JP4081725B2 - Optical pick-up - Google Patents

Description

によつて求めることができる。
【００１１】
従つて収束光入射で主光線Ｌ１に対して垂直な面内に光学軸を含むような方向に光学軸を設定する場合、光束が図７に示すような斜入射になると、光束の入射角度によつて（２）式中における光路長Ｌの変化が顕著に現れることになる。このため従来例では、この変化が位相差として残つてしまい特性を劣化させる原因となつていた。
【００１２】
そこで本実施例では（２）式における次式
【数３】

を光路長Ｌの変化をキヤンセルするように変化させること、すなわちΔｎ・Ｌが一定となるようにすることによつて位相差の問題を解決することを考える。
【００１３】
そこで本実施例では、図２に示すように、水晶の光学軸方向Ａを主光線Ｌ１と垂直な面に対して傾けることにする。このように傾けると、光路が長くなるにつれて光線方向と光学軸方向Ａとのなす角が小さくなり、長軸と短軸との距離差Δｎを小さくすることができる。すなわち楕円体の切断面を次第に円に近づけていくことができるのである。この結果、各光線について光路長Ｌと、楕円体の長軸及び短軸の距離差Δｎとの積Δｎ・Ｌをほぼ一定に保つことができる。
【００１４】
ただし入射側の開口数ＮＡ及び屈折率ｎに対して入射光線の入射角が、次式
【数４】

以上となるように設定することが必要となる。これは水晶を通過する光束の光路長が単調に変化するために必要な条件である。
【００１５】
また実際に機器に応用する場合には位相差が所定の条件を満たすための条件の他に条件が必要となる。例えば光磁気デイスク装置における光学ピツクアツプの半波長板として使用する場合には、（２）式で与えられる位相板の位相差を所定の値（例えば1.5 λ）とする条件に加えて、45〔°〕検波を実現するための条件、楕円の短軸方向と入射光の偏光方向とのなす角が22.5〔°〕となるような要請も生じよう。具体的な計算方法については次項で説明する。
【００１６】
（３）具体的な計算方法
ここでは図３に示すように座標系を設定する。まず初期光学軸方向ベクトルＣ₀ を（０，１，０）とすると、これをｘ軸周りにθ回転、ｚ軸回りにφ回転させると、光学軸方向ベクトルＣは、次式
【数５】

で表すことができる。
【００１７】
ここでは簡単のため屈折光方向を逆向きのベクトル、ベクトルＳ＝（０， sinα， cosα）で表すと、切断面の楕円の短軸方向ベクトルｅ_C は、ベクトルＳとベクトルＣに垂直であることから、次式
【数６】

と表すことができる。
また長軸方向ベクトルｅ_L は、ベクトルＳとベクトルｅ_C に垂直であることから、次式
【数７】

と表すことができる。
【００１８】
ここで長軸方向ベクトルｅ_L と光学軸方向ベクトルＣとのなす角をΘとすると、 cosΘは次式
【数８】

で表される。
するとこの（８）式を用いて、次式
【数９】

で表される屈折率楕円体を初期特性とする位相板における２固有偏光間の屈折率差は、次式
【数１０】

のように求めることができる。
【００１９】
また光路長Ｌ（α）は、次式
【数１１】

で表されることにより、位相板を１次波長板（ 1.5λ板）として用いるための条件は、（10）式と（11）式との積で表される値が、次式
【数１２】

を満たすことである。
【００２０】
また45〔°〕検波を実現するための条件は、短軸方向ベクトルｅ_C と偏光方向ベクトルｘ（＝（１，０，０））とのなす角が22.5〔°〕となる、すなわち次式
【数１３】

を満たすことである。
【００２１】
まず（６）式を解いて、短軸方向ベクトルｅ_C を、次式
【数１４】

のように求める。
これを（７）式に代入して解くと、長軸方向ベクトルｅ_L は、次式
【数１５】

となる。
【００２２】
ここで（15）式を（８）式に代入すると、
【数１６】

が得られ、 cosΘ＝√Ｂであることが分かる。これによりsin²Θ＝（１−Ｂ）であることが分かるため、Ｎ_o 及びＮ_e の値を代入することにより（10）式はＢの式となる。また光路長Ｌ（α）は既知の値αの代入によつて結晶の厚みｌの式となる。これにより（12）式にλを代入すればＢ（すなわちθとφの式）とｌとについての値を求めることができる。
【００２３】
また（13）式のベクトルｅ_C ・ベクトルｘは、次式
【数１７】

で表すことができることにより既知の値αの代入によつてθとφで表される式の値が定まる。
これらを解くことによりθ、φ、ｌの条件が定まる。
【００２４】
例えばこの方法を用いて、屈折角αの範囲が40〔°〕〜50〔°〕（すなわち主光線の屈折角は45〔°〕）となる収束光に適した位相板を設計すると、図４に示すようにその光学特性は従来に比して格段的に良好となることが分かる。因に図はＮ_e が1.547570696 、Ｎ_o が1.538646231 となる水晶に波長λが0.785 〔μｍ〕の光を入射させるときに得られる特性である。
【００２５】
以上の構成によれば、結晶の光学軸が発散光又は収束光の主光線に対して斜めになるように位相板を配置し、このとき結晶内を透過する光束のうち屈折角が大きい光束ほど該光束に垂直な面と光学軸とのなす角が大きくなるようにしたことにより、射出光に位相分布が生じないようにできる。
これにより光学系設計時の自由度を高めることができ、さらには部品点数の削減や製法の単純化・小型化も実現できる。
【００２６】
（４）他の実施例
なお上述の実施例においては、光磁気デイスク装置の光学ピツクアツプを例に位相板を半波長板として用いる場合について述べたが、本発明はこれに限らず、任意の光学系に用いられる位相板（例えば４分の１波長板）を構成する場合に広く適用し得る。
また上述の実施例においては、位相板に収束光を入射する場合について述べたが、本発明はこれに限らず、発散光を入射する場合にも同様に適用し得る。なおこのとき収束光や発散光を位相板に対して斜めに入射する場合について述べ、斜入射のうち特別な例である垂直入射については述べなかつたが、この場合にもθ＝０°とすることで本発明と同様の効果を得ることができる。このように本発明は発散光又は収束光中で位相板を用いる光学系全般に亘つて有効なものである。
【００２７】
【発明の効果】
上述のように本発明によれば、光束を射出し、記録媒体を照明する発光素子と、記録媒体で反射された光束の発散光又は光束の収束光中に配置された位相板と、位相板を透過した光束を受光する受光素子とを光学ピツクアツプに設け、位相板を透過する発散光又は収束光に垂直な面で屈折率楕円体を切断した断面の長軸及び短軸の長さの差と、当該発散光又は当該収束光の光路長との積が一定となるように位相板の結晶の光学軸方向を定めるようにしたことにより、結晶の光学軸が発散光又は収束光の主光線に対して斜めになるように位相板を配置することができるので、光学系設計時の自由度を高めることができ、部品点数の削減や製法の単純化・小型化ができる光学ピツクアツプを実現することができる。
光束を射出し、記録媒体を照明する発光素子と、記録媒体で反射された光束の発散光又は光束の収束光中に配置された位相板と、位相板を透過した光束を受光する受光素子とを光学ピックアップに設け、位相板を透過する発散光又は収束光によつて異なる２つの固有偏光の屈折率の差と、当該発散光又は収束光によつて異なる光路長との積が一定となるよう、結晶の光学軸方向が定められるようにしたことにより、結晶の光学軸が発散光又は収束光の主光線に対して斜めになるように位相板を配置することができるので、光学系設計時の自由度を高めることができ、部品点数の削減や製法の単純化・小型化ができる光学ピツクアツプを実現することができる。
【図面の簡単な説明】
【図１】本発明による光学ピツクアツプの構成を示す略線図である。
【図２】本発明による位相板の光学軸と入射角との関係を示す略線的断面図である。
【図３】座標軸を示す略線図である。
【図４】実施例での光学特性を示す特性曲線図である。
【図５】光学ピツクアツプを示す略線的斜視図である。
【図６】斜入射の説明に供する略線的断面図である。
【図７】斜入射によつて生じる光路差の説明に供する略線的断面図である。
【図８】従来例での光学特性を示す特性曲線図である。
【符号の説明】
１、１１……光学ピツクアツプ、２……デイスク、３……対物レンズ、４……立ち上げミラー、５……コリメータレンズ、６……ビームスプリツタ、７……レーザダイオード、８、１８……半波長板、９……ＰＢＳ差動検光子、Ｌ１……主光線、Ａ……光軸方向。[0001]
【table of contents】
The present invention will be described in the following order.
DETAILED DESCRIPTION OF THE INVENTION Means for Solving the Problems to be Solved by the Prior Art Invention Embodiment (1) Optical Pickup (FIG. 1)
(2) Phase plate (FIGS. 2 and 7)
(3) Specific calculation method (FIGS. 3 and 4)
(4) Effects of other embodiments of the invention
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup, and is suitable for application to an optical pickup used in an optical disk device.
[0003]
[Prior art]
Today, phase plates are used in various optical systems as elements that change the polarization direction. In particular, a half-wave plate, which is one of phase plates, is used in various optical systems including an optical pickup (FIG. 5) of an optical disk device. In general, the incident surface of the half-wave plate 8 is arranged so as to be perpendicular to the principal ray of the reflected light.
[0004]
[Problems to be solved by the invention]
Nowadays, optical disk devices are increasingly miniaturized, and accordingly, the optical pickup is also required to be miniaturized and the number of parts is reduced. As one method for solving such a technical problem, for example, as shown in FIGS. 6A and 6B, a half-wave plate is disposed so that the principal ray L1 is inclined with respect to the crystal plane 8A. Can be considered.
If the incident light is parallel light, set the optical axis of the phase plate so that it is perpendicular to the direction of the light beam after refraction, and set the crystal to an appropriate thickness. This is convenient for downsizing the optical pickup. However, if the incident light is divergent light or convergent light, a distribution occurs in the distance propagating through the crystal in the light flux as shown in FIG. 7 (that is, L3 <L1 <L2), resulting in a phase difference between the two intrinsic polarizations. It cannot be avoided that the distribution occurs.
[0005]
An example of this phase distribution is shown in FIG. This figure shows the phase difference distribution when the chief ray is incident so that the refraction angle of the chief ray is 45 °. As can be seen from the figure, it can be seen that a phase difference of about 10 degrees per refraction angle of 1 degree occurs. As described above, there is a possibility that the optical characteristics may be deteriorated due to the phase difference distribution. Therefore, conventionally, there has been no use in which the principal ray L1 of divergent light or convergent light is incident on the crystal plane obliquely.
Further, even if the principal ray L1 is incident perpendicularly to the crystal plane 8A, there is a problem of phase difference when diverging light or convergent light is incident. .
[0006]
The present invention has been made in consideration of the above points, and an object of the present invention is to propose an optical pickup capable of increasing the degree of freedom in designing an optical system as compared with the prior art.
[0007]
[Means for Solving the Problems]
In order to solve such a problem, in the present invention, a light emitting element that emits a light beam and illuminates the recording medium, a phase plate disposed in a divergent light beam reflected by the recording medium or a convergent light beam, and a phase A light receiving element that receives a light beam that has passed through the plate, and the phase plate has a major axis and a minor axis length of a cross section obtained by cutting the refractive index ellipsoid along a plane perpendicular to the diverging light or the convergent light transmitted through the phase plate. Since the optical axis direction of the crystal is determined so that the product of the difference between the difference and the optical path length of the divergent light or the convergent light is constant, the optical axis of the crystal is the principal ray of the divergent or convergent light. Therefore, the phase plate can be arranged so as to be inclined with respect to the angle, and the degree of freedom in designing the optical system can be increased.
A light emitting element that emits a light beam and illuminates the recording medium; a phase plate disposed in a divergent light beam reflected by the recording medium or a convergent light beam; and a light receiving element that receives the light beam transmitted through the phase plate; The phase plate has a constant product of the difference in refractive index between two intrinsic polarizations that differ depending on the diverging or converging light transmitted through the phase plate and the optical path length that differs depending on the diverging or converging light . The phase plate can be arranged so that the optical axis of the crystal is inclined with respect to the principal ray of diverging light or convergent light. The degree of freedom during system design can be increased.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
[0009]
(1) Optical Pickup FIG. 1 is a block diagram showing the entire structure of the optical pickup 11 shown in FIG. This optical pickup 11 has the same configuration except that the configuration of the optical pickup 1 and the half-wave plate 18 shown in FIG.
Here, the optical pickup 11 condenses the laser light emitted from the laser diode 7 on the disk 2 via the beam splitter 6, the collimator lens 5, the rising mirror 4, and the objective lens 3, and the reflected light is objectively reflected. Recording on the disk 2 by focusing on a polarizing beam splitter (hereinafter referred to as PBS) differential analyzer 9 through a lens 3, a raising mirror 4, a collimator lens 5, a beam splitter 6, and a half-wave plate 18. It is an optical element used to reproduce the recorded information, and is used in many optical disk devices. Here, the conditions required for the half-wave plate 18 will be described taking the conditions generally required for the phase plate as an example.
[0010]
(2) Phase plate The phase difference between the two intrinsic polarizations after transmission with respect to the light incident on the phase plate (progressing as a quartz crystal for the time being) can be considered as follows.
A major axis and a minor axis of an ellipse of a cut surface obtained by cutting a refractive index ellipsoid having an optical axis direction as one major axis (in the case of quartz) along a plane having a normal parallel to the incident ray direction and passing through the center of the ellipsoid. the length of the shaft and n _e and n _o, further when the optical path length of the light beam transmitted through the phase plate is L, the phase difference of each beam has the formula ## EQU2 ##

Can be obtained.
[0011]
Therefore, when the optical axis is set in a direction including the optical axis in a plane perpendicular to the principal ray L1 by convergent light incidence, when the light beam is obliquely incident as shown in FIG. Therefore, the change in the optical path length L in the expression (2) appears remarkably. For this reason, in the conventional example, this change remains as a phase difference and causes deterioration in characteristics.
[0012]
Therefore, in this embodiment, the following formula in the formula (2):

Is considered to solve the phase difference problem by changing the optical path length L so as to cancel the change in the optical path length L, that is, by making Δn · L constant.
[0013]
Therefore, in this embodiment, as shown in FIG. 2, the optical axis direction A of the crystal is inclined with respect to the plane perpendicular to the principal ray L1. By tilting in this way, the angle between the light beam direction and the optical axis direction A becomes smaller as the optical path becomes longer, and the distance difference Δn between the major axis and the minor axis can be reduced. In other words, the cut surface of the ellipsoid can gradually approach a circle. As a result, the product Δn · L of the optical path length L and the distance difference Δn between the major axis and the minor axis of the ellipsoid can be kept substantially constant for each ray.
[0014]
However, the incident angle of the incident light with respect to the numerical aperture NA and the refractive index n on the incident side is expressed by the following equation:

It is necessary to set so that it becomes the above. This is a necessary condition for the optical path length of the light beam passing through the crystal to change monotonously.
[0015]
In addition, when actually applied to a device, a condition is necessary in addition to a condition for the phase difference to satisfy a predetermined condition. For example, when used as a half-wave plate for an optical pickup in a magneto-optical disk apparatus, in addition to the condition that the phase difference of the phase plate given by equation (2) is a predetermined value (for example, 1.5 λ), 45 [° There will also be a requirement to realize detection, such that the angle between the minor axis direction of the ellipse and the polarization direction of the incident light is 22.5 [°]. A specific calculation method will be described in the next section.
[0016]
(3) Specific Calculation Method Here, a coordinate system is set as shown in FIG. First, assuming that the initial optical axis direction vector C ₀ is (0, 1, 0), when this is rotated by θ rotation about the x axis and φ rotation about the z axis, the optical axis direction vector C is expressed by the following equation:

Can be expressed as
[0017]
Here, for the sake of simplicity, when the direction of the refracted light is represented by a reverse vector, vector S = (0, sinα, cosα), the short axis direction vector e _C of the cut plane ellipse is perpendicular to the vector S and the vector C. Therefore, the following formula:

It can be expressed as.
Since the long axis direction vector e _L is perpendicular to the vector S and the vector e _C ,

It can be expressed as.
[0018]
Here, when the angle formed by the major axis direction vector e _L and the optical axis direction vector C is Θ, cos Θ is expressed by the following equation:

It is represented by
Then, using this equation (8),

The refractive index difference between the two intrinsic polarizations in the phase plate having the refractive index ellipsoid represented by

Can be obtained as follows.
[0019]
The optical path length L (α) is expressed by the following equation:

As a condition for using the phase plate as the primary wave plate (1.5λ plate), the value represented by the product of the equations (10) and (11) is expressed by the following equation: ]

Is to satisfy.
[0020]
The condition for realizing 45 [°] detection is that the angle formed by the short-axis direction vector e _C and the polarization direction vector x (= (1, 0, 0)) is 22.5 [°]. [Formula 13]

Is to satisfy.
[0021]
First, the equation (6) is solved, and the minor axis direction vector e _C is expressed by the following equation:

Seek like.
Substituting this into equation (7) and solving, the long axis direction vector e _L is given by

It becomes.
[0022]
Here, substituting equation (15) into equation (8),
[Expression 16]

It can be seen that cosΘ = √B. As a result, it can be seen that sin ² Θ = (1−B). Therefore, by substituting the values of N _o and N _e , equation (10) becomes the equation of B. Further, the optical path length L (α) is obtained by substituting the known value α into the formula of the crystal thickness l. Thus, if λ is substituted into the equation (12), values for B (that is, the equations of θ and φ) and l can be obtained.
[0023]
Further, the vector e _C · vector x in the equation (13) is expressed by the following equation:

The value of the expression represented by θ and φ is determined by substituting the known value α.
By solving these, the conditions of θ, φ, and l are determined.
[0024]
For example, when a phase plate suitable for convergent light having a refraction angle α in the range of 40 [°] to 50 [°] (that is, the principal ray has a refraction angle of 45 [°]) is designed using this method, FIG. It can be seen that the optical characteristics are remarkably better than the conventional one. The figure shows the characteristics obtained when light having a wavelength λ of 0.785 [μm] is incident on a quartz crystal having N _e of 1.547570696 and N _o of 1.538646231.
[0025]
According to the above configuration, the phase plate is arranged so that the optical axis of the crystal is inclined with respect to the principal ray of diverging light or convergent light, and the light beam having a larger refraction angle among the light beams transmitted through the crystal at this time. By making the angle between the surface perpendicular to the light beam and the optical axis large, it is possible to prevent phase distribution from occurring in the emitted light.
As a result, the degree of freedom in designing the optical system can be increased, and further, the number of parts can be reduced and the manufacturing method can be simplified and downsized.
[0026]
(4) Other Embodiments In the above-described embodiments, the case where the phase plate is used as a half-wave plate is described taking the optical pickup of the magneto-optical disk device as an example. However, the present invention is not limited to this, and any optical The present invention can be widely applied when a phase plate (for example, a quarter wave plate) used in the system is configured.
In the above-described embodiments, the case where the convergent light is incident on the phase plate has been described. However, the present invention is not limited to this, and can be similarly applied to the case where divergent light is incident. At this time, the case where the convergent light and the divergent light are incident obliquely on the phase plate is described, and the vertical incident which is a special example of the oblique incident is not described, but in this case also θ = 0 °. Thus, the same effect as the present invention can be obtained. As described above, the present invention is effective over the entire optical system using the phase plate in divergent light or convergent light.
[0027]
【The invention's effect】
As described above, according to the present invention, a light emitting element that emits a light beam and illuminates the recording medium, a phase plate disposed in a divergent light beam reflected by the recording medium or a convergent light beam, and a phase plate The difference between the major axis and minor axis length of the cross section obtained by cutting the refractive index ellipsoid at a plane perpendicular to the diverging light or converging light transmitted through the phase plate is provided in the optical pickup with a light receiving element that receives the light beam transmitted through the phase plate. And the optical axis direction of the crystal of the phase plate so that the product of the diverging light or the optical path length of the converging light is constant, the optical axis of the crystal is the principal ray of the diverging light or converging light. The phase plate can be placed so as to be inclined with respect to the angle, so that the degree of freedom in designing the optical system can be increased, and an optical pickup that can reduce the number of parts and simplify and downsize the manufacturing method is realized. be able to.
A light emitting element that emits a light beam and illuminates the recording medium; a phase plate disposed in a divergent light beam reflected by the recording medium or a convergent light beam; and a light receiving element that receives the light beam transmitted through the phase plate; Is provided in the optical pickup, and the product of the difference in refractive index between two intrinsic polarizations that differ depending on the diverging light or converging light transmitted through the phase plate and the optical path length that differs depending on the diverging light or converging light becomes constant. In this way, the optical axis direction of the crystal can be determined so that the phase plate can be arranged so that the optical axis of the crystal is oblique to the principal ray of diverging light or convergent light. The degree of freedom in time can be increased, and an optical pickup capable of reducing the number of parts and simplifying / miniaturizing the manufacturing method can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the configuration of an optical pickup according to the present invention.
FIG. 2 is a schematic cross-sectional view showing a relationship between an optical axis and an incident angle of a phase plate according to the present invention.
FIG. 3 is a schematic diagram showing coordinate axes.
FIG. 4 is a characteristic curve diagram showing optical characteristics in Examples.
FIG. 5 is a schematic perspective view showing an optical pickup.
FIG. 6 is a schematic cross-sectional view for explaining oblique incidence.
FIG. 7 is a schematic cross-sectional view for explaining an optical path difference caused by oblique incidence.
FIG. 8 is a characteristic curve diagram showing optical characteristics in a conventional example.
[Explanation of symbols]
1, 11 ... Optical pick-up, 2 ... Disc, 3 ... Objective lens, 4 ... Raising mirror, 5 ... Collimator lens, 6 ... Beam splitter, 7 ... Laser diode, 8, 18 ... Half-wave plate, 9 ... PBS differential analyzer, L1 ... chief ray, A ... optical axis direction.

Claims (2)

A light emitting element that emits a luminous flux and illuminates the recording medium;
A phase plate disposed in the divergent light of the light beam reflected by the recording medium or the convergent light of the light beam;
Comprising a light receiving element for receiving the light beam transmitted through the phase plate,
The phase plate is
The difference between the major axis and minor axis length of the cross section obtained by cutting the refractive index ellipsoid at a plane perpendicular to the diverging light or the converging light transmitted through the phase plate, and the optical path length of the diverging light or the converging light An optical pick-up characterized in that the optical axis direction of the crystal is determined so that the product of is constant. A light emitting element that emits a luminous flux and illuminates the recording medium;
A phase plate disposed in the divergent light of the light beam reflected by the recording medium or the convergent light of the light beam;
Comprising a light receiving element for receiving the light beam transmitted through the phase plate,
The phase plate is
The product of the difference in refractive index between two intrinsic polarizations that differ depending on the diverging light or the converging light transmitted through the phase plate and the optical path length that differs depending on the diverging light or the converging light is constant. An optical pickup characterized in that the optical axis direction of the crystal is defined.

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