JP4852271B2 - Optical pickup optical system, optical head, and optical disc apparatus - Google Patents

Description

  The present invention relates to an optical pickup optical system, an optical head, and an optical disc apparatus.

  Conventionally, compatible optical disk devices have been proposed that can play back together optical disks of different types such as CD (Compact Disc) and DVD (Digital Versatile Disc). A CD or DVD (hereinafter collectively referred to as an optical disk) uses a transparent substrate, and an information recording surface is provided on one surface of the transparent substrate. The optical disk has a structure in which two transparent substrates are bonded together with their information recording surfaces facing each other, or such a transparent substrate is a transparent protective substrate, and the information recording surface of the transparent substrate is a protective substrate. It is configured to be pasted so as to face each other.

  When reproducing the information signal stored in the optical disk having such a configuration, the optical disk apparatus needs to focus the laser beam from the light source on the information recording surface of the optical disk via the transparent substrate. The wavelength of the laser beam differs depending on whether it is used on a CD or a DVD as described later. In order to focus the laser beam, an objective lens is usually used in the optical disc apparatus. Here, the thickness of the transparent substrate on which the information recording surface is provided may vary depending on the type of optical disk (difference in the wavelength of the laser beam). For example, the thickness of the transparent substrate used in CD is 1.2 mm, whereas the thickness of the transparent substrate used in DVD can be 0.6 mm.

  In an optical disc apparatus that reproduces optical discs of different types, it is necessary to focus the laser beam on the information recording surface even if the thickness of the transparent substrate varies depending on the type of optical disc. In addition, it has been proposed that a new optical disc apparatus that has been proposed in recent years uses a blue laser having a wavelength of about 400 nm for reproducing information. Therefore, it is expected that such a new optical disc can be used simultaneously with the optical disc apparatus in addition to the CD and the current DVD for backward compatibility.

  As such a compatible optical disk device, it is conceivable to provide an objective lens for each type of optical disk in the pickup and replace the objective lens according to the type of optical disk to be used. Alternatively, it is conceivable that a pickup is provided for each type of optical disk, and the pickup is replaced according to the type of optical disk to be used. However, in order to realize cost reduction and downsizing of the apparatus, it is desirable that the same lens can be used for any kind of optical disk as the objective lens.

  An example of such an objective lens is disclosed in Patent Document 1. The objective lens disclosed in Patent Document 1 is divided into three or more annular lens surfaces in the radial direction. Of these annular lens surfaces, the refractive power of every other annular lens surface is different from the refractive power of every other annular lens surface. Then, every other annular lens surface condenses the laser beam on the information recording surface of an optical disk (DVD) with a thin transparent substrate (0.6 mm), for example. For every other annular lens surface, for example, a laser beam is focused on the information recording surface of an optical disk (CD) on a thick transparent substrate (1.2 mm) for a laser beam having the same wavelength as that of the laser beam. Light up.

  Another example is disclosed in Patent Document 2. In the optical disk device disclosed in Patent Document 2, a laser beam having a short wavelength (635 nm or 650 nm) is used for a DVD having a thin transparent substrate, and a long wavelength (780 nm) is used for a CD having a thick transparent substrate. ) Is used.

  The optical disc apparatus disclosed in Patent Document 2 has an objective lens that is commonly used for these laser beams. The objective lens is formed with a diffractive lens structure in which minute steps of an annular zone are densely provided on one surface of a refractive lens having positive power. Such a diffractive lens structure condenses the diffracted light of the short wavelength laser beam on the DVD of the thin transparent substrate and the diffracted light of the long wavelength laser beam on the information recording surface of the CD of the thick transparent substrate. Designed. Each diffracted light is designed to collect the same order of diffracted light on the information recording surface. The reason for using a laser beam with a short wavelength for DVD is that the recording density of DVD is higher than that of CD, and it is therefore necessary to narrow down the beam spot. As is well known, the size of the light spot is proportional to the wavelength and inversely proportional to the numerical aperture NA.

  In any of the conventional optical disc apparatuses described above, a common objective lens can be used for both DVD and CD. This eliminates the need for means for exchanging the members used for each DVD and CD including the objective lens, which is advantageous in terms of cost and simplification of the configuration.

  In addition, Blu-ray Disc (HD) and HD DVD (High Definition DVD) have emerged as optical recording media following CD and DVD, and an optical pickup device that supports three optical recording media including CD and DVD has been developed. Has been. In such an optical pickup device, it is required to transmit the output of the laser light source to the disk of the recording medium with high efficiency, which is one of the points in the development for realizing it. And an antireflection film formed on an optical component such as an objective lens included in the apparatus.

The wavelength of light used for CD is about 790 nm, the wavelength of light used for DVD is about 655 nm, and the wavelength of light used for Blu-ray Disc and HD DVD is about 405 nm. Therefore, it is desirable to form an antireflection film having optical characteristics such that the reflectance is low in the vicinity of these three wavelengths on the objective lens provided in the optical pickup device. As an antireflection film corresponding to a plurality of wavelengths, Patent Document 3 discloses an antireflection film corresponding to the wavelength of light used in DVDs and Blu-ray discs.
Japanese Patent Laid-Open No. 9-145995 JP 2000-81666 A JP 2005-38581 A

  However, in Patent Document 1, since the annular lens surface used in the objective lens is different for each DVD and CD, there are many portions that are ineffective with respect to the incident laser beam, and the light utilization efficiency is extremely low. .

  Further, in Patent Document 2, since diffracted light by the diffractive lens structure is used, there is a problem that the diffraction efficiency for each of different wavelengths cannot be made 100% at the same time. In this diffractive lens, the diffraction efficiency is 100% at a wavelength approximately halfway between the short wavelength (635 nm or 650 nm) laser beam used for DVD and the long wavelength (780 nm) laser beam used for CD. Thus, the diffraction efficiency is balanced with respect to the used laser beam.

  In Patent Document 2, since a diffractive lens structure is provided on the lens surface, a minute step is required. However, it is easily affected by manufacturing errors, and if the diffractive structure deviates from the design, the diffraction efficiency deteriorates. Will be invited. Thus, the deterioration of the diffraction efficiency and the fact that the diffraction efficiency does not reach 100% means that all of the incident light cannot be condensed on the information recording surface provided on the transparent substrate of the optical disk, This is a light quantity loss.

  In addition, there is a Blu-ray standard that uses a blue laser with a shorter wavelength, which has been proposed recently, but in this case, backward compatibility is also required. In this case, the wavelength difference is larger than that between DVD-CD, and the difference in refractive index between the lenses is also increased based on the wavelength difference. Therefore, in the conventional method as described above, it is more difficult to ensure a good wavefront aberration in any medium.

  Further, the antireflection film described in Patent Document 3 defines light transmittance by focusing on two types of wavelengths, that is, the wavelength of light used for DVDs and the wavelength of light used for Blu-ray discs. In this case, an antireflection film having a low reflectance can be formed in the wavelength region of light used for DVDs and Blu-ray discs. However, in the embodiment disclosed in Patent Document 3, the reflectance in the wavelength region of the light used for the CD is higher than that before the antireflection film is formed, which is a problem when the CD is used as the recording medium. May occur.

  For example, when writing information on a CD at a high writing speed magnification, if an antireflection film having a high reflectance in the wavelength region of light used for the CD is used, there is a high possibility that a writing error will occur. Today, the writing speed magnification of the CD-R drive has been improved, and those having a speed of 52 times or more have appeared, and therefore it is assumed that writing at a high magnification is performed.

  In Patent Document 3, an antireflection film having three or more layers is also assumed. Since the deposition time of the antireflection film in the manufacturing process is proportional to the number of antireflection films, it is preferable that the number of antireflection films is small. However, when a film having low reflectance is formed in two or more types of wavelength regions, if the assumed two types of wavelength regions are relatively close to each other, the spectral reflection characteristic is a two-layer reflection called a V-shaped V coat. Although the protective film can be used, it cannot be said that the wavelength range of light used for Blu-ray discs and the wavelength range of light used for DVDs and CDs are close to each other. is there.

  Further, such an antireflection film is mainly provided on the objective lens surface in the optical pickup device, and the objective lens often uses a plastic material for reasons of optical performance and cost. Although the lens made of a plastic material is vulnerable to heat, the antireflection film is formed by vapor deposition or sputtering, so that the temperature of the film formation surface also increases at this time. As described above, since the film formation time is proportional to the number of layers of the antireflection film, the number of layers of the antireflection film is small for the purpose of reducing the film formation time and adverse effects such as thermal deformation of the film formation surface. It is preferable.

  The present invention has been made to solve such problems, and in a state in which wavefront aberration is reduced as much as possible for each of a plurality of types of optical recording media having different wavelengths to be used, and high light utilization. It is an object of the present invention to provide an optical pickup optical system, an optical head, and an optical disc apparatus that are capable of efficiently condensing a light beam on an information recording surface. In particular, the first object of the present invention is to provide an optical pickup optical system that is optimal for three types of optical information storage media.

In the objective lens according to the present invention, a light beam having a different wavelength λn (n ≧ 3) is incident on at least three types of optical recording media, and the information recording surface provided on the transparent substrate of the optical recording media In an objective lens having a positive power for condensing the light beam by refraction, an incident light beam or an extension line of the incident light beam to the objective lens having each wavelength λn (n ≧ 3) intersects the optical axis. n ≧ 3) and the distance from the point Q where the lens surface far from the optical recording medium of the two lens surfaces of the objective lens intersects the optical axis is Sn (n ≧ 3), and the position of the point Pn is the position of the point Pn When the point Q is located on the opposite side of the optical information recording medium, the sign of the distance Sn is set to plus, and the point Pn is located on the same side as the optical information recording medium with respect to the point Q. Defines the sign of the distance Sn minus the sign of the distance Sn In this case, an incident light beam that satisfies the following equations (1) and (2) is incident,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)
Each light beam is focused on the information recording surface with an RMS wavefront aberration of 0.035λ RMS or less.

Here, it is preferable that the objective lens is divided into sections in which at least one lens surface is divided in the radial direction. Further, the optical path length of the light beam passing through each section of the objective lens is an optical path length passing through another section, and any one of the first light beam, the second light beam, and the third light beam. Are different by approximately 2 mλ (m is an integer), and other light beams may be approximately different by mλ (m is an integer). In particular, the substantially 2mλ (m is an integer) Mixed wavelength is preferably a lambda _1. In a preferred embodiment, the wavelength λ ₁ is around 405 nm, the wavelength λ ₂ is around 655 nm, and the wavelength λ ₃ is around 790 nm.
In particular, it is desirable that S1 and S2 satisfy the following expression.
S1 <0, S3> 0

An optical pickup optical system according to the present invention includes a first light beam having a wavelength λ ₁ corresponding to a _first optical storage medium and a second light beam having a wavelength λ ₂ corresponding to a second optical storage medium. And a third light beam having a wavelength λ ₃ corresponding to the third optical storage medium is incident, the first light beam is condensed on the first optical storage medium by the objective lens, A light beam is condensed on the second optical storage medium, and a third light beam is condensed on the third optical storage medium. The first optical storage medium, the second optical storage medium, and the third optical storage medium In the optical pickup optical system capable of reading the information stored in the optical storage medium, the objective lens is a positive lens that focuses the light beam on the information recording surface provided on the transparent substrate of the optical recording medium by refraction. With the respective wavelengths λ ₁ , λ ₂ and λ ₃ incident light beams to the lens or the extension lines of the incident light beams intersect with the optical axis P ₁ , P ₂ , P ₃ and the optical recording medium of the two lens surfaces of the objective lens S ₁ , S ₂ , S ₃ are distances from the point Q where the lens surface far from the point intersects the optical axis,
When the positions of the points P ₁ , P ₂ , P ₃ are located on the opposite side of the point Q from the optical information recording medium, the signs of the distances S ₁ , S ₂ , S ₃ are set to be positive, When the positions of P ₁ , P ₂ , and P ₃ are located on the same side as the optical information recording medium with respect to the point Q, the signs of the distances S ₁ , S ₂ , and S ₃ are minus and the distances S ₁ , S ₃ , When the signs of S ₂ and S ₃ are defined, the following expressions (1) and (2) are satisfied.
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)

Here, the first, second and third light beams are preferably condensed on the information recording surface with an RMS wavefront aberration of 0.035λRMS or less. The objective lens has at least one lens surface divided into sections divided in the radial direction, the optical path length of the light beam passing through each section is the optical path length passing through the other section, and the first light beam. It is desirable that one of the second light beam and the third light beam is different by about mλ (m is an integer), and the other light beams are different by about 2 mλ (m is an integer). At this time, it is preferable that the wavelength different from about 2 mλ (m is an integer) is λ ₁ . Further, it is desirable that the wavelength λ ₁ is in the vicinity of 405 nm, the wavelength λ ₂ is in the vicinity of 655 nm, and the wavelength λ ₃ is in the vicinity of 790 nm.
An optical head can be configured by the pickup optical system having such a configuration, and an optical disc apparatus can be configured.

  The first light beam, the second light beam, and the third light beam may be “transmit the first light beam and the second light beam, and block the third light beam. It is preferable that the light is transmitted through a wavelength selective filter having an outer region and an inner region that transmits the first light beam, the second light beam, and the third light beam, and is collected before the optical recording medium.

  In particular, it is desirable that the numerical aperture of the light beam after passing through the objective lens is larger in the order of the first light beam, the second light beam, and the third light beam. The wavelength selective filter preferably has a transmittance of 10% or less for the third light beam, and more preferably has a transmittance of 90% or more for the first light beam and the second light beam.

Here, S1 and S3 should satisfy the following formula.
S1 <0, S3> 0
Further, when the magnification m1 of the first light beam and the magnification m3 of the third light beam are satisfied, 0 <m1 ≦ 1/10 and −1 / 10 ≦ m3 <0 are satisfied, and 0 <m1 It is desirable to satisfy ≦ 1/20 and −1 / 20 ≦ m3 <0.

  Here, it is preferable that the wavelength selective filter is formed on a surface of the objective lens. The refractive index of the wavelength selective filter is preferably in the range of 0.9 to 1.1 with respect to the refractive index of the objective lens. Further, it is desirable that the objective lens is formed of a material mainly composed of glass having a refractive index of 1.49 to 1.70.

In another objective lens according to the present invention, a light beam having a different wavelength λn (n ≧ 3) is incident on at least three types of optical recording media, and an information recording surface provided on the transparent substrate of the optical recording media In the objective lens having a positive power for condensing the light beam by refraction, the incident light beam to the objective lens of each wavelength λn (n ≧ 3) or the extension line of the incident light beam intersects the optical axis. Sn (n ≧ 3) is the distance between Pn (n ≧ 3) and the point Q where the lens surface far from the optical recording medium of the two lens surfaces of the objective lens intersects the optical axis, and the position of the point Pn Is located on the opposite side of the optical information recording medium with respect to the point Q, the sign of the distance Sn is set to a plus, and the position of the point Pn is located on the same side as the optical information recording medium with respect to the point Q. In this case, the sign of the distance Sn is minus and the sign of the distance Sn is fixed. When defined, an incident light beam that satisfies the following equations (1) and (2) is incident:
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)
Each light beam is focused on the information recording surface with an RMS wavefront aberration of 0.050λ RMS or less,
The objective lens is formed of a material mainly composed of glass having a refractive index of 1.49 to 1.70 and a heat distortion temperature of 300 degrees or less.

  According to the present invention, a light beam is condensed on an information recording surface with high light utilization efficiency in a state where wavefront aberration is reduced as much as possible for each of a plurality of types of optical recording media having different wavelengths used. It is possible to provide an optical pickup optical system, an optical head, and an optical disc apparatus that can be used.

  The lens according to the present invention is a multi-wavelength lens that uses a plurality of types of monochromatic light, and includes various types such as CD (including CD such as CD-R), DVD, Blu-ray disc, and AOD (Advanced Optical Disc). This is a general-purpose multi-wavelength lens that can be used in a compatible recording / reproducing apparatus that can handle different optical recording media. The multi-wavelength optical system, the optical head, and the optical disc apparatus according to the present invention use such a multi-wavelength lens.

First, the present invention will be schematically described.
Now, with respect to a first optical disk using a transparent substrate having a thickness t ₁ , an objective lens in an optical disk apparatus using the same is corrected for aberration, and an information recording surface provided on the substrate has a wavelength λ ₁ . It is assumed that the laser beam is focused well. An attempt is made to focus a laser beam having a wavelength λ ₂ different from λ _{1 on a} second optical disc using a transparent substrate having a thickness t ₂ in such an optical disc apparatus.

In this case, even if the difference between the wavelengths λ ₁ and λ ₂ of the laser beam, the difference between the thickness t ₂ and the thickness t ₁ of the transparent substrate, or the thickness t ₂ and t ₁ are the same, the wavelengths λ ₁ and λ ₂ are Different. For this reason, spherical aberration due to the difference in thickness of the transparent substrate and chromatic aberration due to the difference in the refractive index of the objective lens due to the difference in the wavelength of the laser beam, or only chromatic aberration occurs, and the information recording surface However, the laser beam is not condensed well.

  The present invention relates to an aspherical shape of an objective lens and an objective lens so that an optical path length passing through an arbitrary light beam height is free from aberration or less in all cases for different types of discs at different wavelengths. Sets the degree of divergence of incident light. As a result, aberrations can be sufficiently reduced for all the optical disks. In addition, in the present invention, since the diffraction effect is not used and only the refracted light beam is used, no light loss due to diffraction efficiency occurs.

  In addition, the lens of one embodiment of the present invention is configured by dividing the lens surface into a plurality of aspheric surfaces, as will be described in detail in the following embodiments.

  Now, a case where a laser beam is focused on the information recording surface 2a of the substrate 2 using the objective lens 1 will be described with reference to FIG. Here, the surface A of the objective lens 1 is a light incident side surface, the surface B is a light emission side surface, and the information recording surface 2a of the substrate 2 is on the side opposite to the objective lens 1 side.

FIG. 1 schematically shows the optical path of the objective lens 1. In FIG. 1, the laser beam incident on the objective lens 1 is parallel light. Therefore, the optical system shown in FIG. 1 is a so-called infinite optical system. Further, FIG. 1 shows a point (condensing point) P ₅ where a light beam passing through a position P ₁ at a distance (light beam height) h in a direction perpendicular to the optical axis OA of the objective lens 1 crosses the optical axis OA. The optical path to reach is schematically shown.

Here, the incident point to the objective lens 1 in this optical path is P ₂ , the exit point from the objective lens 1 is P ₃ , and the incident point to the transparent substrate 2 is P _4. The
Point P ₁ to incident point P ₂ : spatial distance = S _1h , refractive index = n ₁ ,
Entrance point P ₂ to exit point P ₃ : spatial distance = S _2h , refractive index = n ₂ ,
Outgoing point P ₃ to incident point P ₄ : spatial distance = S _3h , refractive index = n ₃ ,
Incident point _{P 4} ~ focal point _{P 5:} CLEARANCE _{= S 4h,} refractive index = _{n 4}
And
At this time, the optical path length L _h from the point P ₁ to the condensing point P ₅ is
L _h = n ₁ × S _1h + n ₂ × S _2h + n ₃ × S _3h + n ₄ × S _4h (3)
It is represented by The optical path length L _h on the optical axis OA is a case where h = 0 in the equation (3).

The equation (3) is intended as applicable for any ray height h, when it is the aberration correction, the focal point P ₅ is within the allowable range of each relative to a light ray with a height h of people any husband It is on the information recording surface 2a. That is, according to the present invention, for example, by using a laser beam having a different wavelength for each of a plurality of substrates having different thicknesses, the chromatic aberration and the spherical aberration cancel each other, so that the focal point P ₅ with respect to an arbitrary ray height h can be obtained. It is intended to be on the information recording surface 2a within each allowable range.

  Moreover, although the parallel light incidence, so-called infinite system has been described, the incident light to the objective lens 1 may be divergent light, that is, a finite system. Furthermore, an infinite system and a finite system may be used separately for different optical recording media and wavelengths. Alternatively, the degree of divergence of incident light may be changed for different optical recording media even in the same finite system. Moreover, the incident light to the objective lens may be convergent light.

For example, assume that 405 nm monochromatic light (λ ₁ ) in HDDVD (AOD), 655 nm monochromatic light (λ ₂ ) in DVD, and 790 nm monochromatic light (λ ₃ ) in CD are used. In such a case, a lens surface in which these wavelengths are used in common can be divided into a plurality of aspherical surfaces. In this method, the optical path length of the arbitrary aspheric surface portion differs from the optical path length of the other aspheric surface portions by almost an integral multiple of the wavelength λ _i of each monochromatic light, and each monochromatic light in each aspheric surface portion is different. Let ΔVd (λ ₁ ) and ΔVd (λ ₂ ) (d is an integer of 1, 2,..., Meaning each aspherical portion).

  At this time, the ratio of the difference between the maximum value and the minimum value of the wavefront aberration of each monochromatic light is 0.4 or more and 2.5 or less, preferably 0.5 or more and 2.0 or less, in any aspheric part. As a result, RMS (Root Mean Square) wavefront aberration within an allowable range can be ensured for the entire lens at all wavelengths. Furthermore, in order to improve the RMS wavefront aberration value for a CD, the incident light beam may be set to be diverging light or set to be an incident light beam having a higher degree of divergence than HDDVD or DVD. Thereby, the spherical aberration due to the increase in the substrate thickness and the spherical aberration due to the increase in the degree of divergence of incident light cancel each other, and the spherical aberration occurring in the CD can be corrected.

Incidentally, the wavefront aberration ray height here a (h) and the optical path length in the case of h = 0 and L _0, when the optical path length of each ray height and L _h, wavefront aberration V _h is the following formula It is represented by (4).
V _h = (L _h −L ₀ ) / λ _i (4)

  In FIG. 2, the wavefront aberrations of HDDVD, DVD, and CD are shown in contrast. In FIG. 2, the horizontal axis represents the light ray height, and the vertical axis represents the wavefront aberration. 2A shows the wavefront aberration of each aspheric part of the HDDVD, FIG. 2B shows the wavefront aberration of each aspheric part of the DVD, and FIG. 2C shows the above formula of each aspheric part of the CD. It shows the required wavefront aberration.

For example, the difference between the maximum value and the minimum value of the wavefront aberration in the aspherical portion in the first region of the aspherical portion is defined by ΔV ₁ (λ ₁ ) and ΔV ₁ (λ ₂ ). In the present invention, as will be clarified in the embodiments of the invention described later, the ratio of the difference between the maximum value and the minimum value of the wavefront aberration at each wavelength is 0.4 or more and 2.5 in any aspherical portion. It is as follows. That is, the present invention has a constant distribution of wavefront aberration at each aspherical portion at any wavelength. The present invention is also different from the conventional method in which the lens surface is configured based on one of the wavelengths and the wavefront aberration is corrected using the phase shift only at the other wavelength.

Further, in multi-wavelength lens of the present invention, even in the region of one of the non-spherical surface portion, the difference between the maximum value and the minimum value of the wavefront aberration of each wavelength, 0.14Ramuda _i or less, preferably, 0.12Ramuda _i Hereinafter, it is more preferably 0.10λ _i or less. For example, as an example when the difference between the maximum value and the minimum value is 0.14λ _i or less, when the wavelength is 790 nm, it is 110.6 nm or less, and when the wavelength is 655 nm, the wavelength is 91.7 nm or less. Can be set to 56.7 nm or less. Thereby, the multi-wavelength lens of the present invention can secure even better optical characteristics at each wavelength.

Furthermore, in the present invention, in the case of a two-wavelength optical system, by using a multi-wavelength lens in which the wavefront aberrations of the respective wavelengths are almost symmetrical, the two wavelengths are balanced, and the RMS wavefront aberration is further reduced. be able to.
As a result, the RMS wavefront aberration is 0.03152λ ₁ RMS for HDDVD and 0.03237λ ₂ RMS for DVD, and the RMS wavefront aberration of HDDVD and the RMD wavefront aberration of DVD are almost equal. Note that the RMS wavefront aberration of the CD is 0.01764λ ₃ RMS, which is smaller than that of HDDVD or DVD.

  For recording and reproduction of a 790 nm CD, the degree of divergence of incident light incident on the objective lens and the object distance for the so-called objective lens in terms of geometric optics can be changed. This is effective as a means for correcting spherical aberration because the spherical aberration that occurs due to the degree of divergence of incident light varies. This is described below in the embodiments of the invention.

  In the embodiment of the present invention described below, incident light beams with wavelengths of 405 nm and 655 nm are infinite systems, and incident light beams with wavelengths of 790 nm are finite systems. That is, the divergence degree of incident light with a wavelength of 405 nm is made the same as the divergence degree of incident light with a wavelength of 790 nm. Here, depending on the wavelength used and the thickness of the substrate, it is possible to determine which wavelength of incident light divergence is to be changed or not to be changed so as to reduce aberration each time. Further, light beams of all wavelengths may be incident as divergent light beams, or conversely, they may be incident as convergent light beams.

The embodiment of the present invention described below makes it possible to form a good light spot on the information recording surface, for example, for any optical disk having a different substrate thickness. Note that the same is not different in thickness of the disc substrate, that is, even if the thickness is different, such as the same wavelength, within the allowable range of people each converging point P ₅ shown in FIG. 1 This is applicable. The present invention is not limited to an optical recording medium, and can also be applied to cases where laser beams having different wavelengths are allowed to pass through the same lens or optical system in optical communication or the like.

The best mode for carrying out the present invention will be specifically described below.
Embodiment 1 of the Invention
In the first embodiment of the invention (first embodiment), three types of optical disks, ie, HDDVD (λ ₁ = 405 nm), DVD (λ ₂ = 655 nm), and CD (λ ₃ = 790 nm) are taken as examples. This will be described with reference to the drawings. The lens in the first embodiment has a refractive index equivalent to that of a plastic resin. However, if the lens material is glass, it may be designed with the refractive index of glass.

  FIG. 3 is a schematic diagram showing an example of the action of the objective lens according to the present invention. FIG. 3 (a) is for HDDVD, FIG. 3 (b) is for DVD, and FIG. 3 (c) is for CD. It is. In FIG. 3, 1 is the objective lens in the first embodiment, 2 is an HDDVD transparent substrate, 3 is a DVD transparent substrate, 4 is a CD transparent substrate, 5 is a stop, and 6 is a wavelength selective stop.

First, in FIG. 3A, the objective lens 1 is provided in an optical head of an optical disk device (not shown), and an HDDVD is mounted on the optical disk device. A laser beam incident as parallel light is condensed by the objective lens 1, thereby recording and reproducing. Here, the thickness t ₁ of the HDDVD substrate 2 is 0.6 mm, and the laser beam 5 at this time is a laser beam having a wavelength λ ₁ = 405 nm as a light beam having a numerical aperture NA = 0.650. Under such conditions, such a laser beam is focused on the information recording surface 2a on the opposite side of the HDDVD substrate 2 from the objective lens 1 side.

FIG. 3B shows a case where a DVD is mounted on the same optical disk device (not shown) as described above, and recording / reproduction is performed using the same objective lens 1. Here, the thickness t ₂ of the DVD substrate 3 is 0.6 mm, and the laser beam 5 at this time is used as a light beam having a wavelength λ ₂ = 655 nm and a numerical aperture NA = 0.628. In HDDVD and DVD, NA is different from 0.650 and 0.628 even though the diameter of the diaphragm 5 is the same between HDDVD and DVD. This is because the refractive index of the objective lens 1 is different because the wavelengths are different from 405 nm and 655 nm, and the focal lengths are different accordingly.

FIG. 3C shows a case where a CD is mounted on the same optical disk apparatus (not shown) as described above, and recording / reproduction is performed using the same objective lens 1. Here, the thickness t ₃ of the CD substrate 4 is 1.2 mm, and the laser beam at this time is incident on the objective lens 1 in the state of divergent light, and has a numerical aperture NA of approximately λ ₃ = 790 nm. = 0.470.

  As shown in FIG. 4, the wavelength selective filter 6 shown in FIGS. 3A, 3B, and 3C includes an inner total light transmission region and an outer CD (790 nm) light blocking region. It is divided. Specifically, a dichroic coating that reflects a light of 750 nm or more with a mask inside may be applied.

  More specifically, for example, a dichroic coat exhibiting spectral transmittance characteristics as shown in FIG. 5 may be applied to the CD light blocking region. Thereby, for example, the wavelength selective filter 6 having the spectral transmittance characteristic shown in FIG. 5 can be obtained for the outer CD light blocking region. Thus, the objective of blocking only the CD light in the outer region and transmitting the DVD and HDDVD light can be achieved. As a result, the NA (numerical aperture) of HDDVD can be 0.650, the NA of DVD can be 0.628, and the NA of CD can be 0.470.

  The spectral transmittance characteristics shown in FIG. 5 are 99% and 0.2% with respect to the ideal state where the transmittance is 100% at a wavelength of 750 nm or less and 0% at a wavelength of 750 nm or more. It is close to the ideal state. If this cannot be realized with actual filter characteristics, for example, the transmittance at a wavelength of 700 nm or less is 90% or more, and the transmittance is about 5% or less at a wavelength of 770 nm or more. Even in this case, there is a side effect that the signal level of the optical disk device is slightly lowered or the CD jitter characteristic is slightly deteriorated, but it is not at a level where it cannot be used but can be used.

In such a first embodiment, the optical path length L _h shown in the above equation (3) is within the allowable value by reducing the aberration with respect to an arbitrary light beam height h in both of the HDDVD, DVD, and CD. The lens surface shape of the objective lens 1 is set so as to obtain such a value. As a result, the aberration of HDDVD, DVD, and CD is reduced well, and a good light spot can be obtained on each information recording surface.

  In the first embodiment, the light incident surface A is divided into a plurality of sections in the radial direction from the optical axis, and the aberration of each section of the HDDVD, DVD, and CD is satisfactorily reduced within an allowable value. It is set as follows.

で表わされる。なお、式（５）での光源高さｈは、ｊ番目の区間でのものである。 The surface shape of the light incident surface A in the first embodiment will be described with reference to FIG. Now, the distance between the points a and b in the j-th section from the optical axis OA side in the light beam height h direction (radial direction) of the light incident surface A is expressed by the following function Z _Aj ,

It is represented by Note that the light source height h in equation (5) is for the j-th section.

In FIG. 6, for the light exit side B of the objective lens 1, let c be the point of the light beam height h, and d be the point on the light exit side B in a direction parallel to the optical axis OA from this point c. At this time, the surface shape of the light emitting side surface B is set so as to be expressed by the following expression by the distance Z _B between the points c and d with respect to an arbitrary light ray height h.

For each of HDDVD, DVD, and CD, the range (range of h) and its constants B, C, K, and A _{4 for} each section in Equation (5) for satisfactorily reducing the aberration within an allowable value. , A ₆ , A ₈ , A ₁₀ , A ₁₂ , A ₁₄ , A ₁₆ are calculated. The calculation results are shown in the table of FIG. Further, the value of each coefficient in the equation (6) is also calculated, and the calculation result is shown in the table of FIG.

  Further, the table of FIG. 9 shows the distance and arrangement between the optical elements in the optical system corresponding to FIG. 3 with respect to the objective lens in HDDVD, DVD, and CD.

The distance between the surface vertices f and e on the optical axis of the objective lens 1, that is, the center thickness t ₀ is 1.94 mm. Further, the refractive index n at the wavelength λ ₁ = 405 nm (Blu-ray Disc) is 1.54972, the refractive index n at the wavelength λ ₂ = 655 nm (DVD) is 1.53, and the refractive index at the wavelength λ ₃ = 790 nm (CD). The rate n is 1.5263653.

The thickness and refractive index of the transparent substrate are 0.6 mm and 1.6235 for the wavelength λ ₁ = 405 nm (Blu-ray disc), and 0.6 mm and the refractive index is 1.5 for the wavelength λ ₂ = 655 nm (DVD). 58, and the wavelength λ ₃ = 790 nm (CD) has a thickness of 1.2 mm and a refractive index of 1.57163.

  In the case of HDDVD having a wavelength of 405 nm, NA is 0.650 and the focal length is 3.1015 mm. In the case of DVD having a wavelength of 655 nm, NA is 0.628 and the focal length is 3.2116 mm. The effective diameter of the incident parallel light beam is φ4.032 for both HDDVD and DVD. Further, the entire surface of the A-side lens with φ4.032 is the HDDVD / DVD common use area. For a CD with a wavelength of 790 nm, the NA is 0.470 and the focal length is 3.2327 mm.

  FIG. 9 shows the object plane for the stop, objective lens, disk and for the objective lens. As shown in FIG. 9, for example, in HDDVD and DVD, the incident light to the objective lens is parallel light, that is, the distance between the object plane for the objective lens and the objective lens is ∞. In the case of an actual optical system, an HDDVD laser and a DVD laser are arranged at the focal position of the collimator lens, and collimator lens emission light is incident on the objective lens as parallel light.

  In the case of CD, the distance from the object surface to the objective lens is 49.4 mm, and divergent light is incident on the objective lens. Also for this CD, in the case of an actual optical system, the distance from the light emitting point of the CD laser to the surface vertex on the light source side of the objective lens may be 49.4 mm. In this case, there is a concern that the optical pickup becomes large.

  In such a case, the collimator lens may be disposed between the CD laser light source and the objective lens, and the position of the light emission point of the CD laser may be disposed closer to the collimator lens than the focal position of the collimator lens. As a result, the light emitted from the CD laser and passed through the collimator lens becomes divergent light and enters the objective lens. At this time, the collimator lens and the CD laser may be arranged so that the incident light to the objective lens is the same as the light incident state emitted from a distance of 49.4 mm without the collimator lens.

  FIG. 9 shows the effective diameter of the diaphragm surface, and a wavelength selective filter as shown in FIG. 4 is used. That is, in FIG. 4, the outer diameter of the total light transmission region = the inner diameter of the CD light blocking region is set to φ3.15, and the outer diameter of the CD light blocking region is set to φ4.032 or more which is larger than the effective diameter of HDDVD or DVD. Specifically, the effective diameter of the diaphragm surface is set to φ4.8, for example, in consideration of the dimensions necessary for holding by the lens frame.

  Regarding the thickness of the wavelength selective filter, it is fundamental that incident light is incident at 0 degree. This incident light beam may be obliquely incident in consideration of the mounting position of parts such as lasers and various mirrors, the variation in accuracy, and the positional deviation in the direction perpendicular to the optical axis of the emission point by the two-wavelength laser or the three-wavelength laser. . Therefore, the thickness of the wavelength selective filter is desirably thin, but is 0.5 mm in the first embodiment.

Regarding the first embodiment shown in FIG. 9, when looking at the relationship between S ₁ , S ₂ , and S ₃ shown in equations (1) and (2),
HDDVD; λ ₁ = 405 nm, S ₁ = ∞,
DVD; λ ₂ = 655 nm, S ₂ = ∞,
CD: λ ₃ = 790 nm, S ₃ = 49.4 mm
Because
405 nm (λ ₁ ) <790 nm (λ ₃ )
And
(1 / S ₁ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 49.4) = 0.0202429
Because
0 <0.0202429, ie (1 / S ₁ ) <(1 / S ₃ )
It becomes.
In other words, in HDDVD and CD,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ )
Is established.

Also,
655 nm (λ ₂ ) <790 nm (λ ₃ )
And
(1 / S ₂ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 49.4) = 0.0202429
Because
0 <0.0202429, ie (1 / S ₂ ) <(1 / S ₃ )
It becomes.
In other words, in DVD and CD,
λ ₂ <λ ₃ and (1 / S ₂ ) <(1 / S ₃ )
Is established.

  Incidentally, the refractive index of the objective lens is close to the refractive index of the plastic resin. For example, when a polyolefin resin, an acrylic resin, or the like is applied, an objective lens may be designed using the refractive index value of the resin to set each aspheric shape and the center thickness of the lens. In particular, polyolefin resins are advantageous in that they do not absorb water even in a high humidity environment, and therefore there is no change in refractive index. The acrylic resin is advantageous in that the transmittance of Blu-ray is good and the change with time of the transmittance in the vicinity of Blu-ray (405 nm) is small.

  Further, it is advantageous to select a material with low birefringence as the material of the objective lens, since it is easy to obtain a good value as wavefront aberration when a lens is manufactured by injection molding or casting. Furthermore, when an acrylic material is used as described above, a change in water absorption rate under a high humidity environment may be a problem. Therefore, when using acrylic materials, it is effective to adjust the humidity in advance and absorb water to some extent, considering the case where it is placed in an environment close to absolutely dry or in an environment close to high humidity. It is. In that case, the design may be performed using the value of the refractive index of the resin in a state where water has been absorbed to some extent after humidity control.

In the equation (6), when the values of the coefficients C, K, A ₄ , A ₆ , A ₈ , A ₁₀ are substituted to obtain the distance Z _B for an arbitrary ray height h (≠ 0), The value is negative. This indicates that the point d on the light emission side surface B is located at the point c, and therefore is located on the emission surface side (left side in FIG. 6) from the surface vertex e through which the optical axis OA of the light emission side surface B passes. . On the other hand, when the distance Z _B is a positive value, it indicates that the distance is on the right side.

(I) Here, as an allowable value of the aberration for evaluating the aberration, the incident laser beam to the objective lens 1 has an incident angle of 0 ° (that is, parallel light parallel to the optical axis OA). , HDDVD (wavelength λ ₁ = 405 nm), DVD (wavelength λ ₂ = 655 nm), CD (wavelength λ ₃ = 790 nm) have an RMS wavefront aberration of 0.035λ, preferably 0.033λ. In the first embodiment, the light exit surface B and the light incident surface A are set to the above-described surface shapes so that the wavefront aberration of HDDVD, DVD, and CD is less than the allowable value.

In the first embodiment, a case where _three different wavelengths λ ₁ , λ _{2, and} λ ₃ are used is shown, but in general, n types (where n is an integer of 2 or more) different wavelengths λ _i (where n is an integer of 2 or more). However, the same applies to the case of using i = 1, 2,..., N).

を満足するようにする。このときの許容値Ｗ_０としては、０．０３５、好ましくは０．０３３以下とする。 (ii) Further, when n types of wavelengths λ _i are used in this way, the RMS wavefront aberration when the incident laser beam of these wavelengths λ _i has an incident angle of 0 ° is denoted by W _i · λ _i. These aberrations are

To be satisfied. The allowable value W _{0 at} this time is 0.035, preferably 0.033 or less.

However, in equation (7), the wavelength of the i-th light beam is λ _i (i = 1, 2,...), The sum of the squares of individual RMS wavefront aberrations over all wavelengths is ΣW _i ² , the wavelength the RMS wavefront aberration of the light beam of λ _i and _{W i} · _{λ i.}

(iii) Alternatively, when using a laser beam of different n types of wavelengths lambda _i, and the maximum of the RMS wavefront aberration among the wavelengths of respective lambda _{i W max,} the minimum RMS wavefront aberration and _{W min,}
1 ≦ W _max / W _min <W _th
And In this case, the allowable value _Wth is 1.8, preferably 1.6, and more preferably 1.4. In the case of the first embodiment, the RMS wavefront aberration _{W min} of either the maximum RMS wavefront aberration _{W max,} and the other is a minimum with the RMS wavefront aberration _{W 2} of RMS wavefront aberration _{W 1} and CD of DVD To do.

で０．０３５以下、好ましい０．０３３以下にもなっている。 FIG. 2 shows a wavefront aberration diagram of the first embodiment. The RMS wavefront aberration of HDDVD is 0.03152λRMS, the RMS wavefront aberration of DVD is 0.03237λRMS, and the RMS wavefront aberration of CD is 0.01764λRMS. These HDDVDs, DVDs, and CDs are 0.035λ RMS or less, and further 0.033λ RMS or less.
For the value of equation (7),

0.035 or less, preferably 0.033 or less.

FIG. 10 shows a light spot diagram of the first embodiment. The light spot diameter at 1 / e ² (= 0.135) relative light intensity is 0.5149 μm for 405 nm HDDVD , 0.8606 μm for 655 nm DVD, and 1.3979 μm for 790 nm CD. .

  Note that the above optical spot diameter is approximately 0.82 × wavelength / NA in an ideal optical system having no aberration, and it is generally desirable for an actual lens to be small. In addition, even if the optical SPOT diameter is too small, other side effects can be considered. Therefore, it is desirable that the value is 0.9 times to 1.03 times the value of 0.82 × wavelength / NA. Furthermore, if the optical spot diameter is too small, side effects such as super-resolution may occur. If the optical spot diameter is too large, the light SPOT condensing characteristic will be poor, and the optical spot will not be narrowed down. Adverse effects on the

Incidentally, the evaluation of the first embodiment is as follows.
In HDDVD (wavelength 405 nm, NA 0.650), 0.82 × wavelength / NA = 0.5109 μm. Since the actual SPOT diameter is 0.5149 μm, it is 1.0078 times the SPOT diameter by 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.
In DVD (wavelength 655 nm, NA 0.628), 0.82 × wavelength / NA = 0.8553 μm. Since the actual SPOT diameter is 0.8606 μm, it is 1.0062 times the SPOT diameter of 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.
For CD (wavelength 790 nm, NA 0.470), 0.82 × wavelength / NA = 1.3783 μm. Since the actual SPOT diameter is 1.3979 μm, it is 1.0142 times the SPOT diameter of 0.82 × wavelength / NA, and is in the range of 0.9 to 1.02.

In this lens, the wavefront aberration is set so as to appear on the + side at the wavelength of DVD655 nm and on the-side at the wavelength of HDDVD405 nm, and both wavefront aberrations are substantially symmetrical.
Note that there is a difference in optical path length between adjacent aspherical surfaces divided about the optical axis, and the difference is approximately an integer multiple corresponding to the DVD and CD wavelengths, and the HDDVD (Blu-ray, 405 nm) wavelength. Is designed so as to be almost twice the integer corresponding to.

  The table of FIG. 11 shows the difference between the approximate optical path length of the first section and the approximate optical path length of the second to ninth sections. The difference between the approximate optical path length of the second to eighth sections and the approximate optical path length of the first section is mλ (m is an integer) for a DVD with a wavelength of 655 nm and a CD with a wavelength of 790 nm. For HDDVD, it is 2 mλ (m is an integer).

Thus, the first embodiment can suppress the aberration within the allowable value. This is because in consideration of each wavelength and each substrate thickness, the lens surface shape is set so that the aberration falls within the allowable value, and the degree of divergence of the incident light to the objective lens is set.
In the first embodiment, it is clear from the graph of the light spot shown in FIG. 10 and the wavefront aberration shown in FIG. 2 that the total aberration is reduced. Further, in the first embodiment, the surface shape of the light incident side surface A of the objective lens 1 is given by the equation (5) and FIG. 7, and the surface shape of the light emitting side surface B is the non-shape shown in the equation (6) and FIG. It is given by the spherical formula. Therefore, a diffractive lens structure such as the conventional lens described above (for example, the lenses in Patent Documents 1 and 2) is not employed. Further, in the first embodiment, almost all light fluxes can be collected with respect to the aperture (NA) necessary for recording or reproduction, so that high light utilization efficiency can be obtained.

  In the first embodiment, three types of optical discs of HDDVD, DVD, and CD are used as examples. However, the present invention is not limited to this, and other types of optical discs may be used. The first embodiment can also be applied to optical discs having the same or different substrate thicknesses. The wavelength of the laser beam used for each of the optical discs is different, and the total aberration is accordingly changed. What is necessary is just to set a lens surface shape so that it may reduce.

Embodiment 2 of the Invention
In the second embodiment (second embodiment) of the invention, a case where the substrate thickness is different and the wavelengths are different from 405 nm, 655 nm, and 790 nm will be described. Specifically, in the second embodiment, when using a so-called Blu-ray or blue laser with a wavelength of 405 nm and a substrate thickness of 0.1 mm, so-called DVD, when the wavelength is 655 nm and the substrate thickness is 0.6 mm, so-called CD, This relates to a case where the wavelength is 790 nm and the substrate thickness is 1.2 mm.

  In the second embodiment, the basic lens configuration is the same as that of the first embodiment shown in FIG. That is, for Blu-ray and DVD, parallel light is incident from the A side and a good light spot is formed on the recording surface of the disk substrate (not shown) on the B side. For CD, divergent light is incident from the A side, and a good light spot is formed on the recording surface of a disk substrate (not shown) on the B side.

A surface of the light source side, the relationship between Z _A and h is represented by the formula (5). The specific numerical value is shown for every section 1-22 in the table | surface of FIG. 12 thru | or FIG. Further, the B surface on the side opposite to the light source and on the disk side represents the relationship between Z _B and h in Expression (6). The specific numerical values are shown in FIG. 12 to 16, R represents a radius of curvature, “small” represents the optical axis side, and “large” represents the side away from the optical axis.

Incidentally, the refractive index of the objective lens of the second embodiment is a value close to the refractive index of glass having a high refractive index, for example, VC89.
Further, the surface vertex f of the optical axis of the objective lens, the distance between the e, that is, the center thickness _{t 0} is 2.076Mm. Further, the refractive index n at the wavelength λ ₁ = 405 nm (Blu-ray Disc) is 1.83164, the refractive index n at the wavelength λ ₂ = 655 nm (DVD) is 1.7911, and λ ₃ = 790 nm (CD). The refractive index n is 1.783555.

The thickness and refractive index of the transparent substrate are 0.6 mm and a refractive index of 1.6235 at a wavelength of λ ₁ = 405 nm (Blu-ray Disc). Furthermore, the wavelength λ ₂ = 655 nm (DVD) has a thickness of 0.6 mm and a refractive index of 1.58, and the wavelength λ ₃ = 790 nm (CD) has a thickness of 1.2 mm and a refractive index of 1.573. Therefore, the difference in refractive index between the wavelength λ ₁ = 405 nm (Blu-ray disc) and the wavelength λ ₂ = 655 nm (DVD) is 0.03 or more, and the wavelength λ ₁ = 405 nm (Blu-ray disc) and the wavelength λ ₃ = 790 nm ( The difference in refractive index between CD) is also 0.03 or more.

  In the case of a 405 nm wavelength Blu-ray disc, the NA is 0.850 and the focal length is 1.765 mm. In the case of a DVD with a wavelength of 655 nm, the NA is 0.600, the focal length is 1.8564 mm, and the wavelength is 790 nm. In the case of CD, NA is 0.469 and the focal length is 1.8745 mm. Further, each aperture diameter is as shown in FIG. 17, and a wavelength selective filter is used for the aperture as in the first embodiment.

  The table of FIG. 17 shows the distance and arrangement between the optical elements in the optical system corresponding to FIGS. 12 to 16 with reference to the objective lens in Blu-ray, DVD and CD. FIG. 17 shows the object plane for the stop, objective lens, disk and for the objective lens. As shown in FIG. 17, for example, in Blu-ray or DVD, the incident light to the objective lens is parallel light, that is, the distance between the object plane and the objective lens for the objective lens is ∞. In the case of an actual optical system, a blue laser and a DVD laser are arranged at the focal position of the collimator lens so that the collimator lens emission light enters the objective lens as parallel light.

  In the case of CD, the objective lens has a distance of 15.5 mm from the object surface to the objective lens, and divergent light is incident on the objective lens. Also for this CD, in the case of an actual optical system, the distance from the light emitting point of the CD laser to the surface vertex on the light source side of the objective lens may be 15.5 mm. In this case, there is a concern that the optical pickup becomes large.

  In such a case, the collimator lens may be disposed between the CD laser light source and the objective lens, and the position of the light emission point of the CD laser may be disposed closer to the collimator lens than the focal position of the collimator lens. As a result, the light emitted from the CD laser and passed through the collimator lens becomes divergent light and enters the objective lens. At this time, the collimator lens and the CD laser may be arranged so that the incident light to the objective lens is the same as the light incident state emitted from a distance of 15.5 mm without the collimator lens.

Regarding the second embodiment shown in FIG. 17, when looking at the relationship between S ₁ , S ₂ , and S ₃ shown in equations (1) and (2),
Blu-ray; λ ₁ = 405 nm S ₁ = ∞,
DVD; λ ₂ = 655 nm S ₂ = ∞,
CD: λ ₃ = 790 nm S ₃ = 15.5 mm
Because
405 nm (λ ₁ ) <790 nm (λ ₃ )
And
(1 / S ₁ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 15.5) = 0.0645516129
So,
0 <0.0645516129, ie (1 / S ₁ ) <(1 / S ₃ )
It becomes.
So in Blu-ray and CD,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ )
Is established.

Also,
655 nm (λ ₂ ) <790 nm (λ ₃ )
And
(1 / S ₂ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 15.5) = 0.0645516129
So,
0 <0.0645516129, ie (1 / S ₂ ) <(1 / S ₃ )
It becomes.
In other words, in DVD and CD,
λ ₂ <λ ₃ and (1 / S ₂ ) <(1 / S ₃ )
Is established.

  As can be seen from FIG. 18, the effective diameter on the A side is φ2.228, that is, 0 to 1.114 in the range of h, that is, the sections 1 to 21 shown in FIG. 18 are used for both DVD and Blu-ray. This is a common use area. On the other hand, the section outside φ2.228, that is, the section larger than 1.114 in the range of h, that is, the section 22 shown in FIG.

However, the wavelength selective filter transmits the 655 nm light of the DVD in both the Blu-ray exclusive use area and the DVD. For this reason, the incident laser beam light is incident, and the incident light becomes so-called flare light having very large aberration on the information recording surface of the DVD, and does not have a harmful effect.
This is also explained from the light spot diagram of FIG.

となり、０．０３５以下、好ましい０．０３３以下にもなっている。 FIG. 19 shows a wavefront aberration diagram of the second embodiment.
As the RMS wavefront aberration value, the RMS wavefront aberration of Blu-ray is 0.02410λRMS, the RMS wavefront aberration of DVD is 0.02753λRMS, and the RMS wavefront aberration of CD is 0.02127λRMS. These Blu-ray Discs, DVDs, and CDs have RMS wavefront aberrations of 0.035λ RMS or less, and further 0.033λ RMS or less.
For the value of equation (7),

Thus, it is 0.035 or less, preferably 0.033 or less.

  FIG. 18 also shows the approximate optical path lengths of the Blu-ray / DVD common use area Nos. 2 to 21 with respect to the approximate optical path length of the first period in each aspherical part shown in FIG. The figure shows how many times the wavelength λ is shifted.

As can be seen from FIG. 18, the second to 21st sections have a difference of 2 mλ for Blu-ray with a wavelength of 405 nm, and a difference of mλ with respect to a DVD with a wavelength of 655 nm and a CD with a wavelength of 790 nm (m is an integer). . This is because the shorter wavelength λ ₁ is between 380 and 430 nm, the longer wavelength λ ₂ is between wavelengths 630 and 680 nm, and λ ₃ is in the vicinity of wavelength 790 nm. It is easy to satisfy this relationship, and it is easy to obtain the favorable wavefront aberration shown in FIG.

  In addition, since the refractive index of the lens is the value shown above, it is easy to obtain an approximate difference in optical path length and good wavefront aberration. Specifically, the difference between the refractive index at 405 nm and the refractive index at 655 nm is 0.04054, and the difference between the refractive index at 405 nm and the refractive index at 790 nm is 0.048085. Since both of these values are larger than 0.03, it is easy to obtain an approximate difference in optical path length and good wavefront aberration.

FIG. 20 shows the difference in wavefront aberration between the 405 nm Blu-ray and the 655 nm DVD and the ratio thereof in each aspherical part shown in FIG.
As shown in FIG. 20, the ratio ΔVd (λ655) / ΔVd (λ405) of the difference between the wavefront aberrations in the common use region of 655 nm and 405 nm is between 0.90 and 1.65. The ratio ΔVd (λ405) / ΔVd (λ655) is between 0.60 and 1.11. The wavefront aberration of each region is also 0.14λ or less at both wavelengths.

FIG. 21 shows a light spot diagram of the second embodiment. As shown in FIG. 21, the diameter of the light spot with a relative light intensity of 1 / e ² (= 0.135) is 0.3836 μm for 405 nm Blu-ray, 0.8570 μm for 655 nm DVD, and 570 nm for 790 nm CD. The light spot shape is 1.4112 μm and has no problem.

When this optical SPOT diameter is compared with the value of 0.82 × wavelength / NA shown in the first embodiment, it is as follows.
In Blu-ray (wavelength 405 nm, NA 0.850), 0.82 × wavelength / NA = 0.907 μm. Since the actual SPOT diameter is 0.3836 μm, it is 0.9818 times the SPOT diameter by 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.

  For DVD (wavelength 655 nm, NA 0.600), 0.82 × wavelength / NA = 0.8952 μm. Since the actual SPOT diameter is 0.8570 μm, it is 0.9574 times the SPOT diameter by 0.82 × wavelength / NA and is in the range of 0.9 to 1.02. In this DVD, the optical spot diameter is about 4% (0.04 times) smaller than that of the ideal lens. This is because DVD light also passes through the Blu-ray dedicated area, and the optical spot diameter is reduced under the influence of the DVD light.

  For CD (wavelength 790 nm, NA 0.469), 0.82 × wavelength / NA = 1.812 μm. Since the actual SPOT diameter is 1.4084 μm, it is 1.0197 times the SPOT diameter of 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.

  In the second embodiment, the wavelength of one monochromatic light is 405 nm and the other is 655 nm and 790 nm, but one may be 380 to 430 nm and the other may be 630 to 680 nm and 770 to 820 nm. In that case, the refractive index becomes a different value, but it may be designed according to that value.

Embodiment 3 of the Invention
In Embodiment 3 (Third Embodiment) of the invention, three types of optical disks, namely, HDDVD (λ ₁ = 408 nm), DVD (λ ₂ = 658 nm), and CD (λ ₃ = 785 nm) are taken as examples. This will be described with reference to the drawings. In the first embodiment, light incident on the objective lens is parallel light on HDDVD and DVD, and divergent light on CD. In the third embodiment, a case will be described in which the incident light beam to the objective lens is convergent light in HDDVD, parallel light in DVD, and divergent light in CD. Note that the configuration of the third embodiment (HDDVD: convergent light incidence) can increase the object distance with respect to the CD as compared with the first embodiment (HDDVD: parallel light incidence). When the diverging light is incident on the objective lens and used, coma aberration generated off the axis is larger than when the parallel light is used. Therefore, when the objective lens is laterally shifted (hereinafter referred to as objective lens shift) in a plane substantially orthogonal to the optical axis for tracking, there arises a problem that the amount of coma aberration increases. The amount of coma aberration is greatly affected by the divergence of light rays. A smaller divergence, that is, a longer object distance, can suppress the amount of coma generated when the objective lens is shifted. Therefore, the configuration of the optical system according to the third embodiment is more advantageous for CD objective lens shift than the configuration of the first embodiment. However, since the converging light is made incident on the HDDVD, coma aberration occurs when the objective lens is shifted even in the HDDVD. Therefore, it is preferable to consider the balance of object distance (magnification) in HDDVD and CD. When the magnification of HDDVD is m1 and the magnification of CD is m3,
0 <m1 ≦ 1/10, −1 / 10 ≦ m3 <0
It is preferable to satisfy
Outside the above range, the amount of coma generated when the objective lens is shifted increases. More preferably, the following range is good.
0 <m1 ≦ 1/20, −1 / 20 ≦ m3 <0
The lens in the third embodiment has a refractive index equivalent to that of a plastic resin. However, when the lens material is glass, it may be designed with the refractive index of glass.

The basic lens configuration of the third embodiment will be described with reference to FIG. 6 shown in the first embodiment. In the third embodiment, in the HDDVD, convergent light is incident from the A side to form a good light spot on the recording surface of a disk substrate (not shown) on the B side. For DVD, parallel light is incident from the A side, and a good light spot is formed on the recording surface of a disk substrate (not shown) on the B side. For the CD, divergent light is incident from the A side to form a good light spot on the recording surface of the disk substrate (not shown) on the B side.
A surface of the light source side, the relationship between Z _A and h is represented by the formula (5). The specific numerical values are shown for each of the sections 1 to 7 in the table of FIG.

Further, the distance between the surface vertices f and e on the optical axis of the objective lens, that is, the center thickness t ₀ is 1.92 mm. Further, the refractive index n at the wavelength λ ₁ = 408 nm (HDDVD) is 1.5229, the refractive index n at the wavelength λ ₂ = 658 nm (DVD) is 1.5048, and λ ₃ = 785 nm (CD). Has a refractive index n of 1.5018.

The thickness and refractive index of the transparent substrate are 0.6 mm and a refractive index of 1.622 at a wavelength λ ₁ = 408 nm (HDDVD). Furthermore, the wavelength λ ₂ = 658 nm (DVD) has a thickness of 0.6 mm and a refractive index of 1.577, and the wavelength λ ₃ = 785 nm (CD) has a thickness of 1.2 mm and a refractive index of 1.5720.

  In the case of an HDDVD having a wavelength of 408 nm, the NA is 0.650 and the focal length is 3.101 mm. In the case of a DVD having a wavelength of 658 nm, the NA is 0.650, the focal length is 3.2059 mm, and the wavelength is 785 nm. In the case of CD, NA is 0.470 and the focal length is 3.2246 mm. Each aperture diameter is as shown in FIG. 23, and a wavelength selective filter is used for the aperture as in the first embodiment.

  The table of FIG. 23 shows the distance and arrangement between the optical elements in the optical system corresponding to FIG. 1 with respect to the objective lens in HDDVD, DVD, and CD. FIG. 23 shows the object plane for the stop, objective lens, disk and for the objective lens. As shown in FIG. 23, for example, in HDDVD, the incident light to the objective lens is convergent light. Therefore, the distance between the object plane and the objective lens for the objective lens can be expressed as a negative value, which is -93.9 mm. In an actual optical system, a collimator lens may be disposed between the HDDVD laser light source and the objective lens, and the position of the light emitting point of the HDDVD laser may be disposed farther from the collimator lens than the focal position of the collimator lens. As a result, the light emitted from the HDDVD laser and passing through the collimator lens becomes convergent light and enters the objective lens. In DVD, the incident light to the objective lens is parallel light, that is, the distance between the object plane and the objective lens for the objective lens is ∞. In the case of an actual optical system, a DVD laser is arranged at the focal position of the collimator lens so that the light emitted from the collimator lens enters the objective lens as parallel light.

  In the case of CD, the distance from the object surface to the objective lens is 98.9 mm, and the lens configuration is such that divergent light enters the objective lens. In the case of an actual optical system, if the distance from the light emitting point of the CD laser to the surface vertex on the light source side of the objective lens is 98.9 mm, the optical pickup becomes large. Therefore, it is preferable to dispose the collimator lens between the CD laser light source and the objective lens, and to arrange the emission point of the CD laser closer to the collimator lens than the focal position of the collimator lens. As a result, the light emitted from the CD laser and passed through the collimator lens becomes divergent light and enters the objective lens. At this time, the collimator lens and the CD laser may be arranged so that the incident light to the objective lens is the same as the light incident state emitted from a distance of 98.9 mm without the collimator lens.

Regarding the third embodiment shown in FIG. 23, the relationship between S ₁ , S ₂ , and S ₃ shown in equations (1) and (2) is as follows.
HDDVD; λ ₁ = 408 nm S ₁ = −93.9 mm,
DVD; λ ₂ = 658 nm S ₂ = ∞,
CD: λ ₃ = 785 nm S ₃ = 113.0 mm
Because
408 nm (λ ₁ ) <785 nm (λ ₃ )
And
(1 / S ₁ ) = (1 / (− 93.9)) = − 0.010649627
(1 / S ₃ ) = (1 / 113.0) = 0.008849557
So,
−0.010649627 <0.008849557, ie (1 / S ₁ ) <(1 / S ₃ )
It becomes.

In other words, in HDDVD and CD,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ )
Is established.
In addition, the magnification m1 of HDDVD is 1 / 31.2, and the magnification m3 of CD is −1 / 34.1, and 0 <m1 ≦ 1/20 and −1 / 20 ≦ m3 <0.
Is satisfied.

Also,
658 nm (λ ₂ ) <785 nm (λ ₃ )
And
(1 / S ₂ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 113.0) = 0.008849557
So,
0 <0.008849557, ie (1 / S ₂ ) <(1 / S ₃ )
It becomes.

In other words, in DVD and CD,
λ ₂ <λ ₃ and (1 / S ₂ ) <(1 / S ₃ )
Is established.

  As can be seen from FIG. 22, the effective diameter on the A side is φ3.8932 mm, that is, 0 to 1.94658 in the range of h, that is, the sections 1 to 6 shown in FIG. 22 are used for both DVD and HDDVD. This is a common use area. On the other hand, the section outside φ3.8932, that is, the section larger than 1.94658 in the range of h, that is, section 7 shown in FIG.

  However, the wavelength selective filter transmits the 408 nm light of HDDVD in both the DVD dedicated use area and HDDVD. Therefore, in the case of HDDVD, the laser beam emitted from the laser enters the HDDVD through the objective lens, but the incident light has a very large aberration on the information recording surface of the HDDVD, so-called flare light. It will not adversely affect.

Alternatively, a wavelength selective filter as shown in FIG. 24 may be used. As shown in FIG. 24, the wavelength selective filter is divided into an inner total light transmission region 61, an intermediate CD (785 nm) light blocking region 62, and an outer HDDVD (408 nm) and CD (785 nm) light blocking region 63. Has been. For example, the intermediate CD light blocking region 62 may be provided with a dichroic coating that reflects light of 750 nm or more, and the outer HDDVD and CD light blocking region 63 may be provided with a dichroic coating that transmits only light of 600 to 700 nm. .
As a result, the NA (numerical aperture) of HDDVD can be 0.650, the NA of DVD can be 0.650, and the NA of CD can be 0.470.

となり、０．０３５以下、好ましい０．０３３以下にもなっている。 FIG. 25 shows a wavefront aberration diagram of the third embodiment.
As the RMS wavefront aberration value, the RMS wavefront aberration of HDDVD is 0.03253λRMS, the RMS wavefront aberration of DVD is 0.03178λRMS, and the RMS wavefront aberration of CD is 0.02091λRMS. These HDDVDs, DVDs, and CDs have RMS wavefront aberrations of 0.035λ RMS or less, and further 0.033λ RMS or less.
For the value of equation (7),

Thus, it is 0.035 or less, preferably 0.033 or less.

  FIG. 26 shows the amount of coma aberration (third order) generated when the objective lens of the first embodiment and the objective lens of the third embodiment have a CD (NA 0.470) objective lens shift of 0.3 mm. In the first embodiment, the incident light to the objective lens is parallel light for HDDVD and DVD, and divergent light for CD, and the object distance of the CD is 49.4 mm. In the third embodiment, the incident light to the objective lens is converged in HDDVD, parallel in DVD, and divergent in CD, so that the object distance of CD is 113.0 mm, which is longer than that in the first embodiment. ing. As a result, as shown in FIG. 26, the amount of coma generated when the objective lens of the CD, which was 0.0469 λRMS in the objective lens of the first embodiment, is 0.0177 λRMS in the objective lens of the third embodiment. Has been reduced.

  In addition, FIG. 27 shows the approximate optical path lengths of the HDDVD / DVD common use area in the second to sixth sections with reference to the approximate optical path length of the first section in each aspherical part shown in FIG. The figure shows how many times the wavelength λ is shifted.

As can be seen from FIG. 27, the second to sixth sections have a difference of 2 mλ for HD DVD having a wavelength of 408 nm, and a difference of mλ for DVD having a wavelength of 658 nm and a CD having a wavelength of 785 nm (m is an integer). . This is because the shorter wavelength λ ₁ is between 380 and 430 nm, the longer wavelength λ ₂ is between 630 and 680 nm, and λ ₃ is near the wavelength of 785 nm. It is easy to satisfy this relationship, and it is easy to obtain the favorable wavefront aberration shown in FIG.

FIG. 28 shows a light spot diagram of the third embodiment. As shown in FIG. 28, the light spot diameter with a relative light intensity of 1 / e ² (= 0.135) is 0.5029 μm for a 408 nm HDDVD, 0.8236 μm for a 658 nm DVD, and a CD of 785 nm. It is 1.3811 μm, and it has a light spot shape with no problem.

When this optical SPOT diameter is compared with the value of 0.82 × wavelength / NA shown in the first embodiment, it is as follows.
In HDDVD (wavelength 408 nm, NA 0.650), 0.82 × wavelength / NA = 0.5147 μm. Since the actual SPOT diameter is 0.5029 μm, it is 0.9771 times the SPOT diameter of 0.82 × wavelength / NA and is in the range of 0.9 to 1.02. In this HDDVD, the optical spot diameter is about 2.3% (0.023 times) smaller than that of the ideal lens. This is because HDDVD light also passes through the DVD dedicated area, and the optical spot diameter is reduced due to the influence.

In DVD (wavelength 658 nm, NA 0.65), 0.82 × wavelength / NA = 0.8301 μm. Since the actual SPOT diameter is 0.8236 μm, it is 0.9922 times the SPOT diameter by 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.
For CD (wavelength 785 nm, NA 0.470), 0.82 × wavelength / NA = 1.696 μm. Since the actual SPOT diameter is 1.3811 μm, it is 1.0084 times the SPOT diameter of 0.82 × wavelength / NA and is in the range of 0.9 to 1.02.

  In the third embodiment, the wavelengths of monochromatic light are 408 nm, 658 nm, and 785 nm, respectively, but may be 380 to 430 nm, 630 to 680 nm, and 770 nm to 820 nm. In that case, the refractive index becomes a different value, but it may be designed according to that value. Further, in the first embodiment, the incident light beam to the objective lens is HDDVD and DVD, collimated light, CD is divergent light, in the third embodiment the incident light beam to the objective lens is HDDVD and convergent light, DVD is parallel light, The CD has a divergent light configuration, but this combination of configurations is not limited to this. For example, it is possible to adopt a configuration of HDDVD convergence, DVD convergence, and CD divergence. Here, if the object distance of HDDVD and DVD is the same, the same photodetector can be used for HDDVD and DVD.

Embodiment 4 of the Invention
The fourth embodiment of the invention (fourth embodiment) is an example in which the substrate thickness is different from the second embodiment and the wavelengths are different from 408 nm, 655 nm, and 790 nm, and the objective shown in the second embodiment is shown. A case where the objective lens is implemented using a lens made of a material different from the lens material will be described.

  In the second embodiment, as a material of the objective lens, for example, glass having a high melting point / heat distortion temperature (Tg = 528 ° C.) such as VC89 is considered. On the other hand, in the fourth embodiment, as a material of the objective lens, for example, a glass having a low melting point and heat distortion temperature (Tg = 288 ° C.) such as K-PG325 manufactured by Sumita Optical Glass Co., Ltd. is considered. The difference in the material of the objective lens between the second embodiment and the fourth embodiment will be described below in comparison.

  A high refractive index glass such as VC89 has a high melting point of 600 ° C. or more, so that it is difficult to engrave a fine structure on the mold surface as a lens mold that can withstand that temperature. Requires a type. Moreover, since it takes a considerable time to lower the temperature to room temperature after molding the lens, there is a problem that productivity per hour is low. On the other hand, a glass having a low refractive index such as K-PG325 has a low melting point of about 300 ° C., so that the same mold as that used for plastic materials can be used as a mold for lens molding. It is easy to engrave a fine structure such as a ring zone in the mold. In addition, since the time required for temperature reduction to room temperature is short, there is an advantage that productivity per hour is high. Hereinafter, a material mainly composed of glass having a refractive index of 1.49 to 1.70 and a heat distortion temperature of 300 ° C. or lower, such as K-PG325, will be referred to as low melting point glass.

  However, the refractive index of the low-melting glass is, for example, 1.49 to 1.70 with respect to a light beam having a wavelength of 408 nm, and there is a difficulty in that the refractive index is small compared to a glass such as a general VC89. Therefore, since the low melting point glass has a small refractive index, it is difficult to optically design a high NA lens. In the present embodiment, the thickness at the center of the lens is set to 2.642 mm, which is larger than usual, to ensure the characteristics when the low melting point glass is used as the material of the objective lens. The refractive index of low-melting glass is almost the same as that of plastic materials. However, low-melting glass has high temperature and humidity characteristics compared to plastic materials. It is advantageous compared to

  Specifically, the fourth embodiment includes a so-called Blu-ray and blue laser using a wavelength of 408 nm and a substrate thickness of 0.0875 mm, a so-called DVD, a wavelength of 655 nm and a substrate thickness of 0.6 mm, and a so-called CD. This relates to a case where the wavelength is 790 nm and the substrate thickness is 1.2 mm, and the low-melting glass is used as the material of the lens.

  In the fourth embodiment, the basic lens configuration is the same as that of the first embodiment shown in FIG. That is, for Blu-ray and DVD, parallel light is incident from the A side and a good light spot is formed on the recording surface of the disk substrate (not shown) on the B side. For CD, divergent light is incident from the A side, and a good light spot is formed on the recording surface of a disk substrate (not shown) on the B side.

A surface of the light source side, the relationship between Z _A and h is represented by the formula (5). The specific numerical values are shown for each section in the tables of FIGS. 29, 30, 31, 32, 33, and 34. Further, the B surface on the side opposite to the light source and on the disk side represents the relationship between Z _B and h in Expression (6). The specific numerical values are shown in FIG. 29 to 35, R represents a radius of curvature, “small” represents the optical axis side, and “large” represents the side away from the optical axis.
Further, the distance between the surface vertices f and e on the optical axis of the objective lens, that is, the center thickness t ₀ is 2.642 mm. Further, the refractive index n at the wavelength λ ₁ = 408 nm (Blu-ray Disc) is 1.5126, the refractive index n at the wavelength λ ₂ = 655 nm (DVD) is 1.4987, and λ ₃ = 790 nm (CD). The refractive index n is 1.4958.

The thickness and refractive index of the transparent substrate are 0.0875 mm and a refractive index of 1.6205 at the wavelength λ ₁ = 408 nm (Blu-ray Disc). Furthermore, the wavelength λ ₂ = 655 nm (DVD) has a thickness of 0.6 mm and a refractive index of 1.5794, and the wavelength λ ₃ = 790 nm (CD) has a thickness of 1.2 mm and a refractive index of 1.5725. Thus, the difference in the respective refractive index of the wavelength lambda ₁ = 408 nm (Blu-ray Disc) and wavelength lambda ₂ = 655 nm (DVD) is 0.03, the wavelength lambda ₁ = 408 nm and (Blu-ray Disc) wavelength lambda ₃ = 790 nm ( The difference in refractive index between CD) is also 0.03 or more.

  In the case of a Blu-ray disc having a wavelength of 408 nm, NA is 0.850 and the focal length is 2.3721 mm. In the case of a DVD having a wavelength of 655 nm, the NA is 0.650, the focal length is 2.4262 mm, and the wavelength is 790 nm. In the case of CD, NA is 0.510 and the focal length is 2.4378 mm. In addition, each aperture diameter is as shown in FIG. 36, and a wavelength selective filter is used for the aperture as in the first embodiment.

  The table of FIG. 36 shows the distance and arrangement between the optical elements in the optical system corresponding to FIGS. 29 to 35 with reference to the objective lens in Blu-ray, DVD, and CD. FIG. 47 represents the object plane for the stop, objective lens, disk and for the objective lens. As shown in FIG. 36, for example, in Blu-ray or DVD, the incident light to the objective lens is parallel light, that is, the distance between the object plane for the objective lens and the objective lens is ∞. In the case of an actual optical system, a blue laser and a DVD laser are arranged at the focal position of the collimator lens so that the light emitted from the collimator lens enters the objective lens as parallel light.

  In the case of CD, the objective lens has a distance from the object plane to the objective lens of 19.35 mm, and divergent light is incident on the objective lens. Also for this CD, in the case of an actual optical system, the distance from the light emitting point of the CD laser to the surface vertex on the light source side of the objective lens may be 19.35 mm. In this case, there is a concern that the optical pickup becomes large.

  In such a case, the collimator lens may be disposed between the CD laser light source and the objective lens, and the position of the light emission point of the CD laser may be disposed closer to the collimator lens than the focal position of the collimator lens. As a result, the light emitted from the CD laser and passed through the collimator lens becomes divergent light and enters the objective lens. At this time, the collimator lens and the CD laser may be arranged so that the incident light to the objective lens is the same as the light incident state emitted from a distance of 19.35 mm without the collimator lens.

With regard to the fourth embodiment shown in FIG. 36, the relationship between S ₁ , S ₂ , and S ₃ shown in equations (1) and (2) is as follows.
Blu-ray; λ ₁ = 408 nm S ₁ = ∞,
DVD; λ ₂ = 655 nm S ₂ = ∞,
CD: λ ₃ = 790 nm S ₃ = 19.35 mm
Because
408 nm (λ ₁ ) <790 nm (λ ₃ )
And
(1 / S ₁ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 19.35) = 0.05168
So,
0 <0.05168, ie (1 / S ₁ ) <(1 / S ₃ )
It becomes.
So in Blu-ray and CD,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ )
Is established.

Also,
655 nm (λ ₂ ) <790 nm (λ ₃ )
And
(1 / S ₂ ) = (1 / ∞) = 0,
(1 / S ₃ ) = (1 / 19.35) = 0.05168
So,
0 <0.05168, ie (1 / S ₂ ) <(1 / S ₃ )
It becomes.
In other words, in DVD and CD,
λ ₂ <λ ₃ and (1 / S ₂ ) <(1 / S ₃ )
Is established.

As can be seen from FIG. 37, the effective diameter on the A side is φ3.153, that is, 0 to 1.5765 in the range of h, that is, the sections 1 to 29 shown in FIG. 37 are used for both DVD and Blu-ray. This is a common use area. On the other hand, a section outside φ3.153, that is, a section larger than 1.5765 in the range of h, that is, section 30 and section 31 shown in FIG.
However, the wavelength selective filter transmits the 655 nm light of the DVD in both the Blu-ray exclusive use area and the DVD. For this reason, incident laser beam light is incident, and the incident light is so-called flare light that has a very large aberration on the information recording surface of the DVD, so that it does not have an adverse effect.

FIG. 38 shows a wavefront aberration diagram of the fourth embodiment.
As the RMS wavefront aberration value, the RMS wavefront aberration of Blu-ray is 0.03210λRMS, the RMS wavefront aberration of DVD is 0.03740λRMS, and the RMS wavefront aberration of CD is 0.04320λRMS. These Blu-ray Discs, DVDs and CDs have RMS wavefront aberrations of 0.045λ RMS or less.
FIG. 37 also shows the approximate optical path lengths of the Blu-ray / DVD common use area Nos. 2 to 29 with reference to the approximate optical path length of the first period in each aspherical portion shown in FIG. The figure shows how many times the wavelength λ is shifted.

As can be seen from FIG. 37, the second to 21st sections have a difference of 2 mλ for Blu-ray with a wavelength of 408 nm, and a difference of mλ with respect to a DVD with a wavelength of 655 nm and a CD with a wavelength of 790 nm (m is an integer). . This is because the shorter wavelength λ ₁ is between 380 and 430 nm, the longer wavelength λ ₂ is between wavelengths 630 and 680 nm, and λ ₃ is in the vicinity of wavelength 790 nm. It is easy to satisfy this relationship, and it is easy to obtain the favorable wavefront aberration shown in FIG.

  In the fourth embodiment, the wavelength of monochromatic light is 408 nm, 655 nm, and 790 nm, but may be 380 to 430 nm, 630 to 680 nm, and 770 nm to 820 nm. In that case, the refractive index becomes a different value, but it may be designed according to that value.

  As described above, in the fourth embodiment, an objective lens with NA = 0.85 is formed of a material mainly composed of glass having a refractive index of 1.49 to 1.70 with respect to a light beam of 408 nm. As such a low refractive index material having characteristics, for example, a material having a low melting point of about 300 degrees can be used, so that it can be manufactured using a normal mold and productivity can be increased.

Embodiment 5 of the Invention
FIG. 39 is a diagram showing a film configuration of a wavelength selective filter (sharp cut filter) by 16-layer coating. As shown in the figure, this wavelength selective filter is configured by laminating SiO ₂ layers and Ta ₂ O ₅ layers alternately on a glass substrate made of BK7. The refractive index shown in FIG. 39 is a value for a light beam of 780 nm. FIG. 40 shows the spectral characteristics of the wavelength selective filter having the configuration shown in FIG. As shown in the figure, the transmittance of the CD wavelength from 780 nm to 790 nm is suppressed to about 1%, and has good characteristics. Note that the spectral characteristics shown in FIG. 40 can be obtained even if the refractive index and film thickness of the film configuration shown in FIG. 39 are about 0.5 to 1% and are shifted due to manufacturing errors. Is possible.

  On the other hand, in the case of a 16-layer coat, the cost is high because the number of coats is large. FIG. 41 is a diagram showing a film configuration of a wavelength selective filter using a 10-layer coat. The refractive index shown in FIG. 41 is a value for a light beam of 810 nm. FIG. 42 shows spectral characteristics of the wavelength selective filter having the configuration shown in FIG. As shown in the figure, since the transmittance of CD wavelength from 780 nm to 790 nm is about 7 to 8%, the characteristics are deteriorated as compared with the 16-layer coated wavelength selective filter, but CD recording / reproduction is performed. It does not adversely affect the characteristics, but is practical enough to withstand. The wavelength selective filter with 10-layer coating can realize cost reduction compared to the wavelength-selective filter with 16-layer coating. Note that the spectral characteristics shown in FIG. 42 can be obtained even if the refractive index and film thickness of the film configuration shown in FIG. Is possible.

  Thus, as a wavelength selective filter, it is preferable that the transmittance | permeability is 10% or less in CD wavelength (770 nm-800 nm). A more preferable range is 5% or less, and an optimal range is 2% or less.

  Moreover, as a wavelength selective filter, it is preferable that the transmittance | permeability is 85% or more in the wavelength (380 nm-700 nm) of Blu-ray or DVD. A more preferable range is 90% or more, and even more preferably 95% or more. The optimum range is 97% or more.

  The wavelength selective filters shown in FIGS. 39 to 42 are formed on the glass substrate separately from the objective lens, but may be formed by coating on one surface of the objective lens. In this case, it is preferable to coat the surface on the disk side close to a flat surface among the surfaces of the objective lens. Thereby, a uniform film can be easily formed. At this time, by making the refractive index of the wavelength selective filter and the refractive index of the objective lens to be coated substantially equal, it becomes possible to realize a design equivalent to the coating design in the embodiments shown in FIGS. Manufacture is easy. In this embodiment, the refractive index of an objective lens such as plastic is 1.54 to 1.55, whereas the refractive index of BK7 is 1.51, and both refractive indexes are substantially equal. The refractive index of the wavelength selective filter is preferably in the range of 0.9 to 1.1 with respect to the refractive index of the objective lens.

Embodiment 6 of the Invention
The sixth embodiment (sixth embodiment) of the invention will be described below in detail with reference to the drawings. In the present embodiment, as an optical component included in the optical pickup device, an objective lens that focuses laser light on an optical storage medium will be described as an example. The optical pickup device is compatible with CDs, DVDs, and Blu-ray discs. The optical component is not limited to an objective lens, and the effect of the present invention can be achieved as long as it is an optical member through which light of three types of wavelength regions passes.

  The antireflection film used in the present invention has a structure in which a high refractive index film and a low refractive index film are sequentially laminated on an optical component. Materials for the high refractive index film include oxides such as aluminum oxide, zirconium oxide, titanium oxide, tantalum oxide, niobium oxide, antimony oxide, cerium oxide, yttrium oxide, hafnium oxide, magnesium oxide, silicon nitride, germanium nitride, etc. At least one selected from nitrides, carbides such as silicon carbide, sulfides such as zinc sulfide, and mixed materials thereof is used as a material for the low refractive index film, such as silicon oxide, magnesium fluoride, aluminum fluoride, fluorine. There is at least one selected from fluorides such as barium fluoride, calcium fluoride, lithium fluoride, sodium fluoride, strontium fluoride, yttrium fluoride, thiolite, cryolite (cryolite), and mixtures thereof . In addition, it is desirable to use oxides, nitrides, carbides, and fluorides in order to improve storage characteristics in a high temperature and high humidity environment.

  The antireflection film of the present invention is produced by, for example, a vacuum film forming method. As the vacuum film forming method, various film forming methods such as a vacuum evaporation method, a sputtering method, a chemical vapor deposition method, and a laser brazing method can be used. When using vacuum deposition, ion is applied to the substrate using ion plating, a cluster ion beam method, or another ion gun that ionizes part of the vapor flow and applies a bias to the substrate to improve film quality. It is effective to use ion-assisted deposition. Examples of the sputtering method include a DC reactive sputtering method, an RF sputtering method, and an ion beam sputtering method. Further, examples of the chemical vapor phase method include a plasma polymerization method, a light-assisted vapor phase method, a thermal decomposition method, and a metal organic chemical vapor phase method. In addition, the film thickness of each refractive index film | membrane can be made into a desired film thickness by changing the vapor deposition time etc. at the time of film formation.

In addition to plastics such as polyolefin resin, cycloolefin resin, methacrylic resin, and polycarbonate resin, optical parts include optical glass such as quartz glass and borosilicate glass, and oxide single crystals such as Al ₂ O ₃ and MgO. , Polycrystalline substrates, fluoride single crystal substrates such as CaF ₂ , MgF ₂ , BaF ₂ , LiF, etc., polycrystalline substrates, chlorides such as NaCl, KBr, KCl, bromide single crystals, other crystal substrates, etc. Any transparent optical material can be applied. The principle of the effect of such an antireflection film will be described with reference to FIG. FIG. 43 is a cross-sectional view schematically showing the objective lens 1 provided with the antireflection film having two layers as described above on the surface. The antireflection film has a high refractive index layer 8 and a low refractive index layer 7.

  The reflected light R when the light beam O enters the objective lens 1 will be described. Here, in FIG. 43, the light beam O is shown with an angle for convenience, but the light beam O is incident in parallel to the optical axis of the objective lens 1. The light beam O first enters the low refractive index layer 7. On the surface of the low refractive index layer 7, the light beam O is divided into transmitted light P and reflected light R1. Next, the transmitted light P enters the high refractive index layer 8 from the low refractive index layer 7. At the boundary between the low refractive index layer 7 and the high refractive index layer 8, the transmitted light P is divided into transmitted light Q and reflected light R2. Then, the transmitted light Q reaches the objective lens 1 from the high refractive index layer 8 and is reflected at the boundary between the high refractive index layer 8 and the objective lens 1 to become reflected light R3.

  Here, at the boundary between the high-refractive index layer 8 and the objective lens 1, there is a light beam incident on the objective lens 1 instead of the reflected light R 3, but the description is omitted because it is not necessary for discussing the reflected light R. . In addition, the reflected light R2 is multiple-reflected at the upper and lower interfaces of the low refractive index layer 7 at a reflectance of about 5%, and the reflected light R3 is multiple reflected at the upper and lower interfaces of the high refractive index layer 8 at a reflectance of about 5%. However, if the light of these components is reflected twice or more, the intensity of the light becomes as weak as 0.25% or less, and is omitted here.

The reflected light R is a composite wave of the reflected lights R1 to R3 shown in FIG. Therefore, the state of the reflected light R changes depending on the phase difference between the reflected lights R1 to R3. The phase difference between the reflected lights R1 and R2 is as follows: the objective lens 1, the refractive index n _H of the high refractive index layer 8, the refractive index n _L of the low refractive index layer 7, and the optics of the high refractive index layer 8 and the low refractive index layer 7. It is defined by the relationship between the film thickness and the wavelength of the light beam O.

  The basic principle will be explained. For example, it is assumed that the optical film thickness of the low refractive index layer 7 is ¼ of the wavelength of the light beam O. In that case, since the phase difference between the reflected light R1 and the reflected light R2 is a half wavelength of the light beam O, the reflected light R1 and the reflected light R2 cancel each other. As a result, only the reflected light R3 becomes the reflected light R. Therefore, in the wavelength region, the ratio of the light intensity of the reflected light R3 to the light beam O becomes the reflectance. On the other hand, if the wavelength of the light beam O is changed while the conditions of the objective lens 1, the high refractive index layer 8, and the low refractive index layer 7 are left unchanged, the reflected lights R1 to R3 may just cancel each other. In the wavelength region, the reflectance is low.

  Utilizing such a principle, reflection occurs at least in the vicinity of the wavelength region corresponding to the wavelength of light used for Blu-ray Disc (about 405 nm) and in the vicinity of the wavelength region corresponding to the wavelength of light used for DVD (about 655 nm). If an antireflection film having a minimum value and having a low reflectance in the wavelength region corresponding to the wavelength of light used for CD (about 790 nm) is formed on the surface of the objective lens 1, the above three types of light It can be expected to have good performance as an optical pickup device corresponding to a storage medium.

Hereinafter, based on the principle described above, an example in which the conditions of the objective lens 1, the high refractive index layer 8, and the low refractive index layer 7 are set and simulated is shown. In the simulation examples and embodiments described below, the film thickness d _L of the film thickness d _H and a low refractive index layer of the high refractive index layer 8 is determined based on the quarter-wave (QW) of a predetermined wavelength . The predetermined wavelength here corresponds to the wavelength of the light beam O shown in FIG. 43, which is 500 nm in the present embodiment. In the present embodiment, n _H d _H (optical film thickness of the high refractive index layer 8) = 1QW, and n _L d _L (optical film thickness of the low refractive index layer 7) = 2QW, as described in FIG. Degree. That is, 225 nm ≦ n _H d _H ≦ 275 nm and 100 nm ≦ n _L d _L ≦ 150 nm.

FIG. 44 shows a simulation of the change in reflectivity in the above-described three types of wavelength regions with respect to the objective lens 1 and the low-refractive index layer 7 as the refractive index of the high-refractive index layer 8 changes. The reflectance values in the respective wavelength regions are the maximum reflectance value at a wavelength of 405 ± 5 nm (hereinafter referred to as λ ₁ ), the maximum reflectance value and the wavelength at a wavelength of 655 ± 20 nm (hereinafter referred to as λ ₂ ). 790 ± 20 nm indicating the minimum value of the reflectance at (hereinafter, lambda ₃ to). N _S represents the refractive index of the objective lens, n _L represents the refractive index of the low refractive index layer, and n _H represents the refractive index of the high refractive index layer.

FIG. 44A shows an example in which APEL (registered trademark) manufactured by Mitsui Chemicals, Inc. is used for the objective lens, and SiO ₂ is used as the low refractive index layer. In the figure, the data surrounded by a thick line shows the most preferable simulation result. FIG. 45 shows a graph of reflectance values with respect to wavelength in the case of n _H = 1.75, 1.85, 1.95 and a single low refractive index layer under the conditions of FIG. As shown in FIG. 45, the reflectance value with respect to the wavelength is a double-humped curve having a minimum value in the vicinity of λ ₁ and λ ₂ , and also shows a low value in λ ₃ . Since the reflectance of the single apel is 4.5%, the reflectance is reduced in a desired wavelength region by providing an antireflection film. Moreover, even if the graph by a low refractive index layer single layer is used as a comparative example, the reflectance is reduced in each of the three types of wavelength regions.

FIG. 44B is a simulation in the case where n _S = 1.70 and other conditions are the same as those in FIG. The same applies to FIG. 44C, which is a simulation in the case of n _S = 1.85. Since n _H must be higher than n _S , the lower limit value of n _{H that} can be used is increased and the amount of data is reduced as compared to the case of n _S = 1.54.

FIG. 46 shows a graph of reflectance values with respect to wavelength in the case of n _H = 1.95, 2.05, 2.15 and a low refractive index layer single layer under the conditions of FIG. As shown in FIG. 46, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . However, compared to the curve shown in FIG. 45, the change in reflectance in the wavelength region from λ ₂ to λ ₃ and beyond is steep, and the reflectance is considerably high at λ ₃ . However, since the reflectance of the objective lens 1 in FIG. 46 is 8.9%, the reflectance is reduced by providing an antireflection film.

44D to 44F show the case where MgF ₂ is used for the low refractive index layer and n _L = 1.38. Also in FIGS. 44D to 44F, simulations are performed in the cases of n _S = 1.54, 1.70, and 1.85. FIG. 47 shows a graph of reflectance values with respect to wavelength in the case of n _H = 1.65, 1.75, 1.85 and a single low refractive index layer in the case of FIG. Similarly to FIG. 45, the value of the reflectance with respect to the wavelength shows a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . However, compared to the curve shown in FIG. 45, the change in reflectance in the wavelength region from λ ₂ to λ ₃ and beyond is sharp when n _H = 1.85, and the reflectance is considerably large at λ ₃ . It shows a high value. However, since the reflectance of the objective lens 1 in FIG. 47 is 4.5%, the reflectance is reduced by providing an antireflection film.

Here, in FIGS. 44A to 44F, data exceeding the case where the reflectance value is a single layer of the low refractive index layer indicates the cell in gray. Furthermore, the data in which the reflectance value exceeds the reflectance of the objective lens alone inverts the cell. That is, it can be said that n _H including a cell shown in gray or a cell shown inverted is not preferable in terms of optical characteristics. 44 (a) to (c), a gray cell appears at n _H = 2.15, and in FIGS. 44 (d) to (f), a gray cell appears at n _H = 1.95. Yes.

The present inventors have discovered that by analyzing the n _H and optimum n _H where gray cell appears in the table shown in FIG. 44 found n _S, the relationship between the value of n _H and n _L. Since the value of n _H where gray cell appears close to a value obtained by squaring the value of n _L, the upper limit of the optimal value of n _H is considered to meet the least _{n H ≦ n L × n L} . The value of n _H is because there must be a higher value than n _S, believed to satisfy at least n _S <n _H.

Therefore, the optimal value of n _H As a minimum, n _S <can be guided n _H ≦ n condition _L × n _L, the optimum value of n _H present in this range a, b and any If it is a constant, it can be derived by (a × n _S + b × n _L × n _L ) / 2. As a result of the simulation analysis as shown in FIG. 44, the values of a and b are preferably in the range of 1.00 ≦ a ≦ 1.4 and 0.65 ≦ b ≦ 1.00. a = 1.21 and b = 0.84. From these, the optimum value of n _H with respect to the values of n _S and n _L can be set as parameter A, and A = (1.21 × n _S + 0.84 × n _L × n _L ) / 2.

In addition, as shown in the table of FIG. 51, the value of n _S increases, narrows the effective range as n _H, the reflectance becomes high at a wavelength near 790nm as shown in Figure 53. In addition, when the value of n _S is decreased, the realizable substance is limited. Therefore, the value of _{n S} is preferably satisfies the condition 1.46 ≦ _{n S} ≦ 1.65. Similarly, when the value of n _L increases, the maximum value of the reflectance existing between λ ₁ and λ ₂ increases, so that the bandwidth with low reflectance is narrowed, and the value of n _L is smaller than 1.3. Then, it becomes difficult to obtain a stable film forming material. Therefore, the value of _{n L} is preferably satisfies the condition 1.3 ≦ _{n L} ≦ 1.55.

  Considering these conditions, specific examples will be described. By applying any one of the three types of AR coating as shown in the following Examples 1, 2, and 3, relatively high transmittance characteristics can be obtained at the above three wavelengths. In addition, since the number of layers is as small as two, AR coating can be performed at a relatively low cost. Examples of these AR coat designs are shown in the table of FIG.

Example 1.
A mixed material of Al ₂ O ₃ (n = 1.68) and ZrO ₂ (n = 2.07) having a refractive index of 1.85 on a lens, a refractive index of 1.53, 135.1 nm (optical film thickness) λ / 2), and SiO ₂ having a refractive index of 1.46 was coated to 85.5 nm (equivalent to optical film thickness λ / 4). The reference wavelength of the AR coating optical film thickness is 500 nm.

Example 2
On the lens, with a refractive index of 1.53, Y ₂ O ₃ with a refractive index of 1.80 is 139 nm (equivalent to an optical film thickness λ / 2), and SiO ₂ with a refractive index of 1.46 is 85.5 nm (optical film thickness λ / 4 equivalent). The reference wavelength of the AR coating optical film thickness is 500 nm.

Example 3
On the lens, with a refractive index of 1.53, SiN with a refractive index of 2.04 is 122.5 nm (equivalent to optical film thickness λ / 2), and SiO ₂ with a refractive index of 1.46 is 85.5 nm (optical film thickness λ / 4 equivalent). The reference wavelength of the AR coating optical film thickness is 500 nm.

  FIG. 49 shows spectral reflectance characteristics per lens surface by the AR coatings of Examples 1 to 3. 49A, 49B, and 49C correspond to the first, second, and third embodiments, respectively. As shown in FIG. 49, it can be seen that in all of FIGS. 49A, 49B, and 49C, the reflectance decreases in the vicinity of 405 nm and in the wavelength region of 650 nm to 790 nm.

Also, the examples shown in FIGS. 48A and 48B also have conditions of 0.9 ≦ n _H /A≦1.1, 0.1 ≦ n _H −n _S , and n _H −n _S ≦ 0. .4 condition is satisfied. Specifically, in FIG. 48A, n _L is 1.46, n _H is 1.85, n _S is 1.53, A is 1.82, n _L d _L is 125 nm, n _H d _H. Is 250 nm, n _H / A is 1.0, and n _H -n _S is 0.32. In FIG. 48B, n _L is 1.46, n _H is 1.80, n _S is 1.53, A is 1.821, n _L d _L is 125 nm, n _H d _H is 250 nm, n _H / a is _1.0, n H -n _S is 0.27. In FIG. 48C, n _L is 1.46, n _H is 2.04, n _S is 1.53, A is 1.82, n _L d _L is 125 nm, n _H d _H is 250 nm, n _H / A is 1.1 and n _H -n _S is 0.51.

In FIG. 50, ZEONEX (registered trademark) manufactured by Nippon Zeon Co., Ltd. is used for the objective lens 1, a mixed film of Al ₂ O ₃ and ZrO ₂ is used for the high refractive index layer 8, and SiO ₂ is used for the low refractive index layer 7. It is Example 4 used. In this case, n _S = 1.525, n _H = 1.83, n _L = 1.46, and A = 1.818. _Further, n _H d _H = _256.3nm, an n _L d _L = 129.3nm. As shown in FIG. 50, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . Further, λ ₃ also shows a low value of 3% or less. Therefore, it can be said that this example is a suitable antireflection film.

FIG. 51 shows Example 5 in which APEL is used for the objective lens 1, MgO is used for the high refractive index layer 8, and MgF ₂ is used for the low refractive index layer 7. In this case, n _S = 1.54, n _H = 1.74, n _L = 1.38, and A = 1.732. Further, n _H d _H = 260.1 nm and n _L d _L = 126.3 nm. As shown in FIG. 51, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . Furthermore, λ ₃ also shows a low value of 3% or less. Therefore, it can be said that this example is a suitable antireflection film.

FIG. 52 is an example 6 in which PMMA (polymethyl methacrylate) is used for the objective lens 1, Y ₂ O ₃ is used for the high refractive index layer 8, and MgF ₂ is used for the low refractive index layer 7. In this case, n _S = 1.49, n _H = 1.78, n _L = 1.38, and A = 1.701. Further, n _H d _H = 253.8 nm and n _L d _L = 129.8 nm. As shown in FIG. 52, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . Furthermore, λ ₃ also shows a low value of 3% or less. Therefore, it can be said that this example is a suitable antireflection film.

FIG. 53 shows an example 7 in which Arton (registered trademark) manufactured by JSR Corporation is used for the objective lens 1, Y ₂ O ₃ is used for the high refractive index layer 8, and SiO ₂ is used for the low refractive index layer 7. In this case, n _S = 1.51, n _H = 1.78, n _L = 1.46, and A = 1.809. _Further, n _H d _H = _258.1nm, an n _L d _L = 123.9nm. As shown in FIG. 53, the reflectance value with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . Furthermore, λ ₃ also shows a low value of 3% or less. Therefore, it can be said that this example is a suitable antireflection film. In the present embodiment, it is greater than the value of the value n _H of the parameters A. At the same time, the value of the minimum value of the reflectance in the vicinity of λ ₁ and λ ₂ is also increased, and it is possible to confirm a change due to an increase in the width between the value of n _{H and} the value of parameter A.

FIG. 54 shows an example 8 in which PMMA is used for the objective lens 1, a mixed film of Al ₂ O ₃ and ZrO ₂ is used for the high refractive index layer 8, and MgF ₂ is used for the low refractive index layer 7. In this case, n _S = 1.49, n _H = 1.83, n _L = 1.38, and A = 1.756. Further, n _H d _H = 250.3 nm and n _L d _L = 132.6 nm. As shown in FIG. 54, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . Furthermore, λ ₃ also shows a low value of 3% or less. Therefore, it can be said that this example is a suitable antireflection film. In the present embodiment, it is smaller than the value of the value n _H of the parameters A. The above width of the value and the value of the parameter A further n _H than Examples 4-7 are open, but it can be confirmed that if the opening of this degree still preferred antireflective film can be formed .

Next, as a comparative example, an example to be excluded if n _H is defined using the parameter A according to the present embodiment within the range defined as a two-layer antireflection film in Patent Document 3. FIG. 55 shows a comparative example 1 in which PC is used for the objective lens 1, ZrO ₂ is used for the high refractive index layer 8, and SiO ₂ is used for the low refractive index layer 7. In this case, n _S = 1.58, n _H = 2.05, n _L = 1.46, and A = 1.851. _Further, n _H d _H = _248.8nm, an n _L d _L = 132.8nm. As shown in FIG. 55, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . However, the reflectance in lambda ₃ and becomes high, since the show a 3% or more, it can be said that it is usable as an antireflective film not preferred.

FIG. 56 is a comparative example 2 in which ZEONEX is used for the objective lens 1, Ta ₂ O ₅ is used for the high refractive index layer 8, and SiO ₂ is used for the low refractive index layer 7. In this case, n _S = 1.525, n _H = 2.14, n _L = 1.46, and A = 1.818. _Further, n _H d _H = _242.0nm, an n _L d _L = 137.0nm. As shown in FIG. 56, the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . However, the reflectance in lambda ₃ and becomes high, since the show a 5% or more, optical characteristics deteriorate in lambda _3, it can be said that not preferable as an antireflection film corresponding to three wavelengths.

Figure 57 is using BK7 the objective lens 1, the TiO ₂ used in the high refractive index layer 8, a comparative example 3 using the SiO ₂ for the low refractive index layer 7. In this case, n _S = 1.52, n _H = 2.30, n _L = 1.46, and A = 1.815. In addition, n _H d _H = 226.0 nm and n _L d _L = 136.0 nm. As shown in FIG. 57, the value of the reflectance with respect to the wavelength has a minimum value in the vicinity of λ _{1 and in the} vicinity of λ ₂ . However, the minimum value in the vicinity of λ ₂ is 2.5% or more, and the value corresponding to λ ₂ is 3% or more. Furthermore, the reflectance exceeds 10% at λ ₃ , and it can be said that it is no longer preferable as an anti-reflection film corresponding to two wavelengths as well as an anti-reflection film corresponding to three wavelengths.

FIG. 58 shows a table summarizing the characteristics of these examples and comparative examples. FIG. 58 shows the values of n _S , n _H , and n _L of Examples 4 to 8 and Comparative Examples 1 to 3, the value of parameter A calculated therefrom, the ratio of the value of n _{H to} the value of parameter A, λ ₁ to The reflectance value at λ ₃ and various analysis values of the optical performance are shown. Here, the reflectance value at each λ represents an average of the reflectance values in the respective λ regions. When the reflectance exceeds 3%, the cell is shown in gray.

As shown in FIG. 58 (a), in Examples 4 to 8, the reflectance value at each λ is at most 2.5%, which is generally a good value. On the other hand, in Comparative Examples 1 to 3, data indicated by a gray cell mainly appears at λ ₃ , indicating that the optical characteristics are not preferable when considered as an antireflection film corresponding to three wavelengths. .

The optical characteristics of the examples and comparative examples will be analyzed in more detail. FIG. 58B shows analysis values calculated from the values of n _S , n _H , and n _L in each of the examples and comparative examples. In Examples 4 to 8, the value of n _H / A falls within the range of 1 ± 0.1, whereas the value of n _H / A in Comparative Examples 1 to 3 is 1.11 or more. Therefore, the condition of 0.9 ≦ n _H /A≦1.1 can be derived. Moreover, in the value of n _H -n _S , Comparative Examples 1 to 3 show higher values than Examples 4 to 8. From these values, the relationship between n _H and n _S satisfies the condition of 0.1 ≦ n _H −n _S ≦ 0.4, more preferably 0.1 ≦ n _H −n _S ≦ 0.35. Is considered preferable.

Furthermore, in each Example and the comparative example, the average value and standard deviation value of the reflectance in λ _{1 to} λ ₃ are shown. These values indicate that the lower the average value, the lower the reflectance at each λ, and the lower the standard deviation value, the less the variation in reflectance at each λ. Therefore, a low average value and a standard deviation value indicate that the reflectance at each λ is stable at a low value, which is a more preferable embodiment. Further, in each example and comparative example, the average value of the square sum of the reflectances in λ _{1 to} λ ₃ is shown as the determination value. As shown in the figure, in Examples 4 to 8, the determination value is 2.11 at the highest, indicating a low value of 3.0 or less, and in Comparative Examples 1 to 3, it is a high value of 4.5 or more. . Therefore, the determination value can be specified to be 3.0 or less, more preferably 2.5 or less.

It is mainly the reflectance at λ ₃ that affects the standard deviation value. As the graphs of the reflectance corresponding to the wavelengths of Examples 4 to 8 and Comparative Examples 1 to 3 show, the reflectances at λ ₁ and λ ₂ are also relatively low in Comparative Examples 1 and 2, and 2 These may be used when used as a wavelength antireflection film. However, in Comparative Examples 1 to 3, the change in reflectance in the wavelength range from λ ₂ to λ ₃ is steep, and the reflectance at λ ₃ becomes high. Therefore, when designing the three-wavelength antireflection film, the goodness of these can be verified by analyzing the standard deviation of the reflectance in λ _{1 to} λ ₃ . In addition, by taking the sum of squares in the determination value, the influence of the above-mentioned λ ₃ can be reflected in the determination value sensitively.

From the analysis result shown in FIG. 58B, when the three-wavelength antireflection film formed on the objective lens 1 is formed of two layers including the high refractive index layer 8 and the low refractive index layer 7, A = (1. 21 × n _S + 0.84 × n _L × n _L ) / 2 is defined, the condition of 0.9 ≦ n _H /A≦1.1 is satisfied, and 0.1 ≦ n _H −n By selecting each member so as to satisfy the condition of _S ≦ 0.4, a suitable three-wavelength antireflection film can be formed.

As described above, according to the present invention, it is possible to provide an antireflection film and an optical component for an optical pickup which have a low reflectance in three types of wavelength regions and are formed of two layers.
In addition, since the antireflection film of the present invention has such a two-layer structure, the film formation time can be reduced and adverse effects such as thermal deformation of the film formation surface can be reduced as compared with a three-layer structure or more. .
In particular, the antireflection film of the present invention is applied to a glass having a low melting point and heat distortion temperature (Tg = 288 ° C.), such as K-PG325 manufactured by Sumita Optical Glass Co., Ltd. described in Embodiment 4 of the invention, that is, a low melting glass. When applied, it is effective from the viewpoint of preventing deformation of the lens surface during film formation. Further, such an antireflection film can exert its effect most when applied to both surfaces or one surface of the three-wavelength compatible lens used in the first and second embodiments of the present invention.

It should be noted that a selection system for each member in the three-wavelength antireflection film formed by two layers can be constructed using the parameter A described in the present embodiment. Such a system includes at least a condition input unit, a calculation unit, a result display unit, a material storage unit, and a control unit. If one or two of n _S , n _H, and n _L are input in the condition input unit, the calculation unit calculates a suitable value of the remaining numerical values based on the conditions of the parameters A and n _H −n _S. Based on the calculation result, a suitable material is selected from the materials stored in the material storage unit. These processes are performed by the control unit.

  Normally, it is difficult to calculate a suitable value for the remaining variables simply by specifying one of the three variables, but by adding a condition of selecting from the materials stored in the material storage unit, 3 By specifying one of the two variables, a suitable value for the remaining two variables can be selected.

As described above, according to the present invention, the aperture (NA) necessary for recording or reproducing by refraction without using a diffractive lens structure for three or more types of optical disks that perform recording or reproducing at different wavelengths. Thus, all the light beams can be condensed at a desired position with as little aberration as possible, and the light utilization efficiency can be further increased.
Further, as can be seen from the above description, the lens according to the present invention can be used in a multi-wavelength optical system that uses a plurality of monochromatic lights and also in an optical system that uses different wavelengths in optical communication or the like. .

Configuration of optical head
The configuration diagram of FIG. 59 shows a configuration example of an optical head using the above-described objective lens according to the present invention. FIG. 59 corresponds to the optical system for an HDDVD (405 nm) disc shown in the first embodiment. As shown in FIG. 59, the optical head 10 in this embodiment includes a blue laser 11, a DVD laser 12, a CD laser 13, and a 3SPOT linear diffraction grating 14, a half prism 15, a collimator lens 16, a half prism 17, and an actuator 181. , 182. In FIG. 59, parts corresponding to those in FIG.

  In FIG. 59, when the DVD disk 2 is recorded or reproduced, the DVD laser 12 is driven. A laser beam having a wavelength of 655 nm generated from the DVD laser 12 is reflected by the half prism 15 and enters the collimator lens 16. The laser beam that passes through the collimator lens 16 and becomes parallel light passes through the half prism 17 and passes through the wavelength selective filter 6. This transmitted light is incident on the objective lens 1 and collected by NA 0.63, and forms a light spot on the information recording surface of the DVD disk 3. Then, the reflected light reflected by the DVD disk 3 becomes parallel light by the objective lens 1 and enters the collimator lens 16.

  The collimator lens 16 converts the parallel light into convergent light and reaches a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuators 181 and 182 are driven.

  When the HDDVD disc 1 is mounted, the blue laser 11 is driven, and a laser beam having a wavelength of 405 nm generated from the blue laser 11 is transmitted through the half prism 15. The transmitted laser beam enters the collimator lens 16 and becomes parallel light after passing through the collimator lens. This parallel light is condensed at NA 0.65 on the information recording surface of the HDDVD disc 1 to form a light SPOT as in the DVD described above.

  Thereafter, the condensed light reaches a photodetector (not shown) as in the DVD described above. The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuators 181 and 182 are driven.

  Next, when the CD disk 4 is loaded, the CD laser 13 is driven. A laser beam having a wavelength of 790 nm generated from the CD laser 13 passes through the linear diffraction grating 14, is reflected by the half prism 17, and enters the wavelength selective filter 6. Since the inside of the wavelength selective filter 6 is the total light transmission region 61, only the light that passes through the inside of the 790 nm light blocking region 62 on the outside passes through as shown in FIG. This transmitted light is incident on the objective lens 1 and collected by NA 0.47, and forms a light spot on the information recording surface of the CD disk 4.

  The reflected light reflected by the CD disk 4 is condensed into convergent light by the objective lens 1 and reaches a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. Note that the tracking error signal in the case of the CD disk 3 is that the laser beam from the CD laser 12 is branched into three beams of zero-order light and soil primary light by the diffraction grating 18, and tracking error is caused by these ± first-order light I try to get a signal.

  The actuators 181 and 182 are driven by the tracking error signal and the focus error signal obtained in this way so that the objective lens 1 is positioned at an appropriate focus position and tracking position in the same manner as the DVD disk 2.

  In the above description, the photodetector is not shown in FIG. 59, but for example, the laser and the photodetector may be arranged in the same package of each laser. Alternatively, a new half prism or the like may be arranged so that the reflected light from the disk is incident on a photodetector arranged at a position different from the laser. In addition, HDDVD (405 nm) and DVD (655 nm) are parallel light, so-called infinite system, the light is incident on the objective lens, and the reflected light from the disk is also parallel light. It is also possible.

  In addition, since the CD is a finite system, the same detector as HDDVD and DVD is difficult in a normal optical system arrangement, and a separate photodetector for CD is necessary. Therefore, for example, it is also possible to provide a diffraction grating that acts as a diffraction grating only for wavelengths near 790 nm for reflected light from a CD disk so that CD light is incident on the same photodetector as HDDVD or DVD. It is.

  Furthermore, the collimator lens 16 is not necessarily required, and the present invention can be applied to a so-called finite optical system. It is also possible to arrange the laser at a position farther from the collimating position of the collimator lens 16 and to make the incident light to the objective lens as convergent light.

1. Configuration of optical head
The configuration diagram of FIG. 60 shows a configuration example of an optical head using the above-described objective lens according to the present invention. 60 corresponds to the optical pickup optical system shown in the second embodiment, that is, in the case of Blu-ray. The configuration of the optical head shown in FIG. 60 is the same optical system arrangement as that in FIG. 59 of HDDVD.
In FIG. 60, when the DVD disk 3 is recorded or reproduced, the DVD laser 12 is driven. A laser beam having a wavelength of 655 nm generated from the DVD laser 12 is reflected by the half prism 15 and enters the collimator lens 16. The laser beam that passes through the collimator lens 16 and becomes parallel light passes through the half prism 17 and passes through the wavelength selective filter 6. This transmitted light is incident on the objective lens 1 and condensed at NA 0.60, and forms a light spot on the information recording surface of the DVD disk 3.

  At this time, as a parallel light beam incident on the objective lens 1, a light beam having an NA of 0.8 or more is incident. However, as in the second embodiment, the light beam side surface of the objective lens 1 has a Blu-ray in the outer region. A dedicated area exists. Therefore, when DVD 655 nm light is incident, the light beam passing through the Blu-ray exclusive area becomes a DVD flare, and does not contribute to image formation on the DVD disk and optical SPOT formation. Therefore, an optical SPOT substantially equivalent to NA 0.60 is formed on the DVD disk 3.

  Then, the reflected light reflected by the DVD disk 3 becomes parallel light by the objective lens 1 and enters the collimator lens 16. The collimator lens 16 converts the parallel light into convergent light and reaches a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuators 181 and 182 are driven.

When a Blu-ray disc is mounted, the blue laser 11 is driven, and a laser beam having a wavelength of 405 nm generated from the blue laser 11 is transmitted through the half prism 15. The transmitted laser beam enters the collimator lens 16 and becomes parallel light after passing through the collimator lens. Thereafter, as with the above-described DVD, NA0. 8 5 condenses to form a light SPOT.

  Thereafter, the condensed light reaches a photodetector (not shown) as in the case of the DVD. The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuators 181 and 182 are driven.

  Next, when the CD disk 4 is loaded, the CD laser 13 is driven. A laser beam having a wavelength of 790 nm generated from the CD laser 13 passes through the linear diffraction grating 14, is reflected by the half prism 17, and enters the wavelength selective filter 6. Since the inside of the wavelength selective filter 6 is the total light transmission region 62, as shown in FIG. 4, only the light passing through the inner portion is transmitted in the 790 nm light blocking region 61 on the outer side. This transmitted light is incident on the objective lens 1 and collected by NA 0.47, and forms a light spot on the information recording surface of the CD disk 4.

The reflected light reflected by the CD disk 4 is condensed into convergent light by the objective lens 1 and reaches a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. Note that the tracking error signal in the case of the CD disk 3 is that the laser beam from the CD laser 12 is branched into three beams of 0th order light and ± 1st order light by the diffraction grating 18, and tracking error is caused by these ± 1st order light. I try to get a signal.
The actuators 181 and 182 are driven by the tracking error signal and the focus error signal obtained in this way so that the objective lens 1 is positioned at an appropriate focus position and tracking position in the same manner as the DVD disk 2.

  In the above description, the photodetector is not shown in FIG. 60. For example, the laser and the photodetector may be arranged in the same package of each laser. Alternatively, a new half prism or the like may be arranged so that the reflected light from the disk is incident on a photodetector arranged at a position different from the laser. In addition, Blu-ray (405 nm) and DVD (655 nm) are parallel light, so-called infinite system, the light is incident on the objective lens, and the reflected light from the disk is also parallel light. It is also possible.

  In addition, since the CD is a finite system, the same detector as HDDVD and DVD is difficult in a normal optical system arrangement, and a separate photodetector for CD is necessary. Therefore, for example, it is also possible to provide a diffraction grating that acts as a diffraction grating only for wavelengths near 790 nm for reflected light from a CD disk so that CD light is incident on the same photodetector as HDDVD or DVD. It is.

  Furthermore, the collimator lens 16 is not necessarily required, and the present invention can be applied to a so-called finite optical system. It is also possible to arrange the laser at a position farther from the collimating position of the collimator lens 16 and to make the incident light to the objective lens as convergent light.

2. Configuration of optical head
The configuration diagram of FIG. 61 shows a configuration example of an optical head using the above-described objective lens according to the present invention. FIG. 61 corresponds to the optical pickup optical system shown in the third embodiment.

  In FIG. 61, when the HDDVD disc 2 is recorded or reproduced, the HDDVD laser 11 is driven. A laser beam having a wavelength of 408 nm generated from the HDDVD laser 11 is reflected by the half prism 15 and enters the collimator lens 16. The laser beam that has passed through the collimator lens 16 and becomes convergent light passes through the half prism 17 and passes through the wavelength selective filter 6. This transmitted light is incident on the objective lens 1 and collected by NA 0.65, and forms a light spot on the information recording surface of the HDDVD disc 2.

  At this time, as a light beam incident on the objective lens 1, a light beam having an NA of 0.65 or more is incident. However, as in the third embodiment, the outer region on the light source side surface of the objective lens 1 is dedicated to DVD. An area exists. For this reason, when 408 nm light of HDDVD is incident, the light beam passing through the DVD dedicated area becomes flare light, and does not contribute to image formation on the HDDVD disc and optical SPOT formation. Therefore, a light spot substantially equivalent to NA 0.65 is formed on the HDDVD disc 2.

  Then, the reflected light reflected by the HDDVD disk 2 becomes divergent light by the objective lens 1 and enters the collimator lens 16. The collimator lens 16 converts the divergent light into convergent light and emits it to a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuator 18 is driven.

  When the DVD disk 3 is mounted, the DVD laser 12 is driven, and a laser beam having a wavelength of 658 nm generated from the DVD laser 12 is transmitted through the half prism 15. The transmitted laser beam enters the collimator lens 16 and becomes parallel light after passing through the collimator lens. Thereafter, similarly to the HDDVD described above, a light spot is formed by condensing on the information recording surface of the DVD disk 3 at NA 0.65.

  Thereafter, the reflected light reflected by the DVD disk 3 becomes parallel light by the objective lens 1 and enters the collimator lens 16. The collimator lens 16 converts the parallel light into convergent light and emits it to a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. A system control circuit (not shown) controls an actuator drive circuit (not shown) so that the objective lens 1 is positioned at an appropriate focus position and tracking position based on the obtained focus error signal and tracking error signal. Then, the actuators 181 and 182 are driven.

  Next, when the CD disk 4 is loaded, the CD laser 13 is driven. A laser beam having a wavelength of 785 nm generated from the CD laser 13 passes through the linear diffraction grating 14, is reflected by the half prism 17, and enters the wavelength selective filter 6. Since the inside of the wavelength selective filter 6 is the total light transmission region 61, as shown in FIG. 4, only the light passing through the inner part is transmitted in the light blocking region 62 on the outer side as shown in FIG. 4. This transmitted light is incident on the objective lens 1 and collected by NA 0.47, and forms a light spot on the information recording surface of the CD disk 4.

  The reflected light reflected by the CD disk 4 is condensed into convergent light by the objective lens 1 and reaches a photodetector (not shown). The detection output signal of the photodetector is supplied to a signal processing circuit (not shown) to obtain an information recording / reproducing signal, a focus error signal, and a tracking error signal. Note that the tracking error signal in the case of the CD disk 3 is obtained by branching the laser beam from the CD laser 12 into three beams of zero-order light and soil primary light by the diffraction grating 18, and these ± first-order light causes tracking error. I try to get a signal.

  The actuator 18 is driven by the tracking error signal and the focus error signal obtained in this way so that the objective lens 1 is positioned at an appropriate focus position and tracking position in the same manner as the DVD disk 2.

In the above description, the photodetector is not shown in FIG. 61. For example, the laser and the photodetector may be arranged in the same package of each laser. Alternatively, a half prism or the like may be newly arranged so that the reflected light from the disk is incident on a photodetector arranged at a position different from the laser.
Furthermore, the present invention can be applied to a lens configuration in which convergent light is incident in the case of HDDVD, convergent light in the case of DVD, and divergent light is incident in the case of CD. Here, if the object distance of HDDVD and DVD is the same, the same photodetector can be used for HDDVD and DVD.

Configuration of optical disk device.
62 is a block diagram showing an embodiment of an optical disk apparatus using an objective lens according to the present invention, in which 20 is an actuator drive circuit, 21 is a signal processing circuit, 22 is a laser drive circuit, 23 is a system control circuit, 24 Is disc discriminating means, and the details corresponding to FIG. 59 and FIG. 60 are omitted because they are the same.

  First, the disc discriminating unit 24 discriminates the type of disc loaded. As the disc discrimination method, a method of detecting the thickness of the substrate of the disc by an optical or mechanical method, a method of detecting an identification mark recorded in advance on the disc or the cartridge of the disc, and the like can be considered. Alternatively, a method may be used in which a disc signal is reproduced assuming a disc thickness and type, and if a normal signal cannot be obtained, it is determined that the disc has a different thickness and type. The disc discrimination result is transmitted from the disc discrimination means 24 to the system control circuit 23.

  When the disc is determined to be a DVD disc, a signal for turning on the DVD laser is transmitted from the system control circuit 23 to the laser drive circuit 22, and the DVD laser 11 is turned on by the laser drive circuit 22. Thereby, in the optical head, the 655 nm laser beam reaches the photodetector as in the embodiment shown in FIG. A detection signal from the photodetector is sent to the signal processing circuit 21 to generate an information recording / reproducing signal, a focus error signal, and a tracking error signal, which are sent to the system control circuit 23.

  The system control circuit 23 controls the actuator drive circuit 20 based on these focus error signal and tracking error signal. Based on this control, the actuator drive circuit 20 drives the actuators 181 and 182 to move the objective lens 1 in the focus direction and the tracking direction. By this so-called servo circuit operation, focus control and tracking control are normally performed, and the above-described circuits and actuators 181 and 182 operate. As a result, the objective lens 1 is positioned at the correct position with respect to the DVD disk 3, and as a result, an information recording / reproducing signal can be obtained satisfactorily.

  When it is determined that the loaded disk is the CD disk 4, a signal for turning on the CD laser 13 is transmitted from the system control circuit 23 to the laser driving circuit 22. As a result, a laser beam having a wavelength of 790 nm is generated from the CD laser 13. The subsequent operation is the same as that of the optical head of FIGS. 59 and 60. This laser beam reaches the photodetector, and each circuit and actuators 181 and 182 are operated as in the case of the DVD disk 2 described above. In operation, a servo operation is performed, and an information recording / reproducing signal can be obtained satisfactorily.

  When it is determined that the loaded disc is a Blu-ray or HDDVD disc, the system control circuit 23 transmits a signal for turning on the blue laser 11 to the laser drive circuit 22. As a result, a laser beam having a wavelength of 405 nm is generated from the blue laser 11. The subsequent operation is the same as that of the optical head of FIGS. 59 and 60. This laser beam reaches the photodetector, and each circuit and actuators 181 and 182 are operated as in the case of the DVD disk 2 described above. In operation, a servo operation is performed, and an information recording / reproducing signal can be obtained satisfactorily.

  FIG. 63 (a) is a top view when the objective lens according to the present invention is manufactured industrially. FIG. 63B is a side view of the lens, and the left half shows a cross section. As shown in the figure, a flange portion 101 is formed on the outer periphery of the lens 1. The flange portion 101 is provided with a flange surface 102 on the optical recording medium side when the lens 1 is attached to the optical disc apparatus and information is read from the optical recording medium. Hereinafter, for the sake of description, the optical recording medium side will be described as the upper side 102 and the opposite side will be described as the lower side 103. The flange portion 101 is located on the outer periphery of the optical function portion of the lens 1 and is formed in a belt shape over the entire periphery. The flange portion 101 does not need to be continuous on the outer periphery, and may have a shape having a notch portion on a part of the outer periphery.

  As shown in FIG. 63B, when viewed from the optical axis direction, the flange surface 102 is formed with a portion that is higher than the upper surface of the optical function portion. Accordingly, even when the lens 1 is placed on a desk or the like with the flange surface 102 side down during work, the flange surface 102 contacts the desk and the optical function unit does not contact the desk. Therefore, damage due to contact of the optical function unit with a desk or the like can be avoided. Further, it is possible to avoid damage caused by the optical recording medium coming into direct contact with the optical function unit after the lens 1 is attached to the optical disc apparatus.

It is a schematic diagram for demonstrating the optical path length in the optical system which consists of an objective lens and the transparent substrate of an optical disk. It is a graph which shows the wavefront aberration of HDDVD, DVD, and CD in 1st Embodiment. It is a schematic diagram which shows embodiment of the objective lens which concerns on this invention. It is a schematic diagram which shows one structural example of a wavelength selective filter. It is a graph which shows the spectral transmittance characteristic of CD light blocking area | region of a wavelength selective filter. It is a schematic diagram which shows a specific example of the lens surface shape of embodiment of this invention. Distance is a table showing coefficients for calculating the Z _A. Distance is a table showing coefficient for calculating Z _B. It is a table | surface which shows the distance between optical elements of the optical system in 1st Embodiment, and arrangement | positioning. It is a graph which shows the calculation result of the light spot with respect to the optical disk from which the kind in 1st Embodiment differs. It is a table | surface which shows the difference of the approximate optical path length of the 2nd-9th section, and the approximate optical path length of the 1st section. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _B and the optical height h. It is a table | surface which shows the distance between optical elements of the optical system in 2nd Embodiment, and arrangement | positioning. It is a table | surface which shows the difference of the approximate optical path length of the 2nd-22nd section, and the approximate optical path length of the 1st section. It is a graph which shows the wavefront aberration of Blu-ray, DVD, and CD in 2nd Embodiment. It is a table | surface which shows the difference and the ratio of the wavefront aberration of Blu-ray and DVD in 2nd Embodiment. It is a graph which shows the calculation result of the light spot with respect to the optical disk from which the kind in 2nd Embodiment differs. Distance is a table showing the relationship between Z _A and the optical height h. It is a table | surface which shows arrangement | positioning of the optical system in 3rd Embodiment. It is a figure which shows the structure of the wavelength-selective filter in 3rd Embodiment. It is a wave aberration diagram of the objective lens in 3rd Embodiment. It is a table | surface which shows the generation amount of the coma aberration in the objective lens of 1st Embodiment and 3rd Embodiment. It is a table | surface which shows the difference of the approximate optical path length of the 2nd-7th section, and the approximate optical path length of the 1st section. It is an optical SPOT figure in 3rd Embodiment. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. Distance is a table showing the relationship between Z _A and the optical height h. It is a table | surface which shows arrangement | positioning of the optical system in 3rd Embodiment. It is a table | surface which shows the difference of the approximate optical path length of the 2nd-31st section, and the approximate optical path length of the 1st section. It is a wave aberration diagram of the objective lens in 4th Embodiment. It is a table | surface which shows the film | membrane structure of the wavelength selective filter concerning 5th Embodiment. It is a figure which shows the spectral characteristics of the wavelength-selective filter concerning 5th Embodiment. It is a table | surface which shows the film | membrane structure of the wavelength selective filter concerning 5th Embodiment. It is a figure which shows the spectral characteristics of the wavelength-selective filter concerning 5th Embodiment. It is sectional drawing explaining the principle of the antireflection film in 6th Embodiment. It is a table | surface which shows the simulation result in 6th Embodiment. It is a graph which shows the change of the reflectance by the wavelength of the simulation result in 6th Embodiment. It is a graph which shows the change of the reflectance by the wavelength of the simulation result in 6th Embodiment. It is a graph which shows the change of the reflectance by the wavelength of the simulation result in 6th Embodiment. It is a table | surface which shows the example of a design of AR coat. It is a graph which shows the spectral reflectance characteristic per lens surface by AR coating. It is a graph which shows Example 4 of this invention. It is a graph which shows Example 5 of this invention. It is a graph which shows Example 6 of this invention. It is a graph which shows Example 7 of this invention. It is a graph which shows Example 8 of this invention. It is a graph which shows the comparative example 1 of this invention. It is a graph which shows the comparative example 2 of this invention. It is a graph which shows the comparative example 3 of this invention. It is a table | surface which shows the analysis result in 5th Embodiment. It is a schematic diagram which shows one structural example of the optical head which concerns on this invention. It is a schematic diagram which shows one structural example of the optical head which concerns on this invention. It is a schematic diagram which shows one structural example of the optical head which concerns on this invention. It is a schematic diagram which shows one structural example of the optical disk apparatus based on this invention. It is the top view and side view which show the external shape of the objective lens concerning this invention.

Explanation of symbols

1 Objective Lens, 2 HDDVD Transparent Substrate, 3 DVD Transparent Substrate 4 CD Transparent Substrate, 5 Aperture, 6 Wavelength Selective Aperture, 7 High Refractive Index Layer 8 Low Refractive Index Layer, 11 Blue Laser, 12 DVD Laser 13 CD Laser, 14 diffraction grating, 15, 17 Half prism 16 Collimator lens, 181, 182 Actuator 20 Actuator drive circuit, 21 Signal processing circuit 22 Laser drive circuit, 23 System control circuit 24 Disc discriminating means

Claims (22)

A light beam having a different wavelength λn (n ≧ 3) is incident on each of at least three types of optical recording media, and the light beam is condensed on the information recording surface provided on the transparent substrate of the optical recording medium by refraction. In an objective lens having a positive power to
The optical recording between the point Pn (n ≧ 3) where the incident light beam to the objective lens of each wavelength λn (n ≧ 3) or the extension line of the incident light beam intersects the optical axis, and the two lens surfaces of the objective lens. The distance from the point Q where the lens surface far from the medium intersects the optical axis is Sn (n ≧ 3),
When the position of the point Pn is located on the side opposite to the optical recording medium with respect to the point Q, the sign of the distance Sn is plus, and the position of the point Pn is on the same side as the optical recording medium with respect to the point Q When the sign of the distance Sn is defined as minus and the sign of the distance Sn is defined, an incident light beam that satisfies the following equations (1) and (2) is incident:
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)
At least one surface of the objective lens is divided into a plurality of concentric sections centered on the optical axis, and an individual aspheric lens surface shape is set in the section,
Adjacent each section, the wavefront aberration + side at a wavelength of lambda _2, the wave front aberration at the wavelength of lambda ₁ is - as it appears to the side, are divided by the step of setting the optical path length difference,
An objective lens, wherein each light beam is focused on the information recording surface with an RMS wavefront aberration of 0.035λ RMS or less. The objective lens according to claim 1, wherein
The optical path length of the light beam passing through each section of the objective lens is substantially the same as the optical path length passing through another section and any one of the first light beam, the second light beam, and the third light beam. An objective lens characterized by being different by 2 mλ (m is an integer) and different by substantially mλ (m is an integer) for other light beams. The objective lens according to claim 2, wherein
The objective lens according to claim ₁ , wherein a wavelength different from about 2 mλ (m is an integer) is λ ₁ . In the objective lens according to claim 1,
The wavelength λ ₁ is around 405 nm,
The wavelength λ ₂ is near 655 nm,
The objective lens, wherein the wavelength lambda ₃ is near 790 nm. In the objective lens according to claim 1,
The objective lens which satisfies the following equation and S ₁ and S _3.
S ₁ <0, S ₃ > 0 Corresponding to a first light beam having a wavelength λ ₁ corresponding to the first optical storage medium, a second light beam having a wavelength λ ₂ corresponding to the second optical storage medium, and a third optical storage medium And the third light beam having the wavelength λ ₃ is made incident, and the first light beam is condensed on the first optical storage medium by the objective lens, and the second light beam is condensed into the second optical storage medium. Information stored in the first optical storage medium, the second optical storage medium, and the third optical storage medium by focusing on the medium and further condensing the third light beam on the third optical storage medium In an optical pickup optical system capable of reading
The objective lens is a lens having a positive power for condensing the light beam by a refraction action on an information recording surface provided on a transparent substrate of the optical storage medium,
At least one surface of the objective lens is divided into a plurality of concentric sections centered on the optical axis, and an individual aspheric lens surface shape is set in the section,
Points P ₁ , P ₂ , and P ₃ where the incident light beam or the extended line of the incident light beam intersects the optical axis of the light beams having the respective wavelengths λ ₁ , λ ₂ , and λ ₃ , and two lenses of the objective lens S ₁ , S ₂ , S ₃ are the distances between the surface of the surface and the point Q where the lens surface far from the optical recording medium intersects the optical axis,
When the positions of the points P ₁ , P ₂ , and P ₃ are located on the opposite side of the optical recording medium with respect to the point Q, the signs of the distances S ₁ , S ₂ , and S ₃ are set to be positive, and the point P ₁ , P ₂ , P ₃ are located on the same side as the optical storage medium with respect to the point Q, the signs of the distances S ₁ , S ₂ , S ₃ are minus and the distances S ₁ , S ₂ , S ₃ is defined, the following expressions (1) and (2) are satisfied ,
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)
At least one surface of the objective lens is divided into a plurality of concentric sections centered on the optical axis, and an individual aspheric lens surface shape is set in the section,
Adjacent each section, the wavefront aberration + side at a wavelength of lambda _2, the wave front aberration at the wavelength of lambda ₁ is - as it appears to the side, are divided by the step of setting the optical path length difference,
An optical pickup optical system , wherein each light beam is focused on the information recording surface with an RMS wavefront aberration of 0.035? RMS or less . The optical pickup optical system according to claim 6,
The optical path length of the light beam passing through each section is approximately mλ (m is about mλ (m is the optical path length through the other section) and any one of the first light beam, the second light beam, and the third light beam. An optical pickup optical system characterized by being different by an integer and different by about 2 mλ (m is an integer) for other light beams. In the optical pickup optical system according to claim 7,
2. The optical pickup optical system according to claim ₁ , wherein the wavelength different from about 2 mλ (m is an integer) is λ ₁ .   An optical head having the optical pickup optical system according to claim 6.   An optical disc apparatus using the optical pickup optical system according to claim 6. The optical pickup optical system according to claim 6,
The wavelength λ ₁ is around 405 nm,
The wavelength λ ₂ is near 655 nm,
Optical pickup system, wherein the wavelength lambda ₃ is near 790 nm. The optical pickup optical system according to claim 6,
The first light beam, the second light beam, and the third light beam are “an outer region that transmits the first light beam and the second light beam and blocks the third light beam. An optical pickup optical system, wherein the optical pickup optical system transmits through the wavelength selective filter having an inner region that transmits the first beam, the second light beam, and the third light beam, and enters the objective lens. In the optical pickup optical system according to claim 12 ,
An optical pickup optical system, wherein the numerical aperture of the light beam after passing through the objective lens is larger in the order of the first light beam, the second light beam, and the third light beam. In the optical pickup optical system according to claim 12 ,
The optical pickup optical system, wherein the wavelength selective filter has a transmittance of 10% or less for the third light beam. In the optical pickup optical system according to claim 12 ,
The optical pickup optical system, wherein the wavelength selective filter has a transmittance of 90% or more for the first light beam and the second light beam. In the optical pickup optical system according to claim 12 ,
The optical pickup system, characterized by satisfying the following formula between S ₁ and S _3.
S ₁ <0, S ₃ > 0 In the optical pickup optical system according to claim 16 ,
An optical pickup optical system satisfying the following equation, where m1 is the magnification of the first light beam and m3 is the magnification of the third light beam.
0 <m1 ≦ 1/10, −1 / 10 ≦ m3 <0 In the optical pickup optical system according to claim 17,
An optical pickup optical system characterized by satisfying 0 <m1 ≦ 1/20 and −1 / 20 ≦ m3 <0. In the optical pickup optical system according to claim 12 ,
The optical pickup optical system, wherein the wavelength selective filter is formed on a surface of the objective lens. In the optical pickup optical system according to claim 12 ,
An optical pickup optical system, wherein the wavelength selective filter has a refractive index in a range of 0.9 to 1.1 with respect to a refractive index of the objective lens. The optical pickup optical system according to claim 6,
The optical pickup optical system, wherein the objective lens is formed of a material mainly composed of glass having a refractive index of 1.49 to 1.70. A light beam having a different wavelength λn (n ≧ 3) is incident on each of at least three types of optical recording media, and the light beams are condensed on the information recording surface provided on the transparent substrate of the optical recording media by refraction. In an objective lens having a positive power to
The optical recording medium out of the two lens surfaces of the objective lens and the point Pn (n ≧ 3) where the incident light beam to the objective lens of each wavelength λn (n ≧ 3) or the extension line of the incident light beam intersects the optical axis Sn (n ≧ 3) is the distance from the point Q where the lens surface far from the point intersects the optical axis,
When the position of the point Pn is located on the opposite side of the optical recording medium with respect to the point Q, the sign of the distance Sn is plus,
When the position of the point Pn is located on the same side as the optical recording medium with respect to the point Q, when the sign of the distance Sn is defined as minus and the sign of the distance Sn is defined, the following equations (1) and (2) An incident light beam that satisfies
λ ₁ <λ ₃ and (1 / S ₁ ) <(1 / S ₃ ) (1)
λ ₂ <λ ₃ (λ ₂ > λ ₁ ) and (1 / S ₂ ) <(1 / S ₃ ) (2)
At least one surface of the objective lens is divided into a plurality of concentric sections centered on the optical axis, and an individual aspheric lens surface shape is set in the section,
Adjacent each section, the wavefront aberration + side at a wavelength of lambda _2, the wave front aberration at the wavelength of lambda ₁ is - as it appears to the side, are divided by the step of setting the optical path length difference,
Each light beam is focused on the information recording surface with an RMS wavefront aberration of 0.050λ RMS or less,
The objective lens is formed of a material mainly composed of glass having a refractive index of 1.49 to 1.70 and a heat distortion temperature of 300 degrees or less.

Images