JPH11142322A - Double refraction measuring apparatus and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double refraction measuring apparatus capable of measuring even the double refraction of an inspected object, having large double refraction exceeding the wavelength of a light source to be used, at a high speed over the entire surface of the object to be inspected with high accuracy. SOLUTION: A double refraction measuring apparatus is equipped with a light source 1 allowing light to be incident on an inspected object 8 in a predetermined polarized state, a polarizing element 11 changing the polarized state of the transmitted light from the inspected object 8, a means 15 for rotating the polarizing element 11 almost around a light advancing direction, a photodetector array 13 consisting of a plurality of photodetectors to receive the light transmitting through the polarizing element 11, optical system 7, 9, 12 expanding the light from the light source 1 to irradiate inspected object 8 and allowing the transmitted light from the inspected object 8 incident on the almost light detecting surface of the photodetector array 13 to form an image, and an operation means 18 for calculating the double refraction of the inspected object 8 from the output detected by photodetectors.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various transparent bodies such as a plastic flat plate such as an optical disk substrate or a plastic lens such as an optical writing lens used for a laser printer, a camera lens, and a pickup lens of an optical disk drive. The present invention relates to a birefringence measuring device and a birefringence measuring method for measuring the birefringence of an object.

[0002]

2. Description of the Related Art As a prior art of a birefringence measuring method, a phase modulation method and a rotation analyzer method are known. In these methods, a transparent object is irradiated with a parallel beam, the transmitted light is received by a light receiving element such as a photodiode, and the change in the polarization state of the transmitted light due to the birefringence of the transparent object is detected. Thereby, the birefringence of the transparent test object is obtained. In the phase modulation method, the irradiation light is phase-modulated using a photoelastic modulator,
Birefringence is determined from the phase difference between the beat signal and the modulation signal of the light transmitted through the transparent test object. In the rotating analyzer method, the transmitted light is received by the light receiving element placed after the analyzer while rotating the analyzer placed after the transparent test object, and the change in output from the light receiving element due to the rotation of the analyzer is obtained. Find birefringence.

In addition, in the birefringence measurement methods described in JP-A-4-58138 and JP-A-7-77490,
The transparent object is irradiated with the expanded parallel light, and the transmitted light
Birefringence surface measurement is performed, in which a two-dimensional sensor such as a CD camera receives light and obtains birefringence of a transparent test object.

[0004]

In the above-described phase modulation method and rotary analyzer method, for example, a so-called point measurement in which a thin parallel beam is irradiated and received by a photodiode, so that the entire surface of the test object is measured. It is necessary to adjust the test object and the measuring device, and when measuring a lens, setting the lens is difficult. On the other hand, JP-A-4-581
38 and JP-A-7-77490 do not require adjustment of the test object or the like because of the surface measurement, but do not need to adjust the size of the light source used for measurement. The measurement of the birefringence phase difference is difficult, and the measurement of the optical writing lens and the like is difficult because the birefringence phase difference becomes large. The difficulty in measuring a large birefringence phase difference is the same as in the above point measurement.

The present invention has been made in view of the above circumstances, and enables high-speed measurement by observing the entire surface of a test object one-dimensionally or two-dimensionally, and at the same time, light receiving arranged in an array. A birefringence measuring device and a birefringence measuring method which can measure a distribution of a large birefringence phase difference exceeding a wavelength of a light source to be used by detecting a difference between measured values of adjacent light receiving elements. The purpose is to provide.

[0006]

In order to achieve the above object, a birefringence measuring apparatus according to the first aspect of the present invention comprises: a light source for causing light to enter a test object in a predetermined polarization state; A polarizing element that changes a polarization state of transmitted light, a unit that rotates the polarizing element substantially around a traveling direction of light, a unit that detects a rotation angle of the polarizing element, and a plurality of light receiving elements. A light-receiving element array that receives light transmitted through the light-emitting element; and irradiating the test object with light expanded from the light source to form an image of transmitted light from the test object substantially on a light-receiving surface of the light-receiving element array. An optical system and a calculation means for obtaining the birefringence of the test object from the output detected by the light receiving element are provided.

The method for measuring birefringence according to the invention of claim 2 is as follows:
Using the birefringent device according to claim 1, detecting a difference in measured values between adjacent light receiving elements of the light receiving element array, thereby detecting a phase jump in a birefringence phase difference distribution (a large position exceeding a predetermined range). In the case where there is a phase difference distribution, a phenomenon that a value different from the original measurement result is output) is corrected.

According to a third aspect of the present invention, there is provided a birefringence measuring apparatus,
Two or more laser light sources having different wavelengths for causing light to be incident on a test object in a predetermined polarization state; a unit for switching irradiation of the test object with light from the two or more laser light sources; A polarizing element for changing a polarization state of light transmitted from an object, a unit for rotating the polarizing element substantially around a traveling direction of light, a unit for detecting a rotation angle of the polarizing element, and a light transmitted through the polarizing element. And a calculating means for obtaining the birefringence of the test object from the output detected by the light receiving element.

According to a fourth aspect of the present invention, there is provided a method for measuring birefringence.
Using the birefringence measurement device according to claim 1 and the birefringence measurement device according to claim 3, the birefringence of the test object is measured by each of the birefringence measurement devices, and the light source wavelength is determined based on the measured values. It is to measure the birefringence phase difference and the distribution of the birefringence phase difference as large as possible.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail based on illustrated embodiments.

FIG. 1 is a schematic structural view of a birefringence measuring apparatus according to an embodiment of the present invention, in which a transparent flat plate having a large birefringence phase difference is used as a test object. In FIG.
A randomly polarized light beam from a HeNe laser 1 as a light source passes through an ND filter 2 for adjusting the amount of light, and is reflected by mirrors 3 and 4. Then, the polarizing plate 5
Is converted into circularly polarized light by the λ / 4 plate 6, then expanded by the beam expander 7, and the object 8 is irradiated with the expanded collimated light of circularly polarized light. After reducing the luminous flux that has become nearly circularly polarized light due to the birefringence of the test object 8 by the beam expander 9, the light beam is converted into nearly linearly polarized elliptically polarized light by the λ / 4 plate 10, and 11, lens 12
The light is received by the CCD camera 13 via the. The position of the lens 12 is adjusted so that an image-forming relationship is established with the vicinity of the test object 8, and a material whose internal birefringence has been sufficiently removed such as a glass lens should be used. preferable. Reference numerals 14 and 15 denote stepping motors for rotating the λ / 4 plate 10 and the polarizing plate 11 around the traveling direction of the light beam, respectively. A rotation origin position sensor (not shown) is attached to the stepping motors 14 and 15. By counting the number of pulses, the rotation angles of the λ / 4 plate 10 and the polarizing plate 11 can be known. Reference numeral 16 denotes a motor driver for driving the stepping motors 14 and 15, and the personal computer 18 and the pulse generator 1
7 and the stepping motors 14 and 15
Drive. Further, image data captured by the CCD camera 13 is taken into the memory of the personal computer 18 through the image input device 19, and based on the image data and the rotation angle data of the stepping motor, the test object 8 is obtained by a predetermined calculation method. Is calculated. The image picked up by the CCD camera 13 may be displayed on the monitor 18a of the personal computer 18 or a dedicated monitor may be separately provided.

The azimuth of the polarizing plate 5 is set in a direction horizontal to the ground, the azimuth of the λ / 4 plate 6 is set to 45 degrees with respect to the ground, and the object 8 is irradiated with circularly polarized light. I do. Before performing the measurement, the azimuth of the λ / 4 plate 10 is set at 45 degrees with respect to the direction parallel to the ground, and the azimuth of the polarizing plate 11 is rotated while the test object 8 is not set. The azimuth angle of the polarizing plate 11 in which the transmitted light intensity from the light source becomes the smallest (the transmitted light becomes the darkest) is determined and stored as the rotation origin in the measurement. In the measurement, first, the test object 8 is set at a predetermined position, and the direction of the λ / 4
In the state of 5 degrees, the polarizing plate 11 is moved (180/180) from the rotation origin.
n) Rotate by degrees. n is the number of measurement points determined in advance. Each time the polarizing plate 11 rotates by (180 / n) degrees, an image captured by the CCD camera 13 is loaded into the memory of the personal computer 18 to obtain rotation angle data of the polarizing plate 11 and n CCD image data. .

Next, the orientation of the λ / 4 plate 10 is set to 0 with respect to the horizontal.
And the CCD camera 13 while rotating the polarizing plate 11 (180 / n) from the rotation origin in the same manner as described above.
Image data captured by the personal computer 1
8 to obtain the rotation angle data of the polarizing plate 11 and the n-th image data. The birefringence of the test object 8 is calculated by the following procedure based on the total 2n image data and the rotation angle data of the polarizing plate 11 obtained as described above.

The state of change of the polarization state in the optical system in the birefringence measuring apparatus shown in FIG. 1 is represented using a Mueller matrix. L is the Mueller matrix of circularly polarized light incident on the test object 8, T is the Mueller matrix of the test object 8,
The Stokes parameter S is determined by defining the Mueller matrix of the 板 plate 10 as Q and the Mueller matrix of the polarizing plate 11 as A.

First, the Stokes parameter S ₄₅ when the azimuth of the λ / 4 plate 10 is set to 45 degrees with respect to the direction parallel to the ground is expressed by the following equation (1), and is detected by each pixel of the CCD camera 13. The light intensity I ₄₅ is as shown in the following equation (2).

[0016]

(Equation 1)

In the equations (1) and (2), θ is the main axis direction of the polarizing plate, δ is the birefringence phase difference of the test object, and φ is the main axis direction of the test object. The light intensity I _{45 in the} expression (2) changes as shown in FIG. 2 with the rotation of the main axis direction of the polarizing plate. Assuming that the reading resolution of the rotation angle of the polarizer is R (the rotation angle corresponding to one pulse of the stepping motor), the phase ψ ₄₅ of the light intensity change due to the rotation of the main axis direction of the polarizing plate is obtained as follows.

[0018]

(Equation 2)

Next, when the azimuth of the λ / 4 plate 10 is set to 0 ° with respect to the direction parallel to the ground, the phase of the light intensity change ψ
_{Find 0} . The Stokes parameter S _{0 at} this time is
Similarly, the following equation (5) is obtained, and the light intensity I ₀ detected at each pixel of the CCD camera 13 is expressed by the following equation (6).

[0020]

(Equation 3)

In equations (5) and (6), θ is the main axis direction of the polarizing plate, δ is the birefringence phase difference of the test object, and φ is the main axis direction of the test object. The phase ψ ₀ of the change in the light intensity I ₀ due to the rotation of the main axis direction of the polarizing plate is obtained as follows in the same manner as the equation (4).

[0022]

(Equation 4)

Here, when the phases ψ ₄₅ and ψ ₀ are obtained by modifying the equations (2) and (6), they are expressed as the following equations (8) and (9).

[0024]

(Equation 5)

From the above equations (4), (7), (8) and (9), the phase difference δ and the principal axis azimuth φ are obtained as the following equations (10) and (11) (claim 1).

[0026]

(Equation 6)

When the measurement is performed by the method described above,
FIG. 3 shows an example of the measurement result of the two-dimensional phase difference distribution. FIG.
FIG. 3A is a two-dimensional image display of the measured values (in which the measured values are replaced with the gradation of the image intensity), and FIG. 3B is a graph showing the measured values in the cross section taken along line AA ′ in FIG. Distribution. (1
The measured value of the phase difference calculated by the equation (0) is obtained as a phase angle. In FIG. 3, the measured value is multiplied by λ / (2 · π) (λ is a light source wavelength) and converted into a unit of length. It is. With respect to the measurement result of FIG. 3, the measured value of the phase difference calculated by the equation (10) is output only in the range of 0 to λ / 4.
When there is a large phase difference distribution exceeding the range of λ / 4, a value different from the original measurement result is output (hereinafter, this is referred to as “phase skip”). For example, the phase difference of 3λ / 8 is originally λ / 8 or −λ /
The phase difference of 8 is output as + λ / 8. Therefore, it is necessary to return the output measurement value to the original phase difference (hereinafter, this is referred to as “phase jump correction”).
4, 5, 6, 7 will be described with reference to FIGS.

In the measurement, the phase difference and the principal axis direction are simultaneously obtained for each pixel of the CCD. However, when the measured value of the phase difference is out of the range of 0 to λ / 4 at the pixel B in FIG. The value of the main axis direction of the pixel B changes by 90 degrees with respect to the direction value. The phase difference can be known from the ratio of the major axis to the minor axis of the elliptically polarized light, and the principal axis direction can be known from the direction of the major axis of the elliptically polarized light. For example, when the phase difference takes a negative value, FIG. As described above, since the major axis and the minor axis of the elliptically polarized light are switched, the principal axis direction changes by 90 degrees between adjacent pixels as described above (the measurement range of the principal axis direction is -90 to 90).
+90 degrees). Therefore, as a first stage of the phase jump correction, a change of the main axis direction by 90 degrees between adjacent pixels is detected, and
By multiplying the measured value of the phase difference in the same pixel by (−1), the range of the measured value of the phase difference can be expanded from −λ / 4 to λ / 4. Next, as a second stage of the phase jump correction, a pixel in which the measured value of the phase difference between adjacent pixels changes by λ / 2 is detected. In this case, the phase difference exceeds one wavelength of the light source wavelength. Then, λ / 2 or -λ / 2 is added to the value of the phase difference of the pixel in which the measured value of the phase difference between adjacent pixels changes by λ / 2. By repeating these processes for all the pixels in the measurement area, it is possible to obtain a measured value exceeding -λ / 4 to λ / 4 over all the pixels (claim 2). FIG. 6 is a flowchart showing an example of the phase jump correction method, and FIG. 7 shows how the measured value distribution changes with the processing of FIG.

According to the birefringence measuring apparatus and the birefringence measuring method using the apparatus according to claims 1 and 2, a distribution of a large birefringence phase difference exceeding the wavelength of a light source to be used can be measured. Since the devices and methods according to the items 1 and 2 are comparative measurements for comparing the measured values of the adjacent pixels, the measured values of the reference phase difference are required to perform the absolute measurement of the phase difference. Therefore, an apparatus and a method for obtaining the reference value of the phase difference will be described below.

FIG. 8 is a schematic structural view of a birefringence measuring apparatus according to a third embodiment of the present invention. In FIG. 8, the semiconductor laser 20 of wavelength λ ₁ or the semiconductor laser 21 of wavelength λ ₂
Is transmitted through an ND filter 23 for adjusting the amount of light, is turned back by mirrors 24 and 25, and is
Is converted into linearly polarized light, and is converted into circularly polarized light by the λ / 4 plate 27. Reference numeral 22 denotes a mirror for switching the light to be irradiated on the test object 28 between the semiconductor laser 20 and the semiconductor laser 21. The mirror 22 is detachable together with a holder (not shown) for the switching. Since the λ / 4 plate has wavelength dependence, the λ / 4 plate 27 can be detached and replaced according to the wavelength of the light source to be used. The light converted into circularly polarized light by the λ / 4 plate 27 is incident on the test object 28 (in the illustrated example, a transparent flat plate having a large birefringence phase difference). The elliptically polarized light due to the birefringence of the test object 28 is converted into elliptically polarized light close to linearly polarized light by the λ / 4 plate 29, and received by the photodiode 31 via the analyzer 30. λ / 4 plate 29 is λ / 4 plate 2
Like FIG. 7, it can be exchanged according to the wavelength of the light source used. Reference numeral 32 denotes a stepping motor for rotating the analyzer 30 substantially around the traveling direction of light.
2, a rotation origin position sensor (not shown) is attached,
By counting the number of pulses of the stepping motor 32, the rotation angle of the analyzer 30 can be known. An AD converter 33 converts an output from the photodiode 31.
After the output from the photodiode 31 is AD-converted,
The data is taken into the personal computer 34.

In the measurement, first, the test object 28 is set at a predetermined position, the stepping motor 32 is rotated, and while detecting the rotation angle of the analyzer 30, (180 / n) degrees (n: the number of measurement points) The output of the photodiode 31 is sampled for each rotation angle of
After the conversion, the data is taken into the personal computer 34 together with the rotation angle data at the time of sampling. The transition of the polarization state of light in this configuration is the same as when the direction of the λ / 4 plate 10 is set at 45 degrees with respect to the direction parallel to the ground in the apparatus according to claim 1 shown in FIG. , The light intensity I obtained by the photodiode 31 is expressed as follows.

[0032]

(Equation 7)

In the equation (12), δ and φ represent the birefringence phase difference and the principal axis direction of the test object 28, respectively, and θ represents the rotation angle of the direction of the analyzer 30. Cos in equation (12)
If the component is α, the sin component is β, and the resolution for reading the angle of the rotary analyzer is R (the rotation angle corresponding to one pulse of the stepping motor 32), the following equation (13) is obtained.

[0034]

(Equation 8)

Then, the birefringence phase difference δ and the principal axis direction φ of the test object can be obtained by using the following equations (14) and (15).

[0036]

(Equation 9)

Here, the phase difference measured using a semiconductor laser of wavelength λ _{1 as} a light source is δ ₁ , the phase difference measured using a semiconductor laser of wavelength λ ₂ is δ _2, and the true value of the phase difference is Δ
FIG. 9 illustrates the relationship among δ ₁ , δ ₂ , and Δ when. Wavelength lambda _1, because the measuring range in case of using the wavelength lambda ₂ are different, the measurement value becomes different when the point deviates from the true value with wavelength lambda _1, wavelength lambda ₂ as shown in FIG. Therefore, δ _1m and δ _2n are calculated while gradually increasing the wave numbers m and n by the following equations (16) and (17), and the wave numbers m and n at which the values of δ _1m and δ _2n are closest are calculated. Once determined, the values of δ _1m and δ _2n at that time become the true values of the phase difference.

Δ _1m = m · λ ₁ −δ ₁ δ _1m = m · λ ₁ + δ ₁ (m = 0,1,2,...) (16) δ _2n = n · λ ₂ −δ ₂ δ _2n = n · λ ₂ + δ ₂ (n = 0,1,2, ...) (17)

The above-described method is a so-called point measurement in which a thin parallel beam is irradiated on the test object, and therefore, only a part of the measured value of the test object can be obtained. Therefore, in order to obtain the true value of the phase difference over the entire test object, the apparatus according to claim 1 shown in FIG. 1 and the apparatus according to claim 3 shown in FIG. 8 are used together.

First, a reference position for measurement is set arbitrarily on the test object, and the true value of the phase difference at that position is determined by the apparatus according to claim 3 shown in FIG. Thereafter, when the distribution of the phase difference over the entire test object is measured by the apparatus according to claim 1 shown in FIG. 1, the position difference of the phase difference with respect to the measured value at the reference position is detected at any position on the test object. Since the change can be detected, the true value of the phase difference is obtained over the entire test object (claim 4).

When the object to be measured is a lens and the measurement is performed by the apparatus according to claim 3 shown in FIG. 8, the light beam is refracted when passing through the lens which is the object and is transmitted. Since the light does not reach the light receiving element, it is preferable to irradiate light near the center of the lens (near the optical axis) so that transmitted light is not refracted, and set that position as a reference position for phase jump correction.

[0042]

As described above, according to the birefringence measuring apparatus and the birefringence measuring method according to the present invention, even a specimen having a large birefringence exceeding the wavelength of the light source used can be used.
High-speed, high-precision measurement is possible over the entire surface of the test object. Further, since the measurement is performed on a surface, it is possible to cope with a case where the test object is a lens.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a birefringence measuring apparatus according to an embodiment of the present invention.

FIG. 2 is a view showing an example of a relationship between a rotation angle of an analyzer (polarizing plate) and light intensity in the apparatus of FIG.

3A and 3B are diagrams showing examples of measurement results of a two-dimensional phase difference distribution by the apparatus of FIG. 1, wherein FIG. 3A shows a two-dimensional image display of measured values (in which the measured values are replaced by image intensity gradations). (B) is a diagram showing a distribution of measured values in a cross section taken along line AA ′ in (a).

4A and 4B are diagrams illustrating an example of a measurement result in the apparatus of FIG. 1, wherein FIG. 4A illustrates a relationship between a measurement position and a phase difference;
(B) is a figure which shows the relationship between a measurement position and a principal axis direction.

FIG. 5 is a diagram illustrating a state in which the major axis and the minor axis of elliptically polarized light are switched between adjacent pixels due to phase skipping.

FIG. 6 is a flowchart illustrating an example of a phase jump correction method.

FIG. 7 is a diagram illustrating a state of a change in a measured value distribution accompanying the correction processing in FIG. 6;

FIG. 8 is a schematic structural view of a birefringence measuring apparatus according to an embodiment of the present invention.

9 is a diagram showing a measurement example in the device of FIG. 8,
It is a figure which shows the relationship between the measured value at the time of measuring a phase difference using two light sources with different wavelengths, and the true value of a phase difference.

[Explanation of symbols]

 1: HeNe laser 2: ND filter 3, 4: mirror 5, 11: polarizing plate 6, 10: λ / 4 plate 7, 9: beam expander 8: test object 12: lens 13: CCD camera 14, 15: Stepping motor 16: Motor driver 17: Pulse generator 18: Personal computer 18a: Monitor 20, 21: Semiconductor laser 22: Mirror for switching light source 23: ND filter 24, 25: Mirror 26: Polarizer 27, 29: λ / 4 plates 28: test object 30: analyzer 31: photodiode 32: stepping motor 33: AD converter 34: personal computer

Claims (4)

[Claims] 1. A light source for causing light to enter a test object in a predetermined polarization state, a polarizing element for changing a polarization state of light transmitted from the test object, and a polarizing element disposed substantially around a traveling direction of light. Means for rotating, means for detecting the rotation angle of the polarizing element, a light receiving element array comprising a plurality of light receiving elements and receiving light transmitted through the polarizing element, and expanding the light from the light source to the test object. An optical system for irradiating an object and forming an image of transmitted light from the test object substantially on a light receiving surface of a light receiving element array;
A birefringence measuring device, comprising: calculating means for obtaining birefringence of the test object from an output detected by the light receiving element. 2. A phase jump in a birefringence phase difference distribution by detecting a difference between measured values of adjacent light receiving elements of a light receiving element array using the birefringent device according to claim 1. A birefringence measurement method, wherein a value different from the original measurement result is output when there is a large phase difference distribution that exceeds the value. 3. A laser light source comprising: two or more laser light sources having different wavelengths for causing light to enter a test object in a predetermined polarization state; and means for switching irradiation of the test object with light from the two or more laser light sources. A polarizing element that changes a polarization state of transmitted light from the test object, a unit that rotates the polarizing element substantially around a traveling direction of light, a unit that detects a rotation angle of the polarizing element, and the polarization unit. A birefringence measuring apparatus, comprising: a light receiving element for receiving light transmitted through the element; and a calculating means for obtaining birefringence of the test object from an output detected by the light receiving element. 4. A birefringence measuring device according to claim 1, and
Using the birefringence measurement device described, the birefringence of the test object is measured by each birefringence measurement device, and based on the measured value, a large birefringence phase difference and a birefringence phase difference exceeding the light source wavelength are obtained. A birefringence measuring method, characterized by measuring the distribution of the birefringence.