Optical head

Details Optical head Optical head

1998-08-05

2002-11-26

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Diffraction

From grating

C359S571000, C385S037000

Reexamination Certificate

active

06487016

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical head for an optical recording and reading medium, and in particular, to devising a thinner optical head.
BACKGROUND OF THE INVENTION
Optical heads are important structural components for reading signals from optical recording media such as compact disks (CDs) or digital video disks (DVDs). Not only signal detection functions but also control mechanisms such as focus servos or tracking servos are necessary for optical heads to read out a signal from an optical recording medium.
FIG. 24
illustrates a typical conventional optical head. As is shown in this drawing, a collimator lens
3
collimates a laser beam
2
, which is emitted from a semiconductor laser
1
serving as the light source, into parallel light. After the laser beam
2
has passed through a focus/track error signal detection element
8
, its optical axis is deflected 90° by a mirror
20
and the light enters an objective lens
4
, which focuses the laser beam
2
on an optical disk
11
. The laser beam is reflected and returns on the same light path. It is turned into parallel light by the objective lens
4
, then reflected by the mirror
20
, and enters the focus/track error signal detection element
8
. When the laser beam
2
enters the focus/track error signal detection element
8
, it is divided into two beams, which are focused on the photo-detectors
13
a
and
13
b
. Thus, regeneration signals and servo signals, i.e. focus error signals and track error signals, can be read.
As can be seen from
FIG. 24
, the height of the optical head can be expressed as the sum of the working distance (WD), the thickness of the objective lens
4
, the space between a lower portion of the objective lens
4
and an upper portion of the mirror
20
(referred to as “lens-mirror space” in the following), and the height lz of the mirror
20
.
When trying to devise a thinner optical head, the minimum total length of WD, lens thickness and the lens-mirror space are for the most part governed by the type of the optical disk
11
. For example, for DVDs, the minimum values for WD, lens thickness and lens-mirror space can be estimated at 1.1 mm each, but the height lz of the mirror
20
has to be larger than a beam diameter w
1
, for example 3 mm. Consequently, in this case, the minimum height of the optical head can be estimated at 6.3 mm, and it is difficult to devise a thinner head.
It is a purpose of the present invention to solve these problems of the prior art and to provide a thinner optical head.
SUMMARY OF THE INVENTION
To achieve this purpose, an optical head according to a first configuration of the present invention comprises a first grating element and a second grating element arranged in that order in a light path between a light source and an objective lens; and a light path alteration member arranged in a light path between the light source and the first grating element or in a light path between the second grating element and the objective lens. In such an optical head, the optical distance between the first grating element and the second grating element can be reduced, so that a lateral shift of the optical axis due to wavelength variations in the light source can be reduced. As a result, the lateral shift of the optical axis from the center of the objective lens can be reduced and a focussing spot with a favorable circular shape can be formed.
In the optical head according to the first configuration of the present invention, it is preferable that the light path alteration member is arranged in the light path between the light source and the first grating element; light emitted from the light source passes through the light path alteration member and then enters the first grating element; light diffracted by the first grating element enters the second grating element; and light diffracted by the second grating element enters the objective lens and is focused on a recording medium. In such a preferable example, light can be deflected toward the objective lens using the second grating element, so that a thinner optical head can be realized.
It is also preferable that the first grating element is a reflection element; the light path alteration member is a first transparent substrate having a first surface; the first grating element and the light path alteration member are arranged so that an angle defined by the optical axis of light emitted from the light source and a normal on the first surface is at least the critical angle, and an angle defined by the normal on the first surface and the optical axis of light diffracted by said first grating element to the second grating element is smaller than the critical angle; light emitted from the light source is reflected from the first surface and enters the first grating element; and light reflected and diffracted by the first grating element passes the first surface and enters the second grating element. In such a preferable example, the optical axis of the light emitted from the light source can be shifted toward the recording medium, so that a thinner optical head can be achieved. In this case, it is even more preferable that the first transparent substrate is a triangular prism having a slanted face, a bottom face and a side face; the slanted face is the first surface; the first grating element is provided on the bottom face; and light emitted from the light source enters the first transparent substrate through the side face. In this specification, a “slanted surface” means a surface that is not substantially perpendicular or parallel with respect to the direction of the light beam. Moreover, in this case, it is preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate, on an upper face of which the second grating element is formed; a multi-layered film is formed on the first surface of the first transparent substrate; and the first transparent substrate and the second transparent substrate are integrated into one component by the multi-layered film. It is also preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate, on an upper face of which the second grating element is formed; and an air gap is provided between the first transparent substrate and the second transparent substrate. It is also preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate on an upper face of which the second grating element is formed; and the second transparent substrate is a triangular prism. It is also preferable that the second grating element is a reflection element; the optical head further comprises a second transparent substrate on a lower face of which the second grating element is formed; a multi-layered film is formed on the first surface of the first transparent substrate; the first transparent substrate and the second transparent substrate are integrated into one component by the multi-layered film; and the first grating element and the second grating element are arranged on the same plane. In these preferable examples, the first grating element and the second grating element can be easily manufactured.
It is preferable that a first incidence angle defined by the optical axis of a laser beam travelling from the light path alteration member to the first grating element and the normal on the first grating element is larger than an outgoing angle defined by the optical axis of diffracted light from the first grating element and the normal on the first grating element, and a second incidence angle defined by the optical axis of the laser beam from the first grating element entering the second grating element and the normal on the second grating element is larger than an outgoing angle defined by the normal on the second grating element and the optical axis of the light diffracted by the second grating element. In this preferable example, beam formation is performed and the light utiliz

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