Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2002-09-26
2004-11-23
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C369S103000, C369S112040, C369S112070
Reexamination Certificate
active
06822771
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical pickup units and optical disk drive units, and more particularly to an optical pickup unit that emits light beams of different wavelengths to the recording surfaces of different types of information recording media and receives reflected lights therefrom, and an optical disk drive unit employing such an optical pickup unit.
2. Description of the Related Art
An optical pickup unit that is designed for both a DVD-type optical recording medium using a 650 nm wavelength &lgr;
1
, such as a digital video disk, and a CD-type optical recording medium using a 780 nm wavelength &lgr;
2
, such as a compact disk, has been put to practical use. Such an optical pickup unit employs, as its light source, a semiconductor laser chip manufactured as a “monolithic chip” emitting light beams of the respective wavelengths &lgr;
1
and &lgr;
2
(hereinafter referred to as a “dual-wavelength monolithic chip”), or a package of different semiconductor laser chips emitting laser beams of the respective wavelengths &lgr;
1
and &lgr;
2
in order to use in common the optical system in the optical path from the light source to the optical recording medium.
In the dual-wavelength monolithic chip, the respective light-emitting parts can be positioned close to each other. It is not easy, however, to position the light-emitting parts relative to each other with accuracy, thus making it difficult to increase the manufacturing yield of the chip. Further, in the case of containing the light source and a “light-receiving element receiving a returning beam” in one package, heat from the dual-wavelength monolithic chip makes it difficult for the light-receiving element to operate at high speed if the light-emitting parts are positioned with a small distance between each other. Therefore, it is difficult to apply the dual-wavelength monolithic chip in the case of recording information on an optical recording medium or reproducing information therefrom at high speed.
On the other hand, in the case of containing semiconductor laser chips of different light emission wavelengths in a single package, the semiconductor laser chips having desired outputs for the respective, wavelengths can be used. Therefore, the best semiconductor laser chip can be used in accordance with the specifications of the optical disk drive unit. Accordingly, a high-speed optical disk drive unit can be realized at low cost.
However, since the semiconductor laser chips are individually mounted in this type of light source, an error is caused in the mounting of the semiconductor laser chips. Therefore, the accuracy of spacing the two light-emitting parts is likely to be decreased. Such a decrease in the spacing accuracy is likely to incur a decrease in the signal detection accuracy of the light-receiving element.
Some optical recording media have substrates of high birefringence. Therefore, in the case of using an optical disk drive unit in common between a plurality of types of optical recording media using different wavelengths, there is the problem of “deterioration in detection signals”, which is a variation caused in detection signals such as reproduction, focus, and tracking signals by the effect of birefringence when information is recorded on or reproduced from an optical recording medium having a substrate of high birefringence.
An optical disk unit uses an information recording medium such as an optical disk. The optical disk unit records information on the information recording medium by focusing a laser beam onto its recording surface on which a spiral track or concentric tracks are formed. The optical disk unit reproduces information from the optical recording medium based on a reflected light from its recording surface. The optical disk unit includes an optical pickup unit as a device for emitting the laser beam into a beam spot on the recording surface of the optical recording medium and receiving the reflected light therefrom.
Normally, the optical pickup unit includes an objective lens guiding a light beam emitted from a light source to the recording surface of the optical recording medium, an optical system guiding the light beam reflected from the recording surface as a returning beam to a predetermined light-receiving position, and a light-receiving element provided at the predetermined light-receiving position. The light-receiving element outputs a signal including not only the reproduced information of data recorded on the recording surface, but also information necessary for controlling the optical pickup unit itself and the position of the objective lens (servo information).
In order to correctly record data at a predetermined position on the recording surface or correctly reproduce data recorded at a predetermined position on the recording surface, it is necessary to form the laser spot at the predetermined position with accuracy. For this purpose, it is required to detect the position at which the laser spot is formed. A variety of methods of detecting a laser spot formation position on the recording surface by using the returning beam reflected therefrom have been proposed, and some of the methods have been put to practice.
The methods of detecting a laser spot formation position on the recording surface can be classified roughly into the following two types. One type uses a returning beam from one beam spot formed on the recording surface. A method of this type is called a one-beam method. The other type uses returning beams from three beam spots formed on the recording surface. A method of this type is called a three-beam method. In the case of using the three-beam method, the light beam emitted from the light source is split into three beams to form the three beam spots on the recording surface.
Of one-beam methods, a so-called push-pull method and a phase difference method are commonly used, while a so-called three-spot method and a differential push-pull method are commonly used three-beam methods.
The push-pull method divides the returning beam into two parts in a direction corresponding to the tangential direction of the track and detects the deviation of a beam spot position (a so-called tracking error signal) from the difference between the amounts of light of the two parts.
The phase difference method detects a tracking error signal based on the rotational change of the intensity pattern of the returning beam. That is, the returning beam is detected by a light-receiving elements divided into four parts, and obtains a phase lead and a phase lag based on signals each representing the sum of the amounts of light received by a corresponding pair of light-receiving element parts positioned diagonally to each other. Thereby, the tracking error signal is obtained.
According to the three-spot method, the light beam emitted from the light source is divided into one main (primary) beam and two sub (secondary) beams. The light beam is emitted so that the main beam and the sub beams are focused onto the recording surface into respective laser spots that are equally spaced a quarter track pitch apart in the tracking directions, which are directions perpendicular to the tangential directions of the track. The returning beams of the two sub beams reflected back from the recording surface are received by their respective light-receiving elements, so that a tracking error signal is detected from the difference between the amounts of light received by the two light-receiving elements.
According to the differential push-pull method, the light beam emitted from the light source is divided into one main beam and two sub beams. The light beam is emitted so that the main beam and the sub beams are focused onto the recording surface into respective laser spots that are equally spaced a half track pitch apart. The returning beams of the main beam and the sub beams reflected back from the recording surface are received by three respective light-receiving elements each divided into two parts. Then, a push-pull signal is obtained in each of the
Funato Hiroyoshi
Oohchida Shigeru
Boutsikaris Leo
Dickstein , Shapiro, Morin & Oshinsky, LLP
Ricoh & Company, Ltd.
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