JP2010169473A - Method of detecting optical axis position of lens and method of measuring eccentricity - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of detecting the optical axis position of a lens for rapidly acquiring the optical axis position of the lens with high accuracy, and to provide a method of measuring lens eccentricity for rapidly acquiring information on eccentricity between an external form center and the axis position with high accuracy. <P>SOLUTION: According to the method of detecting the optical axis position of a lens, concentric epi-illumination is conducted along the optical axis of a lens, and the optical axis position of the lens is detected by measuring the distribution profile (ring image) of light which is totally reflected by the rear face of the lens, goes through reflection N times (N is an integer not less than two), and returns along the optical axis. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lens optical axis position detection method used in an optical apparatus such as an optical pickup, for example. The present invention also relates to a lens eccentricity measuring method that can obtain eccentricity information between the outer shape center and the optical axis position.

  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. An optical pickup device is generally configured by assembling a plurality of optical elements. Therefore, it is desired not only to form a single wavelength imaging spot with a plurality of lenses, but also to form an imaging spot with two or more wavelengths with a plurality of lenses.

  The optical element is preliminarily processed (for example, centering) and incorporated into a lens frame with a single lens configuration or a multiple lens configuration. At that time, the lens is incorporated into the lens frame with the lens outer center as a reference. However, if the center of the optical surface of the lens and the outer center of the lens are decentered, predetermined performance cannot be obtained. In particular, when a plurality of lenses are combined, the influence of eccentricity becomes large.

  Conventionally, a method of detecting and assembling the center of the optical surface using the optical performance of the lens has been proposed (for example, Patent Document 1). This Patent Document 1 discloses an optical pickup device capable of accurately aligning a plurality of transmission optical elements and an alignment method thereof. 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.

JP 2006-120259 A

  In Patent Document 1, since the light passing through one lens (objective lens) forms an imaging spot on a portion other than the surface of the other lens (diffractive optical element), the position of the imaging spot is used for alignment. The mark is separated from the mark by a certain distance along the optical axis direction. Therefore, it is necessary to move the optical microscope in the direction of the optical axis when focusing the observation.

  In this case, 1) If the centering mark is made small to the extent that there is no optical influence, transfer in molding becomes difficult. 2) If the centering mark is made small to the extent that there is no optical influence, observation at a high magnification is necessary, so the depth of focus becomes shallow and the field of view becomes narrow. It takes time to find two of the marks. 3) A means for adjusting the moving direction of the focusing to the optical axis direction is separately required.

  An object of the present invention is to provide a lens optical axis position detection method that can quickly and accurately acquire the optical axis position of a lens without using a centering mark.

  It is another object of the present invention to provide a lens eccentricity measuring method that can quickly and accurately acquire the eccentricity information between the outer shape center and the optical axis position.

  In order to achieve the above object, the lens optical axis position detection method according to the present invention performs coaxial epi-illumination along the optical axis of the lens, totally reflects light on the back surface of the lens, and returns light along the optical axis. The optical axis position of the lens is detected by measuring the shape.

In the present invention, the refractive index of the medium around the lens is n1, the refractive index of the lens material is n2,
sin (θc) = n1 / n2
It is preferable to measure the distribution shape of the light reflected at a reflection angle equal to or greater than the critical angle θc that satisfies

Further, the lens eccentricity measuring method according to the present invention performs coaxial epi-illumination along the optical axis of the lens, and measures the distribution shape of light that is totally reflected on the back surface of the lens and returns along the optical axis. Detecting the optical axis position of
Detecting the outer center of the lens;
Obtaining eccentricity information between the detected optical axis position and the center of the outer shape.

  According to the present invention, since the traveling direction of the light totally reflected on the back surface of the lens is maintained parallel to the traveling direction of the illumination light, the optical axis position of the lens can be quickly determined by measuring the distribution shape of the totally reflected light. And it becomes possible to detect with high precision.

It is a block diagram which shows an example of the optical system which can apply this invention. It is a top view which shows the distribution shape of the light totally reflected by the 2nd surface of the to-be-tested lens ML. FIGS. 3A and 3B show a state in which the inspection lens ML is assembled in the lens barrel 61, FIG. 3A being a cross-sectional view, and FIG. 3B being a plan view. It is a block diagram which shows an example of the interferometer apparatus which measures the optical performance of a lens. It is a flowchart which shows the method of putting the detected lens optical axis in the substantially the same position with respect to the light beam of an interferometer. 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 block diagram which shows the lens fixation by air adsorption | suction.

  FIG. 1 is a block diagram showing an example of an optical system to which the present invention can be applied. The optical system includes a light source (not shown), a beam splitter 55, an imaging lens 56, an imaging element 57 such as a CCD camera, and the like. The test lens ML is disposed below the beam splitter 55 and is positioned so that the optical axis of the test lens ML substantially coincides with the optical axis of the optical system. The test lens ML to which the present invention can be applied is a lens with a high NA that is suitably used for an optical pickup of a CD, DVD, or next-generation DVD (for example, Blu-ray (registered trademark)), that is, the curvature of the optical surface. Is a fairly large lens.

  Parallel illumination light is supplied from the light source, reflected by the beam splitter 55 disposed at an angle of 45 ° with respect to the optical axis, and subjected to coaxial epi-illumination along the optical axis of the test lens ML. The size of the illumination light beam preferably has a larger diameter than the optically processed surface of the test lens ML.

  The illumination light passes through the first surface of the test lens ML and reaches the second surface. At this time, the total reflection condition is satisfied for a part of the illumination light, and after passing through the second surface N times (N is an integer of 2 or more), the final reflected light is emitted in parallel with the incident light. .

  As a total reflection condition on the second surface, a critical angle θc that satisfies sin (θc) = n1 / n2 is defined, where n1 is the refractive index of the medium around the lens and n2 is the refractive index of the lens material. At this time, θi = θo ≧ θc is established where the incident angle of light with respect to the normal direction of the second surface is θi and the reflection angle is θo. For example, as shown in FIG. 1, θi = θo = 45 ° in the case of two reflections, θi = θo = 60 ° in the case of three reflections, θi = θo = 67.5 ° in the case of four reflections, In the case of five reflections, θi = θo = 72 °. In general, in the case of N times reflection (N is an integer of 2 or more), it can be calculated as θi = θo = 90 ° × (N−1) / N.

  The light totally reflected by the second surface is emitted from the first surface of the lens ML to be tested, passes through the beam splitter 55, is imaged by the imaging lens 56, and is converted into an image signal by the imaging device 57.

  FIG. 2 is a plan view showing a distribution shape of light totally reflected by the second surface of the lens ML to be examined. It can be seen that a two-reflection ring image and a three-reflection ring image appear around the center of the lens. The ring image that is reflected three times overlaps with the ring image that is reflected four or more times, so that the ring image has a thick line width.

  In the present invention, the center of the ring image is calculated by performing image processing on the total reflection ring image, and the center position is detected as the optical axis position of the lens ML to be measured. Any method can be used for the image processing. For example, the lens plane image is binarized with an appropriate threshold value, the contour of the ring image is detected, and the optical axis position is calculated from the center of gravity. When detecting the outline of the outer shape, it may be calculated by a pattern matching method. When calculating the optical axis position from the center of gravity, it may be calculated by a mathematical method such as a least square method with respect to a preset contour shape of the template. 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. Furthermore, the above-described detection process may be performed in all image ranges (pixels), or an image only in a range where total reflected light is supposed to return may be cut out and processed only on a part of the image. .

  Furthermore, the outer shape of the lens ML to be examined is also visible in the lens plane image shown in FIG. Accordingly, the outer center of the lens ML can be calculated by performing binarization, contour detection, center of gravity detection, and the like for the outer shape of the lens ML to be tested.

  Therefore, by comparing the optical axis position obtained from the total reflection ring image and the outer shape center obtained from the lens outer shape, and calculating the difference between the two, the eccentricity information between the two is obtained quickly and with high accuracy. be able to.

  FIGS. 3A and 3B show a state in which the demonstration lens ML is assembled in the lens barrel 61, FIG. 3A is a cross-sectional view, and FIG. 3B is a plan view. The lens ML to be tested can be positioned in the same plane as the mounting surface by the lens position adjusting means 60 in a state where the lens ML is in contact with the mounting surface of the lens barrel 61. Therefore, it is possible to accurately match the optical axis position 61C obtained from the ring image of the lens ML to be tested with the outer center of the lens barrel 61 from the same plane image without moving in the optical axis direction. Thereafter, the test lens ML is fixed using an adhesive.

  The lens unit assembled in this way cancels out the influence of the eccentricity of the lens outer center and the optical axis position, and the lens optical axis position and the outer center of the lens barrel exactly match, so Desired optical performance can be achieved when assembled according to the outer shape of the lens frame. Further, even when a plurality of lenses are incorporated in the same lens frame, the same procedure can be used.

  Moreover, when performing the above-mentioned positioning, you may detect the mark K for identifying the direction of a lens previously formed in the to-be-tested lens ML. The mark may be formed at the time of lens molding, or may be processed into a lens before detecting the optical axis position. The mark may be applied with a paint or the like (inkjet or the like).

  In FIG. 3B, the mark K is linear, but the position of the mark is not particularly limited, but it is preferably provided at a position that does not interfere with optical performance and does not become an obstacle during assembly. For example, you may provide in a flange part like FIG.3 (b). Moreover, there is no restriction | limiting in the shape, For example, a linear form like FIG.3 (b) may be sufficient, or a dot form (not shown) may be sufficient. The mark K can be used to always incorporate the lens in a fixed direction with respect to the lens barrel.

  It can also be used as a marker in the eccentric direction between the lens outer shape and the optical axis position, obtained from a planar image.

  FIG. 4 is a configuration diagram illustrating an example of an interferometer apparatus that measures the optical performance of a lens. 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. As shown in FIG. 1, the imaging camera 51 can be configured as an optical system for capturing a ring image totally reflected by the second surface of the lens ML to be measured.

  When interferometric measurement is performed, the position of the lens with respect to the light beam of the interferometer changes every time, and if the position varies in the plane perpendicular to the optical axis direction (X, Y), the position of the reference concave mirror 31 also varies from measurement to measurement. It becomes. In particular, high NA lenses used as pickup lenses for next-generation DVDs are adjusted with an accuracy of several nanometers to several tens of nanometers in order to pursue null fringe, and the position of the concave surface of the original lens is larger than the previous measurement lens. If it is off, no interference fringes are produced as it is, and it takes time to make adjustments for producing the interference fringes again.

  Therefore, by always placing the detected lens optical axis at substantially the same position with respect to the light beam of the interferometer, a clear interference fringe can be obtained every time even if it is continuously measured, and the adjustment amount of the reference concave mirror 31 is small. Therefore, the measurement tact time can be shortened. Furthermore, it is not necessary to extract the interference fringe region for each measurement and set the mask every time, and measurement can be performed with a fixed mask.

  As a method of placing the detected optical axis of the lens at substantially the same position with respect to the light beam of the interferometer, the following method can be considered. As shown in the flowchart of FIG. 5, first, the center of the inner diameter 15a is calculated from the planar image of the stage 15, and is set as the center of the light beam of the interferometer (s1).

  Next, the center of the ring image is calculated from the planar image of the lens ML and set as the optical axis of the lens (s2). By combining these, the detected optical axis of the lens is placed at substantially the same position with respect to the light beam of the interferometer (s3).

  Returning to FIG. 4, when detecting the center of the inner diameter of the stage or detecting the center of the ring image of the lens, the reference concave mirror 31 is moved so that the imaging camera 51 directly faces the lens base 15 and the lens ML to be examined. There is a need.

  6 is an explanatory view showing the retracting operation of the stage 15 or the reference concave mirror 31, and FIGS. 6A, 6C, 6E, and 6G are plan views of the interferometer device of FIG. 4 as viewed from above. FIGS. 6B, 6D, 6F, and 6H are front views thereof.

  6A to 6D show a mode in which the reference concave mirror 31 is moved while the stage 15 is fixed. 6A and 6B, the imaging camera 51 disposed above the stage 15 captures a planar image of the stage 15 and the demonstration lens ML while the reference concave mirror 31 is moved to the side. The center of the inner diameter of the stage and the center of the ring image of the lens are detected from the obtained planar image. Thereafter, as shown in FIGS. 6C and 6D, the reference concave mirror 31 is returned to the original position.

  On the other hand, FIGS. 6E to 6H show a state in which the stage 15 is moved while the reference concave mirror 31 is fixed. 6E and 6F, in a state where the stage 15 is moved to the side, the imaging camera 51 disposed on the stage 15 captures a planar image of the stage 15 and the demonstration lens ML. The center of the inner diameter of the stage and the center of the ring image of the lens are detected from the obtained planar image. Thereafter, as shown in FIGS. 6G and 6H, the reference concave mirror 31 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.

  The following method can be considered as a method of fixing by air adsorption. As shown in FIG. 7, a sealing window 15 b for forming an airtight internal space is provided in the through hole of the stage 15. The sealing window 15b is formed of a transparent material, and is fixed obliquely with respect to the optical axis of the measurement light in order to prevent stray light. By sucking the internal space of the stage 15 via the conduit 15c, the lens ML to be tested can be fixed in close contact with the stage 15, and when the stage 15 is moved, the mounted lens is prevented from shifting. be able to.

  The present invention is extremely useful industrially in that the position of the optical axis of the lens can be acquired quickly and with high accuracy.

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 55 Beam Splitter 56 Imaging Lens 57 Imaging Element ML Test Lens

Claims (3)

  Coaxial epi-illumination is performed along the optical axis of the lens, and the optical axis position of the lens is detected by measuring the distribution shape of the light that is totally reflected on the back surface of the lens and returns along the optical axis. Lens optical axis position detection method. The refractive index of the medium around the lens is n1, the refractive index of the lens material is n2,
sin (θc) = n1 / n2
2. The lens optical axis position detection method according to claim 1, wherein a distribution shape of light reflected at a reflection angle satisfying the above condition is equal to or greater than a critical angle θc. Detecting the optical axis position of the lens by performing coaxial epi-illumination along the optical axis of the lens, measuring the distribution shape of the light that is totally reflected on the back surface of the lens and returning along the optical axis,
Detecting the outer center of the lens;
Obtaining a decentering information between the detected optical axis position and the center of the outer shape.