# US 8,570,849 B2 · Tilt angle adjustment method for objective lens and optical information recording/reproducing apparatus

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- Number: US 8,570,849 B2
- Document ID: US8570849B2
- Status: Granted
- Kind: B2
- Application number: 13/096,337
- Filed: 2011-04-28
- Published: 2011-11-17
- Granted: 2013-10-29
- Earliest claimed: 2010-05-12
- Assignees: Hoya Corporation
- Applicants: Satoshi Inoue; Shuichi Takeuchi
- Inventors: Satoshi Inoue; Shuichi Takeuchi
- Web page: https://patentgrep.com/patent/US8570849B2

## CPC

- G11B 7/0956: {to compensate for tilt, skew, warp or inclination of the disc, i.e. maintain the optical axis at right angles to the disc}
- G11B 7/13927: {during transducing, e.g. to correct for variation of the spherical aberration due to disc tilt or irregularities in the cover layer thickness}
- G11B 2007/0013: {for carriers having multiple discrete layers}
- G11B: INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER (recording measured values in a way that does not require playback through a transducer G01D9/00; recording or playback apparatus using mechanically marked tape, e.g. punched paper tape, or using unit records, e.g. punched or magnetically marked cards G06K; transferring data from one type of record carrier to another G06K1/18; circuits for coupling output of reproducer to radio receiver H04B1/20; gramophone pick-ups or like acoustic electromechanical transducers or circuits therefor H04R)

## Abstract

A method for adjusting a tilt angle of an objective lens with respect to one of a multilayer optical disc having a plurality of recording layers and an optical axis of an optical information recording/reproducing apparatus, wherein a use wavelength λ and a numerical aperture NA are defined by conditions: 390<λ<420 and 0.75<NA<0.90. The method includes determining the tilt angle of the objective lens such that a spot quality defined through a predetermined transparent parallel flat plate matches a predetermined standard, and tilting the objective lens to have the determined tilt angle. When t1 represents a thickness of the predetermined transparent parallel flat plate and t2 represents a design protective layer thickness at which a coma caused when off-axis light is incident on the objective lens becomes smallest, t1 and t2 satisfy: −0.015<t1−t2<0.005.

## Claims

### Claim 1

A method for adjusting a tilt angle of an objective lens with respect to one of a multilayer optical disc having a plurality of recording layers and an optical axis of an optical information recording/reproducing apparatus, wherein the apparatus emits a laser beam having a wavelength λ for the multilayer optical disc, the objective lens has a numerical aperture NA, and the wavelength λ and the numerical aperture NA satisfy the following conditions:
390<λ<420 and
0.75<NA<0.90,
the method comprising:
selecting a predetermined transparent parallel plate to satisfy a condition −0.015<t1−t2<0.005
where t1 represents a thickness of the predetermined transparent parallel flat plate and t2 represents a design protective layer thickness at which a coma caused when off-axis light is incident on the objective lens becomes smallest, t1 and t2 having units of mm
determining the tilt angle of the objective lens by examining if a spot quality of the laser beam defined through the predetermined parallel plate matches a predetermined spot quality standard; and
tilting the objective lens to have the determined tilt angle based on the examining of the spot quality of the laser beam.

### Claim 2 (depends on 1)

The method according to claim 1, further comprising:
observing, through the predetermined transparent parallel flat plate, a wavefront of light from the objective lens,
wherein:
in the determining the tilt angle, the tilt angle is determined based on the observed wavefront;
in the tilting the objective lens, the objective lens is tilted to the determined tilt angle; and
the observing the wavefront, the determining and the tilting are repeated until the observed wavefront matches the predetermined standard.

### Claim 3 (depends on 1)

The method according to claim 1, further comprising:
measuring, through the predetermined transparent parallel flat plate, a spot shape of a beam spot,
wherein:
in the determining the tilt angle, the tilt angle is determined based on the measured spot shape;
in the tilting the objective lens, the objective lens is tilted to the determined tilt angle; and
the measuring the spot shape, the determining and the tilting are repeated until the measured spot shape matches the predetermined standard.

### Claim 4 (depends on 1)

The method according to claim 1, further comprising:
measuring a jitter through the predetermined transparent parallel flat plate,
wherein:
in the determining the tilt angle, the tilt angle is determined based on the measured jitter;
in the tilting the objective lens, the objective lens is tilted to the determined tilt angle; and
the measuring the jitter, the determining and the tilting are repeated until the measured jitter matches the predetermined standard.

### Claim 5 (depends on 1)

The method according to claim 1,
wherein t1 and t2 satisfy a following condition:
−0.015<t1−t2<0.000.

### Claim 6 (depends on 1)

The method according to claim 1,
wherein when tmin represents a smallest protective layer thickness of all of protective layer thicknesses of the plurality of recording layers, and tmax represents a largest protective layer thickness of all of the protective layer thicknesses of the plurality of recording layers, t2satisfies a following condition:
tmin<t2<tmax.

### Claim 7 (depends on 1)

The method according to claim 1,
wherein t1 and t2 respectively satisfy the following conditions:
0.045<t1<0.105; and
0.060<t2<0.100.

### Claim 8 (depends on 1)

The method according to claim 1,
wherein t1 and t2 respectively satisfy the following conditions:
0.070<t1<0.080; and
0.065<t2<0.095.

### Claim 9 (depends on 1)

The method according to claim 1,
wherein a coma component of a wavefront observed through the predetermined transparent parallel flat plate when the spot quality matches the predetermined standard is substantially equal to zero.

### Claim 10

An optical information recording/reproducing apparatus that emits a laser beam having a wavelength λ for a multilayer optical disc, the optical information recording reproducing apparatus comprising:
a light source which emits the light beam having the wavelength λ; and
an objective lens having a numerical aperture NA positioned at a tilt angle, the objective lens being configured to converge the light beam having the wavelength λ onto each recording surface of the multilayer optical disc having a plurality of recording layers the wavelength λ and the numerical aperture NA satisfy the following conditions:
390<λ<420 and
0.75<NA<0.90,
a tilt adjuster that is configured to select a predetermined transparent parallel plate to satisfy a condition −0.015<t1−t2<0.005, where t1 represents a thickness of the predetermined transparent parallel flat plate and t2 represents a design protective layer thickness at which a coma caused when off-axis light is incident on the objective lens becomes smallest, t1 and t2having units of mm, to determine the tilt angle of the objective lens by examining if a spot quality of the laser beam defined through the predetermined parallel plate matches a predetermined spot quality standard, and to tilt the objective lens to the determined tilt angle based upon the examination of the spot quality of the laser beam.

### Claim 11 (depends on 10)

The optical information recording/reproducing apparatus according to claim 10,
further comprising a spherical aberration correcting element configured to correct a spherical aberration which varies depending on a difference in protective layer thickness between the plurality of recording layers of the multilayer optical disc.

### Claim 12 (depends on 11)

The optical information recording/reproducing apparatus according to claim 11,
wherein the spherical aberration correcting element is a coupling lens which changes a diverging angle of the light beam having the wavelength λ emitted by the light source by moving in an optical axis direction.

### Claim 13

An optical information recording/reproducing apparatus that utilizes a multilayer optical disc having a plurality of recording layers, the apparatus comprising:
a light source which emits a light beam having a wavelength λ;
an objective lens configured to converge the light beam having the wavelength λ onto each recording surface of the multilayer optical disc having a plurality of recording layers; and
a tilt adjuster is configured to adjust a tilt angle of the objective lens, and the following condition is satisfied: −0.015<t3−t2<0.005, where t2 represents a design protective layer thickness at which a coma caused when off-axis light emitted by the light source and incident on the objective lens becomes smallest and t3 represents a protective layer thickness of the multilayer optical disc at which a coma caused when information recording or information reproducing is executed for the multilayer optical disc becomes smallest, and t2 and t3 have units of mm.

### Claim 14 (depends on 13)

The optical information recording/reproducing apparatus according to claim 13,
wherein t2 and t3 satisfy a following condition:
−0.015<t3−t2<0.000.

### Claim 15 (depends on 13)

The optical information recording/reproducing apparatus according to claim 13,
further comprising a spherical aberration correcting element configured to correct a spherical aberration which varies depending on a difference in protective layer thickness between the plurality of recording layers of the multilayer optical disc.

### Claim 16 (depends on 15)

The optical information recording/reproducing apparatus according to claim 15,
wherein the spherical aberration correcting element is a coupling lens which changes a diverging angle of the light beam having the wavelength λ emitted by the light source by moving in an optical axis direction.