Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1996-10-30
1999-01-26
Tran, Thang V.
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
369 4426, 369 4435, G11B 709
Patent
active
058645268
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a focusing adjustment or focusing control system and especially to a system used to adjust the focusing of a light beam for the recording and/or reading of information elements on a mobile carrier. It can be applied especially to an instrument for the reading and/or recording of optical or magneto-optical disks.
The invention can be applied to the recording of computer data but it can also be applied to the technique of compact disks as well as video disks.
An optical recording carrier is most usually formed by a reflective surface, in most cases in the form of a disk, having discontinuous and small-sized alterations arranged along tracks. The most common type of carrier at present is the audio disk called a compact disk. In this case, the alterations are micro-troughs or more generally micro-reliefs on the reflective plane surface. These micro-reliefs have a width of about 0.4 micrometers and the axes of two neighbouring tracks are separated by a distance (called the pitch) of 1.6 micrometers.
The presence of these micro-reliefs is detected by means of a machine with a laser whose beam is focused on the reflective surface, the dimension of this beam being minimised so that it reaches only one micro-relief at a time. When the beam reaches the reflective surface, the light is reflected. When the beam reaches a micro-relief, diffraction takes place. In other words, the light is dispersed in every direction. The device is such that for a reflection of the beam on the smooth surface, detectors receive a maximum amount of energy and when this beam reaches a micro-relief, these detectors receive only a part of the light energy owing to the diffraction that has caused part of the beam to escape from the optical system provided between the disk and the detectors.
The disks or other pre-recorded carriers usually have all their information elements in micro-relief form. By contrast, in disks on which recording can be done, it is generally only the pre-recorded information or the preformat that is in micro-relief form, the information being recorded by the forming of other alterations, namely modifications of the reflection coefficient or, for magneto-optical disks, the modification of the magnetisation which, in reading, is expressed by a rotation of polarisation.
The pre-recorded information or preformat defines a track configuration.
The working of the reading and/or recording device is accurate only if the reading laser beam (or recording laser beam) is focused on the reflective surface of the disk.
To enable the permanent focusing of the laser beam on the reflective surface, it is necessary to provide for a servo-control system with a means to measure the divergence in focusing in terms of magnitude and sign.
Known systems can achieve this focusing adjustment function with devices that have a wide dynamic range of detection and provide a wide range of adjustment. These are, for example, continuous type detectors. An example of a detector is given in the French patent No. 74 01283 in which an effect of astigmatism is introduced into the focusing reading beam by means of a cylindrical lens. Four photodetectors are positioned in the form of a cross. When the focusing is set in a normal way, the four photodetectors receive the same quantity of light. When the focusing is displaced in one direction, two diametrically opposite photodetectors receive more light than the other two. The reverse occurs when the focusing is displaced in the opposite direction. The drawback of this type of detector is that its zero adjustment error position may vary in time.
The focusing adjustment may be done also with "sampler" type detectors as described in the French patent No. 2 523 349. In this system, the information carrier has at least one first mark on the side of the axis of an information track and one second mark on the other side of the same axis. Two photodetectors are positioned on either side of a plane containing the axis of the track and receive the light transmitted by the carrier during the scan
REFERENCES:
patent: 4023033 (1977-05-01), Bricot et al.
patent: 4037251 (1977-07-01), Bricot et al.
patent: 4038524 (1977-07-01), Puech et al.
patent: 4142208 (1979-02-01), Oprandi et al.
patent: 4387452 (1983-06-01), Bricot et al.
patent: 4562564 (1985-12-01), Bricot et al.
patent: 4695158 (1987-09-01), Kotaka et al.
patent: 4786794 (1988-11-01), Doi
patent: 4868604 (1989-09-01), Le Carvennec et al.
patent: 5475672 (1995-12-01), Le Carvennec
"Thomson-CSF"
Tran Thang V.
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