METHOD OF OPERATING A NEAR FIELD OPTICAL RECORDING DEVICE AND NEAR FIELD OPTICAL RECORDING DEVICE

FIELD OF THE INVENTION

The invention relates to a method for collision avoidance between a SIL lens in a near field optical recording device and a defect on an optical record carrier cooperating with the near field optical recording device, the near field optical recording device operating in a read or write mode to read data from or write data to the optical record carrier, the near field optical recording device further comprising a collision detection means, comprising steps of:

- Identifying an imminent collision using the collision detection means,

- Moving of the SIL lens in response to a detected imminent collision, - Repositioning of the SIL lens with respect to the optical record carrier within normal operating tolerances for read and write operation.

The invention further relates to a near field optical recording device arranged to cooperate with an optical record carrier and capable of operating in a read mode or a write mode to read data from or write data to the optical record carrier, and comprising, - a SIL lens,

- a means for positioning the SIL lens with respect to the surface of the optical record carrier within normal operating tolerances for read and write operation,

- a collision detection means for detecting a defect on a surface of the optical record carrier, the means for positioning and the collision detection means arranged to cooperate for a movement of the SIL lens in response to an imminent collision between the SIL lens and a defect on the optical record carrier.

BACKGROUND TO THE INVENTION Near field optical recording employing an actuated solid immersion lens (SIL) is a promising technology offering significantly improved storage capacities of more than 100GB per layer and up to 4 layers per optical record carrier. These capacities are achieved by focussing the laser to a smaller spot size, using a blue-violet laser and a very high NA

(numerical aperture) lens, e.g. NA = 1.6. Enhanced signal processing allows reliable read-out of the closely packed data patterns.

Details and experimental results on near field optical recording systems can be found in publications such as F. Zijp et al, ODS 2004, Proc. of SPIE VoI 5380, p209-223 (2004) and C.A.Verschuren et al, ISOM/ODS 2005, Jpn.J.Appl.Phys. VoI 45 No 2B, pl325 (2005). These documents discuss near field read-out of a 50GB first surface disk with NA= 1.9 and a proposal for a cover-layer incident, dual-layer near field system, and also near field recording with a SIL lens on polymer cover-layer protected discs.

The principle of near field recording is based on capture of light reflected from the surface of an optical record carrier by a lens with NA > 1 in the region of evanescent coupling. For NA < 1, light rays with a small angle with respect to the optical axis of the lens propagate as in a conventional far- field system. For angles larger than the critical angle in the SIL, however, the light rays become evanescent in the air outside the SIL, and their amplitudes decrease exponentially in the air outside the SIL. For reliable read-out and recording, the distance between the SIL and the disc (also called the air gap) typically needs to be 25nm, to enable significant evanescent coupling of the focussed light through the air gap. The air gap is strictly controlled using very sensitive gap error signal information which is obtained by polarisation detection of the reflected light. Proper control of the air gap facilitates the recording and reading processes but the air gap is relatively very small compared to other recording device designs. This can lead to difficulties not encountered in far-field systems where the lens and optical record carrier are not in such close proximity.

An issue with near field optical recording is that due to the very small air gap distance, small physical defects on the disc surface, for example particles stuck to the surface or air bubbles in the cover layer, potentially lead to collisions with the SIL. This can cause damage to the lens and can also lead to loss of data and unusable areas on the optical record carrier. Operation of the recording device is also interrupted unexpectedly.

Methods have been proposed to detect such events and to provide work arounds. Such a proposal is outlined in EP 1 136 989 Bl (Optical head apparatus, Pioneer Corporation Tokyo, published 2005). According to this document, the height of a foreign object on the information reading surface of the recording medium is detected and a control system separates the SIL from the optical record carrier to prevent a collision, moving the head of the recording device away from the optical record carrier. The SIL is moved to a higher position than the height of the foreign material. This method is implemented on a case-by-case, ad-hoc basis.

In a typical near field recording device, the movement of the lens back from the optical record carrier consists of a lens movement through a distance of around 20μm to come clear of the defect. The optical record carrier is still spinning, and operation has to be resumed as soon as possible, so the lens must then be pulled in towards the optical record carrier and be restored to the correct height above the optical record carrier for operation. At an average pull-back and pull-in velocity of lmm/s, the minimum excursion time of the lens away from the optical record carrier is around 40ms. For high rotation speeds of 25Hz or more, this corresponds to 1 or more record carrier rotations. In addition, such a movement must be performed per track and a typical defect is likely to cover more than one track. A problem with this prior art method is, therefore, the cycle time required to adjust the SIL lens position in response to a defect and restore normal device operation.

It is an object of the invention to provide a more efficient method of avoiding collisions between the SIL lens and defects on the optical record carrier.

SUMMARY OF THE INVENTION

According to the invention this object is achieved by provision of a method comprising the step of:

- Performing a first radial skip of the SIL lens with respect to the optical record carrier. In this context a radial skip is defined as a displacement of the SIL lens in a radial direction with respect to the optical record carrier.

In prior art methods, each potential collision is treated individually. However, a defect on the surface of the optical record carrier is likely to spread over a number of tracks rather than just one track. For a typical example, a single defect of lOOμm diameter with a 40μm SIL tip diameter already affects (140μm/180nm track pitch) = 778 tracks. By moving the SIL lens in a radial direction with respect to the optical record carrier, it is possible to adjust the distance travelled and skip a chosen number of tracks before replacing the lens in operating position. This avoids unnecessary repetition of remove and replace movements, and increases the efficiency of the device operation. In a further embodiment of the invention, the first radial skip of the SIL lens further comprises movement in a direction so as to increase a distance between the SIL lens and the optical record carrier.

The SIL lens is positioned very close to the optical record carrier during normal operations of read and write. A distance of 25nm is typical. Movement in the radial

direction will move the lens so as to avoid a defect on the optical record carrier. It is additionally advantageous, however, to move the lens away from the optical record carrier so as to increase the distance between the SIL lens and the optical record carrier to provide a buffer distance. This need not involve a complete pull back of the SIL lens but may be a smaller distance to come clear of the defect. For example, a typical smaller removal distance for a typical defect is 20μm. By moving a smaller distance away from the optical record carrier than complete pull back, it is easier and quicker to return the lens to operating conditions. Further the associated radial skip avoids unnecessary repetition of pull-back and pull-in for individual tracks, so the overall efficiency is much improved. In a further embodiment of the invention, the radial movement of the SIL lens is a distance calculated as the SIL tip diameter plus an estimated defect size.

A SIL tip is usually around 40μm in diameter, but can be larger or smaller depending on specific device specifications within a range of lOμm to lOOμm. A typical defect size is lOOμm. For clearance of the defect, a reasonable estimate is the sum of the two quantities.

The lower limit of lOμm is determined by lens fabrication tolerances in case of first-surface configuration of the optical record carrier (no cover layer). For optical record carriers with cover-layer, larger diameters of SIL lens tip are required in order to accommodate the required optical beam, depending on numerical aperture and maximum cover and spacer layer thickness (i.e. multi layers). The upper limit is determined by mechanical tolerances such as disc flatness/waviness. Using active tilt compensation enables the use of larger SIL tip diameters for a given specification of optical record carrier, typically around lOOμm. Although values up to around 150μm could be used in well-controlled conditions, these are not practical for foreseen applications. In a further embodiment of the invention, the method further comprises the steps of:

- Re-checking for an imminent collision using the collision detection means,

- Performing a plurality of radial skips such that the radial movement of the SIL lens for each individual skip with respect to the optical record carrier covers a distance smaller than that covered in the preceding radial skip.

If a defect is larger than expected, further evasive action would be needed for complete safety of the SIL lens. When a further collision is detected close to the position of a first collision detection, this is likely to be related to the same defect. When this is the only collision at this radius (no new defects), it is preferable to make a smaller second radial skip

than the first. This saves time and means less space on the optical record carrier is made redundant. A second radial skip may be estimated at half of the SIL lens diameter, typically 20 μm. If a defect is larger than the typical size, the defect size distribution is usually not so broad that another radial skip equal to the first is justified. Large skips are faster and lead to fewer collisions but skipping tracks costs storage capacity and should normally be minimised. The skipping action can be iterated as often as necessary to clear the defect. The skip distance can be reduced with successive skips if desired to fine tune the overall skip effect and optimise the skipping action.

When another collision is detected at a different tangential position on the track of the optical record carrier, it is quite unlikely that the SIL encounters this defect at the edge closest to the centre of the optical record carrier (as would be the case for normal read or write mode where the SIL would be closely following the track rather than returning to the track after an excursion). Therefore, an intermediate radial skip is preferable, for example given by the SIL diameter plus half the typical defect diameter, typically 40+50=90μm. In a further embodiment of the invention, the method comprises a further step of:

- Storing a skip position in a reference table in non- volatile device memory or on the optical record carrier, the skip position relating to a location on the optical record carrier at which a radial skip has been performed. In a further embodiment of the invention, the method comprises further steps of:

- Associating the skip position with a block number associated with patterning of data storage positions on the optical record carrier.

- Storing the association of position and block number in non- volatile memory.

By storing the skip position, it is possible to avoid the defect region in future movements of the SIL lens. The block number/address corresponding to a defect collision (mostly during writing but also in read mode) can be included in a special table, for example stored on the optical record carrier or in a non- volatile memory of the near field optical recording device. Upon read-out or further writing at a later time, this feature allows the drive to know that the first occurrence of the specific block should be skipped. When recognising the number/address, a lens jump should be initiated, and writing or read-out can be resumed with the repeated, complete block. This enhancement avoids collisions during

read-out by using knowledge of the defect location and thus improves the reliability of the system.

In a refinement, an indicator of the tangential position of the optical record carrier at the time of the collision is temporarily stored. For example, this indicator can be derived from the tacho signal from the motor controller (in a typical set-up, 500 pulses for each revolution, normally used to control the rotation speed). Then, a first radial skip is performed, determined by the SIL tip diameter plus the typical defect size. On a new radius, pull-in and gap control (control of the distance between SIL lens and optical record carrier) is re-started. Preferably, this can be done at a larger air gap than the nominal value. When no collisions are detected, the air gap (closest distance between the SIL lens and the optical record carrier in air) is reduced to the nominal value. When there are (still) no collisions, the defect has been successfully skipped and recording (or read out) can be resumed.

When a series of radial skips are performed, it is possible to combine the information by summing the skip distances to associate one large skip with a position on the optical record carrier. This optimises the process of avoiding the defect, thereby saving time and reducing unnecessary movement of the SIL lens. Further, this additional provision gives radial defect size information (besides the already known defect 'start' location) and allows immediate jumping over the defect, minimising further collisions and providing smoother playback (no defect scanning). In another aspect of the invention, there is provided a near field optical recording device, wherein a means for positioning is arranged for the movement of the SIL lens according to a first radial skip of the SIL lens with respect to the optical record carrier.

The means for positioning determines the position of the SIL lens with respect to the surface of the optical record carrier and is responsible for maintaining the SIL lens within normal operating tolerances for read and write operation.

In this context a radial skip is defined as a displacement of the SIL lens in a radial direction with respect to the optical record carrier.

In prior art methods, each potential collision is treated individually. However, a defect on the surface of the optical record carrier is likely to spread over a number of tracks rather than just one track. For a typical example, a single defect of lOOμm diameter with a 40μm SIL tip diameter already affects (140μm/180nm track pitch) = 778 tracks. By moving the SIL lens in a radial direction with respect to the optical record carrier, it is possible to adjust the distance travelled and skip a chosen number of tracks before replacing the lens in

operating position. This avoids unnecessary repetition of remove and replace movements, and increases the efficiency of the device operation.

In a further embodiment of the invention, the first radial skip further comprises a movement of the SIL lens such that the distance between the SIL lens and the optical record carrier is increased.

The SIL lens is positioned very close to the optical record carrier during normal operations of read and write. A distance of 25nm is typical. Movement in the radial direction will move the lens so as to avoid a defect on the optical record carrier. It is additionally advantageous, however, to move the lens away from the optical record carrier so as to increase the distance between the SIL lens and the optical record carrier to provide a buffer distance. This need not involve a complete pull back of the SIL lens but may be a smaller distance to come clear of the defect. For example, a typical smaller removal distance for a typical defect is 20μm. By moving a smaller distance away from the optical record carrier than complete pull back, it is easier and quicker to return the lens to operating conditions. Further the associated radial skip avoids unnecessary repetition of pull-back and pull-in for individual tracks, so the overall efficiency is much improved.

In a further embodiment of the invention, the first radial skip is a distance calculated as the SIL tip diameter plus a typical defect size.

A SIL tip is usually around 40μm in diameter, but can be larger or smaller depending on specific device specifications within a range of lOμm to lOOμm. A typical defect size is lOOμm. For clearance of the defect, a reasonable estimate is the sum of the two quantities.

The lower limit of lOμm is determined by lens fabrication tolerances in case of first-surface configuration of the optical record carrier (no cover layer). For optical record carriers with cover-layer, larger diameters of SIL lens tip are required in order to accommodate the required optical beam, depending on numerical aperture and maximum cover and spacer layer thickness (i.e. multi layers). The upper limit is determined by mechanical tolerances such as disc flatness/waviness. Using active tilt compensation enables the use of larger SIL tip diameters for a given specification of optical record carrier, typically around lOOμm. Although values up to around 150μm could be used in well-controlled conditions, these are not practical for foreseen applications.

In a further embodiment of the invention, the movement of the SIL lens further provides a plurality of radial skips of the SIL lens such that the radial movement of

the SIL lens for each individual skip with respect to the optical record carrier covers a distance smaller than that covered in the preceding radial skip.

If a defect is larger than expected, further evasive action would be needed for complete safety of the SIL lens. When a further collision is detected close to the position of a first collision detection, this is likely to be related to the same defect. When this is the only collision at this radius (no new defects), it is preferable to make a smaller second radial skip than the first. This saves time and means less space on the optical record carrier is made redundant. A second radial skip may be estimated at half of the SIL lens diameter, typically 20 μm. If a defect is larger than the typical size, the defect size distribution is usually not so broad that another radial skip equal to the first is justified. Large skips are faster and lead to fewer collisions but skipping tracks costs storage capacity and should normally be minimised. The skipping action can be iterated as often as necessary to clear the defect. The skip distance can be reduced with successive skips if desired to fine tune the overall skip effect and optimise the skipping action. When another collision is detected at a different tangential position on the track of the optical record carrier, it is quite unlikely that the SIL encounters this defect at the edge closest to the centre of the optical record carrier (as would be the case for normal read or write mode where the SIL would be closely following the track rather than returning to the track after an excursion). Therefore, an intermediate radial skip is preferable, for example given by the SIL diameter plus half the typical defect diameter, typically 40+50=90μm. In a further embodiment of the invention, the device further comprises a reference table in non- volatile device memory in which a position, inputted from the means for positioning the SIL lens, is stored relating to a location on the optical record carrier at which a radial skip has been performed. In a further embodiment of the invention, the position is further related to the block number associated with the patterning of data storage positions on the optical record carrier by storage of the block number at which a radial skip has been performed in the reference table.

By storing the skip position, it is possible to avoid the defect region in future movements of the SIL lens. The block number/address corresponding to a defect collision (mostly during writing but also in read mode) can be included in a special table, for example stored on the optical record carrier or in a non- volatile memory of the near field optical recording device. Upon read-out or further writing at a later time, this feature allows the drive to know that the first occurrence of the specific block should be skipped. When

recognising the number/address, a lens jump should be initiated, and writing or read-out can be resumed with the repeated, complete block. This enhancement avoids collisions during read-out by using knowledge of the defect location and thus improves the reliability of the system. In a refinement, an indicator of the tangential position of the optical record carrier at the time of the collision is temporarily stored. For example, this indicator can be derived from the tacho signal from the motor controller (in a typical set-up, 500 pulses for each revolution, normally used to control the rotation speed). Then, a first radial skip is performed, determined by the SIL tip diameter plus the typical defect size. On a new radius, pull-in and gap control (control of the distance between SIL lens and optical record carrier) is re-started. Preferably, this can be done at a larger air gap than the nominal value. When no collisions are detected, the air gap (closest distance between the SIL lens and the optical record carrier in air) is reduced to the nominal value. When there are (still) no collisions, the defect has been successfully skipped and recording (or read out) can be resumed. When a series of radial skips are performed, it is possible to combine the information by summing the skip distances to associate one large skip with a position on the optical record carrier. This optimises the process of avoiding the defect, thereby saving time and reducing unnecessary movement of the SIL lens. Further, this additional provision gives radial defect size information (besides the already known defect 'start' location) and allows immediate jumping over the defect, minimising further collisions and providing smoother playback (no defect scanning).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further elucidated by reference to the drawings. Fig. 1 illustrates a method according to the invention

Fig. 2 illustrates the concept of the radial skip according to the invention Fig. 3 illustrates schematically a device according to the invention Fig. 4 illustrates an extended method according to the invention

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts schematically a basic method according to the invention. A near field optical recording device in normal read or write mode has a set-up such that the SIL lens for collecting light information from the optical record carrier is positioned very close to the optical record carrier. Typically this distance, known as the air gap, has a value

of around 25nm and is very carefully controlled. Due to the close proximity of optical record carrier and SIL lens, defects on the surface of the optical record carrier pose a significant problem. Impact between a defect and the SIL lens causes interruption of normal operation and can cause damage to the lens. Identifying imminent collisions using a collision detection means 11 is therefore important. When a collision is imminent, the SIL lens must be moved or diverted away from the defect. In prior art devices this is achieved by a complete pull back of the lens. This costs time, particularly since the pull back is done on an ad-hoc basis and must be repeated as the lens follows tracking on the optical record carrier, and the defect is likely to cover many tracks. By contrast, the invention method step involves moving the SIL lens in response to a detected imminent collision by performing a radial skip 12. This involves a movement of the SIL lens across the tracks of the optical record carrier in a radial direction with respect to the optical record carrier. The radial action permits the control systems in the recording device to maintain the SIL lens closer to the optical record carrier and avoids time-wasting pull back of the lens. Once the defect is cleared, the repositioning of the SIL lens with respect to the optical record carrier within normal tolerances for read and write operation 13 can be done. This proceeds faster and more efficiently than in prior art methods for collision avoidance.

Figure 2 shows an optical record carrier 21, rotating under normal read/write operations in a near field optical recording device (not shown), the direction of rotation of the optical record carrier 21 being indicated by arrow 22. A section 23 of the optical record carrier 21 is enlarged in the figure for clarity. Arrow 24 indicates the initial trajectory of a SIL lens (not shown) as the lens is moved along a track (not shown) of the optical record carrier 21. A defect 25 is present in the projected path of the SIL lens. At closest approach the SIL lens would occupy a position indicated by solid circle 26 and further progress along the track would result in a collision 27. According to the invention, in order to prevent collision and potential damage to the SIL lens, the trajectory of the SIL is altered so that a radial skip is performed. The radial movement is radial in relation to the optical record carrier 21, and is indicated schematically in the figure by arrow 28. After the radial skip, the SIL lens position is as indicated by dashed circle 29. The SIL lens is free from the defect and can be restored to normal operation on a new track (not shown), the updated trajectory of the SIL lens being indicated by arrow 24'.

Figure 3 shows a schematic diagram of a device according to the invention. A near field optical recording device 31 is arranged to cooperate with an optical record carrier 32. A SIL lens 33 is utilised to send and collect a radiation beam (not shown) wherein is

contained information about data patterns (not shown) on the optical record carrier 32. As it is a near field device as opposed to a far field device, the SIL lens 33 and optical record carrier 32 are placed in close proximity across an air gap when in operation, as indicated by arrow 34. The SIL lens displacement is controlled by a means for positioning 35 which inputs positional information to the SIL lens actuators (not shown), as indicated by arrow 36, to adjust the SIL lens 33 with respect to the optical record carrier 32. The near field optical recording device 31 also comprises a collision detection means 37. In the event that a possible collision with a defect on the surface of the optical record carrier 32 is detected, an input (as indicated by arrow 38) is provided to the means for positioning 35 to facilitate movement of the SIL lens. In prior art systems this movement is done by pulling back the SIL lens 33 from the optical record carrier 32. The pull back direction is indicated by arrow V in the figure. According to the invention, however, the movement comprises a radial skip as explained above. The radial skip direction is indicated by arrow R in the figure. This radial skip may be implemented by the means for positioning 35. Alternatively, the device according to the invention may be provided with a specific means for radial skip 39, as shown in the example in the figure, which gives as input (indicated by arrow 40) a movement control for the SIL lens 33.

Figure 4 illustrates a method according to the invention in which it is possible to perform a plurality of radial skips in order to avoid a defect. Method steps consistent with the method outlined in the Figure 1 description above are given the same numbering. The method begins with the identification of an imminent collision 11. The device can locate the SIL lens with respect to the optical record carrier for read and write modes. One option of implementation is the tacho position. In this method, the tacho position corresponding to the position of the SIL lens on the optical record carrier at point of collision can be obtained. The method therefore involves storing the tacho position 41 at this moment. A first radial skip is performed 12 to move the SIL lens away from impact with the defect across tracks on the optical record carrier. This first radial skip can be a pre-defined distance, calculated based on SIL diameter and average defect size which is programmed into a memory or operation of the device. The SIL lens is then pulled in again on a new trajectory and track and normal operation is resumed 13.

In this method, the method is extended to include further checks on the possibility of another collision with the same defect. It is possible that a defect may be of a much larger size than expected, for example, and one radial skip may not be sufficient to completely avoid an impact between the defect and the SIL lens. Thus this method further

includes checking again for imminent collision 42. At this point, the question must be answered by the device, "are there more immediate collisions possible ?" 43

If there are more collisions imminent then the answer is "yes", indicated by arrow Y, and the device proceeds to follow the next method step of performing another, smaller, radial skip 44. The radial skip is not necessarily smaller, it could be larger or the same size as the first radial skip. However, in this method embodiment of the invention it is assumed that the first radial skip will have almost cleared the defect and therefore, based on the statistics of defect sizing which would indicate not many defects are very much larger than average, a smaller radial skip should be sufficient. The method continues with a repeat of step 43 to check for further imminent collisions.

If no more possible collisions are detected then the answer is "no", indicated by arrow N and the device proceeds to follow the next method step of reducing the air gap to level of normal operating conditions 45. As a final step, the new tacho position of the SIL lens is known at point of return to normal operation, and the method then includes the step of storing the new position in memory 46.

It is also possible to calculate and store the overall radial skip connected with a particular defect (not shown) by using the information of starting position from step 41 and ending position from step 46 to provide the overall displacement of the SIL lens.

LIST OF REFERENCE NUMERALS

11. Method step : identifying an imminent collision using collision detection means.

12. Method step : moving of SIL lens in response to a detected imminent collision by performing a first radial skip of the SIL lens with respect to the optical record carrier.

13. Method step : repositioning of the SIL lens with respect to the optical record carrier within normal operating tolerances for read and write operation.

21. Optical record carrier 22. Arrow indicating direction of rotation of the optical record carrier

23. Section of the optical record carrier

24. Arrow indicating initial trajectory of the SIL lens with respect to the optical record carrier

24' Arrow indicating updated trajectory of the SIL lens with respect to the optical record carrier

25. Defect

26. Circle indicating SIL lens position

27. Collision 28. Circle indicating SIL lens position after radial skip

31. Near field optical recording device

32. Optical record carrier

33. SIL lens 34. Arrow indicating air gap

35. Means for positioning

36. Arrow indicating input for control of SIL lens position

37. Collision detection means

38. Arrow indicating input indicating detection of collision 39. Means for radial skip

40. Arrow indicating input for control of SIL lens for radial skip

R arrow indicating radial direction

V arrow indicating vertical pull back direction

41. Method step : storing tacho position

42. Method step : check again for imminent collision

43. Method step : question : imminent collision ? 44. Method step : perform another smaller radial skip

45. Method step : reducing air gap to level of normal operating conditions

46. Method step : storing new position in memory

Y arrow indicating method path for yes answer N arrow indicating method path for no answer