# US 7,411,743 B2 · Multi-wavelength lens, and optical system, optical head and optical disc apparatus using the lens

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- Number: US 7,411,743 B2
- Document ID: US7411743B2
- Status: Granted
- Kind: B2
- Application number: 10/886,655
- Filed: 2004-07-09
- Published: 2005-01-13
- Granted: 2008-08-12
- Earliest claimed: 2003-07-10
- Assignees: Hitachi Maxell, Ltd.
- Inventors: Yasuyuki Sugi
- Web page: https://patentgrep.com/patent/US7411743B2

## CPC

- G11B 7/1374: Objective lenses {(optical objectives per se G02B9/00)}
- G02B 3/04: with continuous faces that are rotationally symmetrical but deviate from a true sphere {, e.g. so called "aspheric" lenses}
- G02B 3/10: Bifocal lenses; Multifocal lenses
- G11B 7/1275: Two or more lasers having different wavelengths
- G11B 7/1367: Stepped phase plates
- G11B 7/13922: {passive}
- G11B 2007/0006: {adapted for scanning different types of carrier, e.g. CD & DVD}
- 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)
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

The multi-wavelength lens for condensing a plurality of kinds of monochromatic light by refraction is disclosed. The lens comprises a common use area for all monochromatic light on at least one lens surface, the area sectioned into a plurality of aspherical zones each of which having a different refractive power; and step portions, each of which formed between adjacent aspherical zones of the plurality of aspherical zones and having a step height Dj (j=1,2,3,4, and so on, in order of closeness to a lens optical axis) in a direction parallel to the lens optical axis. At least half of the step portions satisfy a following formula when a minimum value and a maximum value of Aij for each wavelength λi are MIN(Aij) and MAX(Aij), respectively: MAX(Aij)/MIN(Aij)<3 where, Aij=absolute(Bij−mij), Bij=(absolute(Dj))*(ni−1)/λi−C, ni is a refractive index of a lens for a wavelength λi, mij is an integral number closest to Bij, and C is a corrective term.

## Claims

### Claim 1

A multi-wavelength lens for condensing a plurality of kinds of monochromatic light by refraction, comprising:
a common use area for all monochromatic light on at least one lens surface, the area sectioned into a plurality of aspherical zones each of which having a different refractive power; and
step portions, each of which formed between adjacent aspherical zones of the plurality of aspherical zones and having a step height Dj (j=1, 2, 3, 4, and so on, in order of closeness to a lens optical axis) in a direction parallel to the lens optical axis,
wherein at least half the number of the step heights of the step portions satisfy MAX(Aij)/MIN(Aij)<3,
wherein MIN(Aij) and MAX(Aij) are minimum and maximum values of phase shift amount Aij derived from said step height for each wavelength λi, respectively, Aij=absolute (Bij−mij), Bij=absolute(Dj))*(ni−1)/λi−(NAij2)*K/λi, ni is a refractive index of a lens for a wavelength λi, mij is an integral number closest to Bij, and NAij is a numeral aperture (NA) of each of the aspherical zones defined by each of the step portions for a wavelength λi:
K=0.0004 mm when NAij <0.55
K=0.0005 mm when NAij >=0.55.

### Claim 2 (depends on 1)

The multi-wavelength lens according to claim 1, wherein at least half of the step portions satisfy a following formula:
MAX(Aij)/MIN(Aij)<2.

### Claim 3 (depends on 2)

The multi-wavelength lens according to claim 2, wherein all of the step portions satisfy the formula MAX(Aij)/MIN(Aij)<2.

### Claim 4 (depends on 1)

The multi-wavelength lens according to claim 1, wherein all of the step portions satisfy the formula MAX(Aij)/MIN(Aij)<3.

### Claim 5

A lens having a positive power for condensing light from a light source to form an optical spot on a focus surface, comprising:
a use area on one surface or both surfaces of the lens; and
a nonuse area located outside of the use area,
wherein a diameter of an optical spot obtained when light from the light source is incident on both of the use area and the nonuse are is smaller than a diameter of an optical spot obtained when light from the light source is incident on the use area only,
wherein a root mean square (RMS) wavefront aberration of the positive power is no more than 0.035 λ, and the RMS wavefront aberration satisfies 1≦Wmax/Wmin<1.8,
where Wmax is the maximum RMS wavefront aberration, and Wmin is the minimum wavefront aberration, of the n kinds of different wavelength λi.

### Claim 6 (depends on 5)

The lens according to claim 5, wherein the lens is a multi-wavelength lens for condensing a plurality of kinds of monochromatic light having a plurality of wavelengths, and a nonuse area for a first wavelength serves as a use area for a second wavelength different from the first wavelength.

### Claim 7 (depends on 6)

The multi-wavelength lens according to claim 6, wherein the area serving as the nonuse area for the first wavelength and as the use area for the second wavelength is sectioned into a plurality of zones on one surface or both surfaces of the lens, and a phase shift due to wavefront aberration caused by light having the second wavelength passing through each of the plurality of zones is substantially an integral multiple of each other.