Optical: systems and elements – Lens – With support
Reexamination Certificate
2001-10-17
2003-04-15
Mai, Huy (Department: 2873)
Optical: systems and elements
Lens
With support
C359S824000
Reexamination Certificate
active
06549346
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens assembly, and to an optical head and an optical recording/reproduction apparatus using the objective lens assembly. Specifically, the present invention relates to an objective lens assembly having a high numerical aperture (NA) which is used for optical information processing, optical transmission, etc., and to an optical head and an optical recording/reproduction apparatus using such an objective lens assembly.
2. Description of the Related Art
In recent years, a digital versatile disc (DVD) has been receiving attention as a large storage optical medium because of its high recording density for recording digital information, which is six times higher than that of a compact disc (CD). Along with the increase of information to be stored, an optical recording medium which has a greater recording density than the DVD has been demanded. In order to achieve a higher recording density than the conventional DVD (wavelength: 660 nm, numerical aperture (NA): 0.6), it is necessary to shorten the wavelength of a light source and increase the NA of an objective lens assembly. For example, when using a blue laser which operates at a wavelength of 405 nm as a light source and an objective lens having a NA of 0.85, a recording density achieved is five times greater than that of the above DVD.
Such a high numerical aperture cannot be achieved by a single lens. Thus, an objective lens assembly structured by assembling a plurality of lenses is used in order to obtain a high numerical aperture. However, variations which may occur in the production of respective component lenses of the lens assembly cause aberrations. In order to correct such aberrations, there are various techniques proposed.
Japanese Laid-Open Publication No. 11-203706 proposes a technique for adjusting the relative positions of respective component lenses by moving the component lenses in order to correct aberration.
FIG. 12
shows a structure of a conventional objective lens assembly
80
described in Japanese Laid-Open Publication No. 11-203706. The objective lens assembly
80
includes a first lens
81
, a second lens
82
, a first lens holder
83
for holding the first lens
81
, and a second lens holder
84
for holding the second lens
82
. The first lens holder
83
has the shape of a hollow cylinder. The first lens holder
83
has an end face
83
A which faces the second lens holder
84
. In the end face
83
A, a cylindrical opening
83
B is formed. At a central portion of a bottom face
83
C of the opening
83
B, the first lens holder
83
has a hole
83
D for holding the first lens
81
. The first lens
81
is fit into the hole
83
D such that the optical axis thereof is generally perpendicular to the end face
83
A. The second lens holder
84
also has the shape of a hollow cylinder. The second lens holder
84
has an end face
84
A which faces the end face
83
A of the first lens holder
83
and an end face
84
E which is opposite to the end face
84
A. In the end face
84
E, the second lens holder
84
has a cylindrical opening
84
B whose diameter is larger than that of the opening
83
B. At a central portion of a bottom face
84
C of the opening
84
B, the second lens holder
84
has a hole
84
D which has substantially the same diameter as that of the opening
83
B of the first lens holder
83
. The second lens
82
is retained in the opening
84
B such that the optical axis thereof is generally perpendicular to the end face
84
A.
In the objective lens assembly
80
having such a structure, in order to correct aberrations which may be caused by variations in production, relative positions of the first lens
81
and the second lens
82
are adjusted. Specifically, the relative position of the second lens
82
with respect to the first lens
81
is adjusted by adjusting the relative position of the second lens holder
84
with respect to the first lens holder
83
along the direction indicated by arrow X such that the optical axis of the second lens
82
coincides with the optical axis of the first lens
81
. Then, the second lens
82
which is retained by a, spring (not shown) is moved along the direction indicated by arrow Y by turning a screw (not shown), whereby the relative position of the second lens
82
with respect to the first lens
81
along the direction indicated by arrow Y is adjusted. Thereafter, the first lens holder
83
and the second lens holder
84
are adhered to each other such that the end face
83
A faces the end face
84
A. In this way, the relative position of the second lens
82
with respect to the first lens
81
is adjusted such that aberration of each component lens is corrected. As a result, an objective lens assembly having a high NA can be obtained.
Japanese Laid-Open Publication No. 2000-90473 proposes a structure where component lenses are incorporated into lens holders without strictly adjusting the relative positions of the component lenses, and one of the incorporated lenses is polished so as to correct aberration.
FIG. 13
shows a structure of a conventional objective lens assembly
90
described in Japanese Laid-Open Publication No. 2000-90473. The objective lens assembly
90
includes a first lens
91
, a second lens
92
integrally formed from a plurality of single-lenses, and a lens holder
93
for holding the first lens
91
and the second lens
92
. The lens holder
93
has the shape of a hollow cylinder and has end faces
93
A and
93
B. The lens holder
93
has an opening
93
C which penetrates through the end faces
93
A and
93
B and which has steps inside thereof. The first lens
91
is held in the opening
93
C at the end face
93
A. The second lens
92
is held inside of the opening
93
C. The first lens
91
has an end face
91
A on the opposite side from the second lens
92
in relation to the first lens
91
.
The manner in which the objective lens assembly
90
having such a structure is assembled is described. In the first step, the first lens
91
and the second lens
92
are incorporated in the lens holder
93
without strictly adjusting the relative positions of these lenses. Next, the end face
91
A of the first lens
91
is polished such that a total aberration of the first lens
91
and the second lens
92
is reduced, thereby completing the objective lens assembly
90
. Specifically, aberrations caused due to variations in production of each of the first lens
91
and the second lens
92
and aberrations caused due to variations which may occur when incorporating the first lens
91
and the second lens
92
in the lens holder
93
are corrected by generating aberration by polishing the end face
91
A of the first lens
91
. Thus, when an objective lens assembly is assembled by using this method, a wider production tolerance for each lens component and a wider incorporation tolerance for incorporation into a lens holder can be provided.
Japanese Laid-Open Publication No. 11-174307 proposes an objective lens assembly where two lens holders each incorporating a lens are combined such that one is fit into the other, and the interval between these two lenses can be adjusted.
In adjustment of an objective lens assembly including two lens holders, one of the lens holders is moved with respect to the other such that the relative positions of these two lenses are changed. In this case, it is desirable to move one of the lens holders along five axes (lens interval direction, two directions for lens tilt, and two directions for adjusting the center of the lens), such that the total aberration of the entire objective lens assembly is minimized. One of the known methods for measuring aberration is a method using a Twyman interferometer. A conventional adjustment method for adjusting the relative positions of the two lens holders while measuring the aberration with the Twyman interferometer is described with reference to FIG.
14
.
FIG. 14
shows a structure of a conventional objective lens assembling/adjusting apparatus
1400
. Light is supplied to a half
Kasazumi Ken'ichi
Matsuzaki Keiichi
Mohri Masanari
Nishino Seiji
Saimi Tetsuo
Mai Huy
Matsushita Electric - Industrial Co., Ltd.
Seyrafi Saeed
Snell & Wilmer LLP
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