JP2005158121A - Rotatory power compensation wide-band 1/4-wavelength plate and optical pickup unit using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide-band 1/4-wavelength plate such that influence of rotatory poweris corrected when a 1/4-wavelength plate is constituted by using a birefringent optical material, such as crystal, having rotatory power. <P>SOLUTION: The rotatory power compensation wide-band 1/4-wavelength plate is constituted by forming wavelength plates A and B of the same optical material having rotatory power and placing them one over the other so that the optical axes cross each other, the wide-band 1/4 wavelength plate being characterized in that an optimum value can be obtained by short-time simulation by calculating three approximate expressions (1) to (3) first and performing simulation within a range where an expression (4) or (5) is satisfied, characteristics are very excellent, and influence of rotatory power is corrected. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a wave plate using the birefringence of an optical material, and more particularly, to an optical rotation correcting broadband quarter-wave plate using an optical material having optical activity such as quartz and an optical pickup device using the same.

CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely used as optical recording media for large-capacity information related to music and video, and higher-density and large-capacity optical recording media are being developed. These optical recording media such as CD and DVD perform reproduction or recording using laser light such as linearly polarized light or circularly polarized light, and an optical disk device that performs reproduction / recording is used. In recent years, there has been an increasing demand for miniaturization of the optical disk apparatus itself along with compatibility of CD and DVD.
A DVD has a higher recording density than a CD, and for example, video and audio information for two hours or more can be accommodated in one disc. Along with this, the reproduction wavelength of DVD is 650 nm band shorter than the 780 nm band of CD, and an optical pickup device capable of DVD and CD compatibility necessarily corresponds to these two types of wavelengths. Need arises.
In the optical disk device, two systems of optical pickup devices are provided according to the laser light source that generates light of each wavelength. However, due to recent demands for miniaturization, especially the number of optical system components is reduced. Various attempts have been made to configure the optical pickup device in one system in order to simplify the system.

  Here, polarization of light performed in the optical pickup device will be described. When light is viewed as a wave, the plane including the light traveling direction and the magnetic field is referred to as a polarization plane, while the plane including the light traveling direction and the electric field is referred to as a vibration plane. The light in the case where the directions of the polarization planes are aligned is the polarization, and the polarization in which the polarization plane is limited to one plane is called linear polarization. The linearly polarized light includes P-polarized light having a component that oscillates horizontally and S-polarized light having a component that oscillates vertically with respect to a plane including the incident ray and the normal of the incident surface. In addition, polarized light whose electric field vector viewed at a certain position rotates with time is generally called elliptically polarized light.In particular, when the tip of the electric field vector is projected on a plane perpendicular to the light traveling direction, the trajectory is a circle. This is called circularly polarized light.

A wave plate (also called a phase retardation plate) is an optical element that performs phase modulation of light using the birefringence of an optical material. Among them, a quarter-wave plate that converts linearly polarized light into circularly polarized light is used as a depolarizing plate for an OLPF (Optical Low Pass Filter) used in the above-described optical disk device, digital camera, and the like.
Since the phase difference of the wave plate is a function of the wavelength, there is a characteristic that the phase difference changes as the wavelength used changes. That is, in the case of a CD / DVD compatible type that uses a plurality of wavelengths, or an optical pickup device that uses a plurality of wavelengths in the visible light band selectively / variably, it passes through a wave plate. Since the phase difference changes depending on each wavelength, there arises a problem that the phase difference cannot be maintained at ¼ wavelength.

In order to solve such a problem, for example, as disclosed in Japanese Patent Laid-Open No. 10-068816, a wave plate having a quarter-wave phase difference of birefringent light and a phase difference of birefringent light of 1 are used. It has been proposed that a wave plate having a / 2 wavelength is made of the same material and the two wave plates are bonded so that their optical axes intersect. According to this, a wave plate that functions with a phase difference of almost ¼ wavelength in a certain wavelength range is obtained.
JP-A-10-068816

However, the conventional quarter wavelength plate described above has the following problems. In other words, because the use of the wave plate is premised on preventing discoloration of liquid crystal displays and the like, an example using a polymer film (for example, polycarbonate) as an optical material has been given, but this was applied to an optical pickup device. In this case, in recent high-power lasers used for high-speed writing, the temperature of the pickup itself rises due to the temperature rise, and the polymer film has a large coefficient of linear thermal expansion, so that it expands and generates thermal distortion. As a result, the transmitted wavefront aberration is degraded and the laser cannot be focused on the disk. Furthermore, in recent optical recording media, the light used for the trend toward higher density and larger capacity has advanced to the blue laser (400 nm band wavelength) region. In particular, polycarbonate resin emits light in the ultraviolet region. When received, it is unsuitable for application to an optical pickup device due to its low light resistance that causes chemical change and “yellowing deterioration”.
Therefore, it is considered that quartz or the like is effective as a birefringent optical material having high light resistance.

  However, some birefringent optical materials such as quartz (materials mainly composed of a single crystal) have optical rotatory power, so that the vibration plane of linearly polarized light propagating in the optical axis direction is twisted as the light advances. Therefore, even if a wave plate is configured as disclosed in Japanese Patent Laid-Open No. 10-068816 using quartz or the like as an optical material, it cannot function as a quarter wave plate in a certain wavelength range. It was.

  The present invention has been made in order to solve such problems, and in the construction of a quarter-wave plate using a birefringent optical material having optical activity such as quartz, the influence of optical activity was corrected. An object is to provide a broadband quarter-wave plate.

これによれば、旋光能による影響を低減することができ、広帯域において特性の良い１／４波長板を得ることができる。 In order to solve the above-mentioned problem, the invention of claim 1 of the optical rotatory power-corrected broadband quarter-wave plate according to the present invention comprises two wave plates A and B formed of the same optical material having optical rotatory power. An optical rotation correction broadband quarter-wave plate with the optical axes intersecting each other, the phase difference of the wave plate A is Γ _A , the optical axis azimuth is θ _A , and the optical rotation is 2ρ _A. The angle between the axis and the neutral axis is β _A , the phase difference of the wave plate B is Γ _B , the optical axis azimuth angle is θ _B , the optical rotation power is 2ρ _B , and the angle between the rotation axis and the neutral axis is β _In the case of B, the relationship between the wave plate A and the wave plate B satisfies the following expressions (1) to (3), the optical rotator matrix of the wave plate A is T _AK , and the phase shifter matrix is R _AK and then, when a matrix of optical rotator wavelength plate B T _BK, a matrix of phase shifter and R _BK, the matrix of the incident polarization was I, the following equation (4 Or (5), characterized by being configured to satisfy.

According to this, the influence by the optical rotation can be reduced, and a quarter wavelength plate having good characteristics in a wide band can be obtained.

つまり、請求項１記載の旋光能補正広帯域１／４波長板を光ピックアップ装置に適用することによって、高出力の光線や紫外線に近い領域の波長の光線を扱うことができるようになる。 According to a second aspect of the optical pickup device of the present invention, the first linearly polarized light having the first wavelength and the second linearly polarized light having the second wavelength emitted from the light source pass through the wave plate. In this optical pickup device, the waveplate is composed of two waveplates A and B formed of the same optical material having optical rotation ability, which are superposed so that their optical axes intersect with each other. The wave plate of the wave plate A, wherein the wave plate A has a phase difference of Γ _A , an optical axis azimuth angle of θ _A , an optical rotation power of 2ρ _A , and an angle formed by the rotation axis and the neutral axis is β _A , When the phase difference of _B is Γ _B , the optical axis azimuth angle is θ _B , the optical rotation power is 2ρ _B , and the angle between the rotation axis and the neutral axis is β _B , the wave plate A and the wave plate B relationship satisfies the following formula (1) to (3), a matrix of optical rotator wavelength plate a T _AK, retarders Columns and R _AK, a matrix of optical rotator wavelength plate B T _BK, a matrix of phase shifter and R _BK, when the matrix of the incident polarization and the I, so as to satisfy the following formula (4) or (5) It is characterized by comprising.

That is, by applying the optical rotation correction wide-band quarter-wave plate according to claim 1 to the optical pickup device, it becomes possible to handle high-power light rays and light rays having a wavelength close to ultraviolet rays.

The optical rotatory power-corrected broadband quarter wave plate according to the present invention forms two wave plates A and B having different characteristics from the same optical material having optical rotatory power, and the optical axes of the two wave plates. And the characteristics of the wave plate A are expressed as a phase difference Γ _A , an optical axis azimuth angle θ _A , and an optical rotation power 2ρ _A. the characteristic of the other wavelength plate B, a phase difference gamma _B, the optical axis azimuth angle theta _B, when expressed as a rotatory power 2.rho _B, the relationship between these wave plate a and the wavelength plate B the following conditional expressions (1 ) To (3), and further optimized by the conditional expression (4) or (5) to correct the influence of the optical rotation and correct the wavelength of light over a wide wavelength range. Can be realized. In addition, if an optical pickup device is configured using this optical rotation correction wide-band quarter-wave plate, it can be applied to an optical disk device or the like that handles high-power light rays or light rays having a wavelength close to ultraviolet rays. Become.

なお、ここで「ｎ_ｅ’」は異常光線屈折率、「ｎ_ｏ」は常光線屈折率、「ｎ_Ｒ」は右円偏光屈折率、「ｎ_Ｌ」は左円偏光屈折率、「ｄ」は結晶の厚さを表す。
つまり、合成ベクトルΓ’は直線複屈折性による位相差と円複屈折性による位相差を合成したもので、ポアンカレ球上でＰａＰを回転軸としてΓ’回転する挙動として取り扱うことが出来る。 Before explaining the optical rotatory power-corrected broadband quarter-wave plate according to the present invention, first, the influence of the optical rotatory power, an example of a broadband wavelength plate using an optical material having no optical rotatory power, and an optical material having optical rotatory power An example in which the influence of the optical rotatory power is not corrected in the broadband wavelength plate using the lens will be briefly described.
<Effect of optical rotation>
An optical material having optical activity such as quartz has both linear birefringence (phase difference) and circular birefringence (optical rotation) when light travels through the optical material. This will be explained using the Poincare sphere.
FIG. 1 is a diagram showing an example of the action when light of wavelength λ travels through a quartz crystal.
When this sphere is regarded as the earth, it shows counterclockwise circularly polarized light (R) in the north pole on the polar axis, clockwise circularly polarized light (L) in the south pole, and linearly polarized light on the equator.
As shown in this figure, when taking a vector of a phase difference Γ given from linear birefringence in the neutral axis direction CfCs and a phase difference 2ρ given from circular birefringence in the polar axis direction LR, the resultant vector Γ Let Pa and P be points where 'intersects the Poincare sphere, and the rotation angle around PaP can be expressed by the following equation.
If the angle between the plane always including the neutral axis S1 and the neutral axis S2 and the axis of the PaP is β, β is expressed by the following equation (6) using the phase difference vector Γ and the phase difference vector 2ρ. It is represented by
tan β = 2ρ / Γ (6)
The combined vector Γ ′ is expressed by the following equation (7).

Here, “n _e ” is an extraordinary ray refractive index, “n _o ” is an ordinary ray refractive index, “n _R ” is a right circular polarization refractive index, “n _L ” is a left circular polarization refractive index, and “d”. Represents the thickness of the crystal.
In other words, the combined vector Γ ′ is a combination of the phase difference due to the linear birefringence and the phase difference due to the circular birefringence, and can be handled as a behavior that rotates Γ ′ about the PaP rotation axis on the Poincare sphere.

《Example of broadband wave plate using optical material without optical rotation》
Next, in order to clarify the difference depending on the presence or absence of optical activity, a broadband wavelength plate using a material that does not have optical activity is used as an example of the case where a wavelength plate defined as follows is used. explain.
In a broadband wave plate having a phase difference of approximately 90 deg in the band of wavelengths λ1 to λ2, two wave plates A and B having the following phase difference at a certain wavelength λ in λ1 to λ2 are used in an overlapping manner.
Wave plate A phase difference 180deg
Optical axis orientation θ _A
Wave plate B phase difference 90deg
Optical axis orientation θ _B
The polarization state at this time can be considered as shown in FIG.
FIG. 2 is a diagram for explaining an example of a broadband wave plate using an optical material having no optical rotation ability.
As shown in this figure, the point P _B is output polarization if reaching the Poincare sphere poles (North Pole or South Pole) becomes circularly polarized light.
In order for the point P _B to reach the pole of the Poincare sphere, it is desirable that the optical axis directions θ _A and θ _B in the respective wave plates have the relationship of the following formula (8).
θ _B = 2θ _A +45 (8)
That is, when the wavelength changes between λ1 and λ2, the phase difference between the wave plates A and B changes from 180 deg and 90 deg, respectively. This amount of change is represented by ΔΓ _A and ΔΓ _B.
If the amount of change is a condition that makes the same line segment on the spherical surface connecting the point P _A and the point P _A ′ on the Poincare sphere, the point P _B can always reach the pole.
Here, the points P _A and P _A ′ are approximately connected by a straight line, and the relationship between the phase difference change amount ΔΓ _A , the phase difference change amount ΔΓ _B , and the optical axis direction θ _A is expressed using the cosine theorem. Equation (9) is obtained.
cosΔΓ _B = 1-2 (1-2 cos 2θ _A ) (1-2 cos ΔΓ _A ) (9)
Further, if the wavelength plate A and the wavelength plate B and the material of the same wavelength dispersion, following each of the phase difference is 180 deg, because it is 90deg, the phase difference change amount [Delta] [gamma] _A and the phase difference change amount [Delta] [gamma] _B The relationship of Formula (10) is materialized.
ΔΓ _A = 2ΔΓ _B (10)
If this equation (10) is substituted into the above equation (9), it can be seen that the optical axis azimuth θ _A and the optical axis azimuth θ _B may be θ _A ≈15 deg and θ _B ≈75 deg, respectively.

It was simulated whether or not it functions as a broadband quarter-wave plate under the following conditions obtained from the above results.
Wave plate A phase difference 180deg
Optical axis direction 15deg
Wave plate B phase difference 90deg
Optical axis direction 75deg
However, since this condition includes approximation, Mueller matrix, Jones matrix, etc. were used.
FIG. 3 shows the calculated values and the optimized results under the above conditions with the wavelength λ = 720 nm on the premise of a broadband quarter-wave plate for DVD and CD compatible pickups.
FIG. 3 is a graph showing a simulation result of a broadband wavelength plate using an optical material having no optical rotation. The characteristic curve 1 shows the characteristic based on the calculation result, and the characteristic curve 2 shows the characteristic based on the result when optimization is performed.
When used in an optical pickup, the ellipticity is generally required to be 0.9 or more, but the optimized characteristics shown as characteristic curve 2 in this figure are 650 ± 20 nm, 780 ± used by DVD and CD. An ellipticity of 0.9 or more can be reliably ensured in both bands of 20 nm.

光学軸方位 θ_Ａ
旋光能 ２ρ_Ａ
回転軸と中性軸のなす角 β_Ａ

波長板Ｂ 位相差 Γ_Ｂ’

光学軸方位 θ_Ｂ
旋光能 ２ρ_Ｂ
回転軸と中性軸のなす角 β_Ｂ

前述した関係式（８）、（９）、（１０）に基づき、ポアンカレ球を用いて偏光状態を示したものを図４に示す。
図４は、旋光能を持つ光学材料を用いた帯域波長板において、旋光能を補正しない場合の例を説明するための図である。
この図に示すように、旋光能を持つ場合にあっては、上述した旋光能を持たない光学材料と同じ方法では、点Ｐ_Ｂが極に到達できていないことがわかる。即ち、旋光能を持つ光学材料は、旋光能を持たない材料と同じ方法では波長λ１〜λ２間で概ね１／４波長板として機能しないのである。 << Example of the case where the optical rotatory power is not corrected in a band wavelength plate using an optical material with optical rotatory power >>
Subsequently, in order to show the influence of the optical rotatory power, an optical material having an optical rotatory power is used, and the polarization state is examined by the same method as the optical material having no optical rotatory power described above.
A broadband wave plate having a phase difference of approximately 90 deg in the wavelength λ1 to λ2 band, and two wave plates A and B having the following phase difference at a specific wavelength λ in λ1 to λ2 are overlapped. To use.
Wave plate A phase difference Γ _A '

Optical axis orientation θ _A
Optical rotation 2ρ _A
Angle between rotation axis and neutral axis β _A

Wave plate B phase difference Γ _B '

Optical axis orientation θ _B
Optical rotation 2ρ _B
Angle between the rotation axis and neutral axis β _B

FIG. 4 shows the polarization state using the Poincare sphere based on the relational expressions (8), (9), and (10) described above.
FIG. 4 is a diagram for explaining an example in which the optical rotation is not corrected in the band wavelength plate using the optical material having the optical rotation.
As shown in this figure, it can be seen that the point P _B cannot reach the pole by the same method as that of the optical material having no optical rotatory power described above when having the optical rotatory power. That is, an optical material having optical rotation ability does not function as a quarter wavelength plate between wavelengths λ1 and λ2 in the same manner as a material having no optical rotation ability.

As a more specific example, FIG. 5 shows a simulation example in which a right quartz crystal is used and a quartz wavelength plate cut so that the normal of the main surface is 13 degrees from the Z axis is used.
FIG. 5 is a graph showing a simulation result of the broadband wavelength plate used without correcting the optical rotation of the quartz wavelength plate.
As shown in this figure, the characteristic curve 3 indicating the ellipticity characteristic has an ellipticity of 0.7 or less between wavelengths of 600 nm to 850 nm, which is not at a level practical with an optical pickup.

なお、上記式（１１）において、行列Ｔ及びＲはジョーンズ行列、ミューラ行列のいずれを用いても良いが、以降ミューラ行列を例に説明する。行列Ｔを式（１２）に、行列Ｒを式（１３）にそれぞれ示す。

上記式（１２）において、「ρ」は旋光能、「ｋ」は自然数（１，２，３・・・ｎ）を示す。即ち、旋光子Ｔをｎ分割した行列Ｔ_ｋをミューラ行列にて表している。

上記式（１３）において、「Γ」は位相差、「θ」は光学軸方位角、「ｋ」は自然数（１，２，３・・・ｎ）を示す。即ち、位相子Ｒをｎ分割した行列Ｒ_ｋをミューラ行列にて表している。
図６は、水晶波長板の位相子および旋光子による近似例を説明するためのイメージ図である。
この図に示すように、水晶波長板の作用は、水晶波長板を厚み方向にｎ個の旋光子と位相子に分割し、これらが交互に作用する素子として近似することができる。
ｎは、大きいほど実際の現象に近くなるが、この実験においてｎ＝１０にて、ほぼ実測値と近い値が得られることを確認した。 As described above, if a broadband wavelength plate is configured without considering the optical rotation power of an optical material having optical activity, the wavelength plate cannot obtain a phase difference characteristic with high accuracy in the band.
Therefore, the concept for taking into account the optical rotation will be described.
<< Simulation method of action of optical material with optical rotation >>
A simulation method effective when quartz is used as the birefringent material having optical rotation power will be described.
Assuming that the action of the quartz wavelength plate is a matrix W, W can be approximated by the following equation (11) using a matrix of optical rotators T and phase shifters R.

In the above equation (11), the matrices T and R may be either Jones matrices or Mueller matrices, but the following description will be made by taking the Mueller matrix as an example. The matrix T is shown in Equation (12), and the matrix R is shown in Equation (13).

In the above formula (12), “ρ” represents the optical rotation power, and “k” represents a natural number (1, 2, 3,... N). That is, a matrix T _{k obtained} by dividing the optical rotator T into n is represented by a Mueller matrix.

In the above formula (13), “Γ” represents a phase difference, “θ” represents an optical axis azimuth, and “k” represents a natural number (1, 2, 3,... N). That is, a matrix R _{k obtained} by dividing the phase shifter R into n is represented by a Mueller matrix.
FIG. 6 is an image diagram for explaining an approximation example using a phase shifter and an optical rotator of a quartz wavelength plate.
As shown in this figure, the action of the quartz wave plate can be approximated as an element in which the quartz wave plate is divided into n optical rotators and phase shifters in the thickness direction and these act alternately.
The larger n is, the closer to the actual phenomenon, but in this experiment, it was confirmed that a value almost close to the actual measurement value was obtained when n = 10.

行列Ｔ、Ｒの要素に含まれる旋光能ρ及び位相差Γは波長分散を持つため、行列Ｔ、Ｒも波長分散を持つと考えて良い。波長λ１からλ２の範囲内の、任意の波長において、その波長の旋光能ρ、位相差Γを代入し行列Ｔ、Ｒを求め、これを式（１４）を用いて計算することで、波長λ１からλ２の範囲内の、任意の波長の行列Ｅを求めることが出来る。
すなわち、行列Ｅが示す偏光状態が、ピックアップ用デバイスとして使用する際の許容値である楕円率０．９以上となる様に旋光能ρ、位相差Γ、光学軸方位角θを設定する。
行列Ｅが示す偏光状態を求める方法は幾つか考えられるが、その一つとして行列Ｅの行列要素を用いて求めた。行列Ｅの行列要素Ｓ_Ｅ３が式（４）又は式（５）を満足する様に旋光能ρ、位相差Γ、光学軸方位角θを設定する。

旋光能ρ、位相差Γ、及び光学軸方位角θを適宜、式（４）又は（５）に代入してシミュレーションすることで最適値を探すことも可能であるが、その組合わせが莫大であり計算に時間を要することになる。そのためポアンカレ球を用いて近似的に適当な旋光能ρ、位相差Γ、及び光学軸方位角θを求め、その後、式（４）又は（５）にて演算し最適値を求めた。 Here, consider a case where two quartz wave plates A and B are overlapped and used as a broadband wave plate.
Assuming that the input polarization matrix I is given, the output polarization matrix E can be expressed by the following equation (14).

Since the optical rotation power ρ and the phase difference Γ included in the elements of the matrices T and R have chromatic dispersion, it can be considered that the matrices T and R also have chromatic dispersion. By substituting the optical rotation power ρ and phase difference Γ of the wavelength at any wavelength within the wavelength λ1 to λ2, the matrices T and R are obtained, and this is calculated using the equation (14), thereby calculating the wavelength λ1 A matrix E having an arbitrary wavelength within a range from λ2 to λ2.
That is, the optical rotation power ρ, the phase difference Γ, and the optical axis azimuth angle θ are set so that the polarization state indicated by the matrix E becomes an ellipticity of 0.9 or more, which is an allowable value when used as a pickup device.
There are several methods for obtaining the polarization state indicated by the matrix E, and one of them is obtained using the matrix elements of the matrix E. The optical rotation ρ, the phase difference Γ, and the optical axis azimuth angle θ are set so that the matrix element S _E3 of the matrix E satisfies Expression (4) or Expression (5).

It is possible to search for the optimum value by substituting the optical rotation ρ, phase difference Γ, and optical axis azimuth angle θ into equation (4) or (5) as appropriate, and the combination is enormous. Yes, it takes time to calculate. Therefore, an appropriate optical rotation power ρ, phase difference Γ, and optical axis azimuth angle θ were approximately obtained using a Poincare sphere, and then calculated by the formula (4) or (5) to obtain an optimum value.

光学軸方位 θ_Ａ
旋光能 ２ρ_Ａ
回転軸と中性軸のなす角 β_Ａ

波長板Ｂ 位相差 Γ_Ｂ’

光学軸方位 θ_Ｂ
旋光能 ２ρ_Ｂ
回転軸と中性軸のなす角 β_Ｂ

このときの偏光状態は、ポアンカレ球を用い図７のように考えることができる。
図７は、波長板ＡとＢとを重ねてなる水晶波長板の偏光状態を示す図である。 Hereinafter, the optical rotation correcting broadband quarter-wave plate according to the present invention will be described in detail.
The characteristic point here is that the optical rotation power ρ, the phase difference Γ, and the optical axis azimuth angle θ by approximation are used, and if this is used, the optical rotation correction broadband quarter wavelength according to the present invention is used. Since an expression for specifying the relationship between the two wave plates constituting the plate can be obtained, a high-accuracy broadband quarter-wave plate in which the influence of the optical rotatory power is corrected can be realized very easily.
《Consideration using Poincare sphere of the method of functioning as a quarter of a wide band with optically active material》
In a quartz wavelength plate in which wavelength plates A and B having the following phase difference and optical rotation power are overlapped at a predetermined wavelength λ of wavelengths λ1 to λ2, a broadband in which the phase difference in the band of wavelengths λ1 to λ2 is approximately 90 deg. Consider a waveplate.
Wave plate A phase difference Γ _A '

Optical axis orientation θ _A
Optical rotation 2ρ _A
Angle between rotation axis and neutral axis β _A

Wave plate B phase difference Γ _B '

Optical axis orientation θ _B
Optical rotation 2ρ _B
Angle between the rotation axis and neutral axis β _B

The polarization state at this time can be considered as shown in FIG. 7 using a Poincare sphere.
FIG. 7 is a diagram showing a polarization state of a quartz wavelength plate in which the wavelength plates A and B are overlapped.

First, the phase difference Γ _A and the optical axis azimuth angle θ _A of the wave plate _A will be considered.
FIG. 8 is a diagram for explaining the operation of the wave plate A.
As shown in this figure, the point P _o on the Poincare sphere indicating the polarization state after passing through the wave plate A rotates the axis R _A by the angle β _{A in the} direction of the polar axis S _3. When 180 deg is reached, the point P _A1 is reached. In order to reduce the influence of the change in phase difference, it is desirable that the arrival point is not the position of the point P _A1 but the point P _A2 on the equator. Point P _A2 are that a polar axis S ₃ coordinates the rotation axis R _A intersects the Poincare sphere is a point that the same. If this point it is possible to suppress the change in coordinates of a neutral shaft S ₁ and the neutral axis S ₂ when the phase difference is changed to a minimum.

式（１５）に式（１６）を代入し次式（１７）を得る。

また、光学軸方位θ_Ａは、回転軸Ｒ_Ａが角度β_Ａ傾くことで回転半径が大きくなる。このため実際に寄与する光学軸方位θ_Ａ’は下式（１８）で表される。

ここで、β_Ａは、数度のレベル（僅かな角度）であるため、１−ｃｏｓβ_Ａ≒０と近似し、θ_Ａ’＝θ_Ａとして考慮した。 In order to reach the P _A2 may be corrected by an angle α min than 180deg phase difference as shown in the following equation (15). If the rotation direction about the _RA axis is right rotation, + α is corrected, and if it is left rotation, −α is corrected.
Γ _A '= 180 ± α (15)
Here, from FIG. 8, α is approximately expressed by the following equation (16).

By substituting equation (16) into equation (15), the following equation (17) is obtained.

In addition, the optical axis azimuth θ _A has a larger radius of rotation when the rotation axis R _A is inclined by an angle β _A. Therefore, the optical axis orientation θ _A ′ that actually contributes is expressed by the following formula (18).

Here, since β _A is a level of several degrees (a slight angle), it is approximated as 1-cos β _A ≈0 and considered as θ _A ′ = θ _A.

Next, the phase difference Γ _B ′ and the optical axis azimuth angle θ _B of the other wave plate _B will be considered.
FIG. 9 is a diagram for explaining the operation of the wave plate B.
As shown in this figure, the optical axis azimuth θ _B (not shown) of the wave plate B is inclined by the angle β _{B in the} direction of the polar axis S ₃ . Therefore, it is expressed as the following formula (19).

この式（２１）を式（２０）に代入すると、次の式（２２）のように表すことができる。

Further, electrode from the point P _{A2 (not} shown) (e.g., the polar axis S ₃ direction) to reach the, as shown in the figure, it may be a phase difference between gamma _{B '.}
The phase difference Γ _B is expressed by the following equation (20) using the angle ε.
Γ _B '= 90−ε (20)
Here, the angle ε is approximately expressed by the following equation (21).

If this equation (21) is substituted into equation (20), it can be expressed as the following equation (22).

The above description is summarized below.
That is, the phase difference is Γ _A , the optical axis azimuth is θ _A , the optical rotation is 2ρ _A , the wave plate A having the angle β _A formed by the rotation axis and the neutral axis, the phase difference is Γ _B , and the optical axis azimuth There theta _B, optical rotatory power is 2.rho _B, using the wavelength plate B is the angle beta _B of the rotation axis and the neutral axis, the quartz crystal wave plate formed by superimposing these, the phase difference in the band of wavelengths λ1~λ2 is In order to obtain a broadband quarter-wave plate of approximately 90 degrees, the relationship between the phase difference Γ _A ′, the optical axis azimuth angle θ _A , the phase difference Γ _B ′, and the optical axis azimuth angle θ _B is described below. What is necessary is just to set so that all the formulas (1), (2), and (3) may be satisfied.

そして、式（２）および式（３）は、それぞれ波長板Ａの位相差Γ_Ａと波長板Ｂの位相差Γ_Ｂとを定める関係式である。

これら関係式（１）、（２）、（３）を用いれば、シミュレーションに要する時間を大幅に短縮することができる。しかし、前述した様に関係式（１）、（２）、（３）は近似的に求めたものであるため、より精度の高い計算を行なう必要がある。このため、関係式（１）、（２）、（３）より求められた値を元に上述した式（４）、（５）を用いて各条件の最適化を行なう。 Expression (1) is a relational expression that defines the relationship between the optical axis azimuth angles θ _A and θ _B that the wave plate A and the wave plate B have, respectively.

Expressions (2) and (3) are relational expressions that determine the phase difference Γ _A of the wave plate _A and the phase difference Γ _B of the wave plate B, respectively.

If these relational expressions (1), (2), and (3) are used, the time required for the simulation can be greatly reduced. However, since the relational expressions (1), (2), and (3) are obtained approximately as described above, it is necessary to perform calculation with higher accuracy. For this reason, each condition is optimized using the above-described equations (4) and (5) based on the values obtained from the relational equations (1), (2), and (3).

Here, a specific example is shown.
For example, using a right quartz crystal and two quartz wavelength plates A and B cut so that the ray of the main surface is 13 ° from the Z axis, the wavelength λ = FIG. 10 shows the calculated values and the optimized results under the above-described conditions at 720 nm.
FIG. 10 is a graph showing an example of characteristics of the optical rotation correction wide-band quarter-wave plate according to the present invention.
A characteristic curve 4 shown in this figure shows an example of the characteristics of the optical rotatory power corrected broadband quarter-wave plate after optimization, a characteristic curve 5 shows an example of characteristics based on the calculation results before optimization, and a characteristic curve 6 shows the optical rotation. It is a line which shows the example of a characteristic when not considering performance.
If the characteristic curve 5 and the characteristic curve 6 are compared, it can be confirmed that the characteristic curve 5 based on the calculation result has improved characteristics as compared with the characteristic curve 6 not considering the optical activity.
Further, comparing the characteristic curve 4 and the characteristic curve 5, the characteristic curve 4 optimized by simulation has improved characteristics over the characteristic curve 5 based on the calculation result. The characteristic curve 4 is 650 ± 20 nm, In both bands of 780 ± 20 nm, an ellipticity of 0.9 or more can be secured. Therefore, an extremely excellent quarter-wave plate can be realized for a DVD and CD compatible optical pickup device.

As described above, the optical rotatory power-corrected broadband quarter-wave plate according to the present invention uses a birefringent optical material having optical rotatory power such as quartz by calculating using an approximate expression (relational expressions 1 to 3). Even if it exists, the influence of the optical rotatory power can be reduced, and it can function as a substantially quarter-wave plate in a predetermined wavelength band. Further, if the optimum value is found by simulating from the calculation result of the approximate expression, the calculation time required for the simulation can be reduced, and the optical rotation correction broadband 1 when optimized by the simulation can be obtained. The quarter-wave plate can function as a quarter-wave plate with very good characteristics.
And, by configuring the optical pickup device using the optical rotation correction wide-band quarter-wave plate according to the present invention, even if the light beam is a high-power laser beam, there is little shift in phase difference due to distortion, and Since the light beam can withstand ultraviolet light, an optical pickup device suitable for the optical disk device can be realized.

It is a figure for demonstrating optical rotation, Comprising: It is a figure which shows the example of an effect | action when the light of wavelength (lambda) advances in a quartz crystal. It is a figure for demonstrating the example of the broadband wavelength plate using the optical material which does not have an optical rotatory power. It is a graph which shows the simulation result of the broadband wavelength plate using the optical material which does not have an optical rotation. It is a figure for demonstrating the example when not correcting optical rotatory power in the band wavelength plate using the optical material which has optical rotatory power. It is a graph which shows the simulation result of the broadband wavelength plate used without correcting the optical rotation of a quartz wavelength plate. It is an image figure for demonstrating the approximate example by the phase plate and optical rotator of a quartz wavelength plate. It is a figure which shows the polarization state of the quartz wavelength plate which overlaps the wavelength plates A and B. FIG. It is a figure explaining the effect | action about the wavelength plate. It is a figure explaining the effect | action about the wavelength plate. It is the graph which showed the example of the characteristic of the optical rotation correction broadband 1/4 wavelength plate which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Characteristic curve 2 ... Characteristic curve 3 ... Characteristic curve 4 ... Characteristic curve of wavelength plate based on this invention (after optimization)
5 ... Characteristic curve of wave plate according to the present invention (before optimization)
6 ... characteristic curve S ₁ ... first neutral axis S ₂ ... second neutral axis S ₃ ... polar axis C _S, and _C F ... first neutral axis Point where Poincare sphere intersects Γ ・ ・ ・ Phase difference vector Γ ′ ・ ・ ・ Composite vector (angle)
2ρ: phase difference vector Pa, P: points where composite vector Γ ′ intersects Poincare sphere β, β _A , β _B: angles P _O , P _A , P _A ′, P _B: Poincare [Delta] [gamma] _B point on the sphere, [Delta] [gamma] _a · · · angle 2 [Theta] _a, the angle T involved in 2 [Theta] _B · · · optical axis azimuth, R · · · matrix T _N · · · rotator R _N · · · retarder R _a , R _B ... rotation axis P _A1 , P _A2 ... point on Poincare sphere α ... angle X _A , Y _A , Y _A1 , Y _A2 , Y _B1 , Y _B2 , Y _B3 ... straight line ε ・ ・ ・ Angle

Claims (2)

An optical rotation-corrected broadband quarter-wave plate in which two wave plates A and B formed of the same optical material having optical activity are superposed so that their optical axes intersect each other,
The phase difference of the wave plate A is Γ _A , the optical axis azimuth angle is θ _A , the optical rotation power is 2ρ _A , and the angle between the rotation axis and the neutral axis is β _A ,
When the phase difference of the wave plate B is Γ _B , the optical axis azimuth angle is θ _B , the optical rotation power is 2ρ _B , and the angle between the rotation axis and the neutral axis is β _B ,
The relationship between the wave plate A and the wave plate B satisfies the following expressions (1) to (3),
The optical rotator matrix of the wave plate A is T _AK , and the retarder matrix is R _AK ,
The matrix of optical rotators of the wave plate B is T _BK , the matrix of phase retarders is R _BK ,
When the incident polarization matrix is I,
An optical rotation correcting broadband quarter-wave plate characterized by satisfying the following formula (4) or (5).

In the optical pickup device configured so that the first linearly polarized light having the first wavelength and the second linearly polarized light having the second wavelength emitted from the light source pass through the wave plate,
The wave plate is an optical rotation correction broadband quarter wave plate in which two wave plates A and B formed of the same optical material having optical rotation power are overlapped so that their optical axes intersect each other,
The phase difference of the wave plate A is Γ _A , the optical axis azimuth angle is θ _A , the optical rotation power is 2ρ _A , and the angle between the rotation axis and the neutral axis is β _A ,
When the phase difference of the wave plate B is Γ _B , the optical axis azimuth angle is θ _B , the optical rotation power is 2ρ _B , and the angle between the rotation axis and the neutral axis is β _B ,
The relationship between the wave plate A and the wave plate B satisfies the following expressions (1) to (3),
The optical rotator matrix of the wave plate A is T _AK , and the retarder matrix is R _AK ,
The matrix of optical rotators of the wave plate B is T _BK , the matrix of phase retarders is R _BK ,
When the incident polarization matrix is I,
An optical pickup device configured to satisfy the following expression (4) or (5).

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