Optical head with a phase plate for different types of disks

Details Optical head with a phase plate for different types of disks Optical head with a phase plate for different types of disks

1998-05-28

2001-03-13

Cabeca, John W. (Department: 2752)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S044230, C369S044370, C369S094000, C369S109010, C369S110040

Reexamination Certificate

active

06201780

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical head device, and in particular to an optical head device for the purpose of recording and reproducing in relation to two types of optical recording medium having different substrate thickness.
2. Description of the Related Art
Digital video discs (DVD), which are currently in the process of being developed as a product, have a substrate thickness of 0.6 [mm] as compared 1.2 [mm] in the case of the conventional compact disc (CD). In this situation, there is a demand for an optical head device which will be capable of reproducing both DVDs and CDs.
However, conventional optical head devices are designed in such a manner that the objective lens negates spherical aberration in relation to a disc of a certain thickness. Where a disc of a different thickness is reproduced, spherical aberration remains and it is impossible to reproduce it correctly.
[Conventional Example (1)]
The first example of a conventional optical head device which is capable of reproducing two discs of two different thicknesses is illustrated in FIG. 1 on pp. 460-6 of the
Japanese Journal of Applied Physics
Volume 36 Part 1 No. 1B.
FIG. 19
illustrates the structure of this optical head device. In
FIG. 19
, a first optical system
111
and a second optical system
112
each have a semiconductor laser which outputs the prescribed laser light, and a photosensor which receives light reflected from the disc (optical recording medium). Of these, the wavelength of the semiconductor laser of the first optical system
111
is 650 [nm], while that of the second optical system is 780 [nm].
Meanwhile, number
113
indicates an interference filter. This interference filter
113
works in such a manner as to transmit light of wavelength 650 [nm], while reflecting light of wavelength 780 [nm]. In this way, light emitted from the semiconductor laser of the first optical system
111
passes through the interference filter
113
and is incident upon a hologram
161
. Light which passes through the hologram
161
is incident upon an objective lens
115
in the form of parallel light and converges on a disc (optical recording medium)
116
with a thickness of 0.6 [mm].
Light reflected from the disc
116
passes through the objective lens
115
in the opposite direction and is incident again upon the hologram
161
. Light which passes through the hologram
161
passes through the interference filter
113
and is received by a photosensor within the first optical system
111
.
Similarly, light emitted from the semiconductor laser of the second optical system
112
passes through the interference filter
113
and is incident upon a hologram
161
. First (+) order diffracted light from the hologram
161
is incident upon an objective lens
115
in the form of divergent light and converges on a disc (optical recording medium)
117
with a thickness of 1.2 [mm].
Light reflected from the disc
117
passes through the objective lens
115
in the opposite direction and is incident again upon the hologram
161
. First (+) order diffracted light from the wavelength-selective hologram
161
is reflected by the interference filter
113
and is received by a photosensor within the second optical system
112
.
The objective lens
115
has a spherical aberration which negates the spherical aberration generated when light of wavelength 650 [nm] emitted from the objective lens
115
passes through a substrate with a thickness of 0.6 [mm], while the hologram
161
has a spherical aberration which negates the sum of the spherical aberration of the objective lens
115
and that which is generated in relation to +1st order diffracted light from the hologram
161
when light of wavelength 780 [nm] emitted from the objective lens
115
passes through a substrate with a thickness of 1.2 [mm].
Consequently, light of wavelength 650 [nm] which passes through the hologram
161
converges as a result of the objective lens
115
without aberration on the disc
116
, while +1st order diffracted light of wavelength 780 [nm] converges as a result of the objective lens
115
without aberration on the disc
117
.
FIG. 20
presents a top view and a cross-sectional view of the hologram
161
.
The hologram
161
is structured in such a manner that a concentric hologram pattern is formed on a glass substrate
118
.
Where the cross-section of the hologram pattern
162
is in the form of steps on four levels as in the drawing, and the height of each step is h/2, the refractive index n, and the wavelength of the incident light &lgr;, the transmission efficiency &eegr;
0
and +1st order diffraction efficiency &eegr;
+1
are given by the following formulae.
&eegr;
0
=cos
2
(&phgr;/2)cos
2
(&phgr;/4)  (1)
&eegr;
+1
=(8/&pgr;
2
)sin
2
(&phgr;/2)cos
2
[(&phgr;+&pgr;)/4]  (2)
where, &phgr;=2&pgr;(
n−
1)
h/&lgr;
  (3)
For instance, when h=2.83 [&mgr;m] and n=1.46, since &phgr;=4&pgr; for &lgr;=650 [nm], &eegr;
0
=1, &eegr;
+1
=0.
Similarly, since &phgr;=3.33 &pgr; for &lgr;=780 [nm], &eegr;
0
=0.188, &eegr;
+1
=0.567.
In other words, light of wavelength 650 [nm] emitted from a semiconductor laser all passes through the hologram
161
and heads towards the disc
116
, while 56.7% of light of wavelength 780 [nm] emitted from a semiconductor laser is diffracted by the wavelength-selective hologram
161
as +1st order diffracted light and heads towards the disc
117
.
Moreover, as
FIG. 20
shows, if the effective diameter of the objective lens
115
is
2
a
, the hologram pattern
162
is formed only within an area
2
b
of a diameter smaller than this. Outside the area of diameter
2
b
, light of wavelengths 650 [nm] and 780 [nm] all passes through the hologram
161
.
In other words, with the hologram
116
, light of wavelength 650 [nm] all passes through, while 56.7% of light of wavelength 780 [nm] is diffracted within the area of diameter
2
b
as +1st order diffracted light, and none is diffracted outside the area of diameter
2
b
, where
2
a
and
2
b
are the diameters shown in FIG.
20
(
a
).
Consequently, if the focal length of the objective lens
115
is f, the effective numerical aperture in relation to light of wavelengths 650 [nm] and 780 [nm] is given by a/f and b/f respectively. For example, if f=3 [mm], a=1.8 [mm] and b=1.35 [mm], a/f=0.6 while b/f=0.45.
[Conventional Example (2)]
The second example of a conventional optical head device which is capable of reproducing two discs of two different thicknesses is illustrated in
FIG. 7
on pp. 460-6 of the
Japanese Journal of Applied Physics
Volume 36 Part 1 No. 1B.
FIG. 21
illustrates the structure of this optical head device (conventional example 2).
In
FIG. 21
also, a first optical system
111
and a second optical system
112
each have a semiconductor laser, and a photosensor which receives light reflected from the disc. The wavelength of the semiconductor laser of the first optical system
111
is 650 [nm], while that of the second optical system is 780 [nm]. The interference filter
113
works in such a manner as to transmit light of wavelength 650 [nm], while reflecting light of wavelength 780 [nm].
Light emitted from the semiconductor laser of the first optical system
111
passes through the interference filter
113
and an aperture
163
to be incident upon the objective lens
115
in the form of parallel light and converge on the disc
116
, which has a thickness of 0.6 [mm]. Light reflected from the disc
116
passes in the oppos

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