Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1987-04-29
1989-01-10
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
369 44, G01J 120
Patent
active
047975458
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an optical head device for recording information in an optical information recording medium and reproducing or erasing it therefrom, and more particularly, to an optical head device including an optical system which is free from any spherical aberration and can satisfactorily concentrate light.
2. Description of the Related Art
There has recently been a great deal of activity for realizing an optical head device for recording information in an optical disk and reproducing it therefrom by utilizing an optical system of maximum diffraction which is substantially free from any abberation.
The essential construction of such an optical head device is shown in FIGS. 7(a) to 7(c).
The device includes a semiconductor laser (LD) 1 defining a source of light, a condensing lens 2, a disk 3 having an information recording surface 4 provided with an information track 12, a beam splitter 10, a diffraction grating 14 and a cylindrical lens 15. A beam 7 of light leaving the LD 1 at a point 6 is concentrated by the lens 2 on the disk 3 at a point 5. The beam is also concentrated on a light detector 11 at a point 13.
Referring to the operation of the device, a diverging beam 7 of light is emitted by the LD 1 at the point 6 and converted by the condensing lens 2 to a converging beam. The converging beam passes through a transparent substrate forming the disk 3 and having a thickness d and is concentrated on the information recording surface 4 at the point 5.
The beam which has been reflected by the information recording surface 4 passes through the condensing lens 2, is separated from the beam 7 by the beam splitter 10 and is received by the light detector 11 whereby a light signal is produced.
The device has focus and tracking servo functions to ensure that the point 5 on the disk 3 on which the beam is concentrated by the condensing lens 2 is always located in an information track 12.
The focus and tracking functions of the device shown in FIGS. 7(a) to 7(c) are carried out by an astigmatic method and a twin spot method as will now be described briefly.
Referring first to the astigmatic method, an astigmatic device, such as the cylindrical lens 15, is provided in the path of the reflected beam for rendering it astigmatic. The position of the light detector 11 along the optical axis is adjusted to ensure that the beam radiated on the light detector 11 forms a minimum circle 13 of aberration if the focal point 5 is correctly positioned in the information track 12. The light detector comprises four detector elements 11a to 11d, as shown in FIG. 7(b).
If the optical disk 3 is displaced along the optical axis, the spots on the light detector 11 change their shape froma minimum circle of abberation as shown at 13 by a solid line to an elongated oval shape as shown by broken lines. This change is detected as an electrical focus error signal which is obtained by the calculation of a differential between the total output of one of the two pairs of diagonally disposed detector elements and the total output of the other pair, i.e., (11a+11c)-(11b+11d). This signal causes a focus actuator (not shown) to move the condensing lens 2 to correct any deviation of the focal point 5 from the information recording surface 4 along the optical axis.
According to the twin spot method, the beam 7 of the LD 1 is divided by the diffraction grating 14 disposed across its path into a plurality of beams, i.e., of the zero- and .+-.1-orders and is concentrated on the information track 12 as shown in FIG. 7(c). The beam of the zero-order is correctly radiated on the center of the information track and is used for signal reading and recording purposes. The beams of the .+-.1-order are slightly deviated from the track. The three spots lie in a line which is slightly inclined to the information track 12.
The diffracted beams of the .+-.1-order rays are received by the light detectors 11e and 11f and their difference (11e-11f) provides a tracking error signal which cau
REFERENCES:
patent: 3932700 (1976-01-01), Snopko
patent: 4128847 (1978-12-01), Roullet et al.
patent: 4302830 (1981-11-01), Hamaoka et al.
patent: 4354103 (1982-10-01), Immink et al.
patent: 4458144 (1984-07-01), Reilly et al.
Kondo Mitsushige
Shikama Shinsuke
Toide Eiichi
Mitsubishi Denki & Kabushiki Kaisha
Nelms David C.
Oen William L.
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