WO2008032269A2

WO2008032269A2 - Unstructured multilayer optical discs and recording method

Info

Publication number: WO2008032269A2
Application number: PCT/IB2007/053666
Authority: WO
Inventors: Jacobus Maria Antonius Van Den Eerenbeemd
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-09-13
Filing date: 2007-09-12
Publication date: 2008-03-20
Also published as: TW200834557A; WO2008032269A3

Abstract

A record carrier (2) is disclosed comprising a disc substrate (sb) having one or more recording layers (LO, Ll, L2, L3) on at least one side of the record carrier (2), wherein the one or more recording layers (L0,L2,L2,L3) are unstructured for recording data. A method of recording data on the unstructured recording layer is described. The technique is useful for multilayer optical discs.

Description

Unstructured multilayer optical discs and recording method

Field of the invention

The subject matter relates to multilayer optical discs, and more specifically to unstructured multilayer optical discs.

Background of the invention

US5646932 discloses an optical disc, a tracking error signal-generating apparatus and a tracking control apparatus, in which the tracking error signal can be easily generated in a state in which the track pitch becomes narrower than the diameter of a spot formed by a read laser beam. In the case of multilayer recordable discs, it is required to replicate grooves for tracking purposes. The grooves need to be replicated in very thin separation layers. The separation layers are of the order of microns, which makes it difficult to replicate grooves into the recording layers.

It would be advantageous to have a record carrier that allows easy formation of the recording layers. It would also be advantageous to have a method for recording data on such a record carrier.

Summary of the invention A record carrier is described comprising a disc substrate having one or more recording layers on at least one side of the record carrier, wherein the one or more recording layers are unstructured for recording data.

A method of recording data on a record carrier is described. The record carrier comprises a disc substrate having one or more recording layers on at least one side of the record carrier, wherein the one or more recording layers are unstructured for recording data. The method of recording data comprises rotating the record carrier at a pre-determined speed and writing data blind on the unstructured recording layer using a main spot of a light beam while the record carrier is rotating. The method further comprises capturing the written data by a satellite spot of the light beam upon substantial completion of one revolution of the record carrier. The method further comprises continuing to write the data using the main spot of the light beam while following the captured written data by means of the satellite spot of the light beam.

Brief description of the drawings These and other aspects, features and advantages will be further explained by the following description, by way of example only, with reference to the accompanying drawings, in which same reference numerals indicate same or similar parts, and in which:

Fig. 1 schematically illustrates an example of a four-layer record carrier having a grooveless substrate and recording layers on top of the grooveless substrate; Fig. 2 schematically illustrates another example of a four- layer record carrier having a grooved substrate and recording layers on top of the grooved substrate; Fig. 3 schematically illustrates an exemplary disc drive; and Fig. 4 is a schematic block diagram showing an exemplary photo-detector circuit; Fig. 5 schematically illustrates the positional relationship between a main spot and a side spot in a differential push-pull method;

Fig. 6 shows an exemplary flow chart illustrating steps of the method of recording according to the present subject matter;

Fig. 7 schematically illustrates a method of recording data on an exemplary unstructured record carrier according to the present subject matter; and

Fig. 8 schematically illustrates the determination of the track-pitch according to the present subject matter.

Detailed description of embodiments A record carrier, e.g. DVD, Blu-ray disc, comprises at least one track either in the form of a continuous spiral or in the form of multiple concentric circles, where information may be stored in the form of a data pattern. The record carrier can be of a Recordable (R) or Rewritable (RW) type where information may be stored or recorded, such as DVD+RW, DVD-RW, DVD+R, BD-RE (in single layer and multilayer form). Multilayer recordable discs have grooves for tracking purposes. The grooves need to be replicated in very thin separation layers. The separation layers are of the order of a few tens of micrometers for Blu-ray discs (as per the current BD standard). Generally, for DVD and Blu-ray discs a spiral is defined by replication techniques. This is done by pressure molding the disc using a mold containing the spiral. The spiral may consist of pits in the case of ROM discs or of a continuous groove in the case of recordable and rewritable discs. After molding, several layers are deposited onto the spiral structure. The kind of layers and deposition methods used depend on the disc type. In the case of Blu-ray discs, the spiral in the second, so-called separation layer is made by replicating it from a mold by UV-curing the UV-curable lacquer. This is a difficult process, as the flatness of the separation layer must be guaranteed for readout and recording. Before completing the replication process, the disc needs to be coated with a protection layer. In some cases, (e.g. DVD) the two halves (i.e., the recording layers) are glued together. Further, replication of the spiral (during disc manufacturing) in the separation layer is difficult and troublesome. The separation layer for near- field recording will become much smaller, i.e., only microns thick instead of tens of microns. This might render it difficult to replicate the spiral into the recording layers while keeping the layer thickness within its limits. Hence, it would be advantageous to have a record carrier that allows easy formation of the recording layers.

A record carrier is described comprising a disc substrate having one or more recording layers on at least one side of the record carrier, wherein the one or more recording layers are unstructured for recording data.

It is proposed not to use the pre-defined spiral (pre-embossed structure), but instead use unstructured layers (i.e. flat layers without grooves) for recordable and rewritable discs. This makes the realization of multilayer discs easier. Discs can be produced by merely sputtering and spin coating the recording and separation layers onto a substrate. The substrate itself may still have a pre-defined spiral or may be flat.

Fig. 1 schematically illustrates an example of a four-layer record carrier 2 wherein the disc substrate sb is unstructured (i.e., flat without grooves). As shown in Fig. 1, the four recording layers LO, Ll, L2 and L3 are separated by separation layers spl, sp2, and sp3, respectively. A cover layer cl is disposed over the top recording layer L3 for protection. It can be observed from Fig. 1 that the substrate sb is flat without grooves. The recording layers LO, Ll, L2 and L3 are also unstructured (i.e. flat without grooves). Hence, replication of the recording layers LO, Ll, L2 and L3 becomes easier. Furthermore, replication here refers to the process of replicating the recording layers during the manufacture of the record carrier.

Fig. 2 schematically illustrates another example of a four- layer record carrier 2 wherein the disc substrate sb is grooved. The groove on the substrate is used for tracking a light beam. The substrate is made by replication which means that making a groove in the substrate is easy. With the groove in the substrate tracking is easy and the drive can use the grooved track to do some calibrations (e.g. it can measure how much voltage has to be applied to follow the track over several tracks. This measurement can be used when the drive is writing tracks in the non-grooved layers to adjust the voltage ramp such that it creates the same track pitch as the grooved substrate has). As shown in Fig. 2, the four recording layers LO, Ll, L2 and L3 are separated by separation layers spl, sp2 and sp3, respectively. A cover layer cl is disposed over the top recording layer L3 for protection. It can be observed from Fig. 2 that the recording layers Ll, L2 and L3 are unstructured (i.e. flat without grooves). Hence, replication of the recording layers Ll, L2 and L3 becomes easier.

Fig. 3 schematically illustrates one example of a disc drive 1 (e.g. Blu-ray drive) suitable for writing information on to the record carrier 2 (Cf. Fig. I/Fig. 2) (typically a Blu-ray disc). For rotating the record carrier 2, the disc drive 1 has a motor 4. The motor is typically fixed to a frame defining a rotation axis 5. For receiving and holding the record carrier 2, the disc drive 1 may consist of a turntable or clamping hub 6, which in the case of a spindle motor 4 is mounted on the spindle axis 7 of the motor 4. The disc drive 1 is used for recording/reading data from the record carrier 2.

The disc drive 1 has an optical system 30 for scanning tracks of the record carrier 2 by means of an optical beam. More specifically, the optical system 30 has a light generator 31 (e.g. a laser diode), arranged to generate a light beam 32a. The light beam 32a passes through a beam splitter 33 and an objective lens 34. The objective lens 34 focuses the light beam 32b on the record carrier 2. The light beam 32b reflects from the record carrier 2 (reflected light beam 32c) and passes through the objective lens 34 and the beam splitter 33 (beam 32d) to reach an optical detector 35. For achieving and maintaining correct focusing of the light beam 32b on a desired location (on the record carrier 2), the objective lens 34 is mounted to be axially displaceable. Further, the actuator system 40 of the disc drive 1 includes the following:

1. a radial actuator 41 for controlling the radial position of the objective lens 34;

2. a focus actuator 42 for axially displacing the objective lens 34 with respect to the recording reference plane of the record carrier 2; and

3. a tilt actuator 43 for pivoting the objective lens 34 with respect to the record carrier 2.

It is further noted that the radial actuator 41, the focus actuator 42, and the tilt actuator 43 may be implemented as one integrated 3D-actuator. The disc drive 1 has a control circuit 90 having a first output 92 connected to a control input of the motor 4, a second output 93 coupled to a control input of the radial actuator 41, a third output 94 coupled to a control input of the focus actuator 42, and a fourth output 95 coupled to a control input of the tilt actuator 43. The control circuit 90 is designed to generate the following:

1. at its first output 92, a control signal S_CM for controlling the motor 4;

2. at its second output 93, a control signal S_CR for controlling the radial actuator 41;

3. at its third output 94, a control signal S_CF for controlling the focus actuator 42; and

4. at its fourth output 95, a control signal S_CT for controlling the tilt actuator 43. The control circuit 90 further has a read signal input 91 for receiving a read signal S_R from the optical detector 35.

Fig. 4 illustrates that the optical detector 35 comprises a plurality of detector segments, in this case four detector segments, 35a, 35b, 35c and 35d, capable of providing individual detector signals A, B, C, and D, indicating the amount of light incident on each of the four detector quadrants, respectively. A centerline 37 separates the first and fourth segments 35a and 35d from the second and third segments 35b and 35c. Furthermore, the optical detector 35 comprises two detector segments 35e and 35f capable of providing individual detector signals E and F, indicating the amount of light incident on each of the detector segments (i.e. 35e and 35f). Furthermore, the optical detector 35 comprises two detector segments 35g and 35h capable of providing individual detector signals G and H, indicating the amount of light incident on each of the detector segments (i.e. 35g and 35h). Fig. 4 also illustrates that the read signal input 91 of the control circuit 90 has four inputs 91a, 91b, 91c, and 91d for receiving the individual detector signals A, B, C, and D, respectively. Data and control information is derived from the individual detector signals as will be clear to a person skilled in the art. For instance, a data signal, a tracking signal and a focus error signal can be obtained as follows: Data = (A+ B+ C+ D) Tracking = [(A+D) - (B+C)] - k [(E+G) - (F+H)], where k is a multiplication factor, Focus = (A + C) - (B + D)

Push-pull methods and three beam methods have been employed as servo tracking methods for recording data on the record carrier 2. Among the methods, a typical one is a differential push-pull method. The principle of a differential push-pull method is schematically illustrated in Fig. 5. A light beam generated by the light generator 31 is separated into a main beam and side beams. The main beam is focused into the main spot M and the side beams are focused into the side spots Si and S₂ on the record carrier 2. Further, as shown in Fig. 5, the main spot M is formed by the light receiving areas A to D; the side spot Si is formed by the light receiving areas E and F; and the side spot S₂ is formed by the light receiving areas G and H, on the record carrier 2. Reflected light beams from the main spot M and the side spots Si and S₂ are photo-electrically converted by the optical detector 35 so that push-pull signals can be obtained for the spots M, Si and S₂. The photo-electrically converted signals are used to obtain data and servo signals such as radial, focus and tilt control signals that are required by the disc drive 1. It is to be noted that "main spot" here refers to a central spot or primary spot and, "side spot" refers to a satellite spot or sub-spot or secondary spot. Further, the optical system 30 of the disc drive 1 can be suitably modified to form the main spot M and the side spots Si and S₂. Alternatively, it is possible to derive a tracking signal according to a high- frequency phase detection method. The detector signals from elements 35a and 35c are added and amplified. The amplified output is fed into a slicer. The slicer detects level crossings of the input signal. The detector signals of the elements 35ba and 35d are added and amplified. The amplified output is fed into a slicer. The output signals of the slicers are fed into a phase comparator which produces an output signal dependent on the phase between pulses in the two inputs of the comparator. The output signal of the comparator is low-pass filtered by a filter. The output signal is the tracking signal derived according to the diagonal time difference (DTD) method.

A method 6000 of recording data on an example unstructured record carrier 2 according to the present subject matter is shown in Fig. 6. In step 602, the record carrier 2 is rotated at a pre-determined speed and data blind is written (i.e., without tracking as there is no tracking signal available) on the unstructured (i.e. flat without grooves) recording layer LO (Cf. Fig.l) using a main spot M (Cf. Fig. 5) of a light beam while the record carrier 2 is rotating. During the first rotation the radial servo is actuated in a feed forward manner such that after one revolution the spot has shifted over one track-pitch. In step 604, the written data is captured by a satellite spot Si (Cf. Fig. 5) of the light beam upon substantial completion of one revolution of the record carrier. In step 606, writing the data is continued using the main spot M of the light beam. In step 608, the captured written data is followed by the satellite spot Si (Cf. Fig. 5) of the light beam. The same steps are used to write data on the recording layers Ll, L2 and L3.

In the disclosed method it is proposed to use unstructured (i.e. flat without grooves) recording layers and start writing the spiral "blind". The writing of the spiral blind involves capturing the spiral blind after one revolution and following it by a satellite spot. In essence, one starts writing data without a radial tracking signal. The tracking servo is used in a feed forward manner such that after one revolution the main spot is shifted over one track- pitch. The focus signal is available and while writing the servo loop for focusing is closed. After the first revolution, the written data of the first track is captured by the trailing satellite spot, which will yield a tracking signal that is used to close the tracking servo loop.

Fig. 7 shows a sequence of frames indicating how the disclosed recording method works. The first frame shows an empty (blank) record carrier 2 (Cf. Fig. I/Fig. 2). The second frame shows how writing on the first track takes place while the objective lens 34 (Cf. Fig. 3) make the spot move to the right. The direction of rotation of the record carrier is indicated with the arrow 7A. After the first track has been completed, the first track is captured by the satellite spot S₁, as is shown in the third frame. After completing a full revolution, the final frame is obtained, which shows the tracks written neatly beside one other due to the tracking by the satellite spot S₁, as shown in the fourth frame.

It is to be noted that there should be no external disturbances acting on the objective lens 34 (Cf. fig 3) while the first track is being written, as the objective lens is controlled in an open loop fashion during that time period.

The idea is to start writing on an empty (blank) disc without any structure (i.e. without grooves or without any pre-embossed structure). During the first revolution, of the disc, the actuator 40 (Cf. fig. 3) needs to be actuated such that after the completion of the first revolution, the main spot M (Cf. fig 5A) has advanced about one track. This can be achieved by applying a ramped voltage to the actuator 40 (Cf. Fig. 3). The slope of the ramp is determined by the actuator characteristics together with the desired track pitch. During recording, the focus must be held using the focus actuator 42 (Cf. Fig. 3).

After the first revolution, the first written track is captured by a satellite spot Si (Cf. Fig. 5) which is aligned such that the writing spot is kept at a distance P of one track- pitch (Cf. Fig. 5) while tracking with the satellite spot. This can be achieved by using single- spot push-pull, differential phase detection (DPD) or other tracking signals, and might involve a certain offset depending on the radial distance between the main spot and the satellite spot. The tracking error signal will only be generated, after the first track has been written. Regarding the offsets (focus offset, tilt offset, radial offset), these are parameters which are determined during the manufacturing of optical pickup units (OPU' s) and are placed in a memory. These offsets are due to manufacturing tolerances. One exception to this is the case of near- field recording, where for the focusing action of the actuator, an air- gap signal is used. So, in that case (instead of the focus), the distance between the bottom surface of the lens and the disc is controlled. Focusing the spot onto the recording layer can be achieved by changing the conjugate distance of the objective lens. A signal is required to focus the main spot onto the recording layer. One can use the radial signal for a record carrier with grooves or data. This cannot be used for an unstructured record carrier 2 (i.e., without grooves, Cf. Fig. I/Fig. 2). But, one can still determine a starting point from measuring structured discs during the manufacturing of the OPU. During operation, this can be fine-tuned by considering the quality of the written data. This is similar to running optimal power control used for recording systems. In the system described here, one can use the quality of the tracking signal to adjust the focus after the first track has been written.

Push-pull, differential phase detection (DPD), and differential time detection (DTD) are typically used to generate tracking signals. Push-pull relies on the diffraction of the light in the radial direction and can be used on grooved discs as well as on ROM discs. The diffraction orders in the radial and tangential (i.e., (-1,-1); (-1,1); (1,-1) ^and (1,1) ) directions are used for DPD and DTD. This means that a data pattern is needed to generate these signals. The data pattern is absent on an unwritten grooved disc and hence DPD and DTD cannot be used. In the disclosed method, the tracking signal is generated after the first track has been written, which means that data is present and therefore DPD and DTD can be used. In an embodiment, the track-pitch is determined by the distance between the main spot and the satellite spot and the angle between the line connecting these spots and the tracks The determination of the track-pitch is shown in Fig. 8. Referring to Fig. 8, the distance between the main spot M and the satellite spots Si and S₂ are determined by the track-pitch. The distance d between the main spot M and the satellite spot Si is determined as Sin θ = t_p / d; and t_p = d Sin θ ;

On the same lines, the distance di between the main spot M and the satellite spot S₂ is determined as

Sin Q₁ = t_pi / di; and t_pl = di Sin Q₁ ;

It is to be noted that the track-pitch may not be a constant. In a further embodiment, writing the data blind on the unstructured recording layer includes writing the data blind in a spiral form with an increasing radius from the centre of the record carrier to an outer edge of the record carrier.

In general, the spiral written according to the disclosed method will be eccentric with respect to the hole in the disc. This may pose somewhat stricter demands on the disc and the drive in terms of allowed eccentricity in order to enable a disc to be taken out of and placed back into a recorder and still be readable or writable, and also to be able to read discs written in another recorder.

Although the subject matter has been explained by means of embodiments using four-layer Blu-ray discs, the subject matter is applicable to all types of record carriers, e.g., write-once media and write-many recordable types (DVD-RW, DVD+RW, Blu-ray discs). It is not limited to a two-layer one side disc, i.e., a dual layer disc, and to a two-layer double-side disc, i.e., a dual layer double-side disc. A person skilled in the art can implement the described embodiments of the method of recording data in software or in both hardware and software. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art of practicing the claimed subject matter, from a study of the drawings, the disclosure and the appended claims. The use of the verb "comprise" does not exclude the presence of elements other than those stated in a claim or in the description. The use of the indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps. The Figures and description are to be regarded as illustrative only and do not limit the subject matter.

In summary, a record carrier is disclosed comprising a disc substrate having one or more recording layers on at least one side of the record carrier, wherein the one or more recording layers are unstructured for recording data.

Claims

CLAIMS:

1. A record carrier (2) comprising: a disc substrate (sb) having one or more recording layers (LO,L1,L2,L3) on at least one side of the record carrier (2), wherein the one or more recording layers (LO,L1,L2,L3) are unstructured for recording data.

2. The record carrier as claimed in claim 1, wherein the disc substrate (sb) is grooved or grooveless.

3. The record carrier as claimed in claim 2, further comprising: at least one separation layer (spl,sp2,sp3) disposed between the recording layers (LO,L1,L2,L3) and a cover layer (cl) on the top recording layer (L3).

4. A method of recording data (6000) on a record carrier (2), wherein the record carrier comprises a disc substrate (sb) having one or more recording layers (LO,L1,L2,L3) on at least one side of the record carrier (2), and wherein the one or more recording layers (LO,L1,L2,L3) are unstructured for recording data, the method (6000) comprising: rotating (602) the record carrier (2) at a pre-determined speed; writing data blind (604) on the unstructured recording layer using a main spot (M) of a light beam while the record carrier is rotating; capturing the written data (606) by a satellite spot (Si) of the light beam upon substantial completion of one revolution of the record carrier (2); continuing to write the data (608) using the main spot of the light beam; and following the captured written data by the satellite spot of the light beam.

5. The method as claimed in claim 4, wherein writing the data blind on the unstructured recording layer comprises: writing the data blind in a spiral form with an increasing radius from the center of the record carrier (2) to an outer edge of the record carrier (2).

6. The method of recording data on a record carrier as claimed in claims 4 to 5, further comprising: determining a track-pitch (P) based on a) distance between the main spot (M) of the light beam and the satellite spot (Si or S₂) of the light beam and b) the angle between a line connecting the main spot of the light beam and the satellite spot of the light beam and the tracks.

7. The record carrier as claimed in claim 1, wherein the record carrier is a recordable disc or a rewritable disc.

8. The record carrier as claimed in claim 7, wherein the record carrier is a DVD or a Blu-ray disc.