JP2010169472A - Method of interference measurement - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of interference measurement, facilitating the association of a rotation angle of a lens with an interference measurement result. <P>SOLUTION: This method of interference measurement for measuring the optical characteristics of a lens using an interferometer includes a step of detecting a mark K previously given to the lens; a step of positioning the mark K so as to face in the reference direction of the interferometer by angularly displacing the lens around an optical axis; and a step of performing interference measurement on the positioned lens. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lens interference measurement method used in an optical apparatus such as an optical pickup, for example.

  In recent years, in addition to CD and DVD, next-generation DVDs (for example, Blu-ray (registered trademark), etc.) have been developed as optical recording media, and compatibility that enables recording / playback on different types of recording media There is a need for an optical pickup device having a certain size.

  The aberration of the optical element used in such an optical pickup device includes a directional aberration having no rotational symmetry with respect to the optical axis. Typical aberrations include astigmatism and coma. If these aberrations of the optical element are incorporated in a state that affects the optical system of the pickup, for example, the optical characteristics of the pickup device are deteriorated and sufficient optical performance cannot be obtained.

  In Patent Document 1, when forming an objective lens using a molding die, a marking shape is formed on the molding die itself so that a mark for indicating the direction of astigmatism is transferred outside the effective optical diameter of the objective lens. Is formed in advance.

  Patent Document 2 discloses an optical pickup device capable of accurately aligning a plurality of transmission optical elements and an alignment method therefor. One lens (diffractive optical element) has a centering mark formed on the axis, and the other lens (objective lens) has no centering mark. At the time of assembly, the objective lens is irradiated with illumination light, and the position of the imaging spot is aligned with the centering mark of the diffractive optical element while observing the imaging spot with an optical microscope or the like.

Japanese Patent No. 3204031 JP 2006-120259 A Japanese Patent Laid-Open No. 10-182173

  In Patent Document 1, the temperature distribution due to unevenness in heat conduction due to the distortion and variation at the time of mounting the molding die, the variation due to the clearance of the sliding portion of the molding die, and the degree of oxidation on the surface of the molding die. Due to variations, variations before molding of the lens material, and, for example, in the case of a glass material, unevenness in temperature and density during pressing may occur due to variations in the melted / softened state, surface state, and the like. As a result, axially asymmetrical variations occur in the shape of the optically effective portion (optical functional surface) of the objective lens, which results in variations in the amount of astigmatism. Therefore, even if a marking shape is added to the molding die in advance, astigmatism may still vary due to variations in conditions during molding.

  In particular, in a high NA lens used as a pickup lens for next-generation DVDs, etc., the sensitivity of the amount of coma aberration generated is high with respect to the relative eccentricity of the mold (relative tilt eccentricity, parallel eccentricity). In addition, coma aberration occurs due to variations during mounting of the molding die and variations due to the clearance of the sliding portion of the molding die, and even if the lens is continuously molded in size and direction. , May vary without continuity.

  Further, in processing the molding die, it is difficult to make the outer diameter central axis of the molding die and the axis of the molding surface completely coincide with each other. In many cases, there is a deviation.

  In Patent Document 3, the upper and lower molding dies are pressed from the side surfaces so that the outer diameter central axes coincide with each other. However, in principle, it is impossible to correct this when the outer diameter central axis of the molding die and the axis of the molding surface do not match. Therefore, even if a marking shape is added to the molding die in advance, there is a problem that aberrations vary in the direction other than the direction due to variations in molding conditions and problems at the time of previous molding.

  On the other hand, Patent Document 2 can only adjust for rotationally symmetric aberrations with respect to the optical axis. For this reason, depending on the combination of the directions of astigmatism and coma, the magnitude may increase without canceling each other.

  An object of the present invention is to provide an interference measurement method capable of easily associating a rotation angle of a lens with an interference measurement result.

To achieve the above object, the present invention provides an interference measurement method for measuring optical characteristics of a lens using an interferometer,
Detecting a mark previously given to the lens;
Angularly displacing the lens around the optical axis to position the mark in the reference direction of the interferometer;
Performing interference measurement of the positioned lens.

The present invention also provides an interference measurement method for measuring optical characteristics of a lens using an interferometer,
A step of providing a mark in a predetermined angular displacement direction around the optical axis with the lens positioned;
Measuring the interference of the lens provided with the mark.

  According to the present invention, since the rotation angle around the optical axis can be specified for each individual lens due to the presence of the mark, the rotation angle of the lens can be easily associated with the interference measurement result. As a result, since the residual error of the interferometer depending on the rotation angle of the lens can be taken into account, highly accurate interference measurement can be realized.

It is a block diagram which shows an example of the interferometer apparatus which can apply this invention. It is explanatory drawing which shows the various examples of the to-be-tested lens ML to which the mark K was provided. An example of a planar image of the test lens ML is shown. It is explanatory drawing which shows the retraction | saving operation | movement of the stage 15 or the reference concave mirror 31. FIG. It is a top view which shows the state which arranged the lens in the tray.

  FIG. 1 is a block diagram showing an example of an interferometer apparatus to which the present invention can be applied. This interferometer apparatus is composed of a Twiman-Green type interferometer, and as an optical system, a light source 12, a collimator lens 13, a beam splitter 14 which is a light splitting / synthesizing means, a stage 15 to be examined, and a reference plane. A mirror 16, an imaging lens 17, and a CCD sensor 18 are provided. The interferometer device includes a light source driving circuit 21, a stage driving device 23, a reference mirror scanning D / A conversion circuit 24, a reference mirror moving motor driver 25, and image processing as a drive control system. The apparatus 26 and the computer 27 which controls these operation | movement centrally are provided.

  The light source 12 is configured as a coherent light source such as a semiconductor laser, and emits inspection light having a predetermined wavelength. The light source 12 may be a variable wavelength light source, or may be configured to switch a plurality of light sources that emit light of different wavelengths.

  The light source 12 is controlled by a light source driving circuit 21. When the light source 12 is a semiconductor laser, the light source driving circuit 21 has a built-in high-frequency superposition circuit 21a, and supplies a superposition of a high-frequency current on a DC current, so that the coherence length of inspection light emitted from the light source 12 is increased. Can be adjusted to a desired value.

  The collimator lens 13 converts the light emitted from the light source 12 into a parallel light beam. When using inspection light of different wavelengths, an achromatic lens is used as the collimator lens 13, or the collimator lens 13 is replaced with one corresponding to each wavelength. The collimated inspection light enters the beam splitter 14 via the mirror 30.

  The beam splitter 14 is a parallel plate-like transparent plate, and a semi-transparent film, for example, is formed on the beam splitting surface 14a. The beam splitter 14 reflects a part of the inspection light incident thereon on the beam splitting surface 14a as reference light, and transmits the remaining inspection light as test light.

  The stage 15 to be examined can be driven by a manual mechanism (not shown) or a stage driving device 23, and the subject to be examined is moved in three dimensions and held in place. In the illustrated case, a test lens ML that is a test target is fixed to the test target stage 15. When the object to be examined is a lens as shown in the figure and its imaging characteristics are measured, a reference concave mirror 31 is disposed behind the lens to be examined ML, the light that has passed through the lens to be examined ML is reflected, and again The light passes through the lens ML to be tested, is converted into a substantially parallel light beam, is returned to the beam splitter 14, and interferes with the reference light. When the test lens ML is a lens designed to collect light through a predetermined parallel plane substrate such as an objective lens for an optical disk, a cover glass 32 is provided between the reference concave mirror 31 and the test lens ML. Deploy.

  For example, the reference flat mirror 16 has a reflection film formed on the incident surface 16a. The reference plane mirror 16 is fixed to the alignment device 42 via the piezoelectric element 41. The piezoelectric element 41 can expand and contract in accordance with a control voltage from the D / A conversion circuit 24 as a phase feed mechanism, and can accurately reciprocate the reference mirror in the direction of the optical axis OA in the wavelength order. The alignment device 42 is driven by a manual mechanism or a motor driver 25 to keep the position and posture of the reference plane mirror 16 in the optical axis direction in an appropriate state.

  The imaging lens 17 condenses the test light from the test lens ML and the reference light from the reference plane mirror 16 synthesized through the beam splitter 14 as synthesized light. Although not shown, the imaging lens 17 is provided with a drive mechanism for displacing the imaging lens 17 in the direction of the optical axis OA and the focus state can be adjusted by adjusting the drive mechanism or the like.

  The combined light once condensed by the imaging lens 17 is projected onto the CCD sensor 18 as interference fringes. The interference fringe pattern is output to the image processing device 26 as an electrical signal. This electrical signal is output to the computer 27 as an image signal corresponding to the interference pattern projected on the CCD sensor 18. Although not shown, the CCD sensor 18 is provided with a drive mechanism for moving the CCD sensor 18 in the direction of the optical axis OA, and the imaging magnification by the CCD sensor 18 is adjusted by adjusting the drive mechanism or the like. Can do. The CCD sensor 18 has a camera shutter controlled from the image processing device 26 side. This camera shutter adjusts the accumulation time of the built-in photodiode, and provides an image signal having an appropriate luminance distribution regardless of the incident light intensity.

  The computer 27 controls the light source driving circuit 21 to turn on the light source 12 and adjust the coherence length of the emitted inspection light. Further, the computer 27 can control the movement of the interference fringes projected on the CCD sensor 18 by controlling the D / A conversion circuit 24 and moving the reference plane mirror 16 in the optical axis direction. Stripe position can be controlled. The wavefront and shape of the measurement object can be measured with high accuracy from at least three interference fringes that are phase-controlled.

  Further, the computer 27 controls the motor driver 25 to drive the alignment device 42 to adjust the position of the reference plane mirror 16 in the optical axis direction. That is, the computer 27, the motor driver 25, and the alignment device 42 constitute an optical path difference control unit. This makes it possible to adjust the visibility corresponding to the difference in light and dark intensity of the interference fringes projected on the CCD sensor 18, and to keep the measurement accuracy of the wavefront change due to the test object above a certain level.

  Next, the operation of the interferometer apparatus will be described. First, the test lens ML that is the test target is set on the test target stage 15. Next, by operating the alignment device 42 manually or electrically, the reference plane mirror 16 can be moved along the optical axis OA to adjust the distance L1, and the reference optical path length and the test optical path length can be substantially reduced. Can be equal to

  Next, the light source driving circuit 21 is operated to emit inspection light having a specific wavelength from the light source. At this time, the light source drive circuit 21 adjusts the coherence length of the inspection light by supplying the light source 12 with a high-frequency current superimposed on the DC current. That is, the light source driving circuit 21 and the computer 27 function as coherence adjusting means. Thereby, it is possible to select only necessary test light and generate interference fringes with the reference light.

  Next, the alignment device 42 is finely moved to adjust the visibility of the interference fringes projected on the CCD sensor 18. Thereby, the contrast of the interference pattern detected by the image processing device 26 can be set to a desired value. Before and after this, the brightness of the interference pattern detected by the image processing device 26 is adjusted by appropriately adjusting the camera shutter of the CCD sensor 18. Specifically, the accumulation time of the CCD sensor 18 is adjusted so that the luminance of the interference pattern is a value suitable for the measurement of the interference pattern. At this time, the adjustment of the camera shutter of the CCD sensor 18 can be automated. For example, the image detected by the image processing device 26 is analyzed by the computer 27, the accumulation time of the CCD sensor 18 is set from the average luminance of the image, etc., and a command signal is output to the image processing device 26. Next, a control signal is output from the computer 27 to the D / A conversion circuit 24 to change the piezoelectric element 41. As a result, it is possible to scan the phase of the reference plane mirror 16 and to perform highly accurate wavefront measurement.

  Furthermore, in the present embodiment, an imaging camera 51 for capturing a planar image of the test lens ML placed on the test target stage 15, and an image processing device 52 for processing an image signal from the imaging camera 51, Is installed. The signal processed by the image processing device 52 is transmitted to the computer 27 and displayed on the screen as necessary. The operation of the imaging camera 51 will be described later.

  FIG. 2 is an explanatory diagram showing various examples of the test lens ML to which the mark K is given. The position of the mark is not particularly limited, but it is preferably provided at a position that has little influence on optical performance and does not become an obstacle during assembly. For example, the mark K may be provided on the flange portion 2 on the outer side of the optical processing surface 1 as shown in FIG. 2 (a), or the optical portion 1 of the optical processing surface 1 is optical as shown in FIG. 2 (b). You may provide the mark K in the outer peripheral part with little influence with respect to performance. Moreover, you may provide the mark K in the side part 2a of the flange part 2 like FIG.2 (c). Further, the shape of the mark K is not limited, and may be a linear shape as shown in FIG. 2 (d) or a dotted shape as shown in FIG. 2 (e). The mark K can be given by marking, painting, sealing, forming mark, laser marking, or the like.

  FIG. 3 shows an example of a planar image of the test lens ML. The imaging camera 51 in FIG. 1 has a function of detecting the mark K given to the lens ML to be examined. By performing predetermined image processing on the captured planar image of the lens, for example, the direction (angle θ) of the mark K can be automatically detected based on the N azimuth of the image. At this time, it is preferable to separately detect the direction of the lens and the direction of the mark K. For example, the following procedure can be adopted.

  (1) First, the center position of the lens is detected. After binarizing the lens plane image using an appropriate threshold, the contour of the lens is detected, the center of the contour is calculated from the center of gravity, and this is set as the center position of the lens.

  (2) Next, if necessary, only the image in the vicinity of the mark K is taken out, the area other than the area with the mark K is masked, and unnecessary information is erased. As an erasing mask, an inner diameter mask and an outer diameter mask are used with the lens center position calculated in (1) as a reference.

  (3) Finally, the direction of the mark K is detected. When the mark K is linear, a straight line may be detected by Hough transform with the lens center as a reference, or the mark direction may be detected by a pattern matching method for a template image. Note that noise removal, edge enhancement, contrast enhancement, filter processing, and the like by smoothing, expansion / contraction processing, or the like may be performed before, during, or during these processes.

  By such a procedure, it is possible to detect in which direction the mark K exists as viewed from the lens center position.

  In the interferometer apparatus of FIG. 1, since the reference concave mirror 31 (+ cover glass 32) exists above the stage 15, it is necessary to retract either one when placing the lens.

  FIG. 4 is an explanatory view showing the retracting operation of the stage 15 or the reference concave mirror 31, and FIGS. 4 (a), (c), (e), and (g) are plan views of the interferometer device of FIG. FIGS. 4B, 4D, 4F, and 4H are front views thereof.

  4A to 4D show a mode in which the reference concave mirror 31 is moved while the stage 15 is fixed. 4A and 4B, the imaging camera 51 disposed above the stage 15 captures a planar image of the test lens ML with the reference concave mirror 31 moved to the side. The mark direction is detected from the obtained planar image. Thereafter, as shown in FIGS. 4C and 4D, the reference concave mirror 31 is returned to the original position.

  On the other hand, FIGS. 4E to 4H show a state in which the stage 15 is moved while the reference concave mirror 31 is fixed. 4E and 4F, with the stage 15 moved to the side, the imaging camera 51 disposed above it captures a planar image of the lens ML. The mark direction is detected from the obtained planar image. Thereafter, as shown in FIGS. 4G and 4H, the stage 15 is returned to the original position.

  In order to perform such a retracting operation, a linear moving mechanism (not shown) that allows the reference concave mirror 31 and / or the stage 15 to move sideways is installed inside the interferometer apparatus. When the stage 15 is moved, it is necessary to fix the lens so that the mounted lens does not shift. The lens may be fixed mechanically or may be fixed by air adsorption.

  Next, lens marking will be described. When a mark is given at the time of lens injection molding, the mark is directed in a random direction through the subsequent cleaning process, coating process, appearance inspection, and the like, and the mark direction is not aligned. On the other hand, even when marks are formed at arbitrary positions after lens molding and before interference measurement, the mark directions are not aligned after various processes.

  Therefore, before the marked lens is placed on the lens base of the interferometer, the direction of the mark is detected by visual observation or CCD camera + image processing, and 1) it is manually aligned in one direction using tweezers or the like. ) Hold the lens with the suction pad and align it in one direction by rotating the suction pad (automatic control), or 3) Place it on a separately prepared rotary stage (automatic control) and align it in one direction to interfere. It is preferable to place it on the total lens base. At this time, the direction of the lenses may be aligned for each lens (for each measurement), or the interference measurement may be performed for each lens after aligning the mark directions of a plurality of lenses.

  Instead of placing the lenses one by one on the lens base, as shown in FIG. 5, a plurality of lenses are arranged on the tray, and the tray is inserted under the concave surface of the interferometer, and is horizontally measured for each measurement. You may measure by scanning the tray in the direction. In this case, after detecting the mark direction by visual inspection or CCD camera + image processing, 1) manually align it in one direction using tweezers, etc. 2) grip the lens with the suction pad and rotate the suction pad Interference measurement may be performed by aligning in one direction (automatic control).

  As another method of aligning the direction for each lens (every measurement), the mark direction is detected by visual observation or CCD camera + image processing after being placed on the lens stage of the interferometer, and depending on the direction. The lens base may be rotated and aligned in one direction manually or automatically.

  The mark may be formed immediately before the interference measurement. By providing a mark after positioning around the optical axis of the lens, it is possible to form the mark in the same direction and eliminate the need for mark detection / rotation operations. For example, if marks are formed by inkjet nozzle scanning in a state where lenses before measurement are arranged in a matrix on a tray, the marks can be formed at a high speed in a batch.

  Further, when the lens is transported to the lens base, the mark direction may be slightly shifted during handling. It is difficult to place it with high accuracy by hand.For example, even when handling with a suction pad, there is a gap between the lens, suction pad, and lens base when sucking or when placing the lens after releasing suction. The lens rotates slightly during operation (in the air). As a countermeasure, a mark may be formed in a certain direction after the lens is placed on the lens base of the interferometer. This can improve the accuracy of aligning the mark direction when performing interference measurement.

  Instead of placing the lenses one by one on the lens base, as shown in FIG. 5, a plurality of lenses are arranged on the tray, and the tray is inserted under the concave surface of the interferometer, and is horizontally measured for each measurement. In the case of measuring by scanning the tray in the direction, if the marks are formed by inkjet nozzle scanning with the lenses arranged in a matrix on the tray, the marks can be formed at a high speed in a batch.

  Thus, the presence of the mark makes it possible to specify the rotation angle around the optical axis for each individual lens, so that the rotation angle of the lens can be easily associated with the interference measurement result.

  The result of the interference measurement is a combination of a) lens-specific aberration and b) device error (interferometer-specific aberration). If the mark is placed in an arbitrary direction and measured, and the measurement result is rotated according to the angle, the apparatus error is also rotated together. Therefore, in order to improve the measurement accuracy, it is necessary to unify the direction of the mark so that the influence of the apparatus error on the measurement result is the same.

  Here, b) apparatus error (interferometer-specific aberration) can be broadly divided into two types of causes: “shape error” and “phase of incident light”.

  The shape error is an aberration caused by a shape error due to distortion at the time of manufacturing or assembling an optical component inside the interferometer such as a mirror or a lens. Therefore, the amount generated in the interferometer beam varies from device to device.

  The phase of the incident light is an error that occurs on the light source and wave plate inside the interferometer, and also on the coating surface of the optical component, and the major axis direction of elliptically polarized light and the polarization direction of linearly polarized light differ from the direction of the mark. The measurement results are different. In other words, the lens-specific aberration amount to be measured is also affected by the apparatus error. In particular, when the lens has birefringence, the influence is remarkable. Birefringence occurs due to the stress state inside the lens and the coating surface of the lens surface.

  Since pickup lenses are generally used with circularly polarized light, interference measurement also uses circularly polarized incident light. The laser light is usually emitted as linearly polarized light, passes through a λ / 4 wavelength plate, and is converted into circularly polarized light. However, elliptical polarization may occur due to a setting error of the λ / 4 wavelength plate with respect to the polarization direction or a manufacturing error of the λ / 4 wavelength plate. Furthermore, a phase difference may be generated when transmitting through the optical components inside the interferometer, particularly the coating surface, which may result in elliptical polarization. That is, the phase of the incident light with respect to the lens mark is different.

  Even when measurement is performed using linearly polarized light, the phase of the incident light with respect to the lens mark will be different if there is an angle shift between the XY direction of the interferometer and the polarization direction of the laser light.

  Therefore, the device error due to the shape error is the same in the measurement region of the same position and the same radius, and the device error due to the phase of the incident light can be made constant by aligning the lens mark direction. It is possible to realize high-precision interference measurement that enables relative comparison of lens-specific aberrations.

  The present invention is extremely useful industrially in that high-precision interference measurement can be realized.

DESCRIPTION OF SYMBOLS 1 Optical processing surface 2 Flange part 12 Light source 13 Collimator lens 14 Beam splitter 15 Stage for test 16 Reference plane mirror 17 Imaging lens 18 CCD sensor 21 Light source drive circuit 23 Stage drive device 24 D / A conversion circuit 25 Motor driver 26, 52 Image processing device 27 Computer 31 Reference concave mirror 42 Alignment device 51 Imaging camera ML Test lens

Claims (2)

An interference measurement method for measuring optical characteristics of a lens using an interferometer,
Detecting a mark previously given to the lens;
Angularly displacing the lens around the optical axis to position the mark in the reference direction of the interferometer;
Performing an interference measurement of the positioned lens. An interference measurement method for measuring optical characteristics of a lens using an interferometer,
A step of providing a mark in a predetermined angular displacement direction around the optical axis with the lens positioned;
Performing interference measurement of a lens provided with a mark, and an interference measurement method.

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