Abstract:
An apparatus for making automated measurements of an optical property of a sample includes 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 also includes 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.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art system for obtaining birefringence of a material. 
     FIG. 2 shows another prior art system for obtaining birefringence of a material. 
     FIG. 3 is a schematic of an automated system for measuring an optical property. 
     FIG. 4 illustrates birefringence measurement at a point on a sample. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 illustrates an automated system  30  for measuring an optical property, e.g. birefringence, across a sample. The automated system  30  includes a light source unit  32  and a first polarizer  34 . The first polarizer  34  may be one made by Corning Inc., sold under the trade name Polarcor®. This type of polarizer creates linear polarized light and has a higher light extinction ratio (&lt;10 −5 ) than commonly used sheet polarizers, which have extinction ratios of about (10 −4 ). However, the invention is not limited to this type of polarizer. Sheet polarizer or other types of polarizers, e.g., calcite polarizers, can also be used. The light source unit  32  and the first polarizer  34  are mounted on a vertically movable first translational stage  36 . The first translational stage  36  preferably has the ability to accurately move as little as 1 micron. Translational stages which can be used with the automated system  30  are commercially available and can be purchased, for example, from Newport Company (model # MTMCC1). 
     The automated system  30  further includes a detector end  42 . The detector end  42  comprises a quarter-wave plate  44 , a second polarizer or analyzer  46  oriented in a crossed Nicols fashion with respect to the first polarizer  34 , and a photomultiplier  48 . The second polarizer or analyzer  46  may be one made by Coming Inc., sold under the trade name Polarcor®, or may be other type of polarizer. The wave plate  44  is not limited to quarter-wave plates, but may be a half-wave plate, for example. The detector end  42  is mounted on a second translational stage  50 . The second translational stage  50  can move simultaneously with the first translational stage  36 , while keeping the light source unit  32  and the detector end  42  optically aligned. The translational stages  36  and  50  may also move independently of one another along a selected line. The analyzer  46  is mounted in a rotation stage (not shown) which is also mounted on the second translational stage  50 . The rotation stage has the ability to rotate the analyzer  46  such that the angular position of light extinction can be measured. 
     The automated system  30  also includes a sample holder  40 . In the illustrated embodiment, the sample holder  40  is rotatable and comprises a series of plates with rings (not shown) for holding a sample of a selected shape, e.g., sample  38 . The sample holder  40  further comprises a controllable means (not shown) for rotating the sample  38  preferably around a full circle. In the illustrated embodiment, the sample  38  is a birefringent lens blank which has parallel surfaces. It should be understood that the sample  38  can be any shape or material of a birefringent nature, as long as it can be placed physically in the sample holder  40  and can be rotated. 
     In operation, a light beam  54  from the light source unit  32  enters the polarizer  34 . The light beam in this embodiment is a He-Ne laser beam with a wavelength of 632.8 nanometers, but may be any other type of light beam. A planar polarized light  56  emerges from the first polarizer  34  and enters the sample  38 . Because of the birefringent nature of the sample  40 , when the planar polarized light  56  enters the sample  38 , it splits into two light rays (not shown). The two light rays (not shown) recombine into an elliptically polarized light  58  upon exiting from the sample  38 . The elliptical polarization of the light  58  is caused by the phase difference between the two light rays. 
     The elliptically polarized light  58  then enters the quarter-wave plate  44 , where it is converted into a linearly or nearly linearly polarized ray  60 . This ray  60  then enters the analyzer  46 , which is arranged in a crossed Nicols fashion with respect to the first polarizer  34 . The light beam emerging from the analyzer  46  then enters a photomultiplier  48 , which measures the light intensity. The results are analyzed by a computer  52 . By measuring the angular position at which the light is extinguished as it passes through the two crossed linear polarizers  34  and  46 , the birefringence at a particular point in the sample  38  can be determined. The angular position at which the light is extinguished is obtained by rotating the second polarizer or analyzer  46 , with respect to the first linear polarizer  34 , until the light intensity measured by the photomultiplier  48  diminishes to some minimum value, e.g., zero. The translation stages  36  and  50  are then moved vertically, and the process is repeated until a linear series of points at a particular angular orientation of the sample  38  has been taken. The sample  38  is then rotated through a predetermined angle to the next desired measurement location, and the process is repeated. 
     Data points are sampled by a process which takes parameters, e.g., sample thickness, sample diameter, spatial resolution, entered by a user and uses these parameters to determine the coordinates of all data points lying within the sample coordinate system. The computer  52 , based on the parameters entered by the user, calculates the geometric center of the sample  38 . There is no restriction on the shape of the sample  38  because determination of the geometric center of the sample is a mathematical construct. The geometric center of the sample  38  is used as the origin of a Cartesian coordinate system. The process works by first converting the desired measurement locations on the sample  38  from Cartesian coordinates to polar coordinates. The process then transforms these polar coordinates into command signals. The process may also work directly with the Cartesian coordinates. 
     The command signals automatically rotate the sample holder  40  to the appropriate orientation and move the translational stages  36  and  50  to the appropriate height so that birefringence measurements can be made at the desired measurement locations on the sample  38 . As illustrated in FIG. 4, birefringence may be measured at a data point P on the sample  38  by rotating the sample  38  through Φ degrees so that the data point P lies within the measurement region, i.e., along line segment AB. The translational stages  36  and  50  may then be moved so that the light source unit  32  and the detector end  42  are aligned with the data point P. If the light source unit  32  and the detector end  42  are initially aligned with the center A of the sample  38 , the translational stages  36  and  50  will be moved R units. This process may be repeated for all measurement locations on the sample  38  until the system completely maps the birefringence of the sample. 
     Alternatively, the computer  52  can be programmed to rotate the sample holder  40  and correspondingly move the translational stages  36  and  50  to make measurements along a substantially spiral pattern about the cross-section of the sample  38 . In one example, the amount of angular rotation of the sample holder  40  between successive measurement points can be related to the lateral position of the stages  36  and  50  with respect to the center of the sample  38 , so that substantially constant spatial resolution of measurement can be maintained. 
     When the translational stages  36  and  50  and the sample  38  are at the appropriate height and orientation, respectively, the analyzer  46  is rotated to measure birefringence. The birefringence measurement is recorded and stored on the computer  52 , where the user can either examine it as the measurement is made or view a summary after all the measurements are taken. The translational stages  36  and  50  can be moved, while keeping the sample  38  in the same orientation, so that birefringence measurements can be made along a line extending from the center of the sample  38 . When the translational stages  36  and  50  reach the edge of the sample  38 , the sample  38  can be rotated a predetermined amount and the translational stages  36  and  50  can be moved accordingly to allow measurement at the next measurement location. 
     The process thus described provides advantages in that a single operator can quickly and accurately create a birefringence map of a sample. The process can also eliminate much of the inaccuracy in measuring birefringence manually. 
     While the example embodiment described herein is directed to measurement of the birefringence of a sample, it should be clearly understood that the automated system can measure other types of optical properties, e.g., transmission. The process described above can easily be extended to other measurements, simply by changing some of the elements, the analyzer or light source. It is also possible to perform continuous measurements while keeping spatial resolution constant, which is previously unknown in the prior art. Also, the process can be extended to look at any optical property in which there is a source and a detector. Specifically, the process applies to any property that measures the state of the energy entering a sample and compares it to the state of the energy leaving a sample. 
     Those skilled in the art will appreciate that other embodiments of the invention can be devised which do not depart from the spirit of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.