Self-synchronizing clock source for optical memories

Periodic clock pulses are pre-recorded on each servo track of an optical disk to provide a local self-synchronizing source of clock frequency information for read only or direct read after write optical digital disk memories. In keeping with accepted practices, the servo track or tracks additionally function as references for controlling the radial positioning of a read head or a read/write head of such a memory.

BACKGROUND OF THE INVENTION 
This invention relates to optical memories for digital data and, more 
particularly, to a radial tracking and clock control for digital optical 
disk memories. 
As is known, digital disk memories conventionally require a local source of 
clock frequency information and a radial tracking servo control system. In 
read only memories, the servo control system is used to precisely control 
the radial positioning of a focused read light beam so that the read beam 
is intensity modulated in accordance with the prerecorded serial data on a 
preselected data track or track sector. A video detector then converts the 
intensity modulated beam into an electrical video signal. If the recorded 
data is self clocking, the original digital bit stream is directly 
available from the video signal, typically as the modulation component of 
a carrier frequency signal. Otherwise, however, a synchronized clock 
source is required to supply clock pulses for synchronously sampling the 
video signal (or, more specifically, the demodulated video signal) at the 
appropriate rate to recover the original digital bit stream. Direct read 
after write digital optical disk memories have additional requirements 
because of the write function. In particular, a clock pulse source is 
required in conjunction with the input data so that such data is converted 
while being read out into a digital bit stream having a predetermined bit 
rate, and the radial tracking servo control system is needed for precisely 
controlling the radial positioning of an intensity modulated focused write 
light beam so that the digital data is serially recorded on a preselected 
data track or sector. 
Heretofore, the need for clock frequency information and for radial 
tracking servo control have generally been treated as separate, unrelated 
requirements. For example, in U.S. Pat. No. 4,094,010 it is suggested that 
a groove be "burned" into the disk for radial tracking purposes and that 
an external clock be used to supply the clock pulses needed for writing 
and reading. Another relevant reference is Kenney et al., "An Optical Disk 
Replaces 25 Mag Tapes", IEEE Spectrum, February 1979, p. 33, where it is 
suggested (at p. 37) that digital data be recorded in self clocking form, 
thereby avoiding the need for synchronizing an external clock while data 
is being read from the disk. 
As will be appreciated, the requirement that the external clock be 
synchronized for a read mode operation is one of the basic disadvantages 
of the digital optical disk memory described in the above-identified 
patent. Likewise, the memory described in the above-identified article 
suffers from the data density limitations which are inherent in recording 
digital data in self clocking form. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the disk for a digital optical 
disk memory is initialized by recording clock pulses on one or more servo 
tracks, thereby providing a source of clock frequency information directly 
on the disk. The disk may have a continuous spirally configured servo 
track or a plurality of concentric servo tracks. In either case, the servo 
pitch is selected so that there is space for a predetermined number of 
parallel data tracks on both sides of each servo track or servo track 
convolution. The clock pulses may be recorded on the servo track or tracks 
by an embossing process performed while the disk is being manufactured or 
by a laser writing process carried out at some later time but before any 
data is recorded. 
The radial tracking servo control system typically relies on the 
distinctive parity or encoding of the clock pulses recorded on the servo 
track or tracks of an initialized disk to discriminate the servo tracks 
from the data tracks. Once the radial servo control system has locked onto 
a servo track, radial departures from the servo track are detected to 
provide the servo control system with a departure correcting error signal, 
and the clock pulses are read out to provide clock control for data 
recording or reading. As a result, data to be recorded may be encoded 
using non-self clocking codes, thereby providing a relatively high 
recorded data density.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
While the invention is described in some detail hereinbelow with specific 
reference to a single illustrated embodiment, it is to be understood that 
there is no desire to limit it to that embodiment. On the contrary, the 
intent is to cover all modifications, alternatives and equivalents falling 
within the spirit and scope of the invention as defined by the appended 
claims. 
Turning now to the drawings, and at this point especially to FIG. 1, there 
is a direct read after write optical recorder or disk memory 10 having a 
read/write head 11 for optically recording digital data on and retrieving 
digital data from a threshold sensitive recording medium 12. Typically, 
the recording medium 12 is a removable disk which is rotated (by means not 
shown) during operation at an essentially constant angular velocity in the 
direction of the arrow relative to the read/write head 11. 
Referring for a moment to FIGS. 2 and 3, each recording disk 12 initialized 
to have one or more servo tracks 13 for controlling the radial positioning 
of the read/write head 11 during writing and reading. If the objective is 
to provide random access to relatively short digital data streams, there 
typically are a plurality of concentric servo tracks 13. On the other 
hand, if longer digital data streams are to be accommodated, a single 
servo track 13 having a continuous spiral configuration may be better. In 
any event, the servo track pitch is selected so that each servo track 13 
is straddled by a predetermined number of data tracks 14. For example, as 
illustrated in FIG. 3, the disk 12 is initialized to have a plurality of 
concentric servo tracks 13 with a pitch of approximately 20 .mu.m. That 
provides space to either side of each servo track 13 for four (4) 
concentric data tracks 14 having a pitch of 2 .mu.m or so, while still 
leaving a guard band of about 4 .mu.m between each adjacent set of data 
tracks 14 (i.e., adjacent data channels) as a tolerance allowance for 
errors in positioning the servo tracks 13 and the data tracks 14. Of 
course, if tight tolerances can be maintained over the positioning of the 
servo tracks 13 during the initialization of the disk 12, the guard bands 
may be reduced or even eliminated in the interest of providing further 
space on the disk 12 for additional data tracks 14. As will be evident, 
the above-described format may also be used for a disk having a single 
spirally configured servo track, except that such a disk is limited to a 
single band composed of a continuous, spirally configured servo track and 
one or more similarly configured data tracks. 
Returning to FIG. 1, to record digital data on any given data track 14 of 
the disk 12, the read/write head 11 comprises a relatively high power 
laser 21 which is selectively energized for recording to supply a coherent 
write light beam 22. A relay lens 23 focuses the write beam 22 into an 
acousto-optic modulator 24, and an acoustic wave which is modulated in 
accordance with the digital data to be recorded is launched into the 
modulator 24 to intensity modulate the write beam 22. The intensity 
modulated write beam 22 is first transmitted through a refractive 
galvanometer 25 and then through a dichroic beam splitter 26 where it is 
combined with read beams 32a and 32b. The combined beams 22, 32a and 32b 
are subsequently reflected by a reflective galvanometer 27 to an objective 
lens 28. In keeping with accepted practices, the objective lens 28 is 
moved back and forth relative to the disk 12 by a servo controlled voice 
coil 29 so that the beams 22, 32a and 32b are focused on the disk 12. 
As described in more detail hereinbelow, the reflective galvanometer 27 
references the read/write head 11 to the servo track 13 of a preselected 
band of tracks. Thus, a control signal is applied to the refractive 
galvanometer 25, thereby causing it to rotate in a clockwise or 
counterclockwise direction, as indicated by the arrow, to independently 
align the write beam 22 with any given one of the data tracks 14 of the 
selected band. Accordingly, the intensity modulated write beam 22 serially 
alters the optical properties of the disk 12 lengthwise of the given data 
track 14 in accordance with the digital data which is to be recorded. 
To retrieve previously recorded data from the disk 12 and to provide radial 
tracking and focus control while data is being recorded or retrieved, the 
read/write head 11 also includes a relatively low power laser 31 for 
supplying a coherent light beam 32 which is focused onto a two dimensional 
holographic diffraction grating 33 by a relay lens 34. The grating 33 
diffracts the beam 32 to provide a plurality of essentially equal 
intensity, spaced apart read beams 32a.sub.l . . . 32a.sub.n (collectively 
referred to herein by the general designator 32a) and a pair of 
essentially equal intensity radial tracking and focus control beams 
32b.sub.1 and 32b.sub.2 (collectively referred to by the general 
designator 32b). Typically, the grating 33 is produced using the pulse 
width modulation technique for fabricating a two dimensional holographic 
grating as described in Lee, "High Efficiency Multiple Beam Gratings", 
presented at the Optical Data Symposium, Society of Photographic 
Scientists and Engineers, Jan. 23-26, 1979 (Applied Optics), which is 
hereby incorporated by reference. As will be seen, the diffracted beams 
32a and 32b are refracted by a polarizing beam splitter 35 to a quarter 
(1/4) wavelength plate 36 which, in turn, transmits them to the dichroic 
beam splitter 26. The lasers 21 and 31 are selected to have significantly 
different output wavelengths. Hence, the beam splitter 26 reflects the 
diffracted beams 32a and 32b for application to the disk 12 via the 
galvanometer 27 and the objective lens 28 as previously described. 
More particularly, as shown, the read beams 32a.sub.l . . . 32a.sub.n 
optically read out the servo track 13 and all of the data tracks 14 of any 
selected band of tracks in parallel. In other words, one of the read 
beams-namely, the center one 32a.sub.(n/2+1) -illuminates the servo track 
13 of the selected band-while the others illuminate different ones of the 
associated data tracks 14. To that end, the read beams 32a.sub.1. . . 
32a.sub.n are equidistantly spaced in, say, horizontal alignment at the 
output of the grating 33 so that the objective lens 28 focuses them onto 
the disk 12 in essentially radial alignment and on centers which are more 
or less equidistantly spaced by an amount selected to match the data track 
pitch. The radial tracking and focus control beams 32b.sub.1 and 
32b.sub.2, on the other hand, are spaced above and below, respectively, 
the servo track or center read beam 32a.sub.(n/2+1) at the output of the 
grating 33, but are tilted at a small angle (1.degree.-3.degree.) relative 
to the normal or vertical axis. Specifically, the spacing of and the tilt 
angle for the radial tracking and focus control beams 32b.sub.1 and 
32b.sub.2 are selected so that the objective lens 28 focuses those beams 
onto the disk 12 above and below, respectively, the read beams 32a.sub.l . 
. . 32a.sub.n on opposite sides of but substantially tangential to a line 
which passes through the servo track read beam 32a.sub.(n/2+1) 
tangentially relative to the disk 12. Thus, it is important that the 
radius of curvature of the servo tracks 13 is negligible over the arc 
subtended by the radial tracking and focus control beams 32b.sub.1 and 
32b.sub.2, but otherwise the spacing between those beams can be selected 
to provide maximum sensitivity to radial tracking and focus errors. In the 
interest of completeness, a two dimensional diffraction grating 33 which 
may be used to provide diffracted read beams 32a and radial tracking and 
focus control beams 32b of the foregoing type for a disk 12 having eight 
(8) data tracks 14 per band is shown in FIG. 4. The illustrated grating 
was fabricated using the pulse width modulation technique described in 
Wai-Hon Lee's aforementioned paper. 
As will be recalled, the recording disk 12 is a threshold sensitive 
recording medium. For example, the disk 12 suitably comprises an ablatable 
tellurium based, reflective film which is coated on an optically 
transparent substrate, such as glass or plastic. In that event, the output 
power of the laser 21 and the depth of modulation of the write beam 22 are 
selected so that the intensity of the write beam 22, as measured at the 
disk 12, swings above and below a predetermined ablation threshold level 
for the film as a function of the modulation. Consequently, as shown in 
FIG. 2, the write beam 22 opens small holes in the film to represent the 
data which is to be recorded. In contrast, the output power of the laser 
31 is selected to ensure that the intensities of the read beams 32a and of 
the radial tracking and focus control beams 32b remain well below the 
ablation threshold of the film. Thus, the beams 32a and 32b do not affect 
the optical properties of the disk 12, but are reflected therefrom after 
being intensity modulated in accordance with any prerecorded information 
they happen to scan. 
The reflected write beam 22, read beams 32a and radial tracking and focus 
control beams 32b pass back through the objective lens 28 and then 
serially reflect off the galvanometer 27 and next off the dichroic beam 
splitter 26. From there, the relected beams 22, 32a and 32b are 
sequentially transmitted through the quarter wavelength plate 36 and then 
through the polarizing beam splitter 35 to a mirror 41 which, in turn, 
reflects the beams 22, 32a and 32b to individual detectors of a detector 
array 42. The quarter wavelength plate 36 and the polarizing beam splitter 
35 are relied on to prevent any significant optical feedback to the laser 
31 so that the reflected beams 22, 32a and 32b are efficiently transmitted 
to the detector array 42. 
Referring to FIG. 5, it will be seen that the detector array 42 has 
individual detector elements 43, 44a.sub.l . . . 44a.sub.nl and 44b.sub.1 
and 44b.sub.2 which are positioned to intercept the reflected write beam 
22, read beams 32a.sub.l . . . 32a.sub.nl and radial tracking and focus 
control beams 32b.sub.1 and 32b.sub.2, respectively, so that those beams 
are converted into corresponding video signals. As illustrated in FIG. 1, 
the read beams 32a.sub.l . . . 32a.sub.n are effectively supplied by 
individual point sources (i.e., the diffracted output of the diffraction 
grating 33). It should, however, be understood that those beams could also 
be supplied by a common-line like source (not shown) since the 
segmentation of the detector elements 44a.sub.l . . . 44a.sub.n inherently 
perform a beam separation function. Indeed, the basic disadvantage of 
using a commonline like source for supplying the read beams 32a.sub.l . . 
. 32a.sub.n is expected to be increased crosstalk between those beams. 
In accordance with this invention, as shown in FIG. 2, periodic clock 
pulses are written on each of the servo tracks 13 during the 
initialization of the disk 12. Thus, the tracks 13 function as servo/clock 
tracks. As will be appreciated, the clock pulses may be pre-embossed onto 
the servo tracks 13 during the manufacture of the disk 12. If that 
technique is used, the embossing depth should be controlled to be an odd 
integer multiple of one quarter of the output wavelength of the laser 31 
so that the clock pulses cause intensity modulation of the servo track 
read beam 32a.sub.(n/2+1). Alternatively, the clock pulses may be ablated 
onto the servo track 13 through the use of a laser writing station (not 
shown) having independent radial positioning control means. 
Turning to FIGS. 6A and 6B, once the galvanometer 27 (FIG. 1) has locked 
onto a servo track 13, any further radial movement of the radial tracking 
and focus control beams 32b.sub.1 and 32b.sub.2 will result in an 
imbalance in the optical modulation of those beams. Thus, the optical 
detector elements 44b.sub.1 and 44b.sub.2 are coupled to the inverting and 
non-inverting inputs, respectively, of an operational amplifier 47 to 
provide a corrective error signal for the galvanometer 27. Positive or 
negative radial tracking errors, such as shown in FIGS. 7A and 7C, 
respectively, which are within approximately plus or minus one half of the 
data track pitch of being "on track" (FIG. 7B) may be corrected in this 
manner, but greater errors require a coarser adjustment. 
Thus, as shown in FIG. 8, provision is made for seeking a servo/clock track 
13 once the disk has been positioned (by means not shown) to bring such a 
track 13 within the field of view of the objective lens 28. This provision 
forms no part of the present invention and will, therefore, be discussed 
on a highly simplified level. Indeed, as illustrated, there is a simple 
routine, such as might be carried out under program control. At the outset 
of the routine, a step counter (not shown) is cleared, as indicated at 51, 
and a timer (also not shown) is reset, as indicated at 52. Thereafter, the 
servo track read beam detector 44a.sub.(n/2'1) is monitored, as indicated 
at 53, to determine whether the servo track read beam 32a.sub.(n/2+1) is 
tracking on either a servo/clock track 13 or a data track 14. The timer 
provides a predetermined time-out period, as indicated at 54, for the 
galvanometer 27 (FIG. 1) to stabilize the servo track read beam on one or 
another of the tracks in response to the corrective error signal supplied 
by the operational amplifier 47 (FIG. 6A). Once tracking is achieved, a 
parity check or the like is performed (by means not shown) on the output 
of the servo track read beam detector 44a.sub.(n/2+1) to determine, as 
indicated at 55, whether the read beam 32a.sub.(n/2+1) is tracking on a 
servo/clock track 13 or a data track 14. For example, the clock pulses may 
be recorded with an even parity and data with an odd parity so that a 
simple parity check may be relied on to discriminate between the 
servo/clock tracks 13 and the data tracks 14. If it is determined that the 
servo track read beam 32a.sub.(n/2+1) is tracking on a servo/clock track 
13, the routine is completed and the operational amplifier 47 acquires 
exclusive control over the subsequent positioning of the 27. If, on the 
other hand, it is found that the read beam 32a.sub.(n/2+1) is tracking on 
a data track 14, the step counter is incremented by one, as indicated at 
56, and the accumulated count is then compared, as indicated at 51, 
against a predetermined maximum permissible count equal to, say, the 
number of data tracks 14 per band. If the maximum permissible count is 
reached, a fault occurs. Otherwise, a step-like increment is added, as 
indicated at 58, to the control signal, for the galvanometer 27 to shift 
the servo track read beam 32a.sub.(n/2+1) one track to the right or left. 
Thus, the routine recycles to reset the timer and to then repeat the 
above-described steps in search of a servo/clock track 13. 
Referring to FIG. 9A, the video signals supplied by the detector elements 
44a.sub.l . . . 44a.sub.n (collectively referred to by the general 
designator 44a) in response to the reflected read beams 32a.sub.l . . . 
32a.sub.n, respectively, may be applied to a serial output line 61 by a 
multiplexer 62. If it is desired to separate the clock frequency video 
from the detector 44a.sub.(n/2+1) from the video data provided by the 
other read beam detectors, the multiplexer 62 may be used to multiplex the 
video from such other detectors onto the output line 61. Alternatively, as 
shown in FIG. 9B, the detector elements 44a may be coupled in parallel to 
a channel selector 63 to selectively read out the servo/clock track 13 or 
any one of the data tracks 14 of a selected data channel via a serial 
output line 64. Still other combinations of serial and parallel read outs 
will, of course, be evident. 
CONCLUSION 
In view of the foregoing, it will now be understood that this invention 
provides a local, self-synchronizing source of clock frequency information 
for use in recording digital data on and retrieving such data from optical 
memories, such as optical disks. Due to the self-synchronizing nature of 
the clock source, digital data may be easily recovered, even if it is 
encoded by a non-self clocking code for recording. Furthermore, it will be 
understood that this invention effectively combines the clock source with 
the radial tracking position control function by using the servo track or 
tracks to store the clock pulses.