Optical data storage medium

Abstract: A bi-level optical data storage media is disclosed. The media includes a flat substrate pattern with a regular periodic pattern of lands and grooves, each land and groove having approximately equal widths. Information wobble marks are added to the lands and grooves as deviations in a pathic grooves to a first or second side. The deviation is by a fixed amount identical in magnitude for each wobble mark and having a short duration along the direction of the grooves. Additionally, grooved depths are provided in the grooves as are wobble mark deflection distances being placed along side the grooves. Both the groove depths and the wobble mark deflection distances are chosen to give useable reflectivity signal amplitude from the wobble marks when since during playback. Additionally, first wobble mark encodings are provided that designate a physical location of a length of groove or land as distinguished from other lengths grooves or lands. These are placed at wobble positions with or without a wobble mark, these wobble positions being aligned perpendicular to the direction of the grooves on each grooves and for all grooves in a group of grooves. Furthermore, second wobble mark encodings are provided opposite appropriate the first wobble mark encoding. Lastly, the media further includes a plurality of first and second wobble mark deflections extending over a short length of a groove. These deflections prevent a push/pull position error signal from being bias into indicating an erroneous center position perpendicular to the groove direction for a given average.

Description: The present invention relates in general to an optical data storage medium and to a mass memory optical disk data storage system for use in a data processing system and, more particularly, to a method of formatting embossed header patterns on the optical media, such as a disk. More particularly still, the present invention relates to providing embossed header format on bi-level optical media to reduce adjacent track crosstalk during a header play back operation of the optical disk. Description of the Related Art: One type of optical disk drive, such as those using either ISO standard 130 mm rewritable or write once, read many (CD-WORM) optical disks, uses a push/pull tracking method. These optical disk types have guide grooves or so-called pre-grooves formed previously along with the formation of sector header information on the disk. These grooves are irradiated with a spot of coherent light to sense the center position of either the grooves or the land between grooves. The unbalance of the distribution of the reflection of light defracted from the pre-grooves is used to generate the track position error signal. The track position error signal is fed back to a servo system to control the spot to follow either the center of the grooves or the center of the land between grooves. This method for generating the position error signal is often called the push/pull tracking method. To increase the storage capability of optical disks, the recorded area on the disk has been expanded to include bi-level optical media. Bi-level optical media is media that has'equal land and groove spacing such that data can be recorded on both the lands and the grooves, which allows greater track density on the optical media for a given laser wave length using the push pull tracking method. With this increased track density, the track pitch becomes narrow relative to the read spot size generated by the laser beam and laser optics. This narrow track pitch leads to significant adjacent track signal interference for both the read only header and read/write user information. There are several possible methods for cancelling the adjacent track interference for the read/write user information on the disk. For example bi-level media can be designed to cancel the adjacent track interference of magnetic or reflective change marks by adjusting the depth of the groove of the biplanar media. Magnetic super resolution is another method that can be used to mask the adjacent track interference for magnetically recorded marks. However none of these methods helps to cancel the adjacent track interference for the read only header part of the media format because this information is formed by embossed features rather than by magnetic or reflectivity change marks. Because of this limitation, the master written header for the narrow track pitch bi-level media substrate must be encoded and read in such a way that the sector header information can be reliably read by a single read spot in the presence of significant adjacent track interference. European Patent Application EP-A-0 588 305 reflecting the preamble of claim 1 discloses an optical data storage medium comprising a flat substrate in which a periodic pattern of lands and grooves are formed defining respective paths for bi-level recording tracks, each land and groove having substantially equal widths and said grooves having respective pre-determined depths, said recording medium further comprising: wobble mark encodings formed on alternate tracks, which in one instance includes a first wobble mark comprising a land in a groove in register with a second wobble mark comprising a groove in a land along the length of a track, said wobble marks deviating from the path of the track by an identical fixed amount; said wobble mark encodings being defined in said tracks at a minimum deflection distance from one another, said minumum deflection distance and said groove depths giving a usable reflectivity signal amplitude to a conventional sensing system and sufficient position error signal when defraction light distribution reflected from said grooves is sensed by said sensing system; said wobble mark encodings being grouped to designate track and sector positions, each group comprising a plurality of wobble mark encodings spaced at multiples of said minumum deflection distance. The wobble mark encodings of the prior art on one track are not balanced with the wobble mark encodings on the alternate track. Thus, if the wobble marks are formed in the lands, they do not facilitate reading track centre on the grooves and vice versa. Hence the conventional sensing system has to be adapted to ensure that an erroneous centre position perpendicular to the track direction is not indicated. Accordingly, the present invention is characterised in that each group of wobble mark encodings comprises an equal number of lands in a groove and grooves in a land such that said sensing system is not biassed into indicating an erroneous center position perpendicular to a track direction. The grooves may be circular, spiral, or straight lines and the substrate may be either a disc or rectangularly shaped. The substrate is also coated with a magnetic super resolution comprising a magneto optical recording film. The groove depths are determined primarily by the minimization of adjacent track interference of reflectivity marks from a phase change recording film, an ablative write-once film or from a magneto optical recording film. Typically, the wobble mark information is encoded by the presence (1) or absence (0) of the read-back signal change in amplitude with variable length and at specific locations on the media. Alternatively, the wobble mark information may be encoded by the center position of the read-back signal from individual wobble marks relative to one another. Further still, the wobble mark information may be encoded by leading and trailing edges of the read-back signal from the wobble marks relative to one another. Wobble mark encoding is also intended to be used with a synchronous detection channel for decoding the sector and/or track address information. The encoding uses an N out of M code in which M is greater than N and the read-back detection is done by picking the code word that corresponds to the N largest amplitude signals at the read-back wobble mark center positions for each code word. The wobble mark encoding may use a synchronous detection channel for decoding the sector or track address information or both within a wobble mark pattern. The wobble mark pattern typically includes a radially aligned wobble mark tone pattern for phase synchronizing a data channel clock to be aligned with the subsequent data wobble marks positions. The wobble mark pattern further includes an address mark feature for determining which channel clock cycle is at the first bit position of the wobble mark data and a sequence of synchronously decoded wobble marks that determine a unique sector address on the media. While using the wobble mark patterns, this provides for redundant recordation to allow reliable sector address determination in the presence of media defects. Finally, the land and groove widths typically are approximately a laser full width half maximum diameter for a laser spot used during playback to sense the features on the substrate of the media. A method for mastering the optical data storage medium for low noise is also disclosed. The method includes applying an uniform thickness of photo resist on a master substrate. Next, the method exposes the photo resist to define grooves and wobble mark locations using two or more side by side defraction limited, focused laser spots. These laser spots are aligned so that lines between all spot centers are perpendicular to the direction that the grooves are to be formed. Then, the photo resist is exposed so as to reach the master substrate so that a steep light intensity gradient from the laser spots defines the edges of the grooves. Next, the exposed photo resist is developed such that a top surface land is defined by the under-exposed surface of the photo resist such that a bottom surface of the groove is defined by the master substrate and such that wall edges are steep and as smooth as possible. Finally, the master photo resist topology is replicated to form a substrate for the mass production of the optical media. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 depicts in accordance with a preferred embodiment of the present invention a schematic implementation of an optical system and associated photo diode and preamplifier electronics that can be used to sense the embossed header features that are part of the invention. Figure 2 is a schematic representation of the functions that have to be performed to decode the sequences of embossed header features that are part of the invention. Figure 3 depicts the embossed feature types and waveforms that arise from the electronic sensing of the reflected light from the features of Figure 2. Figure 4 depicts an embossed tone pattern that is sensed by the system in Figures 1 and 2 to produce a constant frequency data detection clock signal. Figure 5 depicts a five tone cycle length of bi-level media that in conjunction with the tone pattern in Figure 4 constitutes an address mark, which allows the system in Figure 2 to align it counters relative to the embossed track address and sector address features that follow the address mark. Figures 6A-6J show radially aligned 2-out-of-5 encoding for sector address digits 0 to 9 such that 1) each code digit will all have the same number of opposite polarity wobbles and 2) such that the code never results in a decrease of detected signal for the desired pattern due to adjacent track interference because the pattern is the same on every groove and aligned perpendicular to the grooves. Figure 7 shows radially aligned 2-out-of-5 encoding for track address digits 0 to 9 such that 1) each code digit and the normal sequential transitions between code digits (including 9 to 0) all have the same number of opposite polarity wobbles and 2) such that the change in code at the transitions never results in a decrease of detected signal for the desired pattern due to adjacent track interference. Figure 8 shows an example of bi-level media being mastered using a pair of side-by-side small diameter spots that allows for 1) the definition of sharp edges to the grooves due to steep intensity gradients at the groove edge and 2) the high resolution wobbling of the spot pair to encode accurately the wobble marks at a high wobble density in order to make the header as short as possible. Detailed Description of the Preferred Embodiment 1. Detecting the Header Figure 1 is a schematic diagram representing an optical system for recording onto and reproducing from information an optical disk 12. The optical system 10 includes a semi-conductor laser 16, which transmits a laser beam through a collimation lens 18 and through a beam splitter 20. The beam splitter 20 separates the light so that a portion continues towards the optical disk 12, and the remaining portion proceeds to the detector portion of the optical system 10. The light portion projected towards the optical disk 12 passes through an objective lens 22 where it is focused to a point sharp enough to reflect off the surface of the optical disk 12 and be retransmitted through the objective lens and reflected off a mirror within the beam splitter 20 to a detector 24. The detector 24 and amplifier 26, which is coupled to detector 24, process the light received from the laser and reflected from the optical disk 12 to convert it to a corresponding electrical signal on line 28. The reflectivity signal on line 28 only contains a non-constant waveform where there is header information. The header information consists of four parts, a sync field, that is a tone, a constant field that, in conjunction with the end of the sync field, gives an exact location of the start of the header, a decimal encoded sector address, and a decimal encoded track address. The reflectivity signal on line 28 goes to signal decoder 30 illustrated in the block diagram of Figure 2. The purpose of the signal decoded is to recover the sector and track address and send the addresses to the microprocessor 46. Specifically the reflectivity signal on line 28 is split and fed to the tone detector 32 and the phase locked oscillator (PLO) and data separator 34. The tone detector 32 senses the presence of a unique embossed tone pattern from the disk, which signals the start of a header to the header gate generator 36. The header gate generator generates a pulse that has a duration equal to the nominal length of the header information. This header gate pulse from the header gate generator 36 controls the PLO and data separator 32 to start processing data from the reflectivity signal on line 28. The PLO generates a detection clock that is in phase lock with the tone in the sync field, which is the very first part of the header. The data separator uses the detection clock from the PLO to decode the 1's and 0's data from the reflectivity signal on line 28. The data separator does this by looking to see if the reflectivity signal is bigger than a threshold value at the time of the detection clock positive transition. The data separator outputs detected data at the positive detection clock transitions of a 0 bit if the value is below the threshold level and a 1 bit otherwise. The detected data is in synchronism with the detection clock for later decoding. The detection clock and detected data then proceeds to the address mark detector 42, which determines the start of the encoded sector and track data, to the sector address detector 38, which determines which sector address is currently being read, and to the track address detector 40, which determines which track is currently being read. The address mark detector 42 starts the timing gate generator 44, which selects which detected data bits are sector and which detected data bits are track information. Finally the decoded digits from the sector address detector 38 and track address detector 40 are sent to microprocessor 46, which processes the signal and provides appropriate output to the user through an appropriate device. Figure 3 illustrates the allowable header mark combinations as found on optical media 12 and as represented by the embossed header patterns 14. Figure 3 also shows the electrical signals that result from these pattern. Specifically Figure 3A illustrates a bi-level media having equal land and groove spacing with all allowable combinations of groove wobble in the direction perpendicular to the grooves. Note that in this drawing it is assumed that the grooves are mastered in the master writing process, although the lands could be mastered using another mastering process. Figure 3B illustrates a sample signal and detection level corresponding to the data stored in the wobble deflections in the track as followed by the laser spot in Figure 3A. 2. Master Writing the Header In a preferred embodiment the mastering on bi-level media meets the following requirements. First, both the land and groove portion of the optical media should be as flat as possible to minimize any spurious surface noise. Second, the sector (or track plus sector number per track) position information should be encoded in the mastering process, such that it can be read when the read spot is positioned on either the lands and the grooves. Lastly, the mastered header data should not distort or bias the push pull position error signal so that it shifts the sensed track center erroneously toward an adjacent track. The mastering of grooves is done by spinning a uniform photo resist onto a master substrate. The photo resist is then heavily exposed to produce spiral grooves by simultaneous rotation and translation of an illumination pattern from a light source such as, for example, a laser. The exposed photo resist is then developed down to the substrate. Both the top of the photo resist and the substrate can be made very flat and hence will generate little noise. Only the walls between the lands and the grooves will cause noise because they are subject to changes in photo resist exposure sensitivity and intensity variations of the exposing spot of light. The groove edges can be made less dependent to these photo resist and light level variations by making the gradient of light as a function of distance perpendicular to the grooves very large. The gradient is determined by the theoretically diffraction limited intensity distribution (Aries function) for a given exposure width. Hence, if a single exposure spot is used to expose the groove in the bi-level media master, then one is not free to make the intensity gradient steeper. The result of a poorly defined groove edge is "groove noise." This problem can be alleviated by exposing the media with two or more radially adjacent smaller diameter spots as shown in Figure 8. The molds used to fabricate the actual media substrates are replicated from this master substrate topology. The principles of mastering the land and grooves of an optical media are well known to those skilled in the art and do not represent any advances put forward by this invention. The wobble track method is used to encode information in each sector header . The illumination pattern is deflected radially while mastering grooves using an acousto-optical modulator to make a wobble mark. Light modulation may be used in conjunction with the deflection to control the shape of the wobble marks. The wobble track method has the characteristic that it simultaneously places a land in the grooves relative to the nominal groove center while putting a groove in the land relative to the nominal land center. The result is that the wobbled track information can be sensed when tracking on either the lands or the grooves in bi-level media. There are two basic variables in the design of the bi-level media, the groove depth and the distance of the wobble relative to the track pitch. The groove depth must be chosen from the range of depths that give reliable generation of the push pull position error signal. The groove depths must also be chosen from the range of depths that give acceptable cancellation of the adjacent track cross talk signal for the user data. The amount of wobble of the land into groove or the groove into land can be adjusted to give an adequate wobble detection signal, ie. to give a large reduction if reflected light where the wobble occurs. The wobble marks in the preferred embodiment are used as a type of pulse position modulation (PPM), but is not limited to this method. For PPM data encoding, the center of the read back pulse in time relative to other like pulses is the information that is recorded on the media. In this case the centers of the marks are used to keep a clock synchronized to the encoded data and the data is detected synchronously. A wobble is a "1" and no-wobble is a "0" as shown in Figure 3B . The encoding of the wobble bits uses three conditions: The first is track deflection toward the outer diameter (OD), track deflection towards the inter diameter (ID), and no deflection. The track wobble is sensed using a reflectivity detection channel as shown in Figure 1. Thus, it is NOT possible to distinguish between the ID and the OD deflection direction of the wobble. The wobble direction, since the detection channel cannot discriminate between ID and OD wobbles, can be used to insure that the push pull position error signal is balanced on average. Preferably, the number and duration of the OD direction and ID direction wobbles should average to zero over about a few microseconds so that the servo" signal is not disturbed on the average . 3. Encoding the Header Information In the preferred embodiment the encoding of the header information on bi-level media meets the following requirements. Firstly, the code words using ID and OD wobbles should average to zero so that there is no push pull position error signal corruption. Secondly, the header encoding should accurately identify the location in time (or space) of the start of the sector. Thirdly, the header encoding should uniquely identify each sector relative to all other sectors. Fourthly, the header encoding should minimize the amount of mark to mark adjacent track interference by the type and placement of each mark reactive to the marks on an adjacent track. Lastly, the encoding should be capable of having redundancy so that adequately reliable decoding can be done in the presence of media defects and noise. The sector header for the preferred embodiment is similar in structure to the header in a conventional 130 mm ISO standard rewritable optical media. It is as follows: where Sync, AM, S and T are a 40 clock period synchronization mark, a 5 clock period address mark, a 5 clock period sector address digit as part of a multi-digit number, and a 5 clock period track address digit as part of a multi-digit number respectively. A track is defined to be one revolution of the disk so that only the track address changes from one adjacent track to another. The sector number will be the same in each radially aligned row, and as such will not experience much adjacent track interference because there is no pattern change radially. The track address number will also be radially aligned, but will change encoding from track to track. Thus the track address digit encoding will have to be done so as to minimize the effects of adjacent track interference. Separation of the sector location address into the track and sector per track addresses allows the sector per track address to be more efficiently coded as compared to the track address part. This separation is optional for the functionality of the preferred embodiment. The sector and track addresses are encoded in triplicate and decoded by majority voting to achieve the required reliability with respect to defects. If the failure rate of an SSTTTT address is P, then the failure rate of the triple redundancy pattern is P2. Interestingly, both the leading and trailing edges of the Sync field is a constant-to-tone and a tone-to-AN signal, can be used to generate the start of sector timing. In effect, there is double redundancy in the address mark. The synchronization field (Sync) in this system is N ID and N OD deflections as shown in Figure 4, which is a schematic representation of a sync encoding. This pattern is used by the tone detector to detect the start of the header, and by the phase locked oscillator (PLO) to generation of a phase locked detection clock, according to the present invention. In this case, N is chosen to be 40 so as to allow adequate time for the header to be sensed and the PLO to lockup. Notice that there is no adjacent track interference in the sync field because all of the information from track to track is the same. The address mark (AM) consists of five consecutive detection clock periods with no wobble marks, as shown in Figure 5, which is a schematic illustration of an AM encoding for the bi-level media header. The AM is used to allow detection of the trailing edge of the Sync field for the purpose of determining the start of the sector. Notice that there is no adjacent track interference in the AM field because all of the information from track to track is the same. The sector number information is encoded in decimal digits. Each digit uses five consecutive header code bits, which are shown in Figures 6A-6J. Figures 6A-6J are schematic diagrams of sector number encoding for the bi-level media header for codes 0 to 9, respectively. Further, the format requires that a track be one complete revolution and that the sector header information be radially aligned from track to track. Notice that there is no adjacent track interference in the sector address field because all of the information from track to track is the same. The sector number code is a two out of five code. It is intended that maximum likelihood detection be used to detect the code words. Maximum likelihood detection can be implemented by sampling the signal amplitude at the header clock positions and calling 1's the 2 out of 5 amplitudes that have the largest values. Maximum likelihood detection minimizes the effects of any residual adjacent track interference. Table I illustrates the way the sector decimal digit code would appear. The track number information is encoded in decimal digits. Each digit uses 5 consecutive header code bits as shown in Figure 7 , which is a schematic diagram of the track number encoding for the bi-level media header for codes 0 to 9. The format requires that a track be one complete revolution and that the track header information be radially aligned from track to track. Notice that there is adjacent track interference in the track address field because all of the information from track to track is the different. The track number code is also a two out of five code. Again, it is intended that maximum likelihood detection be used to detect the code words. Additionally, the adjacent track signal adds constructively with the desired signal, i.e. 0's get lower and 1's get higher in amplitude. Accordingly, this leads to no loss of signal-to-noise ratio because of adjacent track interference. The encoding of the second, third and fourth digits of the TTTT encoding allows each of these positions to have effectively 5 values, not 10. Thus, the number of tracks that can be encoded are 5*5*5*10=1250, which is sufficient to encode all of the tracks in one revolution of the disk. Table II illustrates the way the track decimal digit code would appear.

Claims: An optical data storage medium (12) comprising a flat substrate in which a periodic pattern of lands and grooves are formed defining respective paths for bi-level recording tracks, each land and groove having substantially equal widths and said grooves having respective pre-determined depths, said recording medium further comprising: wobble mark encodings formed on alternate tracks, each comprising a first wobble mark comprising a land in a groove in register with a second wobble mark comprising a groove in a land along the length of a track, said wobble marks deviating from the path of the track by an identical fixed amount; said wobble mark encodings being defined in said tracks at a minimum deflection distance from one another, said minumum deflection distance and said groove depths giving a usable reflectivity signal amplitude to a conventional sensing system and sufficient position error signal when defraction light distribution reflected from said grooves is sensed by said sensing system; said wobble mark encodings being grouped to designate track and sector positions, each group comprising a plurality of wobble mark encodings spaced at multiples of said minumum deflection distance; characterised in that: each group of wobble mark encodings comprises an equal number of lands in a groove and grooves in a land such that said sensing system is not biassed into indicating an erroneous center position perpendicular to a track direction. The storage medium of claim 1 in which said alternate tracks are lands. The storage medium of claim 1 in which said alternate tracks are grooves. The storage medium of claim 1 in which each group comprises two wobble mark encodings. The storage medium of claim 1 in which each sector is encoded by two successive groups. The storage medium of claim 1 in which each track is encoded by four successive groups. The storage medium of claim 1 in which track header information comprises: a radially aligned wobble mark tone pattern for phase synchronising. said sensing system's data channel clock with subsequent wobble mark encodings, an address mark feature for determining which channel clock cycle is at the first bit position of the wobble mark encodings; two groups of wobble mark encodings encoding sector information; and four groups of wobble mark encodings encoding track information. A medium as claimed in claim 7, wherein the synchronization, address mark, track and sector information are encoded redundantly to allow reliable sector address determination in the presence of medium defects.

Inventors: BELSER KARL, BELSER, KARL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES
Application year: 1996
Main classification: G11B7
IPC codes: G11B7, G11B5, G11B11