Optical recording parameter control

FIELD OF THE INVENTION

The invention relates to a device for scanning a record carrier, the record carrier comprising a data layer having substantially parallel tracks, the device comprising an optical head comprising an optical system and a radiation source for generating a radiation spot on a track and a detector for generating detector signals indicative of radiation reflected from the data layer, a power unit for controlling the power of the radiation source, and a control unit coupled to an actuator for, in response to a difference of an error signal and a target value, controlling a parameter of the optical head.

The invention further relates to a method of radiation power control in a device for scanning a record carrier, the record carrier comprising a data layer having substantially parallel tracks, the device comprising an optical head comprising an optical system and a radiation source for generating a radiation spot on a track and a detector for generating detector signals indicative of radiation reflected from the data layer, and a control unit coupled to an actuator for, in response to a difference of an error signal and a target value, controlling a parameter of the optical head, the method comprising controlling the power of the radiation source.

In optical drives, the scanning performance is dependent on parameters of the optical scanning unit, usually called optical head, such as a focus of the optical system, tracking (radial position with respect to the track), or tilt of the optical head, which are required to correctly position the radiation spot, usually from a laser, in focus on the data layer and radially on the desired track to accurately write or detect the marks in the data tracks representing recorded information. Tilt is the angle between an optical axis of the optical head and a perpendicular of the data layer of the record carrier.

The parameters of the optical head are controlled via a control system, usually a servo controlled actuator system for each parameter based on error signals generated by radiation reflected from the record carrier to a detector in the head.

BACKGROUND OF THE INVENTION

A device for scanning an optical record carrier is known from US 6,947,359. An optical disk drive apparatus is arranged for data recording or reproduction from on optical record carrier, such as a CD or DVD. The device has an optical head having a laser and optical elements for generating a laser spot on a track of the record carrier. The focus of the radiation spot on the track is controlled based on a focus error signal by a focus controller and an actuator for moving an objective lens. The device detects a defect of the record carrier from signals from a detector in the optical head. When a defect is detected, a correction signal is generated by a disk physical strain correction signal generator. During the defect the focus controller is clamped, i.e. the input of the focus error signal is interrupted to prevent the objective lens being moved due to erratic focus error signals due to said defect. Before the defect is detected, and clamping starts, the focus error signals already have deviating signal parts. After the defect the optical system may have some defocus. The de focus is decreased by supplying a correction pulse to the focus controller during the defect based on the deviating signal parts. Hence the unwanted defocus due to the defect is decreased.

SUMMARY OF THE INVENTION

The above system has the problem that some parameters of the optical head show unwanted deviations from a desired target value, e.g. remaining defocus due to a defect. The deviations of the parameter control result in reading and/or recording errors.

Therefore it is an object of the invention to provide a device and method for parameter control reducing said errors.

According to a first aspect of the invention the object is achieved with a device as described in the opening paragraph, comprising a power correction unit coupled to the power unit for adding a correction amount of power to the power of the radiation source, the correction amount being dependent on a deviation of the error signal from the target value. According to a second aspect of the invention the object is achieved with a method as described in the opening paragraph, which method comprises adding a correction amount of power to the power of the radiation source, the correction amount being dependent on a deviation of the error signal from the target value.

The effect of the measures is that a remaining deviation of a controlled parameter is detected from a deviation of the error signal from the target value, for example the absolute value of the difference of error signal and target. Subsequently the correction amount of power is added to compensate for the parameter being off target. For example a

remaining amount of defocus, resulting in a less effective writing spot, is compensated by adding the correction amount of power. Advantageously read or write errors due to the parameter being off target are reduced.

The invention is also based on the following recognition. It has been noted that the control of parameters of the optical head in practice is not perfect, in particular in the vicinity of defects. During a defect, such as a dust particle or a scratch on the surface, the power of the radiation source may be increased by the power control system due to detecting a reduced reflected energy, or writing may be interrupted. Surprisingly, even after the defect, writing and/or reading of marks was found to be not optimal. The inventors have seen that a remaining parameter error, like defocus, causes deteriorating effects on marks written after a defect, a so-called comet tail. Such deteriorating effects, due to parameters being off-target, are reduced by adding the correction amount of power. In particular the effective radiation power at the track should be maintained close to its target value. The inventors noted that a parameter error results in a change of the effective radiation power at the track. Hence the actual power produced by the radiation source is set to a different value, usually above the nominal target value, to achieve an effective radiation power on the data layer at the required level. By detecting any remaining parameter error, and subsequently controlling the radiation power to compensate by adding the correction amount of power, the read and/or write errors are reduced. In an embodiment of the device the power correction unit is arranged for detecting a defect of the record carrier, and applying the correction amount of power in a period after the defect. This has the advantage that disturbances of the parameter control caused by the defect are effectively compensated by applying the correction power after the defect. In an embodiment of the device the power correction unit is arranged for adding the correction amount of power proportionally to said deviation. This has the advantage that for a large deviation more correction power is added. Hence any deteriorating effects from small and large disturbances are proportionally compensated.

In an embodiment of the device the power correction unit is arranged for adding the correction amount of power according to a predetermined relationship between the deviation and a decrease of power of the radiation spot at the data layer. The relationship between a deviation and the required additional power is established and used to determine the amount of correction power. This has the advantage that an appropriate amount of correction power is added to accurately compensate a remaining parameter error.

Further preferred embodiments of the device and method according to the invention are given in the appended claims, disclosure of which is incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which

Figure 1 shows an optical recording process in a scanning device, Figure 2 shows a scanning device having power correction,

Figure 3 shows detector and error signals at a defect, Figure 4 shows power correction based on parameter control, and Figure 5 shows a power correction unit and various switching controls. In the Figures, elements which correspond to elements already described have the same reference numerals.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows an optical recording process in a scanning device. The scanning device comprises a turntable 1 and a drive motor 2 for rotating a disc shaped record carrier 4 about an axis 3 in a direction indicated by an arrow 5, and an optical write head 6.

The record carrier 4 comprises a radiation-sensitive recording layer which upon exposure to radiation of sufficiently high intensity is subjected to an optically detectable change, such as for example a change in reflectivity, for forming marks 8 representing information in a track 11. The track 11 may be indicated by a servo pattern for generating servo tracking signals for positioning an optical head opposite the track. The servo pattern may for example be a shallow wobbled groove, usually called a pre-groove, and/or a pattern of indentations, usually called pre-pits or servo pits.

The optical write head 6 is arranged opposite the rotating record carrier. The optical write head 6 comprises a radiation source, for example a solid-state laser, for generating a write beam 13. The power I of the write beam 13 can be modulated in conformity with a control signal Vs in a customary manner. The power I of the write beam 13 varies between a write intensity Iw, which is adequate to bring about detectable changes in the optical properties of the radiation-sensitive record carrier, and an intensity In for producing spaces.

The marks may be in any optically readable form, e.g. in the form of areas with a reflection coefficient different from their surroundings, obtained when recording in materials such as dye, alloy or phase change material, or in the form of areas with a direction of polarization different from their surroundings, obtained when recording in magneto-optical material.

During scanning a beam reflected from the record carrier is modulated in conformity with the information pattern being scanned. The modulation of the read beam can be detected in a customary manner by means of a radiation-sensitive detector which generates a read signal which is indicative of the beam modulation. Figure 2 shows a scanning device having power correction. The device is provided with scanning means for scanning a track on the record carrier 11 which means include a drive unit 21 for rotating the record carrier 11, a head 22, a servo unit 25 for positioning the head 22 on the track, and a control unit 20 for controlling the scanning. The head 22 comprises an optical system of a known type for generating a radiation beam 24 guided through optical elements focused to a radiation spot 23 on a track of a data layer of the record carrier. The radiation beam 24 is generated by a radiation source, e.g. a laser diode. The device has a power unit 29 for controlling the power of the radiation source. The head further comprises a focusing actuator 38 for moving the focus of the radiation beam 24 along the optical axis of said beam under the control of a focus control unit 36. The head further comprises a tracking actuator (not shown) for fine positioning of the spot 23 in a radial direction on the center of the track. The tracking actuator may comprise coils for radially moving an optical element or may alternatively be arranged for changing the angle of a reflecting element. The tracking actuator is driven by actuator signals from the tracking servo unit 25. The record carrier 11 may exhibit a tilt as schematically indicated by arrow

301. For example the tilt may result from a non-flat surface, a non perfect mechanical support, or scanning system offset, etc. A tilt angle 304 is defined at the position of the scanning spot 23, as the angle between an optical axis 302 of the head 22 and a perpendicular 303 of data layer of the record carrier. Note that in practice the tilt angle is about 1 degree or less, and the Figure is not drawn to scale.

The head, or the record carrier support system, may further include a tilt actuator (not shown) for adapting the tilt angle. The tilt actuator is driven by a tilt control unit (not shown) and may be controlled based on a tilt error signal in a servo loop, as is known as such. Detecting tilt is, for example, described in the document "New radial tilt detection

method using only one beam and one four-quadrant detector" by Y. Wang et al. Japanese Journal of Applied Physics, Vol.43, No.l IA, 2004, pp7513-7518. The tilt angle may also be controlled according to detected tilt pattern, e.g. in dependence of a region of the record carrier. During reading the radiation reflected by the information layer is detected by a detector of a usual type, e.g. a four-quadrant diode, in the head 22 for generating detector signals coupled to a front-end unit for generating various scanning signals, including a main scanning signal 33 and error signals 35 for tracking and focusing. The error signals 35 are coupled to the control units 25,36 for controlling said tracking, focusing and tilt actuators. Hence each of the above control units is coupled to a corresponding actuator for, in response to a difference of an error signal and a target value, controlling a parameter of the optical head.

The main scanning signal 33 is processed by read processing unit 30 of a usual type including a demodulator, deformatter and output unit to retrieve the information. The control unit 20 comprises control circuitry, for example a microprocessor, a program memory and control gates. The control unit 20 may also be implemented as a state machine in logic circuits. The control unit may be arranged to perform the power correction adjustment or calibration as discussed below.

The device may be provided with recording means for recording information on a record carrier of a writable or re-writable type. The recording means comprise an input unit 27, a formatter 28 and the power unit 29 and cooperate with the head 22 for generating a write beam of radiation. The formatter 28 is for adding control data and formatting and encoding the data according to the recording format, e.g. by adding error correction codes (ECC), synchronizing patterns, interleaving and channel coding. The formatted data comprise address information and are written to corresponding addressable locations on the record carrier under the control of control unit 20. The formatted data from the output of the formatter 28 is passed to the power unit 29 which drives the laser and controls the laser power for writing the marks in a selected layer.

In an embodiment the recording device is a storage system only, e.g. an optical disc drive for use in a computer. The control unit 20 is arranged to communicate with a processing unit in the host computer system via a standardized interface. Digital data is interfaced to the formatter 28 and the read processing unit 30 directly.

In an embodiment the device is arranged as a stand alone unit, for example a video recording apparatus for consumer use. The control unit 20, or an additional host control

unit included in the device, is arranged to be controlled directly by the user, and to perform the functions of the file management system. The device includes application data processing, e.g. audio and/or video processing circuits. User information is presented on the input unit 27, which may comprise compression means for input signals such as analog audio and/or video, or digital uncompressed audio/video. Suitable compression means are for example described for audio in WO98/16014-A1, and for video in the MPEG2 standard. The input unit 27 processes the audio and/or video to units of information, which are passed to the formatter 28. The read processing unit 30 may comprise suitable audio and/or video decoding units. The device has a power correction unit 31 for generating a power correction signal 34 coupled to the power unit 29 for adding a correction amount of power to the power of the radiation source. The correction amount is dependent on a deviation of the error signal from the target value for a controlled parameter, e.g. focus, tracking, or tilt. In the following sections the power correction for defocus is described in detail. Figure 3 shows detector and error signals at a defect. In a graph of amplitude versus time a lower trace 41 shows a detector signal MIRN (mirrorN) indicative of the total reflection of radiation from the track at the data layer. The signal MIRN has a high frequency component that may be used to derive the data represented by marks in the track during reading. While writing the signal is modulated by the power pattern applied to the radiation source for writing the marks in the track. The Figure shows a part of the track having a defect, the signal part 42 being almost zero. An upper trace 40 in the graph shows a focus error signal FE for the corresponding part of the track. During the defect, the FE signal shows erratic patterns 43. After the defect, a remaining deviation 44 can be seen. Note that the focus servo system controls the focus actuator to its nominal position, but, due to the disturbance of the defect, some deviation remains in the period following the defect.

Figure 4 shows power correction based on parameter control. In a schematical graph of various signals versus time an upper trace 51 shows a focus error signal. A second trace shows a defect detector signal 52, for example a write defect detection (WDD) signal. The signal 52 goes active in a period 57 during a defect. A third trace shows a power level 53 for the radiation source, starting at a nominal power level 58, e.g. a power determined during an optimum power calibration (OPC) process. A fourth trace 54 shows a logical power control signal having a shaded area 60 indicating a period after the defect where correction power 59 is to be applied. The period may be predetermined, e.g. to a known settling time of

the parameter control. Alternatively the end of the period 60 may be determined by monitoring the respective error signal.

The focus error signal shows a normal pattern 55 before the defect, and is suppressed during the defect, for example based on the defect detector signal 52. After the defect a large focus error signal part 56 occurs, having a positive part and a negative part. Subsequently the focus error signal returns to normal operational patterns. During the period 60 after the defect, the focus error signal is used to control the correction power 59 that is added to the nominal power level 58. As shown, the correction power is proportional to a deviation of the focus error signal from its nominal value. Thereto the difference of the focus error signal and a focus target value (assumed to be the level of line 51) is used as an absolute value, e.g. rectified, to control the correction power 59.

Figure 5 shows a power correction unit and various switching controls. The power correction unit 31 has an error signal module 71, a conversion module 72 for generating a power correction signal, and (optionally) a switch 80 controlled by a switch control unit 70. The power correction signal is coupled to an adder 73 which adds a nominal power level signal 77 and the correction power level signal 79 to constitute the actual power level signal 78. Note that in practice the adder may include further operations, like a multiplying factor, level shifting, or adding/subtracting factors, to achieve the required total actual power. In the error signal module 71 an error input signal from a parameter servo control loop, for example a focus error signal FE, is received. Also a clamp signal is received from a clamp unit 75. The error signal processing module 71 generates a deviation signal 84 indicative of the deviation of the error input signal from the target value, for example the absolute value with respect to zero. Subsequently, the power correction unit 31 may add the correction amount of power proportionally to the deviation signal.

The deviation signal 84 may be processed in the conversion module 72 for generating the power correction signal by applying a relationship between the deviation of the error signal and a required correction power level. Thereby the power correction unit 31 adds, via adder 73, the correction amount of power according to the predetermined relationship between the deviation of the parameter error signal and a decrease of power of the radiation spot at the data layer.

Alternatively, the deviation signal 84 may be used as a logical control when exceeding a threshold, or may be transformed into a few levels, each level adding a predetermined correction amount of power. In practice, correction power is in the order of 10

to 20%. The additional power required for a remaining parameter error may be limited to a maximum amount, e.g. +25%, for not exceeding operational limits of the components.

The relationship may be modeled on an actual relation between the remaining error and a corresponding amount of correction power. For example a curve of a focus error signal versus a required correction power level may be used. The predetermined relationship may be determined during design and/or manufacture of the device, and may be preset, or may be programmed later. The power correction unit 31 may be arranged for adjusting the predetermined relationship, for example to accommodate various operational conditions or changes in the components by temperature or aging. In an embodiment of the device, the power correction unit 31 is arranged for adjusting the predetermined relationship based on a calibration process. The process includes determining, at zero deviation, a nominal power level of the radiation source for optimally scanning a track, usually called optimum power control (OPC). The nominal power level may be calibrated, may already be known from earlier calibration, and/or may be updated while recording. Subsequently, the relationship calibration process sets the control unit 36 to cause a non-zero deviation, e.g. a forced amount of defocus. It is noted that the forced level of the focus error signal is used for calculating the relationship, whereas the actual amount of defocus is not relevant. Next, at the non-zero deviation, a second power level of the radiation source for optimally scanning a track is determined. Thereto the OPC process may be repeated. The difference of the second power level and the nominal power level is to be used to calculate the correction power corresponding to the forced error signal level. Further measurements at different forced error signal levels may be acquired similarly for forming an accurate relationship at various off-target levels, which relationship may be not linear as shown by a graph in the conversion unit 72. In an embodiment of the device the power correction unit 31 is arranged for adjusting the predetermined relationship based on operational conditions. For example, the power correction unit 31 may detect one or more of the following conditions: a type of the record carrier to be scanned; a layer of the record carrier to be scanned; or a scanning speed. For each of the relevant conditions a respective adjustment of the relationship is executed. The required adjustment may be stored in a memory in the device, e.g. a lookup table in EEPROM, or may be retrieved from an external source, e.g. stored on the record carrier.

The power correction signal may be coupled always to compensate deviations in the controlled parameter, or may be switched to supply the correction power only in

certain periods with respect to a defect, as indicated by switch 80 controlled by switch control unit 70. The switch control unit 70 may be embodied in one of the following ways.

In a first embodiment the switch control 70 has a filtering and switching unit 74, which receives the deviation signal and derives a logical control signal based on a filtered version of the deviation signal, e.g. being active until the filtered version is below a predetermined threshold as shown near switch control signal 81.

In a second embodiment the switch control 70 has a reflection defect detection unit 75, which receives the MIRN signal and derives a logical switch control signal 82 based on a filtered version of the MIRN signal, e.g. becoming active when the filtered version is below a predetermined threshold, and continuing for a predetermined period after the defect ends, e.g. indicated by a normal level of the MIRN signal. Note that the reflection defect detection unit 75 may also generate the clamp signal, which clamp signal may be used in the control unit of the respective parameter to hold the actual status during a defect.

In a third embodiment the switch control 70 has a write defect detection unit 76, which receives the WDD signal and derives a logical control signal 83 based on a processed version of the WDD signal, e.g. being active for a predetermined time triggered by the WDD signal. The WDD signal may be available from a write control unit in the device, or may be generated based on a suitable combination of detector signals, e.g. based on an amount of reflected energy during writing a mark. In the above examples the power correction unit 31 is arranged for detecting a defect of the record carrier, and subsequently applying the correction amount of power in a period after the defect. The period may be based on various events, e.g. a time delay after MIRN has recovered, after the clamp signal is off, based on FE amplitude, based on duration of the WDD signal, and may even be related to speed of rotation of the record carrier. However, it is noted that the correction power may also be active at all times, or may be triggered on a different event, such as the respective parameter error signal exceeding a predetermined level. Furthermore, in the example the power correction unit 31 may be arranged for, during detecting the defect of the record carrier, clamping the respective parameter control unit or units, for example focus control unit 36. The clamping may include setting the respective error signal to zero, or to its target value, and/or holding drive signal(s) to the actuator(s) to a preceding or predetermined level.

In the development of the above the following was found. Whilst writing across defects on optical discs there are often blank areas behind the defect (dot or mark) on the disc. The length of the blank area behind the mark is quite often related to the size of the

defect thus give a comet like tail to the defect. It was found that the tail was caused by the writing spot being slightly out of focus whilst writing. Using this knowledge it proved to be possible to reduce the size of this tail by the following method:

- determine relation of defocus to power loss, i.e. a focus to power factor. - determine defocus in the blank area, and the position of the blank area

(behind defect)

- increase the power as to compensate for the power loss due to this defocus. In detail the steps of the method include first to calibrate the defocus to power factor. This step may be added to an OPC procedure. Secondly determine when the defect has occurred. Note that detecting defects during writing may already be implemented for allowing a drive to record data again at a different location. Thirdly apply the correction power in dependence of the defocus signal.

Although the invention has been mainly explained by embodiments using focus error for power correction, the invention is also suitable for power correction based on an error signal of any other controlled parameter, like tilt or radial position. Furthermore, the correction amount of power may be negative. For example, the calibration may find a minimum power focus offset point different to the focus offset point actually being used (e.g. due to play-ability settings). This requires that the power is reduced if the focus error, controlling the correction power, passed through the minimum power point point. Also a parameter that is only detected, but not actually controllable via an actuator, may be compensated, when an error signal is provided that indicates the deviation of the parameter from its target. Furthermore the invention can be applied to other record carriers such as rectangular optical cards, magneto -optical discs, multilayer high-density discs, etc, and may be used for any other type of scanning system sensitive to reduced power efficiency due to errors in one or more parameters.

It is noted, that in this document the word 'comprising' does not exclude the presence of other elements or steps than those listed and the word 'a' or 'an' preceding an element does not exclude the presence of a plurality of such elements, that any reference signs do not limit the scope of the claims, that the invention may be implemented by means of both hardware and software, and that several 'means' or 'units' may be represented by the same item of hardware or software, and a processor may fulfill the function of one or more units, possibly in cooperation with hardware elements. Further, the invention is not limited to the embodiments, and the invention lies in each and every novel feature or combination of features described above.