JP2006059517A - Compound optical device and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compound optical device which is inexpensive and permits reduction in man-hours of adjustment work at the time of assembly, and to provide an optical pickup having this compound optical device. <P>SOLUTION: The compound optical device of this invention is used for the optical pickup device having a resin layer provided with a optical path difference-making structure on at least one optical surface of an aspheric lens. Then, a ratio of an optical path length L' when an incident luminous flux passes through the resin layer at the end of an effective diameter corresponding to a required numerical aperture to an optical path length L in the optical axis when the incident luminous flux passes through the resin layer satisfies a formula (1): 0.8≤(L'/L)≤1.2. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a composite optical element and an optical pickup device used in an optical pickup device.

In recent years, in an optical pickup device, a laser light source used as a light source for reproducing information recorded on an optical disc and recording information on the optical disc has been shortened. For example, a blue-violet semiconductor laser, A laser light source having a wavelength of 405 nm such as a blue-violet SHG laser that performs wavelength conversion of an infrared semiconductor laser using harmonic generation is being put into practical use.
When these blue-violet laser light sources are used, when an objective lens having the same numerical aperture (NA) as that of a digital versatile disk (hereinafter abbreviated as DVD) is used, information of 15 to 20 GB is recorded on an optical disk having a diameter of 12 cm. Recording becomes possible, and when the NA of the objective lens is increased to 0.85, 23 to 27 GB of information can be recorded on an optical disk having a diameter of 12 cm. Hereinafter, in this specification, an optical disk and a magneto-optical disk using a blue-violet laser light source are collectively referred to as a “high density optical disk”.

  By the way, two standards are currently proposed as high-density optical disks. One is a Blu-ray disc (hereinafter abbreviated as BD) with an NA0.85 objective lens and a protective layer thickness of 0.1 mm, and the other is an NA0.65 to 0.67 objective lens. This is an HD DVD (hereinafter abbreviated as HD) having a protective layer thickness of 0.6 mm. In view of the possibility that high density optical discs of these two standards will be distributed in the market in the future, compatible optical pickup devices capable of recording / reproducing existing DVDs on any high density optical disc Is important.

As an objective lens that requires a high numerical aperture for a high-density optical disk, for example, a glass-molded aspheric lens combined with a plastic optical element having a diffractive structure to form a two-lens structure is known. . The reason for the two-lens configuration is that, in the case of an aspherical lens having a single glass mold configuration, correction of chromatic aberration due to abrupt wavelength fluctuation (mode hop) of the light source is not sufficient.
The plastic optical element in this configuration is intended to correct chromatic aberration due to wavelength fluctuation of about several nanometers using a diffractive structure and to be compatible with DVD / CD (compact disc) having a different operating wavelength. Such a combination lens composed of at least two types of optical elements is difficult to use in a thin optical pickup device due to an increase in man-hours required for assembly adjustment and an increase in thickness in the optical axis direction. In consideration, it is desirable that the number of objective lenses is one.

By the way, as an application compatible with DVD / CD, an aspheric objective lens having a diffractive structure on the surface and plastic injection molding is known, but an objective lens having a high numerical aperture of NA 0.8 or more is known. If it is made of plastic, it is difficult to guarantee the deterioration of the spherical aberration due to the refractive index fluctuation due to the temperature change.
Further, when an optical path difference providing structure is to be formed on the optical surface using glass as a material, the mold temperature must be raised in order to improve the transferability of the mold. However, in this method, the mold temperature must be raised to at least the glass transition point, and the damage to the mold increases. In addition, since the life of the mold is shortened, it is necessary to frequently replace the mold, resulting in an increase in cost.
In consideration of the above problems, Patent Document 1 below discloses an objective lens for an optical pickup device in which a resin layer is provided on the optical surface of an aspherical lens made of glass and a diffractive structure is provided on the resin layer. Is described.
JP 2000-40247 A

  The technique described in Patent Document 1 is intended to extend the life of the mold by injecting a resin having a low melting temperature or an ultraviolet curable resin into the mold to which the diffraction structure is transferred. When the technology is applied to an objective lens having a large NA group configuration, the difference in the thickness of the resin layer between the vicinity of the optical axis and the effective diameter increases, resulting in the influence of the change in optical path length when the temperature changes. The problem arises that the temperature increases and the temperature characteristics deteriorate.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a composite optical element that takes the above-described problems into consideration and that is inexpensive and that can reduce the number of adjustment operations during assembly, and an optical pickup device having the composite optical element.

In order to solve the above problems, the invention described in claim 1 is a composite optical element for an optical pickup device having a resin layer provided with an optical path difference providing structure on at least one optical surface of an aspheric lens. An optical path length L ′ when the incident light beam passes through the resin layer at the end of the effective diameter corresponding to the required numerical aperture, and an optical path length L on the optical axis when the incident light beam passes through the resin layer. The ratio satisfies the formula (1).
0.8 ≦ (L ′ / L) ≦ 1.2 (1)

As described in claim 1, by setting L ′ / L within the above range, chromatic aberration correction using a diffractive structure provided in the resin layer and spherical aberration caused by a change in the refractive index of the resin layer due to a change in environmental temperature. Correction and coma aberration correction when off-axis light is incident on the objective lens can be appropriately performed.
In particular, by setting L ′ / L within the above range, even when a composite optical element is used as an objective lens and a high numerical aperture such as a high-density optical disk is required, for example, refractive index fluctuation due to temperature change It is possible to suppress the generation amount of spherical aberration due to the above.
When L ′ / L is smaller than the lower limit, correction of chromatic aberration is insufficient. When L ′ / L is larger than the upper limit, correction of generation of spherical aberration and coma when the environmental temperature changes is insufficient. It will be a thing.
In addition, by forming the diffractive structure on the optical surface of the aspherical lens instead of directly on the optical surface, the manufacturing process of the composite optical element can be simplified, and as a result, the composite optical element can be manufactured at low cost. . Also, compared with the case where a diffraction structure is formed on an optical element separate from the aspherical lens and this optical element is combined with the aspherical lens, the number of adjustment work during assembly can be reduced. .

The invention according to claim 2 is the composite optical element according to claim 1, wherein the composite optical element is used for reproducing and / or recording information on an optical information recording medium having a required numerical aperture of 0.8 or more. The thickness of the end portion of the effective diameter corresponding to the required numerical aperture of the resin layer when the light beam having the wavelength λ1 is disposed in the optical path of the light beam of (390 nm ≦ λ1 ≦ 420 nm) and passes through the resin layer. Where t ′ [μm] and the thickness on the optical axis is t [μm], the expression (2) is satisfied.
0.9 ≦ t ′ / t ≦ 2.5 (2)
However, t and t ′ are lines from the point where the light flux having the wavelength λ1 intersects the surface of the resin layer to the point where the straight line drawn parallel to the optical axis intersects the boundary surface between the resin layer and the lens. Refers to the length of a minute.

The thickness t and t ′ of the resin layer provided with the diffractive structure is different from the optical path length (L and L ′) in use, in order to appropriately correct the aberration of the light beam having the wavelength λ1 that has passed through the required numerical aperture. To be determined. If the t ′ / t falls within the range of the expression (2) as in the second aspect, it is possible to minimize aberration deterioration due to the optical path length difference, and to record and reproduce the HD. It can be used as an objective lens for the apparatus.

The invention according to claim 3 is the composite optical element according to claim 1, wherein the composite optical element is used for reproducing and / or recording information on an optical information recording medium having a required numerical aperture of 0.6 or more. The thickness of the end portion of the effective diameter corresponding to the required numerical aperture of the resin layer when the light beam having the wavelength λ1 is disposed in the optical path of the light beam of (390 nm ≦ λ1 ≦ 420 nm) and passes through the resin layer. Is defined as t ′ [μm], and the thickness on the optical axis is defined as t [μm].
1.0 ≦ t ′ / t ≦ 2.0 (3)
However, t and t ′ are lines from the point where the light flux having the wavelength λ1 intersects the surface of the resin layer to the point where the straight line drawn parallel to the optical axis intersects the boundary surface between the resin layer and the lens. Refers to the length of a minute.

  The thickness t and t ′ of the resin layer provided with the diffractive structure is different from the optical path length (L and L ′) in use, in order to appropriately correct the aberration of the light beam having the wavelength λ1 that has passed through the required numerical aperture. To be determined. If the t ′ / t falls within the range of the expression (3) as in the third aspect, the aberration deterioration caused by the optical path length difference can be minimized, and the optical pickup for recording and reproducing the HD It can be used as an objective lens for the apparatus.

  According to a fourth aspect of the present invention, there is provided the composite optical element according to the second or third aspect, wherein the light beam having the wavelength λ1 and at least one light beam having a wavelength different from the wavelength λ1 are each changed to information on different optical information recording media. It has a function of condensing light on the recording surface.

  According to the invention described in claim 4, compatibility is achieved between a high-density optical disk using a light beam having a wavelength λ1 and another optical disk (for example, DVD or CD) using at least one light beam having a different wavelength. it can.

  The invention according to claim 5 is the composite optical element according to any one of claims 1 to 4, wherein the resin is an ultraviolet curable resin.

  In the wavelength range (390 nm to 800 nm) used for a general optical information recording medium, the resin layer does not cause a chemical change by using an ultraviolet curable material for the resin as in claim 5. It is possible to cure by causing an irreversible change at the wavelength.

A sixth aspect of the present invention is the composite optical element according to any one of the first to fifth aspects, wherein the refractive index with respect to the wavelength λ1 of the aspheric lens is n1, and the wavelength λ1 of the cured resin. (4) is satisfied, where n2 is a refractive index with respect to.
(N1 / n2) ≦ 1.2 (4)

  When the refractive index ratio n1 / n2 between the aspherical glass lens serving as the base of the objective lens and the resin is larger than the upper limit value, the refractive index variation due to the temperature change increases, resulting in an increase in spherical aberration.

The invention according to claim 7 is the composite optical element according to any one of claims 1 to 6, wherein a thickness t [μm] of the resin layer on the optical axis satisfies the formula (5). To do.
10 ≦ t ≦ 1000 (5)

  The required thickness of t for correcting chromatic aberration using the optical path difference providing structure and correcting the aberration caused by the wavelength difference of the used wavelength is preferably 10 μm or more. Is 1000 μm or more, spherical aberration is caused by a change in the refractive index of the resin when the environmental temperature changes, and therefore it is preferably within 1000 μm.

  The invention according to claim 8 is the composite optical element according to any one of claims 1 to 7, wherein the aspherical lens is made of plastic.

  When the resin layer is made of a material having an Abbe number of around 30, as in claim 8, for example, by combining a plastic lens with an Abbe number of around 60, the diffraction efficiency in the case of compatibility can be increased. In this case, the spherical aberration deterioration due to the temperature change is worse than the case where the aspherical lens is made of glass, but can be suppressed below the Marechal limit.

  The invention according to claim 9 is the composite optical element according to any one of claims 1 to 7, wherein the aspherical lens is made of glass.

  According to the ninth aspect of the present invention, even if the objective lens has a high NA, if it is made of glass, the amount of change in the refractive index due to the temperature change is small, so that spherical aberration deterioration can be suppressed.

  According to a tenth aspect of the present invention, in the composite optical element according to the ninth aspect, the aspherical lens is a glass mold lens.

  According to the tenth aspect, by manufacturing an aspherical lens with a glass mold, it is possible to easily create an aspherical shape in a short time compared to a polishing and grinding lens.

  An eleventh aspect of the present invention is the composite optical element according to any one of the first to tenth aspects, which is an objective lens of an optical pickup device.

  According to a twelfth aspect of the present invention, the composite optical element according to any one of the first to eleventh aspects is a part of an objective lens in which at least two optical elements are combined.

  A thirteenth aspect of the present invention is the composite optical element according to any one of the first to twelfth aspects, wherein the resin layer is formed on two surfaces of an incident surface and an output surface of the aspherical lens. It is characterized by that.

  A fourteenth aspect of the invention includes the composite optical element according to any one of the first to thirteenth aspects.

  ADVANTAGE OF THE INVENTION According to this invention, the optical pick-up apparatus which is cheap and can reduce the man-hour of the adjustment work at the time of an assembly, and this composite optical element can be obtained.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.
FIG. 1 schematically shows a configuration of an optical pickup apparatus PU that can appropriately record / reproduce information for any of HD (first optical disk), DVD (second optical disk), and CD (third optical disk). FIG. The optical specification of HD is a wavelength λ1 = 407 nm, the thickness t1 of a protective layer (protective substrate) PL1 is 0.6 mm, and the numerical aperture NA1 = 0.65. The optical specification of a DVD is a wavelength λ2 = 655 nm, The thickness t2 of the protective layer PL2 = 0.6 mm and the numerical aperture NA2 = 0.65, and the optical specifications of the CD are the wavelength λ3 = 785 nm, the thickness t3 of the protective layer PL3 = 1.2 mm, and the numerical aperture NA3 = 0.51.
However, the combination of the wavelength, the thickness of the protective layer, and the numerical aperture is not limited to this.

The optical system magnification m1 of the objective lens OBJ (composite optical element) when recording and / or reproducing information with respect to the first optical disk is m1 = 0. That is, the objective lens OBJ in the present embodiment has a configuration in which the first light flux having the wavelength λ1 is incident as parallel light.
Similarly, the optical system magnification m2 of the objective lens OBJ when recording and / or reproducing information with respect to the second optical disk is m2 = 0. That is, the objective lens OBJ in the present embodiment has a configuration in which the second light flux having the wavelength λ2 is incident as parallel light.
The optical system magnification m3 of the objective lens OBJ when recording and / or reproducing information with respect to the third optical disk is m3 <0. That is, the objective lens OBJ in the present embodiment has a finite conjugate system in which the third light flux having the wavelength λ3 is incident as diverging light. In the present invention, the combination of m1, m2, and m3 can be changed as appropriate.

  The optical pickup device PU is a blue-violet semiconductor laser LD1 (first light source) that emits a 407-nm laser beam (first beam) when recording / reproducing information on the HD, and the light for the first beam. Detector PD1, red semiconductor laser LD2 (second light source) that emits a 655 nm laser beam (second beam) when recording / reproducing information on DVD, and photodetector PD2 for the second beam , An infrared semiconductor laser LD3 (third light source) that emits a 785 nm laser beam (third beam) when recording / reproducing information on a CD, a third beam photodetector PD3, A collimating lens COL through which one light beam and a second light beam pass, a coupling lens CUL through which a third light beam passes, and a diffractive structure are formed on the optical surface thereof, and each light beam is transmitted to the information recording surfaces RL1, RL2, R 3 has an aspherical objective lens OBJ having a function of condensing light, a biaxial actuator AC for moving the objective lens OBJ in a predetermined direction, first to fifth beam splitters BS1 to BS5, a beam shaper BSH, and an aperture STO. And sensor lenses SEN1 to SEN3.

When recording / reproducing information with respect to the HD in the optical pickup device PU, first, the blue-violet semiconductor laser LD1 is caused to emit light, as shown by the solid line in FIG. When the divergent light beam emitted from the blue-violet semiconductor laser LD1 passes through the beam shaper BSH, its cross-sectional shape is changed, passes through the first beam splitter BS1 and the second beam splitter BS2, and reaches the collimating lens COL.
Then, when passing through the collimating lens COL, the first light beam is converted into parallel light, passes through the third beam splitter BS3 and the stop STO, reaches the objective lens OBJ, and passes through the first protective layer PL1 by the objective lens OBJ. Thus, the spots are formed on the information recording surface RL1. The objective lens OBJ performs focusing and tracking by a biaxial actuator AC arranged around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL1 passes again through the objective lens OBJ, the third beam splitter BS3, the collimator lens COL, and the second beam splitter BS2, and is branched by the first beam splitter BS1 to be a sensor lens. Astigmatism is given by SEN1 and converges on the light receiving surface of the photodetector PD1. And the information recorded on HD can be read using the output signal of photodetector PD1.

When recording / reproducing information on / from a DVD, first, the red semiconductor laser LD2 is caused to emit light, as shown by the dashed line in FIG. The divergent light beam emitted from the red semiconductor laser LD2 passes through the fourth beam splitter BS4, is reflected by the second beam splitter BS2, and reaches the collimating lens COL.
Then, when passing through the collimating lens COL, the second light beam is converted into parallel light, passes through the third beam splitter BS3 and the stop STO, reaches the objective lens OBJ, and passes through the second protective layer PL2 by the objective lens OBJ. Thus, the spots are formed on the information recording surface RL2. The objective lens OBJ performs focusing and tracking by a biaxial actuator AC arranged around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL2 passes again through the objective lens OBJ, the third beam splitter BS3, and the collimator lens COL, is branched by the second beam splitter BS2, and is further branched by the fourth beam splitter BS4. Then, astigmatism is given by the sensor lens SEN2 and converges on the light receiving surface of the photodetector PD2. Then, information recorded on the DVD can be read using the output signal of the photodetector PD2.

When recording / reproducing information on / from a CD, first, the infrared semiconductor laser LD3 is caused to emit light, as illustrated by the dotted line in FIG. The divergent light beam emitted from the infrared semiconductor laser LD3 passes through the fifth beam splitter BS5 and reaches the coupling lens CUL.
Then, when passing through the coupling lens CUL, the divergence angle of the third light beam is changed, reflected by the third beam splitter BS3, passes through the stop STO, reaches the objective lens OBJ, and reaches the objective lens OBJ. 3 Spots formed on the information recording surface RL3 through the protective layer PL3. The objective lens OBJ performs focusing and tracking by a biaxial actuator AC arranged around the objective lens OBJ.

  The reflected light beam modulated by the information pits on the information recording surface RL3 passes again through the objective lens OBJ, is reflected by the third beam splitter BS3, passes through the coupling lens CUL, and is branched by the fifth beam splitter BS5. Astigmatism is given by the lens SEN3 and converges on the light receiving surface of the photodetector PD3. And the information recorded on CD can be read using the output signal of photodetector PD3.

Next, the configuration of the objective lens OBJ will be described.
As shown in FIG. 2, the objective lens OBJ is a single-piece glass mold lens L in which both the incident surface S1 (optical surface on the light source side) and the exit surface S2 (optical surface on the optical disc side) are aspherical. It has the structure which has the resin layer R which consists of ultraviolet curable resin on entrance plane S1.
Note that the resin layer R may be formed not only by the ultraviolet curable resin but also by insert molding of a thermoplastic resin (by pouring the resin between the mold and the glass mold lens L).
An optical path difference providing structure is formed on the surface of the resin layer R and in a region corresponding to the numerical aperture NA1. In the present embodiment, a diffractive structure DOE having a sawtooth shape in cross section including the optical axis l is formed as the optical path difference providing structure.
Further, the optical surface S2 of the objective lens OBJ is constituted by a refractive surface.

The optical path length L ′ when the light beam A having the wavelength λ1 passes through the resin layer R having the effective diameter corresponding to the required numerical aperture (NA1) and the light beam A having the wavelength λ1 when passing through the resin layer R are shown. The ratio with the optical path length L on the optical axis l is set so as to satisfy the equation (1).
0.8 ≦ (L ′ / L) ≦ 1.2 (1)
L and L ′ are equal to values L1 / n2 and L2 / n2 obtained by dividing L1 and L2 in FIG. 2 by the refractive index n2 with respect to the wavelength λ1 of the resin layer R.
By setting L ′ / L within the above range, chromatic aberration correction using the diffractive structure DOE provided in the resin layer R, correction of spherical aberration due to a change in the refractive index of the resin layer R due to a change in environmental temperature, It is possible to appropriately correct coma aberration when off-axis light is incident on the objective lens OBJ.
When L ′ / L is smaller than the lower limit, correction of chromatic aberration is insufficient. When L ′ / L is larger than the upper limit, correction of occurrence of spherical aberration and coma when the environmental temperature changes is insufficient. It will be a thing.
Note that the expression (1) performs coma aberration correction appropriately in both cases of BD and HD.
0.9 ≦ (L ′ / L) ≦ 1.2
Is a particularly preferred range for BD.

Further, in the present embodiment, as shown in FIG. 2, when the light beam having the wavelength λ1 passes through the resin layer R, the end portion of the effective diameter corresponding to the required numerical aperture (NA1) of the resin layer R is obtained. When the thickness is t ′ [μm] and the thickness on the optical axis l is t [μm], it is set to satisfy the equation (2).
0.9 ≦ t ′ / t ≦ 2.5 (2)
Here, t and t ′ are the points from the point where the light flux of wavelength λ1 intersects the surface of the resin layer R to the point where the straight line drawn parallel to the optical axis l intersects the boundary surface between the resin layer R and the lens L. Refers to the length of the line segment.

The thicknesses t and t ′ of the resin layer R provided with the diffractive structure DOE are different from the optical path lengths (L and L ′) in the usage state, and appropriately correct aberration correction of the light beam having the wavelength λ1 that has passed through the required numerical aperture. Decided to do. If t ′ / t falls within the range of equation (2), aberration deterioration due to the optical path length difference can be minimized and used as an objective lens for an optical pickup device for recording and reproducing HD. It becomes possible.
The thickness t [μm] on the optical axis 1 of the resin layer R is preferably within the range of the formula (5).
10 ≦ t ≦ 1000 (5)

It is preferable that the necessary thickness of t for correcting the chromatic aberration using the diffractive structure DOE as described above and for correcting the aberration due to the wavelength difference of the used wavelength is 10 μm or more, while t is 1000 μm or more. Then, since spherical aberration occurs due to the change in the refractive index of the resin when the environmental temperature changes, it is preferably within 1000 μm. It is more desirable to set t to 50 μm to 150 μm.
Further, when the refractive index of the lens L with respect to the wavelength λ1 is n1, and the refractive index of the cured resin with respect to the wavelength λ1 is n2, it is preferable to set so as to satisfy the expression (4).
(N1 / n2) ≦ 1.2 (4)

When the refractive index ratio n1 / n2 between the aspherical glass lens L serving as the base of the objective lens OBJ and the resin R is larger than the upper limit value, the refractive index fluctuation due to temperature change increases, resulting in an increase in spherical aberration. End up. If the refractive index nd of the glass lens L at the d line is nd = 1.61, the refractive index nd ′ of the resin R at the d line is preferably about nd ′ = 1.54.
In the present embodiment, the composite optical element according to the present invention is applied to a single objective lens. However, the present invention is not limited to this, and is one of objective lenses in which a plurality of lenses are continuously arranged in the direction of the optical axis l. The present invention may be applied to these lenses. In addition, the composite optical element according to the present invention may be applied to an optical element other than the objective lens disposed in the optical path of the light beam having the wavelength λ1, such as a collimator lens.
In addition, the resin layer R is formed only on the incident surface S1 of the aspheric lens L. However, the present invention is not limited to this. For example, the resin layer R is formed only on the output surface S2 or on both the incident surface S1 and the output surface S2. May be.

  Further, as schematically shown in FIG. 3, the optical path difference providing structure is composed of a plurality of annular zones 100, and a diffractive structure in which the cross-sectional shape including the optical axis is a sawtooth shape, or as schematically shown in FIG. In addition, a diffractive structure in which the direction of the step 101 is composed of a plurality of annular zones 102 having the same effective diameter within the effective diameter and the cross-sectional shape including the optical axis is a staircase shape, or as shown in FIG. A diffraction structure composed of a plurality of annular zones 103 formed with a structure, or a plurality of annular zones 105 in which the direction of the step 104 is changed in the middle of the effective diameter as schematically shown in FIG. Some cross-sectional shapes include a staircase shape. The structure schematically shown in FIG. 6 may be a diffractive structure, or may be a structure that gives an optical path difference to a passing light beam but does not give a diffractive action. 3 to 6 schematically show the case where the optical path difference providing structure is formed on a plane, but it may be formed on a spherical surface or an aspherical surface.

  By providing such an optical path difference providing structure in the resin layer, for example, a semiconductor laser in which the spherical aberration is suppressed when the wavelength of the semiconductor laser is changed due to a temperature change or the oscillation wavelength is shifted from the reference wavelength due to a manufacturing error. The recording / reproducing characteristics can be maintained even when the wavelength of the incident light beam is changed instantaneously by suppressing the spherical aberration when the lens is used or by the mode hopping of the laser.

  Further, by using the optical path difference providing structure provided in the objective lens OBJ, chromatic aberration caused by the wavelength difference between the first light flux with wavelength λ1 for HD and the second light flux with wavelength λ2 for DVD, and / or HD The spherical aberration due to the difference in thickness between the protective layer and the protective layer of the DVD can be corrected. Note that the chromatic aberration here refers to a fluctuation in the minimum position of wavefront aberration in the optical axis direction caused by a wavelength difference. For example, the optical path difference providing structure is a diffractive structure that gives a positive diffractive action to at least one of the light fluxes having the wavelengths λ1 and λ2, and this occurs due to the wavelength variation of the light flux that has given the diffractive action. Chromatic aberration can be suppressed.

In addition, an aperture limiting element AP is disposed in the vicinity of the optical surface S1 of the objective lens OBJ as an aperture element for performing aperture limitation corresponding to NA3, and the aperture limiting element AP and the objective lens OBJ are integrated by a biaxial actuator. It is good also as a structure driven by tracking.
In this case, a wavelength selection filter WF having wavelength selectivity of transmittance is formed on the optical surface of the aperture limiting element AP. This wavelength selection filter WF transmits all wavelengths from the first wavelength λ1 to the third wavelength λ3 in the region within NA3, blocks only the third wavelength λ3 in the region from NA3 to NA1, and transmits the first wavelength λ1 and the first wavelength λ1. Since it has wavelength selectivity of transmittance that transmits two wavelengths λ2, aperture restriction corresponding to NA3 can be performed by such wavelength selectivity.
Further, as a method for limiting the aperture, not only a method using the wavelength selection filter WF, but also a method of mechanically switching a diaphragm or a method using a liquid crystal phase control element LCD described later.

The objective lens OBJ is desirably plastic from the viewpoint of light weight and low cost, but may be made of glass in consideration of temperature resistance and light resistance. Currently, the refraction-type glass mold aspherical lens is mainly on the market, but if a low-melting-point glass being developed is used, a glass mold lens provided with a diffractive structure can be manufactured. In addition, while plastics for optical use are being developed, there are materials whose refractive index changes with temperature are small. This is to reduce the refractive index change due to the temperature of the entire resin by mixing inorganic fine particles with the opposite sign of the change in refractive index due to temperature. Some materials have a reduced dispersion, and it is even more effective if they are used in a BD objective lens.
In addition, the optical pickup device PU in the present embodiment is configured to have compatibility between three types of optical disks of high density optical disk (HD) / DVD / CD, but not limited to this, only the blue-violet semiconductor laser LD1 is used. May be configured exclusively for high-density optical discs, or may have only blue-violet semiconductor laser LD1 and red semiconductor laser LD2 and have compatibility between two types of optical discs, high-density optical disc / DVD. .

In this specification, in addition to the above-described BD and HD, an optical disc having a protective film with a thickness of several to several tens of nanometers on the information recording surface, a protective layer or a protective film having a thickness of 0 ( Zero) optical disks are also included in high density optical disks.
In this specification, DVD is a generic term for DVD-series optical disks such as DVD-ROM, DVD-Video, DVD-Audio, DVD-RAM, DVD-R, DVD-RW, DVD + R, DVD + RW, and the like. CD is a general term for CD series optical disks such as CD-ROM, CD-Audio, CD-Video, CD-R, CD-RW and the like.
Further, in the present specification, the condensing optical element corresponds to, for example, members such as an objective lens, a coupling lens, a beam expander, a beam shaper, and a correction plate that constitute a condensing optical system of the optical pickup device.
The objective lens is not limited to a single lens, and a lens group configured by combining a plurality of lenses in the direction of the optical axis l may be used as an optical element.

Next, examples of the composite optical element shown in the above embodiment will be described.
Table 1 shows lens data of Example 1.

As shown in Table 1, in this example, the composite optical element of the present invention is applied to an objective lens.
The objective lens OBJ of this embodiment shown in FIG. 7 is dedicated to BD, and is set to have a focal length f1 = 1.77 mm and an image-side numerical aperture NA = 0.85.

The surface (second surface) of the resin layer R, the incident surface (optical surface on the light source side, third surface) of the lens L, and the exit surface (optical surface on the optical disk side, fourth surface) are expressed in the following formula 1. An aspherical surface that is axisymmetric about the optical axis l and is defined by a mathematical formula that substitutes the coefficient shown in FIG.

Here, x is an axis in the optical axis direction (the light traveling direction is positive), κ is a conic coefficient, and A _2i is an aspheric coefficient.
The diffractive structure DOE of the second surface is represented by an optical path difference added to the transmitted wavefront by this structure. The optical path difference is such that h (mm) is the height in the direction perpendicular to the optical axis, B _2i is the optical path difference function coefficient, n is the diffraction order of the diffracted light having the maximum diffraction efficiency out of the diffracted light of the incident light flux, λ When (nm) is the wavelength of the light beam incident on the diffractive structure and λB (nm) is the manufacturing wavelength of the diffractive structure, the optical path difference function φ () defined by substituting the coefficient shown in Table 1 into the following equation (2) h) Expressed in mm.

また、樹脂層Ｒの有効径の端の部分における厚みｔ´＝０．１８ｍｍ、光軸ｌ上の厚みｔ＝０．１ｍｍに設定されている。 The blazed wavelength λB of the diffractive structure DOE is 1.0 mm.

Further, the thickness t ′ = 0.18 mm at the end portion of the effective diameter of the resin layer R and the thickness t = 0.1 mm on the optical axis l are set.

FIG. 8 is a longitudinal spherical aberration diagram when the wavelength λ1 = 405 nm fluctuates by ± 1 nm, the horizontal axis indicates the amount of spherical aberration generated, and the vertical axis indicates the position corresponding to NA = 0.85. 00. The solid line in FIG. 8 indicates the case where the wavelength is 405 nm, the broken line indicates the case where the wavelength is 404 nm, and the two-dot chain line indicates the case where the wavelength is 406 nm.
FIG. 9 shows, as a comparative example, the resin layer R is removed from the objective lens OBJ shown in FIG. 7, and the entrance surface (third surface) and the exit surface (fourth surface) of the lens L are shown in Equation 1 in Table 1 above. FIG. 5 is a longitudinal spherical aberration diagram in the case of using an objective lens composed of an aspherical surface that is axisymmetric about the optical axis l, which is defined by a mathematical formula into which the coefficient shown is substituted.

  8 and 9, in the objective lens of this example, the spherical aberration is suppressed to within 0.5 μm even when the wavelength varies, so the objective lens is driven by an actuator to follow the wavelength variation and the Marshall limit is reached. Although it can be within 0.07 [λrms], the objective lens of the comparative example has a spherical aberration exceeding 0.1 μm when the wavelength fluctuates. Therefore, even if the objective lens is driven by an actuator, the marshal limit is reached. It can be seen that it cannot be within 0.07 [λrms].

Table 2 shows lens data of Example 2.

As shown in Table 2, in this example, the composite optical element of the present invention is applied to an objective lens.
The objective lens OBJ of this embodiment shown in FIG. 10 is for HD / DVD / CD compatibility, and has a focal length f1 = 1.75 mm and an image-side numerical aperture NA = 0.65. Although not described in Table 2, the thicknesses of the protective layers of DVD and CD are 0.6 mm and 1.2 mm, respectively, the wavelength λ2 = 655 nm of the DVD luminous flux, and the wavelength of the CD luminous flux. λ3 = 785 nm.

The surface (second surface) of the resin layer R, the incident surface (optical surface on the light source side, third surface) of the lens L, and the exit surface (optical surface on the optical disk side, fourth surface) are shown in the above equation 1 in Table 2. The aspherical surface is symmetric with respect to the optical axis and is defined by a mathematical formula in which the coefficient shown is substituted.
The diffractive structure DOE of the second surface is represented by an optical path difference added to the transmitted wavefront by this structure. Such an optical path difference is represented by an optical path difference function φ (h) (mm) defined by substituting the coefficient shown in Table 2 into the above equation (2).

The blazed wavelength λB of the diffractive structure DOE is 1.0 mm.
Further, the thickness t ′ of the end portion of the effective diameter of the resin layer R is set to 0.057 mm, and the thickness t on the optical axis l is set to 0.05 mm.

FIG. 11 is a longitudinal spherical aberration diagram in the case where the wavelength λ1 = 407 nm fluctuates by ± 1 nm. The horizontal axis indicates the amount of spherical aberration, and the vertical axis indicates a position corresponding to NA = 0.85 as 1.00. Yes. A solid line in FIG. 11 indicates a case where the wavelength is 407 nm, a line indicated by a broken line indicates a case where the wavelength is 406 nm, and a line indicated by a two-dot chain line indicates a case where the wavelength is 408 nm.
FIG. 12 shows, as a comparative example, the resin layer R is removed from the objective lens OBJ shown in FIG. 10, and the incident surface (third surface) and the exit surface (fourth surface) of the lens L are expressed in the above equation 1 in Table 2. FIG. 5 is a longitudinal spherical aberration diagram in the case of using an objective lens composed of an aspherical surface that is axisymmetric about the optical axis l, which is defined by a mathematical formula into which the coefficient shown is substituted.

  11 and 12, in the objective lens of the present embodiment, since the spherical aberration is suppressed to within 0.5 μm even at the time of wavelength fluctuation, the objective lens is driven by an actuator to follow the wavelength fluctuation and the Marshallal limit. Although it can be within 0.07 [λrms], the objective lens of the comparative example has a spherical aberration exceeding 0.1 μm when the wavelength fluctuates. Therefore, even if the objective lens is driven by an actuator, the marshal limit is reached. It can be seen that it cannot be within 0.07 [λrms].

It is a principal part top view which shows the structure of an optical pick-up apparatus. It is principal part sectional drawing which shows the structure of an objective lens. It is side view (a) and (b) which show an example of an optical path difference providing structure. It is side view (a) and (b) which show an example of an optical path difference providing structure. It is side view (a) and (b) which show an example of an optical path difference providing structure. It is side view (a) and (b) which show an example of an optical path difference providing structure. It is principal part sectional drawing which shows the structure of the objective lens in an Example. FIG. 6 is a longitudinal spherical aberration diagram when the objective lens in the example is used. It is a longitudinal spherical aberration diagram when the objective lens in the comparative example is used. It is principal part sectional drawing which shows the structure of the objective lens in an Example. FIG. 6 is a longitudinal spherical aberration diagram when the objective lens in the example is used. It is a longitudinal spherical aberration diagram when the objective lens in the comparative example is used.

Explanation of symbols

LD1 Blue-violet semiconductor laser DOE Diffraction structure L Glass mold lens OBJ Objective lens PU Optical pickup device R Resin layer

Claims (14)

In a composite optical element for an optical pickup device having a resin layer provided with an optical path difference providing structure on at least one optical surface of an aspheric lens,
An optical path length L ′ when the incident light beam passes through the resin layer at the end of the effective diameter corresponding to the required numerical aperture, and an optical path length L on the optical axis when the incident light beam passes through the resin layer. A composite optical element characterized in that the ratio satisfies the formula (1).
0.8 ≦ (L ′ / L) ≦ 1.2 (1) The composite optical element is disposed in an optical path of a light beam having a wavelength λ1 (390 nm ≦ λ1 ≦ 420 nm) used for reproducing and / or recording information on an optical information recording medium having a required numerical aperture of 0.8 or more,
When the light flux having the wavelength λ1 passes through the resin layer, the thickness of the end portion of the effective diameter corresponding to the required numerical aperture of the resin layer is t ′ [μm], and the thickness on the optical axis is t [μm. The compound optical element according to claim 1, wherein the formula (2) is satisfied.
0.9 ≦ t ′ / t ≦ 2.5 (2)
However, t and t ′ are lines from the point where the light flux having the wavelength λ1 intersects the surface of the resin layer to the point where the straight line drawn parallel to the optical axis intersects the boundary surface between the resin layer and the lens. Refers to the length of a minute. The composite optical element is disposed in an optical path of a light beam having a wavelength λ1 (390 nm ≦ λ1 ≦ 420 nm) used for reproducing and / or recording information on an optical information recording medium having a required numerical aperture of 0.6 or more,
When the light flux having the wavelength λ1 passes through the resin layer, the thickness of the end portion of the effective diameter corresponding to the required numerical aperture of the resin layer is t ′ [μm], and the thickness on the optical axis is t [μm. The compound optical element according to claim 1, wherein the following expression (3) is satisfied.
1.0 ≦ t ′ / t ≦ 2.0 (3)
However, t and t ′ are lines from the point where the light flux having the wavelength λ1 intersects the surface of the resin layer to the point where the straight line drawn parallel to the optical axis intersects the boundary surface between the resin layer and the lens. Refers to the length of a minute.   4. The method according to claim 2, wherein the light beam having the wavelength λ <b> 1 and at least one light beam having a wavelength different from the wavelength λ <b> 1 are condensed on information recording surfaces of different optical information recording media. The composite optical element described.   The composite optical element according to claim 1, wherein the resin is an ultraviolet curable resin. The formula (4) is satisfied, where n1 is a refractive index of the aspheric lens with respect to the wavelength λ1, and n2 is a refractive index of the cured resin with respect to the wavelength λ1. The composite optical element according to any one of the above.
(N1 / n2) ≦ 1.2 (4) The composite optical element according to claim 1, wherein a thickness t [μm] on the optical axis of the resin layer satisfies the formula (5).
10 ≦ t ≦ 1000 (5)   The composite optical element according to claim 1, wherein the aspherical lens is made of plastic.   The composite optical element according to claim 1, wherein the aspheric lens is made of glass.   The compound optical element according to claim 9, wherein the aspherical lens is a glass mold lens.   The composite optical element according to claim 1, wherein the composite optical element is an objective lens of an optical pickup device.   The composite optical element according to claim 1, wherein the composite optical element is a part of an objective lens in which at least two optical elements are combined.   The composite optical element according to claim 1, wherein the resin layer is formed on two surfaces of an incident surface and an output surface of the aspheric lens.   An optical pickup device comprising the composite optical element according to claim 1.

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