JP4224654B2 - Optical head device - Google Patents

Description

【００４７】
また、輪帯の中心に、直径約２０μｍ、段差ｈ₂＝２．６μｍの円形凹型の位 置決めマーク１１を形成している。
【００４８】
図３に示す位相制御素子においては、ＤＶＤ系光ディスクのＮＡである０．６はＮＡ６に対応し、ＣＤ系光ディスクのＮＡである０．４５はＮＡ５に対応する。段差ｈ₁を１．３μｍ、段差ｈ₂を２．６μｍに設定することで、波長λ₁が６ ５０ｎｍの光に対しては実質的に位相は変化せず、波面収差は図６に示す良好な特性が維持できる。
【００４９】
一方、上記設定された段差ｈ₁、ｈ₂を有する位相制御素子に、ＣＤ系である波長λ₂が７８０ｎｍの光を透過させると、１．６７πと３．３３πの位相差が発 生する。この位相差は、対物レンズと組み合わせて用いるときに、図１１に示す対物レンズの波面収差分布において、対応する輪帯域の波面収差を実効的に０．１７λ₂及び０．３３λ₂だけ差し引くように作用する。即ち、図３に示す位相補正用輪帯１Ａに光を透過させることで、波長７８０ｎｍの光に対してのみ波面収差が低減されることになる。
【００５０】
本実施例においては、図１に示すように、上記仕様の位相制御素子１と対物レンズ２とを組み合わせて、位相補正素子に形成された位置決めマーク１１と対物レンズに形成された位置決めマーク２１が一致するように顕微鏡で観察しながら位置合わせ調整した後、ホルダー３に固定する。これにより、位相制御素子１の中心軸と対物レンズ２の中心軸との偏心が無くなるため、波面収差のＲＭＳは、ＤＶＤ系では０．００１λ₁と変わらず、ＣＤ系ではＮＡが０．４５の光束に対 して０．０４４λ₂に低減できる。
【００５１】
このときのＣＤ系におけるＮＡが０．４５までの波面収差分布を図７に示す。図７によれば、波面収差は残留波面収差の目安であるマレシャル基準値０．０７λを十分下回るため、本光ヘッドによってＤＶＤ系及びＣＤ系何れの光ディスクに対しても安定して再生が可能となる。
【００５２】
また、ＣＤ系におけるＮＡが０．６までの波面収差を図８に示す。図中の実線は本実施例の位相補正用輪帯の溝１７がある場合を示し、点線は溝１７の無い場合を示している。したがって、ＮＡが０．４５以上の領域では、ＣＤ系の大きな波面収差が上記溝１７による段差を形成することによりさらに増大する。このような光束は信号読み出しに寄与しないため、実効的に開口制御が行われることになる。
【００５３】
対物レンズと位相制御素子とを位置決めマークが無い状態でホルダーに固定する場合、各部品の外形加工精度にはそれぞれ±３０μｍ程度の公差が存在するため、最大±１２０μｍ程度の対物レンズと位相制御素子の偏心が発生する。図９に対物レンズと位相制御素子が△ｒだけ偏心したときの波面収差のＲＭＳの変化を示した。これによれば、偏心量△ｒが７０μｍ以上でマレシャル基準値０．０７λを満たさないため、ＣＤ系の光ディスクを安定して再生できない。
【００５４】
また、偏心量を７０μｍ以下に保つため、各部品の外形公差が±１８μｍ以下の部品を選別するとなると、歩留まりの低下及び検査時間の増大に繋がる。さらに、本光ヘッド装置を実際にＤＶＤ系及びＣＤ系光ディスクの情報を再生する光ヘッド装置として用いる場合、光ディスクの変形や傾斜に伴うコマ収差の発生や温度変化による波面収差のＲＭＳの劣化などを考慮すると、理想的な光ディスクに対する波面収差のＲＭＳを０．０５λ以下に保つ必要がある。対物レンズと位相制御素子との偏心が生じやすい従来の構成ではさらに歩留まり低下を招くことになるが、本発明の構成では偏心が抑えられ、特に問題とならない。
【００５５】
次に、この位相制御用の溝を形成した基板と、ビームスプリッタ用ホログラムが形成された基板とを積層して一体化した位相制御素子、及び対物レンズを組み込んだ光ヘッド装置を説明する。この光ヘッド装置の一構成例を図１０に示した。
【００５６】
この場合のホログラムとしては、複屈折性を有する高分子液晶の薄膜に格子状の凹凸部を設け、この高分子液晶の常光屈折率とほぼ等しい屈折率を有する光学的等方性材料で高分子液晶の薄膜の凹凸部を充填した偏光ホログラム６を用いた。この偏光ホログラム６は、入射する光の偏光方向により回折効率が異なるもので、半導体レーザ４１Ａ、４１Ｂから光ディスク４（５）に向かう光の往路では高透過率の偏光方向を利用する。また、光ディスク４（５）からの反射光路となる復路では、λ／４板８により偏光方向を回転させて高回折効率の偏光方向を利用することで、光検出器４２Ａ、４２Ｂに反射光を導くことができる。ここで用いたλ／４板８は、２つの波長λ₁＝６５０ｎｍとλ₂＝７８０ｎｍの平均の波長に対して１／４となる位相差とした。
【００５７】
図１０に示す光ヘッド装置において、ＤＶＤ系光ディスク用の光源である波長６５０ｎｍの半導体レーザ４１Ａと、ＣＤ系光ディスク用の光源である波長７８０ｎｍの半導体レーザ４１Ｂからの出射光は、それぞれのコリメートレンズ４３Ａ、４３Ｂを透過し、波長選択性プリズムミラー４４により光軸を一致させ、本発明の対物レンズ２と、偏光ホログラム６を有する位相制御素子１とがホルダー３に一体化された複合素子１０を透過する。なお、この複合素子１０にはλ／４板８が偏光ホログラム６と位相制御素子１との間に接合されている。そして透過光は、対物レンズ２により光ディスク４（５）の光情報記録媒体面に集光する。この光ディスク４（５）のピット情報を含む反射光は再び複合素子１０を透過して、偏光ホログラム６により光軸をわずかに曲げられ、各光検出器４２Ａ、４２Ｂに到達する。ここで、λ／４板８の役割は上記した通りである。
【００５８】
ここで、ＤＶＤ系の光ディスク用の半導体レーザを４１Ｂ、ＣＤ系の光ディスク用の半導体レーザを４１Ａとしてもよい。この場合、波長選択性プリズムミラー４４の反射特性は上記の場合と異なり、波長６５０ｎｍの光を反射することとなる。
【００５９】
ＣＤ系の光ディスク再生時に、この位相制御素子と対物レンズとを備えた光ヘッド装置を用いることにより、波面収差のＲＭＳを安定して０．０４４λ₂の小 さな値に維持することが可能となる。これによって、光ディスクからの反射光である情報光のノイズが低減され、安定した再生ができる。また、光ヘッド装置の構成部品点数を減らすことができ、光ヘッド装置を小型・軽量化できる。
【００６０】
【発明の効果】
本発明によれば、対物レンズと位相制御素子とは中心軸合わせ用の位置決めマークが形成されており、かつこのマークによって位置決めされて一つのホルダーに固定されることにより、光ヘッド装置の位相制御素子と対物レンズとが偏心することなく一体化されるため、ＤＶＤ系の光ディスクの再生性能を維持したまま、設計値通りのＣＤ系光ディスクの再生性能が安定して得られる。
また、２つの光源からそれぞれ異なる波長の光を出射して、各波長の光が対物レンズと位相制御素子を備えた光学系を透過して光記録媒体上に集光されるとき、位相制御素子表面の溝が一方の波長の光の波面収差のＲＭＳには変化を与えずそのまま光を透過させ、他方の波長の波面収差のＲＭＳを低減することにより、実質的に開口制御が行え、ＤＶＤ系及びＣＤ系の光ディスクの情報を簡単な構成により記録・再生することができる。以て、記録装置の部品点数が減少し、小型・軽量化を図ることができる。
【図面の簡単な説明】
【図１】本発明の光ヘッド装置の位相制御素子及び対物レンズの断面図で、（ａ）はＤＶＤ系光ディスクに用いた場合で、（ｂ）はＣＤ系光ディスクに用いた場合の図である。
【図２】本発明の光ヘッド装置に用いられる対物レンズの一例を示す構成図で、（ａ）は平面図で、（ｂ）は断面図である。
【図３】本発明の光ヘッド装置に用いられる位相制御素子の一例を示す構成図で、（ａ）は平面図で、（ｂ）は断面図である。
【図４】本発明の光ヘッド装置に用いられる位相制御素子がホログラムと一体化された一例を示す断面図である。
【図５】本発明の光ヘッド装置に用いられる位相制御素子がチルト補正素子及びホログラムと一体化された一例を示す断面図である。
【図６】本発明の光ヘッド装置をＤＶＤ系光ディスクに用いた場合の波面収差の一例を示す図である。
【図７】本発明の光ヘッド装置をＣＤ系光ディスクに用いた場合のＮＡが０．４５までの波面収差の一例を示す図である。
【図８】本発明の光ヘッド装置をＣＤ系光ディスクに用いた場合のＮＡが０．６までの波面収差の一例を示す図である。
【図９】本発明の光ヘッド装置において位相制御素子と対物レンズに偏心が存在した場合にＣＤ系光ディスクにおいて生じるＲＭＳ波面収差の一例を示す図である。
【図１０】本発明の光ヘッド装置の構成例を示す概念図である。
【図１１】位相制御素子の無い対物レンズを備えた従来の光ヘッド装置をＣＤ系光ディスクに用いた場合に発生する波面収差の一例を示す図である。
【符号の説明】
１：位相補正素子（位相制御用の基板）
１Ａ：位相補正素子の位相補正用輪帯
２：対物レンズ
３：ホルダー
４：ＤＶＤ用光ディスク
５：ＣＤ用光ディスク
１１：位相補正素子に形成された位置決めマーク
１６：開口数ＮＡ２以下に対応した位相補正素子の位相補正用輪帯
１３：開口数ＮＡ１〜ＮＡ２に対応した位相補正素子の位相補正用輪帯
１４：開口数ＮＡ２〜ＮＡ３に対応した位相補正素子の位相補正用輪帯
１５：開口数ＮＡ３〜ＮＡ４に対応した位相補正素子の位相補正用輪帯
１６：開口数ＮＡ４〜ＮＡ５に対応した位相補正素子の位相補正用輪帯
２１：対物レンズに形成された位置決めマーク
４Ａ、４１Ｂ：半導体レーザ
４２Ａ、４２Ｂ：光検出器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device used for a recording device or a reproducing device for an optical recording medium such as an optical disk.
[0002]
[Prior art]
For recording / reproduction of CD-based optical recording media including CD-R (hereinafter referred to as optical disc), a semiconductor laser having a wavelength of 780 nm as a light source and an objective lens having an NA (numerical aperture) of 0.45 In addition, an optical disk having a thickness of 1.2 mm is used. On the other hand, for recording / reproduction of a DVD-type optical disc, a semiconductor laser having a wavelength of 650 nm, an objective lens having an NA of 0.6, and an optical disc having a thickness of 0.6 mm are used as a light source.
[0003]
Therefore, in order to realize recording and reproduction of both CD and DVD optical discs with a single optical head device, a total of two semiconductor lasers having oscillation wavelengths for each of the CD and DVD systems, and each NA. A total of two corresponding objective lenses are used. However, this system has the disadvantages that the optical system has two systems, so the volume of the optical head device is large, the weight is large, and the assembly process is complicated because of the large number of parts.
[0004]
In order to solve these drawbacks, it has been proposed to synthesize and separate light from semiconductor lasers with different wavelengths by a synthesis separation mirror having wavelength selectivity, and to construct a compact optical head device using the same objective lens. ing. However, as described above, the NA required for the objective lens differs between the CD-type and DVD-type optical discs. Therefore, when recording / reproducing both types of optical discs using the same objective lens, the NA of the objective lens is set to the wavelength. It was necessary to change according to.
[0005]
As a method of changing according to this wavelength, light in two wavelength bands is transmitted in a straight line in the central region of the objective lens including the optical axis, while a wavelength of 650 nm that requires a large NA in the peripheral region of the objective lens not including the optical axis. There is a method in which light is transmitted linearly and light having a wavelength of 780 nm, which may be a small NA, is reflected. In this way, by arranging a wavelength-selective aperture between the optical disk and the light source to selectively lower the linear transmittance in the peripheral region with respect to one wavelength, the wavelength of the optical disk in the CD system and the DVD system. NA is switched for. However, since the optical disc thickness differs between the CD system and the DVD system, it is difficult to sufficiently reduce the generated spherical aberration only by such aperture control (NA control).
[0006]
As conventional means for solving this problem, there are roughly two types of measures. These strategies are outlined, for example, in Optical 28, No. 2, 1999, pages 64-70. That is, an annular step is formed on the surface of the objective lens designed to minimize the wavefront aberration in the DVD system, and the wavefront aberration in the CD system is reduced while suppressing an increase in the wavefront aberration in the DVD system. There are an annular phase correction lens system and a phase control element system in which an element in which a planar surface has an annular shape and a cross-sectional shape has a stepped shape formed on a substrate is provided separately from the objective lens.
[0007]
[Problems to be solved]
However, the following two types of measures also cause the following problems.
First, specific examples of the annular phase correction lens system are described in Japanese Patent Laid-Open Nos. 11-2759 and 11-16190, but both are compared with an objective lens having no phase correction annular zone. Although the RMS (Root Mean Square) of the wavefront aberration of CD is improved, the wavefront aberration of DVD tends to deteriorate. Further, in the case of a glass molded lens having stable characteristics against temperature fluctuations, it is difficult in terms of the manufacturing method to accurately mold a wavelength level phase correction ring zone.
[0008]
In the phase control element method, for example, as described in JP-A-10-334504, the phase control element hardly changes the phase distribution with respect to the DVD wavelength. The objective lens value designed optimally is maintained, and only the RMS wavefront aberration of the CD system is reduced. Therefore, this method is effective for a DVD whose recording / reproducing performance is sensitive to wavefront aberration. However, since the phase control element is separated from the objective lens, there is a problem that the RMS reduction effect of the CD wavefront aberration does not function when the eccentricity between the phase control element and the objective lens exceeds an allowable value. And, with the external processing accuracy of the objective lens, the phase control element and the holder that integrates both components manufactured by the conventional manufacturing method, the eccentricity of the objective lens and the phase control element generated during assembly is stably held below the allowable value. It was difficult.
[0009]
Further, in an optical head device for an optical disk, light emitted from a semiconductor laser is reflected by the optical disk to become return light, and this return light needs to be guided to a light receiving element that is a photodetector using a beam splitter. As the beam splitter, in addition to the above-described combining / separating mirror for combining and separating semiconductor laser beams having different wavelengths, a half mirror for guiding the light of each wavelength separated by the combining / separating mirror to the light receiving element is further provided. Necessary. Therefore, the number of parts of the apparatus increases, the assembly process becomes complicated, and the productivity decreases. Further, since the half mirror is used so as to reflect light in a direction perpendicular to the normal light incident direction, it is difficult to reduce the size of the optical head device.
[0010]
Further, in order to reduce the size of the optical head device, a beam splitter (holographic beam splitter) using a hologram element has been proposed. This holographic beam splitter can guide light to a light receiving element arranged near the semiconductor laser by bending the light traveling direction by diffraction.
If this holographic beam splitter is arranged on the side close to the semiconductor laser between the semiconductor laser and the objective lens, respective hologram elements are required near the semiconductor lasers having two different wavelengths, and the number of parts increases. In particular, when reproducing a DVD-type optical disc, tracking accuracy increases when the hologram element is driven integrally with the objective lens.
[0011]
The present invention has been made to solve the above-described conventional problems, and uses a phase control element and an objective lens, and can stably record and reproduce information on DVD-type and CD-type optical discs with a simple configuration. The object is to provide a device.
[0012]
[Means for Solving the Problems]
The present invention relates to an optical head device that collects light from a light source on an optical recording medium by an optical system including an objective lens and a phase control element, and records or reproduces information. There is provided an optical head device characterized in that a positioning mark for center axis alignment is formed and positioned by this mark and fixed to one holder.
[0013]
In addition, there is provided the above-described optical head device, wherein a concave or convex positioning mark is formed at a position of a central axis on each surface where the phase control element and the objective lens face each other.
[0014]
Further, the surface of the phase control element has a groove having a circular planar shape and a stepped sectional shape, the light source is composed of two light sources, each light source emits light of a different wavelength, When light of a wavelength passes through the optical system and is condensed on the optical recording medium, the groove transmits the light as it is without changing the RMS of the wavefront aberration of the light of one wavelength, and the other wavelength. The above-mentioned optical head device is characterized in that the RMS of the wavefront aberration is reduced.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of an optical head device according to the present invention will be described below in detail with reference to the drawings.
FIG. 1 is a sectional view showing a configuration example of a phase control element and an objective lens used in the optical head device of the present invention. Reference numeral 1 denotes a phase control element, 2 denotes an objective lens, and 3 denotes a holder for incorporating and fixing the phase control element and the objective lens. The objective lens 2 is optimally designed so that the wavefront aberration is minimized with respect to the optical disk 4 for DVD system as an optical recording medium having a thickness of 0.6 mm. FIG. 1 (a) shows a state in which a luminous flux corresponding to NA of 0.6 is condensed on a DVD-type optical disc 4, and FIG. 1 (b) shows a CD as an optical recording medium having a thickness of 1.2 mm. A state in which a luminous flux corresponding to NA of 0.45 is condensed on the optical disk 5 of the system is shown.
[0016]
In FIG. 1, a positioning that is a concave depression in the center axis of the annular zone of the phase control element is used for eccentricity adjustment on each surface where the phase control element 1 and the objective lens 2 face on the optical axis of the optical head indicated by the one-dot chain line. An example is shown in which the mark 11 is also formed with a positioning mark 21 that is a concave depression on the central axis of symmetry of the objective lens. At the time of actual assembly, the eccentricity is adjusted so that the positioning mark 21 at the center of the objective lens coincides with the positioning mark 11 at the center of the annular zone of the phase control element, and then fixed to the holder 3. As a result, the phase control element of the optical head device and the objective lens are integrated without being decentered, and the reproduction performance of the CD optical disk as designed is stably obtained while maintaining the reproduction performance of the DVD optical disk. It is done.
[0017]
As an example of the objective lens 2 used in the optical head device of the present invention, a plan view and a sectional view thereof are shown in FIG. FIG. 3 shows a plan view and a cross-sectional view of the phase control element. The positioning mark 11 formed on the eccentricity-adjustable phase correction element used here and the positioning mark 21 formed on the objective lens are not limited to the concavo-convex shape, and any mark can be used as long as the position can be determined. It may be a shape. In the case of the concave / convex mark, both the objective lens 2 and the phase correction element 3 may be concave or convex. In addition, since a convex mark can be formed in the metal mold | die used at the time of lens shaping | molding, and a concave mark can be formed in the same process at the time of the ring zone processing of the phase control element 2, formation of a concave mark is preferable in any case. The mark shape is circular in FIGS. 2 and 3, but may be any shape such as a square, a triangle, or a + mark.
[0018]
The size of the positioning mark 11 formed on the phase correction element and the positioning mark 21 formed on the objective lens may be small areas that do not significantly deteriorate the transmittance and wavefront aberration of the entire element. Specifically, an outer shape of about several μm to 100 μm is preferable. In order not to affect the wavefront aberration of the DVD system, the depth H of the concavo-convex mark is also set to the DVD system wavelength λ similarly to the phase correction ring zone of the phase control element described later. ₁ And 2π (n−1) H / λ with respect to the refractive index n of the concave / convex forming medium. ₁ It is preferable to process so that the phase difference expressed by can be an integral multiple of 2π.
In addition, the mark position is not necessarily one at the center of the element, but the distance between the objective lens 2 and the phase control element 1 is used when the enlarged image is displayed using an imaging device or the like during assembly adjustment and the mark position is aligned. Is the apex of the convex surface of the shortest objective lens, that is, the center of the optical axis, which is preferable because focusing accuracy can be improved.
[0019]
The objective lens 2 used in the optical head device of the present invention is manufactured by precision molding a base material such as glass or resin using a mold having a surface shape corresponding to a design value. Since the tolerance of wavefront aberration in DVD-based optical discs is stricter than that in CD-based lenses, glass molded lenses with less deterioration of wavefront aberration with respect to temperature changes compared to resin are usually used. Is optimally designed to minimize
[0020]
Next, the phase control element 1 used in the optical head device of the present invention will be described. In FIG. 1, in the conventional case where the phase control element 1 is not used, the objective lens 2 optimally designed so as to minimize the wavefront aberration is used for the DVD optical disk 4 having a thickness of 0.6 mm. When a 1.2 mm CD-based optical disk 5 is reproduced, for example, spherical aberration having a phase difference as shown in FIG. 11 occurs. FIG. 11 shows a cross section of the phase difference distribution, which is actually a three-dimensional, doughnut-shaped peripheral portion and has an annular distribution.
[0021]
Since the aberration is large at the phase difference level shown in FIG. 11 and the information on the CD-type optical disk is difficult to reproduce, the spherical surface shown in FIG. 11 is used by using the phase correction element 1 formed with the phase correction ring zone 1A shown in FIG. Reduce aberrations. This phase correction ring zone 1A has a wavelength λ ₁ In order to prevent the wavefront aberration from deteriorating when using a DVD optical disc of 650 nm, the grooves 12 to 17 having a circular planar shape and a stepped sectional shape are arranged concentrically at predetermined intervals.
[0022]
Where h in the figure ₁ And h ₂ The phase difference Δφ generated by the step h indicated by (the difference in height between adjacent steps) ₁ Can be expressed by equation (1).
Δφ ₁ = 2π (n−1) h / λ ₁ (1)
The phase correction annular zone 1A of the phase correction element is processed so that this phase difference is an integral multiple of 2π. Here, n is the refractive index of the processed substrate. For example, when the refractive index n = 1.5, by setting the step h to an integral multiple of 1.3 μm, the wavelength λ is substantially increased. ₁ It is possible to prevent the phase from changing for light of = 650 nm. In addition, the number of ring zones formed is normally 2-7. Further, the annular zone of the phase correcting annular zone 1A does not have to be a complete circle, and may be somewhat close to an ellipse, and further, the circle may not be connected and may be partially cut off. Further, the groove forming method includes a method combining photolithography and etching, a pressing method, an injection molding method, and the like, and an appropriate method is selected according to the material in which the groove is formed.
[0023]
On the other hand, such a phase correction ring zone 1A has a wavelength λ ₂ = If the light of 780 nm is transmitted, the phase difference Δφ expressed by the equation (2) ₂ Occurs.
Δφ ₂ = 2π (n−1) h / λ ₂ (2)
In this case, a phase difference of 1.67π is generated. As shown in FIG. 3, by forming the phase correction ring zone 1A from a plurality of step-like grooves 12 to 17, the phase can be changed only for light having a wavelength of 780 nm. Utilizing this fact, the spherical aberration can be corrected by controlling the phase of the incident light beam.
[0024]
The substrate of the phase control element 1 used here may be any material as long as it has good light transmission and durability and the refractive index is appropriately selected. For example, in addition to glass, acrylic resin, polycarbonate Polyvinyl chloride may be used. Among these, a glass substrate is preferable because it has high light transmittance and high durability.
[0025]
Also, the required NA value is different between DVD and CD optical discs, and the NA value during playback of CD optical discs is smaller than the DVD NA, so the periphery of objective lens 2 and phase control element 1 is used. It is preferable to limit so as not to. Therefore, an aperture control filter having wavelength selectivity, which transmits light having a wavelength of 650 nm for a DVD optical disk but does not transmit light having a wavelength of 780 nm for a CD optical disk, to the periphery of the phase control element 1 is preferable. Can be used.
[0026]
Further, when reproducing a CD-type optical disc, the optical design is made so that the phase of the transmitted light beam at the periphery of the phase control element 1 is shifted, and aberration is generated so that the light is not effectively condensed on the optical disc surface. Control is possible. In this case, a wavelength-selective aperture control filter can be omitted, and the number of components used can be reduced.
[0027]
Specifically, in the phase control element 1 shown in FIG. 3, also for an annular zone region corresponding to a region that is used in the DVD system and not used in the CD system is greater than 0.45 and less than or equal to 0.6. Then, the groove 17 having an annular plane shape and a stepped sectional shape is formed. The phase difference Δφ generated by the step h of the groove 17 ₁ Is the wavelength λ of the DVD system ₁ = Represented by the above formula (1) for a light beam of 650 nm. This phase difference Δφ ₁ The groove 17 is processed so that becomes an integral multiple of 2π.
At this time, the wavelength λ of the CD system ₂ = NA of the optical system is 0. If the step h is defined such that the difference between the wavefront aberration of 45 or less and the wavefront aberration of NA greater than 0.45 and 0.6 or less is large, the phase control element 1 has effective wavelength selectivity. Acts as an aperture control filter.
[0028]
Further, FIG. 3 shows a ring-shaped groove 17, but it is more preferable to form a diffraction grating in this region with the step h in order to improve wavelength selectivity. In this case, the DVD light beam is transmitted straight and the CD light beam is diffracted, so that the aperture control function substantially similar to that of the conventional dielectric multilayer interference filter system that reflects only the CD light beam is realized. it can.
[0029]
Further, the phase control substrate 1 on which the planar shape of the phase correction annular zone 1A having an annular shape and a cross-sectional shape of the step shape is integrated with the hologram substrate 6 on which the beam splitter hologram 6A is formed. A sectional view of the configuration is shown in FIG. By integrating the phase control substrate 1 and the hologram substrate 6 in this way, the number of components can be reduced, and the size and weight can be reduced. Further, the optical head device can be easily assembled. it can. Further, by integrating, the alignment accuracy between the annular groove of the phase control element and the unevenness of the hologram can be increased, and the production yield can be improved.
[0030]
As the integration method, a hologram may be formed on the back surface of the substrate on which the phase control groove is formed, or the substrate on which the phase control groove is formed and the substrate on which the hologram is formed are laminated. Also good. When the latter lamination is performed, the surface of the substrate on which the phase control groove is formed and the surface on which the hologram grating is formed may be on the same side as shown in FIG. The surfaces may face each other, and conversely, the forming surfaces may face each other.
Moreover, although the hologram which has an unevenness | corrugation with a rectangular cross section was shown in FIG. 4, the hologram of the shape blazed may be sufficient. In that case, since the diffraction efficiency of a specific order can be improved, it is a preferable choice depending on the intended use.
[0031]
This phase control element is further integrated with a wave plate such as a quarter wave plate (hereinafter referred to as a λ / 4 plate) to control the polarization characteristics of light traveling toward the optical disk and reflected light from the optical disk, respectively. it can.
[0032]
In addition, as the hologram to be integrated, a polarization hologram in which a grating-like uneven portion is provided in an optical material having birefringence and the uneven portion is filled with another optically isotropic material may be used. preferable. This polarization hologram is particularly preferable because of its high light utilization efficiency. The refractive index of the optically isotropic material that fills the concavo-convex portion of the optical material having birefringence is either the ordinary refractive index or the extraordinary refractive index of the birefringent optical material when the light utilization efficiency is to be increased. Equal to
[0033]
In the above configuration, the birefringent optical material is provided with a lattice-shaped uneven portion, and the uneven portion is filled with an optically isotropic material. That is, a lattice-shaped uneven portion may be provided in the optically isotropic material, and the uneven portion may be filled with a birefringent optical material. Further, a lattice-like uneven portion may be provided in the birefringent optical material, a lattice-like uneven portion may be provided in the optically isotropic material, and both the uneven portions may be fitted.
[0034]
This polarization hologram has different diffraction efficiencies depending on the polarization direction of incident light. The polarization direction has a high transmittance in the outgoing light path from the light source toward the optical disk, and in the return path of the outgoing light reflected back by the optical disk. It is desirable to rotate the polarization direction with a λ / 4 plate or the like to obtain a polarization direction with high diffraction efficiency. By changing the polarization direction in this way, unnecessary diffracted light in the forward path can be reduced.
[0035]
In addition, when birefringence exists on the substrate of the optical disk, the refractive index of the optically isotropic material is birefringence in order to detect information contained in the reflected light reflected and returned from the optical disk. What is necessary is just to set so that it may not be equal to neither the ordinary-light refractive index nor the extraordinary refractive index of the optical material. In this case, the number of optical disks that can be used increases, and the versatility of the optical head device can be further improved.
[0036]
The polarization hologram is LiNbO _Three It can also be produced using birefringent optical single crystals such as the above, but it is possible to form a lattice-shaped concavo-convex portion on a birefringent organic thin film, particularly a polymer liquid crystal thin film, and use other optically isotropic materials. Filling the lattice portion is preferable from the viewpoints of ease of manufacture and freedom of selection of the refractive index. Furthermore, a polarization hologram produced from an organic thin film is preferable because the material cost and processing cost are low.
Examples of the optically isotropic material include acrylic polymers, epoxy polymers, urethane polymers, and the like.
[0037]
The configuration of the optical head device according to the present invention is not limited to the combination of the above-described phase control element and objective lens. That is, in the configuration of the present invention, the eccentricity between the lens and the optical element can be reduced by accurately adjusting the eccentricity of the optical element with respect to the optical axis center of the lens in the composite element that functions as a composite integrated with the lens. Is effective in degrading the performance of the entire composite element.
[0038]
For example, in an actual optical disc, coma aberration may occur on the optical information recording surface as the optical disc is deformed, and reproduction may be difficult. This is particularly apparent in a DVD system in which the allowable width of wavefront aberration is narrow and the optical disk plate is thin. As a countermeasure, use of a tilt correction element has been proposed. The tilt correction element is an element that performs spatial phase wavefront correction so that coma aberration caused by the above causes is corrected in real time following the rotation of the optical disk.
Specifically, the tilt correction element is obtained by enclosing liquid crystal so that the alignment is aligned on two glass substrates formed by patterning transparent electrodes in a shape effective for coma aberration correction, and applying a voltage to the transparent electrodes. The liquid crystal orientation is changed in real time to form a spatial phase distribution.
[0039]
Similarly to the above-described phase control element, this tilt correction element cannot provide a designed phase correction function when the eccentricity occurs from the central axis of symmetry of the objective lens. After the positioning mark is formed and the eccentricity with the objective lens is adjusted, it may be integrated and fixed.
Therefore, in order to perform stable recording or reproduction of DVD-type and CD-type optical discs, as shown in FIG. 5, the phase control element 1 and the tilt correction element 7 are integrated, and a beam splitter hologram 6A is formed. The hologram substrate 6 may also be integrated to eliminate the eccentricity with the objective lens.
Here, FIG. 5 is a cross-sectional view showing an example in which the phase control element used in the optical head device of the present invention is integrated with the tilt correction element and the hologram. Cross section of a configuration in which a tilt correction element 7 is interposed between a phase control substrate 1 on which a correction ring zone 1A is formed and a hologram substrate 6 on which a beam splitter hologram 6A is formed. FIG.
[0040]
Specifically, in FIG. 5, the tilt correction element 7 is a phase control element 1 in which a phase correction ring zone 1 </ b> A is formed on one surface and a transparent electrode (not shown) patterned on the other surface in contact with the liquid crystal is formed. The liquid crystal 71 is produced by injecting the substrate and the substrate 74 of the tilt correction element 7 into an empty cell formed by a seal 72 so as to maintain a gap of several μm. At this time, an alignment film is applied on the transparent electrode so that the liquid crystal molecules are aligned at the substrate interface. 73 is an electrode terminal for voltage application. Further, the substrate 6 on which the hologram is formed is bonded to the opposite side of the tilt element 7 from the substrate 1.
[0041]
The phase control element 1 and the objective lens 2 described above include at least a light detector that guides light emitted from two light sources having different wavelengths to the optical recording medium and detects reflected light from the optical recording medium. The head device is disposed between the light source and the optical recording medium.
[0042]
Wavefront aberration can be reduced by integrating the phase control element 1 and the objective lens 2 by adjusting the eccentricity and then incorporating the phase control element 1 and the objective lens 2 into the optical head device. Therefore, it is possible to reduce the noise of the signal light that is the reflected light from the optical disk, and the number of components of the optical head device is reduced, the volume is reduced correspondingly, and the process for manufacturing the optical head device is further reduced. The number can be reduced and productivity can be improved.
[0043]
【Example】
Hereinafter, this embodiment will be described with reference to the drawings. The oscillation wavelengths of the two semiconductor lasers used here are λ ₁ = 650 nm and λ ₂ = 780 nm.
[0044]
First, the wavelength λ in a configuration without a phase control element ₁ FIG. 6 shows the wavefront aberration distribution of a glass-molded objective lens that is optimally designed for a DVD-type optical disk with = 650 nm and NA = 0.6. This objective lens has a wavelength λ ₂ When used for a CD-type optical disk with = 780 nm and NA = 0.45, a large spherical aberration shown in FIG. 11 occurs. The calculated RMS value of the wavefront aberration at this time is 0.001λ in the DVD system. ₁ , 0.138λ for CD system ₂ Met. A circular concave positioning mark having a diameter of about 20 μm is formed on the symmetrical central axis of the convex surface of the objective lens opposite to the optical disk.
[0045]
Next, the phase correction ring zone of the phase control element shown in FIG. 3 will be described.
The refractive index n of the glass substrate is 1.5. FIG. 3 shows a step h with respect to the numerical aperture NA1-NA2 region, the numerical aperture NA3-NA4 region, and the numerical aperture NA5-NA6 region. ₁ Is 1.3 μm, and the step h with respect to the numerical aperture NA2 to NA3 region and the numerical aperture NA5 to NA6 region ₂ Is 1.3 μm. Table 1 summarizes the specifications of each phase correction zone.
[0046]
[Table 1]

[0047]
In addition, at the center of the ring zone, the diameter is about 20 μm, the level difference h ₂ = 2.6 μm circular concave positioning mark 11 is formed.
[0048]
In the phase control element shown in FIG. 3, the NA 0.6 of the DVD optical disk corresponds to NA6 and the NA 0.45 of the CD optical disk corresponds to NA5. Step h ₁ 1.3 μm, step h ₂ Is set to 2.6 μm, the wavelength λ ₁ However, the phase does not change substantially for light of 650 nm, and the wavefront aberration can maintain the good characteristics shown in FIG.
[0049]
On the other hand, the set step h ₁ , H ₂ A phase control element having a wavelength λ that is a CD system ₂ When transmitting light of 780 nm, a phase difference of 1.67π and 3.33π occurs. When this phase difference is used in combination with an objective lens, the wavefront aberration of the corresponding annular band is effectively 0.17λ in the wavefront aberration distribution of the objective lens shown in FIG. ₂ And 0.33λ ₂ Only acts to deduct. That is, by transmitting light through the phase correction ring zone 1A shown in FIG. 3, wavefront aberration is reduced only for light having a wavelength of 780 nm.
[0050]
In this embodiment, as shown in FIG. 1, a positioning mark 11 formed on the phase correction element and a positioning mark 21 formed on the objective lens are obtained by combining the phase control element 1 and the objective lens 2 having the above specifications. The alignment is adjusted while observing with a microscope so as to match, and then fixed to the holder 3. As a result, the decentering between the central axis of the phase control element 1 and the central axis of the objective lens 2 is eliminated, so that the RMS of the wavefront aberration is 0.001λ in the DVD system. ₁ In the CD system, it is 0.044λ for a luminous flux with NA of 0.45. ₂ Can be reduced.
[0051]
FIG. 7 shows the wavefront aberration distribution up to NA of 0.45 in the CD system at this time. According to FIG. 7, since the wavefront aberration is well below the Marechal reference value 0.07λ, which is a measure of residual wavefront aberration, the present optical head can be stably reproduced for both DVD and CD optical discs. Become.
[0052]
FIG. 8 shows wavefront aberrations up to NA of 0.6 in the CD system. The solid line in the figure shows the case where there is a groove 17 of the phase correction ring zone of this embodiment, and the dotted line shows the case where there is no groove 17. Therefore, in the region where NA is 0.45 or more, the large wavefront aberration of the CD system is further increased by forming a step due to the groove 17. Since such a light beam does not contribute to signal readout, aperture control is effectively performed.
[0053]
When the objective lens and the phase control element are fixed to the holder without the positioning mark, there is a tolerance of about ± 30 μm for the outline processing accuracy of each part, so the objective lens and phase control element of about ± 120 μm at maximum Eccentricity occurs. FIG. 9 shows the change in RMS of wavefront aberration when the objective lens and the phase control element are decentered by Δr. According to this, since the eccentricity Δr is 70 μm or more and does not satisfy the Marechal reference value 0.07λ, it is not possible to stably reproduce the CD type optical disk.
[0054]
Further, if the parts having an outer tolerance of ± 18 μm or less are selected in order to keep the eccentricity at 70 μm or less, the yield is reduced and the inspection time is increased. Furthermore, when this optical head device is actually used as an optical head device for reproducing information on DVD and CD optical discs, coma aberration due to deformation or tilt of the optical disc, RMS degradation of wavefront aberration due to temperature change, etc. Considering this, it is necessary to keep the RMS of wavefront aberration for an ideal optical disc at 0.05λ or less. The conventional configuration in which eccentricity between the objective lens and the phase control element is likely to occur further causes a decrease in yield, but the configuration of the present invention suppresses the eccentricity and causes no particular problem.
[0055]
Next, an optical head device incorporating a phase control element in which the substrate on which the phase control groove is formed and the substrate on which the beam splitter hologram is formed is laminated and an objective lens is incorporated will be described. One configuration example of this optical head device is shown in FIG.
[0056]
The hologram in this case is a polymer made of an optically isotropic material having a refractive index substantially equal to the ordinary refractive index of the polymer liquid crystal provided with a lattice-like uneven portion on a thin film of polymer liquid crystal having birefringence. A polarization hologram 6 filled with an uneven portion of a liquid crystal thin film was used. This polarization hologram 6 has different diffraction efficiency depending on the polarization direction of incident light, and utilizes a polarization direction with high transmittance in the forward path of light from the semiconductor lasers 41A and 41B to the optical disc 4 (5). In the return path as a reflected light path from the optical disc 4 (5), the polarization direction is rotated by the λ / 4 plate 8 to use the polarization direction with high diffraction efficiency, so that the reflected light is transmitted to the photodetectors 42A and 42B. Can lead. The λ / 4 plate 8 used here has two wavelengths λ. ₁ = 650 nm and λ ₂ = A phase difference of 1/4 with respect to the average wavelength of 780 nm.
[0057]
In the optical head device shown in FIG. 10, light emitted from a semiconductor laser 41A having a wavelength of 650 nm, which is a light source for a DVD optical disk, and a semiconductor laser 41B having a wavelength of 780 nm, which is a light source for a CD optical disk, is emitted from each collimating lens 43A. , 43B, the optical axes are matched by the wavelength selective prism mirror 44, and the objective lens 2 of the present invention and the phase control element 1 having the polarization hologram 6 are transmitted through the composite element 10 integrated in the holder 3. To do. In this composite element 10, a λ / 4 plate 8 is bonded between the polarization hologram 6 and the phase control element 1. The transmitted light is condensed on the surface of the optical information recording medium of the optical disc 4 (5) by the objective lens 2. The reflected light including the pit information of the optical disk 4 (5) is transmitted again through the composite element 10, and the optical axis is slightly bent by the polarization hologram 6, and reaches each photodetector 42A, 42B. Here, the role of the λ / 4 plate 8 is as described above.
[0058]
Here, the semiconductor laser for DVD optical disk may be 41B, and the semiconductor laser for CD optical disk may be 41A. In this case, the reflection characteristic of the wavelength selective prism mirror 44 is different from the above case and reflects light having a wavelength of 650 nm.
[0059]
By using an optical head device including this phase control element and an objective lens when reproducing a CD-type optical disk, the RMS of wavefront aberration is stabilized to 0.044λ. ₂ It is possible to maintain a small value of. As a result, noise of information light, which is reflected light from the optical disc, is reduced, and stable reproduction can be performed. Further, the number of components of the optical head device can be reduced, and the optical head device can be reduced in size and weight.
[0060]
【The invention's effect】
According to the present invention, the objective lens and the phase control element are formed with the positioning mark for center axis alignment, and are positioned by this mark and fixed to one holder, so that the phase control of the optical head device is performed. Since the element and the objective lens are integrated without being decentered, the reproduction performance of the CD optical disk as designed is stably obtained while maintaining the reproduction performance of the DVD optical disk.
In addition, when light of different wavelengths is emitted from the two light sources, and the light of each wavelength is transmitted through an optical system including an objective lens and a phase control element and condensed on the optical recording medium, the phase control element The surface groove allows the light to pass through without changing the RMS of the wavefront aberration of the light of one wavelength and reduces the RMS of the wavefront aberration of the other wavelength, thereby substantially controlling the aperture, and the DVD system In addition, information on a CD-type optical disc can be recorded / reproduced with a simple configuration. As a result, the number of parts of the recording apparatus is reduced, and the size and weight can be reduced.
[Brief description of the drawings]
FIGS. 1A and 1B are cross-sectional views of a phase control element and an objective lens of an optical head device according to the present invention, where FIG. 1A is a diagram when used for a DVD-type optical disc, and FIG. .
2A and 2B are configuration diagrams illustrating an example of an objective lens used in the optical head device of the present invention, in which FIG. 2A is a plan view and FIG. 2B is a cross-sectional view.
3A and 3B are configuration diagrams showing an example of a phase control element used in the optical head device of the present invention, wherein FIG. 3A is a plan view and FIG. 3B is a cross-sectional view.
FIG. 4 is a cross-sectional view showing an example in which a phase control element used in an optical head device of the present invention is integrated with a hologram.
FIG. 5 is a cross-sectional view showing an example in which a phase control element used in the optical head device of the present invention is integrated with a tilt correction element and a hologram.
FIG. 6 is a diagram showing an example of wavefront aberration when the optical head device of the present invention is used in a DVD optical disk.
FIG. 7 is a diagram showing an example of wavefront aberration up to NA of 0.45 when the optical head device of the present invention is used for a CD optical disk.
FIG. 8 is a diagram showing an example of wavefront aberration up to NA of 0.6 when the optical head device of the present invention is used for a CD optical disk.
FIG. 9 is a diagram showing an example of an RMS wavefront aberration that occurs in a CD-based optical disc when the phase control element and the objective lens are decentered in the optical head device of the present invention.
FIG. 10 is a conceptual diagram showing a configuration example of an optical head device of the present invention.
FIG. 11 is a diagram illustrating an example of wavefront aberration that occurs when a conventional optical head device including an objective lens without a phase control element is used for a CD-based optical disc.
[Explanation of symbols]
1: Phase correction element (substrate for phase control)
1A: Phase correction ring for phase correction element
2: Objective lens
3: Holder
4: Optical disc for DVD
5: Optical disc for CD
11: Positioning mark formed on the phase correction element
16: Phase correction ring zone of phase correction element corresponding to numerical aperture NA2 or less
13: Annulus for phase correction of phase correction element corresponding to numerical apertures NA1 to NA2
14: Annulus for phase correction of phase correction element corresponding to numerical aperture NA2 to NA3
15: Annulus for phase correction of phase correction element corresponding to numerical aperture NA3 to NA4
16: Annulus for phase correction of phase correction element corresponding to numerical aperture NA4 to NA5
21: Positioning mark formed on the objective lens
4A, 41B: Semiconductor laser
42A, 42B: Photodetector

Claims (3)

In an optical head device that collects light from a light source on an optical recording medium by an optical system including an objective lens and a phase control element, and records or reproduces information, the objective lens and the phase control element are used for center axis alignment. The optical head device is characterized in that a positioning mark is formed and is positioned by this mark and fixed to one holder. 2. The optical head device according to claim 1, wherein a concave or convex positioning mark is formed at a position of a central axis on each surface where the phase control element and the objective lens face each other. The surface of the phase control element has a groove having a circular planar shape and a step-shaped cross section. The light source includes two light sources, each light source emits light having a different wavelength, and When the light passes through the optical system and is condensed on the optical recording medium, the groove transmits the light as it is without changing the RMS of the wavefront aberration of the light of one wavelength, and the wavefront of the other wavelength. The optical head device according to claim 1, wherein an RMS of aberration is reduced.

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