WO2008015631A2

WO2008015631A2 - Method and device for focus capture on a specific layer of an optical storage medium

Info

Publication number: WO2008015631A2
Application number: PCT/IB2007/053004
Authority: WO
Inventors: Mohamad-Hussein El Husseini; Stefan Geusens; Jacobus A. Wisse
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-08-02
Filing date: 2007-07-30
Publication date: 2008-02-07
Also published as: WO2008015631A3; TW200832385A

Abstract

A method and device for focus capture on a specific layer of an optical storage medium are disclosed. The optical storage medium has multiple information storage layers (L0, L1). The method is performed in device for reading from and/or writing to the optical storage medium. The method comprises identifying passing of a first information storage layer (L0) of said multiple information storage layers (L1, L1) during a ramping procedure of an optical pick-up unit in said device, and enabling a focus locking procedure to a specific information storage layer (L1) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

Description

Method and device for focus capture on a specific layer of an optical storage medium

FIELD OF THE INVENTION

This invention pertains in general to the field of optical storage media. More particularly the invention relates to a method and device for focus capture on a specific layer of an optical storage medium having multiple storage layers.

BACKGROUND OF THE INVENTION

Optical storage media are widespread and used for storing very large amounts of digital information, which represents for instance music, video, images or digital data. After CD (Compact Disks) and DVD (Digital Versatile Disks), BD (Blu-ray disks) is the next generation in optical storage industry. The first generation BD drives are required to robustly handle dual layers BD disks (BD-DL disks). The term LO is herein used for the inner layer of the two layers, and the term Ll is used for the outer layer. In these BD drives, the main servo task is to keep the laser beam focused on one of the data layers at a time, while following the radial tracks. But in the first place, the drive is able to capture focus on both layers, while focus capture is adjusted to one of the specific layers.

One approach for that is to capture focus on a random layer, in practice this would be the easiest layer to capture, and then jump to the required layer, if this is different than the captured layer. However, amongst other drawbacks, the start-up time of this method becomes long. A second approach is to capture focus directly on the required layer as direct focus capture on a required layer has some advantages, for instance the ones given below.

However, currently, there are no methods implemented yet for directly capturing focus on the outer layer (Ll) of BD-DL disks, as this is difficult due to spread on the substrate signal for BDs. Still, in some situations it is desired to capture focus on Ll directly. These situations include, but are not limited to:

Time saving: by capturing on Ll directly (when required), the read or write access time to Ll is smaller. The parameter of read or write access time is usually an important criterion in optical drives classification performance and should be as short as possible.

Lifetime issues: for spherical aberration correction, caused by larger Numerical Aperture and smaller substrate layer thickness, a special optical lens called the "collimator" is used in the optical pick-up unit (OPU) of some optical disk drives, like the BD drives. With the state of the art method, the collimator lens is first set to LO, and then mechanically moved to Ll before or during each layer jump to Ll. By capturing directly on Ll (when required), the collimator displacement would be avoided, enhancing therefore the collimator lifetime and, hence, the system/drive mean time between failures (MTBF). Especially under severe conditions, such as high operating temperature, MTBF can be increased considerably by a direct focus capture method.

Thermal management requirements: in order to reduce power consumption, the drive is forced into power-down state in case of prolonged inactivity. The drive is required to recover immediately to the last known layer, meaning that capturing focus directly on Ll if this was the last known layer. If focus is first captured on LO and then a focus jump is made to Ll, this causes inconvenient time delays when powering-up again. Some Thermal Management modes of disk drives require powering ON/OFF the drive, or at least part of the drive, several times per minute and this mode can last for hours depending on the drive temperature. This puts a heavy burden on the lifetime of the drive. Hence, there is a need for an advantageous method allowing robustly and directly capture focus on the outer layer Ll of Blu-ray Dual Layer (BD-DL) disks and corresponding devices configured to access BD-DL disks, and in particular such a drive allowing for increased flexibility for quickly accessing the outer information layers on such a disk, combined with cost-effectiveness would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above-mentioned problems by providing a method, device and a computer-readable medium according to the appended patent claims.

According to one aspect of the invention, a method is provided, for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers in a device for reading from and/or writing to said optical storage medium, said method comprising identifying passing of a first information storage layer of said multiple information storage layers during a ramping procedure of an optical pick-up unit in said device, and enabling a focus locking procedure to a specific information storage layer of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure. According to an embodiment, the identifying passing of a first information storage layer may comprise comparing a focus error signal value (FE) during said ramping procedure in said device to a tunable first threshold FE check level), and when said focus error signal value (FE) is larger than or equal to said first threshold (FE check level) repeating said comparing until said focus error signal value (FE) is smaller than or equal to a second predefined threshold (FE noise level) corresponding to the noise immunity level of the focus error signal, thereby accounting for noise and hysteresis, and enabling said focus locking procedure to said specific information storage layer (Ll) when said focus error signal value (FE) is smaller than or equal to a third predefined threshold (FE capture level) of the focus error signal. According to an embodiment, the method may further comprise, before enabling said focus locking procedure, in parallel to said repeating said comparing until said focus error signal value (FE) is smaller than or equal to said second predefined threshold (FE noise level), waiting until said focus error signal value (FE) is smaller than or equal to said second predefined threshold (FE noise level) if said focus error signal value (FE) is larger than or equal to said first threshold (FE check level), and subsequently returning to said comparing a focus error signal value during said ramping procedure.

According to an embodiment, the first information storage layer may be an inner information storage layer and said specific information storage layer is an outer information storage layer. According to an embodiment, the ramping procedure may be a ramping down procedure.

According to an embodiment, the focus locking procedure may allow robustly and directly capture focus on the outer layer Ll of a Blu-ray Dual Layer (BD-DL) disk.

According to another aspect of the invention, a device is provided for reading from and/or writing to an optical storage medium, comprising a controlling unit for focus capture on a specific layer of the optical storage medium, said optical storage medium having multiple information storage layers (LO, Ll), said device comprising an identifying unit for identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and a focus locking unit for enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

According to a further aspect of the invention, a computer-readable medium having embodied thereon a computer program for processing by a computer is provided for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers in a device for reading from and/or writing to said optical storage medium, the computer program comprising an identifying code segment for identifying passing of a first information storage layer of said multiple information storage layers during a ramping procedure of an optical pick-up unit in said device, and a focus locking code segment for enabling a focus locking procedure to a specific information storage layer of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

According to an embodiment the computer program of the above aspect of the invention may enable carrying out of the method according to the above other aspect of the invention.

The present invention has, amongst others, the advantages over the prior art that it provides fast access times to the outer layer of BD-DL disks, and increases lifetime and reliability of corresponding disk drives.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which Figure IA is a view of a dual layer optical disk;

Figure IB is a perspective view of the dual layer disk of Figure IA;

Figures 2A and 2B are schematic illustrations of a device for accessing a multi layer optical storage disk according to an embodiment of the invention;

Figure 3 illustrates the principle of conventional focus capturing: Focus capture is performed during ramping-down and capture is always done first on LO - A layer jump is then performed in order to focus on layer Ll;

Figure 4 illustrates focus error (FE), and mirror signal (MIR) signals waveforms in ramping-up and ramping-down, for referring to a basic description of an embodiment of the method according to the invention; Figure 5 illustrates double-crossing of layer LO caused by Vertical Disk Deviation (VDD);

Figure 6 illustrates another embodiment of the method according to the invention taking into account the double-crossing of layer LO caused by VDD, shown in Figure 5;

Figure 7 is a flowchart illustrating a method according to an embodiment of the invention; and

Figure 8 is a schematic illustration of a computer readable medium comprising program code segments according to another embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

The following description focuses on an embodiment of the present invention applicable to a BD-DL optical medium. However, the case of a BD-DL optical medium is only given for an easier understanding of the invention. The same principle applies to multi- layer optical media. Furthermore, according to specific embodiments, other optical media as Blu-ray disks, such as HD-DVDs or holographical media having multiple layers are equally within the scope of some embodiments.

Figures IA and IB, illustrate schematically a BD-DL disk 1. Disk 1 has two information layers LO, Ll. More precisely, Figure IA illustrates one of the two information layers of the Blu-ray disk 1. As is illustrated, these layers are formed of a single continuous spiral pattern 2, which in the case of pre-mastered disks comprises pits and plane areas. The spiral pattern 2 forms a plurality of essentially concentric circular tracks 3. An information layer having the BD format has about 110,000 tracks. Furthermore, disk 1 has a center opening 5 for engagement with a drive spindle to rotate the optical disk 1. For illustrative purposes the proportions shown in Figures IA and IB are not to scale.

Optical storage drives are devices that can extract data from optical disk media (CD, DVD, BD). A more detailed description of an optical storage drive is given below with reference to Figures 2 A and 2B. According to an embodiment of the invention, a device in form of a disk drive

40 for optical media that is illustrated in Figure 2A is provided, which is adapted to direct focus capture on a desired layer, e.g. the outer layer of a dual-layer optical medium such as a BD-DL disk. The device is for instance a drive of a computer or a consumer player for disks 10. In order to position a disk in drive 40 for operation therein, the drive 40 has for instance a tray 41 for inserting disks into the drive 40. Alternatively, the drive 40 may have a disk changer system into which a plurality of disks can be inserted for automatically accessing one disk at a time, or alternatively the disk may be housed inside a cartridge means that protects the disk from being accessed outside the drive, wherein the cartridge replaces the above- mentioned tray of the drive. Furthermore, the drive 40 has a processing device 42 (inside the housing of the illustration) for processing software controlling the drive 40.

More precisely, a disk drive 48,49 in the form of a spindle motor 48 and a rotatable spindle 49 is adapted to rotate the optical disk 10 in a direction indicated by arrow 50 in Figure 2B, in a manner which is well known in the art. An optical pick-up unit (OPU) 44 is positioned close to the surface opposite the cover side 15 of the optical disk 10 and is movable in a radial direction of the optical disk 10, as is indicated by the arrow 45 in Figure 2B. The OPU operates to irradiate the optical disk 10 with a beam of laser light, detects reflections from the optical disk, produces a readout signal in response thereof and provides this signal for further processing. When accessing information from the disk, the optical disk 10 will be kept in rotation by the disk drive, i.e. the spindle motor 48 and the spindle 49.

The laser pickup unit 44 comprises mechanical drive means 46 for causing the optical assembly or optical read device 47 of the laser pickup unit 44 to move radially along the surface of the optical disk 10 in the direction of arrow 45 indicated in Figure 2B between different radial positions. However, such mechanical drive means 46 are well known per se in the technical field, and it is left to the skilled person to choose the suitable mechanical and electrical components, such as an electric motor and a mechanical carriage arrangement, depending on an actual implementation. The output signal from the OPU 44 is a signal 51 that arises from the scattering, absorption and reflection of the actual pit and land pattern and/or the pre-existing wobble signal present in a track on an information layer of the disk 10. A processing device, such as a processor, 42 of the drive 40 may be implemented by any commercially available microprocessor. Alternatively, another suitable type of electronic logic circuitry, for instance an Application-Specific Integrated Circuit (ASIC) or a Field- Programmable Gate Array (FPGA) may substitute the processor 42. Correspondingly, further components, such as memory, input devices and output device of the drive (not illustrated) may all be implemented by commercially available components and are not described in any detail herein.

The processor 42 controls the function of drive 40. For instance, the processor controls the rotational speed of motor 48, the radial position and focus position of the pickup unit 44, and receives the read-out signal 51 for further processing, e.g. for decoding and sending to an audio-visual unit for presentation of audio-visual data read from disk 10.

In summary, data is written on the disk following the spiral pre-groove or prewritten pattern 2 and the laser-based optical pick-up unit (OPU) reads or writes the data from or onto the spiral track 2, respectively. A fraction of the laser beam emitted by the OPU is reflected by the disk media and collected by a suitable set of photodiodes (mounted also on the OPU). From the collected light, the optical drive is able to extract position information in form of error tracking signals, namely for focus error (FE) and for radial error (RE). These signals allow the drive to focus the laser beam precisely on the desired spot while following the required track. Also, a measure of the global light intensity is extracted from the reflected light and called herewith the mirror signal (MIR). Hitherto focus capturing is based on FE and MIR signals.

In order to capture focus on BD-DL disks, a vertical scan by the focused laser beam is performed. More precisely, the focus actuator, which is situated in the optical pick- up unit (OPU), initiates a ramping-up and ramping-down procedure in order to obtain valid FE and MIR signals. The procedure is illustrated with in Figure 1, showing the mechanism of focus capturing. Focus capture is performed during ramping-down and capture is always done first on LO. A layer jump is then performed in order to focus on layer Ll. Hence, the first focus capture is done on LO during the ramping-down state, mainly because during ramping-up there are pre-reflection on the cover layer and noisy MIR and/or FE signals, which make the capture during ramping-up difficult and unreliable. This focus capture procedure is referred to as the "state of the art" in focus capturing. Focusing on Ll is done by first capturing focus on LO (always in ramping-down) and then jumping to Ll.

As mentioned above, it is desired to provide an advantageous method allowing robustly and directly capture focus on the outer layer Ll of Blu-ray Dual Layer (BD-DL) disks, as well as corresponding devices. Furthermore, as already mentioned, it is desired that focus capture can take place as quickly and as completely as possible.

According to an embodiment, a method is provided in order to assure a robust direct focus capture on the outer layer Ll of BD dual layers disks. A detailed embodiment of the invention is given, with reference to Figure 4 showing FE and MIR signal waveforms in ramping-up and ramping-down, respectively, while reading a BD-DL disk.

The top part of Figure 4 shows the signals in the "ramping-up" state. Details of interest here are the presence of detectable FE signals (called S curve) combined with a good level of light reflection (MIR). In the ramping-up, the drive sees first the reflection from the cover layer, then from the layer Ll and finally from LO (c.f. Figure 1). As mentioned before, the problem with the ramping-up state is that the reflection on the cover-layer is not reliable, i.e. sometimes this reflection is detectable, while it at other times is not detectable. For this reason, a focus capture on Ll in the ramping-up state is not reliable.

Capturing focus on Ll may also be performed in the ramping-down state. The bottom part of Figure 4 shows exactly this situation. The drive will see first the reflection (and the S-FE curve) of layer LO, then Ll and last the cover layer. Here, the S curve of layer LO is reliably detectable, even when the collimator lens providing spherical aberration correction is set to layer Ll, as is the case in Figure 4, for example.

In order to capture focus directly on Ll, the first reflection, i.e. the S-FE curve of LO, is discarded, i.e. not taken into account. For this the following procedure is applied: 1. set a tunable FE check level and compare FE to that level. When FE is larger than or equal to (>) FE check level, then: 2. wait till FE is smaller than or equal to (<) FE noise level , which accounts for noise and hysteresis, then

3. if FE is smaller than or equal to (<) FE capture level, then enable capture focus procedure.

This procedure ensures that the first S curve, of layer LO, is ignored and the focus capture only occurs on Ll, wherein

FE check level is a first predefined threshold of the FE signal,

FE noise level is a second predefined threshold corresponding to the noise immunity level of the FE signal, and

FE capture level is a third predefined threshold of the FE signal.

According to a further embodiment, an extension is added to make the method immune to "Vertical Disk Deviation", hence adding further robustness. The method according to the above-described embodiment is already robust enough to yield good results. However, in certain cases, the method may be more advantageous when being even more robust. For instance due to double-crossing of layer LO, caused by vertical deviation of the disk combined with a difficulty in tuning the ramping-down speed of the actuator relative to the VDD of the disk, may result in decreased robustness of the above described method. This sort of situation is illustrated in Figure 5, showing double-crossing of layer LO caused by VDD. In this case, the method described above will capture focus on LO and not on Ll, as required.

A correction for this is now explained with reference to Figure 6 illustrating the embodiment taking into account a double-crossing of layer LO caused by VDD. More precisely, in order to account for double-crossing of LO, the following extension is added to the above-mentioned steps:

4. if FE is larger than or equal to (>) FE check level, then:

5. wait till FE is smaller than or equal to (<) FE noise level, to account for noise and hysteresis, then 6. restart the whole procedure at step 1.

Steps 3 and 4 will run in parallel.

A flowchart of the method for direct focus capture on Ll is shown in Figure 7.

It takes into account double-crossing of LO, wherein the elements of the flowchart represent the fo Ho wing steps :

100 Start of method - Last known layer is Ll . After a fast reduced power a disk drive is to recover to layer Ll;

110 Ramp down? - Check status of focus unit ramping procedure - If ramping procedure is ramping down, continue with step 120, otherwise loop back to step 110, i.e. wait until ramp procedure is ramp down;

120 Set a tunable FE check level and compare FE to that level. When FE is larger than or equal to (>) FE check level, continue with step 130 - otherwise loop back to step

110, i.e. continue ramping down

130 Wait until FE is smaller than or equal to (<) FE noise level, which accounts for noise and hysteresis, then continue with step 140;

140 If FE is smaller than or equal to (<) FE capture level - If yes, enable capture focus procedure in step 170, otherwise proceed to step 150;

150 Check if FE is larger than or equal to (>) FE check level - If yes, proceed to step 160 - otherwise go back to step 140; 160 Wait until FE is smaller than or equal to (<) FE noise level, to account for noise and hysteresis, then restart the whole method at step 100;

170 Start focus capture at present position, i.e. to layer Ll; and

180 End of method. According to an embodiment, a method for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers LO, Ll in a device 40 for reading from and/or writing to said optical storage medium 11 is provided. The method comprises identifying passing of a first information storage layer LO of said multiple information storage layers LO, Ll during a ramping procedure of an optical pick-up unit in said device, and enabling a focus locking procedure to a specific information storage layer Ll of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

For adapting the drive for performing the above method, the following measures are added to a conventional apparatus. The software of the drive is adapted to handle the direct focus capture method. More specifically, an embodiment may comprise a device 40 for reading from and/or writing to an optical storage medium 10 comprising a controlling unit 42 for focus capture on a specific layer of the optical storage medium, said optical storage medium having multiple information storage layers LO, Ll. The device comprises an identifying unit for identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and a focus locking unit for enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure. An embodiment of the software is illustrated in Figure 8, wherein the program is stored on a computer-readable medium 80 for processing by a processing device 42 of a drive 40 for playing and/or recording of an optical disk 10. The computer program comprises code segments 81, 82. More precisely, the computer program is configured for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers (LO, Ll) in a device for reading from and/or writing to said optical storage medium. The computer program comprises an identifying code segment (81) for identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and a focus locking code segment (82) for enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

Finally, the mirror signal (MIR) signals waveforms may be used to further improve robustness of the presented method, if so desired. In more detail, the MIR signal may be used to identify a layer or cover, as is evident e.g. from Fig. 4. However, the MIR signal alone does not provide sufficient information for identifying a specific characteristic, like a layer or cover. The mirror signal (MIR) signal may be used in addition of the above described method.

Applications and use of the above described method, device and computer program are various and include exemplary fields such as computer drives for optical media, consumer players and recorders for optical media, such as BD optical storage drives or BD recordable video recorders, etc.

The present invention has been described above with reference to specific embodiments. However, other embodiments than the preferred above are equally possible within the scope of the appended claims, e.g. different number of layers of the optical media, different types of media than those described above, etc.

The invention may be implemented in any suitable form including hardware, software, firmware or any combination of these. However, preferably, the invention is implemented as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit, or may be physically and functionally distributed between different units and processors. In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS:

1. A method for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers (LO, Ll) in a device for reading from and/or writing to said optical storage medium, said method comprising identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

2. The method according to claim 1, wherein identifying passing of a first information storage layer (LO) comprises: comparing a focus error signal value (FE) during said ramping procedure in said device to a tunable first threshold (FE check level), and when said focus error signal value (FE) is larger than or equal to said first threshold (FE check level) repeating said comparing until said focus error signal value (FE) is smaller than or equal to a second predefined threshold (FE noise level) corresponding to the noise immunity level of the focus error signal, thereby accounting for noise and hysteresis, and enabling said focus locking procedure to said specific information storage layer (Ll) when said focus error signal value (FE) is smaller than or equal to a third predefined threshold (FE capture level) of the focus error signal.

3. The method according to claim 2, further comprising, before enabling said focus locking procedure, in parallel to said repeating said comparing until said focus error signal value (FE) is smaller than or equal to said second predefined threshold (FE noise level), waiting until said focus error signal value (FE) is smaller than or equal to said second predefined threshold (FE noise level) if said focus error signal value (FE) is larger than or equal to said first threshold (FE check level), and subsequently returning to said comparing a focus error signal value (FE) during said ramping procedure.

4. The method according to claim 1, wherein said first information storage layer is an inner information storage layer (LO) and said specific information storage layer (Ll) is an outer information storage layer (Ll).

5. The method according to claim 1, wherein said ramping procedure is a ramping down procedure.

6. The method according to claim 1, wherein said focus locking procedure allowing robustly and directly capture focus on the outer layer Ll of a Blu-ray Dual Layer (BD-DL) disk.

7. A device (40) for reading from and/or writing to an optical storage medium

(10) comprising a controlling unit (42) for focus capture on a specific layer of the optical storage medium, said optical storage medium having multiple information storage layers (LO, Ll), said device comprising an identifying unit for identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and a focus locking unit for enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

8. A computer-readable medium (80) having embodied thereon a computer program for processing by a computer (42) for controlling focus capture on a specific layer of an optical storage medium having multiple information storage layers (LO, Ll) in a device for reading from and/or writing to said optical storage medium, the computer program comprising an identifying code segment (81) for identifying passing of a first information storage layer (LO) of said multiple information storage layers (LO, Ll) during a ramping procedure of an optical pick-up unit in said device, and a focus locking code segment (82) for enabling a focus locking procedure to a specific information storage layer (Ll) of the multiple layers of said optical storage medium at a position passed subsequently during said ramping procedure.

9. The computer program of claim 8 enabling carrying out of a method according to claims 1-6.