WO2006075257A1

WO2006075257A1 - Device and method for an inverse write strategy for a phase change optical storage medium

Info

Publication number: WO2006075257A1
Application number: PCT/IB2006/050012
Authority: WO
Inventors: Robertus A. J. Van Kollenburg
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-01-12
Filing date: 2006-01-03
Publication date: 2006-07-20
Also published as: TW200639834A

Abstract

The present invention relates to a recording device and method for recording an information signal on a phase change layer of an optical storage medium. The device comprises erasing means and writing means arranged to apply a first and a second electromagnetic beam, respectively, to the phase change layer along a track within the optical storage medium. Control means are associated with the erasing means and writing means, to control the first and second electromagnetic beams so that the first electromagnetic beam has an erase power level Pe adapted to bringing the phase change layer over at least a portion into an amorphous state, and so that, the second electromagnetic beam has a write power level Pw adapted to bring the phase change layer into a crystalline state while being guided along the track and being modulated according to the information signal so that crystalline domains are formed.

Description

Device and method for an inverse write strategy for a phase change optical storage medium

The present invention relates to a recording device (recorder) and a method for recording an information signal on a phase change (PC) layer of an optical storage medium (disk).

PC recording is applied in case of several rewritable optical disks such as CD- RW, DVD-RAM, DVD-RW, DVD+RW, and BD-RE. PC layers commonly comprise a PC material, which typically is an alloy with a durable polycrystalline structure sandwiched in a stack between two dielectric ZnS-SiO2 layers (recording stack). PC materials for rewritable discs presently fall into two categories, namely materials such as ternary Ge2Sb2Te5 alloys, in which crystal growth is initiated by nucleation within the material, and fast-growth materials such as doped Sb-Te alloys (e.g. AgInSbTe or GeInSbTe), in which crystallization of amorphous area is initiated by the surrounding crystalline environment. For land and groove recording formats such as DVD-RAM, Ge2Sb2Te5 PC materials have been preferred because of their excellent direct-overwrite (DOW) capability. For groove-only recording formats such as DVD+RW, developed to maintain backward compatibility with DVD-Video and DVD-ROM players, the crystallization mechanism of Ge2Sb2Te5 alloys, however, provides insufficient linear density to achieve the required data capacity. The DVD+RW format was made possible by the use of doped Sb-Te alloys. Their crystallization mechanism allows data to be written with much higher linear density, enabling the DVD data capacity of 4.7 Gbyte to be achieved. During recrystallization, however, doped Sb-Te alloys form a mixture of crystalline phases which are generally assumed to have a negative impact on direct overwrite cyclability.

While the disk is moved with respect to a focused electromagnetic laser beam the latter modulated by the information signal which is to be recorded (information signal herein shall be understood as a signal carrying any kind of information including user data, control data, identification data, or the like) will principally be absorbed along a given track in the recording stack by the PC material. As a result, the alloy which in its ground state has a crystalline phase is locally heated. When the temperature exceeds its melting point (about 500° C to 700° C) the PC material converts to an amorphous phase. Rapid heat dissipation through adjacent dielectric layers causes a fast cooling of the alloy, thereby stabilizing the amorphous phase. Thus, written marks remain along the track.

Applying a laser beam with a reduced power allows to erase written marks. Thereby, the PC layer is locally heated to a temperature of about 200° C inducing a phase change back to the crystalline phase (annealing). Since the atoms must be kept at elevated temperatures long enough to re-crystallize, crystallization is a rather slow process although amorphization in PC media can be very rapid.

The PC layer shows different indices of refraction in its crystalline phase and its amorphous phase (written state). The index of refraction mismatch between the PC layer and the adjacent dielectric layers, therefore, causes different reflectivity of the crystalline phase (ground state) and the amorphous phase. Whereas the PC layer, or more precisely the recording stack backed by a metal mirror, has a high reflectivity in the crystalline phase, its reflectivity is reduced in the amorphous phase. A reading beam focused on said recording stack is reflected by the recording stack with different intensity depending on whether it strikes a written mark (pit) or an unwritten area (space).

On the way to faster re-writing (i.e. erasing and writing) on PC optical disks and to higher data capacity a variety of problems must be solved. One of these problems mainly refers to the write strategy. At a given crystallization time, for example, it must be ensured that the linear velocity of the storage medium relative to the laser beam matches so that old data, i.e. amorphous marks, from a previous recording can be completely erased and overwritten. Otherwise thermal interference occurs, for example, after one mark is written, and a second mark is written next to it before the first has time to cool sufficiently. The residual heat can cause changes in the separation between the two marks, and in mark length which leads to jitter problems. It is to be noted that the jitter (the standard deviation of the time variation of the binarized data passed through the read channel) of the leading and the trailing edges of the marks measured relative to the PLL clock according to present standards shall not exceed 8.0% of the Channel bit clock period.

On the other hand, remaining old data fragments cause a high noise level which especially causes problems for recorders that use defect management, such as recorders implementing the Mount Rainier standard where a high noise level can lead to many bad block replacements, for example. The noise level, in particular, reduces the optical contrast, i.e. the attenuation of the reflected light when the reading incident beam strikes an amorphous mark normalized by the crystalline state reflection, and thus the best signal modulation. Current write strategies use a multi-pulse writing technique to overcome some of the above problems, so that a single mark is created by a multiple- short-pulse laser beam. When a short pulse hits, the PC layer melts at a low energy level so that when the pulse ends the PC layer material cools quickly and becomes stable in its amorphous state. Several pulses are repeated to form a long string of small marks close to each other, representing a single long recording mark.

The write strategy using a repetition time of one pulse per channel clock period is called a IT strategy. As recording speed increases, however, it becomes more difficult for the heat of one mark to dissipate before the next mark is written. Therefore, if the speed is increased, even with the IT strategy, it will be impossible to assure sufficient time to stabilize the amorphous state. For example, known PC layers need a cooling time of about 5ns while at 16-fold speed the time period will be 2.4 ns. Therefore, sophisticated 2T or 3T pulse strategies, block writing strategies with a low-power continuous laser beam or even multiple beam DOW strategies with a leading beam for re-crystallizing and a second beam for (over)writing are being investigated.

The present invention solves the above problems in an alternative way. According to a first aspect of the invention this object is achieved by a recording device as described in the opening paragraph comprising erasing means arranged to apply a first electromagnetic beam to the phase change layer along a track within the optical storage medium, writing means arranged to apply a second electromagnetic beam to the phase change layer along the track within the optical storage medium, and control means associated with said erasing means and said writing means arranged to control the first and second electromagnetic beams, so that the first electromagnetic beam has an erase power level P_e adapted to bring the phase change layer into an amorphous state while being guided along the track over at least a portion to be recorded, and so that, the second electromagnetic beam has a write power level P_w adapted to bring the phase change layer into a crystalline state while being guided along the track over said portion and being modulated according to the information signal so that crystalline domains are formed within said portion to be recorded. While in all state of the art recorders a PC storage medium, e.g. a DVD or BD, is erased by applying a laser beam with sufficient energy to locally heat the PC layer to its annealing temperature and keep it at this elevated temperature long enough to recrystallize, according to the invention the recorder is arranged to erase old data on the disk by amorphization of the PC layer along the track over the entire portion to be recorded.

In contrast, writing according to the invention means re-crystallization of domains along the amorphous track in order to form a pattern that corresponds to the information signal instead of melting the PC material to form such an inverse domain pattern. In other words, the present invention utilizes an inverse erase and write strategy. Since the entire track portion is first melted old data are reliably and quickly destroyed without leaving any fragments. Consequently, the high noise level problem will be minimized. Another advantage is that also the thermal interference occurring between two subsequently written adjacent domains or marks can be minimized during writing, since the write power and the transmitted energy is much lower and the heat crosstalk between adjacent tracks or marks can be significantly decreased. This reduces the jitter problem mentioned above.

It is to be noted that the PC layer material in its ground state has a crystalline phase. Therefore, first writing requires pre-amorphization. According to a second aspect of the invention which constitutes a further development of the first aspect, said erasing means comprise a leading radiation source for generating the first electromagnetic beam and said writing means comprise a trailing radiation source for generating the second electromagnetic beam.

By utilizing two separate radiation sources, such as laser diodes, the recording device can erase and write data to the storage medium in a single sweep. Optical means such as lenses, beam splitters, and the like may be designed as shared components and/or can be provided separately along the light paths of the first and second beams.

According to a third aspect of the invention which constitutes a further development of the first aspect said recording device comprises a common radiation source shared by said erasing means and said writing means for generating the first and second electromagnetic beams.

According to this embodiment the radiation source may be a single beam laser diode. In this case appropriate means, such as a beam splitter, lenses or the like, may be provided for generating beams at erase power level P_e and write power level P_w, respectively, on the basis of time and/or energy division. Preferably, the radiation source is a multiple beam laser diode generating at least two beams at different output powers levels. The first and second beams may further be divided in space by appropriate means guiding the beams along different optical paths. According to a fourth aspect of the invention, which constitutes a further development of the third aspect, said control means are arranged to subsequently actuate said common radiation source in order to generate the first electromagnetic beam while being guided along the track over at least said portion to be recorded and afterwards to generate the second electromagnetic beam while being guided along the track over said portion.

Instead of separating the two electromagnetic beams in space by using different sources and/or light paths, according to this embodiment there is a separation in time. The laser beam is scanned twice along the track in the portion to be recorded. During the first scan old data are erased by melting the PC material with the first laser beam having an erase power level P_e. During the second scan the second laser beam having a write power level P_w and being modulated in accordance with the information data to be written warms up the PC material in domains until it reaches its annealing or re-crystallization temperature. In comparison, this procedure may take some more time than erasing and writing in a single sweep but, in particular when old data are overwritten, its reliability increases. According to a fifth aspect of the invention, which constitutes a further development of any one of the first to fourth aspects, said control means are arranged to control the second electromagnetic beam such that, in response to the information signal, the crystalline domains within said portion are assigned to spaces and amorphous intervals between consecutive crystalline domains within said portion are assigned to pits. Domains and intervals between consecutive domains within the portion to be recorded represent the information signal. The marks recorded by the recording device according to the invention is the same as that which can be obtained by a device utilizing a straightforward rewriting process where starting from a (re)crystallized track pits are written in the form of amorphous domains leaving crystalline spaces in between. Thus, the result is in accordance with given CD, DVD and BD standards.

According to a sixth aspect of the invention the object is further achieved by a method as described in the opening paragraph comprising irradiating a first electromagnetic beam along a track onto at least a portion of the phase change layer to be recorded, thereby bringing said portion along the track into an amorphous state and subsequently irradiating a second electromagnetic beam modulated according to the information signal along the track onto said portion, thereby forming crystalline domains along the track within said portion.

Subsequently herein includes different write strategies, namely, subsequent in time and/or subsequent in location. According to a seventh aspect of the invention which constitutes a further development of the sixth aspect the first and second electromagnetic beams are irradiated simultaneously (at the same time) and in series (to different locations) on said portion and domains, respectively. According to an eighth aspect of the invention, which constitutes a further development of the sixth aspect, the first electromagnetic beam is irradiated along the track on the complete portion before the second electromagnetic beam is irradiated along the track on said domains within the whole portion.

According to a ninth aspect of the invention, which constitutes a further development of any one of the sixth to eighth aspects, the crystalline domains within said portion are assigned to spaces and said amorphous intervals between consecutive domains within said portion are assigned to pits.

Again, the same result can be obtained by this method as that obtained by a straightforward rewriting process where starting from a (re)crystallized track pits are written in the form of amorphous domains leaving crystalline spaces in between. Thus, the result is in accordance with given CD, DVD and BD standards.

According to a tenth aspect of the invention, which constitutes a further development of any one of the sixth to ninth aspects, the method further comprises modulating the erase power level P_e of the first electromagnetic beam according to additional information, the modulation having a frequency lower than the frequency of the modulation according to the information signal.

In order to achieve a good erase result the temperature in the PC has to exceed the melting temperature of the PC material. Nevertheless, the erase result will not be perfect, in particular there is always a significant reflection also at amorphous regions of the PC layer. Hence, the optical contrast and thus the read out signal modulation never reaches 100%. According to present standards the modulation must be bigger than or equal to 60%. By modulating the erase power level P_e and/or the write strategy, this read out signal modulation can be influenced, e.g. in an interval between 65% and 70%. If this variation is small enough, it will hardly influence the optical properties, so that it does not disturb the information signal read out. However, read-out of the additional information can be achieved e.g. using peak detection means (integrating means) and a low-pass filter.

In this way, while erasing old data along the track in at least a portion of the disk it is possible to vary slightly the erasing power level P_e of the first electromagnetic laser beam, thereby storing additional information. This variation leads to slightly fluctuating optical properties of the amorphous PC layer along the track. After writing the crystalline domains according to the information signal to be recorded, the fluctuation can still be observed if its frequency is below that of the information signal corresponding pit and space frequency.

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings in which Figs. IA, IB show schematic diagrams of the recording device according to a first embodiment of the invention; and

Fig. 2 shows a schematic diagram of the recording device according to a second embodiment of the invention.

The recording device 100 according to Fig. IA and IB has a most simple design. Erasing means and writing means 110 share the same radiation source 112 and the same optical elements, for simplicity represented by lens 114, only. Accordingly, the first and second electromagnetic beams from the common radiation source 112 up to the spot on an optical disk they are focused at, have the same light paths 116, 117. The rewritable optical phase change disk 120 is composed of several layers in a known manner. The beams are focused at the recording stack, for simplification represented by a single phase change layer 122.

While the disk 120 rotates in the direction of arrow 124 driven by a motor (not shown) the laser beam is tracked in a radial direction by commonly known means such as a multiple-array detector and control means (not shown) for generating a tracking signal in order to follow the track on the disk. In Fig. IA the erasing beam is illustrated deleting old data represented by amorphous domains 126 (pits) and crystalline intervals 127 (spaces) in a first scan by overwriting the entire track or at least a pre-selected or preset portion thereof with a continuous amorphous mark 128. Subsequently, after the old data are completely overwritten, crystalline domains 130 are formed within the amorphous mark 128 by applying the modulated writing beam in a second scan, see Fig. IB. Together with the remaining amorphous intervals 132 the crystalline domains 130 represent the new data. The modulation of the writing beam in accordance with an information signal will be described next. Control means 150 are arranged to receive the information signal 152. In the control means a pattern generator 154 is provided connected with its input to a first output of a current source 156. Through this channel the current source 156 produces a first current adapted to generate a laser beam having the appropriate write power level P_w. The pattern generator 154 transforms the information signal 152 into a control signal S₀ comprising sequences of write pulses in conformity with the desired pattern representing the information to be recorded. The current source 156 further has a second output delivering a second current which, when fed to the radiation source 112 through a control signal S₀, will result in the electromagnetic beam having an appropriate erase power level P_e. The second output current is input into modulation means 158 for generating a control signal S₀ on the basis of the erase power level P_e. The modulation means 158 modulate the erase power Pe in accordance with an input additional information signal 160. A switch unit 162 can select the output current of the pattern generator and the modulation means. Although the erasing means and writing means in the above embodiment share the same radiation source and some or all of the optical elements, although these components are physically the same, the erasing means and writing means at least differ functionally depending on the control signal S₀.

In the embodiment of the recording device 200 according to Fig. 2, two radiation sources are provided instead of a common radiation source shared by the erasing means and the writing means. A leading radiation source 212 assigned to the erasing means generates the first electromagnetic beam 216 in accordance with the control signal S_ce. A trailing radiation source 213 assigned to the writing means generates the second electromagnetic beam 217 in accordance with the control signal S_cw- The erasing means and the writing means share a common lens 214. Other optical means (not shown for simplicity reasons) may be shared as well in order to minimize the dimensions of the erasing and writing means. The optical paths of both laser beams are locally separated. The first (erasing) beam 216 with respect to the direction of rotation indicated by arrow 224 (i.e. with respect to the location of its focus) hits the disk first. In the illustration the erasing beam thereby deletes old data represented by amorphous domains 226 (pits) and crystalline intervals 227 (spaces) by overwriting the entire track or at least a pre-selected or preset portion thereof with a continuous amorphous mark 228. At the same time and at a succeeding location of the disk 220 with respect to the direction of rotation 224, the second (writing) beam 217 forms crystalline domains 230 within the amorphous mark 228 modulated according to the input information signal 252. Together with the remaining amorphous intervals 232 the crystalline domains 230 represent the new data. The locations of both beam foci can be separated by a predetermined parallel offset along the track, as illustrated in Fig. 2, and/or by a predetermined lateral offset of a number of tracks. The separation can be chosen depending on the thermal properties of the PC layer such as cooling time and the like.

Control means 250 according to the second embodiment are composed of essentially the same components as before. A pattern generator 254 provided in the control means receives the input information signal 252 as well as a first output of a current source 256 providing a first current adapted to generate a laser beam having the appropriate write power level P_w. The control signal S_cw is generated by the pattern generator in conformity with the desired pattern representing the information to be recorded. The second output of the current source 256 delivers a second current adapted to generate a laser beam having the appropriate erase power level P_e to modulation means 258 for generating a control signal S_ce. Thereby, modulation means 258 modulate the erase power P_e in accordance with an input additional information signal 260. The output current of the pattern generator 254 and the modulation means 256 are input into a switch unit 262 which selectively applies the erase power control signal S_ce to the leading radiation source 214 of the erasing means and the write power control signal S_cw to the trailing radiation source 213 of the writing means.

It will be apparent to those skilled in the art that in the above embodiments of the recording device according to the invention the control means may be composed in a different way. For example, it may be realized without modulation means for additional information. More power levels may be provided by further outputs of the current source 156 or by several power sources and by extending the switch unit 162 in order to generate, for example, beams at bias power level or any kind of intermediate power level. The control means may also comprise setting means for setting the current of output B of the current source in dependence on the recording speed. The control means can further be designed to output a control signal such that the first and/or second beams are pulsed beams. In this way pulsed and/or continuous write and erase strategies can be implemented.

Also, the erasing means and/or writing means are not limited to the above embodiments. As already mentioned the optical elements along the light path(s) may comprise a mirror, a beam splitter, focusing means, tracking means and a one or more lenses or a lens array. Particularly some of these components may be arranged in parallel with the rotating disk in order to gain a minimum installation height. Appropriate measures will be apparent to those skilled in the art.

Claims

CLAIMS:

1. Recording device for recording an information signal on a phase change layer of an optical storage medium, the device comprising erasing means arranged to apply a first electromagnetic beam to the phase change layer along a track within the optical storage medium, writing means arranged to apply a second electromagnetic beam to the phase change layer along the track within the optical storage medium, and control means associated with said erasing means and said writing means arranged to control the first and second electromagnetic beams, so that the first electromagnetic beam has an erase power level P_e adapted to bring the phase change layer into an amorphous state while being guided along the track over at least a portion to be recorded, and so that the second electromagnetic beam has a write power level P_w adapted to bring the phase change layer into a crystalline state while being guided along the track over said portion and being modulated according to the information signal so that crystalline domains are formed within said portion to be recorded.

2. Recording device according to claim 1, wherein said erasing means comprise a leading radiation source for generating the first electromagnetic beam and said writing means comprise a trailing radiation source for generating the second electromagnetic beam.

3. Recording device according to claim 1, comprising a common radiation source shared by said erasing means and said writing means for generating the first and second electromagnetic beams.

4. Recording device according to claim 3, wherein said control means are arranged to subsequently actuate said common radiation source in order to generate the first electromagnetic beam while being guided along the track over at least said portion to be recorded and afterwards to generate the second electromagnetic beam while being guided along the track over said portion.

5. Recording device according to any one of the preceding claims, wherein said control means are arranged to control the second electromagnetic beam so that, in response to the information signal, the crystalline domains within said portion are assigned to spaces and amorphous intervals between consecutive crystalline domains within said portion are assigned to pits.

6. Method of recording an information signal to a phase change layer of an optical storage medium, the method comprising irradiating a first electromagnetic beam along a track on at least a portion of the phase change layer to be recorded, thereby bringing said portion along the track into an amorphous state and subsequently irradiating a second electromagnetic beam modulated according to the information signal along the track on said portion, thereby forming crystalline domains along the track within said portion.

7. Method according to claim 6, wherein the first and second electromagnetic beams are irradiated simultaneously and in series on said portion and domains, respectively.

8. Method according to claim 6, wherein the first electromagnetic beam is irradiated along the track on the complete portion before the second electromagnetic beam is irradiated along the track on said domains within the whole portion.

9. Method according to any one of the preceding claims, wherein, the second electromagnetic beam is irradiated in a pattern so that crystalline domains within said portion are assigned to spaces and amorphous intervals between consecutive domains within said portion are assigned to pits.

10. Method according to any one of the preceding claims, comprising modulation of the erase power level P_e of the first electromagnetic beam according to additional information, the modulation having a frequency lower than the frequency of the modulation according to the information signal.