WO2005041173A2

WO2005041173A2 - Optical storage disc

Info

Publication number: WO2005041173A2
Application number: PCT/IB2004/052042
Authority: WO
Inventors: Christopher Busch; Michael A. H. Van Der Aa
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-10-22
Filing date: 2004-10-11
Publication date: 2005-05-06
Also published as: WO2005041173A3; TW200519930A

Abstract

An optical disc (1; 10), suitable for optical registration of information, has a diameter (D) in the range of approximately 5 cm to approximately 8 cm, and has a thickness (H) in the range of approximately 0.1 mm to approximately 0.8 mm. In one embodiment, the disc has a diameter (D) of approximately 5 cm and a thickness (H) of approximately 0.3 mm. In another embodiment, the disc has a diameter (D) of approximately 8 cm and a thickness (H) of approximately 0.8 mm. The disc may have a central hub section (11) having a thickness (H1) approximately equal to 1.2 mm.

Description

Optical storage disc

FIELD OF THE INVENTION

The present invention relates in general to optical discs.

BACKGROUND OF THE INVENTION As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writeable type, where information may be stored by a user. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in more detail.

Basically, for a user, optical discs have two aspects of major importance: the storage capacity and the data rate, i.e. the speed at which data can be read out. Smaller dimensions of the data pattern allow for an increase in storage capacity and for an increase in data rate, but optical considerations pose a lower limit to the dimensions of the data pattern, e.g. pit size. At a fixed pit size, the data rate can be increased by increasing the rotational speed at which the disc is played, and disc drive manufacturers have indeed developed disc drives capable of playing discs at speeds higher than the nominal speed fixed in the format. However, the rotational speed of the disc can not be increased indefinitely. As the speed increases, power dissipation increases, acoustical noise increases, mechanical vibrations increase, etc. At present, these problems seem to limit the rotational speed of the current 12 cm optical discs to about 160-180 Hz. In the case of a BD system (Blu-Ray Disc), this corresponds to an upper limit of the data rate of about 350 Mb/s.

Even though such data rate is thus possible from a mechanical point of view, the associated acoustical noise makes it very unattractive for a user to actually use this possibility. A general objective of the present invention is to eliminate or at least mitigate these problems.

More particularly, the present invention aims to provide an optical disc having a novel design allowing for higher data rates without the above problems, or at least having the problems mitigated.

Also, the present invention aims to provide an optical disc having a novel design allowing for similar data rate capabilities as current discs, yet at reduced acoustical noise.

SUMMARY OF THE INVENTION

An obvious approach to attaining the above objectives would be to increase the disc diameter. After all, the data rate is proportional to the linear velocity, which, at the same angular velocity, increases with increasing radius. Thus, in the outer parts of the disc, higher data rates would become possible or, conversely, the same data rate would become possible at reduced rotational speed and thereby reduced noise.

However, the approach of the present invention is quite the opposite. According to an important aspect of the present invention, an optical disc has a diameter in the range of approximately 5 cm to approximately 8 cm. According to a further important aspect of the present invention, an optical disc has a thickness in the range of approximately 0.1 mm to approximately 0.8 mm. At first sight, it would seem that this results in a disc having a lower data rate. However, the smaller dimensions of the inventive disc as compared to current discs (12 cm diameter, 1.2 mm thickness) allow inter alia for higher rotational speeds without the associated disadvantages; the rotational speed of the discs can be chosen at such a high value that the data rate is actually increased.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of preferred embodiments of the disc according to the present invention with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:

Figure 1A is a schematic top view of an optical disc according to a first embodiment of the present invention; Figure IB is a schematic cross section according to the line B-B in figure 1A; figure 2 A is a schematic top view of an optical disc according to a second embodiment of the present invention;

Figure 2B is a schematic cross section according to the line B-B in figure 2A.

DESCRIPTION OF THE INVENTION

Figure 1A is a schematic top view of an optical disc 1 according to a first embodiment of the present invention, and figure IB is a cross section according to the line B-B in figure 1A. A central centring opening is indicated at 2. The disc 1 has a diameter D and a thickness H. The diameter D is selected in the range of approximately 5 cm to approximately 8 cm; the thickness H is selected in the range of approximately 0.1 mm to approximately 0.8 mm. In a first special embodiment, the diameter D is equal to approximately 5 cm and the thickness H is equal to approximately 0.3 mm. In a second special embodiment, the diameter D is equal to approximately 8 cm and the thickness H is equal to approximately 0.8 mm.

The disc 1 comprises at least one storage layer 3 (only one layer being shown in the figures), which layer may contain information in the form of a data pattern. The disc may be read-only, but the disc may also be a writeable type, so that information may be written to the storage layer 3. In a preferred embodiment, in case the disc comprises only one storage layer, the storage layer 3 is located halfway of the disc thickness H, i.e. sandwiched between two material layers of approximately 0.5H, as illustrated in figure IB.

In an alternative preferred embodiment, the storage layer 3 is located closer to the light receiving surface of the disc, i.e. the surface which will be irradiated by a light beam for reading and or writing the disc. One advantage of such design is that tilt margins are increased.

In comparison with present-day standard discs, such as CD and DVD (diameter 12 cm, thickness 1.2 mm), the disc 1 according to the present invention has the following advantages:

* reduced costs: in view of the reduced size, the disc 1 requires less material. * reduced power consumption: in view of the reduced inertia, less energy is required for spinning-up and spinning-down. Further, in view of the reduced diameter, the disc suffers less from air friction. The reduced power consumption is especially important in portable, battery-supplied disc drive applications, which apply a burst-access system.

* reduced wear and tear of disc motor: in view of the reduced inertia, the disc motor of a disc drive is loaded less.

* reduced acoustical noise: in view of the reduced inertia, excitation of vibrations is reduced.

* higher rotational frequencies are possible without striking eigenmodes: the eigenfrequencies of the ground vibrational mode (so-called umbrella mode) are shifted to higher frequencies (beyond 350 Hz for the 5 cm disc).

* reduced height of disc drive: in view of the reduced thickness of the disc, it is possible to apply a disc drive with reduced height.

* reduced internal stress: in view of the reduced diameter, the internal stresses of the disc during rotation are reduced.

* reduced seek time: due to the reduced diameter, the average distance to be covered by a seek action is reduced, hence the average seek time is reduced.

Table 1 below lists some disc parameters of the inventive disc as well as of the standard disc, for purposes of comparison to illustrate the improvement in many aspects.

In the above table, the maximal tangential membrane stress, i.e. the stress occurring in tangential direction at the inner radius of the disc, obeys the following formula: σ_max = p (2 π )²((l-v)r_inner²+(3+v)r_outer²)/4 where p is the material density, is the rotational frequency, v is the Poisson ratio, rinner is the inner radius, and router is the outer radius.

In the calculations of the above illustrative examples, the inner radius is kept constant at 7.5 mm.

At radius r, for reading data at a certain data rate and thus a certain linear velocity, a certain disc rotation frequency is needed. In the case of a smaller disc, the stress occurring at the same rotational frequency is reduced quadratically with the reduction of the diameter. From the point of view of stress, it is possible to set a higher rotational frequency before the disc suffers the same stress, thus it is possible to increase the data rate.

In the above table, the disc stiffness S is defined according to the following formula: -E-h³ ~ ( Vl¹-v ^{v 2}) )-r 'outer ² wherein r_oute_r is the outer radius of the disc, v is the Poisson ratio (equal to 0.4 in good approximation) h is the thickness of the disc; E is Young's modulus of the disc material. α is a constant, determined by the ratio of inner disc radius and outer disc radius Current standard discs, having a diameter of 12 cm and a thickness of 1.2 mm, which are made from poly carbonate, have a stiffness S₀ of approximately 260 N/m. If the diameter is reduced to 5 cm while the material is kept the same, the disc stiffness remains the same if the thickness is reduced to 0.67 mm. In the case of the 8 cm disc, the thickness would be 0.9 mm. Selecting a material like SAN, the thickness can be further reduced while still achieving the same stiffness. Preferably, however, the stiffness is increased with respect to the current standard discs, thus allowing to increase the rotational speed without encountering disc eigenmodes. Thus, preferably, the disc according to the present invention has a thickness selected to have a disc stiffness higher than S₀.

The data given in the above table 1 are based on the assumption that the disc according to the invention is manufactured from the same material as the current discs, i.e. polycarbonate. Polycarbonate is a good choice of material as regards optical properties. Preferably, however, the disc in accordance with the present invention is used in combination with a format like, for instance, Blu-Ray, where the light only has to travel through a cover layer of the disc. In that case, the requirements of the optical properties of the disc substrate can be relaxed or even neglected, and the choice of material may purely be based on the mechanical properties and cost considerations. In the table 2 below, important parameters of alternative materials are compared with those of polycarbonate.

Herein, as figure-of-merit FOM, the specific stiffness E/p is used, normalized to one for polycarbonate. This parameter determines several mechanical characteristics of the medium, among which the eigenfrequencies of the disc vibration modes. The higher this parameter, the less "sagging" occurs under the influence of the disc's own weight, and the higher the fundamental frequency will be. The above table 2 illustrates the preferred choice of SAN as substrate material for the optical disc, because, as compared to polycarbonate, the mass and inertia are reduced by 10% and the figure-of-merit increases by a factor 1.8. An alternatively preferred material is LCP carbon, which has a very high figure-of-merit, but disadvantages of this material are its anisotropic material properties and its relatively high price. Figure 2A is a schematic top view of an optical disc 11 according to a second embodiment of the present invention, and figure 2B is a cross section according to the line B-B in figure IB. The disc 10 has a central hub section 11 having a thickness HI and a diameter Dl, and an outer recording section 12 having a diameter D2 and a thickness H2. The diameter D2 and the thickness H2 of the outer section 12 can be identical to the diameter D and thickness H, respectively, of the disc 1 described above. The thickness HI of the hub section 11 is approximately equal to 1.2 mm, substantially equal to the standard thickness of existing discs. The diameter Dl of the hub section 11 is approximately equal to 33 mm. As was already mentioned in respect of the disc 1 of figure 1, the disc 11 comprises at least one storage layer 3; in figure 2B, only one storage layer is indicated.

The disc 11 has substantially the same advantages as described above for the disc 1. The thicker hub section 11 slightly adds to the material costs, but, since it is located close to the rotational center of the disc, it hardly adds to the inertia and to the internal stress. The main advantage is to be seen in the fact that it is possible to use existing hub clamping constructions. This means that, for new disc drives, it is not necessary to develop new clamping constructions. Also, it means that the new disc of the invention can be played in existing disc drives. Also, it means that new disc drives, specially developed for the new disc, are capable of handling existing discs having a standard thickness of 1.2 mm, at least, if they allow the insertion of discs having a diameter of 12 cm.

The central hub section 11 has a lower surface 21 and an upper surface 31. The outer recording section 12 has a lower surface 22 and an upper surface 32. Preferably, and as shown in figure 2B, the lower surface 21 of the central hub section 11 is flush with the lower surface 22 of the outer recording section 12, to further increase the compatibility between current standard discs and the disc proposed by the present invention.

In the case of a disc having only one storage layer 3, the storage layer may preferably be located halfway between the lower surface 22 and an upper surface 32 of the outer recording section 12, as illustrated in figure 2B.

However, generally speaking, the position of the storage layer 3 is not critical. The same applies to the embodiment of figure 1. In principle, the light receiving surface of the disc may be either the lower surface 22 of the outer recording section 12 or the upper surface 32 of the outer recording section 12. However, from a mechanical point of view, it is preferred that the light receiving surface of the disc is the lower surface 22. In such case, therefore, in said alternative preferred embodiment, the storage layer 3 is located closer to lower surface 22. In figure 2B, it is shown that the storage layer 3 may extend in the hub section 11. If the light receiving surface of the disc is the upper surface 32 of the outer recording section 12, the storage layer 3 should preferably be confined to the outer recording section 12.

It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.

Claims

CLAIMS:

1. Optical disc (1; 10), suitable for optical registration of information, having a diameter (D) in the range of approximately 5 cm to approximately 8 cm, and having a thickness (H) in the range of approximately 0.3 mm to approximately 0.8 mm.

2. Disc according to claim 1, having a diameter (D) of approximately 5 cm and having a thickness (H) of approximately 0.3 mm.

3. Disc according to claim 1, having a diameter (D) of approximately 8 cm and having a thickness (H) of approximately 0.8 mm.

4. Disc according to claim 1, comprising: a central hub section (11) having a thickness (HI) and a diameter (Dl); an outer recording section (12) having a diameter (D2) and a thickness (H2); wherein the diameter (Dl) of the central hub section (11) is smaller than the diameter (D2) of the outer recording section (12), and wherein the thickness (HI) of the central hub section (11) is larger than the thickness (H2) of the outer recording section (12).

5. Disc according to claim 4, wherein the diameter (Dl) of the central hub section (11) is substantially equal to about 33 mm, and wherein the thickness (HI) of the central hub section (11) is approximately equal to 1.2 mm.

6. Disc according to claim 4, wherein a lower surface (21) of the central hub section (11) is substantially flush with a lower surface (22) of the outer recording section (12).

7. Disc according to claim 1, having a thickness such that its stiffness S is higher than 260 N/m.

8. Disc according to claim 1, having a figure of merit higher than 1, preferably in the order of 2 or higher, more preferably higher than 20.

9. Disc according to claim 1, comprising a substrate manufactured from SAN.