JPH09311271A - Objective lens and optical pickup device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct chromatic aberration by making numerical aperture equal to or above a specified value by respectively providing phase type diffraction type lenses on both surfaces of a single lens whose first and second surfaces are aspherical and satisfying a specified condition. SOLUTION: The phase type diffraction type lenses 4 and 5 are added to both surfaces 2 and 3 of the single lens 1 whose both surfaces are the aspherical surfaces and which is constituted of glass material having high refractive index and high dispersion, so that an objective lens having the high numerical aperture of >=0.7 is realized. In the case that center wavelength is set as λ2 and wavelength is used from shortest wavelength λ1 to longest wavelength λ3 , and when the refractive indexes of the glass material forming the lenses 1, 4 and 5 at each wavelength λ1 , λ2 or λ3 are respectively set as n1 -n3 , and the Abbe's number V of the glass material at this area is set as V=(n2 -1)/(n1 -n3 ), and the Abbe's number VHOE of the lenes 4 and 5 is set as VHOE=λ2 /(λ1 -λ3 ), the expression of (1+(VHOE/V).(n2 -1)>0.572 is valid.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technical field relating to an objective lens used in various optical devices and an optical pickup device constructed by using the objective lens.

[0002]

2. Description of the Related Art Conventionally, an optical disc, which is an optical recording medium, has been used for storing information signals (data) such as moving images, sounds, computer data and the like. Further, this optical disc is widely used because of its good mass productivity and low cost. There is a strong demand for high density and large capacity of recorded information signals for this optical disc, and in recent years, this demand has become even stronger. To increase the recording density of the information signal recorded on the optical disc,
It is effective to shorten the wavelength of the light beam used for reading the information signal and to use a lens having a high numerical aperture (NA) as an objective lens for condensing the light beam on the optical disc.

In reproducing a so-called "digital video disc (DVD)" in which moving image information is converted into a digital signal and recorded, an optical pickup device having an objective lens of NA 0.6 is used. This objective lens is made of a transparent synthetic resin material, using an injection molding means using a mold.
Alternatively, it is formed by glass molding. In addition, in this "digital video disc",
In order to reduce the influence of coma aberration due to the tilt of the optical disc, the thickness of the disc substrate made of a transparent material is 0.6
It is made in mm. The thickness of the disc substrate of this "digital video disc" is the thickness of the disc substrate of a so-called "compact disc (CD)" on which audio information converted into a digital signal is recorded or a magneto-optical disc (1.2 mm). ) And half of it.

However, it is necessary to make a single-lens aspherical lens having a higher NA (ie, 0.6 or more) than the objective lens used in the above-mentioned "digital video disc", when processing and molding a mold. It is said to be almost impossible because eccentricity control becomes difficult.
That is, in processing a die for molding the objective lens, the inclination of the lens surface exceeds 45 degrees (less than 45 degrees with respect to the optical axis) due to the size of the tip of the cutting blade (diamond tool). ), It is difficult to process an aspherical mold. Further, when the curvature of the lens surface becomes tight, "sag" (depth along the direction of the axial direction from the apex of the lens to the outermost circumference) becomes large, which makes it difficult to cut the die. Under such circumstances, there has been no report so far that a lens having a numerical aperture of 0.7 or more is manufactured as a single lens.

Further, one of the noise factors in reproducing an optical disk is a problem of mode hopping due to laser output fluctuation. When the laser wavelength fluctuates, the focus position moves, causing defocus (focus error). Particularly in a magneto-optical disk, mode hop is likely to occur because reading and writing are frequently switched. On the other hand, an attempt to correct chromatic aberration by using a diffractive integrated lens is disclosed in, for example, US Pat. No. 5,349,47.
No. 1 "Hybrid Refractive Diffractive Achromatic Lens for Optical Data Storage System"("HYBRID REFRAC
TIVE / DIFFRACTIVE ACHROMATIC LENS FOROPTICAL DATA S
TORAGE SYSTEM "G. Micael Morris et al. USPatent N
umber 5,349,471) and the 41st Spring Society of Applied Physics Proceedings 29a-L-11 (1994) "Diffractive chromatic aberration correction objective lens for magneto-optical disks" (Koichi Maruyama, Makoto Iwashiro,
Shunichiro Wakamiya, Ryota Ogawa) or "56th Autumn Applied Physics Society Proceedings 29a-ZA-11 (1995)" Glass Achromatic Objective Lens with Integrated Diffractive Lens ".
(Michihiro Yamagata, Yasuhiro Tanaka, Yoshinori Shirato, Yoshiyuki Shimizu, Masaaki Sunohara). These proposals are aimed at correction of chromatic aberration, and relate to a single lens having a diffractive lens on one surface, and to a lens having a numerical aperture of about 0.55.

The diffractive lens-integrated chromatic aberration correction lens itself is described in 1988 as "Hybrid diffractive-refractive lenses and ac".
hromats "Thomas Stone and Nicholas George, Appl.Op
t. 27, 2960 (1988)) and the like. However, processing of these diffractive lenses,
It is difficult to manufacture, and until recently, it was not possible to manufacture a lens that can be put to practical use. However,
Recently, the phase type diffractive lens can be manufactured by diamond turning on the mold, like an ordinary aspherical lens, so that it can be manufactured integrally with a plastic or glass molded aspherical lens. became. As a result, it has become possible to manufacture a lens in which a phase type diffractive lens is integrated at a cost substantially equal to that of a conventional aspherical lens.

The principle of correcting chromatic aberration while maintaining the small size and light weight by combining the diffractive optical element with the refractive optical element is as follows. First, in general,
The achromatic condition is derived as follows. If _two thin lenses having focal lengths f ₁ and f ₂ are arranged at an interval d, the focal length f of the synthetic lens is

[0008]

[Equation 2]

[0009] Take the total derivative of both sides,
If d = 0,

[0010]

(Equation 3)

To obtain If the radii of curvature of the first and second surfaces of the thin lens are r ₁ and r ₂ , and the refractive index of the glass material is n,

[0012]

(Equation 4)

Then, this is differentiated with respect to n to obtain the following equation.

[0014]

(Equation 5)

Here, the Abbe number V is defined by the following equation. Generally, what is often used is that the d-line for visible light is the center. In the following, the subscripts D, F,
N having C is d line (589.6 nm), f
It is a refractive index with respect to the spectrum line of the line (486.1 nm) and the c line (656.3 nm). These values may be replaced appropriately for the wavelength range of the light source.

[0016]

(Equation 6)

Therefore,

[0018]

(Equation 7)

From this [Formula 7] and the above [Formula 3],

[0020]

(Equation 8)

From the above [Formula 2] and the above [Formula 8],

[0022]

[Equation 9]

Is obtained. This [Formula 9] is the achromatic condition of the compound lens.

The Abbe number of the diffractive lens is obtained as follows. The phase transfer function t (x, y) of an ideal diffractive lens is as follows for a design wavelength λ ₀ and a design focal length f ₀ :

[0025]

(Equation 10)

## EQU1 ## This diffractive lens has a wavelength λ (= c
(Light velocity) / ν (frequency)) monochromatic plane wave exp [+ i
2πνt] is incident, the transmitted wavefront is

[0027]

[Equation 11]

It is shown by. Where f is a function of wavelength,

[0029]

(Equation 12)

## EQU1 ## Especially,

[0031]

(Equation 13)

And the power of the ordinary lens Φ (λ)
Is

[0033]

[Equation 14]

Is as follows. Similarly, the effective refractive index n _z ^eff of the diffractive lens at the wavelength λ _z can be determined. That is,

[0035]

(Equation 15)

Is as follows. Here, C ₀ is a constant determined by design. From [Equation 13] and [Equation 15] above,

[0037]

(Equation 16)

From this [Equation 16] and the above [Equation 6], the Abbe number V _HOE of the diffractive lens is

[0039]

[Equation 17]

Is calculated. By substituting this [Equation 17] and the Abbe number of the refraction lens into the above [Equation 9], the ratio of the focal lengths of the diffractive lens and the refraction lens necessary for achromatization can be obtained. The Abbe number of glass is always positive, 2
It is distributed between about 5 and 70. Therefore, in a normal achromatic lens (convex lens), chromatic aberration correction is performed using a convex lens of crown glass and a concave lens of flint glass (f ₁ > 0, f ₂ <0). However, in chromatic aberration correction using a diffractive lens and a refractive lens,
Since V _HOE <0, chromatic aberration can be corrected by combining the refractive convex lens and the diffractive convex lens. Therefore, there is an advantage that the power of each lens can be weak, that is, the chromatic aberration can be corrected while the curvature is loose, and the process of manufacturing the mold becomes easy. Also, for visible light −
The Abbe number of 3.452 of the diffractive lens has an order of magnitude different from that of ordinary glass even when viewed as an absolute value. That is, it can be seen that the dispersion of the diffractive lens is much larger than that of the refractive lens.

[0041]

A lens having a high numerical aperture of 0.7 or more and an achromatic lens can be realized by combining a plurality of lenses. However, it has not been possible to realize this simultaneously with one lens.
Recently, to solve these problems, two lenses, an aspherical lens (rear lens) and a hemispherical lens (front lens), are used, and the hemispherical lens is made to closely adhere to the optical disk. , A solid immersion lens in which the numerical aperture (NA) is increased by the refractive index (about 1.5 times) of the hemispherical lens based on the same principle as the immersion lens used in a microscope. ing. However, even this solid immersion lens has a problem of chromatic aberration caused by mode hopping when a semiconductor laser is used as a light source. Further, in this solid liquid immersion type lens, it is advantageous in terms of design that the power of the rear lens is increased.

The thickness of the disk substrate of an optical disk such as a so-called "compact disk (CD)" which has been conventionally used is 1.2 mm as described above. However, in the future, there is a high possibility that high-density format optical disks having thinner disk substrates will be used. This is because the thinner the disc substrate is, the smaller the noise factor such as coma aberration caused by the disc skew can be reduced. The influence of the thickness of the disc substrate becomes more and more significant as a light source with a shorter wavelength and an objective lens with a higher numerical aperture are used to increase the density of recorded information signals. Here, in an optical pickup device that reads an information signal from an optical disc, it is an important issue to ensure compatibility between an optical disc of a conventional format and an optical disc of a future format. If the thickness of the disk substrate is different, the spherical aberration caused by the thickness changes, so that compatibility cannot be ensured as it is.

Regarding this subject, the 56th Japan Society of Applied Physics Preliminary Collection (No 3.p956-957, SPIE VOL.2338 Optical Data
In Storage (1994) p282-288), it is reported that a hologram is formed in the center of the objective lens and the 0th-order light and the 1st-order light read the discs of different formats (ISOM'95 Post- deadline Paper Technical Digest
P-38-39). Further, the applicant of the present application has proposed an "optical disk device using a bifocal lens" that achieves the same effect as the above proposal by using ± first-order light.

Further, forming the signal recording layer of the optical disc into a multi-layer structure is an effective means for dramatically increasing the information recording amount. However, there is a problem how to correct the difference in the amount of spherical aberration caused by the difference in the thickness of the disc substrate corresponding to each signal recording layer. For these,
Although some proposals have been made, these proposals require a plurality of optical pickup devices or a movable part, which leads to complication of the device configuration or is required. However, there was a problem that a large numerical aperture could not be secured.

The present invention is proposed in view of the above-mentioned circumstances, and an objective lens having a numerical aperture of 0.7 or more and chromatic aberration is corrected is used as a diffractive lens-integrated both aspherical surface. This is to solve the problem of realizing it as a single lens.

[0046]

In order to solve the above-mentioned problems, the objective lens according to the present invention is a single-lens lens in which both the first surface and the second surface are aspherical surfaces. each has a phase type diffraction lens, the wavelength lambda ₁ of the glass material forming the lens in the case of using the shortest wavelength lambda ₁ to the maximum wavelength lambda ₃ a center wavelength as lambda _2, lambda _2,
The refractive indices at λ ₃ are n ₁ , n ₂ , and n ₃ , respectively, and the Abbe number V of the glass material in this region is V = (n ₂ −1) / (n ₁ −
n ₃ ), the Abbe number V _HOE of the phase type diffraction lens is V _HOE =
If λ ₂ / (λ ₁ −λ ₃ ), then (1+ (V _HOE / V)) · (n ₂ −1)> 0.572 is established, the chromatic aberration is corrected, and the numerical aperture is Is 0.7 or more.

Further, according to the present invention, in the above objective lens, the chromatic aberration is excessively corrected by the phase type diffractive lens so that the focal length becomes short for a short wavelength.

Further, according to the present invention, in the above objective lens, the phase type diffractive lens has an aspherical phase term, and the position of the minimum pitch portion of the phase type diffractive lens is between the center and the peripheral edge of the lens. It is supposed to be the position.

According to the present invention, in the above objective lens, the depth of each groove constituting the phase type diffraction lens is
The depth is such that the optical path difference is an integral multiple of 10 or less with respect to the wavelength.

Further, according to the present invention, in the above-mentioned objective lens, each of a plurality of diffracted lights generated by the phase type diffractive lens formed on the first surface or the second surface has an optical specification different from each other. It is intended to be used for recording and reproduction on a recording medium.

Further, according to the present invention, the above objective lens is combined with a hemispherical lens to form a solid immersion type objective lens.

In the optical pickup device according to the present invention, the light source, the objective lens for condensing the light beam emitted from the light source on the signal recording surface of the optical recording medium, and the reflected light beam of the light beam by the optical recording medium. The objective lens is a single-lens lens in which both the first surface and the second surface are aspherical surfaces, and each has a phase-type diffractive lens on each of the both surfaces. when used in the shortest wavelength lambda ₁ to the maximum wavelength lambda ₃ a center wavelength as lambda _2, respective wavelength lambda ₁ of the glass material forming the lens,
The refractive indices at λ ₂ and λ ₃ are n ₁ , n ₂ , and n ₃ , respectively, and the Abbe number V of the glass material in this region is V = (n ₂ −1) / (n ₁ −
n ₃ ), the Abbe number V _HOE of the phase type diffractive lens is V
_{If HOE} = λ ₂ / (λ ₁ −λ ₃ ), then (1+ (V _HOE / V)) · (n ₂ −1)> 0.572 holds, and the chromatic aberration is corrected and The numerical aperture is set to 0.7 or more.

Further, according to the present invention, in the above optical pickup device, a skew servo mechanism for correcting coma aberration caused by the fact that the optical axis of the objective lens and the signal recording surface of the optical recording medium are not perpendicular is provided. It was done.

Furthermore, the present invention is the above optical pickup device, wherein the optical recording medium has a substrate made of a transparent material and having a thickness of 0.6 m.
It is intended to collect the light flux on the signal recording surface through this substrate by using a material of m or less.

[0055]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the objective lens and the optical pickup device according to the present invention will be described below in the following order.

[1] Outline of the Invention [2] Correction of Chromatic Aberration [3] Pitch of Grooves Constituting Diffractive Lens [4] Structure of Optical Pickup Device [5] Application to Solid Liquid Immersion Lens [6] Recording / reproduction on optical discs with different specifications

[1] Outline of the Present Invention As a result of intensive studies for solving the above-mentioned problems, the present inventor has shown that, as shown in FIG. 3, a double-sided aspherical surface made of a glass material having a high refractive index and a high dispersion is used. By adding the phase type diffractive lenses 4 and 5 to both surfaces 2 and 3 of the single lens 1, it has been found that an objective lens having a high numerical aperture of 0.7 or more can be realized. Further, in order to widen the pitch (machining pitch) of the grooves forming the diffractive lens, the power is evenly distributed to the diffractive lenses on both sides, and the aspherical surface term of the diffractive lens is utilized to reduce the minimum pitch of the grooves. We came to have the knowledge that it can be expanded. Further, if such an objective lens is used, it is possible to configure an optical pickup device for reading and writing an information signal with respect to an optical disc having a disc substrate having a thickness of 0.6 mm or less, if necessary. Further, if the skew servo mechanism is used together, an optical pickup device for reading / writing information signals from / on an optical disc for recording information at a higher density can be constructed. Furthermore, by combining this objective lens with a hemispherical lens, a solid immersion lens with a high numerical aperture can be realized.

Even if the curvature of each surface of the refractive lens and the glass material forming the refractive lens are not changed, the power of the lens can be increased by forming the diffractive lens on the surface of the refractive lens, thus increasing the aperture. The number (NA) can be increased.

There is a great merit in integrally molding a diffractive lens including an aspherical coefficient on the aspherical lens surface by molding. At first glance, it seems that even if an aspherical surface is placed on an aspherical surface, it is still an aspherical surface, but this has a great effect. That is, as shown in FIG. 21, in a normal refracting lens, if the curvature of the surface (= 1 / R) is changed, the incident angle θ and sag z (incident position from the apex of the lens to the same ray) are also changed. Depth measured parallel to the optical axis up to) changes. But,
If the diffractive lens 4 is formed on this surface, the thickness of the diffractive lens 4 is 0. Therefore, if the grating period at that position is changed, only the emission angle θ ′ from this surface changes. That is, the emission angle θ ′ and the sag z can be freely changed independently by the base curvature as the refraction type lens and the grating period as the diffraction type lens. Therefore, the degree of freedom in design is increased.

In order to remove coma from the lens, the sine condition must be satisfied. The sine condition for the afocal system is given by the following equation, as shown in FIG.

(H / f) = sinU where f: focal length, h: incident height, and U: exit angle.

In order to completely satisfy the sine condition in the thick system, it is necessary to dispose the diffractive lens 5 on the second surface as shown in FIG. That is, when considering the optical path of light that is incident on the lens having the focal length f at the incident height h, if this light satisfies the sine condition, the emission angle U is
= Sin ⁻¹ (h / f). Therefore, the exit point of the lens must be C. Therefore, if the incident position A is determined, the optical path ACF (F is
The focus) is uniquely determined. However, in the refraction type or lens, not all light rays can pass through the optical path satisfying this condition, and this shift causes coma. However, if the diffractive lens 5 is provided on the second surface of the lens 1, only the direction of the light beam can be freely changed as described above, so that the sine condition can be completely satisfied.

Further, in the refraction type diffractive integrated objective lens according to the present invention, spherical aberration and coma can be corrected up to high order by the degree of freedom of design obtained by the diffractive lens. it can. In general, coma and the like caused by decentering are functions of lower-order spherical aberration and coma. If these low-order aberrations are suppressed, naturally, the coma aberration generated by decentering also decreases. Alternatively, by optimizing the balance of the generated amounts, it is possible to design so as to cancel each other out. Therefore,
In the refraction-type diffractive integrated lens, the tolerance for decentering can be widened.

[2] Correction of chromatic aberration Here, the problem is the large chromatic aberration of the diffractive lens. Even if the refractive lens and the diffractive lens are integrated (hybridized), if the ratio of the power of the diffractive lens and the power of the refractive lens does not substantially satisfy the achromatic condition, large chromatic aberration will occur. Will occur. As described above, the power ratio of each lens is determined by the dispersion of the glass material forming the refractive lens. The greater the dispersion of this glass material, the higher the ratio of the power of the diffractive lens to the power of the refractive lens. That is, strong power can be applied to the diffractive lens. If the power ratio of each lens satisfies the achromatic condition, chromatic aberration can be corrected by combining a refractive convex lens with a diffractive convex lens. Therefore, the higher the glass material is, the higher the power can be increased by the diffractive lens while correcting the chromatic aberration. However, even if the achromatic condition is slightly exceeded, if the chromatic aberration is within a practical range, the power of the diffractive lens is increased to perform overcorrection, and the numerical aperture (N
You may raise A). In this case, the focal length becomes shorter as the wavelength becomes shorter.

Further, a glass material having a high dispersion generally has a high refractive index. The numerical aperture (NA) of a lens is
Given as follows:

[0066]

(Equation 18)

In this [Equation 18], a, f, n,
R ₁ and R ₂ represent the following.

A: pupil radius of lens f; focal length of lens n; refractive index R ₁ ; radius of curvature of first surface R ₂ ; radius of curvature of second surface That is, the refractive index of the glass material forming the refractive lens is high The numerical aperture (NA) can be increased without changing the curvature. From this point as well, when the diffractive lens is integrally formed with the refractive lens, it is desirable to use a glass material having a high refractive index and a high dispersion. Further, the diffraction efficiency of the diffractive lens decreases as the incident angle increases. for that reason,
The efficiency is better when the curvature of the base curved surface is as small as possible. From this viewpoint, it is desirable that the radii of curvature R ₁ and R _{2 in} the above [Formula 18] be as large as possible.

Then, how much the numerical aperture (NA) can be raised by changing the glass material becomes a problem. Here, two lenses L ₁ (refractive index n ₁ , Abbe number V ₁ ) and L ₂ (refractive index n ₂ , Abbe number V ₂ ) having the same radius of curvature R ₁ , R ₂ and lens diameter a on both sides of the lens. think of. Then, the numerical apertures NA ₁ and NA ₂ of these two lenses are

[0070]

[Equation 19]

Is obtained. Here, a chromatic aberration is corrected by adding a diffractive lens to one lens L ₂ . At this time, the focal length f _HOE of this diffractive lens is

[0072]

(Equation 20)

Is obtained. Therefore, the total power of the integrated lens of the refractive lens and the diffractive lens is

[0074]

(Equation 21)

It is Therefore, the numerical aperture NA of the entire lens
_achromat is

[0076]

(Equation 22)

It becomes The ratio Q of this numerical aperture NA _achromat to the NA ₁ of the other lens L ₁ is

[0078]

(Equation 23)

Is as follows. A refraction-type diffractive integrated lens with a numerical aperture (NA) of 0.7 or more has a numerical aperture (NA) of 0.6.
In order to manufacture with the same mold processing technique as used in the following conventional lens processing, Q ≧ 1.17 (= 0.7 /
A glass material having a refractive index and an Abbe number of 0.6) (ratio Q is 1.17 or more) may be used.

As the lens L _{1 used} as a reference here, as shown in FIG. 1, for example, an objective lens for a so-called video disc or an objective lens for “digital video disc (DVD)” is used. It is enough to think.
As shown in [Table 1] below, these lenses have the highest numerical aperture (NA) among commercially available lenses and lenses that have been prototyped, and both have actual manufacturing results. There is something. Therefore, the curvature of these lenses can be considered as a measure of the curvature that can be sufficiently created by the current processing technology.

[0081]

[Table 1]

In the case of increasing the numerical aperture (NA) by integrally forming a diffractive lens on a refractive lens having the same curvature as the objective lens for the video disk,

[0083]

(Equation 24)

If the above condition is satisfied, it is understood that the glass material is a glass material that can be used to form an achromatic refraction-diffraction integrated lens with a numerical aperture (NA) of 0.7 or more. Also, "Digital
In the case of increasing the numerical aperture (NA) by integrally forming a diffractive lens on a refractive lens having the same curvature as the objective lens for “Video Disc (DVD)”,

[0085]

(Equation 25)

If founded, it will be understood that the glass material is a glass material capable of producing an achromatic refracting-diffraction integrated lens with a numerical aperture (NA) of 0.7 or more. However, the Abbe number of the diffractive lens is a function of wavelength here. As described above, the commonly used one is d for visible light.
589.6nm, 486.1nm, 65 centered on the line
V _d using the refractive index for the 6.3 nm spectral line
It is. In a lens that uses a semiconductor laser (LD) as a light source, the Abbe number may be appropriately replaced with respect to the wavelength range of the light flux emitted by this semiconductor laser.
The same applies to other refractive indices and Abbe numbers. However, as a guideline for selecting a glass material, a simple guideline is more convenient. At present, the emission wavelength of semiconductor lasers is being shortened in the visible region from red to green and blue. Considering these, a sufficient standard can be obtained even if the Abbe number that is usually used is considered. Therefore, based on the case of the objective lens for the above video disc,

[0087]

(Equation 26)

It is understood that a glass material satisfying the above conditions may be used. If this is examined for glass materials often used in actual glass molds, for example, in the glass materials manufactured by "HOYA", BSC7 and FCD1 are glass materials with low dispersion and low refractive index that are often used as glass materials for aspherical glass mold lenses. However, the condition of the above [Formula 26] is not satisfied. B
Both aCD5 and BaCD16 are often used as a glass material for an objective lens, but they satisfy the condition of [Equation 26] above, but are based on the curvature of the objective lens for "Digital Video Disc (DVD)". If so, the condition that can be created is not satisfied. Therefore, it can be understood that these glass materials are not preferable materials, although it may be possible to form a lens of refraction type diffractive type. On the other hand, LaF20, LaF81, NbFD81,
Glass materials having a high refractive index and a high dispersion, such as FDS3, FD13, and LaC13, satisfy these conditions well. Many of these materials have excellent productivity and are ideal for mass production.
Since it has a lot of chromatic aberration, it has been used only for applications such as optical communication. Therefore, if these glass materials can be used for an objective lens for an optical disk or a magneto-optical disk by achromatic refraction-diffraction achromatism, productivity can be improved and a large effect can be expected from the viewpoint of mass production cost. Most plastic optical materials do not satisfy these conditions.

[3] Regarding the pitch of the grooves forming the diffractive lens, there is a problem in considering how much the numerical aperture (NA) of the lens can be increased by using such a glass material. What becomes is the pitch of the grooves forming the diffractive lens. If an attempt is made to increase the power of a diffractive lens, the pitch of the grooves will become narrower and the processing will be difficult for the same lens diameter.

The power of the diffractive lens is determined from the above-described achromatic condition by determining the numerical aperture and glass material of the entire lens. Therefore, even if the lenses have the same numerical aperture, the pitches of the diffractive lenses are different if the glass materials are different.
Therefore, the optimum glass material must be determined so that both the refraction-type aspherical lens and the diffractive-type lens can be processed.

Regarding the groove pitch, the following discussion is valid. First, consider the case where the diffractive lens is formed only on the first surface of the refractive lens as shown in FIG. Ignoring the aspherical term of the diffractive lens, as shown in FIG. 10 to FIG. 12 and FIG. The following estimation can be made by considering it as a diffractive optical element having a function of transmitting only the light of (4) and condensing the diffracted light at one point. As shown in FIG. 17, what is mainly considered in the present invention is a blazed phase type diffractive lens, and the phase type diffractive lens has the density type Fresnel zone plate shown in FIG. As shown in FIG. 15 and FIG. 16, expanded through a phase type Fresnel zone plate,
It is based on the same principle as the concentration-type Fresnel zone plate, and even if it is replaced in this way, the generality is not lost. The phase type Fresnel zone plate is
A function of converging the diffracted light at one point by setting every other Fresnel ring zone 8 and 9 as a region having a different thickness so that the phase of the transmitted light is different for each Fresnel ring zone 8 and 9. It is a diffractive optical element having Here, in particular, FIG.
As shown in 0, odd-numbered ring zones 7 that start in the central disk
What is made transparent is called a positive (concentration type) zone plate, and as shown in FIG. 11, what made even numbered ring zones 7 transparent is called a negative (concentration type) zone plate. These positive and negative (density type) zone plates function as an aplanatic convex lens and a concave lens, respectively. The outer radius s _n (n = 1, 2, 3, ...) Of the n-th transparent ring 7 of this concentration type Fresnel zone plate is given by the following equation.

[0092]

[Equation 27]

(In this [Equation 27], the above equation is for a positive zone plate, and the following equation is for a negative zone plate.) Also, the width of each ring zone is

[0094]

[Equation 28]

It is (In this [Equation 28], the above equation is for the positive zone plate and the following equation is for the negative zone plate.) FIG. 17 shows what is considered in the present invention. As described above, this is a phase-type diffractive convex lens, and this phase-type diffractive convex lens is a region corresponding to one phase period in which the transmission ring zone 7 and the light-shielding ring zone 6 in the positive (density type) zone plate are combined one by one. Then, it can be considered that a phase difference is imparted to the transmitted light due to the blazed (tilted groove) step. Therefore, in the region where the blaze pitch on the outer peripheral side is small, the blaze pitch ds forming the phase type diffractive lens is

[0096]

(Equation 29)

It is considered that The numerical aperture (NA) of only the diffractive lens, which is one of the refractive lens and the diffractive lens that constitute the achromatic lens, is 5% to 15% of the numerical aperture (NA) of all lenses for commonly used glass materials. is there. Therefore, in order to achieve the numerical aperture (NA) of 0.7, 0.035 to 0.105 of this 0.7 is the numerical aperture (NA) of only the diffractive lens. If the effective diameter of the lens is 5 mm, the focal length is about 25 mm.
That is to say 70 mm. The outermost peripheral portion with an effective diameter of 5 mm has a wavelength of 635 nm.

[0098]

[Equation 30]

Therefore, it corresponds approximately to the 71st to 197th ring zones. At this time, the pitch of the blaze of the outermost periphery is

[0100]

[Equation 31]

Is as follows. Even at the shortest pitch, it is about 10 times the wavelength, so the scalar theory is valid. When the blaze pitch is shorter than the shortest pitch, the diffraction efficiency changes depending on the polarization direction, so that skew noise has direction dependency, and noise easily occurs when recording / reproducing on a magneto-optical disk. There are many. Therefore, it is desirable that the blaze pitch is at least 5 μm, and practically 10 μm or more. From this, in the objective lens according to the present invention, it is desirable to use a means for relaxing (widening) the blaze pitch by some means. It can be seen that by relaxing (widening) the pitch of the blaze, it is possible to suppress the influence on the polarized light, and the objective lens can be sufficiently applied to the recording / reproducing with respect to the magneto-optical disk. Also, from the viewpoint of the current processing technology, this pitch is quite narrow as the pitch of the grid to be processed on the curved surface. Therefore, if this pitch can be relaxed, it will be a great advantage in manufacturing.

In order to relax the manufacturing conditions of this diffractive lens, it is possible to disperse the power by disposing diffractive lenses 4 and 5 on both surfaces 2 and 3 of the refractive lens 1 as shown in FIG. It is valid. Ideally, the focal lengths of the diffractive lenses 4 and 5 on both sides are both twice as long as the diffractive lens 4 in the case where the diffractive lens is formed only on the first surface as shown in FIG. Is a solution that maximizes the pitch of the blaze. At this time, if the diameter of the lens is constant, the number of ring zones is about half that in the case where the diffractive lens is formed only on the first surface according to the above [Formula 27]. From this fact and the above [Formula 29], the shortest pitch of the blaze is about twice as large as the case where the diffractive lens is formed only on the first surface. However, in actual design, so-called bending is necessary from the viewpoint of aberration correction, so if the ratio of the focal lengths of the diffractive lenses on both sides is in the range of 0.5 to 2.0, it is sufficient.
The effect of dividing the power of the diffractive lens on both sides can be obtained. In this case, this objective lens has a diffractive lens on both sides, and it seems that the diffraction efficiency of the diffractive lens becomes a problem, but at present, the diffractive lens has a sufficiently high design wavelength (90 There is no problem because it is said that a diffraction efficiency of (% or more) can be obtained. Further, in recent years, the output of a semiconductor laser serving as a light source has been improved, and the efficiency of light output can be sufficiently compensated by a method such as employing a polarization optical system using a polarization beam splitter.

The diffraction efficiency of the diffraction grating depends on the angle between the grating direction and the polarization direction. Further, when the angle of incidence on the optical disc or the angle of incidence on the lens is large, the reflectance differs depending on the polarization direction. For this reason, when a linearly polarized light beam is incident on the diffractive lens, the intensity distribution on the pupil becomes non-uniform, and the spot may become asymmetric. Such nonuniformity of the intensity distribution on the pupil affects the recording / reproducing characteristics when recording / reproducing information signals to / from the magneto-optical disk. In order to avoid such an influence, the linearly polarized light beam from the light source such as the semiconductor laser incident on the diffractive lens may be circularly polarized in advance. To make linearly polarized light into circularly polarized light,
It suffices that a retarder such as a quarter-wave plate is transmitted.

Further, in the objective lens according to the present invention, there can be considered some methods of expanding the minimum pitch of the blaze while keeping the achromatic condition by keeping the power of the diffractive lens constant. That is, as a method of relaxing the pitch of the blaze, firstly, it is considered to utilize the aspherical coefficient of the diffractive lens. As shown in FIG. 5, if a higher-order aspherical coefficient with an opposite sign is added so that the inclination of the phase difference due to the diffractive lens at the outermost circumference of the lens becomes gentle, this aspherical coefficient becomes Since it is not related to the chromatic aberration of
As shown in FIG. 4, the blaze pitch can be loosened as compared with the case where the aspherical coefficient of the diffractive lens is not utilized. In this case, the diffractive lens has a high-order aspherical phase term, and the minimum pitch position of the blaze is located inside the lens, that is, between the outermost circumference of the lens and the center of the lens. Such an aspherical coefficient may be repeated by the aspherical coefficient of the refractive lens having the same surface. However, in this case, high-order chromatic aberration occurs, and it is necessary to keep this within a sufficiently allowable range.

Secondly, as shown in FIGS. 6 to 9, the depth of each groove of the phase type diffractive lens is set to a depth such that an optical path difference that is an integral multiple of the wavelength is generated. It is known that the pitch of the blaze can be widened (1961, Journal of Optical Society of America, "The Phase Fresnel.
Lens "(K. Miyamoto," The Phase Fresnel Lens ", Jou
rnal of the OpticalSociety of America, vol.51, No.
1, Jan.1961, p.17)). However, in this case, if the depth of each groove is deepened, it finally returns to the refraction type Fresnel lens. Further, if the depth of each groove is increased, the sensitivity of the change of the diffraction efficiency to the wavelength fluctuation becomes high. In addition, the manufacturing error of the depth of each groove becomes large in the manufacturing. From this point of view, it is practically desirable that the depth of each groove is not more than about 10 times the wavelength (in FIG. 6, the depth of each groove of the phase type diffractive lens is 1 wavelength). 7 shows the case where the depth is such that the optical path difference is doubled.Similarly, in Fig. 7, the depth of each groove of the phase type diffractive lens is such that the optical path difference is twice the wavelength. 8 shows a case where the depth of each groove of the phase type diffractive lens is such that an optical path difference of 3 times the wavelength occurs, and FIG. 9 shows a case of the phase diffractive lens. The case where the depth of each groove is such that an optical path difference of four times the wavelength occurs is shown).

[4] Constitution of Optical Pickup Device By using the objective lens as described above, an optical pickup device for recording / reproducing information on / from an optical disc on which information signals are recorded at high density can be realized. Here, a problem is coma aberration caused by the tilt of the optical disc. The coma aberration generated by the disc skew at the time of reproducing the optical disc increases due to the shortening of the wavelength and the increase of the NA, and becomes a major noise factor. In increasing the density of recorded information signals on an optical disc, coma aberration has recently attracted attention as a major problem of optical systems. The third and fifth coma aberrations are
Proportional to disk skew. Disk skew θ (ra
3rd and 5th coma aberration coefficients (W ₃₁ , W
₅₁ ) is

[0107]

(Equation 32)

Is given by In this [Formula 32], t, N, and NA represent the following.

T: Thickness of disc substrate N: Refractive index of disc substrate NA; Objective lens numerical aperture For coma aberration caused by tilt of the optical disc,
There are two possible solutions. The first solution is active deskew (see "Fast
Disk skew servo for optical
"Disk Pickup"("Fast Disk Skew Servo fo
r Optical Disk Pickup "N. Eguchi et al., Techinical
Digest of SOM'94 (1994) P.83) and 1995, 42nd Spring Society of Applied Physics Proceedings 29a-T-3, 4 "Tilt correction by collimating optics" Makoto Itonaga, Yuichi Hasegawa, Takayuki Yoshida. , Kunihisa Matsuzaki, Yasuo Haji). Such skew correction is particularly effective when the thickness of the disc substrate is equal to or larger than that of the "digital video disc (DVD)".

The principle of skew correction here is as follows, for example. That is, as shown in FIG. 23, two compensating optical elements (skew plate (convex) 12 and skew plate (concave) 13) are arranged in an optical system having coma, and they are arranged in opposite directions. The aberration is corrected by moving it to generate coma with positive and negative signs. The optical system converts the light beam emitted by the semiconductor laser 19 into a parallel light beam by the collimator lens 18, reflects the parallel light beam by the beam splitter 15, and the quarter wavelength plate 14 and the two compensating optical elements 13, It is an optical system which is made incident on the objective lens (hybrid lens) 1 via 12. The objective lens 1 focuses the incident light flux on the signal recording layer of the optical disc 23. The optical disk 23 has a central portion supported by a drive shaft 24 of a spindle motor 22 and is rotated by the spindle motor 22. The light flux is the optical disc 2
It is reflected by the signal recording layer 3 and returns to the beam splitter 15 through the objective lens 1, the compensation optical elements 12 and 13, and the quarter wavelength plate 14. Here, the light flux reflected by the signal recording layer of the optical disc 23 passes through the beam splitter 15, and is received by a detector (photodetector) 17 via a detection system lens 16. The skew plates 12 and 13 are
Based on the detection result of the skew sensor 20 that detects the skew of the optical disc 23 with respect to the optical axis of the objective lens 1, the optical disc 23 is moved through the servo circuit 21 in a direction orthogonal to the optical axis of the optical system. The skew sensor 20 includes, for example, a light emitting element such as a light emitting diode that irradiates the optical disc 23 with a light flux, and a photodetector that detects reflected light of the light flux. As shown, the amount of tilt of the optical disk 23 in the radial direction is detected.

The coma aberration mentioned here includes up to the fifth order, and its azimuth does not matter. The two skew plates (compensation optical elements) 12 and 13 have convex and concave aspherical surfaces given by ± αR ⁴ (R is a normalized pupil radius and α is a fourth-order aspherical surface coefficient), respectively. An element having the same or an element having a wavefront conversion function equivalent thereto. For example, this aspherical shape can be approximated by a quantized step shape. It is also possible to make it by combining a gradient index lens and spherical polishing. The optimum movement operation for these skew plates 12 and 13 is
With respect to the azimuth of the generated coma, the two skew plates 12 and 13 are set in the opposite directions so that the optical disc 23
It is given by moving in proportion to the skew angle of.
By arranging such a correction optical element in the optical path of the optical pickup device, it is possible to follow the correction of coma aberration caused by the rotational shake or warpage of the optical disc 23 up to a high band. That is, the servo circuit 2
1 calculates the amount of coma aberration that occurs based on the detection result obtained by the skew sensor 20, and moves the skew plates 12 and 13 in the optimum direction by a moving drive mechanism (not shown) according to the calculation result. Move the optimal distance.

Another solution is to thin the disk substrate. That is, as for the coma aberration generated in the disc substrate, the generation amount due to the skew is reduced as the disc substrate is made thinner. By combining these solutions, an optical pickup device using an objective lens having a high numerical aperture (NA) can be constructed.

[5] Application to solid immersion lens As described above, a lens with a high numerical aperture of 0.7 or more and an achromatic lens can be realized by combining a plurality of lenses. . However, it has not been possible to realize these simultaneously as a single lens. Recently, in order to solve this problem, a hemispherical lens (front lens) has a flat surface portion closely attached to an optical disk, and an aspherical lens (rear lens) is arranged behind the hemispherical lens to provide a microscope. According to the same principle as the immersion lens of, only the refractive index of the hemispherical lens (that is, about 1.5 times)
A solid immersion lens having an increased numerical aperture (NA) has been proposed. However, even in this solid immersion lens, the chromatic aberration at the time of mode hopping in the semiconductor laser as the light source becomes a problem. Further, it is advantageous in terms of tolerance that the numerical aperture (NA) of the rear lens is increased and the refractive index of the front lens is lowered. Therefore, by using the refraction-type diffractive integrated objective lens according to the present invention as the rear lens, it is possible to configure a solid immersion lens having a high numerical aperture.

[6] Recording / reproducing to / from optical disks of different specifications Recording / reproducing to / from optical disks of different specifications by using a plurality of diffracted lights generated by one surface of the refraction / diffraction integral objective lens according to the present invention. You can That is, in optical discs having different specifications and having different thicknesses, it is necessary to correct the spherical aberration with respect to the thickness of each disc substrate. Further, in an optical disc having different specifications adapted to an objective lens having a different numerical aperture, it is necessary to limit the effective diameter of the objective lens according to the numerical aperture. In the objective lens according to the present invention, the first surface of the diffractive lens is corrected by correcting the spherical aberration according to the difference of the disc substrate and adjusting the numerical aperture of the objective lens to the optical disc of each specification. This can be performed by properly using the 0th-order light and the 1st-order light or the ± 1st-order light generated by.

[0115]

EXAMPLES Specific examples of the objective lens according to the present invention will be described below in detail with reference to the drawings. In the following description, regarding the shape of the lens,
It is defined according to the optical design software “CODEV”. The aspherical shapes are x, h, c, k, A, B,
When C and D are defined as follows, x: distance from the tangent plane of the aspherical vertex of the point on the aspherical surface whose height from the optical axis is h: h: height from the optical axis c: non Curvature of spherical apex (= 1 / R) k: Conical constant A: Fourth-order aspherical coefficient B: Sixth-order aspherical coefficient C: Eighth-order aspherical coefficient D: Tenth-order aspherical coefficient And when

[0116]

[Expression 33]

Is represented by Further, the diffractive lens is classified into an amplitude type and a phase type, but what is considered in the present invention is a blazed type, as shown in FIG. is there. This is the same as with general holograms, using the polar coordinate polynomial on the substrate as the coefficient of the aspherical phase shift on each surface when the two point light sources at the time of manufacture are at infinity. specify. Here, the coefficient of the polynomial is the optical path difference (OP
D) is given in mm. That is, the optical path difference due to diffraction at the point of height R from the optical axis on the surface of the diffractive lens is defined by the following equation. That is,

[0118]

(Equation 34)

It is The actual shape changes intermittently as shown in FIG. 19 to cause diffraction. That is, since the optical path difference generated between the optical path in the medium having the refractive index n and the optical path in the air is given by t (n-1), the step d of each ring zone of the diffractive lens has a diffraction reference wavelength of λ nm. When this is done, d of the following equation or an integral multiple thereof.

[0120]

(Equation 35)

In other words, the surface shape is provided with a depth that results from the remainder obtained by dividing the optical path difference OPD by the wavelength λ as the optical path difference.

[Example 1] BaCD16 ("HOYA")
Example of design using (manufactured by Mfg. Co., Ltd.) The objective lens according to the present invention can be designed using a glass material that is commonly classified as SK. As an example, BaCD
Design examples using 16 (manufactured by "HOYA") are shown in [Table 2] and [Table 3] below. In addition, the shape of the lens is shown in FIG.
Shows the spherical aberration (LONGITUDINAL SPHERICAL ABER: 6
15nm, 635nm, 655nm) and astigmatism (AS
FIG. 25 shows a TIGMATIC FIELD CURVES), and FIG. 26 shows a lateral aberration diagram. Note that, in FIG. 25 and the following description, the graph of spherical aberration also shows the state of chromatic aberration (due to the difference in graph for each wavelength (615 nm, 635 nm, 655 nm)).

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0126]

[Table 2]

[0127]

[Table 3]

[Example 2] Design example using BaCD5 (manufactured by "HOYA") A lens according to the present invention can be designed using a glass material commonly called SK. As an example, BaCD5
A design example using (manufactured by “HOYA”) is shown in [Table 4] below.
And [Table 5]. In addition, the shape of the lens is shown in FIG. 27, and the spherical aberration (LONGITUDINAL SPHERICAL ABER: 615
nm, 635 nm, 655 nm) and astigmatism (ASTIGM
Fig. 28 shows the ATIC FIELD CURVES) and Fig. 2 shows the lateral aberration diagram.
9 shows.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0132]

[Table 4]

[0133]

[Table 5]

[Example 3] Design example using LaC13 (manufactured by "HOYA") A lens according to the present invention can be designed using a glass material generally classified as LaK. As an example, LaC13
A design example using (manufactured by "HOYA") is shown in [Table 6] below.
And [Table 7]. The shape of the lens is shown in FIG. 30, and the spherical aberration (LONGITUDINAL SPHERICAL ABER: 615
nm, 635 nm, 655 nm) and astigmatism (ASTIGM
Fig. 31 shows the ATIC FIELD CURVES) and Fig. 3 shows the lateral aberration diagram.
It is shown in FIG. The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0137]

[Table 6]

[0138]

[Table 7]

[Example 4] Design example using FD13 (manufactured by "HOYA") A lens according to the present invention can be designed using a glass material generally classified as SF. As an example, FD13
A design example using (manufactured by "HOYA") is shown in [Table 8] below.
And [Table 9]. In addition, the shape of the lens is shown in FIG. 33, and the spherical aberration (LONGITUDINAL SPHERICAL ABER: 615
nm, 635 nm, 655 nm) and astigmatism (ASTIGM
Fig. 34 shows the ATIC FIELD CURVES) and Fig. 3 shows the lateral aberration diagram.
It is shown in FIG.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0143]

[Table 8]

[0144]

[Table 9]

[Example 5] Design example using FDS3 (manufactured by "HOYA") A lens according to the present invention can be designed using a glass material generally classified as SFS. FDS3 as an example
A design example using (manufactured by “HOYA”) is shown in the following [Table 1
0] and [Table 11]. Moreover, the shape of the lens is shown in FIG.
6 shows the spherical aberration (LONGITUDINAL SPHERICAL ABER:
615 nm, 635 nm, 655 nm) and astigmatism (ASTIGMATIC FIELD CURVES) are shown in FIG. 37, and a lateral aberration diagram is shown in FIG. 38. The diffraction reference wavelength is 635 nm. Design wavelength is 635 nm (615 to 655 nm)
It is. The numerical aperture is 0.7.

[0146]

[Table 10]

[0147]

[Table 11]

Example 6 LaF20 (“SCHOT
Example of design using "T" company) The lens according to the present invention can be designed using a glass material that is commonly classified as LaF. As an example, LaF20
Design examples using (manufactured by "SCHOTT") are shown in [Table 12] and [Table 13] below. In addition, the shape of the lens is shown in FIG. 39, and the spherical aberration (LONGITUDINAL S
PHERICAL ABER: 615nm, 635n
m, 655 nm) and astigmatism (ASTIGMATIC FIELD CUR
VES) is shown in FIG. 40, and a lateral aberration diagram is shown in FIG.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0152]

[Table 12]

[0153]

[Table 13]

Example 7 Design Example Using LaF81 (manufactured by “HOYA”) A lens according to the present invention can be designed by using a glass material commonly classified as LaF. As an example, LaF81
A design example using (manufactured by “HOYA”) is shown in the following [Table 1
4] and [Table 15]. Also, the shape of the lens is shown in FIG.
2 shows the spherical aberration (LONGITUDINAL SPHERICAL ABER:
615 nm, 635 nm, 655 nm) and astigmatism (ASTIGMATIC FIELD CURVES) are shown in FIG. 43, and a lateral aberration diagram is shown in FIG.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0158]

[Table 14]

[0159]

[Table 15]

[Embodiment 8] NbFD82 ("HOYA")
Example of design using (made by a company) The lens according to the present invention can be designed by using a glass material that is generally classified as NbSF. As an example, NbFD
Design examples using 82 (manufactured by "HOYA") are shown in [Table 16] and [Table 17] below. The shape of the lens is shown in Fig. 45, and the spherical aberration (LONGITUDINAL SPHERICAL ABE
R: 615 nm, 635 nm, 655 nm) and astigmatism (ASTIGMATIC FIELD CURVES) are shown in FIG. 46, and a lateral aberration diagram is shown in FIG. 47.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.7.

[0164]

[Table 16]

[0165]

[Table 17]

[Embodiment 9] Design Example in which Numerical Aperture is Raised to 0.9 So far, the design example of the objective lens having a numerical aperture of about 0.7 has been shown. Designs of a degree are possible in the objective according to the invention. An example of this design is shown in [Table 18] and [Table 19] below. The lens shape is shown in Fig. 48, and spherical aberration (LONGITUD
INAL SPHERICAL ABER: 615nm, 635nm, 65
5 nm) and astigmatism (ASTIGMATIC FIELD CURVES) are shown in FIG. 49, and a lateral aberration diagram is shown in FIG.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (615 to 65).
5 nm).

The numerical aperture is 0.9.

[0170]

[Table 18]

[0171]

[Table 19]

Example 10 Design Example of Objective Lens for Blue Semiconductor Laser such as GaN In recent years, a short wavelength semiconductor laser has been developed. The present invention can provide an objective lens that can achieve higher density recording by combining with these short wavelength light sources. Recently, 410 using GaN
Room temperature oscillations of semiconductor lasers of nm have been reported. Design examples when this semiconductor laser is used as a light source are shown in [Table 20] and [Table 21] below. In addition, the shape of the lens is shown in Fig. 51, and spherical aberration (LONGITUDINAL SPHERICAL A
BER: 400 nm, 410 nm, 420 nm) and astigmatism (ASTIGMATIC FIELD CURVES) are shown in FIG. 52, and a lateral aberration diagram is shown in FIG.

The diffraction reference wavelength is 410 nm.

The design wavelength is 410 nm (430 to 41).
0 nm).

The numerical aperture is 0.75.

[0176]

[Table 20]

[0177]

[Table 21]

Example 11 Design Example of Objective Lens for Green Semiconductor Laser such as ZnSe It has been reported that a room temperature oscillation of a semiconductor laser having an emission wavelength of 515 nm using ZnSe is carried out. Design examples using this semiconductor laser as a light source are shown in [Table 22] and [Table 2] below.
3]. Also, the shape of the lens is shown in FIG. 54, and spherical aberration (LONGITUDINAL SPHERICAL ABER: 505 nm, 5
15nm, 525nm) and astigmatism (ASTIGMATIC FIE
LD CURVES) is shown in FIG. 55, and a lateral aberration diagram is shown in FIG.

The diffraction reference wavelength is 515 nm.

The design wavelength is 515 nm.

The numerical aperture is 0.75.

[0182]

[Table 22]

[0183]

[Table 23]

Example 12 Design Example of High Numerical Aperture Bifocal Lens Using 0th-Order Light and 1st-Order Light In the objective lens according to the present invention, a plurality of diffractive lenses are formed by the first surface. Each of the diffracted lights can be used for recording / reproducing with respect to optical disks having different specifications. For example, the 0th-order light and the 1st-order light generated by the first surface of the diffractive lens can be used for recording / reproduction of an optical disc having the following format. Design examples in this case are shown in [Table 24], [Table 25] and [Table 26] below. The shape of the lens is shown in FIGS. 57 (position 1) and 61 (position 2), and the transfer function (modulation transfer function: MTF) of this objective lens is shown in FIG. 58 (position 1).
62 (position 2), the spherical aberration (LONGIT
UDINAL SPHERICAL ABER: 625nm, 635nm, 6
45nm) and astigmatism (ASTIGMATIC FIELD CURVES)
Fig. 59 (position 1) and Fig. 63 (position 2)
FIG. 60 shows the lateral aberration diagram in FIG. 60 (position 1) and FIG.
Shown in (position 2).

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (625 nm to 645 nm).

The numerical apertures are 0.75 and 0.45.

[0188]

[Table 24]

[0189]

[Table 25]

[0190]

[Table 26]

Example 13 High NA Double Focus Lens Design Example Using ± 1st Order Light In the objective lens according to the present invention, each of a plurality of diffracted lights generated by the first surface of the diffractive lens is used. ,
It can be used for recording and reproducing optical discs of different specifications. For example, the ± first-order light generated by the first surface of the diffractive lens can be used for recording / reproduction of an optical disc having the following format. Design examples in this case are shown in [Table 27], [Table 28] and [Table 2] below.
9]. Also, the shape of the lens is shown in FIG. 65 (position 1) and FIG. 68 (position 2), and the spherical aberration (LO
NGITUDINAL SPHERICAL ABER: 625nm, 635n
m, 645 nm) and astigmatism (ASTIGMATIC FIELD CUR
66 (position 1) and FIG. 69 (position 2), and lateral aberration diagrams are shown in FIG. 67 (position 1) and FIG. 70 (position 2).

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm (625 nm to 645 nm).

The numerical apertures are 0.75 and 0.45.

[0195]

[Table 27]

[0196]

[Table 28]

[0197]

[Table 29]

Example 14 Design Example in which the Refractive Diffractive Integrated Lens According to the Present Invention is Used as a Rear Lens of a Solid Liquid Immersion Lens A flat portion of a hemispherical lens (front lens) is formed on an optical disc. Adhere closely, and place an aspherical lens on the rear side of this hemispherical lens, and by the same principle as the liquid immersion lens of a microscope, open by the refractive index of the hemispherical lens (about 1.5 times). A solid immersion lens that raises the number (NA) has been proposed.
Here again, chromatic aberration caused by mode hopping in a semiconductor laser as a light source is a problem. Further, the higher power of the rear lens is advantageous in terms of design tolerance. Therefore, it is effective to use the refractive diffractive integrated lens according to the present invention as the aspherical lens (rear lens). Design examples in this case are shown in [Table 30] and [Table 31] below. Also, the lens shape is shown in FIG. 71, and spherical aberration (LONGITUD
INAL SPHERICAL ABER) and astigmatism (ASTIGMATIC FIEL
D CURVES) is shown in FIG. 72, and a lateral aberration diagram is shown in FIG. 73.

The diffraction reference wavelength is 635 nm.

The design wavelength is 635 nm.

The numerical aperture is 1.0.

[0202]

[Table 30]

[0203]

[Table 31]

[0204]

As described above, the objective lens according to the present invention is a single lens in which both the first surface and the second surface are aspherical surfaces, and the phase type diffractive lens is provided on each of the both surfaces. Since it has the numerical aperture, it is a refraction / diffraction integral type objective lens in which the chromatic aberration is corrected.

Therefore, when this objective lens is applied to an optical pickup device for recording / reproducing information signals on / from an optical disc, it is possible to achieve high resolution and reduce the influence of mode hop noise due to chromatic aberration correction. The information recording can be realized.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the shape of a double-sided aspherical refractive lens.

FIG. 2 is a vertical cross-sectional view showing the shape of a double-sided aspherical refractive lens in which a diffractive lens is integrally formed on one surface.

FIG. 3 is a vertical cross-sectional view showing the shape of an objective lens according to the present invention, which is a refraction type lens of double-sided aspherical surface in which diffractive lenses are integrally formed on both sides.

FIG. 4 is a graph showing a wavefront phase according to a spherical coefficient of a diffractive lens.

FIG. 5 is a graph showing relaxation of pitch due to an aspherical coefficient of a diffractive lens.

FIG. 6 is an enlarged longitudinal sectional view of an essential part showing a sectional shape of a diffractive lens using first-order diffracted light.

FIG. 7 is an enlarged vertical cross-sectional view of a main part showing a cross-sectional shape of a diffractive lens using second-order diffracted light.

FIG. 8 is an enlarged vertical sectional view of an essential part showing a sectional shape of a diffractive lens using third-order diffracted light.

FIG. 9 is an enlarged longitudinal sectional view of an essential part showing a sectional shape of a diffractive lens using fourth-order diffracted light.

FIG. 10 is a front view showing the structure of a positive zone plate.

FIG. 11 is a front view showing the configuration of a negative zone plate.

FIG. 12 is a front view showing the structure of a concentration type zone plate.

FIG. 13 is a front view showing a configuration of a phase type zone plate.

FIG. 14 is a side view showing an image formation state in the density type zone plate.

FIG. 15 is a side view showing an image formation state in the phase type zone plate.

FIG. 16 is an enlarged vertical sectional view showing the shape of a phase type Fresnel zone plate.

FIG. 17 is an enlarged vertical sectional view showing the shape of a blazed phase-type diffractive lens.

FIG. 18 is a graph showing the relationship between the shape (first shape) of the diffractive lens and the transmitted wavefront.

FIG. 19 is a graph showing the relationship between the shape (second shape) of the diffractive lens and the transmitted wavefront.

FIG. 20 is a side view of a lens for explaining a sine condition of an afocal system.

FIG. 21 is an enlarged longitudinal cross-sectional view of a main part of the lens for showing the function of the diffractive lens.

FIG. 22 is a vertical cross-sectional view of a lens for showing the function of the diffractive lens.

FIG. 23 is a side view showing the configuration of the optical pickup device according to the present invention, which is configured using the objective lens according to the present invention.

FIG. 24 is a side view showing the shape of the objective lens according to Example 1 of the present invention.

FIG. 25 is a graph showing spherical aberration and astigmatism in the first example of the objective lens according to the present invention.

FIG. 26 is a graph showing lateral aberration in the first example of the objective lens according to the present invention.

FIG. 27 is a side view showing the shape of the objective lens according to Example 2 of the present invention.

FIG. 28 is a graph showing spherical aberration and astigmatism in the second example of the objective lens according to the present invention.

FIG. 29 is a graph showing lateral aberration in the second example of the objective lens according to the present invention.

FIG. 30 is a side view showing the shape of the objective lens according to the third example of the present invention.

FIG. 31 is a graph showing spherical aberration and astigmatism in the third example of the objective lens according to the present invention.

FIG. 32 is a graph showing lateral aberration in the third example of the objective lens according to the present invention.

FIG. 33 is a side view showing the shape of the objective lens according to Example 4 of the present invention.

FIG. 34 is a graph showing spherical aberration and astigmatism in the fourth example of the objective lens according to the present invention.

FIG. 35 is a graph showing lateral aberration in the fourth example of the objective lens according to the present invention.

FIG. 36 is a side view showing the shape of the objective lens according to the fifth example of the present invention.

FIG. 37 is a graph showing spherical aberration and astigmatism in the fifth example of the objective lens according to the present invention.

FIG. 38 is a graph showing lateral aberration in the fifth example of the objective lens according to the present invention.

FIG. 39 is a side view showing the shape of the objective lens according to the sixth example of the present invention.

FIG. 40 is a graph showing spherical aberration and astigmatism in the sixth example of the objective lens according to the present invention.

FIG. 41 is a graph showing lateral aberration in the sixth example of the objective lens according to the present invention.

FIG. 42 is a side view showing the shape of the objective lens according to the seventh example of the present invention.

FIG. 43 is a graph showing spherical aberration and astigmatism in the seventh example of the objective lens according to the present invention.

FIG. 44 is a graph showing lateral aberration in the seventh example of the objective lens according to the present invention.

FIG. 45 is a side view showing the shape of the objective lens according to Example 8 of the present invention.

FIG. 46 is a graph showing spherical aberration and astigmatism in the eighth example of the objective lens according to the present invention.

FIG. 47 is a graph showing transverse aberration in the eighth example of the objective lens according to the present invention.

FIG. 48 is a side view showing the shape of the objective lens according to Example 9 of the present invention.

FIG. 49 is a graph showing spherical aberration and astigmatism in a ninth example of the objective lens according to the present invention.

FIG. 50 is a graph showing lateral aberration in the ninth example of the objective lens according to the present invention.

FIG. 51 is a side view showing the shape of the objective lens according to the tenth embodiment of the present invention.

52 is a graph showing spherical aberration and astigmatism in the tenth example of the objective lens according to the present invention. FIG.

FIG. 53 is a graph showing lateral aberration in the tenth example of the objective lens according to the present invention.

FIG. 54 is a side view showing the shape of an objective lens according to Example 11 of the present invention.

FIG. 55 is a graph showing spherical aberration and astigmatism in the eleventh example of the objective lens according to the present invention.

FIG. 56 is a graph showing lateral aberration in the eleventh example of the objective lens according to the present invention.

FIG. 57 is a side view showing the shape of an objective lens according to a twelfth embodiment (position 1) of the present invention.

FIG. 58 is a graph showing a transfer function (modulation transfer function: MTF) in the twelfth embodiment (position 1) of the objective lens according to the present invention.

FIG. 59 is a graph showing spherical aberration and astigmatism in the twelfth embodiment (position 1) of the objective lens according to the present invention.

FIG. 60 is a graph showing lateral aberration in the twelfth embodiment (position 1) of the objective lens according to the present invention.

FIG. 61 is a side view showing the shape of an objective lens according to a twelfth embodiment (position 2) of the present invention.

FIG. 62 is a graph showing a transfer function (modulation transfer function: MTF) in a twelfth embodiment (position 2) of the objective lens according to the present invention.

FIG. 63 is a graph showing spherical aberration and astigmatism in the 12th example (position 2) of the objective lens according to the present invention.

FIG. 64 is a graph showing lateral aberration in the twelfth embodiment (position 2) of the objective lens according to the present invention.

FIG. 65 is a side view showing the shape of the objective lens according to the thirteenth embodiment (position 1) of the present invention.

66 is a graph showing spherical aberration and astigmatism in the thirteenth embodiment (position 1) of the objective lens according to the present invention. FIG.

67 is a graph showing lateral aberration in the thirteenth embodiment (position 1) of the objective lens according to the present invention. FIG.

FIG. 68 is a side view showing the shape of the objective lens according to the thirteenth embodiment (position 2) of the present invention.

69 is a graph showing spherical aberration and astigmatism in the thirteenth embodiment (position 2) of the objective lens according to the present invention. FIG.

70 is a graph showing lateral aberration in the thirteenth embodiment (position 2) of the objective lens according to the present invention. FIG.

71 is a side view showing the shape of the objective lens according to the fourteenth embodiment of the present invention. FIG.

72 is a graph showing spherical aberration and astigmatism in the fourteenth example of the objective lens according to the present invention. FIG.

FIG. 73 is a graph showing lateral aberration in the fourteenth example of the objective lens according to the present invention.

[Explanation of symbols]

1 double-sided aspherical lens, 2 1st surface, 3 2nd surface, 4,
5 diffractive lenses, 12 skew plates (convex), 1
3 Skew plate (concave), 17 Detector, 19
Semiconductor laser, 20 skew sensor, 21 servo circuit, 23 optical disk

Claims (9)

[Claims] 1. A single lens having aspherical surfaces on both the first and second surfaces, each having a phase type diffractive lens on each surface, and having a center wavelength of λ _{2 and} a shortest wavelength λ ₁ To the longest wavelength λ ₃ , the refractive index of the glass material forming this lens at the respective wavelengths λ ₁ , λ ₂ and λ ₃ is n ₁ , n ₂ and n ₃ , respectively. Abbe number V of the glass material is V
= (N ₂ −1) / (n ₁ −n ₃ ), if the Abbe number V _HOE of the above phase type diffraction lens is V _HOE = λ ₂ / (λ ₁ −λ ₃ ), then And the chromatic aberration is corrected and the numerical aperture is 0.7 or more. 2. The objective lens according to claim 1, wherein the chromatic aberration is excessively corrected by the phase type diffractive lens so that the focal length becomes short for a short wavelength. 3. The phase type diffractive lens has an aspherical phase term, and the position of the minimum pitch portion of the phase diffractive lens is located between the center and the peripheral edge of the lens. Objective lens. 4. The objective lens according to claim 1, wherein the depth of each groove forming the phase type diffractive lens is such that an optical path difference that is an integral multiple of 10 or less with respect to the wavelength occurs. 5. A plurality of diffracted lights generated by a phase type diffractive lens formed on the first surface or the second surface are used for recording / reproducing with respect to optical recording media having different specifications. The objective lens according to claim 1. 6. The objective lens according to claim 1, which is combined with a hemispherical lens to form a solid immersion type objective lens. 7. A light source, an objective lens for condensing a light beam emitted from the light source on a signal recording surface of an optical recording medium, and a light detecting means for detecting a reflected light beam of the light beam by the optical recording medium. The objective lens is a single-lens lens in which both the first surface and the second surface are aspherical surfaces, and phase-diffractive lenses are provided on both surfaces, and the shortest wavelength λ with a center wavelength of λ _{2. When used from 1 to the} longest wavelength λ ₃ , the refractive index of the glass material forming this lens at the respective wavelengths λ ₁ , λ ₂ and λ ₃ is n ₁ , n ₂ and n ₃ , respectively. Abbe number V of the glass material is V = (n ₂ −1) / (n ₁ −n ₃ ), and Abbe number V _HOE of the phase type diffraction lens is V _HOE = λ ₂ /
If (λ ₁ −λ ₃ ), then (1+ (V _HOE / V)) · (n ₂ −1)> 0.572 is established, the chromatic aberration is corrected, and the numerical aperture is 0. An optical pick-up device that is 7 or more. 8. The optical pickup device according to claim 7, further comprising a skew servo mechanism for correcting coma aberration caused by the fact that the optical axis of the objective lens and the signal recording surface of the optical recording medium are not perpendicular. 9. An optical recording medium having a substrate made of a transparent material and having a thickness of 0.6 mm or less is used as the optical recording medium, and a light flux is transmitted through the substrate to a signal recording surface. The optical pickup device according to claim 7, which is configured to collect light.