light. Further, the elliptically polarized light, which is the reflected light from the measuring object W, is separated into horizontal and vertical components by the BDP 12 and passed through the inside of the λ / 4 wavelength plate 44, so that each is further inclined by 45 ° to be circularly polarized. .

  The BDP 12 totally transmits the reached linearly polarized light and directs it to the downstream photoelastic modulator 45. In addition, when the polarized light reflected and returned from the measurement object W is transmitted, linearly polarized light having the same polarization plane as that at the time of incidence (the first polarized light A, which is a vertical component) is returned to the same optical path and transmitted through the measurement object W. In this process, only the component (horizontal component second polarization B) whose polarization state has changed under the influence of the stress acting on the measurement object W is extracted and output in a direction different from the first polarization A. That is, the first polarized light A and the second polarized light B are output from different positions on the same surface of the BDP 12. The BDP 12 corresponds to the separation means of the present invention.

  The photoelastic modulator 45 has been described in detail in the above embodiment, and as shown in FIG. 1, a piezoelectric element 47 is provided at one end of a disk-shaped optical element 46, and a multi-end facing the piezoelectric element 47 is a frame. 48 and supported. That is, the photoelastic modulator 45 modulates the linearly polarized light toward the measurement object W to make elliptically polarized light, and reflects the reflection from the measurement object W and is influenced by the stress acting on the measurement object W. The elliptically polarized light whose orientation is slightly rotated around the optical axis is totally transmitted as it is.

  Since the photoelastic modulator 45 has a birefringence change amount determined in advance according to the optical element 46, the value of the detection signal Is of the light intensity of the second polarized light B detected by the second polarized light detection unit 9 is used. Increase That is, the photoelastic modulator 45 functions as the second optical means of the present invention.

  The mounting table 8 is formed with an opening H so as to allow light to pass through the central portion while holding the edge portion of the rectangular measuring object W.

  The second polarization detection unit 9 includes a lens 9a, a plate-like object 16 in which a pinhole 15 is formed, and a second photodiode 9b. That is, the second polarization B of the vertical component whose optical path is changed by the third polarization beam splitter 43 is condensed by the lens 9a, and only the polarized light that is reflected from the focal point and returned by the pinhole 15 is extracted. This polarized light is received by the second photodiode 9 b, converted into a light intensity detection signal Is, and transmitted to the control unit 23. The second polarization detection unit 9 corresponds to the first detection means of the present invention.

  The first polarization detection unit 3 includes a first polarization beam splitter 10, a second non-polarization beam splitter 17 </ b> A, a plate-like object 20 in which a pinhole 19 is formed, and a second photodiode 21. The first polarization detection unit 3 corresponds to the second detection means of the present invention.

  The second non-polarizing beam splitter 17A divides the light into two in the direction orthogonal to the straight traveling direction. That is, the light reaching from the first polarization beam splitter 10 is branched. Each light is directed to the first polarized photodiode 21 and the parallelism detector 22.

  The first photodiode 21 receives the first polarized light A that has passed through the pinhole 19, converts it into a light intensity detection signal Ir, and transmits it to the control unit 23.

  The parallelism detection unit 22 detects the state of occurrence of bending or warping of the measurement object W held on the mounting table 8. As shown in FIG. 6, the parallelism detection unit 22 includes four photodiodes 22 a to 22 d arranged adjacent to each other in a two-dimensional array, and the optical axis of linearly polarized light reaching from the second non-polarizing beam splitter 17 is Located at the center point C adjacent to each other, the light is received evenly across the four photodiodes 22a to 22d. The linearly polarized light received by each of the photodiodes 22 a to 22 d is converted into a light intensity detection signal and transmitted to the control unit 23. That is, the parallelism detection unit 22 detects the deviation of the optical path of the reflected light. The parallelism detection unit 22 corresponds to the detection means of the present invention.

  The control unit 23 includes an arithmetic processing unit 24, a drive control unit 25, an operation unit 26, and the like. Each configuration will be specifically described below.

  The arithmetic processing unit 24 mainly performs two types of processing. As a first process, the polarized light reflected and returned from the focal plane due to the influence of the warp of the measurement object W held on the mounting table 8 is both photodiodes 9b of the first and second polarization detectors 9 and 3. , 21 to calculate the correction amount of the optical path deviation. As a second process, an unknown parameter among the amount of change in birefringence, the photoelastic coefficient, and the thickness of the glass substrate generated by the stress acting on the predetermined glass substrate W1 or W2 of the measurement object W, and the predetermined The difference and the direction of the main stress acting on the glass substrate are obtained. These specific processes will be described in detail in the operation description. The arithmetic processing unit 24 corresponds to the arithmetic means of the present invention.

  The drive control unit 25 transmits a drive signal based on the conditions set by the operation unit 26 to the rotation drive mechanism 27 and the actuator 28. That is, the rotation drive mechanism 27 controls the optical system unit 5 to rotate about a predetermined rotation angle around the optical axis of the initial light from the light source 1. Further, the mounting table 8 is tilted on the XY plane. The driving mechanism 26 corresponds to the rotating means of the present invention, and the actuator 28 corresponds to the driving means.

  The control unit 23 including these components controls the operation of each drive mechanism and the like based on initial setting conditions in addition to the above processing.

  Next, with respect to the operation and processing of a round of measuring the direction in which the difference between the main stress acting on each glass substrate W1, W2 of the measuring object W and the difference in the principal stress acts using the above-described embodiment apparatus, Description will be made along the flowchart shown in FIG. In addition, the case where the stress is acting on both the glass substrates W1 and W2 which comprise the measuring object W is demonstrated as an example.

  The operator operates the operation unit 26 to set and input measurement conditions such as the total thickness of the measurement target W, the rotation angle of the optical system unit 5 and the number of measurements (step S1). In the case of this embodiment, the rotation angle is a difference between main stresses acting on the glass substrates W1 and W2 of the object W to be measured at three positions of the reference 0 °, 45 °, and 90 ° and measurement in the direction. Set to do.

  When the condition setting is completed, the measuring object W is placed and held on the placing table 8, and each drive mechanism is controlled to be ready for measurement. At this time, the movable table 7 is operated, and the focal position of the objective lens 6 is adjusted to the outermost surface of the measurement object W. When these measurement conditions are satisfied, the operator performs test irradiation (step S2). At this time, linearly polarized light is irradiated from the light source 1 toward the measurement object W, and the first polarized light A that is reflected back is received by the parallelism detector 22, and a detection signal of light intensity for each of the photodiodes 22a-22d is received. It transmits to the arithmetic processing part 24 of the control unit 23.

  The arithmetic processing unit 24 determines whether or not there is a deviation in the optical path from the light source 1 to the measurement object W from the light intensity value and the average value for each of the photodiodes 22a to 22d (step S3). When it is confirmed that there is a deviation of the optical path, the arithmetic processing unit 24 obtains a correction amount for correcting the warp caused by the influence of the warp of the measurement object W and performs signal conversion (step S4). Based on this correction signal, the drive control unit 25 operates the actuator 28 to tilt the mounting table 8 so that the reflected light is evenly applied to the photodiodes 22a to 22d of the parallelism detector 22 ( Step S5).

  When the optical path deviation correction process is completed, test irradiation is performed again. If the optical path deviation is eliminated at this time, the first first measurement at a predetermined set angle is started (step S6). If the optical path deviation is not eliminated, the blur correction process from step S2 is repeated.

  In the first measurement, the measurement is started in a state where the optical system unit 5 is aligned with the reference 0 ° of the X and Y axes. The horizontal component laser light output from the light source 1 is totally transmitted through the first polarizing beam splitter 10 </ b> A and reaches the polarizing plate 40. When the light passes through the polarizing plate 40, the polarization plane of the light rotates 45 ° and travels toward the linear polarizing plate 41.

  The light transmitted through the linear polarizing plate 41 becomes 45-degree linearly polarized light and travels toward the λ / 2 wavelength plate 42. When passing through the λ / 2 wavelength plate 42, the plane of polarization of the linearly polarized light is returned by 45 ° to return to the linearly polarized light of the original horizontal component.

  This linearly polarized light is totally transmitted through the third polarizing beam splitter 43 and is converted into circularly polarized light by 45 ° in the process of passing through the next λ / 4 wavelength plate 44.

  This circularly polarized light becomes a vertical component in the process of passing through the λ / 4 wavelength plate 13 of the optical system unit 5, passes through the BDP 12 as it is, and passes through the photoelastic modulator 45. At this time, the linearly polarized light composed of the vertical component is changed to elliptically polarized light and condensed by the objective lens 6 to reach the focal plane of a predetermined layer.

  While irradiating this light, the movable base 7 is advanced by a distance corresponding to the amount obtained by dividing the thickness of the measuring object W by the amount of refraction, and the first polarized light A is reflected back by the front and back surfaces of the glass substrates W1 and W2. The second polarized light B is received by the first and second photodiodes 9b and 21. These changes in light intensity are converted into detection signals Is and Ir in real time and transmitted to the arithmetic processing unit 24.

  At this time, when stress is applied to the glass substrates W1 and W2, the second polarized light B is included in the reflected light that is reflected and returned from each focal plane. That is, as shown in FIG. 8, the reflected light R1 reflected and returned at the contact interface (focal plane P1) between the surface of the glass substrate 1 and the air layer, the air layer in the minute gap between the glass substrate W1 and the glass substrate W2, and Reflected light R2 reflected and returned at the contact interface (focal plane P2) with the back surface of the glass substrate W1, reflected light R3 reflected and returned at the contact interface (focal plane P3) between the air layer and the surface of the glass substrate W2, and Each of the reflected light R4 reflected and returned by the contact interface (focal plane P4) between the glass substrate W2 and the mounting table 8 includes a component of the detection signal Is of the light intensity of the second polarized light B.

  The reflected light reflected and returned from each focal plane P1 to P4 is returned to the same optical path as the initial light. When the reflected light passes through the layer on which the stress acts and is reflected by a predetermined focal plane and returned, it is returned as a polarization component different from the incident light by photoelasticity. This reflected light passes through the objective lens 6 and the photoelastic modulator 45 as it is, and passes through the BDP 12. At this time, the first polarized light A having a vertical component returning from the initial optical path and the second polarized light B having a horizontal component whose polarization plane has changed are separated. The first polarization A returns to the initial optical path, and the second polarization B is output to another optical path.

  The first polarization A is composed of a vertical component, is circularly polarized by the λ / 4 wavelength plate 13 in the initial optical path, and then becomes a horizontal component by the λ / 4 wavelength plate 44, and the entire amount of the third polarization beam splitter 43 is changed. To Penetrate. Furthermore, the first polarized light A is totally transmitted through the λ / 2 wavelength plate 42 and the linear polarizing plate 41 to become 45 ° linearly polarized light, and when it is transmitted through the Faraday Rotator 40, it is rotated 45 ° to become only the vertical component. Totally reflected toward the first polarization beam splitter 3.

  The first polarized light A that has reached the first polarizing beam splitter 3 is totally reflected in the direction of the first photodiode 21 that is orthogonal to the light of the initial horizontal component, and travels toward the second non-polarizing beam splitter 17A. This second non-polarizing beam splitter 17A branches the polarized light toward the first photodiode 21 and the parallelism detector 22. The detection signals received by the first photodiode 21 and the parallelism detector 22 are transmitted to the arithmetic processing unit 24 of the control unit 23.

  The second polarized light B is composed of a horizontal component, is transmitted through the λ / 4 wave plate 13 and is inclined by 45 ° to be circularly polarized light, is further transmitted through the λ / 4 wave plate 44 and is further inclined by 45 °, and is polarized with a vertical component. To be. The entire amount of the second polarized light B is directed to the second photodiode 9b in the direction orthogonal to the initial optical path by the third beam splitter 43. In this process, only polarized light reflected and returned from the focal plane by the pinhole 15 is extracted and received by the second photodiode 9b. This light intensity detection signal Is is transmitted to the arithmetic processing unit 24 of the control unit 23.

  Here, the light intensity of the second polarized light B reflected and returned from the focal planes P1 to P4 received by the second photodiode 9b appears as three peaks as shown in FIG.

  In this embodiment, since there are four focal planes P1 to P4, four peaks should occur, but appear as three peaks. This phenomenon is detected by separating the reflected lights R2 and R3 which are reflected by the focal planes P2 and P3 and are reflected by the light reflected by the glass surfaces before and after the air layer, and the intensity of the light is increased. It was not possible, and it was obtained as a new finding that it was synthesized. The solid line shown in FIG. 9 is a photoelastic signal to be detected, and the broken line is a signal of reflected light that is reflected by the measurement object W and returned.

  Therefore, the arithmetic processing unit 24 first detects the detection signal and the focal plane included in the combined reflected light that is reflected by the focal planes P2 and P3 and returned from the detection signals from the first and second photodiodes 9b and 21. A detection signal reflected and returned at P2 is extracted, and the values of both detection signals Is are corrected. That is, the detection signal Is of the second polarization B is corrected by Is / Ir divided by the detection signal Ir of the first polarization A (hereinafter referred to as a corrected elasticity signal). Then, the corrected light intensity value and the positional information of the rotation angle of the optical system unit 5 at the time of measurement are stored in association with each other.

  When the first measurement is completed, the drive control unit 25 performs the second measurement of the same process as the first measurement by rotating the optical system unit 45 around the optical axis at the same time while returning the movable base 7 to the measurement start position. (Step S7) Further, when the second measurement is completed, the third measurement is continued (Step S8).

When the first measurement to the third measurement are finished, the arithmetic processing unit 24 obtains the difference between the principal stresses acting on the glass substrates W1 and W2 and the direction thereof (step S9). Specifically, the arithmetic processing unit 24 organizes the stored light intensity values and position information of the three rotation angles as a group of data for the same focal plane, and converts the light intensity values for each data group into the XY plane. From the plotted state, the corrected elastic signal (which is the same as the amount of birefringence) obtained when the optical system unit 5 is rotated once from this plotted state and the rotation angle (θ) at that time When the birefringence amount of the sample is smaller than 0.01 rad, a model of [(φ / 2) sin2 (α−θ) + c] ² (hereinafter simply referred to as “model”) is approximated by the method of least squares. The main stress difference (φ / 2) and its direction (α) are obtained from the obtained model. Here, c is the amount of birefringence of an optical member including a lens that transmits polarized light. In addition, the measurement of the difference (φ / 2) and the direction (α) of the main stress acting on the glass substrates W1 and W2 of the measurement target W is completed. Note that the amount of birefringence of the sample is not limited to a value smaller than 0.01 rad, and can be changed as appropriate according to the shape of the photoelastic modulator.

  According to the photoelasticity measuring apparatus having the above-described configuration, the objective lens 6 is moved forward by the layer thickness of the measuring object W by focusing on the surface of the glass substrate W1 of the measuring object W, and the glass substrates W1 and W2 are moved forward. Of the reflected light reflected and returned from the focal planes P1 to P4 by the displacement of the focal point with respect to the surface of the glass substrate W1 and W2 in the process of passing through the glass substrates W1 and W2, the polarization due to the effect of stress acting on both glass substrates W1 and W2 Only the second polarization B of the vertical component generated by the state change can be separated and extracted by the BDP 12. By passing the second polarized light B separated by the BDP 12 through the pinhole 15, the glass substrate W1 is reflected back from the back surface (focal plane P2 and the front surface (focal plane P3) of the glass substrate W2) of the glass substrate W1. Only the second polarized light B affected by the stress and the second polarized light B affected by the stress of the glass substrate W2 reflected and returned from the back surface (focal plane P4) of the glass substrate W2 can be extracted. Then, by using the confocal optical system, it is possible to measure the second polarized light affected by the stress acting on an arbitrary glass substrate (layer).

  Then, the optical system unit 5 is rotated around the optical axis, and the light intensity values of the group of second polarized light B acquired under the same conditions from at least three predetermined rotation angles are arranged for each focal plane, and onto the XY plane. The model is approximated by the least square method from the corrected elastic signal obtained when the optical system unit 5 is rotated once from this plotted state and the rotation angle. That is, the main stress difference (φ / 2) and its direction (α) can be specified from this model.

  Further, by using the parallelism detector 22, the optical path shift of the measurement light can be corrected, and the first and second polarized light A and B can be received with high accuracy by the first and second photodiodes 21 and 9b. .

  Furthermore, the apparatus according to the present embodiment can be downsized by using a semiconductor laser as the light source 1 and using the photoelastic modulator 45 as the optical system unit 5 to reduce the number of components of the optical system unit 5. . Accordingly, a small and low output rotational drive mechanism 27 that rotates the optical system unit 5 around the optical axis can be used.

  Further, by using the first polarization beam splitter 10 for the first polarization detection unit 3, all of the laser light output from the light source 1 can be directed to the measurement object. Moreover, the reflected light reflected from the measurement object W can be detected by the first photodiode 21 without returning to the light source 1 side. That is, the instability of the light source due to the collision between the output initial light and the reflected light can be suppressed, and the difference between the main stresses and the measurement of the direction can be accurately performed.

  Furthermore, since the light from the light source 1 is composed of a horizontal component, the light from the light source 1 can be incident 100% on the measurement object W by being transmitted through the first polarization beam splitter 10, and the measurement object W The reflected light from 100% can be used. As a result, detection accuracy can be improved.

  The present invention is not limited to the above-described embodiment, and can be modified as follows.

  (1) The photoelastic modulator 45 of the above embodiment may be configured such that the piezoelectric element 47 itself is weighted by a weight or a weight to increase the inertial mass of the piezoelectric element 47. According to this configuration, the resonance frequency of the photoelastic modulator 45 can be arbitrarily adjusted in a range lower than the resonance frequency of the optical element 46.

  (2) In the above embodiment, as shown in FIG. 10, the reflected light from the measurement object W is incident on the photoelastic modulator 30, and the light intensity of the polarization component of the reflected light is modulated at a predetermined frequency. The frequency after modulation is input to the lock-in amplifier 31 as a reference signal, and the detection signal Is of only two polarizations B from the second photodiode 9 b is input to the lock-in amplifier 31. At this time, the DC component is removed from the detection signal Is of the second polarized light B, and only the AC component is extracted. Therefore, it is possible to improve the calculation accuracy of the amount of change in birefringence. Further, this may be a method in which the elastic signal Is of FIG. 5 is input to the lock-in amplifier and the output is input to the arithmetic processing unit 24.

  For example, the amount of change in birefringence of the front glass substrate W1 in the above embodiment can be expressed by the following equation (1) when the amount of birefringence of the sample is smaller than 0.01 rad.

Change amount of birefringence of front glass substrate W1 = K (ρ ² + ρφsin [2 (α−θ)]) (1)

  Here, ρ is the amount of birefringence of the photoelastic modulator 45, a minute amount, α is the direction of the principal stress difference (φ / 2), and e is the glass substrate W1 for detecting the direction and magnitude of the birefringence. It is assumed that the rotation angle, φ / 2, is a birefringence amount of W1 and a square value is negligibly small.

  When the reflected light is passed through the lock-in amplifier 31, the amount of change in birefringence of the front glass substrate can be expressed by the following equation (2).

Birefringence of front glass = Kρ ₀ × φsin [2 (α-θ)]) (2)

Here, the photoelastic modulator is expressed by the following equation, ρ (t) = ρ ₀ sin (ω ₀ t) by introducing its birefringence amount ρ ₀ , its frequency ω ₀ , and time t. Since the lock-in amplifier is used, the square term of ρ is not output from the lock-in amplifier. On the other hand, the reflected signal side Ir is expressed as Ir = K using the notation on the elastic signal side.

  Therefore, the corrected elasticity signal (= Is / Ir) can be expressed by the following equation.

Corrected elastic signal = ρ ₀ × φsin [2 (α-θ)])

In an actual optical system, an offset c is added to the above formula,
Correcting acoustic signal _{= ρ 0 × φsin [2 (} α-θ)]) + c ... (3)
It is expressed as

Here, ρ ₀ is known, but actually, it is calculated from a calibration experiment for determining the relationship between the magnitude of the corrected elastic signal when a tensile force is applied to the sample with a known force and the principal stress difference obtained from the tensile force. .

  That is, only the signal components of f and a including information on birefringence that is an AC component are extracted. Thereafter, similarly to the above-described embodiment, the extracted signal components are arranged for each rotation angle, the light intensity values are plotted on the XY plane, and obtained when the optical system unit 5 is rotated once from this plotted state. The corrected elastic signal to be obtained is approximated by the formula (3) by the method of least squares. Then, the difference (φ / 2) and the direction (α) of the main stress are obtained from the result. This method makes it possible to distinguish between compression and tension from the experimental results. Further, by including the objective lens 6 in the rotating system 5, the birefringence of the objective lens 6 can be ignored.

  (3) In the above-described embodiment device and the modification device, the optical system unit 5 is rotated around the optical axis. However, the mounting table 8 may be rotated.

  (4) In each of the above-described embodiments, the measurement object W provided with the two glass substrates W1 and W2 provided with a gap is used. However, the measurement object W is not limited to this form, and the gap is eliminated. A plurality of measurement objects having permeability may be stacked in close contact. For example, there are combinations of measurement objects having different refractive indexes, such as glass substrates and glass substrates and films.

  (5) In each of the above embodiments, the second non-polarizing beam splitter 17 and the parallelism detector 22 may be omitted when the parallelism of the measurement object W is maintained.

  (6) In each of the above embodiments, the mounting table 8 having the opening H formed in the center is used. However, the mounting table 8 may be formed of a flat object in which the opening H is not formed.

  (7) In the first and second embodiments, other than the detection target such as scattering generated when the second polarized light B is reflected or transmitted by another optical member on the optical path until the second polarized light B is detected by the second photodiode 9b. It is preferable that a light shielding plate for removing stray light is provided on the upstream side of the lens 9a.

  As shown in FIG. 11, the light shielding plate 52 has an arc-shaped opening 53 formed at a position radially away from the center of the plate on the disk, and the second polarized light B to be measured is the opening 53. Pass through. That is, in the embodiment apparatus, the BDP 12 rotates around the optical axis of the first polarized light A that returns through the same optical path as the incident light of the BDP 12. Along with this rotation, the second polarized light B output from a position separated from the same plane as the first polarized light A of the BDP 12 passes through an arc orbit around the first polarized light A. Therefore, the opening 53 of the light shielding plate 52 is formed along the trajectory along which the second polarized light B moves. In the above embodiment, since the optical system unit 5 is rotated around the optical axis in the range of 0 to 90 °, the opening 53 of the light shielding plate 52 is formed in a substantially semicircular arc shape. Note that if the rotation angle is changed, the circumference of the opening 53 is also appropriately changed.

It is a perspective view which shows the photoelastic modulator which concerns on an Example. It is a top view which shows a photoelastic modulator. It is XX arrow sectional drawing of the photoelastic modulator shown in FIG. It is a figure which shows the vibration mode of the photoelastic modulator which concerns on an Example. It is a figure which shows schematic structure of the photoelasticity measuring apparatus which concerns on an Example. It is a top view which shows the light reception state of the polarization by a parallelism detection part. It is a flowchart which shows the process and operation | movement of a round which measures the difference of main stress, and its direction. It is a figure which shows the state of the reflected light reflected on each focal plane of a measurement object. It is a figure which shows the detection state of the light intensity of a 2nd polarization | polarized-light. It is a figure which shows the structure of a modification apparatus. It is a top view of a light-shielding plate. It is a top view which shows the conventional photoelastic modulator. It is a figure which shows the vibration mode of the conventional photoelastic modulator. It is a figure which shows the vibration mode of the conventional photoelastic modulator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Light source 3 ... 1st polarization inspection part 5 ... Optical system unit 6 ... Objective lens 7 ... Movable base 8 ... Mounting base 9 ... 2nd polarization detection part 10 ... 1st non-polarization beam splitter 12 ... BDP
DESCRIPTION OF SYMBOLS 13 ... (lambda) / 4 wavelength plate 15, 19 ... Pinhole 23 ... Control unit 24 ... Arithmetic processing part 27 ... Drive mechanism 28 ... Actuator 40 ... Polarizing plate 41 ... Linearly polarizing plate 42 ... λ / 2 wavelength plate 43 ... Third polarization Beam splitter 44 ... λ / 4 wave plate 45 ... Photoelastic modulator 46 ... Optical element 47 ... Piezoelectric element 48 ... Frame 49 ... Signal generator 50 ... Adhesive

Claims (6)

A photoelastic modulator that changes the phase difference of two orthogonal components of the polarization of transmitted light,
An optical element that transmits light;
Vibration generating means for applying vibration from a direction perpendicular to the light incident surface while in contact with the optical element;
A frame that contacts and supports a surface facing the contact surface with the vibration generating means in the optical means;
A photoelastic modulator characterized by comprising: The photoelastic modulator according to claim 1, wherein
The vibration generating means imparts a vibration having a frequency lower than the resonance frequency of the optical element alone to the optical element. The photoelastic modulator according to claim 1 or 2,
A photoelastic modulator characterized by being configured to increase the inertial mass by applying a weight to the vibration generating means. Holding means for holding a measuring object having a permeability composed of a plurality of layers;
Irradiating means for irradiating light toward the measurement object;
A first optical means for transmitting the light and converting it into polarized light composed of polarization components in two orthogonal directions;
A lens that transmits the polarized light and focuses on the interface of the predetermined layer of the measurement object;
Of the polarized light reflected from the focal plane of the measurement object and transmitted through the photoelastic modulator, the first polarized light returning the initial optical path and the second polarized light orthogonal to the first polarized light are separated into separate optical paths. Separating means for outputting,
A member formed with a pinhole that allows only the polarized light reflected from the focal plane to pass back among the separated second polarized light; and
First detection means for detecting the light intensity of polarized light that has passed through the pinhole;
Moving means for relatively moving the lens and the holding means back and forth along the optical axis direction of polarized light;
Rotating means for relatively rotating the holding means and the first optical means around an optical axis;
The polarized light irradiated to the measurement object by the rotating means and the measurement object are rotated relative to each other around the optical axis, and the light intensity of the polarized light detected by the first detection means at least every three predetermined positions. Is obtained based on the position information and the birefringence amount (c) of the optical member including the lens that transmits the polarized light, and [(φ / 2) sin2 (α−θ) + c] Calculating means for approximating the model ² and calculating the difference (φ / 2) and the direction (α) of the principal stress acting on each layer of the measurement object from the model When,
The polarized light that is disposed between the lens and the separating unit, modulates the polarized light that passes through the separating unit and travels toward the lens, and the polarized light that is reflected by the measurement object and transmitted through the lens and returns to the separating unit. The photoelastic modulator according to claim 3,
A photoelasticity measuring device comprising: The photoelasticity measuring apparatus according to claim 4,
A member formed with a pinhole that allows only the polarized light reflected from the focal plane to pass back among the first polarized light separated by the separating means;
Second detection means for detecting the light intensity of the polarized light that has passed through the pinhole,
The photoelasticity measuring apparatus, wherein the computing means divides the light intensity of the polarized light detected by the first detection means by the light intensity of the polarized light detected by the second detection means. In the photoelasticity measuring apparatus according to claim 4 or 5,
The photoelasticity measuring apparatus, wherein the rotating means is an optical element that transmits polarized light from the first optical means toward the measurement object and rotates the light around the optical axis.

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