JP4457499B2 - Objective lens for optical pickup device and optical pickup device - Google Patents

Description

【００５６】
また、この対物レンズの断面図を図４に示し、その球面収差図をＤＶＤの場合を図５（ａ）に、ＣＤの場合を図５（ｂ）に示す。
【００５７】
また、ＮＡＬ１における段差部１３ａの深さが６．４５μｍであり、金型の回折輪帯に対応する金型面を刃先半径４μｍのバイトで加工し、１次回折光を用いたときの第１面における回折効率を次の表２に示す。
【００５８】
【表２】

【００５９】
比較例として、第１面を同様の仕様で全面を回折面とした場合の回折効率は、ＤＶＤ８８％、ＣＤ８９％であり、この比較例の全面回折レンズと比較すると、本実施例は回折効率が高く、光量をより利用可能であることが分かる。特に、ＤＶＤでは、ストレール比が１００％であり、レーザ利用効率が優れていることが分かる。
【００６０】
次に、実施例２及び実施例３について説明する。実施例２，３は、実施例１と同様のＤＶＤとＣＤとの互換可能な対物レンズであって、図４とほぼ同様の断面を有し、図１と図６とを組み合わせた形状であって、回折輪帯の光軸に近い屈折面との境界における段差部１３ａ（図１）及び光軸から遠い屈折面との境界における段差部２３ａ（図６）を有している。回折面は、実施例１と同様の回折レリーフをはずしたマクロ的な形状を示す母非球面と、上述の式による光路差函数とで表す。また、非球面も上述の式で表される。
【００６１】
実施例２の対物レンズは、光源波長λ１＝６５５ｎｍのとき、焦点距離ｆ＝３．３６、像側開口数＝０．６０（必要開口数ＮＡ１＝０．６０）である。
【００６２】
光源波長λ２＝７８５ｎｍのとき、焦点距離ｆ＝３．３８、像側開口数＝０．６０（必要開口数ＮＡ２＝０．５０）である。
【００６３】
実施例２のレンズデータは次の表３の通りである。
【００６４】
【表３】

【００６５】
また、ＮＡＨ１における段差部（図６の段差部２３ａに相当する）の深さは２．７５２μｍであり、ＮＡＬ１における段差部（図１の段差部１３ａに相当する）の深さは６．４４７μｍである。なお、段差部の深さの符号は、その境界部において外側の部分が内側の部分よりも像側（光情報記録媒体側）に変位しているときを正とする。実施例２の対物レンズの球面収差図をＤＶＤの場合を図７（ａ）に、ＣＤの場合を図７（ｂ）に示す。
【００６６】
また、実施例３の対物レンズは、光源波長λ１＝６６０ｎｍのとき、焦点距離ｆ＝３．３６、像側開口数＝０．６０（必要開口数ＮＡ１＝０．６０）である。
【００６７】
光源波長λ２＝７９４ｎｍのとき、焦点距離ｆ＝３．３８、像側開口数＝０．６０（必要開口数ＮＡ２＝０．４５）である。
【００６８】
実施例３のレンズデータは次の表４の通りである。
【００６９】
【表４】

【００７０】
また、ＮＡＨ１における段差部（図６の段差部２３ａに相当する）の深さは−２．７８４μｍであり、ＮＡＬ１における段差部（図１の段差部１３ａに相当する）の深さは６．３７８μｍである。なお、段差部の深さの符号は、その境界部において外側の部分が内側の部分よりも像側（光情報記録媒体側）に変位しているときを正とする。実施例３の対物レンズの球面収差図をＤＶＤの場合を図８（ａ）に、ＣＤの場合を図８（ｂ）に示す。なお、表３及び表４では１０のべき乗の表現にＥ（またはｅ）を用いて、例えば、Ｅ−０２（＝１０^−２）のように表している。
【００７１】
以上の実施例においては、対物レンズの１つの面を３つの分割面（領域）に構成し、中間領域に回折構造を備えた分割面としたが、分割面数はこれに限定されることなく、更に３以上の分割面を設けた構成としてもよく、例えば、回折構造を備えた領域内が更に複数の分割面に構成されてもよく、また中心領域等の回折構造を備えていない屈折面の領域が更に分割面に構成されていてもよい。
【００７２】
【発明の効果】
本発明の光ピックアップ装置用対物レンズ及びその対物レンズを備える光ピックアップ装置によれば、光源からの光量を十分に利用することができ、情報記録媒体からの光の誤検出も生ぜず、製造コストの上昇を抑えることができる。
【図面の簡単な説明】
【図１】本実施の形態の対物レンズの模式的な上半分の断面図である。
【図２】図１の要部を拡大した断面図である。
【図３】図１の対物レンズを備える本実施の形態の光ピックアップ装置の概略図である。
【図４】実施例１の対物レンズの断面図である。
【図５】実施例１の対物レンズに関するＤＶＤの場合の球面収差図（ａ）、ＣＤの場合の球面収差図（ｂ）である。
【図６】本実施の形態の別の対物レンズの模式的な上半分の断面図である。
【図７】実施例２の対物レンズに関するＤＶＤの場合の球面収差図（ａ）、ＣＤの場合の球面収差図（ｂ）である。。
【図８】実施例３の対物レンズに関するＤＶＤの場合の球面収差図（ａ）、ＣＤの場合の球面収差図（ｂ）である。
【図９】図１の対物レンズに関するＤＶＤの場合の球面収差を模式的に示す図である。
【符号の説明】
１０ 対物レンズ
１０ａ 第１面
１１、１２ 屈折面
１３ 回折輪帯
１３ａ 段差部
２０ 対物レンズ
２０ａ 第１面
２１、２２ 屈折面
２３ 回折輪帯
２３ａ 段差部
１１１ 第１の半導体レーザ
１１２ 第２の半導体レーザ
２００ 光ディスク（情報記録媒体）
３００ 光検出器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens and an optical pickup device used for recording or reproducing information on an information recording medium in the optical pickup device.
[0002]
[Prior art]
At least an objective lens is commonly used as an optical pickup device capable of recording and / or reproducing information for a plurality of information recording media (for example, optical discs) such as CDs and DVDs having different recording densities and transparent substrate thicknesses. Thus, it is desired to reduce the size and cost of the apparatus, and various objective lenses and optical pickup apparatuses have been proposed.
[0003]
For example, the objective lens is provided with a notch as a special ring zone in a ring shape, and one surface of the objective lens is configured as a plurality of divided surfaces (for example, three divided surfaces), and a part of the plurality of divided surfaces (for example, light) The first divided surface on the axis side) can be used for recording and / or reproducing information on two information recording media having different recording densities and transparent substrate thicknesses, and a part of the remaining divided surfaces ( For example, the second divided surface adjacent to the first divided surface is made mainly usable for recording and / or reproducing information on one information recording medium (for example, an information recording medium having a smaller required numerical aperture). The remaining (eg, the third divided surface adjacent to the second divided surface) can be used mainly for recording and / or reproducing information on the other information recording medium (for example, the information recording medium having a larger required numerical aperture). It is known to do. One example thereof is that described in JP-A-11-96585. However, in the objective lens provided with the ring-shaped notch as described above, erroneous detection may occur in the focus signal. For example, of the three divided surfaces provided on one surface of the objective lens, the light beam that has passed through the second divided surface at the time of DVD recording or reproduction is projected onto the sensor by the second divided surface whose spherical aberration has been corrected for CD. In some cases, the light is defocused and condensed and erroneously detected as a focus signal.
[0004]
Japanese Patent Laid-Open No. 10-283668 discloses that a hologram type ring lens is used so that the hologram portion is not diffracted with respect to 650 nm wavelength light for DVD (using 0th order light), and 780 nm wavelength light for CD is used. Describes an optical pickup device that is diffracted into primary light to act.
[0005]
However, in this way, the objective lens is provided with a dividing surface, and one of the dividing surfaces uses a zero-order light at one wavelength and a first-order light at the other wavelength, or a diffraction surface using a first-order light. In some cases, the diffraction efficiency becomes low and the amount of light from the light source cannot be fully utilized, and erroneous detection occurs in the focus signal.
[0006]
In addition, providing a diffractive surface on the entire surface of the objective lens is not preferable from the viewpoint of an increase in manufacturing cost because the cost of manufacturing the mold is high.
[0007]
[Problems to be solved by the invention]
In view of the above-described problems of the prior art, the present invention can fully utilize the amount of light from the light source, can prevent erroneous detection of light from the information recording medium, and can suppress an increase in manufacturing cost. An object is to provide an objective lens for an optical pickup device and an optical pickup device including the objective lens.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, an objective lens for an optical pickup device according to the present invention has a lens surface provided with a region without a diffractive structure across a region with a diffractive structure in a direction from the optical axis side toward the outside. The diffractive structure has the same order (excluding the 0th order light) with respect to light beams of at least two wavelengths (λ1, λ2) having different orders of diffracted light that generate the maximum amount of diffracted light. It is characterized by being. The “same order” means that the orders are the same, including positive and negative signs.
[0009]
The position where the region having the diffractive structure is provided is the wavelength of the light source used in the optical pickup device using the objective lens, the thickness of the transparent substrate of the information recording medium, the information recording density of the information recording medium, etc. And is determined according to the required numerical aperture of the objective lens determined for each information recording medium.
[0010]
For example, in the optical pickup device, a numerical aperture required for the objective lens when reproducing or recording two information recording media is different, and a region having the diffractive structure in the vicinity of the smaller numerical aperture is provided. It is desirable to set. As a result, it is possible to collect light with an appropriate spot diameter having diffraction-limited performance on an information recording medium having a smaller numerical aperture, and conversely, a larger required numerical aperture. On the other side of the information recording medium, it is possible to contribute to image formation in a region that is not normally used and may be focused by defocusing to cause erroneous detection of the focus signal, and has a diffraction structure. By providing a region that does not have a diffractive structure between the two regions, the light amount loss can be reduced compared to a diffractive structure that generates a light amount loss for different wavelengths, so the amount of light from the light source that contributes to information recording or reproduction can be reduced. Utilization efficiency can be increased, and erroneous detection of the focus signal can be prevented, so that the performance of the optical pickup device can be improved.
[0011]
In addition, as described above, by providing the region without the diffractive structure across the region with the diffractive structure, the light quantity utilization efficiency is improved as compared with the case where the diffractive structure is provided over the entire surface, and the diffraction ring zone. Since the number is small, it is possible to shorten the man-hours for processing the mold for forming the lens, and to reduce the manufacturing cost of the mold.
[0012]
In addition, the use efficiency of the light quantity is further improved by forming a region having no diffractive structure as a refractive surface in a region including the optical axis on the lens surface.
[0013]
In the objective lens for an optical pickup device according to the present invention, the optical pickup device includes a first light source having a wavelength λ1 and a second light source having a wavelength λ2 (λ1 <λ2). A second light source emits a first light beam for reproducing or recording information on a first optical information recording medium having a transparent substrate thickness t1, and a second light source emits second optical information having a transparent substrate thickness t2. Optical information of the objective lens necessary for recording and / or reproducing the first optical information recording medium with the first light source by emitting a second light beam for reproducing or recording information on the recording medium A required numerical aperture on the recording medium side is NA1, and a required numerical aperture on the optical information recording medium side of the objective lens necessary for recording and / or reproducing the second optical information recording medium with the second light source is set to NA1. When NA2 (NA2 <NA1), the pair The lens has a region having a diffraction structure rotationally symmetric with respect to an optical axis on at least one surface, and the first optical information recording medium is recorded and / or reproduced when the first light source is used for recording and / or reproduction. The Nth-order diffracted light (N is an integer other than 0) from the region having the diffractive structure is used, and the diffractive structure is provided when the second optical information recording medium is recorded and / or reproduced by the second light source. Using the Mth order diffracted light (M = N) from the region, the Nth order diffracted light from the circumference farthest from the optical axis of the region having the diffractive structure of the light beam from the first light source is The N-th order diffracted light from the circumference closest to the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAH1 on the information recording medium side. When the numerical aperture on the recording medium side is converted into a light beam of NAL1,
NAH1 <NA1
(1/3) NA2 <NAL1 <NA2
It is characterized by satisfying.
[0014]
In particular, in a DVD / CD compatible optical pickup device, a light source having a wavelength of 655 ± 30 nm is used for DVD, and a light source having a wavelength of 785 ± 30 nm is used for CD. In the system to
0.45 ≦ NAH1 ≦ 0.56 and 0.3 ≦ NAL1 ≦ 0.45
It is preferable that More preferably, the position of the region having the diffractive structure is an image-side numerical aperture of the objective lens of 0.3 to 0.5, and more preferably 0.35 to 0.47.
[0015]
Further, 0.05 ≦ (NAH1-NAL1) ≦ 0.20 is preferable, and 0.07 ≦ (NAH1-NAL1) ≦ 0.13 is further preferable.
[0016]
As a specific design, for example, in a DVD / CD compatible optical Bicup device, up to the numerical aperture required for a DVD excluding a region having a diffractive structure (that is, a central region and a peripheral region) Aspherical shape with well-corrected spherical aberration compared to DVD, and diffracted by correcting spherical aberration that deteriorates due to the difference in thickness of the transparent substrate with respect to a CD having a transparent substrate thickness different from that of DVD. An aspherical shape in which the area near the numerical aperture necessary for the CD (ie, the intermediate area) is corrected with good spherical aberration with respect to the DVD so that focusing with an appropriate spot diameter with limit performance can be performed. As a mother aspherical surface having a different aspherical shape, a spherical aberration with respect to a DVD deteriorates only with this mother aspherical surface. A diffraction structure is provided in this region, and the first-order diffracted light from that region is coupled to the DVD. Spherical aberration is well corrected contribute to, first-order diffracted light by the region against CD can be created by designing the diffractive structure so as to contribute to the image formation. Of course, the objective lens of the present invention can be designed by various applications without being limited to such a design. Note that the DVD / CD compatible optical Bicup device is capable of at least one of information recording and reproduction with respect to at least one type of DVD and information with respect to at least one type of CD. This is an optical pickup device capable of at least one of recording and reproduction. Examples of various CDs include CD-R, CD-RW, CD-Video, and CD-ROM. Examples of DVD include DVD-ROM, DVD-RAM, DVD-R, and DVD-RW. .
[0017]
Further, N = 1, and it is preferable to use first-order diffracted light for both the first and second information recording media.
[0018]
In addition, at the boundary between the region having the diffractive structure and the region not having the diffractive structure closest to the optical axis, the light having the wavelength λ1 is transmitted using the first light source and the first optical information recording medium. The phase shift of the wavefront transmitted through the transparent substrate having the thickness t1 is preferably λ1 / 10 or less, and more preferably λ1 / 20 or less.
[0019]
In addition, a step portion is provided on the most optical axis side circumference of the region having the diffractive structure, and the depth of the step portion is such that the optical path difference generated at the boundary with the refracting surface is λ1 and λ2. It is preferably set to be an integer multiple, so that the phase shift due to the step portion can be made substantially zero at each wavelength of λ1 and λ2.
[0020]
Specifically, the diffractive structure has a refracting surface on the optical axis side so as to have a step portion at the most optical axis side circumference of the region having the diffractive structure, and the depth of the step portion is not less than 4 μm and not more than 10 μm. The shape can be depressed or protruded.
[0021]
On the other hand, the region having the diffractive structure also has a step portion on the circumference farthest from the optical axis, and in this step portion, the refracting surface far from the optical axis is depressed or protruded with respect to the diffractive structure. Can be shaped. With this structure, it is possible to reduce the deterioration of wavefront aberration when the temperature changes. That is, a step is provided in the optical axis direction at the boundary between the region having the diffractive structure and the region not having the diffractive structure farthest from the optical axis, and the step is 1 μm or more and 10 μm or less in the optical axis direction. It can be made to have a level difference. When the step portion is 1 μm or more, the Strehl ratio on the second optical information recording medium (for example, CD) side can be increased, and when it is 10 μm or less, the temperature characteristics relating to wavefront aberration do not deteriorate. Note that the step portion provided in the optical axis direction may of course be provided with an inclination with respect to the optical axis direction. In this case as well, the step in the optical axis direction may be 1 μm or more and 10 μm or less. In particular, the stepped portion is preferably a step parallel to the optical axis direction.
[0022]
An optical pickup device according to the present invention includes a first light source that emits a first light beam having a wavelength λ1 for reproducing or recording information on a first optical information recording medium having a transparent substrate thickness t1. A second light source for emitting a second light beam having a wavelength λ2 (λ1 <λ2) for reproducing or recording information on the second optical information recording medium having a thickness t2 of the transparent substrate; A light detector for detecting light from the information recording medium 2 and a lens surface provided with a region not provided with a diffractive structure across a region provided with a diffractive structure in a direction from the optical axis side toward the outside; The diffractive structure is an objective lens having a shape having the same order (excluding the 0th order) with respect to light beams of at least two wavelengths (λ1, λ2) having different orders of diffracted light that generate the maximum amount of diffracted light. It is characterized by comprising.
[0023]
Further, another optical pickup device according to the present invention provides a first light source that emits a first light flux having a wavelength λ1 for reproducing or recording information on a first optical information recording medium having a transparent substrate thickness t1. A second light source that emits a second light beam having a wavelength λ2 (λ1 <λ2) for reproducing or recording information on the second optical information recording medium having a transparent substrate thickness t2. And a photodetector for detecting light from the second information recording medium, and an objective lens having a region having a diffraction structure rotationally symmetric with respect to the optical axis on at least one surface, NA1 is a required numerical aperture on the optical information recording medium side of the objective lens required for recording and / or reproducing one optical information recording medium with the first light source, and the second optical information recording medium is Necessary for recording and / or playback with a second light source The required numerical aperture on the optical information recording medium side of the objective lens is NA2 (NA2 <NA1), and the diffraction structure is formed when the first optical information recording medium is recorded and / or reproduced by the first light source. N-order diffracted light (N is an integer other than 0) from the provided area is used to record and / or reproduce the second optical information recording medium with the second light source from the area provided with the diffractive structure. The Nth order diffracted light (M = N) of the light beam from the first light source and the Nth order diffracted light from the circumference farthest from the optical axis of the region having the diffractive structure is an optical information recording medium. N-th order diffracted light from the circumference closest to the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAH1 on the optical information recording medium side Is converted to a NAL1 luminous flux,
NAH1 <NA1
(1/3) NA2 <NAL1 <NA2
It is characterized by satisfying.
[0024]
The objective lens of the above-described optical pickup device can obtain the same effect by using an objective lens having the above-described features. In the present invention, as the first and second optical information recording media, for example, various CDs such as CD, CD-R, CD-RW, CD-Video, CD-ROM, DVD, DVD-ROM, DVD-ROM, etc. Various types of DVDs such as RAM, DVD-R, DVD-RW, etc., and disk-shaped information recording media such as MDs are included, but new high-density information recording media with higher recording density are also included.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view of the upper half of an objective lens for an optical pickup device showing an embodiment of the present invention. As shown in FIG. 1, the objective lens 10 includes a refracting surface 11 near the optical axis p on the first surface 10 a, and a diffraction ring zone 13 provided in a rotationally symmetric manner in an outer peripheral region away from the optical axis p of the refracting surface 11. And a refracting surface 12 provided on the outer periphery of the diffraction zone 13.
[0026]
The diffraction ring zone 13 is provided in the numerical aperture NAL1 to NAH1 range, and corrects spherical aberration by using the wavelength difference of the light source for two types of optical information recording media having different thicknesses of the transparent substrate. Is.
[0027]
As an example, in the case of corresponding to a DVD having a required numerical aperture of 0.6 at a wavelength of 655 nm and a CD having a required numerical aperture of 0.5 at a wavelength of 785 nm, the NA0. The diffraction ring zone 13 is provided in the range of 37 to 0.50.
[0028]
This refracting surface 11 with NA of 0.37 or less has no loss due to diffraction efficiency, and 100% of the amount of light can be used. When the NA is as small as 0.37 or less, the influence on the spot diameter and wavefront aberration necessary for imaging performance is small even if the spherical aberration is not completely corrected, and the recording / reproduction is performed due to the large amount of light. Accuracy can be increased, and erroneous detection of the focus signal can be prevented.
[0029]
Also, when mass-producing objective lenses by mold molding, if the production volume is large, a large number of molds themselves are required, which increases the processing man-hours, especially for diffractive lenses, it takes time to process the mold corresponding to the diffraction zone. However, the structure of the intermediate diffraction ring zone has a smaller number of ring zones than the entire surface diffraction structure and the number of processing steps can be shortened, so that the mold cost can be reduced and the manufacturing cost can be reduced.
[0030]
Further, at the boundary between the diffraction zone 13 and the refractive surface 11 on the optical axis p side, the light of wavelength λ1 and the light of wavelength λ1 are transmitted through the transparent substrate of thickness t1 using a DVD (transparent substrate thickness t1). The phase shift of the wave front is preferably λ1 / 10 or less, more preferably λ1 / 20 or less. Specifically, when a diffraction ring zone is formed on the objective lens based on lens data as in the examples described later, the phase difference between the diffraction ring zone 13 and the refracting surface 11 is almost zero in a DVD. Further, it can be realized by finely adjusting the position of the diffraction zone 13 in the optical axis p direction. That is, as shown in FIG. 9, wavefront aberration occurs in the diffraction zone 13 due to the difference between the blazed wavelength and the used wavelength (λ1), but the average wavefront of the diffraction zone 13 causes a phase shift from the wavefront of the refractive surface 12. What is necessary is just to determine the position of the optical axis p direction of the diffraction ring zone 13 so that it may not exist. As shown in FIG. 9, adjustment is made so that the average wavefront aberration of the diffracting zone is λ1 / 10 or less with respect to the wavefront aberration of the refracting portion (refractive surface).
[0031]
Next, the step portion 13a at the boundary between the refracting surface 11 and the diffraction zone 13 will be described with reference to FIG. The dimension (depth) of the step portion 13a is set so that the optical path difference generated at the boundary with the refracting surface by the step portion 13a is approximately an integral multiple of λ1 and λ2. That is, when a light beam having a wavelength λ1 of 655 nm is incident from the first surface 10a of the objective lens 10, the depth of the step portion 13a is determined so as to produce an optical path difference of 6λ1 as shown in FIG. When the wavelength λ2 is 785 nm, since λ1: λ2 is approximately 5: 6, an optical path difference of approximately 5λ2 is generated with respect to the wavelength λ2, and no phase difference is generated in both λ1 and λ2. In addition, by appropriately setting the dimension (depth) of the step portion 13a, it is possible to reduce the deterioration of the wavefront aberration when the temperature changes. That is, the depth of the step portion 13a is preferably 4 μm to 10 μm.
[0032]
Next, another objective lens for an optical pickup device of the present embodiment will be described with reference to FIG. FIG. 6 is a schematic cross-sectional view of the upper half of an objective lens for an optical pickup device showing a second embodiment of the present invention.
[0033]
As shown in FIG. 6, the objective lens 20 includes a refracting surface 21 close to the optical axis p on the first surface 20 a, and a diffraction ring zone 23 provided rotationally symmetrically in an outer peripheral region away from the optical axis p of the refracting surface 21. And a refracting surface 22 provided on the outer periphery of the diffraction ring zone 23, and can be used in the same manner as the objective lens of FIG. In the objective lens 20, a step portion 23 a is provided in the optical axis p direction with respect to the mother aspherical surface of the diffraction ring zone 23 at the boundary between the diffraction ring zone 23 and the refractive surface 22 on the side away from the optical axis p. .
[0034]
By providing such a step portion 23a with an appropriate step in the optical axis direction, the Strehl ratio on the CD side can be increased. The step amount of the step portion 23a is a value substantially according to the following equation so that the optical path difference caused by the step at the wavelength λ1 of DVD is an integral multiple of the wavelength.
[0035]
m · λ1 / (cos θ−n · cos θ ′)
(M is an integer, θ is the tilt angle of incident light, θ ′ is the tilt angle of outgoing light, and n is the refractive index)
[0036]
On the CD side, spherical aberration is caused so that the light beam passing through the refracting surface 22 outside the diffraction zone 23 becomes flare. At this time, this flare portion is in relation to the Strehl ratio of the imaging spot for CD. It was found to have an impact. That is, when the step amount is changed so that the integer m in the above formula takes a different integer value, the integer of the wavelength λ1 at the boundary between the diffraction zone 23 and the refractive surface 22 with respect to the wavelength λ1 of DVD. The phase changes by a factor of 2, and the Strehl ratio does not change. On the other hand, for the CD wavelength λ2, a fraction from an integral multiple of λ2 occurs in the phase change at the boundary, and at this time, the contribution of the Strehl ratio from the refracting portion 22 varies. If the value of the integer m is appropriately taken, the Strehl ratio can be increased. The step amount (depth) b of the step portion 23a (the distance in the optical axis p direction at the boundary between the mother aspheric surface and the refracting surface 22 indicated by the broken line of the diffraction ring zone 23) is preferably 1 μm or more and 10 μm or less.
[0037]
In FIGS. 1 and 6, the diffraction zones 13 and 23 are positioned so as to be recessed with respect to the refracting surfaces 11 and 22, but may be positioned so as to protrude, and the step portions 13a and 23a are preferable. The dimension (depth) can be applied to both a depressed shape and a protruding shape in terms of an absolute value.
[0038]
Next, the optical pickup device according to the present embodiment provided with the above-described objective lens will be specifically described with reference to FIG.
[0039]
The optical pickup device shown in FIG. 3 includes a first semiconductor laser 111 that is a first light source for reproducing a first optical disk, and a second semiconductor laser 112 that is a second light source for reproducing a second optical disk. And have.
[0040]
First, when reproducing the first optical disk, a beam is emitted from the first semiconductor laser 111, and the emitted light beam is transmitted through a beam splitter 190 which is a means for synthesizing the emitted light from both semiconductor lasers 111 and 112. The light passes through the polarizing beam splitter 120, the collimator 13, and the quarter wavelength plate 14, and becomes a circularly polarized parallel light beam. This light beam is focused by the diaphragm 17 and focused on the information recording surface 220 by the objective lens 10 via the transparent substrate 210 of the first optical disk 200.
[0041]
Then, the light beam modulated and reflected by the information bit on the information recording surface 220 is transmitted again through the objective lens 10, the diaphragm 17, the quarter wavelength plate 14, and the collimator 13, and enters the polarization beam splitter 120, where it is reflected. Then, astigmatism is given by the cylindrical lens 18 and incident on the photodetector 300, and a read signal of information recorded on the first optical disc 200 is obtained using the output signal.
[0042]
In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape and position of the spot on the photodetector 300. Based on this detection, the two-dimensional actuator 150 moves the objective lens 10 so that the light beam from the first semiconductor laser 111 forms an image on the recording surface 220 of the first optical disk 200, and the light beam from the semiconductor laser 111. The objective lens 10 is moved so as to form an image on a predetermined track.
[0043]
When reproducing the second optical disk, a beam is emitted from the second semiconductor laser 112, and the emitted light beam is reflected by the beam splitter 190, which is a light combining unit, and is polarized similarly to the light beam from the first semiconductor 111. The light is condensed on the information recording surface 220 via the transparent substrate 210 of the second optical disk 200 through the beam splitter 120, the collimator 13, the quarter wavelength plate 14, the diaphragm 17, and the objective lens 10.
[0044]
Then, the light beam modulated and reflected by the information pits on the information recording surface 220 is detected again through the objective lens 10, the diaphragm 17, the quarter wavelength plate 14, the collimator 13, the polarization beam splitter 120, and the cylindrical lens 180. A signal for reading information recorded on the second optical disc 200 is obtained using the output signal.
[0045]
Further, as in the case of the first optical disc, the spot shape change on the photodetector 300 and the light quantity change due to the position change are detected, and focus detection and track detection are performed. The objective lens 10 is moved for tracking.
[0046]
An optical pickup apparatus using an optical system using a two-wavelength semiconductor laser in which the semiconductor laser 111 and the semiconductor laser 112 are unitized, and a light source-detector module in which the semiconductor laser 111, the semiconductor laser 112, and the photodetector are unitized. The objective lens is also suitable. It is also possible to apply to a pickup optical system having a plurality of light sources known to those skilled in the art.
[0047]
【Example】
Next, Embodiment 1 of an objective lens compatible with DVD and CD corresponding to FIG. 1 will be described.
[0048]
The first surface 10a of the objective lens 10 shown in FIG. 1 has a central region (11) corresponding to a numerical aperture NAL1 or less and a peripheral region (12) corresponding to a numerical aperture NAH1 or more, which are refractive aspheric surfaces. The intermediate area corresponding to a numerical value NAL1 or more and a numerical aperture NAH1 or less is the diffractive surface 13. The second surface opposite to the first surface is a refractive aspheric surface.
[0049]
The diffractive surface is represented by a mother aspherical surface showing a macroscopic shape with the diffraction relief removed and an optical path difference function. The optical path difference function represents the optical path difference added by the diffraction surface to the diffracted light of the reference wavelength, and a diffraction ring zone is provided every time the value of the optical path difference function changes by mλ (m is the diffraction order).
[0050]
The optical path difference function Φ (h) is expressed by the following equation.
Φ (h) = b0 + b2 * h2 + b4 * h4 + b6 * h6 + (mm)
Where h: distance from the optical axis, b0, b2, b4, b6,...: Coefficients of the optical path difference function.
[0051]
An aspherical surface is expressed by the following equation.
x = (h2 / r) (1 + √ (1- (1 + k) h2 / r2) + A0 + A2h2 + A4h4 + A6h6 +.
Where A0, A2, A4, A6,...: Aspherical coefficient, k: conical coefficient, r: paraxial radius of curvature, r, d, n are the curvature radius, surface spacing, and refractive index of the lens. To express.
[0052]
When the light source wavelength is λ1 = 655 nm, the focal length f = 3.36 and the image-side numerical aperture = 0.60 (required numerical aperture NA1 = 0.60).
[0053]
When the light source wavelength is λ2 = 785 nm, the focal length f = 3.38 and the image-side numerical aperture = 0.60 (required numerical aperture NA2 = 0.50).
[0054]
The lens data of Example 1 is as shown in Table 1 below.
[0055]
[Table 1]

[0056]
FIG. 4 shows a cross-sectional view of the objective lens, and FIG. 5A shows the spherical aberration diagram in the case of DVD and FIG. 5B shows the case of CD.
[0057]
Further, the depth of the stepped portion 13a in the NAL1 is 6.45 μm, and the mold surface corresponding to the diffraction ring zone of the mold is processed with a cutting tool having a cutting edge radius of 4 μm, and the first surface when the first-order diffracted light is used. The diffraction efficiency is shown in Table 2 below.
[0058]
[Table 2]

[0059]
As a comparative example, the diffraction efficiency when the first surface has the same specifications and the entire surface is a diffractive surface is DVD 88% and CD 89%. Compared with the full surface diffractive lens of this comparative example, this example has a diffraction efficiency of It is high and it turns out that light quantity can be utilized more. In particular, in DVD, the Strehl ratio is 100%, and it can be seen that the laser utilization efficiency is excellent.
[0060]
Next, Example 2 and Example 3 will be described. Examples 2 and 3 are objective lenses that are compatible with DVD and CD as in Example 1, and have substantially the same cross section as in FIG. 4, and are a combination of FIG. 1 and FIG. Thus, it has a step 13a (FIG. 1) at the boundary with the refracting surface close to the optical axis of the diffraction zone and a step 23a (FIG. 6) at the boundary with the refracting surface far from the optical axis. The diffractive surface is represented by a mother aspherical surface having a macroscopic shape with the same diffraction relief as in the first embodiment, and an optical path difference function according to the above formula. An aspheric surface is also expressed by the above formula.
[0061]
The objective lens of Example 2 has a focal length f = 3.36 and an image-side numerical aperture = 0.60 (necessary numerical aperture NA1 = 0.60) when the light source wavelength λ1 = 655 nm.
[0062]
When the light source wavelength is λ2 = 785 nm, the focal length f = 3.38 and the image-side numerical aperture = 0.60 (required numerical aperture NA2 = 0.50).
[0063]
The lens data of Example 2 is as shown in Table 3 below.
[0064]
[Table 3]

[0065]
Further, the depth of the step portion (corresponding to the step portion 23a in FIG. 6) in NAH1 is 2.752 μm, and the depth of the step portion (corresponding to the step portion 13a in FIG. 1) in NAL1 is 6.447 μm. is there. It should be noted that the sign of the depth of the step portion is positive when the outer portion of the boundary portion is displaced to the image side (optical information recording medium side) relative to the inner portion. The spherical aberration diagram of the objective lens of Example 2 is shown in FIG. 7A for DVD and FIG. 7B for CD.
[0066]
The objective lens of Example 3 has a focal length f = 3.36 and an image-side numerical aperture = 0.60 (necessary numerical aperture NA1 = 0.60) when the light source wavelength λ1 = 660 nm.
[0067]
When the light source wavelength is λ2 = 794 nm, the focal length f = 3.38 and the image-side numerical aperture = 0.60 (necessary numerical aperture NA2 = 0.45).
[0068]
The lens data of Example 3 is as shown in Table 4 below.
[0069]
[Table 4]

[0070]
Further, the depth of the stepped portion (corresponding to the stepped portion 23a in FIG. 6) in NAH1 is −2.784 μm, and the depth of the stepped portion in NAL1 (corresponding to the stepped portion 13a in FIG. 1) is 6.378 μm. It is. Note that the sign of the depth of the step portion is positive when the outer portion of the boundary portion is displaced to the image side (optical information recording medium side) than the inner portion. The spherical aberration diagram of the objective lens of Example 3 is shown in FIG. 8A for DVD and FIG. 8B for CD. In Tables 3 and 4, E (or e) is used to express a power of 10, for example, E-02 (= 10 ^-2 ).
[0071]
In the above embodiment, one surface of the objective lens is configured as three divided surfaces (regions) and a divided surface having a diffractive structure in the intermediate region. However, the number of divided surfaces is not limited to this. Further, a configuration in which three or more dividing surfaces are further provided, for example, a region having a diffractive structure may be further formed into a plurality of dividing surfaces, and a refracting surface not having a diffractive structure such as a central region. These areas may be further formed on a dividing surface.
[0072]
【The invention's effect】
According to the objective lens for an optical pickup device of the present invention and the optical pickup device including the objective lens, the light amount from the light source can be sufficiently utilized, and no erroneous detection of light from the information recording medium occurs, resulting in a manufacturing cost. Can be suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an upper half of an objective lens according to the present embodiment.
FIG. 2 is an enlarged cross-sectional view of a main part of FIG.
3 is a schematic diagram of an optical pickup device of the present embodiment including the objective lens of FIG.
4 is a sectional view of an objective lens according to Example 1. FIG.
5A is a spherical aberration diagram in the case of DVD related to the objective lens of Example 1, and FIG. 5B is a spherical aberration diagram in the case of CD.
FIG. 6 is a schematic cross-sectional view of the upper half of another objective lens according to the present embodiment.
FIG. 7A is a spherical aberration diagram in the case of DVD related to the objective lens of Example 2, and FIG. 7B is a spherical aberration diagram in the case of CD. .
FIG. 8A is a spherical aberration diagram in the case of DVD related to the objective lens of Example 3, and FIG. 8B is a spherical aberration diagram in the case of CD.
FIG. 9 is a diagram schematically showing spherical aberration in the case of a DVD related to the objective lens shown in FIG. 1;
[Explanation of symbols]
10 Objective lens
10a first side
11, 12 Refractive surface
13 Diffraction ring zone
13a Step part
20 Objective lens
20a first side
21, 22 Refractive surface
23 Diffraction Ring Zone
23a Stepped part
111 First semiconductor laser
112 Second semiconductor laser
200 Optical disc (information recording medium)
300 photodetector

Claims (21)

In the direction from the optical axis side to the outside, a lens surface provided with a region not provided with a diffractive structure across a region provided with a diffractive structure,
The diffractive structure has a shape that has the same order (excluding the 0th order) with respect to light beams having at least two wavelengths (λ1, λ2) having different orders of diffracted light that generate the maximum amount of diffracted light. An objective lens for an optical pickup device. 2. The objective lens for an optical pickup device according to claim 1, wherein a region not including the diffractive structure is formed in a region including the optical axis on the lens surface. The objective lens for an optical pickup device according to claim 1, wherein the same order is a first order. An objective lens for an optical pickup device, wherein the optical pickup device includes a first light source having a wavelength λ1 and a second light source having a wavelength λ2 (λ1 <λ2).
The first light source emits a first light beam for reproducing or recording information on the first optical information recording medium having a transparent substrate thickness t1.
The second light source emits a second light beam for reproducing or recording information on the second optical information recording medium having a transparent substrate thickness t2.
NA1 is a required numerical aperture on the optical information recording medium side of the objective lens required for recording and / or reproducing the first optical information recording medium with the first light source,
When the required numerical aperture on the optical information recording medium side of the objective lens necessary for recording and / or reproducing the second optical information recording medium with the second light source is NA2 (NA2 <NA1),
The objective lens has a region having a diffraction structure rotationally symmetric with respect to the optical axis on at least one surface;
When recording and / or reproducing the first optical information recording medium with the first light source, N-order diffracted light (N is an integer other than 0) from the region having the diffractive structure is used.
When recording and / or reproducing the second optical information recording medium with the second light source, M-order diffracted light (M = N) from the region having the diffractive structure is used,
The Nth order diffracted light from the circumference farthest from the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAH1 on the optical information recording medium side,
N-th order diffracted light from the circumference closest to the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAL1 on the optical information recording medium side. ,
NAH1 <NA1
(1/3) NA2 <NAL1 <NA2
An objective lens for an optical pickup device, characterized in that: The objective lens for an optical pickup device according to claim 4, wherein the NAH1 and the NAL1 satisfy the following expression.
0.45 ≦ NAH1 ≦ 0.56
0.3 ≦ NAL1 ≦ 0.45 6. The objective lens for an optical pickup device according to claim 4, wherein the N is N = 1. A step is provided in the optical axis direction at the boundary between the region having the diffractive structure and the region not having the diffractive structure farthest from the optical axis, and the step is 1 μm or more and 10 μm or less in the optical axis direction. The objective lens for an optical pickup device according to claim 4, wherein: At the boundary between the region having the diffractive structure and the region not having the diffractive structure closest to the optical axis, the light having the wavelength λ1 is the thickness using the first light source and the first optical information recording medium. 7. The objective lens for an optical pickup device according to claim 4, wherein a phase shift of a wavefront transmitted through a transparent substrate having a thickness t1 is λ1 / 10 or less. The region having the diffractive structure has a step portion on the most optical axis side circumference, and the depth of the step portion is an optical path difference generated at the boundary with the refracting surface by the step portion, which is almost an integer of λ1 and λ2. The objective lens for an optical pickup device according to claim 1, wherein the objective lens is set to be doubled. 10. The method according to claim 1, further comprising a step portion at a circumference of the most optical axis side of the region having the diffractive structure, wherein the depth of the step portion is 4 μm or more and 10 μm or less. Objective lens for optical pickup device as described in 1. A first light source that emits a first light beam having a wavelength λ1 for reproducing or recording information on the first optical information recording medium having a thickness t1 of the transparent substrate;
A second light source that emits a second light beam having a wavelength λ2 (λ1 <λ2) for reproducing or recording information on a second optical information recording medium having a transparent substrate thickness t2.
A photodetector for detecting light from the first and second information recording media;
In a direction from the optical axis side to the outside, a lens surface having a region not provided with a diffractive structure sandwiching a region provided with a diffractive structure is provided, and the diffractive structure has a maximum diffraction with respect to the first light flux. An objective lens having a shape in which the order of the diffracted light that generates the amount of light and the order of the diffracted light that generates the maximum amount of diffracted light with respect to the second light flux are the same order (excluding the 0th order light) An optical pickup device comprising: 12. The optical pickup device according to claim 11, wherein a region not including the diffractive structure is formed in a region including the optical axis on the lens surface. 13. The optical pickup device according to claim 11, wherein the same order is a first order. A first light source that emits a first light beam having a wavelength λ1 for reproducing or recording information on the first optical information recording medium having a thickness t1 of the transparent substrate;
A second light source that emits a second light beam having a wavelength λ2 (λ1 <λ2) for reproducing or recording information on a second optical information recording medium having a transparent substrate thickness t2.
A photodetector for detecting light from the first and second information recording media;
An objective lens having a region having a diffractive structure rotationally symmetric with respect to the optical axis on at least one surface;
NA1 is a required numerical aperture on the optical information recording medium side of the objective lens required for recording and / or reproducing the first optical information recording medium with the first light source,
The required numerical aperture on the optical information recording medium side of the objective lens required for recording and / or reproducing the second optical information recording medium with the second light source is NA2 (NA2 <NA1),
When recording and / or reproducing the first optical information recording medium with the first light source, N-order diffracted light (N is an integer other than 0) from the region having the diffractive structure is used.
When recording and / or reproducing the second optical information recording medium with the second light source, M-order diffracted light (M = N) from the region having the diffractive structure is used,
The Nth order diffracted light from the circumference farthest from the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAH1 on the optical information recording medium side,
The Nth-order diffracted light from the circumference closest to the optical axis of the region having the diffractive structure of the light beam from the first light source is converted into a light beam having a numerical aperture of NAL1 on the optical information recording medium side In addition,
NAH1 <NA1
(1/3) NA2 <NAL1 <NA2
An optical pickup device satisfying the requirements. 15. The optical pickup device according to claim 14, wherein the NAH1 and the NAL1 satisfy the following expression.
0.45 ≦ NAH1 ≦ 0.56
0.3 ≦ NAL1 ≦ 0.45 16. The optical pickup device according to claim 14, wherein the N is N = 1. A step is provided in the optical axis direction at the boundary between the region having the diffractive structure and the region not having the diffractive structure farthest from the optical axis, and the step is 1 μm or more and 10 μm or less in the optical axis direction. The optical pickup device according to claim 14, 15 or 16. At the boundary between the region having the diffractive structure and the region not having the diffractive structure closest to the optical axis, the light having the wavelength λ1 is the thickness using the first light source and the first optical information recording medium. 17. The optical pickup device according to claim 14, 15 or 16, wherein a phase shift of a wavefront transmitted through a transparent substrate having a length t1 is λ1 / 10 or less. The region having the diffractive structure has a step portion on the most optical axis side circumference, and the depth of the step portion is an optical path difference generated at the boundary with the refracting surface by the step portion, which is almost an integer of λ1 and λ2. The optical pickup device according to claim 11, wherein the optical pickup device is set to be doubled. 20. The method according to claim 11, further comprising a step portion at a circumference of the optical axis side of the region having the diffractive structure, wherein the depth of the step portion is 4 μm or more and 10 μm or less. The optical pickup device described in 1. 21. The optical pickup device according to claim 11, wherein the first optical information recording medium is a DVD and the second optical information recording medium is a CD.

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