Optical pickup and optical device

Details Optical pickup and optical device Optical pickup and optical device

1999-10-18

2001-11-20

Huber, Paul W. (Department: 2651)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S121000

Reexamination Certificate

active

06320837

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical pickup and optical device for use in a playback unit for an information recording medium such as an optical disc and more particularly to an optical pickup and optical device preferable for a compatible playback system for digital versatile disc (DVD) and compact disc-write once (CD-R).
2. Description of the Related Art
Currently, a DVD system has been proposed and marketed, and its prevalence has started, the DVD system having a higher density than an optical compact disc (CD) which has already prevailed as a consumer product. In DVD player as a playback unit of this system, compatible playback with CD is indispensable to avoid an overlapping of the devices and complexity of operation upon use. Likewise, the DVD player is also required to be compatible with the CD-R which can be played back by the CD player. A technology for playing back discs of various standards has been developed and further, simplification and reduction of cost for achieving it are problems to be solved.
For the aforementioned CD-R, a laser beam source having 780 nm band laser beam source different from 650 nm band for the DVD is required because reflectivity of an information recording medium has a high wave length dependency and therefore, an optical pickup containing a light source for two wave lengths (wavelengths) is necessary.
On the other hand, integration of optical system of the optical pickup has been demanded for the purpose of cost reduction and size reduction, and for this integration of the optical system, making a common optical axis of light from a light source having two wave lengths is effective. This is because by making common the optical axis, the optical path can be made single, and optical parts can be used in common and the like advantages exist.
As an optical system satisfying these requirements, such a structure in which diffraction gratings are disposed on the same optical axis and diffracted lights having each wave length (wavelength) are received by respective light receiving devices (photodetectors) can be considered.
However, it is very difficult to design such a diffraction grating as to change optical paths of lights having different wave lengths appropriately and to be provided with lens power (lens action). The reason is that because characteristics such as diffracting angle of the diffraction grating is determined depending on a mathematical relation between its cyclic structure and wave length of light, accordingly the diffracting angle changes largely with respect to a different wave length and further characteristics about optical path change and lens power (lens action) also change largely.
Thus, if lights having different wave lengths are emitted to the same diffraction grating, an optimum light receiving plane differs depending on the above-described characteristic change, so that sharing of the light receiving device (photodetector) substrate cannot be achieved, thereby obstructing a further integration of the optical system.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been achieved to solve the above problem and therefore, it is an object of the invention to provide an optical pickup and optical device in which optical axes of two different kinds of lights are made common and diffracted lights of lights having different wave lengths can be received by a single light receiving device substrate, thereby contributing to integration of an optical system.
To achieve the above object, according to a first aspect of the present invention, there is provided an optical pickup for irradiating light to an information recording medium and reading information by using a reflection light from the information recording medium, the optical pickup comprising: a first diffraction grating disposed substantially perpendicular to an incident optical axis of the reflection light; a second diffraction grating disposed substantially perpendicular to an incident optical axis of the reflection light and on a face different from a face on which the first diffraction grating is provided; and a light receiving device substrate having light receiving plane on the same level, on which the reflection light from the information recording medium impinges and for receiving ±primary-order (±1st-order) diffracted light diffracted/divided by the first diffraction grating and the second diffraction grating; wherein a point where the first diffraction grating and the incident optical axis intersect is named as first intersection point, a length from the first intersection point to point where +primary-order (+1st-order) diffracted light diffracted by the first diffraction grating intersects the light receiving plane is named as first-a(
1
a
) optical path length, a length from the first intersection point to a point where −primary-order (−1st-order) diffracted light diffracted by the first diffraction grating intersects the light receiving plane is named as first-b(
1
b
) optical path length, and an average of the first-a(
1
a
) optical path length and first-b(
1
b
) optical path length is named as first average optical path length, a point where the second diffraction point and the incident optical axis intersect is named as second intersection point, a length from the second intersection point to a point where +primary-order (+1st-order) diffracted light diffracted by the second diffraction grating intersects the light receiving plane is named as second-a(
2
a
) optical path length, a length from the second intersection point to a point where −primary-order (−1st-order) diffracted light diffracted by the second diffraction grating intersects the light receiving plane is named as second-b(
2
b
) optical path length, and an average of the second-a(
2
a
) optical path length and second-b(
2
b
) optical path length is names as second average optical path length, the first diffraction grating, the second diffraction grating and the light receiving device substrate being disposed such that a first curve in which the first average optical path length is constant, a second curve in which the second average optical path length is constant and an optical axis after a reflection light from the information recording meclum passes through the first and second diffraction gratings as 0-order (0th-order) light intersects at a single point or substantially single point.
According to the present invention, the intersection point between the first curve, the second curve and the optical axis or the substantial intersecting point is a converging point of light passing through the first and second diffraction gratings as 0-order (0th-order) light. Even if the light receiving device substrate is disposed in any way within the above condition, the first-a(
1
a
) optical path length and the first-b(
1
b
) optical path length are equal with respect to the first average optical path length and the second-a(
2
a
) optical path length and second-b(
2
b
) optical path length are equal with respect to the second average optical path length. Alternatively, always one thereof increases by the same distance and the other decreases by the same distance. Thus, the light receiving plane of the light receiving device substrate can be set up as an optimum light receiving plane for the ±primary-order diffracted light by the first diffraction grating and ±primary-order diffracted light by the second diffraction grating. Optical axes of two different kinds of lights (for example, wave length) are made common and diffracted lights having different kinds (for example, wave length) of light are received by a single light receiving device substrate, thereby contributing to integration of an optical system.
Further, to achieve the above object, according to a second aspect of the invention, there is provided an optical pickup for irradiating light to an information recording medium and reading information by using a reflection light from the information re

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