Optical data storage system with sector servo

Abstract: An optical disk 10 has groups of features (such as depressed areas) arranged in a servo pattern disposed in a plurality of circumferentially spaced sectors, and with a flat, non-grooved surface inbetween. Each even sector 15b includes a plurality of features formed in an arrangement which is the complement and/or reversal in order circumferentially of their arrangement in the intervening odd sectors 15a. A single photodetector 36 senses the change in amplitude of reflected light as a spot from a laser passes over one of the features and generates position sensing signals corresponding to the pattern. At least three phases of the position sensing signal are generated for each sector. Each phase is determined by edges on the disk offset circumferentially with respect to each other and separated radially from each other by a critical distance. Errors caused by variations in pattern size and/or radial velocity of an optical head as it moves generally radially of the disk are reduced.

Description: Field of the Invention This invention relates to sector servo systems for optical disks and, more particularly, to such systems wherein the servo patterns provide automatic compensation for variations in pattern size that may occur during fabrication of the optical disk and/or for variations in radial velocity of an optical head as it moves generally radially of the disk. Background of the Invention Optical disk drives in current use employ grooved disks and split photodetectors to provide position sensing information. The grooves introduce noise into the data channel, reducing data capacity. Due to thermal and other effects, the area of the spot imaged on the photodetector may not be equally split between the two detector halves. Also, grooved disks do not contain track-type information. To overcome these and other problems, sector servo systems have been proposed with circumferentially spaced sector patterns comprising "features" (embossments or depressions) that extend radially from the inner to the outer diameter of the data band. The data recording surface of the data band is flat between the sector pattern so there is no signal degradation. The servo pattern edges (and hence diffraction orders) used for tracking error signals (TES) are separated in time, directed onto a single (rather than split) photodetector, and used to calculate the TES. Offsets due to tilt, objective lens translation, and thermal shift are eliminated because there is no split detector. Finally, there is no coupling between the tracking features and focus error signal (FES) because the laser beam can be focused on the flat data recording area and turned off when over the sector area containing position information. A consortium of Japanese manufacturers is proposing for low end optical drives a sector servo system in which the position of "di-bits" is varied to encode track type information. It uses di-bits in the form of dots, thereby providing a track error signal of very limited linearity. Also, it provides no means to compensate for radial velocity error. During radial seek operations, spots are sensed as being closer; as a result of this distortion of the velocity in relation to the change in radial distance, the radial distance is not accurately sensed. Finally, it provides no means to automatically compensate for variations in pattern size that may occur during fabrication of the disk. In the commonly owned copending application, U.S. Ser. No. 07/392,035, filed August 8, 1989 (EPA 412738; Docket TU9-89-011), a sector servo system is disclosed which employs a multi-phase servo pattern that provides both fine tracking and coarse seeking position information. The fine tracking pattern provides, for each phase, sets of uniformly spaced edges whose positions are fixed by the track pitch and cannot be varied. While this system works satisfactorily, the track error signal has limited linearity and, due to the nonvariable relationship of the edges, it is not possible to optimize the edge spacing to minimize sensitivity to tilt and to variations in spot size. Disclosure of the Invention Accordingly, the invention provides an optical disk comprising a substrate with a plurality of circumferentially spaced servo sectors to provide position information, characterised by comprising alternating first of second types of sector said first type of sector comprising a servo pattern including a plurality of features in a preselected ordered pattern which constitutes a permuted rearrangement of their arrangement in said second type of sector. Such a disk can be used with an optical data storage system having laser means for directing a spot of coherent light at selectable tracks on the disk during rotation of the disk; and a photodetector for sensing the change in amplitude of reflected light as the spot passes over one of said features, thereby generating position sensing signals corresponding to the pattern. The sector servo system has two interleaved sets of servo patterns, those of one set being a permutation (i.e., a reversal and/or the complement) of those of the other set, thereby providing a tracking error signal which automatically compensates for variations in radial velocity of an optical head as it moves generally radially of the disk and/or cancels the effects of process variations during fabrication of the disk. Each phase of the sector servo pattern has two edges that are independently variable. Thus a sector servo system is provided for an optical disk that has groups of features (such as depressed areas) arranged in a preselected position sensing servo pattern. A single photodetector senses the change in amplitude of reflected light as a spot from a laser passes over one of the features and generates position sensing signals corresponding to the pattern. At least three phases of the position sensing signal are generated. Each phase is determined by at least one pair of edges offset circumferentially with respect to each other and separated radially from each other by a critical distance that is independent of the pitch of the recording tracks on the disk so as to provide a track error signal that is substantially linear within a desired range to either side of a zero-crossing point. The critical distance does not change despite variations in servo pattern size during fabrication. The disk has a flat, non-grooved surface between a plurality of circumferentially spaced sectors. Each even sector includes a plurality of features formed in an arrangement which is the complement and/or reversal in order circumferentially of their arrangement in the intervening odd sectors to cancel simultaneously, by averaging the position sensing signals from adjacent sectors, errors caused by variations in pattern size and/or radial velocity of an optical head as it moves generally radially of the disk. Brief Description of the Drawings Fig. 1 is a plan view of an optical disk showing servo and data sectors; Fig. 2 is a schematic view of an optical system for use with an optical disk having a sector servo pattern; Fig. 3 is a schematic view of circuitry for detecting sector servo position information; Fig. 4 depicts a three-phase servo pattern and the signals derived therefrom; Fig. 5 is a diagram depicting how the sampling times for the three phases of the position sensing signal are determined; Fig. 6 is a diagram depicting the relationships between the various features of the sector servo pattern; Fig. 7 depicts a three-phase servo pattern which has pattern features in the even-numbered sectors which are the complement of those in the intervening odd-numbered sectors to compensate for errors due to pattern expansion during photolithic exposure while the disk is being fabricated; Fig. 8 depicts a three-phase servo pattern which has pattern features in the even-numbered sectors which are reversed with respect to those of the intervening odd-numbered sectors to correct for errors due to variations in radial velocity of an optical head as it moves generally radially of the disk; Fig. 9 depicts a three-phase servo pattern which has pattern features in the even-numbered sectors which are both the complement of and reversed with respect to those in the intervening odd-numbered sectors to cancel errors due to both pattern size and radial velocity; Fig. 10 is a diagram depicting how even and odd sectors are determined; and Fig. 11 is a diagram showing how the fine tracking and coarse seeking track and sector information is encoded. Detailed Description As illustrated in Fig. 1, the optical disk 10 embodying the invention comprises a plurality of uniformly circumferentially spaced servo sectors 11 that are separated by flat, non-grooved data sectors 12. Sectors 11 and 12 extend from the inner diameter 13 to the outer diameter 14 of the data band. The disk has a plurality of concentric (or, if preferred, spiral) data recording tracks (not shown) between the inner and outer diameters 13,14. Each servo sector 11 comprises a pattern 15 consisting of a plurality of "features" in the form of either depressed areas or raised areas in the otherwise flat surface of the disk. For purposes of this application, the features are assumed to be depressed areas. These depressed areas are deep enough (e.g., 1/4 the wavelength of the laser beam) to cause the reflected light to change in amplitude as a laser spot passes over the depressed areas. The servo patterns are designed to give a linear fine tracking error signal to either side of the center of a track as a function of radial position. According to a feature of the invention, and as will be described in greater detail presently, the sector servo pattern 15b in the even-numbered servo sectors II, IV, VI is a permutation of the sector servo pattern 15a in the odd numbered sectors I, III, V, except that a wedge-shaped synchronization feature or bar 16 is always located at the leading edge of each sector. The. expression "permutation of a sector servo pattern" as herein used, is intended generically to mean a preselected ordered rearrangement of the pattern, such as a reversal or the complement or both a reversal and the complement of the pattern. These servo patterns 15a,b are designed to be as insensitive as possible to variations in laser spot size, radial velocity, pattern shape width and to media tilt. The patterns 15a,b also give coarse track-type information for the purposes of counting tracks when seeking and selecting the proper track center from a fine tracking signal. In Fig. 2 is illustrated an optical system that may be used for sensing the servo pattern. A control circuit 20 responds to a timing signal in line 21 modulated by feedback from a laser power control diode 22 to control the voltage level applied via line 23 to control current to a laser 24. The beam of light from laser 24 is collimated at 25 and directed via a beamsplitter 26 and objective lens 27 to focus a spot onto the sector servo pattern 15. From pattern 15 the focused light is reflected back through lens 27 to beamsplitter 26, which directs it via a beamsplitter 28 , lens 29 and a beamsplitter 30 to a single photodetector 31. The signal from photodetector 31 is amplified by an amplifier 32 to provide a bulk reflectivity signal in line 33 that is used to decode information from the sector servo patterns 15a,b. Some of the light passing through beamsplitter 30 is directed by a polarizing beamsplitter 34 onto a horizontal polarization detector 35 and onto a vertical polarization detector 36 for detecting magneto-optic data. Signals from detectors 35,36 are amplified at 37,38, respectively, and then subtracted at 39 to provide a data signal in line 40. The system also includes a conventional astigmatic lens 41 and quad focus error signal detector 42 that serve as the transducer for generating the focus error signal. As illustrated in Fig. 3, the bulk reflectivity signal developed by the optical system of Fig. 2 is applied via branches of line 33 to three pairs 43,44,45 of sample and hold circuits and to a peak detector 46. When the laser spot hits the synchronization bar 16 of a pattern 15, detector 46 is triggered to provide an output to a timing control circuit 47. Circuit 47 provides in a six-channel bus 48 sample time pulses T-1A, T-1B, T-2A, T-2B, T-3A and T-3B that, as best shown in Fig. 5, are delayed equal increments of time from the instant the laser spot hits synchronization bar 16 for controlling the sample and hold circuits 43,44,45. Assuming the same edge spacing is used as with grooved media, a sampled tracking error signal (TES) can be calculated as follows. The sample and hold circuits 43a,b respond to the sequential sample pulses T-A1 and T-B1, respectively, to provide time-spaced outputs A and B, respectively, to two summing circuits 49a,50a. Circuit 49a provides an output equal to (A-B); whereas circuit 50a provides an output equal to (A+B). A divider circuit 51a divides the (A-B) output of circuit 49a by the (A+B) output of circuit 50a to compute, for phase 1 of the three-phase fine tracking error signal, a value equal to (A-B)/(A+B). Sample and hold circuits 44a,b output time-spaced signals A and B, respectively, to summing circuits 49b, 50b for causing divider circuit 51b to compute for phase 2 of the three-phase track error signal a value equal to (A-B)/A+B . Similarly, sample and hold circuits 45a,b and circuits 49c, 50c and 51c operate to compute the value for phase 3 of the track error signal. The (A-B)/(A+B) outputs of each divider circuit 51a,b,c for the respective phases is connected to a respective comparator 52a,b,c and also in parallel to an analog switch 53. Each comparator 52a,b,c produces as an output a logic level "1" if the input level is positive and a logic level of "0" otherwise. The outputs of comparators 52a,b,c are each connected as inputs to signal selection logic circuitry 54 that produces outputs that control the analog switch 53. Switch 53 selects between the signals from divider circuits 51a,b,c according to the following selection algorithm: The conditions where either (i) the outputs of 51a,b,c are all positive making all outputs of 52a,b,c "1", or (ii) the outputs of 51a,b,c are all negative making all outputs of 52a,b,c "0" are error conditions and do not occur in normal operation. The timing control circuit 47 produces a signal 60 to control a sample and hold circuit 55 that samples the selected phase signal from switch 53 and holds it during the time between sectors. Signal selection logic circuit 54 also produces incrementing and decrementing signals for a track counter 56, as shown in the following tabulation: The N-line bus from track counter 56 controls a digital-to-analog converter (DAC) 57. The output of DAC 57 changes voltage in steps whose difference is equal to the peak to peak excursion range of the selected phase signals from the sample and hold circuit 55. The sampled phase signal from circuit 55 and the output from DAC 57 are added together by an adder 58 to produce a linear fine tracking error signal over several adjacent tracks. An even/odd sector decoder 61 controls the order of the timing pulses coming from the timing control circuit 47, such that order reversal of the sector pattern can be accounted for. The even/odd indication of the previous sector is used to control the order of the timing pulses that appear on bus 48. If previous sector type was encoded as an even sector, then the next sector to be decoded will be an odd sector. The sample and hold circuits in Fig. 3 are activated in the order 43a, 43b, 44a, 44b, 45a and 45b. If previous sector type was encoded as an odd sector, then the next sector to be decoded will be an even sector. The sample and hold circuits in Fig. 3 are activated in the order 45a, 45b, 44a, 44b, 43a and 43b. Lastly, the track type decoder 59 measures the distance in time between the synchronization feature 16 and the features of pattern 15a or 15b that give coarse track type information. The sector servo pattern illustrated in Fig. 4 generates three phases of a fine tracking position sensing signal. As illustrated, the pattern comprises a series of depressed areas with pairs of parallel edges; however, in actuality, the areas have one pair of concentric edges with a fixed angular width circumferentially and equal radial lengths. But since the radius of curvature is so large in relation to the angular width of the edges, they may for all practical purposes be considered as parallel. Each phase is defined by two pairs X,X′ and Y,Y′ of concentric edges offset circumferentially with respect to each other but with corresponding edges X,Y and X′,Y′ being separated radially from each other by a critical distance Z that is independent of the pitch of the recording tracks. The bulk reflectivity signals 33 at the center position of the edges are denoted as A1, B1, A2, B2, A3 and B3. These signals are sampled at the specified times T-A1, T-B1, T-A2, T-B2, T-A3 and T-B3 as shown in Fig. 5. The. (A1-B1)/(A1+B1), (A2-B2)/(A2+B2) and (A3-B3)/(A3+B3) values for phases 1, 2 and 3, respectively, of the track error signals as a function of radial distance are shown as the signals labelled P1, P2 and P3. These signals will have a zero crossing defining a track center C at the location exactly half way between the X and Y or X′ and Y′ edges. The separation of edges Z can be specified to optimize the linearity of the tracking error signal phase to plus or minus one-half of the track pitch. The optimal separation will depend on the track pitch specified. At the right of Fig. 4, switched phase voltages are depicted which denote the active phase of the tracking error signal. For example, line Q shows that maximum linearity is achieved during phase 2, during which the signal crosses the zero point along line P2. Fig. 6 illustrates the basic pattern and gives as an example the dimensions of each radial feature with respect to the full width, half maximum laser spot sizes. Figs. 7, 8 and 9 show pattern variations between the odd-numbered sectors I, III and V and the even-numbered sectors II, IV and VI in Fig. 1. As will be explained more fully presently, Fig. 7 shows a pattern that compensates for variations in pattern size during media fabrication; Fig. 8 shows a pattern variation that compensates for variations in radial velocity; and Fig. 9 shows the patterns in Figs. 7 and 8 combined into one pattern that compensates for variations in both pattern size and radial velocity. Fig. 7 illustrates odd-numbered sectors on the left and even-numbered sectors on the right. The pattern on the right is the complement of the pattern on the left. Variations in pattern size occurring during media fabrication are illustrated by the dotted lines making the depressions wider than intended. Note that the critical distance Z is not changed by such expansion of the pattern shapes. However, the track centers corresponding to the features on the top edges X and Y of the pattern move up towards the outer diameter 14 as the pattern expands. Similarly, the track centers corresponding to the features on the bottom edges X′ and Y′ of the pattern move down toward the inner diameter 13 as the pattern expands. This effect repeats every three tracks in the original pattern shown in the left side of Fig. 7. The apparent track center shiftings for the pattern on the right in Fig. 7 are exactly the opposite of those for the pattern on the left. The even and odd sectors are decoded alternately so that the offset errors are effectively averaged since the control system cannot respond to such rapid changes in the tracking error signal (TES) values. Hence, the pattern shown in Fig. 7 compensates for small variations in pattern size. Fig. 8 illustrates odd-numbered sectors (e.g., I, III or V) on the left and even-numbered sectors (e.g., II, IV or VI) on the right. The pattern on the left is the mirror image of the pattern on the right such that the order of the features is exactly reversed. Variations in radial velocity of the laser spot as disk 10 turns is illustrated by a circular spot that moves from left to right across the pattern. Motion of the spot toward the outer diameter 14 is denoted by the dash line moving upward from left to right; whereas motion toward the inner diameter 13 is denoted by the dash line moving downward left to right. The slopes of these dashed lines showing the laser spot motion are highly exaggerated for purposes of illustrating the effect. Thus, assuming the spot moves toward the outer diameter 14 in the left (odd sector) servo pattern, the center of the spot will appear to be at A1 and B1; i.e., farther from the edges X,X′ and Y,Y′, than if the spot remained at a constant radial location. This distortion causes the slope of the tracking error signal (TES) as a function of radial distance to be smaller than normal. On the other hand, assuming the spot moves toward the inner diameter 13 in the left pattern, the spot will appear as at A1′ and B1′; i.e., to be nearer to edges X,X′ and Y,Y′ than if the spot remained at a constant radial location. This distortion causes the slope of the TES signal as a function of radial distance to be larger than normal. This slope is called the TES gain. The apparent edge shiftings for the even sector pattern on the right are exactly the opposite to those for the pattern on the left. Motion of the spot toward the outer diameter 14 causes the edges to appear closer together, and motion toward inner diameter 13 causes the edges to appear father apart. According to a feature of the invention, for small radial velocities, the changes in TES gain corresponding to these edge motions are equal and opposite between the left and right patterns. The even and odd sectors are decoded alternately so that the TES gain errors are effectively averaged because the control system cannot respond to such rapid changes in the TES values. Hence, the pattern shown in Fig. 8 compensates for small variations in radial velocity of an optical head as it moves generally radially of the disk. Fig. 9 depicts a servo pattern that combines the function of those of Figs. 7 and 8. The pattern in the even-numbered sectors are both the complement and a reversal in sequence of the pattern in the odd-numbered sectors to thereby compensate for both variations in pattern size during fabrication and in radial velocity. Fig. 10 illustrates the encoding of the odd and even sector information using times T-odd and T-even. Each time the laser spot, while focused on the pattern, crosses an edge, there is a pulse. These pulses are caused by the sharp decrease in the reflected beam due to interference cancellation of the signals from the flat surface and the surface of the feature. The times used to encode the information are those between these pulses. The times T-odd and T-even are measured from the leading edge of the synchronization bar 16 to the leading edge of the feature immediately following the fine tracking pattern. Referring now to Fig. 3, the timing control circuit 47 quantizes the time following the synchronisation bar 16 into discrete intervals, and the odd/even decoder 61 classifies the times at which the pulse corresponding to the even or odd edge occurs into an even or odd output, respectively. The even or odd output of decoder 61 is held in a latch (not shown) and used to control the sequence of timing for the sample and hold circuits 43, 44 and 45 over bus 48. The synchronization bar 16 and the even/odd encoding radial edges may be located in a variety of positions relative to the fine tracking pattern. Therefore, it is important that the times be measured from leading edge to leading edge or alternately from trailing edge to trailing edge of bar 16 and of the edges to be decoded so that variations in pattern size during fabrication do not cause a change in the distance and hence time between the synchronization edge of bar 16 and the odd/even encoding feature. Fig. 11 illustrates the encoding of a coarse track pattern into several digits that, when combined, represent coarse track type information over a range of many tracks. Each time that the laser spot, while focused on the pattern, crosses an edge, there is as earlier described a sharp decrease in the reflected beam due to interference cancellation of the signals from the flat surface and the surface of the feature. The times used to encode the information are those between these edge triggered pulses. The times T-D2-0 and T-D2-1 are measured from the leading edge of the synchronisation feature 16 to the leading edge of the second feature following the fine tracking pattern. The times T-D3-0, T-D3-1 and T-D3-2 are measured from the trailing edge of synchronization feature 16 to the trailing edge of the first feature following the fine tracking pattern; whereas the times T-D1-0 and T-D1-1 are measured from the trailing edge of the synchronization feature to the trailing edge of the second feature following the fine tracking pattern. As explained in connection with Fig. 10, the timing control circuit 47 quantizes the time following the synchronization bar 16 into discrete intervals; however, the track type decoder 59 (Fig. 3) now classifies the times at which the pulse corresponding to the track type digit edge occurs into a track number. Again, the synchronization bar 16 and the even/odd encoding radial edges may be located in a variety of positions relative to the fine tracking pattern. Therefore, it is important that the times be measured from leading edge to leading edge or alternately from trailing edge to trailing edge of bar 16 and of the edges to be decoded so that variations in pattern size during fabrication do not cause a change in the distance and hence time between the synchronization edge and the track type encoding features.

Claims: An optical disk (10) comprising or substrate with a plurality of circumferentially spaced servo sectors (11) to provide position characterised by information to comprising alternating first and second types of sector, said first type of sector (15b) comprising a servo pattern including a plurality of features in a preselected ordered pattern which constitutes a permuted rearrangement of their arrangement in said second type of sector (15a). The optical disk of claim 1, wherein all the even-numbered sectors are said first type of sector, and all the odd-number sectors alternating therewith are said second type of sector. The optical disk of claim 2, wherein each even-numbered sector includes a plurality of features formed in an arrangement which is the complement of their arrangement in the intervening odd-numbered sectors, whereby averaging position information sensed from adjacent sectors compensates for errors caused by variations in pattern size that may occur during fabrication of the disk. The optical disk of claim 2 or 3, wherein each even-numbered sector includes a plurality of features formed in an arrangement which is the reversal in order circumferentially of their arrangement in the intervening odd-numbered sectors, whereby averaging position information sensed from adjacent sectors compensates for errors caused by variations in radial velocity of an optical head as it moves generally radially of the disk. The optical disk of any preceding claim, wherein the features are depressions in said optical disk having a depth of the order of about 1/4 the wavelength of the light used to read information from said optical disk. The optical disk of any preceding claim, wherein each servo pattern includes a synchronization feature (16) which establishes the time when the position information from each successive circumferential portion of the pattern is to be sampled. The optical disk of any preceding claim having a plurality of tracks, wherein the servo pattern provides coarse track-type information for counting the tracks during a seek operation by encoding the track number as the time difference between either leading-to-leading or trailing-to-trailing edges of position information features. The optical disk of any preceding claim having a plurality of tracks, wherein each sector includes at least three phases of position information, each of which phases for a particular sector is determined by adjacent concentric feature edges on the disk offset circumferentially with respect to each other and separated radially from each other by a critical distance that is independent of the pitch of said tracks so as to provide a track error signal that is substantially linear. The optical disk of claim 8, wherein said desired range extends from between about plus and minus one-half of the track pitch from a track centre. The optical disk of claim 8 or 9, wherein each of the three phases for a particular sector is determined by two pairs of equally radially spaced concentric edges on the disk, the concentric edges of each pair being offset circumferentially with respect to corresponding edges of the other pair, and the edges of each pair being separated radially from the edges of the other pair by said critical distance. The optical disk of claim 8, 9 or 10, wherein the features corresponding to the three phases are side-by-side and each shifted radially by one-third of the pitch of the pattern with respect to an adjacent phase. An optical data storage system comprising: an optical storage disk (10) as claimed in any preceding claim; laser means (24) for directing a spot of coherent light at the disk during rotation of the disk to read information therefrom; and a photodetector (36) for sensing the change in amplitude of reflected light as the spot passes over one of said features, thereby generating position sensing signals corresponding to the pattern of position information. The system of claim 12, wherein a single photodetector provides two pulse signals separated in time according to the circumferential distance between a synchronization feature and a track number encoding feature, and including: means for decoding (59) the time intervals between the leading edge of the synchronization feature and the leading edge of the feature defining the odd digits of a track number, or the trailing edge of the synchronization feature and the trailing edge of the feature defining the even digits of a track number in order to determine the track number. The system of claim 12 or 13, wherein the single photodetector provides two amplitude signals separated in time according to the circumferential length of two adjacent ones of said features, and including means for decoding (61) the even-numbered and odd-numbered sectors and reversing the order of the signals if the sector is even-numbered; means for calculating (43, 49, 50) a track error signal by computing the difference in said two signals divided by the sum of said two signals; means for selecting (52, 53, 54) which of the three phases is in its linear range; means for incrementing or decrementing a counter (56) as the spot moves from one phase to the adjacent phase; and means for converting (57) the counter output to an amplitude signal which together with the selected phase signal provides a linear tracking error signal over many tracks. The system of claim 12, 13, or 14 wherein said position sensing signals include fine tracking signals generated from each phase of the position information, and the pattern provides even/odd sector type information for determining the time order of decoding the fine tracking signals of each phase in the even and odd-numbered sectors.

Inventors: BELSER KARL ARNOLD, BELSER, KARL ARNOLD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G11B7
IPC codes: G11B7, G11B23