Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2000-03-02
2001-05-08
Neyzari, Ali (Department: 2651)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S013010
Reexamination Certificate
active
06229783
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to an optical recording medium or support having two superimposed levels, as well as the corresponding recording device and reading process. This medium can be in random form, but in most cases it is a disk. The present invention relates to all optical recording types, namely reading or writing, writable and/or rewritable procedures.
The invention can e.g. be used in the optical disk recording field, e.g. for a second generation, high recording capacity DVD-RAM (2.6 to gigabytes per disk).
2. Prior Art
Essentially two types of optical disks are known, namely those using the phase change of a solid material (crystalline phase or amorphous phase) and those using the magnetooptical properties of certain materials, particularly the polarization rotation of a light beam. Intense research has been carried out for a considerable number of years on phase change optical disks. These disks operate on the principle according to which it is possible to pass in a reversible manner a material from an amorphous state to a crystalline state, as a function of the intensity and duration of a laser spot applied thereto. This principle also permits a direct overwriting of new informations on already written informations.
The recording area of a phase change disk is in the form shown in the attached FIG.
1
. It is possible to see amorphous areas Za distributed between crystalline areas Zc, said amorphous areas representing the recorded binary information.
Such informations are read optically. A laser spot, generally guided by a groove, scans the surface of the disk and the reflecting wave is transmitted to a detection system. Thus,
FIG. 2
shows a disk
10
illuminated by a laser
12
through a focussing lens
14
. The reflected light passes through a cubic beam splitter
16
. The exit pupil
18
covers a detector having several quadrants, generally four quadrants A, B, C, D. This detector generally comprises photodiodes (4 in the present case).
The intensity distribution in the exit pupil is a function of the reflection coefficients of the amorphous and crystalline areas illuminated by the laser spot. These coefficients are two complex numbers, which can be written:
{circumflex over (r)}=r.exp j&phgr;
where {circumflex over (r)} is the amplitude reflection coefficient and &phgr; the phase of the reflected wave. The intensity reflection of a light beam on the centre is equal to r
2
.
FIG. 3
shows in greater detail the illuminated area of the disk, the laser spot carrying the reference
20
. It partly covers an amorphous area Za. The relative displacement of the spot with respect to the area Za takes place in the direction of the arrow. In practice, the spot is fixed and the disk rotates. The intensity and phase of the reflected wave will depend on the illuminated amorphous and crystalline areas and will consequently vary during the relative displacement of the spot.
It is then possible to envisage two recorded information detection modes:
a) The most standard detection mode in phase change optical recording is a so-called sum mode, in which the sum is formed of the intensity detected by the four photodiodes arranged in the detector having four quadrants. This sum signal Ss is shown in the left-hand part of FIG.
4
. In this case, the phase difference information is not taken into account.
b) In another detection mode, known as the differential mode, the difference between the signals supplied by two groups of two photodiodes arranged symmetrically (or more generally two groups of N photodiodes) is determined. The shape of this differential signal, i.e. Sd, is shown in the right-hand part of FIG.
4
. In this case, the reading signal is proportional to the phase difference between the waves reflected respectively by the amorphous areas and by the crystalline areas illuminated by the reading spot. This detection mode, unlike the first, is sensitive to the passage of the spot from a crystalline area to an amorphous area and changes sign for an inverse transition. The amplitude of the transition is proportional to the quantity:
Δ
S
=
2
log
2
π
|
r
a
||
r
c
|
sin
(
φ
c
-
φ
a
)
where a and c relate to the amorphous and crystalline states.
No confusion is made between the differential signal referred to hereinbefore and another signal, also used in this procedure, and which is also obtained by difference or subtraction and which is known as the push-pull signal. This signal is not used for the detection-of recorded informations, but is used for tracking the track to be read. This track diffracts the incident laser beam and when the latter moves slightly away from the track, there is an unbalance between the diffraction intensities in orders +1 and −1. This unbalance is used for producing a push-pull signal on appropriate reception photodiodes.
Using A, B, C and D for designating the four signals supplied by a photodetector having four quadrants, the signal used in differential reading is the quantity (A+B)−(C+D), whereas the push-pull signal will be the (A+C)−(B+D) signal.
The values of the amplitude reflection coefficients r
c
and r
a
from the crystalline and amorphous areas can be adjusted by inserting the phase change material in a multilayer interferential system. The function of these supplementary layers or films is not limited to the optical aspect, but is accompanied by a protective function. They also influence the dynamics of thermal phenomena during recording.
The differential reading of phase change disks is particularly interesting for two reasons:
it firstly permits a greater flexibility of the structure of the system of films, which makes it possible to envisage high optical transmission stacks still having phase differences and consequently powerful signals;
it then permits the production of powerful signals on a single level.
However, this reading method has scarcely been developed up to now, because it moves away from the conventional procedure adopted for optical or compact disks of the preceding generation, i.e. that of reading in the “sum” mode.
It is true that the differential method suffers from a disadvantage. In the case of phase change optical recording, it is particularly difficult to accurately check or control the length of the recorded area (because a fluctuation known as jitter exists), which leads to a deterioration of the spectrum of the differential signal. However, solutions for obviating this disadvantage have been proposed [2]. The reading signals of magnetooptical disks result from a polarization rotation of the reading beam following reflection on the support. This rotation is a function of the magnetization direction of the domains illuminated by the laser spot. At present, the magnetooptical method is the only method requiring polarized optics. The signals read are also of a differential nature, but unlike in phase change recording, the signal is obtained by two supplementary photodiodes, which separate the two polarization directions.
FIG. 5
shows the corresponding reading device. The disk
30
is still illuminated by a laser
32
through a focussing lens
34
. A cubic beam splitter
36
returns the light reflected by the disk to another cubic beam splitter
38
, which makes it possible to split the polarizations. The parallel polarization component strikes a first detector
40
, whilst the perpendicular polarization component strikes a second detector
42
.
The areas of the disk are subdivided, as illustrated in
FIG. 6
, into positive magnetization areas Z
+
, where the rotation of the polarization is positive (+&thgr;) and negative magnetization areas Z
−
, where the polarization rotation is negative (−&thgr;).
Under these conditions, the difference between the signals received by the two detectors
40
and
42
is proportional to the projection of the polarization component of the light reflected by the disk to the perpendicular direction,
Bechevet Bernard
Bruneau Jean-Michel
Comissariat a l'Energie Atomique
Neyzari Ali
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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