WO2007113749A1

WO2007113749A1 - Method of layer jumping in an optical recording device and device for layer jumping in an optical recording device

Info

Publication number: WO2007113749A1
Application number: PCT/IB2007/051110
Authority: WO
Inventors: Joris H. J. Geurts
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-04-04
Filing date: 2007-03-29
Publication date: 2007-10-11
Also published as: TW200809819A

Abstract

The invention comprises a method and a device to facilitate layer jumping in an optical recording device, the device characteristics including a low depth of field associated with the light spot used to obtain information from a multi-layer optical record carrier. The invention separates the tasks of focussing on a layer of an optical record carrier and moving the depth of field of the light spot. This maintains safe operation of the optical recording device while facilitating reading out from, or writing to, different layers of the optical record carrier.

Description

Method of layer jumping in an optical recording device and device for layer jumping in an optical recording device

FIELD OF THE INVENTION

The invention relates to the field of optical recording devices, in particular to optical recording devices which are arranged to cooperate, in reading and writing processes for example, with optical record carriers comprising a plurality of layers for data storage. The invention further relates to the means and methods used to focus light from the optical record carrier onto a chosen data storage layer of the optical record carrier. In particular, the invention relates to a method of layer jumping in an optical recording device, for reading from or writing to an optical record carrier comprising a first layer and a second layer for data storage, the optical recording device comprising a focus loop and an optical pickup unit (OPU), comprising an actuator assembly comprising an actuator lens and having a depth of field.

BACKGROUND OF THE INVENTION

Optical recording devices utilise light to transfer information between the optical recording device and an optical record carrier. The transfer of information via an optical head may be a write process, where data is added to the optical record carrier either temporarily or permanently, or a read process, where information is gathered from data already stored on the optical record carrier. General information on the optical heads used in optical recording devices may be found in the "Encyclopaedia of Optical Engineering, DOI: 10.1081/E-EOE 120009664, 2003, Marcel Dekker Inc." The sections on tracking actuator and servo mechanisms, particularly focus servo mechanisms, are of special relevance to this invention.

The light is focussed to a light spot and made incident on the optical record carrier by the optics in the optical path of the optical recording device, and the light spot facilitates transfer of the information to and from the optical record carrier. Usually the light spot has a "best" most tightly focussed position and around that a depth of field. The depth of field comprises areas of light, which when incident on, and reflected or diffracted from, an optical record carrier, give information of adequate quality which can be used in the optical recording device. Outside these regions, no useful information is obtained from the light incident on the optical record carrier. The depth of field is often determined by the collimator lens component of the optical path in the optical recording device. The collimator lens increases the range for focussing, especially for optical recording devices with a high numerical aperture. It follows, therefore, that the positioning of the light spot relative to the optical record carrier must be performed accurately. In particular, the light spot must be located at an optimum position relative to the layer where data is to be written or to be read. In the case of an optical record carrier comprising a plurality of layers, it must be possible to select the correct layer. To keep the light spot that is incident on the data layer of the optical record carrier in focus and following the track system of the optical record carrier, the objective lens (responsible for focussing the light to a spot) must actively follow the movements of the optical record carrier. In the optical recording device both vertical and radial movements must be actively followed and this is usually achieved using an electromechanical actuator driven by servo signals from a servo system with a feedback control. This control system allows fast and stable tracking with minimum error. To facilitate tracking of vertical movements of the record carrier, the objective lens often has a moveable lens component mounted in an actuator assembly, this lens then being referred to as an actuator lens. An example of this is a near- field optical recording device.

For focussing control, only vertical movements of the optical record carrier and the actuator are considered here. Known prior art methods for focussing control include focus synchronous repetitive control (focus SRC) wherein monitoring the focus movements during several revolutions of the optical record carrier allows a repetitive pattern to be observed, information which can then be used as a feed forward signal for 'near future' control of focus. Use of a focus loop is also a common method of controlling the focus position of the light spot. Detected deviations in vertical focussing lead to error signals that serve as input for a servo loop. The servo loop forces the focus servo control to follow any excursions of the optical record carrier in real time.

Also applicable to the optical recording device is the concept of layer jumping of the light spot (and thus focus) for optical record carriers which comprise a plurality of layers. In known systems, a change from focussing on one data layer of the optical record carrier to focussing on another data layer is achieved using the actuator. The optical recording device is at first focussed on a particular data layer. In order to move the focus to another data layer, the actuator (and actuator lens) must move in the vertical direction. During this movement the focus error signals generated by the servo mechanism are monitored and the so-called layer jump is ended when the focus error signal indicates that the desired data layer has been reached. The actuator then stops the movement and maintains the lens in the required position with respect to the optical record carrier.

A problem with this known layer jumping technique is that in some optical recording devices, such as a system with a very high numerical aperture, the depth of field of the optical components is very limited which makes it difficult to perform a layer jump. In order to have accurate focus of the light spot on a data layer, the focus error signals generated during the layer jump must be accurate also. For a suitable focus error signal to be formed the layer must be within the usable depth of field of the light spot. The depth of field of the light spot must be sufficient to cover both data layers if a layer jump is to be performed successfully. Consequently in optical recording devices with a limited depth of field it is difficult to perform a layer jump.

SUMMARY OF THE INVENTION It is an object of the invention to enable layer jumping in a system with limited depth of field by provision of a method of layer jumping.

The method of layer jumping according to the invention comprises the following steps of: focussing on the first layer using the actuator assembly; opening the focus loop; relocating a depth of field from a first position where the first layer is overlapped by the depth of field to a second position where the second layer is overlapped by the depth of field and the first layer is not overlapped by the depth of field; moving of the actuator lens until focus is caught at the second layer; closing the focus loop.

By using this method, the control mechanisms in the optical recording device open the focus loop to maintain focus position at a level set for the first layer of the optical record carrier. Meanwhile the depth of field of the light spot is moved to a position suitable for catching focus on the second layer. Once this is done, focus control can be continued as usual by closing the focus loop.

This maintenance of focus position by opening the focus loop means that the lens closest to the optical record carrier (the actuator lens) is kept at a normal working distance from the surface of the optical record carrier. In some optical recording devices, e.g. a BluRay system, the free working distance (the distance between lens and optical record carrier) is smaller than for other systems due to the higher numerical aperture. When the BluRay system is not in focus, the chances of the lens hitting the disc are increased. Thus the method according to the invention also offers protection from accidental damage during layer jumping to both the optical recording device and the optical record carrier. In an embodiment of the method, the optical recording device comprises a collimator lens for relocation of the depth of field.

In a further embodiment of the method, the optical pickup unit comprises the collimator lens for relocation of the depth of field.

A collimator lens is a component of the optical path of the optical recording device which is often used in shaping the beam of light to a uniform collimated beam as it passes along the optical path towards the objective lens for focussing. For devices comprising a collimator lens with a high depth of field (and thus a smaller numerical aperture), the collimator lens is located such that the actuator (and actuator lens) in 'rest' position point the focus position of the light spot so that it is between the two data layers of the optical record carrier. Without moving the collimator lens, the system can catch focus on either the first or second data layer because the depth of field of the light spot is large enough to cover both data layers and focus information can thus be generated for further positioning. For a low depth of field collimator lens, a fixed collimator position can only allow the depth of field of the light spot to cover one layer and therefore focus catch is not possible on both layers. In the invention, the collimator lens is permitted to move. During the movement of the collimator lens, the system will loose focus on the starting data layer. In order to avoid problems due to this, other focus measures, such as the focus loop mechanism, are ignored until the collimator is displaced sufficiently to allow the depth of field of the light spot to be close to the second data layer so that the device can catch focus on the second data layer. In a further embodiment of the method, the method comprises additional steps of: using the means for focus synchronous repetitive control as a supplementary control for focussing on the first layer; focussing on a surface of the optical record carrier, using focus synchronous repetitive control only, after opening the focus loop; focussing using focus synchronous repetitive control and the closed focus loop, once focus is relocated towards capture of the second layer, using the means for focus synchronous repetitive control as a supplementary control for focussing on the second layer. Focus SRC is a method of gathering information on the focus movements required to maintain correct focus on an optical record carrier over several revolutions of that carrier and then using the information gathered to feed forward information to affect near future movements of the focus control. As the information is taken directly from an individual disc, it accurately follows the unique focussing requirements for that disc. It is thus more accurate in effect than maintaining focus at a prescribed fixed level and reacting to required changes in real time (as happens in focus loop control). By utilising focus SRC, the layer jumping method becomes more efficient and the risk of collision damage to optical recording device and optical record carrier is further minimised. The invention may also be applied to an optical recording device. Known optical recording devices are often used for reading from or writing to an optical record carrier comprising a first layer and a second layer for data storage, the optical recording device comprising an actuator assembly comprising an actuator lens and having a depth of field and further comprising a focussing means arranged for focus catch within the depth of field. Focus catch within the depth of field ensures that the layer to be written to or read from remains in the correct position relative to the light spot for the write or read processes to take place.

In an embodiment of the optical recording device, the actuator assembly further comprises a layer jumping means comprising a first means to reposition the depth of field to cover only the first or the second layer, a second means to maintain focus on the layer within the depth of field.

Provision of a layer jumping means, which separates the two functions of moving the depth of field and independently maintaining a set position level, allows operation of the optical recording device (e.g. spinning of the optical record carrier) to continue in a safe way while the depth of focus is moved from one layer to another layer. Usually the positioning of the actuator lens close to the disc relies on signals returned from light located in the depth of focus region. Thus if this region were to be moved without compensating for the effect on the signals, then the possibility of contact between optical record carrier and lens would be increased. In a further embodiment of the optical recording device, the first means comprises a collimator lens.

A collimator lens is a component of the optical path of the optical recording device which is often used in shaping the beam of light to a uniform collimated beam as it passes along the optical path towards the objective lens for focussing. For devices comprising a collimator lens with a high depth of field (and thus a smaller numerical aperture), the collimator lens is located such that the actuator (and actuator lens) in 'rest' position places the focus position of the light spot so that it is between the two data layers of the optical record carrier. Without moving the collimator lens, the system can catch focus on either the first or second data layer because the depth of field of the light spot is large enough to cover both data layers and focus information can thus be generated for further positioning. For a low depth of field collimator lens, a fixed collimator position can only allow the depth of field of the light spot to cover one layer and therefore focus catch is not possible on both layers. Thus in the invention, the collimator lens is permitted to move. During the movement of the collimator lens, the system will loose focus on the starting data layer. In order to avoid this, other focus measures, such as the focus loop mechanism, are used to compensate until the collimator is displaced sufficiently to allow the depth of field of the light spot to be close to the second data layer so that the system can catch focus on the second data layer.

In a further embodiment of the optical recording device, the second means comprises a focus loop switchable between an open setting and a closed setting.

The control mechanism in the optical recording device opens the focus loop to maintain focus position at a level set for the first layer of the optical record carrier. Meanwhile the depth of field of the light spot is moved to a position suitable for catching focus on the second layer. Once this is done, focus control can be continued as usual by closing the focus loop.

In a further embodiment of the optical recording device, the second means further comprises a means for focus synchronous repetitive control.

Focus synchronous repetitive control (focus SRC) is a method of gathering information on the focus movements required to maintain correct focus on an optical record carrier over several revolutions of that carrier and then using the information gathered to feed forward information to affect near future movements of the focus control. As the information is taken directly from an individual disc, it accurately follows the unique focussing requirements for that disc. It is thus more accurate in effect than maintaining focus at a prescribed fixed level and reacting to required changes in real time (as happens in focus loop control). By utilising focus SRC, the layer jumping method becomes more efficient and the risk of collision damage to optical recording device and optical record carrier is further minimised. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further elucidated by reference to the figures: Fig.l shows a schematic representation of the optical path of an optical recording device. Fig.2 shows a first method according to the invention.

Fig.3 shows a second method according to the invention. Fig.4 shows a device according to the invention.

Fig.5, parts A, B and C, shows the method of the invention applied to a dual layer optical record carrier.

Figure 1 shows a simplified layout of the optical path of an optical recording device, as known from the state of the art. The travel of the light through the optical path starts at the light source, usually a laser 1. The light passes through a grating 2 where sub- beams used in various control operations, such as radial tracking, are formed. A beam splitter 3 directs the light towards a collimator lens 4 and the light then passes to an objective lens 5 before being made incident on an optical record carrier 6. In a read mode of the optical recording device light interacting with data present on the optical record carrier 6 is diffracted back along the optical path, passing through lenses 5 and 4 and beam splitter 3, and then being focussed by lens 7 onto a detector 8.

In prior art systems the collimator lens 4 is usually fixed in position and is used to collimate the beam travelling through the path and to determine the depth of focus of a light spot (not shown), focussed by the objective lens 5 and used for read and write processes at the optical record carrier 6. The objective lens 5 is shown here as a single lens, but may in fact be formed of two or more components. Part or all of the objective lens may be located in an actuator assembly (not shown). The actuator assembly is used to position the objective lens 5 above the optical record carrier 6 at an optimum distance, and to move the lens according to fluctuations in the optical record carrier surface. The actuator assembly is also used to move focus position between layers if the optical record carrier 6 comprises more than one layer.

Figure 1 illustrates a simplified version of the optical path of an optical recording device. The components are mentioned in generic terms. Different specific examples of each component are possible. For example, the laser 1 often has a wavelength depending on the device application - CD, DVD, BD etc. In another example, the detector 8 can take different forms or be split into several detector sections.

Figure 2 illustrates a method according to the invention whereby layer jumping is facilitated for optical recording devices comprising a low depth of field. In conventional devices, the depth of field of the light spot is designed to cover the data layers of an optical record carrier so that focus can be caught on any of the layers. Often the depth of field is determined by the collimator lens. For a device with a low depth of field, this coverage of data layers by the depth of field is no longer possible. The method according to the invention provides for stable position control of the objective or actuator lens with respect to the optical record carrier while allowing focus to be moved to another data layer. First the optical recording device is focussed on a data layer of an optical record carrier, as in normal operation. The optical recording device has a low depth of field and so while focussing on this first layer there is no overlap of the depth of field with a second layer. This focussing is achieved by use of the actuator assembly 21. Focussing is conventionally done using a focus loop where focus signals describing the topography of the optical record carrier are fed to the servo mechanisms of the optical recording device in real time to adjust the height of the objective lens above the optical record carrier. By opening this focus loop 22, the height of the lens above the optical record carrier remains steady. While the distance between lens and optical record carrier is kept steady, it is now possible to move the depth of field of the light spot away from the first data layer and close to another data layer on the optical record carrier 23, without loosing control of the height of the lens above the optical record carrier. With the depth of field now in the vicinity of the second layer, it is possible to move the actuator to catch focus on the second layer 24. It is then possible to close the focus loop 25, retaining the new position of focus on the second layer, and thereby regain the functionality of varying the height of the lens above the optical record carrier in response to changes in the topography of the optical record carrier.

Figure 3 illustrates another embodiment of the method according to the invention, based on that method described in connection with Figure 2 above, which comprises extra steps for even more precise control of the height of the lens above the optical record carrier. This extended method comprises all the steps 21 to 25 as outlined above, and four more steps in addition. The additional method steps relate to the use of focus synchronous control (focus SRC), wherein monitoring the focus movements of several revolutions of the optical record carrier allows a repetitive behaviour to be observed, information which can then be used as a feed forward signal for 'near future' control of focus. When focus SRC is used as a supplementary control for focussing on a first layer 31 this focus control adds precision to the task of maintaining a correct position and distance between the actuator lens and the optical record carrier. The focus SRC is then used to maintain this position when the focus loop is opened 32. Here there is the advantage that the focus movements are based on acquired history of previous focus movements over several revolutions of the optical record carrier, and are thus actual responses in anticipation of required movement rather than real time reactions. Thus the focus SRC response is more accurate. Focus SRC is also used during the capture of the second layer as focus is relocated towards that layer and focus loop is once more closed 33. Once the movement of the depth of field has been performed and the optical recording device is now focussed on the second layer with closed focus loop, focus SRC can be used as an additional focus measure 34.

Figure 4 illustrates a device according to the invention. Optical recording devices usually comprise an optical pickup unit (OPU) 41. This mechanism houses the actuator lens 46 (which may form part of the objective lens of the device) and the actuator assembly 45, which is required to move the lens according to focus requirements so that focus can be maintained on the optical record carrier 47. By way of the invention, this actuator assembly also comprises a layer jumping means 42. The layer jumping 42 means comprises two components, namely, a first means 43 to reposition depth of field to cover only a first layer or a second layer of the optical record carrier and a second means 44 to maintain focus on the layer of the optical record carrier 47 which is within the depth of field. An example of the first means 43 would be a collimator lens with a low depth of field. Examples of the second means 44 include focus control loops and focus SRC devices. Input to the actuator assembly 45 from the second means 44, to maintain focus on the layer of the optical record carrier 47 which is within the depth of field, is indicated by the arrow 48. The solid arrows 49, 49' and 49" schematically indicate the passage of light along the optical path and directed to the optical record carrier 47.

Figure 5 further elucidates the method of the invention in terms of an example where the invention is applied to an optical record carrier comprising two data layers.

In Figure 5 A, the first data layer 51 lies under the second data layer 52, the two layers forming part of the information storage capacity of the optical record carrier (not shown). A light beam 53 from the optical recording device (not shown) is focussed to a light spot 54 on the first data layer 51. The depth of field 55 over which the light spot may be positioned and still be usable is indicated by the dashed area. In effect this is the depth of focus of the collimator lens (not shown). It is clear from the figure that the focus range only overlaps the first data layer 51, and does not overlap the second data layer 52. The situation shown would schematically represent the relative arrangements of the light spot with the optical recording device for the optical recording device in read mode under conventional focus servo control (not shown), for example, indicated by the dashed line 57. It may then happen that the optical recording device is required to access the second data layer 52, to read further data for example. The optical recording device cannot immediately catch focus on the second data layer 52 as the depth of field 55 does not cover this area. Figure 5B illustrates the positions of the focus range 55 and the light spot 54 according to the next steps of the invention, to allow the second data layer 52 to be available. The focus loop is opened (not shown) to maintain the hardware in the current focus position with respect to the data layers. The depth of field 55 is then relocated to a position such that it covers the second data layer 52. This is done by adjusting the position of the collimator lens (not shown). The light spot 54 as seen by the first data layer 51 becomes less defined and more of a fuzzy spot 56. Once the depth of field 55 is correctly positioned with respect to the second data layer 52, focus can be caught on the second data layer 52. The light spot 54 is then positioned on the second data layer 52 as shown in Figure 5C, in the correct location for the performance of read and write operations by the optical recording device (not shown) and with focus again controlled by conventional focus servo (not shown), indicated by the dashed line 57.

List of reference numerals :

1. Laser

2. Grating 3. Beam splitter

4. Collimator lens

5. Objective lens

6. Optical record carrier

7. Lens 8. Detector

21. Method step 1^st embodiment of the invention

22. Method step 1^st embodiment of the invention

23. Method step 1^st embodiment of the invention 24. Method step 1^st embodiment of the invention

25. Method step 1^st embodiment of the invention

31. Additional method step 2^nd embodiment of the invention

32. Additional method step 2^nd embodiment of the invention 33. Additional method step 2^nd embodiment of the invention

34. Additional method step 2^nd embodiment of the invention

41. Optical pickup unit (OPU)

42. Layer jumping means 43. First means

44. Second means

45. Actuator assembly

46. Actuator Lens

47. Optical record carrier 48. arrow indicating input to actuator assembly

49. 49, 49', and 49" arrows indicating passage of light 51. First data layer

52. Second data layer

53. Light beam

54. Light spot

55. Depth of field

56. Fuzzy spot

57. Dashed line indicating read process

Claims

CLAIMS:

1. A method of layer jumping in an optical recording device, for reading from or writing to an optical record carrier (47) comprising a first layer (51) and a second layer (52) for data storage, the optical recording device comprising a focus loop and an optical pickup unit (41), comprising an actuator assembly (45) comprising an actuator lens (46) and having a depth of field (55), the method comprising the following steps of: focussing (21) on the first layer (51) using the actuator assembly (45); opening the focus loop (22); relocating (23) a depth of field (55) from a first position where the first layer (51) is overlapped by the depth of field (55) to a second position where the second layer (52) is overlapped by the depth of field (55) and the first layer (51) is not overlapped by the depth of field (55); moving (24) of the actuator lens (46) until focus is caught at the second layer (52); closing the focus loop (25).

2. A method of layer jumping as claimed in claim 1, wherein the optical recording device comprises a collimator lens (4) for relocation of the depth of field (55).

3. A method of layer jumping as claimed in claim 2, wherein the optical pickup unit (41) comprises the collimator lens (4) for relocation of the depth of field (55).

4. A method of layer jumping in an optical recording device, as claimed in claim 1 , the optical recording device further comprising a means for focus synchronous repetitive control, where the method comprises the additional steps of: using (31) the means for focus synchronous repetitive control as a supplementary control for focussing on the first layer (51); focussing on a surface of the optical record carrier, using focus synchronous repetitive control only, after opening the focus loop (32); focussing (33) using focus synchronous repetitive control and the closed focus loop, once focus is relocated towards capture of the second layer (52), using (34) the means for focus synchronous repetitive control as a supplementary control for focussing on the second layer (52).

5. An optical recording device, for reading from or writing to an optical record carrier (47) comprising a first layer (51) and a second layer (52) for data storage, the optical recording device comprising an optical pickup unit (41), comprising an actuator assembly (45) comprising an actuator lens (46) and having a depth of field (55) and further comprising a focussing means arranged for focus catch within the depth of field, characterised in that, the actuator assembly (45) further comprises a layer jumping means (42) comprising a first means (43) to reposition the depth of field (55) to cover only the first (51) or the second (52) layer, a second means (44) to maintain focus on the layer within the depth of field (55).

6. An optical recording device as claimed in claim 5, wherein the first means

(43) comprises a collimator lens (4).

7. An optical recording device as claimed in claim 5, wherein the second means

(44) comprises a focus loop switchable between an open setting and a closed setting.

8. An optical recording device as claimed in claim 5, wherein the second means

(44) comprises a means for focus synchronous repetitive control.