JP2010244648A - Parallel flat plate for inspection, inspection method of objective lens for optical recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correctly inspect an objective lens for an optical recording medium with a simple configuration. <P>SOLUTION: In the inspection method of the objective lens for the optical recording medium, a parallel flat plate for inspection instead of a protective layer of the optical recording medium is inserted in an optical path of light condensed by the objective lens when the objective lens of the optical recording medium which is so designed that light emitted from a light source is condensed in a prescribed position of the optical recording medium via a light transmissible protective layer of the optical recording medium is inspected. As the parallel flat plate, a parallel flat plate which has a refractive index different from that of the protective layer and whose thickness is set so as to correct the spherical aberration caused by difference between refractive indices of the parallel flat plate and the protective layer is used. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for inspecting a parallel plate for inspection and an objective lens for an optical recording medium. More specifically, the present invention relates to at least one of information recording and reproduction by converging light emitted from a light source onto an optical recording medium. A parallel plate for inspection used in place of the protective layer of the optical recording medium when inspecting an objective lens for the optical recording medium used in the optical pickup device, and an optical recording medium for inspection using the parallel plate The present invention relates to an inspection method for an objective lens.

  Conventionally, an optical recording medium such as a DVD (digital versatile disk) or a CD (compact disk) has been used to record audio information, video information, or computer data information. Data recording / reproduction on the optical recording medium is performed by condensing light from the light source onto the optical recording medium by an objective lens mounted on the optical pickup device.

  In recent years, with the rapid increase in the amount of information to be handled, there is an increasing demand for high density optical recording media. To increase the density of optical recording media, it is effective to shorten the wavelength of the light used and to increase the numerical aperture (hereinafter also referred to as NA) of the pickup objective lens used in the optical pickup device. Is known. Recently, by using a semiconductor laser with an output wavelength of about 405 nm as a light source and using an objective lens with a high NA, a BD (Blu-ray disc) capable of recording about 25 GB of information on one layer on one side has come to practical use. ing. This BD is completely different from the above-mentioned DVD and CD in the NA standard, and the current standard for NA is 0.85.

  On the other hand, a recording layer on which information is recorded in an optical recording medium is covered with a translucent protective layer having a predetermined thickness. For example, the current standard for the thickness of the protective layer of BD is 0.1 mm. When the thickness of the protective layer of the optical recording medium changes, the amount of aberration changes. Therefore, the presence of the protective layer of the optical recording medium cannot be ignored in the design and inspection of the objective lens for the optical recording medium.

  Patent Document 1 describes a spherical aberration measuring device that measures the spherical aberration of an optical pickup device using a cover glass having the same thickness and refractive index as a protective layer (light transmission layer) of an optical recording medium. In this spherical aberration measuring apparatus, the light from the light source is incident on the objective lens after passing through the spherical aberration compensating element, and the light condensed by the objective lens is converged on the cover glass. Patent Document 1 describes that the distance between two lens elements constituting a spherical aberration compensating element is changed so as to correct spherical aberration caused by a cover glass thickness error.

JP 2004-251783 A

  As described above, in the inspection of the objective lens, it is necessary to consider the protective layer of the optical recording medium. Therefore, for the inspection that substitutes the protective layer of the optical recording medium in the optical path of the light condensed by the objective lens. The parallel plate is inserted and inspected. In Patent Document 1, a cover glass is used as the parallel plate for inspection. However, it is not always practical to use a parallel plate for inspection having the same thickness and refractive index as the protective layer of the optical recording medium as described in Patent Document 1. This is because a material that is currently widely used as a protective layer of an optical recording medium cannot be said to be suitable for inspection from the viewpoint of water absorption and birefringence. In order to ensure the reliability of inspection, a material different from the protective layer of the optical recording medium is selected as the material of the parallel plate for inspection. In that case, the parallel plate for inspection and the optical recording medium The refractive index of the protective layer becomes different, and there arises a problem that aberration such as spherical aberration due to the difference in refractive index between the two occurs.

  In the case where a member having a different refractive index is substituted as described above, a method of adjusting the thickness so that the optical optical path lengths coincide is generally known. The optical path length is the product of the refractive index and the geometric distance. However, when an inspection parallel plate whose thickness is adjusted by this method is used, for an objective lens having a large NA, the optical path length in the direction that coincides with the optical axis of the objective lens is the same. Since the optical path lengths in the parallel plate of the light beam having a large NA transmitted through the peripheral portion of the lens do not coincide with each other, a non-negligible amount of spherical aberration occurs. Therefore, when a method of simply making the optical path length in the optical axis direction the same for a high NA lens such as a BD objective lens, the parallel plate for inspection and the optical recording medium are protected. The effect of spherical aberration due to the difference in refractive index with the layer becomes large, and there arises a problem that inspection cannot be performed correctly.

  In order to correct the spherical aberration caused by the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium, it is conceivable to use a spherical aberration compensation element as described in Patent Document 1. In that case, not only the spherical aberration compensation element, but also a drive mechanism and a control mechanism for changing the distance between the lens elements constituting the spherical aberration compensation element are required, which complicates the device configuration, There arises a problem of increasing the size and cost.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an inspection parallel plate for inspection and an objective lens inspection method for an optical recording medium that enable appropriate inspection with a simple configuration. It is.

  The parallel plate for inspection according to the present invention inspects an objective lens designed to condense light emitted from a light source onto a predetermined position of the optical recording medium through a transparent protective layer of the optical recording medium. Therefore, a parallel plate for inspection inserted in the optical path of the light condensed by the objective lens instead of the protective layer of the optical recording medium, having a refractive index different from that of the protective layer, The thickness is set so as to correct spherical aberration caused by different refractive indexes.

  The method for inspecting an objective lens for an optical recording medium according to the present invention is designed to condense light emitted from a light source at a predetermined position of the optical recording medium through a light-transmitting protective layer of the optical recording medium. When inspecting an objective lens for an optical recording medium, an optical recording medium for inspecting by inserting a parallel plate for inspection instead of a protective layer of the optical recording medium into the optical path of the light condensed by the objective lens The parallel plate has a refractive index different from that of the protective layer, and the thickness is corrected so as to correct spherical aberration caused by the difference in refractive index between the parallel plate and the protective layer. A set parallel plate is used.

In the inspection method of the parallel plate for inspection and the objective lens for an optical recording medium according to the present invention, the thickness and refractive index of the protective layer are Dc and Nc, respectively, and the thickness and refractive index of the parallel plate are Dm and Nm, respectively. It is preferable that the following conditional expression (1) is satisfied.
−0.05 ≦ 0.3141 × (Nm / Nc) + 0.68701− (Dm / Dc) ≦ 0.05 (1)

  In the inspection method of the parallel plate for inspection and the objective lens for the optical recording medium of the present invention, the thickness of the parallel plate is set so that the spherical aberration of the optical system composed of the objective lens and the parallel plate is minimized. Preferably it is.

  The inspection method of the parallel plate for inspection and the objective lens for an optical recording medium of the present invention is preferably applied to an objective lens having a numerical aperture of 0.8 or more on the optical recording medium side.

  In the inspection method of the parallel plate for inspection and the objective lens for the optical recording medium of the present invention, the parallel plate may be made of a glass material or a plastic material.

  According to the parallel plate for inspection of the present invention, even when an objective lens is inspected using a parallel plate having a refractive index different from that of the protective layer of the optical recording medium, instead of the protective layer of the optical recording medium, the difference in refractive index between the two. Since the thickness of the parallel plate is set so as to correct the spherical aberration caused by the refractive index, the influence of the spherical aberration caused by the refractive index difference can be reduced and properly inspected. . Further, only by setting the thickness of the parallel plate for inspection, it is not necessary to separately provide a means for compensating for the spherical aberration caused by the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium. Therefore, the apparatus configuration can be simplified.

  According to the inspection method of the objective lens for the optical recording medium of the present invention, even when the objective lens is inspected using a parallel plate having a refractive index different from that of the protective layer of the optical recording medium instead of the protective layer of the optical recording medium, Since this parallel plate uses a thickness set so as to correct the spherical aberration caused by the difference in refractive index between the two, the influence of the spherical aberration caused by the difference in refractive index is reduced. Can be properly tested. Further, only by setting the thickness of the parallel plate for inspection, it is not necessary to separately provide a means for compensating for the spherical aberration caused by the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium. Therefore, the apparatus configuration can be simplified.

  In the inspection method of the parallel plate for inspection of the present invention or the objective lens for the optical recording medium of the present invention, the thickness and refractive index of the protective layer of the optical recording medium are Dc and Nc, respectively, and the thickness and refractive index of the parallel plate are In the case where Dm and Nm are set to satisfy the above conditional expression (1), the influence of the spherical aberration due to the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium is further reduced. And can be inspected more appropriately.

  In the inspection method of the parallel plate for inspection of the present invention or the objective lens for the optical recording medium of the present invention, the thickness of the parallel plate is set so that the spherical aberration of the optical system composed of the objective lens and the parallel plate is minimized. In this case, since the influence of spherical aberration due to the difference in refractive index between the parallel plate for inspection and the protective layer of the optical recording medium can be minimized, the inspection can be performed more appropriately.

  When the inspection method for the parallel plate for inspection of the present invention or the objective lens for the optical recording medium of the present invention is applied to an objective lens having a numerical aperture of 0.8 or more on the optical recording medium side, the objective lens described above is used. Compared with the method of matching the optical path length in the direction matching the optical axis, the influence of the spherical aberration caused by the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium can be reduced. Thus, the effect of the present invention becomes more remarkable, and the inspection can be performed more appropriately.

  In the inspection method of the parallel plate for inspection of the present invention or the objective lens for the optical recording medium of the present invention, when the parallel plate is made of a glass material, it is less affected by the environment such as temperature change and humidity change, The reliability of the inspection can be improved, and the inspection can be performed more appropriately.

  In the inspection method of the parallel plate for inspection of the present invention or the objective lens for the optical recording medium of the present invention, when the parallel plate is made of a plastic material, it is hard to break compared to the glass material, and is easy to handle, As with the case of using a glass material, the inspection can be performed more appropriately as compared with the method of matching the optical optical path length in the direction matching the optical axis of the objective lens with a simple configuration.

1 is a schematic configuration diagram of an inspection apparatus according to an embodiment of the present invention. The figure which shows the numerical range of the Example of this invention

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Embodiments described below include an inspection method for inspecting an objective lens for an optical recording medium used in an optical pickup apparatus that performs at least one of recording and reproduction of information by focusing light on the optical recording medium, and the inspection The present invention relates to a parallel plate used in the method. FIG. 1 is a schematic configuration diagram of an inspection apparatus for inspecting an objective lens for an optical recording medium according to an embodiment of the present invention.

  An inspection apparatus 100 shown in FIG. 1 inspects a lens 1 as a subject using a Fizeau interferometer. In addition, illustration of the holder holding each member is abbreviate | omitted in FIG. As shown in FIG. 1, in the inspection apparatus 100, a lens 1, a parallel plate 2 for inspection, and a reflection mirror 3 are arranged in order, and the opposite side of the parallel plate 2 for inspection of the lens 1. A reference plate 4 having a reference surface, a beam splitter 5, a collimator lens 6, and an inspection light source 7 are arranged in this optical path in order from the side closer to the lens 1. An imaging element 9 is disposed in the bent optical path formed by the beam splitter 5, the imaging element 9 is connected to the calculation unit 10, and the calculation unit 10 is connected to the display unit 11.

  For example, when a Blu-ray disc is used as an optical recording medium, a semiconductor laser that emits light having a wavelength of 405 nm can be used as the inspection light source 7. The light emitted from the inspection light source 7 is converted into parallel light by the collimator lens 6. Of the parallel light, the light transmitted through the beam splitter 5 and transmitted through the reference plate 4 enters the lens 1 and is condensed.

  The lens 1 is an objective lens for an optical recording medium mounted on the optical pickup device. In the optical pickup device, the lens 1 functions to condense light emitted from a light source onto a recording layer on which information of an optical recording medium is recorded. Since the recording layer of the optical recording medium is covered with a translucent protective layer having a predetermined thickness, the lens 1 collects light emitted from the light source at a predetermined position of the optical recording medium via the protective layer. Designed to light up. Therefore, it is necessary to consider the protective layer of the optical recording medium also during the inspection, and the inspection parallel plate 2 serving as a substitute for the protective layer of the optical recording medium is inserted into the optical path of the light condensed by the objective lens. Perform an inspection. The detailed configuration of the parallel plate 2 for inspection will be described later.

  The reflection mirror 3 is a concave mirror for reflecting the light transmitted through the lens 1 and the parallel plate 2 for inspection and traveling the same optical path in the reverse direction. Therefore, the light that has been transmitted through the reference plate 4, collected by the lens 1, and transmitted through the parallel plate 2 for inspection forms a condensing point with the smallest beam diameter, and then becomes divergent light and becomes the reflecting mirror 3. Is reflected by the reflecting mirror 3 and travels in the opposite direction. After passing through the same condensing point, it passes through the parallel plate 2 for inspection and exits the lens 1 to become parallel light. The parallel light emitted from the lens 1 passes through the reference plate 4 and enters the beam splitter 5, and the light reflected by the beam splitter 5 enters the image sensor 9.

  On the other hand, part of the light emitted from the inspection light source 7 and incident on the reference plate 4 via the collimator lens 6 and the beam splitter 5 is reflected by the reference surface of the reference plate 4 and passes through the lens 1. Instead, the light re-enters the beam splitter 5 and is reflected by the beam splitter 5 enters the image sensor 9.

  The light passing through the lens 1 and the light reflected by the reference surface of the reference plate 4 interfere to form an interference fringe, and the formed interference fringe can be observed by the image sensor 9. The image pickup device 9 is composed of, for example, a CCD (Charge Coupled Device) or the like, converts the interference fringes into an electric signal, and outputs the electric signal to the calculation unit 10. The arithmetic unit 10 analyzes the interference fringes and outputs the analysis result to the display unit 11. The arithmetic unit 10 calculates an aberration of the optical system including the lens 1 and the parallel plate 2 for inspection, for example, RMS of axial wavefront aberration. The display unit 11 includes, for example, a monitor and displays an interference fringe image, an aberration value calculated by the calculation unit 10, and the like.

  In the inspection apparatus 100 shown in FIG. 1, the inspection parallel plate 2 is disposed between the condensing point and the lens 1. Instead, the light condensed by the lens 1 is condensed. You may make it arrange | position the parallel plate 2 for a test | inspection between the optical path after passing a point, for example, between a condensing point and the reflective mirror 3. FIG.

  In the above embodiment, the inspection apparatus using the Fizeau interferometer has been described as an example. However, in the inspection of the present invention, another type of interferometer may be used, or a means other than the interferometer. May be used for inspection.

  Next, the parallel plate 2 for inspection will be described in detail. The parallel plate 2 for inspection used in the present embodiment is made of a material different from that of the protective layer of the optical recording medium and has a different refractive index. When the refractive index differs between the parallel plate for inspection and the protective layer of the optical recording medium, the inspection parallel plate having the same thickness as the protective layer of the optical recording medium is simply used instead of the protective layer of the optical recording medium. Then, the aberration resulting from the refractive index difference of both will generate | occur | produce. Therefore, in the present invention, inspection is performed using a parallel plate whose thickness is set so as to correct spherical aberration caused by the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium.

  Below, an example of the procedure at the time of setting the thickness of the parallel plate for a test | inspection is demonstrated. First, an objective lens for an optical recording medium and an optical recording medium designed to condense light emitted from a light source of an optical pickup device at a predetermined position of the optical recording medium via a protective layer of the optical recording medium Lens data of an optical system (hereinafter referred to as a first optical system) composed of the protective layer is prepared. In the following description, this objective lens is also simply referred to as a lens. The protective layer of the optical recording medium is also simply referred to as a protective layer.

  Here, “preparing lens data” may be either performing lens design to obtain lens data, or obtaining lens data of already designed lenses. “Lens data” refers to data relating to the shape, refractive index, etc. of the lens. For example, the radius of curvature of the lens surface, the center thickness, the refractive index, and the aspherical surface obtained when designing a lens using general-purpose lens design software. Data such as coefficients. In general, in the first optical system, an optical system in which an objective lens and a protective layer of an optical recording medium are combined is configured using optical simulation software, and the objective lens is optimized and designed to obtain desired performance. Is.

  As the next step, an optical system in which the value corresponding to the refractive index of the protective layer in the lens data of the first optical system is replaced with the refractive index value of the parallel plate for inspection (hereinafter referred to as the second optical system). System)) using optical simulation software. At this time, only the value of the refractive index is replaced, and the thickness is not changed. In the second optical system, the parallel plate having the same refractive index as that of the parallel plate for inspection and having the same thickness as the protective layer is hereinafter referred to as a parallel plate for calculation for convenience. That is, the second optical system is an optical system including an objective lens and a parallel plate for calculation. In the second optical system, aberration is generated due to a difference in refractive index between the protective layer of the optical recording medium and the parallel plate for calculation. Note that the parallel plate for calculation can be changed in thickness for correcting spherical aberration, and has the same thickness as that of the protective layer in the second optical system. In the system, it is treated that the thickness can be different from that of the second optical system.

  Therefore, as the next step, an optical system (hereinafter referred to as a third optical system) using only the thickness of the parallel plate for calculation in the lens data of the second optical system as a variable is configured using optical simulation software. Then, the spherical aberration of the third optical system is corrected using only the thickness of the parallel plate for calculation as a variable. In this correction, the parameters relating to the lens and the refractive index of the parallel plate for calculation are not changed. Such correction calculation can be performed by using known optical simulation software.

  The method for correcting the spherical aberration is not particularly limited, and any method can be used as long as it can substantially correct the spherical aberration. For example, on the optical simulation software, even if the spherical aberration is not specified as the correction target, if it is specified and corrected to reduce the axial wavefront aberration, the spherical aberration can be substantially corrected. Such correction is considered to correct spherical aberration. Alternatively, if correction is performed by specifying an aberration or physical quantity different from the spherical aberration as a correction target, and as a result, the spherical aberration is also corrected, it can be considered that the spherical aberration is corrected.

  Since the parallel plate for calculation corrects spherical aberration by changing its thickness, the thickness of the parallel plate for calculation takes a predetermined value when the correction of spherical aberration is completed. That is, as a result of the correction, the thickness of the calculation parallel plate is determined, and an optical system (hereinafter referred to as a fourth optical system) composed of the calculation parallel plate and the objective lens with the determined thickness is obtained on the optical simulation software. . The thickness of the parallel plate for calculation at this time is a thickness set so as to correct the spherical aberration. The spherical aberration of the fourth optical system is smaller than the spherical aberration of the second optical system. Whether or not the spherical aberration is small may be determined, for example, by comparing RMS (Root Mean Square) values of the on-axis wavefront aberration.

  Finally, a parallel plate for inspection having the same thickness as the parallel plate for calculation of the fourth optical system is prepared, and the first optical system is used in the inspection apparatus 100 shown in FIG. 1 by using this parallel plate for inspection. The objective lens produced based on the lens data is inspected.

  As described above, in this embodiment, the inspection parallel plate whose thickness is adjusted to correct the spherical aberration caused by the refractive index difference between the protective layer of the optical recording medium and the inspection parallel plate. Therefore, the influence of spherical aberration caused by the difference in refractive index between the protective layer of the recording medium and the parallel plate for inspection can be reduced, and the inspection can be performed appropriately.

  If the spherical aberration of the fourth optical system is smaller than the spherical aberration of the second optical system, it can be considered that the spherical aberration has been corrected. The thickness of the parallel plate for calculation is set by correcting so that the spherical aberration is minimized. In this case, the influence of spherical aberration caused by the difference in refractive index between the protective layer of the recording medium and the parallel plate for inspection can be minimized, and inspection can be performed more appropriately. In particular, when inspecting a high NA objective lens having an NA of 0.8 or more on the optical recording medium side of the objective lens, the calculation parallel plate is corrected so that the spherical aberration of the fourth optical system is minimized. It is preferable to set the thickness.

  Since the refractive index changes depending on the temperature, when designing the first optical system or configuring the second optical system, a predetermined temperature is set, and the refractive index at the predetermined temperature is set. In addition, when inspecting the objective lens, it is preferable to inspect at the same temperature as the predetermined temperature. The predetermined temperature to be set may be determined based on, for example, the temperature around a general objective lens when the objective lens is used in the optical pickup device.

As a preferred embodiment, when the thickness and refractive index of the protective layer of the optical recording medium are Dc and Nc, respectively, and the thickness and refractive index of the parallel plate for inspection are Dm and Nm, respectively, the following conditional expression (1) It is comprised so that it may satisfy | fill, and as a more preferable aspect, it is comprising so that the following conditional expression (2) may be satisfy | filled. In particular, when inspecting an objective lens having a high NA with an NA of 0.8 or more on the optical recording medium side of the objective lens, it is preferable that the following conditional expression (1) or (2) is satisfied.
−0.05 ≦ 0.3141 × (Nm / Nc) + 0.68701− (Dm / Dc) ≦ 0.05 (1)
−0.01 ≦ 0.3141 × (Nm / Nc) + 0.687001− (Dm / Dc) ≦ 0.01 (2)

  By satisfying the conditional expression (1), it is possible to properly inspect by reducing the influence of spherical aberration caused by the refractive index difference between the protective layer of the optical recording medium and the parallel plate for inspection. Further, by satisfying the conditional expression (2), it is possible to appropriately inspect by reducing the influence of spherical aberration caused by the refractive index difference between the protective layer of the optical recording medium and the parallel plate for inspection. it can.

  As a material of the parallel plate 2 for inspection, either glass or plastic can be used. Glass is advantageous in that it has a high degree of freedom in material selection and high reliability in consideration of characteristics such as water absorption and birefringence. Plastics are generally less expensive than glass, and have the advantages of being harder to break and easier to handle than glass.

  When the parallel plate 2 for inspection is made of plastic, it is preferable to use a material having a small birefringence and a material having a low water absorption rate in order to ensure the reliability of the inspection. More specifically, the birefringence of the parallel plate 2 for inspection is preferably 40 nm or less at a thickness of 1 mm, and more preferably 25 nm or less at a thickness of 1 mm. Further, the water absorption rate of the parallel plate 2 for inspection is preferably 0.1% or less, and more preferably less than 0.01%. As a plastic material of the parallel plate 2 for inspection, for example, a cycloolefin polymer or a cycloolefin copolymer can be used. Specific examples of the copolymer include Apel (registered trademark, manufactured by Mitsui Chemicals, Inc.).

  Next, data of specific numerical examples of the present invention will be described. In Examples 1 to 10 shown below, the thickness of the parallel plate for inspection is set by correcting the spherical aberration to be the minimum by using the procedure for constructing the first to fourth optical systems described above. It is a thing. In the first to fourth embodiments, the fifth to ninth embodiments, and the tenth embodiment, different first optical systems are used.

  Table 1 shows lens data of the first optical system used in Examples 1 to 4. The design wavelength of this optical system is 405 nm, and the NA of the objective lens on the optical recording medium side is 0.85. The surface numbers in Table 1 are assigned such that the surface on the light source side is 1, and the surface number increases sequentially from the light source side toward the optical recording medium side. The first surface and the second surface correspond to the objective lens, and The surface and the fourth surface correspond to the protective layer. The unit of the surface interval is mm. The optical system of Table 1 assumes a two-layer Blu-ray disc having two recording layers as an optical recording medium. Therefore, the thickness of the protective layer for lens data in Table 1 is the same as that of a two-layer Blu-ray disc. On the other hand, the thickness set in order to exhibit suitable performance is used.

Both surfaces of the objective lens shown in Table 1 are aspheric surfaces, and the aspheric surfaces are described in the column of the radius of curvature of the first surface and the second surface in Table 1. Under the heading of the aspheric coefficient in Table 1, the curvature C, the aspheric coefficient K, and Ai (i = 3 to 20) in the vicinity of the optical axis of each surface are shown. The symbol C in Table 1 is positive when the lens surface is convex on the light source side and negative when the lens surface is convex on the optical recording medium side. In the numerical values of the respective aspheric coefficients in Table 1, “E−n” (n: integer) means “× 10 ⁻ⁿ ”, and “E + 00” means “× 10 ⁰ ”. Each aspheric coefficient is defined by the following aspheric expression.
Zd = C · H ² / {1+ (1−K · C ² · H ² ) ^1/2 } + ΣAiH ⁱ
However,
Zd: Depth of aspheric surface (length of perpendicular drawn from a point on the aspherical surface of height H to a plane perpendicular to the optical axis where the aspherical vertex contacts)
H: Height (distance from the optical axis to the lens surface)
C: curvature K near the optical axis, Ai (i = 3 to 20): aspheric coefficient

  Also, below the aspheric coefficient in Table 1, the focal length, back focus, effective diameters and effective radii of the first and second surfaces at a wavelength of 405 nm and NA of 0.85 are shown.

  Table 2 shows the lens data of the first optical system used in Examples 5 to 9. The design wavelength of this optical system is 405 nm, and the NA of the objective lens on the optical recording medium side is 0.85. The surface numbers in Table 2 are assigned such that the surface on the light source side is 1, and the surface number increases sequentially from the light source side toward the optical recording medium side. The first surface and the second surface correspond to the objective lens, and The surface and the fourth surface correspond to the protective layer. The unit of the surface interval is mm. The optical system of Table 2 assumes a single-layer Blu-ray disc with one recording layer as an optical recording medium.

  Both surfaces of the objective lens shown in Table 2 are aspheric surfaces, and the aspheric surface is described in the column of the radius of curvature of the first surface and the second surface in Table 2. Under the heading of the aspheric coefficient in Table 2, the curvature C, the aspheric coefficient K, and Ai (i = 3 to 20) in the vicinity of the optical axis of each surface are shown. The sign of the curvature C near the optical axis and the definition of each aspheric coefficient are the same as those described in Table 1 above. Further, below the aspheric coefficient in Table 2, the focal length, back focus, effective diameters and effective radii of the first and second surfaces at a wavelength of 405 nm and NA of 0.85 are shown.

  Table 3 shows lens data and the like of the first optical system used in Example 10. The design wavelength of this optical system is 404.7 nm, and the NA of the objective lens on the optical recording medium side is 0.85. The surface numbers in Table 3 are assigned such that the surface on the light source side is 1, and the surface numbers increase sequentially from the light source side toward the optical recording medium side. The first surface and the second surface correspond to the objective lens. The surface and the fourth surface correspond to the protective layer. The unit of the surface interval is mm. The optical system of Table 3 assumes a single-layer Blu-ray disc with one recording layer as an optical recording medium.

  Both surfaces of the objective lens shown in Table 3 are aspheric surfaces, and the aspheric surfaces are described in the column of the radius of curvature of the first surface and the second surface in Table 3. Under the heading of the aspheric coefficient in Table 3, the curvature C, the aspheric coefficient K, and Ai (i = 3 to 20) in the vicinity of the optical axis of each surface are shown. The sign of the curvature C near the optical axis and the definition of each aspheric coefficient are the same as those described in Table 1 above. Also, below the aspheric coefficient in Table 3, the focal length of the objective lens at the wavelength of 404.7 nm and NA of 0.85, the back focus, the effective diameters and effective radii of the first and second surfaces are shown.

  Table 4 shows various data according to Examples 1-10. “Material of parallel plate” in Table 4 is the material of the parallel plate for inspection used in each example (same as the material of the parallel plate for calculation), N-SK5 in Examples 1 and 5, Example 2 N-LAK7 in Example 8, N-BK7 in Example 3, N-SK11 in Example 6, N-LAK21 in Example 7, and N-SK16 in Example 10 are all glass materials manufactured by SCHOTT. Zeonex 330R of Example 4 is a plastic material manufactured by Zeon Corporation, and APL5514ML of Example 9 is a plastic material manufactured by Mitsui Chemicals.

  The “refractive index Nm of the parallel plate” in Table 4 is the refractive index of the parallel plate for each inspection, and the “thickness Dm of the parallel plate” is set as a result of correction so that the spherical aberration is minimized. 4 is the thickness of the parallel plate for calculation of the optical system 4, that is, the thickness of the parallel plate for inspection used for inspection. In Table 4, “refractive index Nc of the protective layer” is the refractive index of the protective layer of the first optical system, and “thickness Dc of the protective layer” is the thickness of the protective layer of the first optical system. All thickness units are mm.

  “Aberration of first optical system”, “Aberration of second optical system”, and “Aberration of fourth optical system” in Table 4 are all RMS of axial wavefront aberration, and the unit of axial wavefront aberration is Wavelength (λ). In the optical system of the present embodiment, it can be considered that the spherical aberration is corrected if the value of the axial wavefront aberration is small. In Table 4, since the aberration of the second optical system is larger than that of the first optical system, spherical aberration occurs due to the refractive index difference between the parallel plate for inspection and the protective layer of the optical recording medium. You can see that The aberration of the fourth optical system is smaller than the aberration of the second optical system, and the aberration of the fourth optical system is approximately the same as or close to the aberration of the first optical system. Therefore, by appropriately setting the thickness of the parallel plate for inspection, the spherical aberration caused by the difference in refractive index between the parallel plate for inspection and the protective layer of the optical recording medium is corrected well. I understand. Therefore, if the inspection is performed using the inspection parallel plate having the thickness set in this manner, the influence of the spherical aberration caused by the difference in refractive index between the inspection parallel plate and the protective layer of the optical recording medium is reduced. It can be made very small and can be properly inspected.

  “Value of conditional expression” in Table 4 is a value obtained by calculating 0.3141 × (Nm / Nc) + 0.68701− (Dm / Dc) using Nm, Dm, Nc, and Dc of each of Examples 1 to 10. That is, it is a value corresponding to the conditional expressions (1) and (2) described above. FIG. 2 shows points corresponding to Nm, Dm, Nc, and Dc of Examples 1 to 10, with Nm / Nc on the horizontal axis and Dm / Dc on the vertical axis. The range surrounded by the two dashed lines in FIG. 2 means the numerical range of the conditional expression (1), and the range surrounded by two broken lines in FIG. 2 means the numerical range of the conditional expression (2). . From Table 4 and FIG. 2, it can be seen that all of Examples 1 to 10 satisfy the conditional expressions (1) and (2).

  The present invention has been described with reference to the embodiment and examples. However, the present invention is not limited to the above embodiment and example, and various modifications can be made. For example, the above embodiment assumes a Blu-ray disc as an optical recording medium. However, as an optical recording medium, a CD, DVD, LD or other optical recording medium for short wavelength light, for example, so-called AOD (HD-DVD) is used. The present invention can also be applied when using a disk or the like.

DESCRIPTION OF SYMBOLS 1 Lens 2 Parallel plate for inspection 3 Reflecting mirror 4 Reference plate 5 Beam splitter 6 Collimator lens 7 Light source for inspection 9 Imaging element 10 Calculation part 11 Display part 100 Inspection apparatus

Claims (12)

In order to inspect the objective lens designed to focus the light emitted from the light source through the transparent protective layer of the optical recording medium on a predetermined position of the optical recording medium, the objective lens collects the light. A parallel plate for inspection inserted in the optical path of the emitted light instead of the protective layer,
A parallel plate for inspection having a refractive index different from that of the protective layer and having a thickness set so as to correct spherical aberration caused by a refractive index different from that of the protective layer. The following conditional expression (1) is satisfied, where the thickness and refractive index of the protective layer are Dc and Nc, respectively, and the thickness and refractive index of the parallel plate are Dm and Nm, respectively. Parallel plate for inspection.
−0.05 ≦ 0.3141 × (Nm / Nc) + 0.68701− (Dm / Dc) ≦ 0.05 (1)   3. The parallel plate for inspection according to claim 1, wherein the thickness is set so that spherical aberration of an optical system comprising the objective lens and the parallel plate is minimized.   The parallel plate for inspection according to any one of claims 1 to 3, wherein a numerical aperture of the objective lens on the optical recording medium side is 0.8 or more.   It consists of glass material, The parallel plate for a test | inspection as described in any one of Claim 1 to 4 characterized by the above-mentioned.   It consists of a plastic material, The parallel plate for a test | inspection as described in any one of Claim 1 to 4 characterized by the above-mentioned. When inspecting an objective lens for an optical recording medium designed to focus light emitted from a light source on a predetermined position of the optical recording medium via a light-transmitting protective layer of the optical recording medium, An inspection method for an objective lens for an optical recording medium, in which an inspection parallel plate instead of the protective layer is inserted into the optical path of light collected by the objective lens for inspection,
As the parallel plate, a parallel plate having a refractive index different from that of the protective layer and having a thickness set so as to correct spherical aberration generated due to a difference in refractive index between the parallel plate and the protective layer. An inspection method for an objective lens for an optical recording medium, characterized by being used. The following conditional expression (1) is satisfied, where the thickness and refractive index of the protective layer are Dc and Nc, respectively, and the thickness and refractive index of the parallel plate are Dm and Nm, respectively. Inspection method of objective lens for optical recording medium.
−0.05 ≦ 0.3141 × (Nm / Nc) + 0.68701− (Dm / Dc) ≦ 0.05 (1)   9. The objective lens for an optical recording medium according to claim 7, wherein a thickness of the parallel plate is set so that spherical aberration of an optical system comprising the objective lens and the parallel plate is minimized. Inspection method.   10. The objective lens inspection method for an optical recording medium according to claim 7, wherein the numerical aperture of the objective lens on the optical recording medium side is 0.8 or more.   The method for inspecting an objective lens for an optical recording medium according to claim 7, wherein the parallel plate is made of a glass material.   11. The objective lens inspection method for an optical recording medium according to claim 7, wherein the parallel plate is made of a plastic material.

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