Optics: measuring and testing – By polarized light examination – With birefringent element
Reexamination Certificate
1999-12-09
2001-11-13
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With birefringent element
C356S426000
Reexamination Certificate
active
06317209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates generally to systems for measuring birefringence or other optical property, e.g., transmission, of a sample of material.
2. Background Art
Birefringence, or double refraction, is a phenomenon that occurs in materials characterized by two indices of refraction. Typically, birefringent materials are optically anisotropic substances, e.g., calcite and quartz. Although, some isotropic materials, e.g., glass and plastic, become birefringent when subjected to stress. When a beam of light enters a birefringent material, the beam splits into two polarized rays traveling with different velocities, corresponding to two different angles of refraction. One ray, called an ordinary ray, is characterized by an index of refraction that is the same in all directions. The second ray, called an extraordinary ray, travels with different speeds in different directions and hence is characterized by an index of refraction that varies with the direction of propagation. If the light entering the birefringent material is unpolarized or linearly polarized, the ordinary and extraordinary rays will have the same velocity along one direction, called the optic axis. The ordinary and extraordinary rays recombine upon exiting the material.
Birefringent materials can change the polarization state of a light passing through them. Therefore, the ability to accurately determine the birefringence of a sample is important, especially in high performance optics, e.g., ophthalmic lenses, laser optics, and optical fibers, where a change in the polarization state of light can cause dramatic changes in optical performance. When a linearly polarized light passes through a birefringent sample, the sample rotates the direction of polarization through some angle. By measuring this angle of rotation, the birefringence of the sample, i.e., the difference between the highest and lowest indices of refraction of the sample, can be determined. Typically, the sample is placed between two crossed linear polarizers. The birefringence at a given point about the cross section of the sample is then determined by measuring the angular position, with respect to the first linear polarizer, at which the light emerging from the sample is extinguished as it passes through the second linear polarizer.
Various other methods are known for determining birefringence. One example of a known method is disclosed in U.S. Pat. No. 5,257,092 issued to Noguchi et al. As shown in
FIG. 1
, an optical source unit
2
emits a linearly polarized light beam, which passes through a quarter-wave plate
4
. The quarter-wave plate
4
converts the beam emitted by the optical source
2
to circularly polarized light, which then passes through the birefringent sample
6
, where the light emerges elliptically polarized. This emergent light then passes through a second quarter-wave plate
8
which converts the light to near-linear polarized light. The light then passes through a rotatable analyzer
10
. Birefringence is determined by measuring the angle of the analyzer
10
with respect to the source
2
at which light is extinguished. The method disclosed by this patent uses circularly polarized light rather than linearly polarized light because, in the samples used, birefringence had to be measured in all directions. If linearly polarized light is used, there inherently will be a direction in which no birefringence occurs, i.e., the optic axis.
Another example of a method for measuring birefringence is disclosed in U.S. Pat. No. 5,587,793 issued to Nakai et al. As illustrated in
FIG. 2
, a sample
12
is placed between a circular polarizer
14
and a circular analyzer
16
and arranged in an optical path between a light source
18
and an optical receiver
20
. The circular polarizer
14
is a combination of a polarizer
22
and a quarter-wave plate
24
, and the circular analyzer
16
is a combination of a quarter-wave plate
26
and an analyzer
28
. The circular analyzer
16
is arranged in a crossed Nicols fashion with respect to the circular polarizer. A crossed Nicols fashion refers to the arrangement of the polarizers such that their polarization axes are set
90
degrees from one another. In this method, monochromatic parallel beams emitted from the light source
18
are converted into circularly polarized light by the circular polarizer
22
and projected onto sample
12
. The light beams then pass through the circular analyzer
16
to be detected by the optical receiver
20
.
The birefringence of the sample may vary from location to location across the sample. Thus, in order to describe the birefringence of a sample, birefringence at a number of points along or distributed on the surface of the sample is measured. One procedure used in industry includes taking a measurement at one position on the cross section of a sample and then manually moving the sample e.g., by using a lab jack, so that the measurement is made at another test point on the cross section. The measurements are repeated at numerous test points about the cross section of the sample to generate a birefringence map. Because mapping requires a large number of points, mapping the sample manually is a difficult and time-consuming task. In some cases, the actual measurement is also performed manually, with the operator having to determine the actual angle of light extinction. Therefore, the accuracy of these measurements can fluctuate from operator to operator.
SUMMARY OF THE INVENTION
One aspect of the invention is an apparatus for making automated measurements of an optical property of a sample. The apparatus comprises a first stage which is movable along a predetermined line, a second stage for holding the sample, and a third stage which is movable along a predetermined line, correspondingly to the motion of the first stage. A light source is mounted on the first stage, and a light detector is mounted on the third stage. The second stage rotates the sample to a selected rotary position. The apparatus further comprises a controller for coordinating movement of the first, second, and third stages such that the light source, the sample, and the light detector are optically aligned.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
REFERENCES:
patent: 3811775 (1974-05-01), Abu-Saud
patent: 4626100 (1986-12-01), Johnson
patent: 5028774 (1991-07-01), Yoshizawa et al.
patent: 5257092 (1993-10-01), Noguchi et al.
patent: 5587793 (1996-12-01), Nakai et al.
patent: 6025906 (2000-02-01), Hepburn et al.
patent: 1 067 096 (2001-01-01), None
patent: 63-82345 (1988-09-01), None
patent: 02159540 (1998-12-01), None
Baoliang Wang & Patrick M. Troccolo, Measurement of Residual Birefringence In Photomask Blanks, 19thAnnual BACUS Symposium on Photomask Technology, Monterey, Calif, Sep. 15-17, 1999, SPIE vol. 3873.
Corning Incorporated
Evans F. L.
Murphy Edward F.
Smith Zandra V.
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