Optical pickup compatible with a plurality of types of optical disks having different thicknesses

TECHNICAL FIELD

The present invention relates to optical pickup devices. More particularly, the present invention relates to an optical pickup device that directs a laser beam of sufficient intensity for signal recording onto an optical recording medium to record and/or reproduce a signal onto/from a plurality of types of optical recording media.

BACKGROUND ART

Optical disks of approximately 1.2 mm in thickness to read out information using a semiconductor laser such as a CD-ROM (Compact Disk-Read Only Memory) are proposed. By carrying out focus servo and tracking servo on the objective lens for pickup with respect to this type of optical disk, a laser beam is directed to a pit train on the signal recording plane from which a signal is reproduced.

A CD-R (Compact Disk-Recordable) is available that has a recording density identical to that of the CD and that allows recording only once. A laser beam of 780 nm in wavelength is employed in recording and reproducting signals thereof.

Recently, the density is further increased to record a motion picture for a long period of time. For example, a DVD (Digital Video Disk) that records 4.7 Gbytes of information on one plane of an optical disk having a diameter of 12 cm that is identical to that of the CD-ROM is commercially available. The thickness of a DVD is approximately 0.6 mm. By fixing these planes together, 9.4 Gbytes of information can be recorded in one disk.

Attention is focused on a magneto-optical recording medium as a rewritable recording medium of great storage capacity and high reliability. The magneto-optical recording media are now applied as computer memories and the like. Standardization of a magneto-optical recording medium having a storage capacity of 6.0 Gbytes (AS-MO (Advanced Storage Magneto Optical Disk) standard) is in progress to be provided for actual usage. This magneto-optical recording medium has the signal reproduced by the MSR (Magnetically Induced Super Resolution) method. More specifically, a laser beam is projected to transfer the magnetic domain of the recording layer of the magneto-optical recording medium to a reproduction layer and also forming a detection window in the reproduction layer to allow detection of only the transferred magnetic domain. The transferred magnetic domain is detected from the formed detection window. A laser beam of 600-700 nm in wavelength is employed for recording and/or reproducing a signal onto and/or from the magneto-optical recording medium.

It is expected that there will be the coexistence of CDs, CD-Rs, DVDs and magneto-optical recording media in the future. The need arises for an optical pickup device that can reproduce information from such optical disks and that can record a signal onto a recordable optical disk. WO 98/19303 discloses an optical pickup device that allows compatible reproduction between a CD-R and a DVD.

The proposed CD-R/DVD compatible pickup includes a semiconductor laser generating a laser beam of 635 nm in wavelength for reproduction of a DVD and a semiconductor laser generating a laser beam of 780 nm in wavelength for recording and reproduction of a CD-R. When a signal is to be recorded onto or reproduced from a CD-R using a laser beam of 780 nm in wavelength, the laser beam is diffracted and a desired diffracted light thereof, for example only the first order light, is introduced into the objective lens to collect light in order to correct aberration caused by difference in the thickness of the substrate.

Therefore, the zero order light or minus first order light could not be used effectively. There was a problem that a laser beam sufficient in intensity for recording could not be obtained at the signal recording plane of the CD-R.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an optical pickup device capable of recording and/or reproducing a signal onto/from a plurality of types of optical disks of different thickness, suppressing laser beam loss to the minimum.

According to an aspect of the present invention, an optical pickup device recording and/or reproducing a signal onto/from a first optical disk and a second optical disk thicker than the first optical disk includes a light source, an objective lens, and an optical device. The light source generates a laser beam. The objective lens is located opposite to the first and second optical disks. The optical device is arranged between the light source and the objective lens to transmit the laser beam from the light source straightforwardly during recording or reproduction of the first optical disk, and bending substantially the entire laser beam from the light source and increasing the diameter thereof to guide the center portion of the laser beam towards the objective lens and the peripheral portion of the laser beam outside the objective lens during recording or reproduction of the second disk.

In the optical pickup device, the optical device bends substantially the entire laser beam from the light source so that only the center portion of the laser beam is guided to the objective lens during recording or reproduction of the second optical disk, so that most of the laser beam can be used effectively with the exception that the peripheral portion is lost. A signal can be recorded onto the first and second optical disks or a signal can be reproduced from the first and second optical disks while suppressing loss of the laser beam at the minimum.

Preferably, the optical device includes a first optical member and a second optical member. The first optical member has a first refractive index. The second optical member is in contact with the first optical member, and has the first refractive index during recording or reproduction of the first optical disk, and has a second refractive index differing from the first refractive index during recording or reproduction of the second optical disk. During recording or reproduction of the first optical disk, the entire optical device has the first refractive index. Therefore, the laser beam from the light source is transmitted straightforwardly. In contrast, the first and second optical members have different refraction indexes during recording or reproduction of the second optical disk. Therefore, the optical device diffracts or refracts the laser beam from the light source.

Further preferably, the light source generates a first laser beam having a first wavelength during recording or reproduction of the first optical disk, and generates a second laser beam having a second wavelength differing from the first wavelength during recording or reproduction of the second optical disk. The first optical member has a first refractive index for the first and second wavelengths. The second optical member has the first refractive index for the first wavelength and the second refractive index for the second wavelength. Since the refractive index of the second optical member changes according to the wavelength, the laser beam can be transmitted straightforwardly or bent without mechanical switching.

Also preferably, the first optical member includes a hologram formed to come into contact with the second optical member. Therefore, the optical device diffracts the laser beam by interference during recording or reproduction of the second optical disk.

Further preferably, the first optical member is arranged at the light source side. The second optical member is arranged at the objective lens side. The first refractive index is higher than the second refractive index. The hologram includes a plurality of annular projections formed concentrically. The pitch of the annular projections become smaller as towards the outer circumference. Therefore, the optical device diffracts the laser beam at a greater angle as towards the circumference.

Preferably, each of the annular projections has a triangular cross section radially. Therefore, the optical device can diffract the incident laser beam in a desired direction without generating 0 order or −1 order diffracted light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

shows a structure of an optical pickup device of the present invention.

FIG. 2

is a prospective view of the optical device of FIG.

1

.

FIGS. 3A and 3B

are a sectional view and a plan view, respectively, of the optical device of FIG.

2

.

FIGS. 4A and 4B

are diagrams to describe the property of the optical device of

FIG. 1

when the wavelength is 635 nm and 780 nm, respectively, and

FIG. 4C

is a diagram to describe the property of the optical device having a hologram of a stepped shape in cross section.

FIG. 5

is a diagram to describe the path of a laser beam of 780 nm in wavelength from the optical device to the objective lens.

FIG. 6

is a diagram to describe the recording or reproduction operation of an optical disk having a substrate thickness of 0.6 mm using the optical pickup device of FIG.

1

.

FIG. 7

is a diagram to describe the recording or reproduction operation of an optical disk having a substrate thickness of 1.2 mm using the optical pickup device of FIG.

1

.

FIG. 8

is a prospective view of another optical device of the present invention.

FIG. 9

is a cross sectional view of the optical device of FIG.

8

.

FIG. 10

shows a structure of another optical pickup device of the present invention.

FIG. 11

is a cross sectional view of the optical device of FIG.

10

.

FIG. 12

is a diagram to describe the property of the optical device of

FIG. 10

when voltage is applied.

FIG. 13

is a diagram to describe the property of the optical device of

FIG. 10

when voltage is not applied.

FIG. 14

is a diagram to describe the recording or reproduction operation of an optical disk having a substrate thickness of 0.6 mm using the optical pickup device of FIG.

10

.

FIG. 15

is a diagram to describe the reproduction operation of an optical disk having a substrate thickness of 1.2 mm using the optical pickup device of FIG.

10

.

FIG. 16

shows a structure of a further another optical pickup device of the present invention.

FIG. 17

is a diagram to describe the rising mirror of FIG.

16

.

FIG. 18

is a perspective view of a laser beam emitted from a laser light source, rendered parallel by the collimator lens, and entering the objective lens.

FIG. 19

shows the intensity distribution of the laser beam of FIG.

18

.

FIG. 20

shows the intensity distribution of the laser beam along cross section X-X′ of FIG.

18

.

FIG. 21

shows the intensity distribution of the laser beam along cross section Y-Y′ of FIG.

18

.

FIG. 22

is a diagram to describe the rim strength when the priority is given on the efficiency.

FIG. 23

shows the intensity distribution of the laser beam along cross section X—X of FIG.

22

.

FIG. 24

shows the intensity distribution of the laser beam along cross section Y-Y′ of FIG.

22

.

FIG. 25

is a diagram to describe the rim intensity when priority is given on the spot size.

FIG. 26

shows the intensity distribution of the laser beam along cross section X-X′ of FIG.

25

.

FIG. 27

shows the intensity distribution of the laser beam along cross section Y-Y′ of FIG.

25

.

FIG. 28

is a diagram to describe the rim intensity suitable for an optical pickup device of eightfold speed.

FIG. 29

shows the intensity distribution of a laser beam along cross section X-X′ of FIG.

28

.

FIG. 30

shows the intensity distribution of a laser beam along cross section Y—Y of FIG.

28

.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding components have the same reference characters allotted, and their description will not be repeated.

Referring to

FIG. 1

, an optical pickup device

10

of the present invention includes a laser light source

1

, a collimator lens

2

, a polarization beam splitter

3

, a half-wave plate

4

, a rising mirror

5

, an optical device

6

, an objective lens

7

, a collective lens

8

, and a photodetector

9

.

Laser light source

1

includes a first semiconductor laser

1

A generating a laser beam of 635 nm in wavelength (tolerance±15 nm, the same applies hereinafter), and a second semiconductor laser

1

B generating a laser beam of 780 nm in wavelength (tolerance±15 nm, the same applies hereinafter). A laser drive circuit not shown selectively drives first semiconductor laser

1

A and second semiconductor laser

1

B to selectively generate a laser beam of 635 nm in wavelength and a laser beam of 780 nm in wavelength.

Collimator lens

2

renders the laser beam from laser light source

1

parallel. Polarization beam splitter

3

transmits the laser beam from collimator lens

2

, and reflects the reflected light from an optical disk

11

(or

110

) towards photodetector

9

. Half-wave plate

4

rotates the plane of polarization of the laser beam 90 degrees and transmits the laser beam. Rising mirror

5

reflects the laser beam passing through half-wave plate

4

towards optical disk

11

(or

110

).

Optical device

6

transmits the laser beam of 635 nm in wavelength straightforwardly into objective lens

7

while maintaining the incident intensity, and diffracts the laser beam of 780 nm in wavelength towards a desired direction to increase the diameter and enter the center portion into objective lens

7

and bend the peripheral portion outside objective lens

7

.

Objective lens

7

is located opposite to optical disk

11

(or

110

) to focus the laser beam from optical device

6

to direct the laser beam onto a signal recording plane

11

a

(or

110

a

) of optical disk

11

(or

110

). Objective lens

7

is designed corresponding to an optical disk

11

having a substrate thickness of 0.6 mm. The numerical aperture is 0.6 (tolerance ±0.05). Collective lens

8

collects the laser beam reflected at polarization beam splitter

3

. Photo detector

9

detects the laser beam collected by collective lens

8

.

Optical pickup device

10

reproduces a signal from a DVD

11

having a substrate thickness of 0.6 mm, and records/reproduces a signal to/from a CD-R

110

having a substrate thickness of 1.2 mm. When a signal is to be reproduced from DVD

11

, a laser beam having a wavelength of 635 nm is output from laser light source

1

. When a signal is to be recorded/reproduced with respect to CD-R, a laser beam having a wavelength of 780 nm is generated from laser light source

1

.

Particularly in the case where a signal is to be recorded on CD-R

110

as will be described afterwards, optical pickup device

10

has a beam focused on a signal recording plane

110

a

of CD-R

110

for recording with little degradation in the power of the laser beam of 780 nm in wavelength emitted from second semiconductor laser

1

B.

Details of optical device

6

will be described hereinafter with reference to

FIGS. 2

,

3

A and

3

B.

Referring to

FIG. 2

, optical device

6

includes a first optical member

60

formed of a transmittive substrate such as of glass, and a second optical member

61

formed to cover first optical member

60

. A plurality of annular projections

602

are formed at a predetermined distance concentrically about an optical axis L

0

on the main surface of first optical member

60

. Projections

602

form a hologram

601

. Projection

602

is formed of, for example, TiO

2

, and has the same refractive index of 2.3 with respect to the laser beam of 635 nm in wavelength and the laser beam of 780 nm in wavelength. Second optical member

61

is formed of, for example, silicon nitride (SiN), silicon carbide (SiC), and has the refractive index of 2.3 for the laser beam of 635 nm in wavelength and the refractive index of 1.8 for the laser beam of 780 nm in wavelength.

The cross sectional structure of optical device

6

at an arbitrary plane including optical axis L

0

will be described with reference to FIG.

3

A. Projections

602

having the shape of a right triangle are formed symmetrically with respect to optical axis L

0

at a predetermined interval at the surface of first optical member

60

. Projections

602

have the height of 0.337 μm, the interval of 296.43 μm at the innermost circumferential region and 31.256 μm at the outermost circumferential region. The pitch becomes gradually narrower from the inner circumference towards the outer circumference. Second optical member

61

is formed to come into contact with the main surface of first optical member

60

forming hologram

601

. First optical member

60

is arranged at the light source

1

side whereas second optical member

61

is arranged at the objective lens

7

side.

FIG. 3B

is a plan view of the structure of optical device

6

. Hologram

601

is formed of a plurality annular projections

602

concentrically at the surface of transmittive substrate

60

. It is apparent that the pitch of annular projections

602

becomes smaller as towards the outer circumference. The hologram lens provided by Aerial Imaging Corporation (U.S.A.) can be employed as first optical member

60

.

The optical property of optical device

6

will be described with reference to

FIGS. 4A and 4B

. Referring to

FIG. 4A

, projection

602

and second optical member

61

both have a refractive index of 2.3 with respect to the laser beam of 635 nm in wavelength. Therefore, laser beam LB

1

of 635 nm in wavelength that enters optical device

6

is directly transmitted as laser beam LB

1

without being diffracted at optical device

6

. As a result, the laser beam of 635 nm in wavelength will not be reduced in power even if transmitted through optical device

6

.

Referring to

FIG. 4B

, projection

602

has a refractive index of 2.3 and second optical member

61

has a refractive index of 1.8 with respect to the laser beam of 780 nm in wavelength. It is to be noted that projection

602

has a gentle slope

603

at the interface with second optical member

61

. Therefore, laser beam LB

2

of 780 nm in wavelength that enters optical device

6

is diffracted towards the outer side from the optical axis via slope

603

when entering second optical member

61

from first optical member

60

to be transmitted as diffracted light LB

3

from optical device

6

.

FIG. 4C

corresponds to the case where first optical member

60

has step-graded projections 40 at the interface with second optical member

61

. Laser beam LB

2

of 780 nm in wavelength is diffracted into 0 order light LB

20

, +first order light LB

21

and −1 order light LB

22

when entering second optical member

61

from first optical member

60

. Therefore, the power of the laser beam will be reduced when only one of the three diffracted light LB

20

, LB

21

and LB

22

is employed. However, gentle slope

603

of projection

602

causes laser beam LB

2

of 780 nm in wavelength to be diffracted almost 100% to first order light LB

3

by optical device

6

. Therefore, there is little reduction in the power of laser beam LB

2

passing through optical device

6

.

Referring to

FIG. 5

, laser beam LB

2

of 780 nm in wavelength incident to optical device

6

is diffracted in a desired direction to be transmitted as diffracted light LB

3

from optical device

6

. As to diffracted light LB

3

passing through optical device

6

, peripheral portion LB

3

EX does not enter objective lens

7

. Only a predetermined center portion LB

3

IN enters projection lens

7

. Therefore, optical device

6

works on laser beam LB

2

of 780 nm in wavelength to diffract the laser beam to diffracted light LB

3

of a desired direction, and directs only predetermined center portion LB

3

IN into objective lens

7

. Since the distance between respective annular projections

602

of optical device

6

becomes gradually smaller from the center towards the periphery, the angle of diffraction differs between the center portion and the peripheral portion. Optical device

6

functions in a manner of diffracting laser beam using a lens.

The efficiency η

m

of m order diffracted light by the hologram is generally represented by the following equation (1).

\begin{matrix} η_{m} = {&LeftBracketingBar; \frac{1}{T} \int_{0}^{T} A (x) \exp {ⅈ ψ (x)} \exp (- ⅈ \frac{2 π mx}{T}) ⅆ x &RightBracketingBar;}^{2} & (1) \end{matrix}

where T is the hologram cycle (here, the pitch of annular projections

602

), A(x) is the transmittance, x is the position on the hologram, ψ(x) is the phase difference function, and m is the order.

Since hologram

601

is the kinoform type formed of a plurality of annular projections

602

having a triangular cross section, the phase difference function ψ(x) is represented by the following equation (2).

\begin{matrix} ψ (x) = \frac{d Δ n}{T} \times \frac{2 π}{λ} \times x, m = 1, A (x) = 1 & (2) \end{matrix}

In equation (2), d is the height of projection

602

, Δn is the difference in the refractive index, and λ is the wavelength.

When A(x)=1, the diffraction efficiency of the first order light is represented by the following equation (3).

\begin{matrix} \begin{matrix} η_{i} = {&LeftBracketingBar; \frac{1}{T} \int_{0}^{T} \exp (ⅈ \frac{d Δ n}{T} \frac{2 π}{λ} x) \exp (- ⅈ \frac{2 π x}{T}) ⅆ x &RightBracketingBar;}^{2} \\ = {&LeftBracketingBar; \frac{1}{T} \int_{0}^{T} \exp {ⅈ \frac{2 π x (\frac{d Δ n}{λ} - 1)}{T}} ⅆ x &RightBracketingBar;}^{2} (\frac{d Δ n}{λ} - 1 = A) \\ = {&LeftBracketingBar; \frac{1}{T} \int_{0}^{T} (\cos ⅈ \frac{2 π xA}{T} + ⅈ \sin ⅈ \frac{2 π xA}{T}) ⅆ x &RightBracketingBar;}^{2} \\ = {&LeftBracketingBar; \frac{1}{T} \times \frac{T}{2 π A} {{[\sin \frac{2 π xA}{T}]}_{0}^{T} + {ⅈ [- \cos \frac{2 π xA}{T}]}_{0}^{T}} &RightBracketingBar;}^{2} \\ = {&LeftBracketingBar; \frac{1}{2 π A} {(\sin 2 π A - 0) + ⅈ (- \cos 2 π A + 1)} &RightBracketingBar;}^{2} \\ = \frac{1}{{(2 π A)}^{2}} (\sin^{2} 2 π A + \cos^{2} 2 π A - 2 \cos 2 π A + 1) \\ = \frac{2 - 2 \cos 2 π A}{{(2 π A)}^{2}} \\ = \frac{4 \sin^{2} π A}{{(2 π A)}^{2}} \\ = \frac{\sin^{2} π A}{{(π A)}^{2}} (\sin c (x) = \frac{\sin (π x)}{π x}) \\ = \sin c^{2} A \\ = \sin c^{2} (\frac{d Δ n}{λ} - 1) \\ = \sin c^{2} (\frac{d Δ n - λ}{λ}) \end{matrix} & (3) \end{matrix}

It is apparent from the above that optical device

6

converts laser beam LB

2

of 780 nm in wavelength into first order diffracted light LB

3

at high efficiency. Since the pitch of annular projections

602

becomes smaller towards the outer circumference, the laser beam passing through the center of optical device

6

travels straightforwardly while the laser beam at the periphery is bent at a greater angle as towards the outer circumference. Therefore, the center portion of laser beam LB

2

enters objective lens

7

whereas the peripheral portion of laser beam LB

2

is deviated from objective lens

7

. Thus, optical device

6

can diffract substantially the entire laser beam LB

2

of 780 nm in a desired direction to enter objective lens

7

except for the peripheral portion thereof.

The operation of reproducing a signal from DVD

11

having a substrate thickness of 0.6 mm using optical pickup device

10

will be described with reference to FIG.

6

. In the case of reproducing a signal from DVD

11

, first semiconductor laser

1

A of laser light source

1

is selectively driven. The laser beam of 635 nm in wavelength emitted from laser light source

1

is rendered parallel by collimator lens

2

, and passes through polarization beam splitter

3

. The light from polarization beam splitter

3

has its plane of polarization rotated 90 degrees by half-wave plate

4

to enter rising mirror

5

. Here, the laser beam passes through polarization beam splitter

3

and half-wave plate

4

at the transmittance of approximately 98%. Therefore, there is little reduction in power by the passage through polarization beam splitter

3

and half-wave plate

4

.

The laser beam incident on rising mirror

5

is reflected almost 100% and enters optical device

6

. The laser beam passes through optical device

6

while maintaining its incident intensity and then enters objective lens

7

. The laser beam is focused by objective lens

7

to be projected on signal recording plane

11

a

of DVD

11

. The light reflected from signal recording plane

11

a

returns to half-wave plate

4

through objective lens

7

, optical device

6

and rising mirror

5

. The light has its plane of polarization rotates 90 degrees at half-wave plate

4

, and then enters polarization beam splitter

3

. The reflected light incident on polarization beam splitter

3

is reflected almost 100% at polarization beam splitter

3

to enter collective lens

8

since the plane of polarization is rotated 180 degrees than the case where the light enters polarization beam splitter

3

from collimator lens

2

. The light in collective lens

8

is collected and directed to photodetector

9

for detection.

By using optical pickup device

10

, a signal can be recorded and/or reproduced onto/from a magneto-optical recording medium that is a recordable optical disk. In this case, a signal can be recorded with the intensity of light equal to the level of the light immediately output from first semiconductor laser

1

A.

Referring to

FIG. 7

, the operation of recording and/or reproducing a signal onto and/or from a CD-R having a substrate thickness of 1.2 mm using optical pickup device

10

will be described hereinafter. In the recording and/or reproduction operation with respect to a CD-R, second semiconductor laser

1

B of laser light source

1

is selectively driven.

First, the signal recording operation will be described. When a signal is to be recorded on a CD-R

110

, a strong laser beam of 70 mW is output from second semiconductor laser

1

B. The laser beam of 780 nm in wavelength output from laser light source

1

is rendered parallel by collimator lens

2

and passes through polarization beam splitter

3

. At half-wave plate

4

, the plane of polarization of the light is rotated 90 degrees and then enters rising mirror

5

. The laser beam passes through polarization beam splitter

3

and half-wave plate

4

at the transmittance of approximately 98%. Therefore, there is little reduction in the power during passage of polarization beam splitter

3

and half-wave plate

4

.

The laser beam entering rising mirror

5

is reflected almost 100% and enters optical device

6

. The laser beam in optical device

6

is diffracted while maintaining its incident intensity. Only the predetermined inner portion of the laser beam enters objective lens

7

. The laser beam in objective lens

7

is focused and projected onto signal recording plane

110

a

of CD-R

110

. It is to be noted that the laser beam is modulated by the record signal. Therefore, a modulated laser beam of 780 nm in wavelength is projected onto signal recording plane

110

a

, whereby a signal is recorded.

The laser beam of 780 nm in wavelength emitted from second semiconductor laser

1

B at the power of 70 mW has its intensity reduced by approximately 2% by passage through polarization beam splitter

3

and half-wave plate

4

, and enters optical device

6

. The laser beam is diffracted and transmitted through optical device

6

while maintaining its intensity. Only the predetermined center portion of light enters objective lens

7

. The predetermined center portion is the region where the effective numerical aperture of objective lens

7

of the numerical aperture of 0.6 is in the range of 0.50-0.53. When the effective light flux of the laser beam of 780 nm in wavelength is 4.46 mm, the diameter of the predetermined center portion where the effective numerical aperture of objective lens

7

is in the range of 0.50-0.53 is 3.2-3.4 mm. Therefore, the intensity of the laser beam incident on objective lens

7

is 70 mW ×0.98 ×(center portion diameter effectively used/effective light flux of laser beam) =49˜52 mW. Therefore, by using optical pickup device

10

, a laser beam of 780 nm in wavelength can be projected on signal recording plane

110

a of CD-R

110

with little reduction in the intensity level to that right after emission from second semiconductor laser

1

B. Therefore, signal recording can be carried out correctly.

Next, the operation of signal reproduction will be described. When a signal is to be reproduced from CD-R

110

, a laser beam of 12 mW is emitted from second semiconductor laser

1

B. The laser beam of 780 nm emitted from laser source

1

is directed to signal recording plane

110

of CD-R

110

with little reduction in the intensity, as described above. The reflected light from signal recording plane

110

a is guided to photodetector

9

as described with reference to

FIG. 6

, and a signal is reproduced.

Another optical device

80

employed in optical pickup device

10

will be described with reference to FIG.

8

. Optical device

80

includes a transparent first optical member

810

and a transparent second optical member

801

. First optical member

810

is arranged at the objective lens

7

side, and has a concave curve plane

802

in contact with second optical member

801

. Second optical member

801

is arranged at the light source

1

side, and has a convex curve plane

802

in contact with first optical member

810

. Second optical member

802

has a refractive index of 2.3 for a laser beam of 635 nm in wavelength and a refractive index of 1.8 for a laser beam of 780 nm in wavelength. First optical member

810

has the same refractive index of 2.3 for a laser beam of 635 nm in wavelength and a laser beam of 780 nm in wavelength. Second optical member

801

is formed of, for example, SiN. First optical member is formed of, for example, TiO

2

.

The cross sectional shape of optical device

80

at an arbitrary plane including optical axis L

0

will be described with reference to FIG.

9

. The interface

802

between first optical member

810

and second optical member

801

has a dome-like aspheric surface protruding towards first optical member

810

. Although interface

802

maybe a spheric surface, it is desirable to correct the spheric surface slightly to reduce aberration. Since first and second optical members

810

and

801

have the same refractive index 2.3 with respect to a laser beam of 635 nm in wavelength, the laser beam of 635 nm will not be diffracted by optical device

80

and is transmitted directly through optical device

80

. In contrast, the second optical member

801

has a refractive index of 1.8 and the first optical member

810

has a refractive index of 2.3 with respect to a laser beam of 780 nm in wavelength. Since the interface

802

between first and second optical members

810

and

801

has a dome-like aspheric surface as described above, laser beam LB

2

of 780 nm in wavelength is diffracted outwards from the optical axis at optical device

80

and output therefrom as diffracted light LB

4

. In this case, the intensity of laser beam LB

2

is substantially equal to that of diffracted light LB

4

.

Reproduction of a DVD and recording and/or reproduction of a CD-R can be carried out as described above even in the case where optical device

80

is used instead of optical device

6

of optical pickup device

10

.

Specific examples of first optical members

601

and

810

and second optical members

61

and

801

forming optical devices

6

and

80

are not limited to those described above. In the case of optical device

6

, first and second optical members

60

and

61

have the same first refractive index n

1

with respect to a laser beam of 635 nm in wavelength, and first optical member

60

and second optical member

61

have a first refractive index n

1

and a second refractive index n

2

smaller than the first refractive index n

1

, respectively, with respect to a laser beam of 780 nm in wavelength.

When in the case of optical device

80

, first and second optical members

810

and

801

have the same first refractive index n

1

for a laser beam of 635 nm in wavelength, and first and second optical members

810

and

801

have the first refractive index n

1

and the second refractive index n

2

smaller than the first refractive index n

1

, respectively, with respect to the laser beam of 780 nm.

Although the substrate portion of the first optical member

60

and projection

602

are formed separately in optical device

6

, they may be formed integrally of the same material. Similarly, optical device

80

may have the substrate portion of second optical member

801

and the dome-like projection of second apparatus member

801

formed integrally of the same material.

In other words, optical devices

6

and

80

are arbitrary as long as a laser beam can be diffracted selectively in a desired direction while maintaining incident intensity corresponding to the wavelength of the laser beam.

Also, the semiconductor laser mounted in optical pickup device

10

is not limited to that emitting a laser beam of 635 nm in wavelength and a laser beam of 780 nm in wavelength. A semiconductor laser that emits laser beams of two other different wavelengths can be employed.

Furthermore, the optical pickup device of the present invention is not limited to that emitting laser beams of two different wavelengths. One that emits a laser beam of one wavelength can be used.

Referring to

FIG. 10

, a structure of another optical pickup device

20

according to the present invention will be described. Optical pickup device

20

has a structure similar to that of optical pickup device

10

, provided that a laser beam source

100

and an optical device

200

are employed instead of laser light source

1

and optical device

6

, respectively. Laser light source

100

generates only a laser beam of 635 nm in wavelength.

The structure of optical device

200

will be described with reference to the cross sectional view of FIG.

11

. Optical device

200

includes a first optical member

60

and a second optical member

21

. Second optical member

21

includes a first transparent electrode

203

formed on annular projections

602

, a TN (Twisted Nematic) type liquid crystal

204

formed thereon, a second transparent electrode

205

formed further thereon, and a transmittive substrate

206

. First optical member

60

is identical to that described previously.

TN type liquid crystal

204

is sealed between first and second transparent electrodes

203

and

205

so that the molecular arrangement is not twisted 90 degrees. Therefore, the plane of polarization of the laser beam passing through TN type liquid crystal

204

will not be rotated 90 degrees.

The optical property of optical device

200

will be described with reference to

FIGS. 12 and 13

. Annular projections

203

forming a hologram have a refractive index of 1.7 with respect to a laser beam of 635 nm in wavelength. TN liquid crystal

204

has a refractive index of 1.5 when voltage is not applied across first and second transparent electrodes

203

and

205

, and has a refractive index of 1.7 when voltage is applied.

Referring to

FIG. 12

, the laser beam of 635 nm in wavelength directly passes through optical device

200

when voltage is applied to TN liquid crystal

204

since projections

602

forming the hologram and TN liquid crystal

204

have the same refractive index 1.7.

Referring to

FIG. 13

, the laser beam of 635 nm in wavelength is diffracted in a desired direction to be transmitted from optical device

200

when voltage is not applied to liquid crystal

204

since projections

602

forming the hologram and TN type liquid crystal

204

have the refractive index of 1.7 and 1.5, respectively, and projection

602

has a gentle slope.

Optical device

200

diffracts the laser beam in a desired direction while maintaining the incident intensity by selectively applying voltage to TN liquid crystal

204

irrespective of the wavelength of the laser beam.

Referring to

FIG. 14

, reproduction of DVD

11

having a substrate thickness of 0.6 mm will be described. In reproducing a signal from DVD

11

, voltage is applied to first and second transparent electrodes

203

and

205

of optical device

200

. As a result, the laser beam of 635 nm in wavelength emitted from laser light source

100

is rendered parallel by collimator lens

2

and passes through polarization beam splitter

3

. The laser beam has its plane of polarization rotated 90 degrees by half-wave plate

4

and enters rising mirror

5

. There is little reduction in the power of the laser beam by the passage of polarization beam splitter

3

and half-wave plate

4

since the laser beam is transmitted through polarization beam splitter

3

and half-wave plate

4

at the transmittance of approximately 98%.

The laser beam incident to rising mirror

5

is reflected almost 100% to enter optical device

200

. The laser beam incident to optical device

200

is directly transmitted maintaining the incident intensity to enter objective lens

7

. The laser beam in objective lens

7

is focused to be projected on signal recording plane

11

a

of DVD

11

. The light reflected from signal recording plane

11

a

passes through objective lens

7

, optical device

200

and rising mirror

5

to return to half-wave plate

4

. The light has its plane of polarization rotated 90 degrees at half-wave plate

4

and then enters polarization beam splitter

3

. The reflected light entering polarization beam splitter

3

is reflected almost 100% thereat to enter collective lens

8

since the plane of polarization is rotated 180 degrees than the case where the light beam enters polarization beam splitter

3

from collimator lens

2

. Then, the light is focused at collective lens

8

to be directed to photodetector

9

for detection.

Reproduction from CD

110

having a substrate thickness of 1.2 mm will be described with reference to FIG.

15

. In reproducing a signal from CD

110

, voltage is not applied to first and second transparent electrodes

203

and

205

of optical device

200

. As a result, the laser beam of 635 nm in wavelength emitted from laser light source

100

is rendered parallel by collimator lens

2

and passes through polarization beam splitter

3

. The light has its plane of polarization rotated 90 degrees at half-wave plate

4

to enter rising mirror

5

. The laser beam is hardly reduced in power by passage through polarization beam splitter

3

and half-wave plate

4

since the laser beam is transmitted through polarization beam splitter

3

and half-wave plate

4

at the transmittance of approximately 98%.

The laser beam incident to rising mirror is reflected almost 100% and enters optical device

200

. The laser beam incident to optical device

200

is diffracted in a desired direction while maintaining the incident intensity. Only the predetermined center portion of the laser beam enters objective lens

7

. The incident laser beam to objective lens

7

is collected at objective lens

7

to be projected on signal recording plane

110

a

of CD

110

. In this case, the diameter of the predetermined center portion is determined so that the effective numerical aperture of objective lens

7

is in the range of 0.3 to 0.4. As a result, the laser beam of 635 nm in wavelength is projected onto signal recording plane

110

a

of CD

110

having a substrate thickness of 1.2 mm with almost no aberration. The light reflected from signal recording plane

110

a

is detected by photodetector

9

in a manner similar to that described above.

Referring to

FIG. 16

, an optical pickup device

30

which is an improvement of optical pickup device

10

of

FIG. 1

will be described. Optical pickup device

30

includes a rising mirror

50

instead of rising mirror

5

of optical pickup device

10

. The remaining structure is similar to that of optical pickup device

10

. The details of rising mirror

50

disclosed in Japanese Patent Application No. 10-257130 will be described briefly hereinafter.

Referring to

FIG. 17

, rising mirror

50

includes a thin film

501

that sets the optical axes of two laser beams LB

1

and LB

2

in coincidence at its surface. Since laser light source

1

has a first semiconductor laser

1

A and a second semiconductor laser

1

B, the optical axes of laser beams LB

1

and LB

2

emitted from the two semiconductor lasers will be deviated from each other. Therefore, it is necessary to set the optical axes of the two laser beams LB

1

and LB

2

in coincidence in order to carry out recording and reproduction of a signal correctly.

Optical pickup device

30

uses rising mirror

50

including a thin film

50

to set the optical axis of laser beam LB

1

of 635 nm in wavelength and the optical axis of laser beam LB

2

of 780 nm in wavelength in coincidence.

Laser beam LB

1

of 635 nm in wavelength is reflected at a first plane

5011

of thin film

501

of rising mirror

50

. Laser beam LB

2

of wavelength 780 nm is refracted at first plane

5011

of thin film

501

of rising mirror

50

and reflected at a second plane

5012

to be refracted again at first plane

5011

to be output from rising mirror

50

as a laser beam having an optical axis identical to that of the reflected light of the laser beam of 635 nm in wavelength.

The passage of rising mirror

50

allows the optical axes of the laser beams LB

1

and LB

2

of the two wavelengths to match each other without reduction in the intensity of the laser beam. Therefore, a signal can be recorded and/or reproduced more accurately.

Optical devices

6

,

80

and

200

described above are located at an arbitrary position between laser light source

1

and objective lens

7

.

The above-described optical pickup devices

10

,

20

and

30

of the present invention diverts the circumferential portion of the laser beam outside objective lens

7

in the recording and reproducing operation with respect to CD-Rn

110

using optical devices

6

,

80

and

200

. This means that there is a relatively large loss. In order to output a laser beam having sufficient power from objective lens

7

, the output power of laser light sources

1

and

100

or the numerical aperture of collimator lens

2

and objective lens

7

should be increased. However, increase thereof has a limit. A greater power is required for the laser beam output from objective lens

7

as the data reading or writing speed increases. Also, it is to be noted that the effective region of collimator lens

2

and objective lens

7

is a true circle whereas the laser beam output from laser light source

1

has a cross section of an ellipse, not a true circle. If the entire laser beam in the direction of the longer diameter is made to be incident on the effective region of collimator lens

2

or objective lens

7

, there will be an effective region that is not used in collimator lens

2

and objective lens

7

in the direction of the shorter diameter. If the laser beam in the direction of the shorter diameter is made to be incident on the entire effective region of collimator lens

2

or objective lens

7

, the laser beam in the direction of the longer diameter will be wasted partially. In general, if the design of the focal length of collimator lens

2

is made short so that laser light source

1

or

100

is located close to collimator lens

2

, the power of the laser beam output from objective lens

7

will become greater. However, the spot diameter of the laser beam formed on optical disk

11

(or

110

) will become too large. Therefore, the rim intensity defined below must be set appropriately in order to obtain sufficient output power while meeting various recording or reproduction conditions.

As shown in

FIG. 18

, the laser beam output from the laser light source is increased in diameter to enter the collimator lens. The laser beam is rendered parallel by the collimator lens, and then enters the objective lens. Here, the broadening angle θ// in the direction of the shorter diameter is smaller than the broadening angle θ⊥ in the direction of the longer diameter.

The intensity of the laser beam corresponds to a Gaussian distribution as shown in FIG.

19

. The intensity is highest at the center and becomes lower towards the outer circumference. Since the laser intensity forms a Gaussian distribution, a laser beam of at least a predetermined intensity will be used in practical usage. If the maximum laser power is 100%, the rim intensity I % is defined when laser of at least I % is used.

Since the laser beam has a cross section of an ellipse, the intensity distribution is abrupt in the shorter diameter direction (X-X′) as shown in FIG.

20

and gentle in the longer diameter direction (Y-Y′) as shown in FIG.

21

. Since the effective region of the objective lens corresponds to a true circle, the rim intensity becomes smaller in the shorter diameter direction as shown in FIG.

20

and higher in the longer diameter direction as shown in FIG.

21

.

The simulation result of the output power when the kinoform type optical device

6

of the present invention shown in

FIG. 3A

is employed will be described in comparison with the output power when the conventional stepped optical device shown in

FIG. 4C

is employed.

First, an example of the rim intensity when priority is given on efficiency will be described. As shown in

FIG. 22

, a laser light source having a broadening angle of 7.5° in the shorter diameter direction and a broadening angle of 17° in the longer diameter direction is used. A collimator lens having a numerical aperture NA of 0.15 with a focal length f of 9 mm is employed. The case is considered where the rim intensity is set to 0.6% in the shorter diameter direction as shown in FIG.

23

and to 36.6% in the longer diameter direction as shown in FIG.

24

.

By setting the effective numerical aperture NA of the objective lens to 0.5, the output power of the laser light source to 70 mW and the other parameters to appropriate values as shown in Table 1 below, the output power from the objective lens becomes 49.90 mW when the kinoform type optical device

6

is employed.

TABLE 1

Priority on Efficiency (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

f = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.18

2. Collimator

NA 0.15

f = 9 mm

Transmittance

Optical

2.81 times

lens

95%

Magnification

Lens bond

20.48 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

82.24%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

71.28%

98%

CW output

49.90 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

0.57%

θ//

7. Rising mirror

Transmittance

Rim intensity

36.57%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

0.6%

36.6%

Eclipse

1.60

0.71

coefficient

Expected spot

1.460 μm

1.384 μm

diameter

If the effective numerical aperture NA of the objective lens is increased to 0.53 as shown in Table 2, the output power is boosted to 51.65 mW.

TABLE 2

Priority on Efficiency (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

f = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

NA

0.19

2. Collimator

NA 0.15

f = 9 mm

Transmittance

Optical

2.81 times

lens

95%

Magnification

Lens bond

21.72 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

85.13%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

73.79%

98%

CW output

51.65 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

0.30%

θ//

7. Rising mirror

Transmittance

Rim intensity

32.24%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

0.3%

32.2%

Eclipse

1.70

0.76

coefficient

Expected spot

1.378 μm

1.314 μm

diameter

When the conventional stepped optical device is employed under conditions identical to those of Table 1 as in Table 3 below, the output power is degraded to 39.92 mW.

TABLE 3

Priority on Efficiency (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

f = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.18

2. Collimator

NA 0.15

f = 9 mm

Transmittance

Optical

2.81 times

lens

95%

Magnification

Lens bond

20.48 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

82.24%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

57.03%

98%

CW output

39.92 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

0.57%

θ//

7. Rising mirror

Transmittance

Rim intensity

36.57%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

0.6%

36.6%

Eclipse

1.60

0.71

coefficient

Expected spot

1.460 μm

1.384 μm

diameter

When the conventional stepped optical device is employed under the conditions identical to those of Table 2 as in Table 4 below, the output power is degraded to 41.32 mW.

TABLE 4

Priority on Efficiency (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

f = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

lens NA

0.19

2. Collimator

NA 0.15

f = 9 mm

Transmittance

Optical

2.81 times

lens

95%

Magnification

Lens bond

21.72 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

85.13%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

59.03%

98%

CW output

41.32 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

0.30%

θ//

7. Rising mirror

Transmittance

Rim intensity

32.24%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

0.3%

32.2%

Eclipse

1.70

0.76

coefficient

Expected spot

1.378 μm

1.314 μm

diameter

The rim intensity when priority is given on the spot size will be described here. As shown in

FIG. 25

, a laser light source having a broadening angle of 7.5° in the shorter diameter direction and 17° in the longer diameter direction is employed. A collimator lens having an effective numerical aperture NA of 0.15 at the focal length f of 20 mm is employed. The case is considered where the rim intensity is set to 35.4% in the shorter diameter direction as shown in FIG.

26

and to 81.7% in the longer diameter direction as shown in FIG.

27

.

By setting the effective numerical aperture NA of the objective lens to 0.5, the output power of the laser light source to 70 mW and the other parameters to appropriate values as shown in Table 5 below, the output power of the objective lens becomes 20.41 mW when using the kinoform type optical device

6

. Although the output power becomes lower than that corresponding to Table 1 where priority is given on the efficiency, the spot size becomes smaller than that of Table 1.

TABLE 5

Priority on spot size (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

f = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.08

2. Collimator

NA 0.15

f = 20 mm

Transmittance

Optical

6.25 times

lens

95%

Magnification

Lens bond

9.18 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

33.64%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

29.15%

98%

CW output

20.41 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

35.42%

θ//

7. Rising mirror

Transmittance

Rim intensity

81.71%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

35.4%

81.7%

Eclipse

0.72

0.32

coefficient

Expected spot

1.386 μm

1.313 μm

diameter

If the effective numerical aperture NA of the objective lens is increased to 0.53 of the objective lens as shown in Table 6, the output power increases to 22.05 mW. Although this output power becomes lower than that of Table 2 where priority is given on the efficiency, the spot size becomes smaller than that of Table 2.

TABLE 6

Priority on spot size (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

f = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

lens NA

0.08

2. Collimator

NA 0.15

f = 20 mm

Transmittance

Optical

6.25 times

lens

95%

Magnification

Lens bond

9.73 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

36.37%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

31.52%

98%

CW output

22.06 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

31.15%

θ//

7. Rising mirror

Transmittance

Rim intensity

79.69%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

31.1%

79.7%

Eclipse

0.76

0.34

coefficient

Expected spot

1.316 μm

1.241 μm

diameter

When a conventional stepped optical device is employed under conditions identical to those of the above Table 5 as in Table 7 below, the output power is reduced to 16.33 mW.

TABLE 7

Priority on spot size (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

f = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.08

2. Collimator

NA 0.15

f = 20 mm

Transmittance

Optical

6.25 times

lens

95%

Magnification

Lens bond

9.18 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

33.64%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

23.32%

98%

CW output

16.33 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

35.42%

θ//

7. Rising mirror

Transmittance

Rim intensity

81.71%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

35.4%

81.7%

Eclipse

0.72

0.32

coefficient

Expected spot

1.386 μm

1.313 μm

diameter

When a conventional stepped optical device is employed under conditions identical to those of the above Table 6 as in Table 8 below, the output power is reduced to 17.65 mW.

TABLE 8

Priority on spot size (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

f = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

lens NA

0.08

2. Collimator

NA 0.15

f = 20 mm

Transmittance

Optical

6.25 times

lens

95%

Magnification

Lens bond

9.73 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

36.37%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

25.22%

98%

CW output

17.65 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

31.15%

θ//

7. Rising mirror

Transmittance

Rim intensity

79.69%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

31.1%

79.7%

Eclipse

0.76

0.34

coefficient

Expected spot

1.316 μm

1.241 μm

diameter

Next, an example of the rim intensity suitable for an eightfold-speed optical pickup device will be described. As shown in

FIG. 28

, a laser light source having a broadening angle of 7.5° in the shorter diameter direction and 17° in the longer diameter direction is employed. A collimator lens having an effective numerical aperture NA of 0.15 with a focal length f of 15 mm is employed. The case is considered where the rim intensity is set to 15.8% in the shorter diameter direction as shown in FIG.

29

and to 69.8% in the longer diameter direction as shown in FIG.

30

.

When the effective numerical aperture NA of the objective lens is set to 0.5, the output power of the laser beam set to 70 mW, and the other parameters set to appropriate values as shown in Table 9, the output power from the objective lens becomes 30.62 mW when using the kinoform type optical device

6

. Since recording and reproduction at the eightfold speed is possible if the output power is at least 30 mW, operation thereof is possible in this case.

TABLE 9

Eightfold-speed pickup (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

F = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.11

2. Collimator

NA 0.15

f = 15 mm

Transmittance

Optical

4.69 times

lens

95%

Magnification

Lens bond

12.25 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

50.47%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

43.75%

98%

CW output

30.62 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

15.75%

θ//

7. Rising mirror

Transmittance

Rim intensity

69.79%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

15.8%

69.8%

Eclipse

0.96

0.43

coefficient

Expected spot

1.428 μm

1.326 μm

diameter

When the effective numerical aperture NA of the objective lens is increased to 0.53 as shown in Table 10, the output power from the objective lens becomes 33.32 mW. Recording and reproduction at the eightfold speed is possible in this case.

TABLE 10

Eightfold-speed pickup (New HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

F = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

lens NA

0.11

2. Collimator

NA 0.15

f = 15 mm

Transmittance

Optical

4.69 times

lens

95%

Magnification

Lens bond

12.98 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

54.92%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

47.60%

98%

CW output

33.32 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 100%

100%

power

Rim intensity

12.52%

θ//

7. Rising mirror

Transmittance

Rim intensity

66.74%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

12.5%

66.7%

Eclipse

1.02

0.45

coefficient

Expected spot

1.354 μm

1.255 μm

diameter

When a conventional stepped optical device is employed under conditions identical to those of Table 9 as in Table 11 below, the output power from the objective lens is reduced to 24.50 mW. In this case, recording and reproduction at eightfold speed is not possible.

TABLE 11

Eightfold-speed pickup (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.5

f = 3.2

Transmittance

Objective lens

3.20 mm

95%

effective

diameter

Collimator

At least

lens NA

0.11

2. Collimator

NA 0.15

f = 15 mm

Transmittance

Optical

4.69 times

lens

95%

Magnification

Lens bond

12.25 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

50.47%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

35.00%

98%

CW output

24.50 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

15.75%

θ//

7. Rising mirror

Transmittance

Rim intensity

69.79%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

15.8%

69.8%

Eclipse

0.96

0.43

coefficient

Expected spot

1.428 μm

1.326 μm

diameter

When a conventional stepped optical device is employed under conditions identical to those of Table 10 as in Table 12 below, the output power from the objective lens is reduced to 26.66 mW. In this case, recording and reproduction at eightfold speed is not possible.

TABLE 12

Eightfold-speed pickup (Conventional HOE)

Pickup Design

Calculation Condition

Calculated value

1. Objective lens

NA 0.53

f = 3.2

Transmittance

Objective lens

3.39 mm

95%

effective

diameter

Collimator

At least

lens NA

0.11

2. Collimator

NA 0.15

f = 15 mm

Transmittance

Optical

4.69 times

lens

95%

Magnification

Lens bond

12.98 deg

angle

3. Laser

Wavelength

CW 70 mW

Pulse 0 mW

Lens bond

54.92%

780 nm

efficiency

θ// 7.5 deg

θ ⊥ 17 deg

effective

7.50 deg

θ//angle

effective θ ⊥

17.00 deg

angle

4. Beam

θ// 1 time

θ ⊥ 1 time

Transmittance

formation

100%

magnification

5. Beam splitter

Tp 100%

Transmittance

Pick efficiency

38.08%

98%

CW output

26.66 mW

power

6. HOE

Spectral

Transmittance

Pulse output

0.00 mW

ratio 80%

100%

power

Rim intensity

12.52%

θ//

7. Rising mirror

Transmittance

Rim intensity

66.74%

98%

θ ⊥

θ//direction

θ ⊥ direction

Rim intensity

12.5%

66.7%

Eclipse

1.02

0.45

coefficient

Expected spot

1.354 μm

1.255 μm

diameter

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Number	Name	Date	Kind
6084843	Abe et al.	Jul 2000	A
6166854	Katsuma	Dec 2000	A

Number	Date	Country
8-278477	Oct 1996	JP
10-27373	Jan 1998	JP
10-143903	May 1998	JP
10-143908	May 1998	JP
10-228664	Aug 1998	JP
10-283662	Oct 1998	JP
11-7653	Jan 1999	JP
WO9819303	May 1998	WO

Optical pickup compatible with a plurality of types of optical disks having different thicknesses

Information

Patent Number

Date Filed

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Inventors

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Agents

CPC

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Abstract

Description

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PCT Information

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