WO2008110960A1

WO2008110960A1 - A method of writing data to an optical record carrier and an optical recording device for writing data to an optical record carrier

Info

Publication number: WO2008110960A1
Application number: PCT/IB2008/050779
Authority: WO
Inventors: Ronald J. A. Van Den Oetelaar; Donato Pasquariello
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-12
Filing date: 2008-03-04
Publication date: 2008-09-18
Also published as: TW200847149A

Abstract

The invention concerns a method and a device for the optimisation of the light power of a beam used for write or read operations of an optical recording device arranged to cooperate with an optical record carrier. The optical record carrier is of a type supporting non-sequential data sequences. The invention allows deconvolution of a beam detected after interaction with partially recorded data layers of the optical record carrier in terms of a reflective component from the data layer being written or read and a transmittive component from layers through which the light beam has passed.

Description

A method of writing data to an optical record carrier and an optical recording device for writing data to an optical record carrier

FIELD OF THE INVENTION

The invention relates to the field of optical recording devices and, more particularly, to using such a device for the function of writing data to an optical record carrier.

BACKGROUND OF THE INVENTION :

Optical recording devices are well known in the art for reading data from and writing data to optical record carriers. The storage of information on such an optical record carrier has developed to take account of the ever increasing need to store larger quantities of data. In this respect, several solutions have been developed, including the use of short- wavelength lasers for smaller read/write spots and thus higher data resolution, and also including the development of multi-layer optical record carriers.

In the first multi-layer optical record carriers data was recorded on tracks on a first layer and then the recording process continued sequentially on the next layer. More recently, many emerging optical storage formats, such as Blu-ray disc (BD), support Random Recording Mode (RRM). In RRM, data can be written randomly at any unrecorded cluster on any layer. Thus for the latter recording technique a pattern exists across a data layer of recorded and unrecorded sectors.

When writing to lower layers, light must pass through one or more data layers to reach the lower layer. Such a layer may contain recorded data or it may be unrecorded, or in the case of RRM it may have a mixture of recorded and unrecorded sectors within an area. The recording characteristics of the layer affect the transmission of the light for write operation on the next layer. A recorded sector will transmit more light than an unrecorded sector. Or put another way, the power required to write correctly on a lower layer will be higher if the layer above it is unrecorded. This phenomenon is discussed in United States patent applications US 2002/0126602 Al and US 2002/0136122 Al.

A problem with such transmission issues is that the light power required to write on a data layer is dependent on the recording state of the other data layers through which the light must pass to reach the data layer to be recorded, and this recording state is variable. Optimum write powers exist for different types of optical record carriers. An optimum write power has a best setting and an associated range within which the power can fluctuate and yet correct recording will still be achieved. A problem in writing to a data layer is that the transmitting layer state can have so much effect on the transmission of the light beam that the light arriving at the data layer to be recorded is beyond the beam power tolerance levels for good data recording thereby producing poor quality written data on the data layer.

This issue is addressed in US 2002/0126602 Al. Here it is suggested to use a corrected value for recording power, at positions where the transmitting layer throws a shadow on the lower layer, due to the change in transmittivity of the transmitting layer. Thus proper recording is maintained during writing. The corrected value may be determined by measuring the reflection level difference of an empty track in the other information layer when the recording is effected through a non-recorded area and a recorded area of at least one information layer. This may be done, for example, during an initial Optimum Power

Calibration (OPC) procedure when trail recordings are performed. The corrected value can then be determined on the basis of the measured reflection level differences. As an alternative, the corrected value may be determined by reading a corresponding specification provided on said record carrier. A suitable corrected value for the recording power is thus determined in advance so as to be used in cases where recording is effected through a non- written portion of the upper information layer. In order to access information on transmission values for parts of the upper layer, a table of contents or a prescan function is suggested. A control unit determines or obtains a corrected value from an output system and corrects the power of the radiation source during writing or recording depending on the transmission characteristic of the upper layer. Throughout the document the effect of the transmission state of the upper layer is measured by detection and analysis of the light beam which is reflected from the lower layer (and therefore has passed twice through the upper layer).

A problem with this method is that there is no provision for power optimisation on a data layer which has already been recorded or partially recorded, the recording characteristic affecting the writing or reading of data at the data layer. SUMMARY OF THE INVENTION :

It is an object of the invention to improve the quality of data written to a lower data layer of an optical record carrier which is partially recorded through an upper data layer which is also partially recorded. This object is achieved according to the invention by provision of a method for optimisation of light power of a beam during reading or writing of an optical record carrier comprising a first data layer and a second data layer comprising recorded data, the method comprising the steps of:

- transmitting the beam through the first data layer, - focussing the beam at the second data layer,

- reflecting the beam at the second data layer,

- retransmitting the beam, after reflection by the second layer, through the first data layer,

- detecting the beam, the method further comprising the steps of :

- deconvo luting the detected beam into a deconvo luted signal to distinguish a first light signal resulting from the reflection of the beam at the second data layer from a second light signal resulting from the transmission of the beam through the first data layer,

- optimising of the light power based on the deconvo luted signal. In the invention, the method includes steps of focussing the light beam to the second data layer where the write or read operation is to take place. This involves the transmission of the light beam through at least one layer on the optical record carrier (for example the first data layer) and the return of the light beam through this layer a second time after reflection at the data layer. The light beam now contains both reflection and transmission information. This permits deconvo lution of the reflection profile R of a rewritable medium in terms of transmission changes T due to recorded and unrecorded regions in the transmitting layer or layers and reflection changes F due to recorded clusters in the data layer.

R= T (deconvo lution symbol) F

Separating the reflection changes and the transmission changes allows the optimum calibration of the light power to obtain the desired read and write mode conditions. In a further embodiment of the invention, the deconvo luted signal is the second light signal. This deconvo luted signal then equates to the effects of transmission of light through the data layers only. It is the transmission effect for which the power optimisation procedure is designed. The correction of the light power should only be in response to transmission, this being the actual power of the light used for the reading or writing function before the light is reflected back towards the detector. The deconvolution makes this correction possible.

In a further embodiment of the invention, the method further comprises a step of : - monitoring the detector signal for a change with a gradient

- initiating deconvolution based on the presence of the change with a gradient. The beam detected at the detector, which provides the basic information for decrease or increase of the power of the light, has a more abrupt change characteristic when reflection changes due to recorded regions or clusters on the second data layer are present. Thus by detecting an abrupt change it is possible to know to deconvo lute the beam.

A change due to reflection at the operation data layer is sharp because the light beam is focussed on this layer, so when a change in reflection state occurs it is sudden and affects most of the beam. Thus the gradient of the beam change is steep. In comparison, transmission effects take place where the beam is more defocused thus a wider cross-section averages out any effect of a layer.

The reflection changes due to recorded regions or clusters on the data layer produce a change effect in the detected light which is typically at least 20% stronger than changes due to transmission.

In a further embodiment of the invention, the method further comprises a step of :

- removing from the detector signal a calibrated reflection response representing the first light signal when value of the change with a gradient exceeds a threshold value.

In prior art methods, the reflection response for different situations can be calibrated. This can be done for different layers on the optical recording device. This can also be done for different recording states. The calibrated reflection response forms part of the prior art and comprises the difference in light reflection which is detected at the detector when the light passes over a change in the state of the data layer. The detected beam changes more sharply due to reflection changes at the second data layer than due to transmission changes at the first layer through which the light beam must pass to reach the second data layer. This is due in part to the fact that the beam is defocused at the first data layer and the beam consequently has a relatively large cross- sectional area compared to the focussed profile at the second data layer. A larger cross- sectional area makes the beam less sensitive to changes in transmission as these effects are effectively averaged over the area of the beam. Thus changes in the beam used for power optimisation which are due to transmission effects will take place more gradually and with a slower gradient than changes due to reflection. By comparing the gradient against another known value which lies between the two extremes due to reflection and transmission effects, it is possible to determine if a deconvolution of the detected beam should take place. The chosen threshold value can also be directly related to information already known as to the steepness of gradients due to reflection changes.

In further embodiments of the invention, the threshold value, or a series of threshold values, may be stored as information on the optical record carrier (for example, in an area already containing other optical record carrier information). Alternatively, the threshold or thresholds may be stored in a non- volatile memory of the optical recording device.

In a further embodiment of the invention, the gradient value used is the absolute value of the gradient function.

For signal processing reasons, or due to the nature of the reflections, it can be advantageous to use the absolute value of the gradient and not take account of sign information.

- initiating deconvolution based on a change in the intensity of the detected beam.

Changes induced in the detected beam may be positive or negative depending on the characteristics of the optical record carrier layer because this is the reflecting medium. If the reflection change is positive or negative when the beam goes from an unrecorded to a recorded area of the data layer this means that the optical record carrier has a High-to-Low (HtL) or Low-to-High (LtH) design, respectively. The reflection change in the disc depends on the optical thickness of the layers in the disc. A HtL disc is designed so that the unrecorded areas have higher reflection than recorded areas. A LtH optical record carrier is designed so that unrecorded areas have lower reflection than the recorded areas. For a rewritable optical record carrier the difference between a HtL and LtH is, in principle, the thickness of the dielectric layers in the stack.

Another embodiment of the invention comprises an optical recording device, arranged to cooperate with an optical record carrier for read and write operations, the optical record carrier comprising a first data layer and a second data layer on which data has already been recorded, a beam with a light power being transmitted through the first data layer to be focussed and reflected at the second data layer and then transmitted a second time through the first data layer, the optical recording device further comprising - a detector to provide a detector signal,

- a control unit arranged for optimisation of the light power, based on a control unit input signal, derived from the detector signal, characterised in that the optical recording device further comprises: - a deconvolution means coupled to the detector and arranged for creating a deconvo luted signal distinguishing in the detector signal between a first light signal due to the reflected light at the second data layer and a second light signal due to light transmitted through the first data layer, the deconvolution means being coupled to the control unit to provide the control unit input signal based on the deconvoluted signal. In the invention, the method includes steps of focussing the light beam to the second data layer where the write or read operation is to take place. This involves the transmission of the light beam through at least one layer on the optical record carrier (for example the first data layer) and the return of the light beam through this layer a second time after reflection at the data layer. The light beam now contains both reflection and transmission information. This permits deconvolution of the reflection profile R of a rewritable medium in terms of transmission changes T due to recorded and unrecorded regions in the transmitting layer or layers and reflection changes F due to recorded clusters in the data layer.

R= T (deconvolution symbol) F

In a further embodiment of the invention, the deconvolution means further comprises a monitoring means to detect a change with a gradient in the detector signal. The beam detected at the detector, which provides the basic information for decrease or increase of the power of the light, has a more abrupt change characteristic when reflection changes due to recorded regions or clusters on the second data layer are present. Thus by detecting an abrupt change it is possible to know to deconvo lute the beam.

In a further embodiment of the invention, wherein the means for deconvolution further comprises removal means for removing a calibrated reflection response representing the first light signal from the detector signal when the monitoring means detects the change with a gradient exceeding a threshold value.

In prior art methods, the reflection response for different situations can be calibrated. This can be done for different layers on the optical recording device. This can also be done for different recording states. The calibrated reflection response forms part of the prior art and comprises the difference in light reflection which is detected at the detector when the light passes over a change in the state of the data layer.

The detected beam changes more sharply due to reflection changes at the second data layer than due to transmission changes at the first layer through which the light beam must pass to reach the second data layer. This is due in part to the fact that the beam is defocused at the first data layer and the beam consequently has a relatively large cross- sectional area compared to the focussed profile at the second data layer. A larger cross- sectional area makes the beam less sensitive to changes in transmission as these effects are effectively averaged over the area of the beam. Thus changes in the beam used for power optimisation which are due to transmission effects will take place more gradually and with a slower gradient than changes due to reflection. By comparing the gradient against another known value which lies between the two extremes due to reflection and transmission effects, it is possible to determine if a deconvolution of the detected beam should take place. The chosen threshold value can also be directly related to information already known as to the steepness of gradients due to reflection changes. In further embodiments of the invention, the threshold value, or a series of threshold values, may be stored as information on the optical record carrier (for example, in an area already containing other optical record carrier information). Alternatively, the threshold or thresholds may be stored in a non- volatile memory of the optical recording device. In a further embodiment of the invention, the gradient is an absolute gradient.

Or signal processing reasons, or due to the nature of the reflections, it can be advantageous to use the absolute value of the gradient and not take account of sign information.

In a further embodiment of the invention, the device further comprises a monitoring means to detect a change in an intensity of the detected beam.

In a further embodiment of the invention, the means for deconvolution further comprises removal means for removing a calibrated reflection response representing the first light signal from the detector signal when the monitoring means detects the change in intensity exceeding a target value. Changes induced in the detected beam may be positive or negative depending on the characteristics of the optical record carrier layer because this is the reflecting medium. If the reflection change is positive or negative when the beam goes from an unrecorded to a recorded area of the data layer this means that the optical record carrier has a High-to-Low (HtL) or Low-to-High (LtH) design, respectively. The reflection change in the disc depends on the optical thickness of the layers in the disc. A HtL disc is designed so that the unrecorded areas have higher reflection than recorded areas. A LtH optical record carrier is designed so that unrecorded areas have lower reflection than the recorded areas. For a rewritable optical record carrier the difference between a HtL and LtH is, in principle, the thickness of the dielectric layers in the stack. BRIEF DESCRIPTION OF THE DRAWINGS :

The invention will now be further elucidated with reference to the drawings :

Fig. 1 : illustrative example of the relative intensity noise as a function of read power

Fig. 2 : illustrative example of how the repeated read varies with read power on a single layer BD-RE disc

Fig. 3 : illustrates a section through an optical record carrier with several data layers Fig. 4 : illustrates the effect of transmission differences on a readout signal from a dual layer BD-RE disc

Fig. 5 : illustrates the change in reflection due to the change in transmission of the upper layer.

Fig. 6 : illustrates the change in reflection due to transition from unrecorded to recorded area on the lower layer.

Fig. 7 : illustration of one embodiment of the method for optimisation of light power of a beam during reading or writing of an optical record carrier.

DETAILED DESCRIPTION OF THE INVENTION For rewriteable optical storage media and optical record carriers, recorded clusters are often overwritten. Recorded clusters in a data layer will exhibit (usually) less reflection than unrecorded clusters. As an example, the reflection from a data layer differs 50% between unrecorded and recorded areas (in the case of rewritable dual layer DVD and BD). Transmission of a layer is also affected by the recording status of a region. In another example, the transmission through a data layer differs in the range of 5% to 20% between unrecorded and recorded areas. Therefore the overall profile of a light beam detected after interaction with the optical record carrier becomes a convolution of reflection profile (from the second layer where data is to be written) and transmission profile (from the first layer or layers through which the light passes), modified by the characteristics due to the recording of the data layer. The beam change can be in the range of 10% to 50% but will typically be at least 20% stronger for reflection than for transmission, and the reflection change (usually a decrease) will also be more abrupt, due to the focussing difference between the data layer where the beam is focussed and other layers where the beam is out of focus. The two effects can be distinguished and deconvo luted according to the invention, which is applicable to device or method operation for overwriting data clusters on recorded media. Reflection profiling (predetermining what happens to the reflection characteristics of a layer under specific recording conditions) of a data layer can be used to correct for the optical transmission profile of the other layers. An example where such recording is used is Random Recording Mode (RRM).

The difference in optical transmission between recorded and unrecorded states depends on the specific design of the stack of the optical record carrier. In some cases, it is theoretically possible to design a stack with virtually no transmission difference between recorded and unrecorded states. However in practice this is extremely difficult or nearly impossible to achieve, since other considerations, such as thermal effects, must be taken into account. This is exemplified for high-speed optical discs used in high-speed recording. In high-speed recording, power margins for recording are very small, making it important to use the optimum write power.

In prior art methods for power optimisation the power of the light beam used for write or read operations is determined on the basis of calibrated measurements of the light returned for different recording states on the disc of the layer or layers through which the light beam must pass before accessing the target data layer. For example, an unrecorded cluster can be used as an optical power correction (OPC) area by writing marks in this cluster at a number of different power settings. The marks are subsequently read back, and the power corresponding to the best mark quality is chosen as optimum power. However this is a time- consuming process, which cannot be used for all applications e.g. real-time recording of video at high data rates. Furthermore, the marks for OPC reduce the user storage capacity of the optical medium.

An unrecorded region in a layer will allow more light to pass than a recorded layer or region. Thus light will be attenuated by a recorded region and less light will be available at the data layer where the write or read operations must take place. In order to compensate for the reduction, the power of the light beam must be increased to an optimum level so that write or read operations are within the required tolerences for quality. The light is then further attenuated when the beam reflects back from the data layer and passes through the attenuating recorded layer for a second time before being detected. For write mode in the prior art, the data layer is initially unrecorded and for read mode the data layer is recorded. Thus the initial state of the data layer does not change during operation. The power optimisation is therefore purely based on transmission effects due to the layer or layers adjacent to the data layer where operation must take place. For discs which are re-recordable or erasable, however, the above situation is too limited and prior art methods fail. The data layer to be written to or read from will have recording characteristics which include regions where some processing not present in the prior art situation has taken place. For example, the write operation may have to take place on a part of the data layer which has already been recorded. In this region, the reflection of the light at the data layer will be different for the unrecorded regions and the previously recorded regions. These differences will change the light arriving back at the detector and will be added to the transmission changes effected by the surrounding layers. Thus the reflection and transmission changes are coupled together and the detector can only receive the total light information. Changes made to the power of the light become a result of both reflection and transmission instead of only transmission thereby rendering the power correction inaccurate and adversely affecting the correct operation of the power optimisation procedure in write and read modes.

As an example of issues which may arise, Figure 1 shows an illustrative example of the relative intensity noise as a function of read power. It is clear from the figure that the lowest values of noise (best read conditions) occur for optimum values of read power in a region indicated by double arrow 11. For lower read power (indicated by arrow 12), increased noise levels are seen and jitter rises, whereas at higher powers (indicated by arrow 13) a problem of repeated read is seen. If the transmission through the upper layer reduces the laser power (for example when reading out data through a recorded layer in a dual layer rewritable disc) it is important to compensate for this power loss, otherwise the signal to noise ratio (SNR) of the detector is reduced which results in increased jitter. The repeated read is an important parameter for both recordable and rewriteable discs and is extremely sensitive to power variations. An illustrative example of how the repeated read varies with read power on a single layer BD-RE disc is shown in Fig. 2. It is important that read power should be calibrated correctly when reading out the lower layer in a dual layer disc where the upper layer is partly written. This is an application suited to the invention.

Fig 3 illustrates a section through an optical record carrier 35 with several data layers. The optical record carrier layers are partially recorded according to RRM. The light is focussed by an objective lens 30 on the data layer 31 as this is where a recording operation will take place. The light beam 34 is defocused at the other data layers 32 and 33. Recorded 36 and unrecorded 37 sections are shown on the data layers 31, 32 and 33, defined by different shadings. The light beam, illustrated by the beam sections 34A, 34B, 34C and 34D, interacts with the different data layers 31, 32 and 33 in different ways depending on the presence or absence of data or recording marks on a layer at the points of interaction. Each interaction will modify the beam depending on whether the beam encounters recorded or unrecorded sections. Especially the intensity of the beam is altered. Beam section 34A encounters one recorded region and one unrecorded region on its way to the data layer 31. Beam section 34B encounters two recorded regions. Beam sections 34C and 34D both encounter two unrecorded regions. The detected beam is a sum of all beam sections.

In addition, the beam will be reflected (not shown) from the data layer 31 and the intensity will thus be modulated by the reflection characteristics of the data layer 31 as well as by the transmission effects of the upper layers as described above. For correct power optimisation of the light power, to reach the data layer 31 and perform a write operation, all of the transmission effects just described must be known for correction and the reflection effects must be accounted for.

Figure 4 illustrates the effect of transmission differences on a readout signal from a dual layer BD-RE disc. The data is being read through a top layer which is either fully recorded (indicated by arrow 41) or fully unrecorded (indicated by arrow 42). The difference in the readout signal between top layer recorded or unrecorded can be seen. There is a clear difference in signal strength 43 and a sharp transition region 44.

In rewriteable optical storage media, recorded data clusters may be directly overwritten (without having first been erased). These clusters in the lower layer will exhibit less reflection than unrecorded clusters (in high-to-low recording; visa versa in low-to-high recording).

The ideas discussed above are illustrated in Figures 5 and 6. Figure 5 illustrates the change in reflection due to the change in transmission of the upper layer. Figure 6 illustrates the change in reflection due to transition from unrecorded to recorded clusters on the lower layer.

Figure 7 illustrates one embodiment of the invention applied as a method of optimisation of light power according to the explanations outlined above. Here the object of the method is to optimise light power during reading or writing of an optical record carrier comprising a first data layer and a second data layer comprising recorded data. The beam is first transmitted through the first data layer 71 in order to reach a second data layer to be written or read. For effective operation at the second data layer the beam is focussed at this layer 72. The second data layer reflects the beam back 73 and the beam is directed such that it passes again through the first data layer 74. Thus the beam is transmitted twice by the first data layer with two transmission effects incorporated into the light properties. The beam, having been reflected and transmitted, is then detected 75. Deconvo luting the detected beam into a deconvo luted signal to distinguish a first light signal resulting from the reflection of the beam at the second data layer from a second light signal resulting from the transmission of the beam through the first data layer 76 allows separation of the reflective and transmittive effects. Based on the deconvo luted signal, optimisation of the light power can be done 77.

LIST OF REFERENCE NUMERALS :

11 double arrow indicating optimum values of read power

12 arrow indicating values of read power corresponding to increased jitter 13 arrow indicating values of read power corresponding to repeated read problem

30 objective lens

31 second data layer 32 a first data layer

33 a first data layer

34A, 34B, 34C and 34D beam sections

35 optical record carrier

36 recorded section 37 unrecorded section

41 arrow illustrating signal from recorded region

42 arrow illustrating signal from unrecorded region

43 difference in signal strength 44 sharp transition

71 method step

72 method step

73 method step 74 method step

75 method step

76 method step

77 method step

Claims

CLAIMS :

1. A method for optimisation of light power of a beam during reading or writing of an optical record carrier comprising a first data layer and a second data layer comprising recorded data, the method comprising the steps of:

- transmitting the beam through the first data layer - focussing the beam at the second data layer

- reflecting the beam at the second data layer,

- detecting the beam, the method further comprising the steps of :

- optimising of the light power based on the deconvo luted signal.

2. A method as claimed in claim 1, wherein the deconvo luted signal is the second light signal.

3. A method as claimed in claim 1, further comprising the step of : - monitoring the detector signal for a change with a gradient

- initiating deconvo lution based on the presence of the change with a gradient.

4. A method as claimed in claim 2, further comprising the steps of :

5. A method as claimed in claim 4, further comprising the step of : - storing the threshold value in an information area of the optical record carrier.

6. A method as claimed in claim 4, further comprising the step of : - storing the threshold value in a non- volatile memory of an optical recording device comprising means to optimise light power and cooperating with the optical record carrier.

7. A method as claimed in claim 3 wherein the gradient is an absolute gradient.

8. A method as claimed in claim 1, further comprising the step of :

9. An optical recording device, arranged to cooperate with an optical record carrier for read and write operations, the optical record carrier comprising a first data layer and a second data layer on which data has already been recorded, a beam with a light power being transmitted through the first data layer to be focussed and reflected at the second data layer and then transmitted a second time through the first data layer, the optical recording device further comprising

- a detector to provide a detector signal,

- a control unit arranged for optimisation of the light power, based on a control unit input signal, derived from the detector signal, characterised in that the optical recording device further comprises:

- a deconvolution means coupled to the detector and arranged for creating a deconvo luted signal distinguishing in the detector signal between a first light signal due to the reflected light at the second data layer and a second light signal due to light transmitted through the first data layer, the deconvolution means being coupled to the control unit to provide the control unit input signal based on the deconvoluted signal.

10. A method as claimed in claim 9, wherein the deconvoluted signal is the second light signal.

11. An optical recording device as claimed in claim 9, wherein the deconvolution means further comprises a monitoring means to detect a change with a gradient in the detector signal.

12. An optical recording device as claimed in claim 11, wherein the means for deconvolution further comprises removal means for removing a calibrated reflection response representing the first light signal from the detector signal when the monitoring means detects the change with a gradient exceeding a threshold value.

13. An optical recording device as claimed in claim 11 or 12, wherein the gradient is an absolute gradient.

14. An optical recording device as claimed in claim 12 wherein the threshold value is stored in a non- volatile memory of the optical recording device.

15. An optical recording device as claimed in claim 9, wherein the device further comprises a monitoring means to detect a change in an intensity of the detected beam.

16. An optical recording device as claimed in claim 15, wherein the means for deconvolution further comprises removal means for removing a calibrated reflection response representing the first light signal from the detector signal when the monitoring means detects the change in intensity exceeding a target value.