JP2009180554A - Interferometer, measuring method, and manufacturing method of optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interferometer which measures with high accuracy an optical performance of an object optical element and also has a long life. <P>SOLUTION: The interferometer 100 which measures optical performances of the object optical elements TS and TB as interference fringes by using light L from a light source 101a has an imaging element 109 which images the interference fringes, a reference wavefront changing means 106 which changes a reference wavefront to be used for generating the interference fringes for each object optical element, a magnification adjusting optical system 108 which adjusts magnification so that the diameters of the interference fringes in the imaging element may become equal among the object optical elements in a plurality different in a diameter, and a control unit 110 which controls an output of the light source so that the optical intensity of the light in the imaging element may become smaller for the object optical element having a large diameter than for the object optical element having a small diameter. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an interferometer, a measuring method, and a method for manufacturing an optical element.

  Laser interferometers that measure the optical performance (for example, surface shape and aberration) of an optical element as interference fringes using laser light are conventionally known. In recent years, as the optical element is required to have high optical performance, improvement in measurement accuracy of the laser interferometer is required more and more, and the life extension of the laser interferometer itself is also required from the viewpoint of cost.

If the light intensity (light quantity) in the image sensor that captures the interference fringes is too low, the contrast of the temporal and spatial interference fringes will decrease and the measurement accuracy will deteriorate. If it is too strong, the image sensor will be saturated and Phase information cannot be obtained, resulting in missing measurement data. For this reason, it is important to maintain the light intensity in the imaging device within a predetermined range. Japanese Patent Application Laid-Open No. 2004-228561 performs adjustment of a variable transmittance filter and diaphragm and position adjustment of a computer generated hologram based on the maximum value or the minimum value of the amount of light in the image sensor at the time of phase shift. Thereby, the incident light quantity to an image pick-up element does not exceed the saturation value of an image pick-up element, and measurement data without omission can be obtained.
JP-A-8-159725

  However, a conventional laser interferometer has not been proposed with a focus on extending the life of an optical fiber that introduces a laser as a light source and laser light into the optical system of the interferometer.

  Accordingly, an object of the present invention is to provide an interferometer that measures the optical performance of the optical element to be measured with high accuracy and has a long lifetime.

  An interferometer according to one aspect of the present invention is an interferometer that measures the optical performance of an optical element to be detected as an interference fringe using light from a light source, the imaging element that images the interference fringe, and the interference Reference wavefront changing means for changing the reference wavefront used for generating the fringe according to each optical element to be tested, and a magnification for adjusting the magnification of the interference fringe in the image sensor according to the size of the optical element to be tested And an adjustment optical system, and a control unit that controls the output of the light source so that the light intensity of the light in the imaging element with respect to the test optical element having a larger diameter is smaller than that of the test optical element having a smaller diameter. It is characterized by.

  According to the present invention, it is possible to provide a long-life interferometer while measuring the optical performance of the optical element to be measured with high accuracy.

  A Fizeau interferometer 100 as an interferometer according to an embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a principal optical path diagram of the Fizeau interferometer 100. The Fizeau interferometer 100 arranges the reference surface (Fizeau surface) and the test surface on the same optical axis so that the reference light path and the test light path overlap each other, and the reflected light from each surface interferes. The Fizeau interferometer 100 measures the optical performance (surface shape and aberration) of the optical element to be detected as interference fringes (interference pattern) using light from the light source.

  The Fizeau interferometer 100 includes an illumination device 101, a Fizeau interference optical system 102, and a control unit 110.

  The illumination device 101 includes a light source 101 a and an optical fiber 101 b that guides the laser light L from the light source 101 a to the Fizeau interference optical system 102.

  The light source 101a is a coherent light source, and the light source 101a in this embodiment is a laser. The type of laser is not limited, and the laser of this embodiment is, for example, a gas laser. The output of the light source 101a is controlled by the control unit 110 so that the light intensity (light quantity) of the laser light can be adjusted. In addition, as long as it has coherence, a light source is not limited to a laser, You may use white light and monochromatic light.

  The optical fiber 101b has a central core material (core) and an outer cladding, and the refractive index of the core is set higher than that of the cladding, and the laser beam is propagated around the core by total reflection and refraction. . Both the core and the clad are made of quartz glass or plastic, which has a very high transmittance for laser light. In this embodiment, divergent light is emitted from the optical fiber 101b, but if necessary, a lens that diverges the laser light L may be provided at the subsequent stage of the optical fiber 101b. The diverging light or lens and the collimator lens 105 described later constitute a beam expander that provides a predetermined beam diameter.

  The Fizeau interference optical system 102 includes a beam splitter 104, a collimator lens 105, stages 106 and 107, a magnification adjustment optical system 108, and an image sensor 109.

  The beam splitter 104 transmits the incident light incident from the optical fiber 101b, and reflects the reflected light from the test surface and the reference surface to the magnification adjusting optical system 108.

  As described above, the collimator lens 105 converts divergent light into parallel light (light beam 103). As will be described later, the Fizeau interferometer 100 can measure the optical performance of a plurality of test optical elements having different diameters. For this reason, the light beam 103 is set to a size necessary for measurement of a test optical element having a large diameter. However, in this case, since the test optical element having a small diameter does not use the peripheral portion of the light beam 103, the illumination light is wasted.

  Nevertheless, the present embodiment maintains the diameter of the light beam 103 for each optical element to be tested. That is, the light beam 103 has a common diameter for a plurality of optical elements to be tested. This is due to the following reason. First, it is conceivable that different magnification adjustment optical systems (beam expanders) can be exchanged for each test surface. However, a different magnification adjustment optical system for each test surface is manufactured in addition to the magnification adjustment optical system 108 described later. This leads to an increase in cost and the measurement reproducibility may be further deteriorated before and after replacement. If a variable magnification optical system is provided, it is not necessary to manufacture a different magnification adjustment optical system for each test surface, but it is difficult to design aberration correction for different test surfaces. Because there is no.

  In this embodiment, the light after passing through the beam splitter 104 is collimated by the collimator lens 105, but in another embodiment, the light collimated by the collimator lens 105 is guided to the beam splitter 104. To do.

  The stage 106 is equipped with a null optical system RS or RB that generates a null wavefront that is perpendicular to the test surface and incident in the same phase in order to obtain effective interference fringes. The stage 107 mounts a test optical element TS or TB. The stages 106 and 107 can position the null optical system and the test optical element at appropriate positions for forming interference fringes. The stages 106 and 107 have a piezo element (not shown) for performing a phase shift method (fringe scan method), and are configured to be movable minutely along the optical axis OA.

  In this embodiment, a null optical system RS or RB having a reference surface RSa or RBa, and a test optical element (subject) TS or TB having a test surface TSa or TBa are provided at the subsequent stage of the collimator lens 105. It is arranged. When the light flux 103 is incident on the test surface, a test wavefront is generated, and when the light beam 103 is incident on the reference surface, a reference wavefront is generated. The null optical system includes a Fizeau lens having a reference surface and one or a plurality of lenses that form a null wavefront together with the Fizeau lens, but FIG. The test optical element is a target on which the interferometer should measure the optical performance such as the surface shape and aberration, but FIG. 1 shows only a part thereof.

  The Fizeau interferometer 100 can measure the optical performance of a plurality of test optical elements having different diameters. FIG. 1 shows a plurality of test optical elements, a test optical element TS having a test surface TSa having a small diameter, and a test optical element TB having a test surface TBa having a large diameter. Yes. In practice, however, any one optical element to be tested is selectively placed on the stage 107.

  In the case of measuring the optical performance of the test optical element TS having the test surface TSa having a small diameter indicated by the dotted line in FIG. 1, the null optical system RS having the reference surface RSa having the small diameter indicated by the dotted line is selected as the null optical system. Is done. Further, in the case of measuring the optical performance of a test optical element TB having a test surface TBa having a large diameter indicated by a solid line in FIG. 1, a null optical system RB having a reference surface RBa having a large diameter indicated by a solid line as a null optical system. Is selected. Thus, in practice, any one null optical system is selectively disposed on the stage 106. Since the null optical system includes a reference surface, the stage 106 functions as a reference wavefront changing unit that changes the reference wavefront used for generating the interference fringes in accordance with each optical element to be detected.

  The magnification adjusting optical system 108 changes the beam diameter of the interference light emitted from the beam splitter 104 by changing the magnification. The magnification adjustment optical system 108 also has a function of forming an image of the test surface on the image sensor 109. The magnification adjusting optical system 108 is disposed immediately in front of the image sensor 109 in order to improve the spatial resolution in the image sensor 109 when measuring test optical elements having different diameters. The magnification adjustment optical system 108 adjusts the magnification so that the diameter of the interference fringe image in the image sensor 109 is equal to each test optical element and becomes the maximum diameter measurable by the image sensor 109.

The imaging element 109 is a two-dimensional image imaging device configured with a CCD camera. The imaging element 109 images an interference fringe (interference pattern) generated by interference between a reference wavefront generated from the reference surface and a detected wavefront generated from the detected surface. The image sensor 109 transmits the captured interference fringe image to the control unit 110. Specifically, the image sensor 109 converts interference fringe information into an electrical signal, and analyzes the interference fringe information with a computer (not shown). Further, the computer can calculate the phase distribution of the interference fringes with high accuracy by performing arithmetic processing based on the phase shift method while the null optical system or the optical element to be tested is moving.

  In the conventional interferometer, the output of the laser is constant. However, the light intensity (light quantity) per unit area in the image sensor 109 differs depending on the diameter of the optical element to be tested. When measuring the test optical element TS having a small diameter, the peripheral portion of the light beam is not used as compared with the case of measuring the test optical element TB having a large diameter. The light intensity per unit area in the image pickup device 109 is reduced. Therefore, the conventional interferometer sets the output of the laser so that the interference light after passing through the magnification adjustment optical system 108 has a light quantity capable of measuring the optical performance of the optical element having the smallest diameter in the image sensor 109. However, it was maintained even for optical elements to be tested having other diameters. When the light amount is adjusted according to the test optical element having the smallest diameter, the light amount in the image sensor 109 with respect to the test optical element having other diameters is always more than necessary. In other words, the life of the laser is shortened by the amount of extra energy released, and the life of the optical fiber that introduces the laser light into the interference optical system is shortened.

  Therefore, the control unit 110 of the present embodiment controls the output (power) of the light source 101a of the illumination device 101 based on the size of the optical element to be tested, and controls the light intensity (light quantity) received by the image sensor 109. ing. Specifically, the control unit 110 outputs the light source 101a so that the light intensity of the laser beam in the image sensor 109 used for the test optical element TB having a larger diameter than that of the test optical element TS having a smaller diameter is reduced. To control. Preferably, the control unit 110 causes the light intensity at the image sensor 109 to be a constant value that is equal to or greater than the minimum value that can be imaged in a state where the image sensor 109 maintains the desired accuracy in all of the plurality of optical elements to be tested. That is, the output of the light source 101a is controlled as given by the following equation.

The minimum value that can be imaged while the image sensor 109 maintains the desired accuracy is determined such that the ratio of effective phase information and invalid phase information in each pixel of the image sensor is 0.001% or more, for example. Is the value to be Here, when controlling the light intensity, spatial bright and dark stripes may be created using the stages 106 and 107, and the maximum value of the spatial bright and dark stripes may be grasped as the light intensity. Alternatively, a light and dark stripe may be created temporally using a piezo element, and the maximum value of the temporal light and dark stripe may be grasped as the light intensity. Furthermore, you may grasp | ascertain the maximum value of the light and dark stripes obtained with the image pick-up element 109 as light intensity. The control unit 110 has a memory (not shown) that stores various necessary data.

  The control unit 110 may adjust the output of the light source 101 a based on the design value of the effective light beam diameter in the Fizeau interference optical system 102. In this case, the control unit 110 inputs the design value of the effective light beam diameter of light different for each optical element to be tested, and the light source 110a based on the design value of the effective light beam diameter input to the input unit 110a. And a control unit 110b for controlling the output of. The effective light beam diameter required for the reference surface in accordance with the diameter and shape of the test surface is determined by the design of the null optical system that forms the null wavefront, and the output of the light source 101a is controlled as given by the following equation.

  Hereinafter, a method of adjusting the output of the light source 101a using the interferometer 100 will be described with reference to FIG. First, the null optical system RS or RB is mounted on the stage 106, the test optical element TS or TB is mounted on the stage 107, the position of the reference surface RSa or RBa or the test surface TSa or TBa is adjusted, and interference fringes are generated. (Step 1). Next, the light intensity at the image sensor 109 of the interference fringes defined as described above is measured (step 2). Next, based on the design value of the test optical element diameter or effective beam diameter, the output of the light source 101a is controlled so that the light intensity of the interference fringes in the image sensor 109 becomes a constant value for all the test optical elements. (Step 3).

  Hereinafter, the operation of the interferometer 100 will be described. A part of the light beam 103 that has passed through the collimator lens 105 is reflected by the reference surface RSa or RBa to become reflected light 103RS or 103RB, and the remaining light passes through the reference surface RSa or RBa and reaches the test surface TSa or TBa. The reflected light becomes reflected light 103TS or 103TB. The reflected light 103TS or 103TB returns to the original optical path and interferes to reach the beam splitter 104. The interference light between the reference surface RSa and the test surface TSa is reflected by the beam splitter 104 to become interference light 103S, and the interference light between the reference surface RBa and the test surface TBa is reflected by the beam splitter 104 to become interference light 103L. Each interference light is then guided to the image sensor 109 via the magnification adjustment optical system 108. Thus, the reference surface RSa or RBa is a light dividing means (amplitude dividing), and is a superposing means. There is only air or a vacuum between the reference surface RSa or RBa and the test surface TSa or TBa, and the optical path on the light source side is common to the reference surface RSa or RBa. The difference between the reference surface RSa or RBa, which is an ideal surface, and the test surface TSa or TBa becomes an interference fringe and is grasped by the image sensor 109.

  A manufacturing method for manufacturing an optical element using the interferometer 100 will be described below with reference to FIG. First, the null optical system RS or RB is mounted on the stage 106, the test optical element TS or TB is mounted on the stage 107, and the surface shape of the test surface TSa or TBa is measured (step 11). Next, it is determined whether the surface shape of the test surface TSa or TBa is within the designed allowable range (step 12). If it is within the allowable range, the process is terminated. If not within the allowable range, the surface of the test surface TSa or TBa is processed based on the design value, measurement result, and allowable range regarding the test surface TSa or TBa (step 13).

  In the present embodiment, the reference surface and the test surface are expressed in specific shapes, but the same effect can be obtained even with a general aspheric shape. The present invention is applicable not only to Fizeau interferometers but also to Twyman Green interferometers, Michelson interferometers, shearing interferometers, Mach-Zehnder interferometers, and the like.

  If the interferometer 100 of this embodiment is used, the lifetime of a light source (for example, a laser) and a light transmission member (for example, laser light are introduced into the interference optical system when measuring the optical performance of an optical element such as a lens or a mirror. The lifetime of the optical fiber) can be extended.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist.

It is an optical path figure of the interferometer of one Example of this invention. It is a flowchart for demonstrating the method to adjust the output of the light source in the interferometer shown in FIG. It is a flowchart for demonstrating the manufacturing method of an optical element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Optical fiber 102 Fizeau type interference optical system 104 Beam splitter 105 Collimator lens 106, 107 Stage 108 Magnification adjustment optical system 109 Image sensor 110 Control unit

Claims (10)

An interferometer that measures the optical performance of the optical element under test as interference fringes using light from a light source,
An image sensor that images the interference fringes;
Reference wavefront changing means for changing the reference wavefront used to generate the interference fringes according to each test optical element;
A magnification adjustment optical system that adjusts the magnification so that the diameter of the interference fringes in the imaging element is equal among a plurality of optical elements to be measured having different diameters;
A control unit for controlling the output of the light source so that the light intensity of the light in the imaging element with respect to the test optical element having a larger diameter than the test optical element having a small diameter is reduced;
An interferometer characterized by comprising: The magnification adjustment optical system changes the magnification so that the diameter of the interference fringes in the image sensor is the same for each optical element to be tested.
The interferometer according to claim 1, wherein the control unit controls the output of the light source so that light intensity in the image sensor is the same for each optical element to be tested. A collimator lens that converts light from the light source into parallel light;
The interferometer according to claim 1 or 2, wherein a beam diameter of the parallel light is maintained for each optical element to be tested. The control unit is
An input unit for inputting a design value of an effective luminous flux diameter of the light different for each optical element to be tested;
A control unit for controlling the output of the light source based on a design value of the effective light beam diameter input to the input unit;
The interferometer according to claim 1, comprising: The light source is a laser;
The interferometer according to any one of claims 1 to 4, further comprising an optical fiber for introducing laser light from the laser.   The interferometer according to any one of claims 1 to 5, wherein the interferometer is a Fizeau interferometer. A measuring step of measuring the surface shape of the optical element using the interferometer according to any one of claims 1 to 6;
A processing step of processing the surface of the optical element based on the result of the measurement step;
A method for producing an optical element, comprising: In an interferometer that measures the optical performance of a test optical element as an interference fringe using light from a light source using an imaging device, a reference wavefront used to generate the interference fringe is matched to each test optical element Steps to change,
Changing the magnification so that the diameters of the interference fringes in the imaging element are equal among a plurality of optical elements to be measured having different diameters;
Controlling the output of the light source so that the light intensity of the light in the imaging element relative to the test optical element having a larger diameter than the test optical element having a smaller diameter is reduced;
A measuring method characterized by comprising: The step of changing the magnification changes the magnification so that the diameter of the interference fringes in the imaging element is the same for each optical element to be tested.
The measurement method according to claim 8, wherein the step of controlling the output adjusts the output so that light intensity in the imaging element is the same for each optical element to be tested.   The measurement method according to claim 8 or 9, wherein the output control step controls the output of the light source based on a design value of an effective luminous flux diameter of the light that is different for each optical element to be tested.