JP3566504B2 - Optical device for 2-aperture bifocal light reproduction - Google Patents

Description

ただし、ｎ_０ はレンズの光軸の位置での屈折率、ｎ_ｒ は光軸から距離ｒの位置での屈折率、およびＡはレンズの材料によって定まる定数である。
【００１３】
本発明では、この屈折率分布レンズに対して、ＤＶＤ再生用レーザ光Ｒ_Ｄ はレンズの光軸と平行に、ＣＤ再生用レーザ光Ｒ_Ｃ はレンズの光軸に対して次式で示される傾き−θ_０ をもってそれぞれ入射させるものとする。この様子を図２に示す。
【数２】

ただし、ｒ_０ はレンズの半径である。
【００１４】
屈折率分布レンズに光軸に対して傾き−θ_０ で入射したレーザ光は、屈折率分布レンズ内で全反射を繰り返すことで進行していく。屈折率分布レンズは入射端面上の光軸からの半径方向の距離によって屈折率が異なるので、一般的に光軸に対して角度−θ_０ で入射する光のうち、屈折率分布レンズ中を伝搬することのできるレーザ光は光軸から次式で示される半径ｒ_１ までの範囲内に限られる。この様子を図３に示す。
【数３】

【００１５】
ここで、情報を読み取る対物レンズの開口は、ＣＤ再生用のレンズでは０．４５、ＤＶＤ再生用のレンズでは０．６となっているが、開口はレンズ径に依存するため、ＤＶＤ再生用のレーザ光に対する開口が０．６になるよう設計した場合、ＣＤ再生用のレーザ光に対する開口を０．４５とするには、ＣＤ再生用のレーザ光の入射する領域をＤＶＤ再生用レーザ光の入射する領域の７５％とすればよいことになる。従って、屈折率分布レンズの開口を０．６として設計した上で、ｒ_１ ＝０．７５ｒ_０ が成り立つようにパラメータを設定すれば（２）式における−θ_０ で入射した光のうち、光軸から０．７５ｒ_０ までの領域に入射した光のみ伝搬し、ＤＶＤ再生用のレーザ光に対しては０．６の開口が得られ、ＣＤ再生用のレーザ光に対しては０．４５の開口が得られることになる。
【００１６】
次に、２焦点のための条件について検討する。
２焦点を実現するための、屈折率分布レンズの長さ方向の断面図とレーザ光の光路を図４に示す。
また、屈折率分布レンズをレーザ光出射端面側から見た外観図を図５に示す。本発明においては、前述したように、屈折率分布レンズ中に２種類の光路を形成するために、レーザ光の入射端面が円形平面の１個の屈折率分布レンズ中にレンズ長の長い部分と短い部分とを有しているような構造とする。この場合、入射端面が平面であることから、屈折率分布レンズの出射端面側にＤＶＤ再生用レーザ光の出射端面Ｏ_Ｄ とＣＤ再生用レーザ光の出射端面Ｏ_Ｃ とを設けた構造となる。入射端面ＩよりＤＶＤ再生用レーザ光の出射端面Ｏ_Ｄ までの距離Ｚ_Ｄ （ＤＶＤ再生用の屈折率分布レンズの長さと言う）と、入射端面ＩよりＣＤ再生用レーザ光の出射端面Ｏ_Ｃ までの距離Ｚ_Ｃ （ＣＤ再生用の屈折率分布レンズの長さと言う）との差、すなわち図４に示す距離Δ_１ は次式で与えられる。
【数４】

である。
【００１７】
ＤＶＤ再生用レーザ光の出射端面Ｏ_Ｄ から主点Ｈ_Ｄ までの距離ｈ_Ｄ は、次式で与えられる。
【数５】

ただし、Ｚ_Ｄ はＤＶＤ再生用の屈折率分布レンズの長さである。
同様にＣＤ再生用レーザ光の出射端面Ｏ_Ｃ から主点Ｈ_Ｃ までの距離ｈ_Ｃ は、次式で与えられる。
【数６】

ただし、Ｚ_Ｃ はＣＤ再生用の屈折率分布レンズの長さである。
【００１８】
また、ＤＶＤ再生用の屈折率分布レンズの長さＺ_Ｄ に対する焦点距離ｆ_Ｄ は、次式で与えられる。
【数７】

また、同様に、ＣＤ再生用の屈折率分布レンズの長さＺ_Ｃ に対する焦点距離ｆ_ｃ は、次式で与えられる。
【数８】

【００１９】
（５）， （７） 式より、ＤＶＤ再生用レーザ光の出射端面Ｏ_Ｄ から同レーザ光に対する焦点（集光点）Ｆ_Ｄ までの距離Ｌ_Ｄ は、次式で与えられる。
【数９】

同様に、 （６）， （８） 式より、ＣＤ再生用レーザ光の出射端面Ｏ_Ｃ から同レーザ光に対する焦点（集光点）Ｆ_Ｃ までの距離Ｌ_Ｃ は、次式で与えられる。
【数１０】

【００２０】
ＤＶＤ再生用レーザ光の出射端面Ｏ_Ｄ を基準とした場合、 （９）， （１０） 式より、ＤＶＤ再生用レーザ光に対する焦点Ｆ_Ｄ とＣＤ再生用レーザ光に対する焦点Ｆ_Ｃ との間の距離は次式で与えられる。
【数１１】

【００２１】
すなわち、（４）式のように距離Δ_１ を設定することにより、ＤＶＤ用レーザ光に対する焦点Ｆ_Ｄ とＣＤ用レーザ光に対する焦点Ｆ_Ｃ との間の距離｜Ｆ_Ｃ −Ｆ_Ｄ ｜が、δ（ＤＶＤとＣＤの基盤厚の差＝０．６ｍｍ）と等しくなるのでＤＶＤとＣＤの双方の再生が可能となる。
【００２２】
ここで、図４中の距離Δ_２ （ＤＶＤ再生用レーザ光を遮ることのない範囲）は次式で与えられる。
【数１２】

である。
【００２３】
すなわち、（１２）式のように距離Δ_２ を決めれば、ｒ_０ ＋ｒ_２Ｄより距離Δ_２ が小さいので、屈折率分布レンズはＤＶＤ再生用のレーザ光を遮ることが無い。また、ｒ_０ ＋ｒ_２Ｃより距離Δ_２ が大きいので、ＣＤ再生用のレーザ光は全て出射端面から出射することになる。つまり、ＤＶＤ再生用のレーザ光とＣＤ再生用のレーザ光の各々を使用した場合に、それらいずれのレーザ光をも遮ること無くＤＶＤおよびＣＤを再生することが可能になる。
【００２４】
最後に、屈折率分布レンズの長さＺ_Ｄ に関しては、ＤＶＤとＣＤの基盤厚の差δを０．６ｍｍとし、屈折率分布レンズの光軸における屈折率ｎ_０ を１．５とした場合、
【外１】

は、（１２）式の条件から次式で与えられる。
【数１３】

以上から、ＤＶＤ再生用の屈折率分布レンズの長さＺ_Ｄ に対する条件は、Ｚ_Ｄ を
【数１４】

のように定めればよい。
【００２５】
以上の説明は、ＤＶＤ再生とＣＤ再生とに共用できる再生用光学系を念頭において説明したが、２開口２焦点が要求される光再生用光学装置であれば、本発明の適用範囲がＤＶＤとＣＤに限られるものでないことは言うまでもない。
【００２６】
【発明の効果】
以上説明したように、本発明によれば、部分的に２種類の長さをもつ１個の屈折率分布レンズのみを対物レンズとして使用し、その屈折率分布レンズにＤＶＤ再生用レーザ光とＣＤ再生用レーザ光をレンズ光軸に対して異なる傾きをもって入射させ、また屈折率分布レンズを、レーザ光の入射端は平面とし、レンズ中にレンズ長の長い部分と短い部分とを有する構造にすることにより、２開口２焦点光再生用光学装置の実現を可能にした。
【図面の簡単な説明】
【図１】本発明２開口２焦点光再生用光学装置の一実施形態を示している。
【図２】ＤＶＤ再生用レーザ光とＣＤ再生用レーザ光のそれぞれについてレーザ光がレンズに入射する様子を示している。
【図３】屈折率分布レンズ中におけるレーザ光の伝搬可能範囲を示している。
【図４】屈折率分布レンズの長さ方向の断面図とレーザ光の光路を示している。
【図５】屈折率分布レンズをレーザ光出射端面側から見た外観図を示している。
【図６】従来の２開口２焦点光再生用光学装置を示している。
【符号の説明】
１ ＤＶＤ用レーザモジュール
２ ＣＤ用レーザモジュール
３ プリズム
４ コリメートレンズ
５ 立ち上げミラー（反射鏡）
６ ホログラム光学素子
７ 対物レンズ
８ ＤＶＤ
９ ＣＤ
１０ 屈折率分布レンズ
θ_０ 屈折率分布レンズの光軸に対するＣＤ再生用レーザ光の傾き
ｒ_０ 屈折率分布レンズの半径
ｒ_１ 入射端面における入射レーザ光の光軸からの距離
Ｉ 屈折率分布レンズの入射端面
Ｚ_Ｄ ＤＶＤ再生用の屈折率分布レンズの長さ
Ｚ_Ｃ ＣＤ再生用の屈折率分布レンズの長さ
ｒ_２Ｄ ＤＶＤ再生用レーザ光のうち、屈折率分布レンズの入射端面Ｉ上の光軸から−ｒ_０ の位置に入射したレーザ光が、入射端面ＩよりＺ_Ｃ の位置での光軸からの位置（ｒ_２Ｄ＜０）
ｒ_２Ｃ ＣＤ再生用レーザ光のうち、屈折率分布レンズの入射端面Ｉ上の光軸からｒ_１ の位置に入射したレーザ光が、入射端面ＩよりＺ_Ｃ の位置での光軸からの位置（ｒ_２Ｃ＜０）
Ｈ_Ｄ ＤＶＤ再生用の屈折率分布レンズの長さＺ_Ｄ に対する主点位置
Ｈ_Ｃ ＣＤ再生用の屈折率分布レンズの長さＺ_Ｃ に対する主点位置
Ｏ_Ｄ ＤＶＤ再生用の屈折率分布レンズの出射端面
Ｏ_Ｃ ＣＤ再生用の屈折率分布レンズの出射端面
ｈ_Ｄ ＤＶＤ再生用の屈折率分布レンズの出射端面Ｏ_Ｄ からＨ_Ｄ までの距離
ｈ_Ｃ ＣＤ再生用の屈折率分布レンズの出射端面Ｏ_Ｃ からＨ_Ｃ までの距離
Ｆ_Ｄ ＤＶＤ再生用の屈折率分布レンズの長さＺ_Ｄ に対する焦点位置
Ｆ_Ｃ ＣＤ再生用の屈折率分布レンズの長さＺ_Ｃ に対する焦点位置
ｆ_Ｄ Ｈ_Ｄ からＦ_Ｄ までの距離
ｆ_Ｃ Ｈ_Ｃ からＦ_Ｃ までの距離
Ｌ_Ｄ Ｏ_Ｄ からＦ_Ｄ までの距離
Ｌ_Ｃ Ｏ_Ｃ からＦ_Ｃ までの距離
δ Ｆ_Ｃ からＦ_Ｄ までの距離
Δ_１ Ｏ_Ｄ からＯ_Ｃ までの距離
Δ_２ 屈折率分布レンズの長さがＺ_Ｃ である領域長[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical reproducing optical device for reading recorded information from an optical recording medium, and more particularly, to a two-aperture 2 that enables reading of recorded information from two types of optical recording media having different standards for aperture and focus. The present invention relates to an optical device for reproducing focused light.
[0002]
[Prior art]
For example, since the standards are different from each other, DVDs (digital video discs) (short focus at an aperture of 0.6) and CDs (compact discs) (long focus at an aperture of 0.45) are optical recording media recorded by different recording methods. ) Can be reproduced by one optical device, it is necessary to realize the reproducing optical device as one having two apertures and two focal points. FIG. 6 shows a conventional example in which the optical device for reproducing light has two apertures and two focal points.
[0003]
Of the two optical paths of the laser light shown in FIG. 6, the solid line is for reading recorded information from the DVD 8 generated from the laser module (wavelength 650 nm) 1, and the broken line is for the laser module (wavelength 780 nm) 2. The optical paths of the generated laser beams for reading recorded information from the CD 9 are shown. That is, in FIG. 6, each laser beam having a wavelength of 650 nm and 780 nm (the wavelength of the laser beam for DVD and CD reproduction, respectively) is a prism 3, a collimating lens 4, a rising mirror (reflecting mirror) 5, The light is focused on the information recording surfaces of the DVD 8 and the CD 9 through the hologram optical element 6 and the objective lens 7, respectively. Here, since the substrate thicknesses of the DVD and CD are 0.6 mm and 1.2 mm, respectively, the information recording surface of the CD is 0.6 mm farther.
[0004]
As described above, in the related art, the optical path is formed by using the hologram optical element 6. Here, the hologram optical element 6 has a concentric hologram pattern formed on the surface of the element. This concentric hologram pattern has the power of a concave lens for + 1st-order diffracted light. The hologram pattern is designed so that most of the hologram pattern is diffracted with respect to a laser beam having a wavelength of 780 nm for reproducing a CD. Furthermore, in the hologram pattern, the focal point of the first-order diffracted light of the laser beam having a wavelength of 780 nm for reproducing the CD is more than the focal point of the transmitted light (0-order light) (the focal point for reproducing the DVD). It is designed to be 6 mm away.
[0005]
On the other hand, since the objective lens 7 is designed for reproducing a disk having an aperture of 0.6 and a substrate thickness of 0.6 mm, that is, for reproducing a DVD, a laser beam having a wavelength of 650 nm for reproducing the DVD, which is almost transmitted through the hologram optical element 6, The light is condensed on the information recording surface of a disc having a base thickness of 0.6 mm. In this way, two types of focus are realized. Further, the hologram pattern is provided only inside a circular region having a diameter smaller than the effective diameter of the objective lens. At this time, the laser beam having a wavelength of 650 nm for DVD reproduction passes through both the inside and the outside of the circular area on which the hologram pattern is formed, so that the corresponding aperture is 0.6 determined by the effective diameter of the objective lens 7. Become.
[0006]
In addition, as shown in FIG. 6, the beam of the laser beam for reproducing a CD having a wavelength of 780 nm is narrowed down so as to correspond to a circular area on which a hologram pattern is formed, and is diffracted only inside this area. A good opening will be the diameter of the circular area. That is, two types of apertures have been realized by designing a circular region so that the aperture for a laser beam having a wavelength of 780 nm for CD reproduction is 0.45.
[0007]
[Problems to be solved by the invention]
As described above, the conventional two-aperture bifocal light reproducing optical device that can be shared by a DVD and a CD is a hologram optical element 6 having a hologram pattern on the element surface as a component for realizing a two-aperture bifocal. And the objective lens 7 (see FIG. 6). This not only means that the number of parts is increased, but also the required aperture (0.6 and 0.45 for DVD and CD, respectively) and the required focus (CD is less than .0 for DVD). Both of them (hologram optical element 6 and objective lens 7) must be designed (and adjusted at the time of installation) to satisfy the above condition, including their respective positions. This is not always easy. Did not.
[0008]
An object of the present invention is to provide a two-aperture bifocal light reproducing optical device that does not include such a problem to be solved.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, a two-aperture bifocal light reproducing optical device according to the present invention is a two-aperture bifocal light reproducing optical device capable of reading recorded information from two types of optical recording media having different standards. The optical device includes a refractive index distribution lens whose refractive index distribution changes in a radial direction as an objective lens of the device, and two types of information reading lasers respectively corresponding to the two types of optical recording media. In addition to realizing two types of apertures by making the inclination of light to the refractive index distribution lens with respect to the lens optical axis different from each other, two types of optical paths having different optical path lengths are formed in the lens. Is what you do.
[0010]
Further, in the optical apparatus for reproducing the two-point bifocal light of the present invention, since the two types of optical paths having different optical path lengths are formed, the refractive index distribution lens is configured such that the incident end face of the laser beam has a circular flat surface. It is characterized in that each of the lenses has a long lens portion and a short lens portion.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments with reference to the accompanying drawings.
First, FIG. 1 shows an embodiment of an optical device for reproducing two-port bifocal light of the present invention.
Compared with the optical device shown in FIG. 1 and the conventional optical device shown in FIG. 6, the hologram element 6 and the objective lens which have been pointed out as problems to be solved by the present inventors in the conventional device. 7 (see FIG. 6) is replaced by a gradient index lens 10 (see FIG. 1), and is indicated by a dashed line, for reading recorded information of CD 9 generated from a laser module (wavelength 780 nm) 2. In FIG. 6 (conventional), the direction of the light output of the laser beam is perpendicular to the element surface of the hologram element 6 (that is, parallel to the optical axis), whereas in FIG. 1 (the present invention), the refractive index is 6 is different from that in FIG. 6 in that the light is incident on the optical axis of the distribution lens 10 at an angle.
[0012]
Therefore, in the following, the refractive index distribution lens (indicated by reference numeral 10 in FIG. 1) which characterizes the present invention will be described in detail based on these differences.
The refractive index distribution lens (the laser light incident end surface is a circular flat surface) is a lens whose refractive index distribution in the radial direction is expressed by the following equation using the distance r from the lens optical axis as a parameter.
(Equation 1)

However, n ₀ is the refractive index at the position of the optical axis of the lens, n _r is the refractive index at the position of distance r from the optical axis, and A is a constant determined by the material of the lens.
[0013]
In the present invention, the DVD reproduction laser beam _RD is parallel to the optical axis of the lens, and the CD reproduction laser beam _RC is inclined with respect to the optical axis of the lens by the following equation. It is assumed that the light is incident at −θ ₀ . This is shown in FIG.
(Equation 2)

Here, r ₀ is the radius of the lens.
[0014]
The laser light incident at an inclination - [theta] ₀ with respect to the optical axis on the refractive index distribution lens, progresses by repeating total reflection at the refractive index distribution within the lens. Since the refractive index of the refractive index distribution lens varies depending on the radial distance from the optical axis on the incident end surface, the light incident on the optical axis at an angle of -θ ₀ generally propagates through the refractive index distribution lens. the laser beam can be is limited to the range from the optical axis to the radius r ₁ represented by the following formula. This is shown in FIG.
(Equation 3)

[0015]
Here, the aperture of the objective lens for reading information is 0.45 for the lens for CD reproduction and 0.6 for the lens for DVD reproduction. However, since the aperture depends on the lens diameter, the aperture for the DVD reproduction is large. In the case where the aperture for the laser beam is designed to be 0.6, in order to set the aperture for the laser beam for CD reproduction to be 0.45, the area where the laser beam for CD reproduction is incident is the area where the laser beam for DVD reproduction is incident. That is, it is sufficient to set 75% of the region to be processed. Therefore, if the aperture of the refractive index distribution lens is designed to be 0.6 and the parameters are set so that r ₁ = 0.75r ₀ , the light out of the light incident at −θ ₀ in equation (2) propagates only the light incident on the area from the axis to 0.75 r _0, for the laser beam for DVD reproduction obtained opening of 0.6, 0.45 of the laser light for CD playback An opening will be obtained.
[0016]
Next, the conditions for two focal points will be discussed.
FIG. 4 shows a sectional view of the refractive index distribution lens in the length direction and an optical path of the laser beam for realizing the bifocal point.
FIG. 5 is an external view of the refractive index distribution lens viewed from the laser light emitting end face side. In the present invention, as described above, in order to form two types of optical paths in the refractive index distribution lens, a laser beam incident end face has a long portion in one refractive index distribution lens having a circular flat surface. The structure has a short part. In this case, the incident end face from being a plane, the exit end face O _C and the provided structure of the exit end face emitting end face of the DVD reproducing laser beam on the side O _D and CD playback laser beam of the refractive index distribution lens. The distance Z _D to exit end face O _D than the incident end face I DVD playback laser beam _(DVD referred to the length of the gradient index lens for reproduction) to the exit end face O _C than the incident end face I CD playback laser beam the difference between the distance Z _{C (CD} say the length of the gradient index lens for reproduction), that is, the distance delta ₁ shown in FIG. 4 is given by the following equation.
(Equation 4)

It is.
[0017]
Distance _{h D} from the exit end face _{O D} of the DVD reproducing laser beam to the principal point _{H D} is given by the following equation.
(Equation 5)

However, Z _D is the length of the gradient index lens for DVD playback.
Similarly the distance h _C from the exit end face O _C of CD playback laser light to the principal point H _C is given by the following equation.
(Equation 6)

However, Z _C is the length of the gradient index lens for CD playback.
[0018]
Further, the focal length f _D to the length Z _D of the gradient index lens for DVD playback is given by the following equation.
(Equation 7)

Similarly, the focal length f _c with respect to the length Z _C of the gradient index lens for CD reproduction is given by the following equation.
(Equation 8)

[0019]
(5) and (7), the distance _{L D} from the exit end face _{O D} of the DVD reproducing laser beam to a focal point (condensing point) _{F D} for the laser beam is given by the following equation.
(Equation 9)

Similarly, (6), (8) from the equation, the distance _{L C} from the exit end face _{O C} of CD playback laser beam to a focal point (condensing point) _{F C} against the laser beam is given by the following equation.
(Equation 10)

[0020]
If relative to the emitting end face _{O D} of the DVD playback laser beam (9), the distance between the focal point _{F C} for (10) than the focus _{F D} and CD playback laser beam for DVD playback laser beam Is given by the following equation.
(Equation 11)

[0021]
That is, by setting the distance delta ₁ as equation (4), the distance between the focal point _{F C} to the focal _{F D} and the CD laser beam for the DVD laser beam _| F C -F _{D |} is, [delta] (Difference between base thickness of DVD and CD = 0.6 mm), so that both DVD and CD can be reproduced.
[0022]
Here, the distance Δ ₂ in FIG. 4 (the range in which laser light for DVD reproduction is not blocked) is given by the following equation.
(Equation 12)

It is.
[0023]
That is, if determined the distance delta ₂ as _(12), since than r 0 _{+ r 2D} distance delta ₂ is small, the refractive index lens is never block the laser beam for DVD playback. Further, since r 0 ₊ r distance delta ₂ is greater than _2C, the laser light for CD playback will be emitted from all exit end face. That is, when each of the laser light for DVD reproduction and the laser light for CD reproduction is used, it is possible to reproduce the DVD and CD without blocking any of the laser lights.
[0024]
Finally, with respect to the length Z _D of the gradient index lens, if DVD and CD of the difference in base thickness δ and 0.6 mm, the refractive index n ₀ at the optical axis of the refractive index distribution lens is 1.5,
[Outside 1]

Is given by the following equation from the condition of equation (12).
(Equation 13)

From the above, the condition for the length _{Z D} of the gradient index lens for DVD playback, Equation 14] The _{Z D}

It may be determined as follows.
[0025]
The above description has been made in consideration of a reproduction optical system that can be used for both DVD reproduction and CD reproduction. However, if the optical reproduction optical device requires two apertures and two focal points, the applicable range of the present invention is DVD and Needless to say, it is not limited to CDs.
[0026]
【The invention's effect】
As described above, according to the present invention, only one refractive index distribution lens partially having two kinds of lengths is used as an objective lens, and a DVD reproducing laser beam and a CD A laser beam for reproduction is incident with a different inclination with respect to the optical axis of the lens, and the refractive index distribution lens has a structure in which the incident end of the laser beam is flat and the lens has a long lens portion and a short lens portion. This has made it possible to realize a two-aperture bifocal light reproducing optical device.
[Brief description of the drawings]
FIG. 1 shows an embodiment of a two-aperture bifocal light reproducing optical device of the present invention.
FIG. 2 shows how laser light is incident on a lens for each of laser light for DVD reproduction and laser light for CD reproduction.
FIG. 3 shows a propagable range of laser light in a refractive index distribution lens.
FIG. 4 shows a longitudinal sectional view of a refractive index distribution lens and an optical path of laser light.
FIG. 5 is an external view of the refractive index distribution lens as viewed from a laser light emitting end face side.
FIG. 6 shows a conventional two-aperture bifocal light reproducing optical device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 DVD laser module 2 CD laser module 3 Prism 4 Collimating lens 5 Start-up mirror (reflection mirror)
6 Hologram optical element 7 Objective lens 8 DVD
9 CD
10 Refractive index distribution lens θ ₀ Slope of the laser beam for CD reproduction with respect to the optical axis of the refractive index distribution lens r ₀ Radius r of the refractive index distribution lens ₁ Distance from the optical axis of the incident laser light at the incident end face I of the refractive index distribution lens of the incident end face Z _D DVD length r _{2D DVD} playback laser beam length Z _C CD refractive index lens for reproduction of the refractive index distribution lens for reproduction, the optical axis on the incident end surface I of the gradient index lens from a laser light incident on the position of -r ₀ is the position of the optical axis at the position of _{Z C} than the incident end face I _(r 2D <0)
Of the r _2C CD reproducing laser light, the laser light incident on the position of r ₁ from the optical axis on the incident end face I of the refractive index distribution lens is the position from the optical axis at the position Z _C from the incident end face I ( r _2C <0)
Emission of H D _DVD principal point H _C CD principal point O _D DVD refractive index lens for reproduction to the length Z _C of the gradient index lens for reproduction to the length Z _D of the gradient index lens for reproduction facet O _C CD emitting end face of the gradient index lens for reproducing h _D DVD emission end face of the distance h _C CD refractive index lens for reproduction from the exit end face O _D refractive index lens for reproduction to H _D O _C from a distance _F D DVD focal position with respect to the length _{Z C} of the focal position _F C CD refractive index lens for reproduction to the length _{Z D} of the gradient index lens for reproduction _f D _{H D} to _{H C} until _{F D} Distance f _CH H _C to F _C distance L _D O _D to F _D distance L _C O _C to F _C distance δ F _C to F _D distance Δ ₁ O _D to O _C Distance Δ ₂ The length of the gradient index lens is Z Area length that is _C

Claims (2)

What is claimed is: 1. A two-aperture bifocal light reproducing optical device capable of reading recorded information from two types of optical recording media having different standards, wherein the optical device has a refractive index distribution in a radial direction as an objective lens of the device. A gradient index lens that is changing, and two types of information reading laser beams respectively corresponding to the two types of optical recording media have different inclinations with respect to the optical axis of the lens toward the gradient index lens. A two-aperture bifocal light reproducing optical device, wherein two types of apertures are realized, and two types of optical paths having different optical path lengths are formed in the lens. 2. The optical system for reproducing two-aperture bifocal light according to claim 1, wherein, in order to form the two types of optical paths having different optical path lengths, the refractive index distribution lens has an incident end face of the laser beam having a circular flat surface. A two-aperture bifocal light reproducing optical device, wherein one lens has a long lens portion and a short lens portion.

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