JP2009244227A - Light wave interference measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light wave interference measuring method capable of easily and inexpensively measuring the surface shape of an aspherical optical element in a short time. <P>SOLUTION: Each light flux from a light source 11 which is halved by a transmission/reflection splitting surface 13a is coupled again by respective corresponding spherical reference lenses 15, 25 in the carrying state of wavefront information corresponding to each surface shape of an inspection aspherical lens 17 and a reference aspherical lens 27, and a wavefront error of the inspection aspherical lens 17 to the reference aspherical lens 27 is formed on an imaging surface of an interferometer CCD camera 31 as interference fringe information. A deformable mirror 42 for correcting the wavefront shape of incident second parallel light fluxes (correcting the wavefront information of the reference aspherical lens 27) is provided between a beam splitter 13 and a high-NA spherical reference lens 25, and the corrected value is controlled by a control operation part 51. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention particularly relates to an optical interference measuring apparatus used for measuring the aspherical shape of an aspherical lens.

  In recent years, there is a strong demand for measuring the aspheric surface shape of an aspherical optical element with high accuracy, particularly in the fields of lens design and manufacturing.

  As a technique related to a highly accurate measurement method of an aspherical shape, in a Fizeau type interferometer, a reference reflecting element having a reference aspherical surface as a standard for the measured aspherical surface is placed close to the measured aspherical surface. The measured aspheric surface is arranged on the basis of interference fringes obtained by optical interference between the reference light reflected by the reference reflecting element and returning to the measured aspheric surface and the object light reflected on the measured aspheric surface. A so-called interference fringe scanning method is known in which the shape is measured and the interference fringes are scanned during the measurement (see Patent Document 1 below).

  Furthermore, as a technique related to a highly accurate measurement method of an aspherical shape, a so-called point scan method as disclosed in Patent Documents 2 and 3 below, and an aperture synthesis method as described in Patent Document 4 below and the like. A method of using is known.

JP 2004-532990 A JP-A-8-146018 JP 2001-133244 A USP 6,956,657

  However, in the method described in the above-mentioned patent documents, particularly in the above-mentioned patent document 1, etc., the reason that the deviation of each optical axis between the reference aspheric surface (aspheric reference surface) and the measured aspheric surface greatly affects the measurement. Therefore, good interference fringes cannot be obtained simultaneously for the entire region of the test surface having an aspherical shape. Eventually, in order to obtain interference fringe information for the entire test surface, it is necessary to repeat imaging every time interference fringe information appears for each region, and to perform processing such as combining a number of these captured interference fringe information. The operation of acquiring interference fringe information becomes extremely complicated.

  Therefore, all of the methods described in the above patent documents require enormous measurement time.

  Further, the device described in Patent Document 1 has a high manufacturing cost of the device, and the device described in Patent Document 4 has a problem that the device configuration is complicated.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical interference measuring apparatus capable of measuring the surface shape of an aspherical optical element easily and at low cost in a short time. It is.

The optical interference measuring apparatus according to the present invention is
The light beam from the light source is divided into two by the light beam separating / synthesizing means, and one is used as the first light beam directed toward the subject, and the other as the second light beam directed toward the reference body.
An imaging body arranged at a predetermined position by combining the return light of the first light flux from the subject and the return light of the second light flux from the reference body to form interference light by the light beam separating and combining means. In the light wave interference measurement apparatus for forming an interference fringe image based on the surface shape information of the subject on the top,
The subject is an aspherical optical element whose surface shape is to be measured, and the reference body is an aspherical optical element having a shape to be a standard of the subject;
The first light beam from the light beam separation / synthesis unit is incident on the surface of the subject and the first light beam reflected from the surface of the subject is interposed between the light beam separation / synthesis unit and the subject. And a first spherical reference lens having a surface facing the subject as a first reference spherical surface,
Between the light beam separation / synthesis unit and the reference body, the second light beam from the light beam separation / synthesis unit is incident on the surface of the reference body and the second light beam reflected from the surface of the reference body And a second spherical reference lens having a second reference spherical surface having the same curvature as the first reference spherical surface on the surface facing the reference body.
At least on the optical path of the first luminous flux between the luminous flux separating / combining means and the first spherical reference lens, and on the optical path of the second luminous flux between the luminous flux separating / combining means and the second spherical reference lens. On the other hand, a deformable mirror for correcting the wavefront shape of the incident first light beam or the second light beam and emitting it is provided.

  In this case, the wavefront shape of the incident first light beam or second light beam is corrected so that the fringe density of the interference fringe image obtained by the imaging body based on the surface shape information of the subject is lowered. It is preferable that a controller for deforming and driving the surface shape of the deformable mirror is provided.

  Moreover, it is preferable that the light wave interference measuring apparatus is an equal optical path length Michelson type.

In this case, a separation surface of the light beam separation / combination means for separating and combining the first light beam and the second light beam by light reflection and light transmission is provided on one surface of the beam splitter,
It is preferable that the beam splitter has a plate shape having a wedge shape in cross section.

  Further, in the optical path of the light beam emitted to the one surface side of the beam splitter, the first light beam and the beam splitter between any of the spherical reference lenses arranged in the optical path and the beam splitter It is preferable that a compensation plate for compensating for the difference in optical path length of the second light beam is provided.

  The reference spherical surface is a spherical surface serving as an aspherical base representing the reference body surface shape. That is, the spherical surface having the curvature (or radius of curvature) as the value of curvature C (or radius of curvature R) when the aspheric surface is expressed by a well-known aspherical expression is used.

  In the light wave interference measuring apparatus according to the present invention, the light beam from the light source is divided into two by the light beam separation / combination means, and one is used as the first light beam directed toward the subject and the other as the second light beam directed toward the reference body. Moreover, the subject surface shape information can be obtained as interference fringe information by combining the first light beam reflected from the subject and the second light beam reflected from the reference body by the light beam separation and synthesis means. I am doing so.

  The interference light combined by the light beam separation / combination means is the difference between the reflected light of the first light beam on the surface of the subject and the reflected light of the second light beam on the surface of the reference body that is the reference for the shape of the subject surface. However, the reflected light of the first luminous flux has information based on the shape difference between the subject surface and the first reference spherical surface (hereinafter referred to as shape difference information), while the second light is reflected on the second light beam. Since the reflected light of the light beam has shape difference information between the surface of the reference body and the second reference spherical surface, the interference fringes formed by the interference of both reflected light beams are basically based on the above two shape difference information. It will be a thing.

  If it is a normal aspherical shape, the fringe density can be made sufficiently small even with the above-described configuration, and good optical interference measurement can be performed. Such a configuration has already been invented by the present inventor and has already been disclosed to the Patent Office in Japanese Patent Application No. 2007-340136 (filed on Dec. 28, 2007).

  However, depending on the aspherical shape of the subject, the difference from the spherical shape may be locally increased. In such a case, the fringe density in the problematic part may be increased.

  Therefore, in the light wave interference measuring apparatus according to the present invention, the light beam separation / combination means and the second spherical reference lens on the optical path of the first light flux between the light flux separation / combination means and the first spherical reference lens. A deformable mirror that corrects the wavefront shape of the incident first light beam or the second light beam and emits it is disposed at least on the optical path of the second light beam. In general, since the deformable mirror has a small movable range, it is difficult to deform the light flux wavefront of the reference light only to that extent to correspond to the light flux wavefront from the aspherical shape of the test surface. However, in the present invention, as described above, the first stage of adjustment is performed by the two reference spherical surfaces. Therefore, even with the first stage of adjustment, the portion where the fringe density is locally increased is as follows. Since it is only necessary to correct the wavefront of the light beam with a deformable mirror, it can be finely adjusted, so that the fringe density is sufficiently small even for an aspherical shape where the difference from the spherical shape increases locally. It is possible to perform good optical interference measurement by applying it to various types of aspherical objects for general purposes.

  Therefore, according to the light wave interference measuring apparatus according to the present invention, the surface shape of the aspherical optical element can be measured easily and at low cost in a short time.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing a configuration of a lightwave interference measuring apparatus according to an embodiment of the present invention.

  As shown in FIG. 1, the apparatus 10 includes a light source 11, a collimating lens 12 that converts a light beam from the light source 11 into a parallel light beam, and a beam that divides the parallel light beam from the collimating lens 12 by a transmission / reflection separation surface 13a. The first parallel light beam reflected by the splitter 13 and the transmission / reflection separation surface 13a is irradiated onto the test aspheric lens 17, and the reflected light from the test aspheric lens 17 is transmitted / reflection separation surface. The high NA spherical standard lens (first spherical standard lens) 15 to be returned to 13a and the second parallel light beam transmitted through the transmission / reflection separation surface 13a are irradiated on the reference aspheric lens 27 and the reference aspheric lens. A high-NA spherical reference lens (second spherical reference lens) 25 that returns the reflected light from 27 to the transmission / reflection separation surface 13a, and the test aspheric lens 1 on the transmission / reflection separation surface 13a. The interferometer CCD camera 31 that captures interference fringes generated by the interference between the reflected light from the reference light and the reflected light from the reference aspheric lens 27, and the interferometer CCD camera 31 captures the interference fringes resulting from the combined reflected light. An imaging lens 29 that forms an image on the surface is provided.

  In addition, the apparatus 10 corrects the wavefront shape of the incident second parallel light beam (the second parallel light beam incident from the beam splitter 13 direction and the second parallel light beam incident from the high NA spherical reference lens 25 direction). A deformable mirror 42 is provided between the beam splitter 13 and the high NA spherical reference lens 25.

  In addition, a piezo element 41 is attached to the deformable mirror 42, and a known phase shift method can be employed.

  Further, the apparatus 10 includes a control calculation unit 51 including a computer. The control calculation unit 51 receives imaged interference fringe information and calculates and analyzes the aspheric surface shape of the aspheric lens 17 to be examined based on the interference fringe information. Based on the fringe information, a drive signal for deforming the surface shape of the deformable mirror 42 is output for a portion having a high fringe density so as to correct the wavefront shape of the second parallel light flux so as to reduce the fringe density. Is.

  The deformable mirror 42 is a reflection-type light modulation device, and is configured to change the spatial light phase distribution according to the drive signal. That is, the positions and inclinations of a large number of mirror devices can be varied according to the applied voltage, and the curved surface formed by the large number of mirror devices can also be deformed. By inputting the drive signal to the deformable mirror 42, the wavefront shape of the incident second parallel light beam can be changed by a minute amount. By doing so, it is possible to make the fringe density sufficiently small even for an aspherical shape in which the difference from the spherical shape locally increases. Further, the wavefront from the reference aspheric lens 27 can be corrected to an ideal shape without processing the reference aspheric lens 27 with extremely high accuracy.

  In FIG. 1, the wavefront corrected by the deformable mirror 42 is schematically drawn.

  Further, in the apparatus 10, the beam splitter 13 is configured such that the opposing surfaces are not parallel to each other in order to prevent noise interference fringes from being generated by the reflected light from the surface opposite to the transmission / reflection separation surface 13a. Thus, it is configured in a wedge shape.

  In addition, the number of times the light beam passes through the beam splitter 13 is one for the first parallel light beam, but three times for the second parallel light beam. In order to match, the compensating plate 14 is provided and is configured by substantially the same wedge-shaped body arranged so as to be in the same wedge direction as the beam splitter 13.

  Further, the high NA spherical reference lens (first spherical reference lens) 15 and the high NA spherical reference lens (second spherical reference lens) 25 schematically depict only main parts, and each concave surface has a reference spherical surface 15a. 25a, which are formed to have the same curvature.

Hereinafter, the operation of the embodiment device of the present invention will be described.
In general, the test aspheric lens 17 has a slight shape error with respect to the reference aspheric lens 27 serving as a shape standard. The shape error of the aspheric lens 17 is quantitatively measured based on the surface shape of the reference aspheric lens 27.

  That is, the first light beam directed to the aspheric lens 17 to be tested out of the light beams from the light source 11 divided into two at the transmission / reflection separation surface 13 a is the aspheric lens 17 to be tested and the high NA spherical reference lens 15. Returning to the transmission / reflection separation surface 13a in a state where wavefront information corresponding to the surface shape difference from the first reference spherical surface 15a is carried. On the other hand, of the light beams divided into two at the transmission / reflection separation surface 13a, the second light beam directed toward the reference aspheric lens 27 is the second standard of the reference aspheric lens 27 and the high NA spherical standard lens 25. Returning to the transmission / reflection separation surface 13a in a state where the wavefront information corresponding to the surface shape difference from the spherical surface 25a is corrected by the deformable mirror 42 is carried. The two light fluxes returning to the transmission / reflection separation surface 13a are combined at the transmission / reflection separation surface 13a, and the relative shape error information of the aspheric lens 17 to be tested with respect to the reference aspheric lens 27 is used as interference fringe information. It is formed on the imaging surface of the interferometer CCD camera 31.

However, as described above, the second light flux reflected by the reference aspheric lens 27 is given by the wavefront information based on the surface shape difference between the reference aspheric lens 27 and the second reference spherical surface 25a and the deformable mirror 42. Therefore, when calculating the above-described interference fringe information, the wavefront information provided by the deformable mirror 42 is subtracted from the actually obtained interference fringe information. Therefore, it is important to analyze the shape information of the subject.
As a result, even a portion having a high fringe density can be significantly reduced, and therefore, the shapes of all effective regions can be simultaneously and satisfactorily acquired for various types of aspheric shapes.

  Therefore, the measurement analysis time can be greatly reduced compared to the conventional interference fringe shape measurement of aspherical optical elements, which must be combined with each other after partially obtaining the interference fringe information of each region of the subject. Is possible. Also, the analysis software can be simplified, and the cost required for measurement can be reduced.

  In addition, the apparatus according to the present embodiment is configured as a Michelson type capable of constructing an equal optical path length type, and the difference in optical path length generated according to the passage of the light beam in the beam splitter 13 using the compensation plate 14 is calculated. Since it compensates, the output light from the light source 11 can be made into low coherence light. As a result, generation of noise interference fringes based on reflected light from other optical surfaces in the optical path can be avoided.

  The compensation plate 14 is formed of the same glass material as that of the beam splitter 13 and has the same refractive index and dispersion value. The shape is also formed in the same way. However, the refractive index, the dispersion value, and the shape of the compensation plate 14 do not necessarily match those of the beam splitter 13. In short, the optical paths of both the optical paths generated by inserting the beam splitter 13 in the optical path. Any device capable of compensating for the difference in length may be used.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment, A mode can be variously changed.

  For example, in the above embodiment, the deformable mirror 42 is provided between the beam splitter 13 and the high NA spherical reference lens 25. However, in place of or in addition to this, the beam splitter 13 and the high NA are provided. Even if a deformable mirror similar to that of the spherical reference lens 15 is provided, the same effect as that of the above embodiment can be obtained.

  Further, in the above embodiment, the subject has an aspherical shape in which the difference from the spherical shape locally increases, and the wavefront from the subject is reduced in order to reduce the fringe density for that portion. Although the case where a deformable mirror is used for correction is described, the correction of the wavefront by the deformable mirror in the device of the present invention is not limited to this. For example, when the reference aspherical lens 27 is not processed with high accuracy, the wavefront from the reference aspherical lens 27 may be corrected to an ideal shape by a deformable mirror. The manufacturing cost of the reference aspheric lens can be reduced.

  In the above-described embodiment, measurement is performed using a low coherent light beam, and thereby a highly accurate measurement result can be obtained. Measurement may be performed. In this case, it is not necessary to match the optical path lengths of the two optical paths of the optical system with high accuracy, and it becomes possible to simplify the adjustment of the optical system.

  Further, the subject is not limited to the above-mentioned aspherical lens, and can naturally be used for measurement of a spherical lens. For example, the surface shape (spherical surface, aspherical surface) of an optical element other than a lens such as a reflecting mirror is used. , Free-form surface, etc.).

Schematic which shows the structure of the lightwave interference measuring apparatus which concerns on embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Light wave interference measuring device 11 Light source 12 Collimator lens 13 Beam splitter 13a Transmission / reflection separation surface 14 Compensation plate 15 High NA spherical reference lens (first spherical reference lens)
15a, 25a Reference spherical surface 17 Tested aspheric lens 25 High NA spherical reference lens (second spherical reference lens)
27 Reference Aspherical Lens 29 Imaging Lens 31 Interferometer CCD Camera 41 Piezo Element 42 Deformable Mirror 51 Control Calculation Unit

Claims (5)

The light beam from the light source is divided into two by the light beam separating / synthesizing means, and one is used as the first light beam directed toward the subject, and the other as the second light beam directed toward the reference body.
An imaging body arranged at a predetermined position by combining the return light of the first light flux from the subject and the return light of the second light flux from the reference body to form interference light by the light beam separating and combining means. In the light wave interference measurement apparatus for forming an interference fringe image based on the surface shape information of the subject on the top,
The subject is an aspherical optical element whose surface shape is to be measured, and the reference body is an aspherical optical element having a shape to be a standard of the subject;
The first light beam from the light beam separation / synthesis unit is incident on the surface of the subject and the first light beam reflected from the surface of the subject is interposed between the light beam separation / synthesis unit and the subject. And a first spherical reference lens having a surface facing the subject as a first reference spherical surface,
Between the light beam separation / synthesis unit and the reference body, the second light beam from the light beam separation / synthesis unit is incident on the surface of the reference body and the second light beam reflected from the surface of the reference body And a second spherical reference lens having a second reference spherical surface having the same curvature as the first reference spherical surface on the surface facing the reference body.
At least on the optical path of the first luminous flux between the luminous flux separating / combining means and the first spherical reference lens, and on the optical path of the second luminous flux between the luminous flux separating / combining means and the second spherical reference lens. On the other hand, a deformable mirror for correcting and emitting the wavefront shape of the incident first light flux or the second light flux is provided.   The deformable so as to correct the wavefront shape of the incident first light beam or the second light beam so that the fringe density of the interference fringe image obtained by the imaging body based on the surface shape information of the subject is lowered. 2. The optical interference measuring apparatus according to claim 1, further comprising a controller for driving to deform the surface shape of the mirror.   3. The light wave interference measuring apparatus according to claim 1, wherein the apparatus is of equal optical path length type Michelson type. A separation surface of the light beam separation / combination means for separating and combining the first light flux and the second light flux by light reflection and light transmission is provided on one surface of the beam splitter,
4. The light wave interference measuring apparatus according to claim 1, wherein the beam splitter is configured in a plate shape having a wedge shape in cross section. An optical path of the light beam emitted to the one surface side of the beam splitter, and the optical path of the first light beam and the second light beam between the spherical reference lens disposed in the optical path and the beam splitter. 5. The optical interference measuring apparatus according to claim 4, further comprising a compensating plate for compensating for the difference in length.

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