JPH10153500A - Method and device for measuring photoelastic constant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for easily measuring the photoelastic constant of a sample to be measured having a light-transmissible material, including the wavelength dependency (wavelength dispersion). SOLUTION: The light for measurement which is phase-modulated by a photoelastic modulator 15, is applied to a sample to be measured 21, and the external force generating, a stress in a state which makes its intensity an direction understandable is added to the same by a pressing means. The light having passed through the sample to be measured, is detected by a photo detecting means 19, while rotating the sample to be measured 21 which is irradiated by the light and to which the external force is added, in a condition that an axis of rotation is agreed with an optical axis of the measuring beam. The double refraction quantity of the sample to be measured 21 is calculated on the basis of the detected light. The photoelasticity constant of the sample to be measured 21, is measured on the basis of the calculated refraction quantity and the stress.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the photoelastic constant of a translucent material and its dispersion with high accuracy, and an apparatus suitable for carrying out the method.

[0002]

2. Description of the Related Art In recent years, an optical system utilizing polarized light, that is, a field of application of a polarized optical system has been rapidly expanding. In order to obtain desired characteristics in such a polarization optical system, a technique capable of controlling the polarization characteristics with high precision is important.

[0003] By the way, when a stress is generated by applying a force to a homogeneous isotropic light-transmitting material such as glass, optical anisotropy occurs, and as with certain types of crystals, Becomes birefringent. This is called a photoelastic effect, and the amount of birefringence generated per unit stress and per unit optical path in a translucent material is defined as a photoelastic constant. As a matter of fact, when a light-transmitting material such as glass is incorporated into various optical systems, the heat stress and mechanical external stress applied to the light-transmitting material cannot be zero. Therefore, in the light-transmitting material, a photoelastic effect occurs due to the above-mentioned stress, and birefringence occurs due to the photoelastic effect. The birefringence generated in this way is one of the causes of deteriorating the polarization characteristics of the polarization optical system incorporating the light-transmitting material. Therefore, in order to control the polarization characteristics with high precision by the polarization optical system, a technique capable of measuring the photoelastic constant of the translucent material with high precision is required.

As one of conventional methods for measuring the photoelastic constant of a light-transmitting material, there is, for example, a method disclosed in Japanese Patent Application Laid-Open No. 4-64444. In this method, the amount of birefringence and the principal axis direction of the birefringence of a sample without external stress are obtained with high accuracy by a method that applies the principle of the optical heterodyne method, and then the external stress is applied by the same method. The birefringence amount and the principal axis azimuth of birefringence of the sample are determined with high accuracy, and then the photoelastic constant is determined based on the obtained data (for example, page 7, left upper column, line 14 to upper right column, page 7). 4
line). According to this method, 1 × 10 ⁻⁴ (nm / cm) /
(Kgf / cm ² ) or less can be measured (for example, page 6, lower right column, bottom line to page 7, upper left column, line 3).

On the other hand, as one of the conventional methods for measuring the amount of birefringence of a material having birefringence with high accuracy, there is a method disclosed in, for example, JP-A-63-82345.
This irradiates the sample with phase-modulated measurement light,
The sample is rotated so that the rotation axis coincides with the optical axis of the measurement light, and the amount of birefringence of the sample is measured based on the light transmitted through the sample in that state (for example, Japanese Patent Application Laid-Open Nos. H11-19710 and H10-203).
Example of page). According to this method, the birefringence is set to ± 0.01.
It can be expected that the measurement can be performed with an accuracy of nm (for example, page 2, lower right column, lines 6 to 9). Compared to the point that birefringence could only be measured with an accuracy of about ± 0.1 nm by a general polarization compensation method in which a DUT and a Babinet Soleil phase compensator were arranged between orthogonal polarizers, This method disclosed in JP-A-82345 was an excellent method for measuring birefringence.

[0006]

However, JP-A-63-82345 only describes a method for measuring the birefringence of a translucent material having birefringence in the first place. There is no description of any technique that can measure the birefringence of a light-transmitting material that does not normally exhibit birefringence, such as glass, and also any technique that can measure the photoelastic constant of such a light-transmitting material. There is no suggestion.

In the method disclosed in Japanese Patent Application Laid-Open No. 4-64444, it is necessary to use a device for generating a beat signal such as a horizontal Zeeman laser device as a light source (for example, page 4, lower left column, second line). Since the horizontal Zeeman laser device itself is expensive, there is a problem that the measuring device cannot be easily manufactured by the method disclosed in Japanese Patent Application Laid-Open No. 4-64444. Furthermore, due to the use of a lateral Zeeman laser device, the wavelength of the measurement light is practically limited to a single wavelength, and there is a problem that it is impossible to measure the photoelastic constant at a desired wavelength. The photoelastic constant of general optical glass often used for a lens, a substrate, a prism, and the like of a polarization optical system has dispersion and shows a different value depending on the wavelength. Therefore, it is essential to measure the photoelastic constant with high accuracy in consideration of the wavelength dependency (wavelength dispersion) and to recognize the value at the wavelength of the light to be used in developing a system using a polarization optical system. . However, JP-A-4-64
The method disclosed in Japanese Patent No. 44 is not preferable because the photoelastic constant cannot be measured in consideration of the wavelength dispersion.

There is a need for a method capable of easily measuring the photoelastic constant of a light-transmitting material and including the wavelength dependency (wavelength dispersion) and an apparatus suitable for carrying out the method.

[0009]

According to the invention of a method for measuring a photoelastic constant of the present application, a light-transmitting sample to be measured is irradiated with a phase-modulated measuring light and the intensity is measured. And applying an external force to generate a stress in a state in which the direction is known, the amount of birefringence determined by detecting the polarization state of light transmitted through the sample to be measured in the state where the light irradiation and the external force are applied, The photoelastic constant of the sample to be measured is measured based on the stress.

According to the present invention, the sample to be measured in a state where an external force is applied is irradiated with the phase-modulated light (hereinafter, also referred to as phase-modulated light). The sample to be measured in the generated state is transmitted. Therefore, even if the sample to be measured does not exhibit birefringence in a normal state, the measurement in the state where birefringence occurs due to the photoelastic effect during measurement by the method of the present invention. The phase modulated light will be transmitted through the sample. On the other hand, the measurement light, which has been phase-modulated at a specific phase amplitude and frequency, is transmitted through the sample to be measured, and an alternating current (ω) component and a direct current (DC) component in a specific polarization direction are detected from the transmitted light. It is known that birefringence can be measured by calculating these results (for example, Document I: Optical Technology Contact Vol. 27, No. 3 P. 27 (1989; details will be described later). It is also known that a photoelastic constant can be obtained from the birefringence generated when a stress is caused to cause a photoelastic effect if the stress is known. The refraction is caused by the photoelastic effect, and in the present invention, an external force is applied to the sample to be measured so that the direction and intensity of the stress generated in the sample can be known. , It is possible to obtain the photoelastic constant.

In the case of the present invention, since the phase modulated light is used, the light source is not particularly limited. This means that it is possible to use a light source having a wide band that generates at least the wavelength light for which the wavelength dependency of the photoelastic constant is to be measured, and to perform the measurement by dispersing the light to a predetermined wavelength width by the spectral means. Therefore, wavelength dependence of photoelastic constant (wavelength dispersion of photoelastic constant)
Can be easily measured.

According to the invention of the photoelastic constant measuring apparatus of the present application, there is provided an apparatus for measuring a photoelastic constant of a translucent sample to be measured, wherein the wavelength dependence of the photoelastic constant is measured. A light source that generates at least the wavelength light to be measured, a spectral unit that splits the light from the light source into light having a predetermined wavelength width, and phase-modulates the spectral light to generate measurement light, and the measurement light is used as the sample to be measured. Phase modulating means for irradiating the sample to be measured, pressing means for applying a stress in a state where the strength and direction are known to the sample to be measured, and A sample rotating means for rotating in a state coinciding with the optical axis of the light, a light detecting means for detecting light transmitted through the sample to be measured, and Birefringence calculation for calculating the birefringence of the sample to be measured Means, characterized in that comprising a photoelastic constant calculating means for calculating the photoelastic constant of the sample under test based on the calculated amount of birefringence and the stress.

According to this photoelastic constant measuring apparatus, since the pressure means is provided, a stress can be generated in the sample to be measured in a state in which the strength and the direction are known. Further, since the light source, the spectroscopy means, and the phase modulation means are provided, it is possible to irradiate the target sample in a stress-generated state with light having a desired wavelength and phase modulated. Furthermore, since the sample rotation means, light detection means, birefringence amount calculation means and photoelastic constant calculation means are provided, obtaining the birefringence amount of the measurement sample,
The photoelastic constant of the sample to be measured can be obtained from the obtained birefringence and the stress. Therefore, the method for measuring a photoelastic constant according to this application can be easily implemented.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a photoelastic constant measuring method and a photoelastic constant measuring apparatus of the present application will be described below with reference to the drawings. However, the drawings used in the description are merely schematic representations so that the present invention can be understood. Also, in each of the drawings, the same components are denoted by the same reference numerals, and duplicate description thereof may be omitted.

In this embodiment, a sample to be measured is irradiated with measurement light that has been phase-modulated at a specific phase amplitude and frequency. In addition, the measurement light is irradiated with an external force applied to the measured sample so that the magnitude and intensity of the stress generated in the measured sample can be understood. Then, an AC (ω) component and a DC (DC) component in a specific polarization direction are detected from the transmitted light transmitted through the sample under the condition of the light irradiation and the external force, and the results are calculated. This measures the birefringence. Then, the photoelastic constant of the sample to be measured with respect to the measurement light is determined from the stress and the birefringence. This will be described more specifically with reference to FIG. FIG. 1 is a diagram showing an example of a basic measurement system for implementing the photoelastic constant measurement method of the present invention. That is,
Light source 11, polarizer 13, photoelastic modulator (PEM) 1
5, an analyzer 17 having a polarization plane rotated by 45 degrees with respect to the polarizer 13, a light detection unit 19, and a pressurizing unit 23 for applying an external force to the sample 21 to be measured. FIG. The components 11 to 19 are arranged in this order along the optical axis of the light for measurement. The sample 21 to be measured is inserted between the photoelastic modulator 15 and the analyzer 17.

In this measuring system, light emitted from the light source 11 is converted into linearly polarized light by the polarizer 13 and further subjected to phase modulation by the photoelastic modulator 15. This light is applied to the sample 21 to be measured. The light transmitted through the sample to be measured is detected by the light detecting means 19 after passing through the analyzer 17 whose polarization plane is rotated by 45 degrees with respect to the polarizer 13.

When the sample 21 to be measured is a sample having birefringence having a fast axis in a specific direction, the sample 21
Is rotated in a plane perpendicular to the measurement optical axis, the light detecting means 19
The ω component and DC component of the signal detected at
1 as a function of the rotation angle. Specifically, assuming that the ω component and the DC component of the light detected by the light detection unit 19 are represented by Iω (θ) and I _DC (θ), respectively, the modulation frequency of the photoelastic modulator 15 is ω , Modulation amplitude A = 13
When the angle is 7.79 degrees, the relational expressions shown in Expressions (1) and (2) hold. These equations (1) and (2) are described in, for example, Document I (Optical Technology Contact Vol. 27, No. 3,
P. 27 (1989)).

[0018]

(Equation 1)

In Equations (1) and (2), ρ is the amplitude transmittance of the sample 21 to be measured, Δφ is the birefringence (phase difference) generated in the sample 21 to be measured, and J ₁ (A) is the Bessel function. The first order coefficient, t, indicates time.

The amplitude transmittance ρ and the birefringence Δφ are obtained from the equations (1) and (2) according to the equations (3) and (4), respectively. Note that these equations (3) and (4) are also described, for example, in Document I (Optical Technology Contact V).
ol. 27, no. 3, p. 27 (1989)).

[0021]

(Equation 2)

Here, I _DC (0) is a DC component when θ = 0, and has a maximum value. I _DC (π / 4), Iω
(Π / 4) are I _DC and Iω when θ = (π / 4), respectively.
It is.

Therefore, during the measurement, the sample 21 to be measured is rotated on a plane perpendicular to the optical axis of the measurement to produce a state in which I _DC is maximized, and a state in which the sample 21 is rotated by π / 4 with respect to that state. Ω component and DC component of the output from the detector 19 in each state, that is, Iω (π / 4) and I _DC (0),
I _DC (π / 4) is measured, and these are calculated by the equations (3) and (4).
, The amount of birefringence of the sample to be measured can be obtained.

The point at which the amount of birefringence is determined as described above is described in, for example, Document I as described above. However, in the present invention, the amount of birefringence is measured in a state where an external force is applied to the sample to be measured by the pressing means 23. That is, the amount of birefringence caused by the photoelastic effect is measured in the present invention. Here, the photoelastic constant will be described in a general form as follows.

The refractive index of the translucent material when a stress is generated can be represented by a so-called refractive index ellipsoid. At this time, the main refractive index axis of the refractive index ellipsoid coincides with the main stress axis. Generally the principal refractive index respectively and _{n 1, n 2, n 3} , the main stress σ
_{If they are 1} , σ ₂ , and σ ₃ (those having common indices are in the same direction), a relationship such as Expression (5) is established between them.

[0026]

(Equation 3)

Here, C ₁ and C ₂ are light wavelengths and constants specific to the substance of the light-transmitting material, and n ₀ is a refractive index when there is no stress.

Various optical glasses widely used as translucent materials are said to exhibit elastic properties until just before breaking under stress, and optically satisfying the above formula (5). It is said to exhibit satisfactory properties, that is, properties in which the amount of birefringence changes in proportion to the magnitude of the stress. Therefore, when light is incident on such a translucent material, if the coordinates are set so that the direction is the same as σ3 in the equation (5), the incident light vibrates in the σ1 and σ2 directions, that is, mutually. The light is split into two linearly polarized light beams whose surfaces are perpendicular to each other. In addition, when light is emitted from the light-transmitting material on which light is incident as described above, a refractive index difference (n1, n2) in each principal stress direction is generated. The amount of birefringence Δφ that can be expressed as follows.

[0029]

(Equation 4)

Here, in equation (6), λ is the wavelength of light,
L is the light transmission thickness of the translucent material, and C = C1-C2 is called the photoelastic constant.

Considering the case where a uniaxial stress having a known magnitude of σ1 = 0 is applied, equation (6) gives Δφ = (2
π / λ) · C · σ ₂ · L, which is further expanded into an expression for the photoelastic constant C to obtain the following expression (7).

[0032]

(Equation 5)

As can be seen from equation (7), the photoelastic constant is the birefringence Δφ measured by the method of the present invention.
Can be obtained by dividing the stress value generated in the sample to be measured by the light transmission thickness L of the sample to be measured. Since the stress value and the light transmission thickness L of the sample to be measured are known as measurement conditions, it is understood that the photoelastic constant can be obtained according to the method of the present invention. Moreover, as the light source 11, it is possible to use a light source having a wide band that generates at least the wavelength light whose wavelength dependence of the photoelastic constant is to be measured. Then, the light from the light source can be split into a predetermined wavelength width by the splitting means and used as measurement light. Therefore, the wavelength dependence of the photoelastic constant can be easily measured.

[0034]

Next, the present invention will be further described with reference to examples. The following examples are conscious of an optical system using light in a wavelength region centered on a visible region, which is generally being developed, and can measure a photoelastic constant at a wavelength of 300 to 1000 nm. Here is an example.

First, the configuration of the photoelastic constant measuring device used in the embodiment by the present inventor will be described with reference to FIG. However, in FIG. 2, the same components as those shown in FIG. 1 are denoted by the same reference numerals.

The photoelastic constant measuring apparatus used in the embodiment is the same as that of FIG. 1, the light source 11, the polarizer 13, the photoelastic modulator 15, the analyzer 17, the light detecting means 19 and the pressing means 2.
It has three.

Here, as the light source 11, a light source (also referred to as a continuous light source) that generates at least the wavelength light whose wavelength dependence of the photoelastic constant is to be measured is used. A continuous light source here means a light source that exhibits a continuous spectrum if you want to completely measure the photoelastic constant for each arbitrary wavelength.
It is not limited to a light source showing a continuous spectrum as long as it is a light source showing an emission line at several wavelengths at which the measurement of the photoelastic constant is desired. Here, a halogen lamp, which is more stable and cheaper than various discharge tubes, was used as the light source 11. In FIG. 2, reference numeral 11a denotes a power supply for driving the light source 11.

As the photoelastic modulator 15, a PEM-
90 (made by HINDS INSTRUMENTS, USA) was used.
The photoelastic modulator 15 uses quartz glass as a medium. In FIG. 2, reference numeral 15a denotes an AC voltage generator for supplying an AC voltage to the photoelastic modulator 15.

Each of the polarizer 13 and the analyzer 17 includes:
In order to obtain higher measurement accuracy, those having a large extinction ratio are preferable. Although not limited to this, the extinction ratio is 1 here.
A 0000: 1 Gran Thompson prism was used.

In this case, a photomultiplier tube (photomultiplier) having high sensitivity in the visible to near-ultraviolet region was used as the light detecting means 19. However, when the setting of the sample 21 or the parallelism of the two measurement light transmitting surfaces of the sample 21 is poor,
When the sample to be measured 21 is rotated by the rotating means 35 (described later), the beam is blurred, which causes the sensitivity unevenness of the light receiving surface. In such a case, a transmission diffusion plate, an integrating sphere, and the like are preferably provided between the sample 21 to be measured and the photomultiplier tube 19 as necessary. In FIG. 2, reference numeral 19a denotes a power supply for driving the photomultiplier tube 19, and 19b denotes a preamplifier for amplifying the output of the photomultiplier tube 19.

The pressing means 23 has a pressing portion 23a for applying an external force to the sample to be measured, a function of holding the sample 21 to be measured, and a function of pressing the sample 21 by the external force from the pressing portion 23a. A holding unit 23b, a pressure / voltage converter (also called a load cell) 23c for monitoring the strength of an external force from the pressurizing unit 23a, a power supply 23d for driving the pressure / voltage converter 23c, This is a means having a configuration including a DC voltmeter 23e for detecting the output voltage of the pressure / voltage converter 23c. FIG. 3 shows a more specific configuration example of the pressing means 23. In the example shown in FIG.
The configuration is such that the pressing portion 23a and the holding portion 23b are inserted into a housing portion 25f (not shown in FIG. 2). Pressurizing section 23a,
Each of the holding portion 23b and the housing 23f is made of metal. When the pressurizing portion 23a is pressed from a direction that does not hinder the irradiation of the measurement light, an external force is applied to the measured sample 21 via the holding portion 23b, so that a compressive stress is generated in the measured sample 21. 25 g of silicon rubber is inserted between the sample 21 to be measured and the holder 23b so that the stress is uniformly generated in the sample 21 to be measured.

Further, the photoelastic constant measuring apparatus includes a light source 1
A collimator 31 for converting the light emitted from 1 into parallel light,
It is provided between the light source 11 and the polarizer 13. Since the halogen lamp is used as the light source, the performance of the collimator greatly affects the measurement accuracy as compared with the case where the laser is used as the light source. This point is taken into consideration when selecting a collimator.
It is desirable that the collimator 31 has a zoom function if it is a reflection type or a refraction type in order to cope with light of an arbitrary wavelength. When there is a concern about the problem of stray light, it is preferable to provide a stop at an arbitrary position between the light source 11 and the sample 21.

Further, the photoelastic constant measuring apparatus includes a light source 1
A spectral filter 33 for dispersing one light into light having a desired wavelength width is provided between the collimator 31 and the polarizer 13.
The spectral characteristics of the spectral filter 33 are determined in consideration of required wavelength accuracy, wavelength resolution, and the like. However, it is necessary to use a spectral filter having a transmission wavelength width that is not affected by the wavelength dispersion of the photoelastic constant of the quartz glass used as the medium of the photoelastic modulator 15. In this embodiment, considering that an appropriate amount of light can be obtained after passing through the spectral filter 33, a spectral filter having a spectral characteristic with a half-width of the center wavelength of about 10 nm was used.
In addition, a large number of spectral filters having such a configuration and having different center wavelengths are prepared, and these can be switched and used at appropriate times by the spectral filter switching means 33a. The spectral filter switching means 33a can be constituted by, for example, means for appropriately switching, by a stepping motor, a disk in which each position on the disk is a spectral filter having different spectral characteristics.

The photoelastic constant measuring apparatus further comprises a sample rotating means 35. The measured sample rotating means 35 is a state in which the measured sample 21 is pressed by the pressing means 23,
In addition, the rotation is performed with the rotation axis coinciding with the optical axis of the measurement light. For example, the means 35 can be constituted by a stepping motor and a suitable rotating mechanism.

Further, in this photoelastic constant measuring apparatus, the output of the photomultiplier tube 19 (specifically, the output of the preamplifier 19b)
An AC / DC separator 37 for separating an alternating current (ω) component and a direct current (DC) component from the A / C component, and a lock-in state between the ω component separated from the output of the photomultiplier tube 19 and the AC voltage generator 15a. Lock-in amplifier 39 for securing, DC voltmeter 41 for measuring the separated DC component, and computer 4 functioning as a control unit of the photoelastic constant measuring device.
3 and so on. The power source 11a for the light source, the spectral filter switching unit 33a, the power source 23d for the pressure / voltage converter, the sample rotating unit 35, the DC voltmeter 23e, the lock-in amplifier 39, and the DC voltmeter 41 are connected to the computer 43, respectively. is there.

In this photoelastic constant measuring apparatus, the amount of birefringence can be calculated by the personal computer 43 calculating the output of the light detecting means 19 and information on the rotation angle of the sample by the sample rotating means 35 ( Birefringence amount calculation means). Further, the photoelastic constant can be calculated by the personal computer 43 calculating the stress generated in the sample 21 measured from the external force applied by the pressurizing means 21 and the calculated birefringence (photoelastic constant calculation). means).

Next, a measurement procedure and a measurement result of the photoelastic constant will be described. First, the sample to be measured will be described. In order to generate a uniaxial uniform stress in the sample 21 to be measured, as shown in FIG. 4, each side constituting the pressing surface with respect to the dimension a along the pressing direction P of the sample 21 to be measured. Must be increased to some extent. In addition, of the sides b and c constituting the surface to be pressed, the dimension along the optical axis L of the measurement light, here, the dimension b is the transmission thickness of the measurement light. The measurement accuracy is improved. On the other hand, if the dimensions of the sides b and c constituting the surface to be pressed are too large, it becomes impossible to efficiently generate stress on the sample to be measured. Further, since it is necessary to rotate the sample to be measured 21 in a pressurized state, it is preferable that the sample to be measured 21 is small because the rotation mechanism is simplified. However, it is necessary to reduce the beam diameter of the measurement light as the sample 21 to be measured is reduced. However, if the beam diameter of the measurement light is too small, the amount of the measurement light decreases due to the nature of the collimator 31. This may cause a decrease in measurement accuracy. Furthermore, as the beam diameter is smaller, the influence of dust and the like on the measurement accuracy occurs. For these reasons, the sample 21 to be measured has a rectangular parallelepiped shape and the above-mentioned b, a, c
10 × 15 × 20m in the order of b, a, c
m. In addition, each of the surface of the sample 21 to be irradiated with the measurement light and the surface on the transmission side (a × c
If the parallelism of the two surfaces defined by (15 × 20) is poor, the beam of the measuring light beam becomes large when the sample to be measured is rotated at the time of measurement, which causes unevenness in sensitivity of the light detecting means 19. . Therefore, the parallelism is set to such a degree that it can be suppressed. Although not limited to this, in this example, the two surfaces were polished so that the parallelism was 1 minute, thereby obtaining a sample 21 to be measured.

On the other hand, the beam diameter of the measuring light is
To about 5 mm. Further, the spectral filter 33 is switched to a filter capable of transmitting a desired wavelength by using the spectral filter switching means 33a. The photoelastic modulator 15 is controlled by the AC voltage generator 15a so that the modulation amplitude A of the photoelastic modulator 15 becomes 137.79 degrees and the modulation frequency ω becomes 50 KHz. Modulation amplitude A is 13
The reason why the angle is set to 7.79 degrees is that in this case, the relations of the above equations (1) and (2) are satisfied. The reason why the modulation frequency ω is set to 50 KHz is that the photoelastic modulator 1
This is because the elastic deformation of the quartz glass, which is the medium used in 5, is good, and the signal passing through the photomultiplier tube 19 can be easily detected by the lock-in amplifier 39. The AC voltage generator 15a also sends a reference signal to the lock-in amplifier 39.

The sample 21 to be measured is pressed by the pressing means 23. Since the pressing force at this time appears in the output of the pressure / voltage converter 23c, the computer 43 takes it in via the DC voltmeter 23e. The computer 43 calculates the stress generated in the sample to be measured by the pressurization.

The output of the photomultiplier tube 19 is separated by an AC / DC separator 37 into a ω component and a DC component. The ω component is input to the computer 43 via the lock-in amplifier 39, and the DC component is input to the computer 43 via the DC voltmeter 41 and after being converted into a digital signal by an A / D converter (not shown). The computer 43 calculates the correlation between the ω component and the DC component from the photomultiplier tube 19, the rotation angle of the sample 21 to be rotated, and the signal measured from the pressure / voltage converter 23c. The photoelastic constant is calculated based on the stresses (1) to (7) above.

Next, specific measurement results will be described. Four types of measured samples 1 to 4 were prepared as the measured samples. Since residual internal strain causes a decrease in measurement accuracy, each of the samples to be measured was annealed before measuring the photoelastic constant. The samples to be measured were selected from optical glasses having a relatively small photoelastic constant. Specifically, the measured samples 1 and 2
Each is a flint glass having a high PbO (lead oxide) content, and the samples 3 and 4 to be measured are fluoride phosphate glasses.

First, the sample 1 to be measured is used. Then, in a state where an external force is applied to the sample 1 to be measured (a state in which stress is generated in the sample to be measured), and the sample to be measured is
The light transmitted through the sample to be measured is detected by the detection unit 19 while rotating the sample. In addition, the ω component Iω (θ) and the DC component I _DC (θ) of the detection light, that is, the above-described I _DC (0), I _DC (π / 4), and Iω (π / 4) are measured. The external force applied to the sample to be measured is
Here, the stress generated in the sample to be measured is from zero to about 50 N /
It was set so that there were 6 levels between cm ² . Next, birefringence generated in the sample to be measured is calculated based on Iω (θ) and I _DC (θ) obtained in the above measurement and the above equations (3) and (4). Also, this series of processing is performed by measuring light of various wavelengths as the measurement light, specifically, the center wavelength is 365 nm, 405 nm, 435 nm, 510 nm,
This was performed for each case where each light of 635 nm was used. When the angle for giving I _DC (0) is known, it is not necessary to perform the transmitted light measurement while rotating the sample to be measured by the rotating means 35. The transmitted light may be measured at this known angle, the sample may be rotated by an angle shifted by π / 4 from the position, and then the transmitted light may be measured again.

Next, using the calculated birefringence and stress, and using the least squares method, an approximate straight line of the relationship between the two is created. FIG. 5 also shows this approximate straight line and the amount of birefringence obtained above. In FIG. 5, the stress (N / cm ² ) generated in the sample to be measured is plotted on the horizontal axis, and the birefringence (nm) is plotted on the vertical axis.
/ Cm). In FIG. 5, black squares indicate the case where light having a center wavelength of 365 nm is used as measurement light, and black circles indicate that the center wavelength is 405 as the measurement light.
In the case of using light of nm, black triangles indicate the case where light with a central wavelength of 435 nm is used as measurement light, and the black diamonds indicate the case where light of a center wavelength of 510 nm is used as measurement light. The white square marks are obtained when light having a center wavelength of 635 nm is used as measurement light.

As can be seen from FIG. 5, for any wavelength, the relationship between stress and birefringence could be approximated to a straight line. In addition, R ² (mean square error) = 0.9
The error was 98 to 0.996. The reproducibility of the measurement when the same measurement was performed again was also very high. Here, Δφ in the above equation (6) is birefringence,
Since σ ₂ −σ ₁ is the stress, the slope of each approximate straight line shown in FIG. 5 can be said to be the photoelastic constant for each wavelength. It should be noted that all of the straight lines obtained here are R ² (mean square error) = 0.998 to 0.996, and therefore, according to the measuring method of the present invention, regardless of the magnitude of the stress, This means that the photoelastic constant can be measured with high accuracy. For example, it can be said that even when R ² = 0.996, the photoelastic constant can be measured with a variation of about 0.4%. Therefore, it can be said that even if the photoelastic constant is as small as about 0.05 cm ² / N, the photoelastic constant can be measured with a variation of about 0.4%. This can be said to be comparable to the method disclosed in JP-A-4-6444. However, JP
The method disclosed in Japanese Patent No. 6444 cannot measure the birefringence of a light-transmitting material that does not normally show birefringence, such as glass, but the present invention makes it possible.

Next, the photoelastic coefficient of each of the four kinds of samples to be measured is actually measured by the measuring method according to the present invention. Then, the change of the photoelastic constant with respect to the wavelength of light was plotted. FIG. 6 shows the result. FIG. 6 shows that the dispersion of the photoelastic constant of the optical glass greatly differs depending on the difference in the composition of the optical glass. In addition, it can be seen that there is a glass whose photoelastic constant changes greatly depending on the wavelength. This clearly indicates that in an optical system that requires high-precision polarization control, it is necessary to reflect the photoelastic constant of the transmissive material at the wavelength actually used in the design. And, using the method of the present invention,
In designing the optical system, it is possible to easily perform the design in consideration of the wavelength dependence of the photoelastic constant.

[0056]

As described in detail above, the present invention provides
In measuring the photoelastic constant, a method of measuring birefringence using a phase-modulated measurement light (photoelastic modulation method),
Since the method of pressurizing the sample to be measured is used, the photoelastic constant of the sample to be measured having a light-transmitting property can be measured with higher accuracy than before and at a desired wavelength of light. Development and selection of optical glass used in optical systems using highly controlled polarized light by performing measurements according to the present invention on various transmissive materials and measuring the photoelastic constant including dispersion characteristics in detail Can help. Further, in developing a device using the optical system, it is possible to give a specification such as how much the stress acting on the glass should be suppressed due to various causes.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a basic idea of a photoelastic constant measuring method according to the present invention.

FIG. 2 is an explanatory diagram of an embodiment of a photoelastic constant measuring device.

FIG. 3 is an explanatory diagram of a pressing unit.

FIG. 4 is an explanatory diagram of a sample to be measured.

FIG. 5 is an explanatory diagram of the example, and particularly shows a relationship between stress and birefringence when the wavelength of measurement light is used as a parameter.

FIG. 6 is an explanatory diagram of an example, particularly a diagram showing a relationship between a measurement light wavelength and a photoelastic constant for each sample having a different composition.

[Explanation of symbols]

 11: Light source 13: Polarizer 15: Photoelastic modulator 17: Analyzer 19: Photodetector (photomultiplier tube) 21: Sample to be measured 23: Pressurizer 23a: Pressurizer 23b: Holder (Measurement Sample holder) 23c: Pressure / voltage converter 23d: Power supply for pressure / voltage converter 23e: DC voltmeter

Claims (6)

[Claims] An external force is applied to a light-transmitting sample to be measured by irradiating the sample with phase-modulated light for measurement and generating a stress whose intensity and direction are known. Measuring the photoelastic constant of the sample to be measured based on the amount of birefringence obtained by detecting the polarization state of light transmitted through the sample to be measured in a state where the stress is applied, and the stress. The method of measuring the photoelastic constant. 2. A light-transmitting sample to be measured is irradiated with a phase-modulated light for measurement and an external force is applied so as to generate a stress whose intensity and direction are known. The sample to be measured in the added state is rotated with the rotation axis coinciding with the optical axis of the measurement light, and the sample to be measured is opposed to the light in at least two states. Detecting light transmitted through the sample, calculating a birefringence amount of the sample to be measured based on the detected light, and photoelasticity of the sample to be measured based on the calculated birefringence amount and the stress. A method for measuring a photoelastic constant, comprising measuring a constant. 3. The method for measuring a photoelastic constant according to claim 1, wherein a light source that generates at least light having a wavelength at which the wavelength dependence of the photoelastic constant is to be measured is used, and the light from the light source is separated by spectroscopic means. Disperse the light having a predetermined wavelength width, apply phase modulation to the light, generate the light for measurement, measure the photoelastic constant using the generated measurement light, and measure the wavelength dependence of these series of processes. A method for measuring a photoelastic constant, wherein the method is performed for each wavelength desired. 4. The method for measuring a photoelastic constant according to claim 3, wherein a halogen lamp is used as the light source. 5. An apparatus for measuring a photoelastic constant of a translucent sample to be measured, wherein the light source generates at least light having a wavelength at which the wavelength dependence of the photoelastic constant is to be measured, and light from the light source. A spectroscopic means for dispersing the light into light having a predetermined wavelength width, a phase modulation means for applying a phase modulation to the spectral light to generate a measuring light and irradiating the measuring light with the measuring light, Pressurizing means for applying an external force so as to generate a stress in a state in which the measurement light and the direction are known; Sample rotating means for detecting, light detecting means for detecting light transmitted through the sample to be measured, and calculating the amount of birefringence of the sample to be measured based on the light detected by the light detecting means Birefringence amount calculating means, and the calculated birefringence Photoelastic constant measuring apparatus characterized by equipped the photoelastic constant calculating means for calculating the photoelastic constant of the sample under test based on the amount and the stress. 6. The photoelastic constant measuring apparatus according to claim 5, wherein the light source is a halogen lamp.