Optical disk having an index mark

In an optical recording disk having groove-like guide tracks, a rotation index mark is provided to indicate a rotation starting point where the rotation index mark is constructed of a grooved-like phase structure. The rotation index mark may be formed in a track which is located separated from the information recording tracks, and may be detected optically without using a laser light beam. The rotation index mark is used to control the rotation of the disk as well as writing and reading operation of an optical disk recording/reproducing apparatus so that various information, such as video signals, can be accurately recorded and reproduced. In the case of recording digital signals, the guide tracks may be divided into a plurality of sector regions, and sector index marks corresponding to respective sector regions may also be formed together with the rotation index mark. Furthermore, an address index mark indicative of the position of track address region radially formed in the guide tracks may also be added.

BACKGROUND OF THE INVENTION 
This invention relates generally to optical disks for recording and 
reproducing various information signals, such as a video signal, thereon 
and therefrom by means of an optical disk recording/reproducing apparatus. 
Various types of optical disk recording/reproducing apparatus have been 
known hitherto, and in one type thereof an optical disk, coated with a 
recording material by spraying or vapor deposition, is rotated, and a tiny 
spot of laser light beam, which is focussed so that the diameter is less 
than 1 micrometer, is applied on the optical disk. The intensity of the 
laser light to be emitted from a laser light source is modulated by a 
recording signal so that recording of a video signal or a digital signal 
can be effected in real-time operation as an optical characteristic change 
such as variation in refractive index, for instance, phase variation by 
concave-convexo portions on the optical disk, or variation in reflection 
factor or in transmittance by light and shade variation or formation of 
holes. The optical characteristic change on the disk will be detected to 
reproduce prerecorded information. 
In such apparatus, structure of an optical disk has been known where 
recording tracks are formed on an optical disk in advance and then a 
recording material is formed thereon by spraying or vapor deposition, as a 
method for actualizing high density of recording tracks, partial writing 
or erasing at random places, and for removing influence due to vibrations 
of the disk. Recording or writing is effected by recording information on 
the groove-like guide tracks, which have been dug or grooved on the 
surface of an optical disk, in such a manner that tracking control is 
effected so as to follow the zigzag motion caused by the eccentricity of 
the guide tracks with the above-mentioned tiny spot focussed on the 
groove-like track. Since the track density of the guide tracks is defined 
by the pitch of the guide tracks to be formed irrespective of the 
eccentricity of the optical disk, a disk of an extremely high density can 
be manufactured. For instance, a disk having a diameter of 30 centimeters 
can contain 20,000 to 40,000 coaxial tracks. Since such a large capacity 
optical disk is capable of containing 20,000 to 40,000 frames of TV 
(television) pictures, address information is necessary to selectively 
pickup necessary information. The address information is assigned to all 
the tracks as serial numbers from the inner-most track to the outer-most 
track or vice versa. 
The address data, of respective tracks are aligned radially where the 
address datum of each track is formed in the form of an arc subtending a 
given angle with respect to the center of the disk. Therefore, it is 
necessary to prevent other information from being recorded on the region 
carrying the address data. Furthermore, it is also necessary to control 
the rotation of the optical disk by matching the phase of the vertical 
synchronous signal of a TV picture signal with the phase of the address 
information region, so that a TV picture signal is recorded after the end 
of the address information region in the case of recording a single frame 
of a TV picture, for example. To this end, an optical disk requires a mark 
indicative of the position of the starting point of rotation. Although the 
address information region may be used as such a mark, it is not practical 
to do so because a detection signal, obtained as a result of searching a 
track by moving the tiny spot of light at high speed, suffers from 
irregularity due to noises. 
On the other hand, since digital information does not have a predetermined 
length as does a TV picture, a problem would occur if only address 
information is provided. Therefore, each track is divided into a plurality 
of information regions for controlling the digital information. This 
information region is called a sector, and respective sectors are numbered 
so that serial numbers are assigned to consecutive sectors in each track. 
With this arrangement, each sector in the optical disk can be readily 
designated by its track address and sector address. 
It is advantageous to form each of the track address information and the 
sector address information as a groove-like portion on the optical disk 
together with the guide tracks in view of manufacturing technique inasmuch 
as optical disks having guide tracks can be manufactured by stamping in 
the same process for manufacturing VLP (trademark) disks. 
The above-mentioned index mark has been provided hitherto by adhering a 
piece of aluminum film or vapor-depositing a mirror surface. Therefore, 
the positional accuracy of the index mark on an optical disk is not very 
high, while productivity of optical disks is low. 
SUMMARY OF THE INVENTION 
The present invention has been developed in order to remove the various 
above-mentioned disadvantages and drawbacks inherent to the conventional 
optical disks. 
It is, therefore, an object of the present invention to provide a new and 
useful optical disk having an index mark indicative of a starting point of 
the rotation of the disk where the index mark can be readily formed 
together with groove-like guide tracks by stamping, so that the positional 
accuracy of the index mark is very high while productivity of optical 
disks can be improved. 
According to a feature of the present invention, other index marks 
indicative of sectors of an optical disk and indicative of the position of 
address information, may also be formed in the same manner as the first 
mentioned index mark. 
According to another feature of the present invention, the various index 
marks mentioned in the above can be detected by using an incoherent light 
source, such as a light-emitting diode, without using a laser light 
source. 
In accordance with the present invention there is provided an optical disk 
for optically recording and reproducing information thereon and therefrom, 
comprising: a substrate; a recording layer deposited on at least one side 
of said substrate; and a rotation index mark indicative of a starting 
point of the rotation of said disk, said index mark being constructed of a 
groove-like phase structure within said substrate and said recording layer 
.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the structure of groove-like guide tracks formed in an optical 
disk to which the present invention is adapted. In FIG. 1, grooves 2a, 2b, 
2c . . . 2e are made on one side R of an optical disk 1 where each groove 
has a width W, a pitch P and a depth D. The grooves 2a to 2e constitute 
guide grooves, and are formed coaxially or spirally. Adjacent grooves are 
separated by flat portions 3a to 3d each provided between adjacent 
grooves. A recording material is vapor deposited onto the side R so as 
form a recording layer 4. A tiny spot lights from a suitable light beam 
source is applied from the side of a substrate 6 in this example so as to 
be focussed on the side R to record information. The portion where 
information is recorded or written is the bottom (i.e. the upper side in 
FIG. 1) of respective grooves of the tracks 2a to 2e. An example of pits 
of light and shade is shown to be recorded at the bottom of the guide 
track 2a. 
An embodiment of the present invention is shown in FIG. 2 which have the 
same guide tracks as shown in FIG. 1. The embodiment will be described 
with reference to a top plan view of FIG. 2, a cross-sectional view of 
FIG. 3A, a top plan view of FIG. 3B corresponding to FIG. 3A, another 
cross-sectional view of FIG. 4, and perspective views of FIGS. 5A and 5B 
showing the structure of an index mark. The cross-sections of FIGS. 3A and 
4 are views respectively taken along the lines IIIA--IIIA and IV--IV of 
FIG. 2. 
The optical disk 1 of FIG. 2 comprises a track region 7 having groove-like 
guide tracks, track address regions 8 each formed in each of the guide 
tracks, and an index mark 9 indicative of a point of rotation start. The 
track address regions 8 are aligned radially with respect to the center O 
of the optical disk 1 so that each track address region 8 has a form of an 
arc or sector having an angle .alpha.. In detail, the track address 
information is recorded as concave-convexo pits in the form of FM or PE 
(pulse-encoded) modulated signals in each track defined between lines CO 
an C'O. 
FIG. 3A is an enlarged cross-sectional view of the disk 1 of FIG. 2 for 
showing the structure of the track address region 8, and a top plan view 
corresponding to FIG. 3A is shown in FIG. 3B . The depth D of the grooves 
is selected to be one eighth of the wavelength in view of the detection of 
error signal and reading operation by reflection. The address information 
is formed as repetitive concave grooves in the form of pits having a depth 
of D, and will be read out as change in the amount of reflecting light 
diffracted in accordance with the presence and absence of the groove. 
FIG. 4 is a cross-sectional view taken along the line IV--IV of FIG. 2, 
showing the track region 7, the index mark 9 and a flat region 10 
therebetween. The index mark 9 comprises a plurality of grooves having the 
same pitch P and groove width W as those of the guide tracks 2a, 2b . . .. 
Namely, the index mark 9 corresponds to the combination of a plurality of 
guide tracks each having a given length as shown in FIG. 2. 
FIGS. 5A and 5B respectively show examples of the index mark 9 of FIGS. 2 
and 4. In the example of FIG. 5A, the index mark 9 is actualized by 
forming a plurality of guide tracks having a uniform depth D, where the 
guide tracks are formed coaxially throughout an angle .beta. with respect 
to the center of the disk 1 so that the index mark 9 takes a form of a 
sector. In the example of FIG. 5B, the index mark 9 comprises repetitive 
pits of a given period "q" in place of each uniform groove of FIG. 5A. 
Therefore, the amount of light reflected is greater in the arrangement of 
FIG. 5B than that of FIG. 5A. The pit period or pitch is selected to the 
order of the period of information pits. The pit period may also be 
selected to the order of the resolving power of an index mark detector, 
which will be described hereafter, while the pits may be aligned radially 
as shown in FIG. 5B so that the index mark 9 can be accurately and 
effectively reproduced. Since the index mark detector reproduces a signal 
having the pit period "q", it is possible to detect a rotation start 
signal from which undesirable influence, such as variation in reflection 
factor of the optical disk, vibrations of the disk, and external light 
interference, have been eliminated, by extracting this frequency 
component. 
FIG. 6 is a block diagram of a reading circuit for detecting the index mark 
9 from the optical disk 1. The reading circuit comprises an index mark 
detector 11 having a light-emitting element 12 and a light-receiving 
element 15. For instance, an optical reflective sensor HEDS-1000 
manufactured by Hewlet Packard may be used. The light-emitting element 12 
may be customary light-emitting diode (LED), and a constant D.C. current 
is applied via a bleeder resistor R1 from a voltage source 13 so that the 
LED 12 emits light continuously. The output light from the light-emitting 
element 12 is diaphragmed by a diaphragming lens 14a to produce a spot on 
the optical disk 1. The light is reflected at the optical disk 1 and is 
then condensed by a condensing lens 14b having a limited aperture so as to 
image a spot on a junction of the light-receiving element 15. 
Since the reflected light from the optical disk 1 is scattered around by 
the index mark 9 whose surface has been made coarse by the concave-convexo 
arrangement, the amount of incident light on the light-receiving element 
15 decreases compared to reflected light from other than the index mark 9. 
The change in the light amount becomes the change in the output current of 
the light-receiving element 15, where the output signal is then amplified 
by a transistor 16. A resistor R2 functions as a load of the transistor 
16, and the output signal of the transistor 16 is fed back via a feedback 
resistor Rf to determine the voltage amplification factor of the 
transistor 16. The output signal "a" of the transistor 16 is fed via a 
band pass filter 90 to an amplifier 17 to be amplified therein, and then 
the output signal of the amplifier 17 is applied to a comparator 18 so 
that the waveform is shaped. The comparator 18 is also responsive to a 
reference voltage VR from a voltage source 19. The output signal of the 
comparator 18 will be used as a rotation starting point or position signal 
"c". This signal "c" indicative of a starting point of the disk rotation 
may be used to detect the angular position of the track address regions 8. 
In order to detect the angular position of the track address regions 8, 
the index mark 9 is located at a predetermined point having a given angle 
with the radially aligned track address regions 8 with respect to the 
center of the disk 1. The angular difference between the index mark and 
the track address regions 8 correspond to a given period of time, and 
therefore, this difference will be offset electrically. 
FIGS. 7A-7C are waveform charts showing various signals in the circuit of 
FIG. 6. The output signal "a" of the transistor 16, shown at FIG. 7A has 
amplitude variation due to the surface vibrations of the optical disk 1 
and the change in the reflection factor, and this amplitude variation or 
fluctuation can be removed by limiting the frequency range by means of the 
band pass filter 90 (see the waveform of the output signal "b" of the 
amplifier 17, shown at FIG. 7B). As a result, only the reflected light 
component from the index mark 9 can be derived. The output signal "b" of 
the amplifier 17 is shaped by the comparator 18 to obtain the pulse train 
signal "c" having a desired waveform as shown in FIG. 7C. 
Since the index mark 9 is formed by making the surface of the optical disk 
1 coarse in the order of 1 micrometer in the same manner as the dimension 
of the guide tracks, the index mark 9 can be readily detected by an 
incoherent light source, such as a light-emitting diode, without using a 
laser light source. Therefore, the index mark 9 can be formed 
simultaneously with the guide tracks when cutting the original or master 
disk. 
The above-described first embodiment may be suitable for recording and 
reproducing video signals, and a second embodiment of an optical disk 
which is suitable for digital signals will be described hereafter. FIG. 8 
is a top plan view of the second embodiment optical disk which comprises a 
plurality of rotation index marks 20 corresponding to the index mark 9 of 
FIG. 2 and a plurality of sector index marks M1 to M8 indicative of 
positions of sector regions of the disk 1. These two kinds of index marks 
20 and Ml to M8 are circularly arranged on a single band. The disk 1 
comprises a recording track region 7 having groove-like guide tracks, a 
track address region A formed on the guide tracks in advance, and eight 
sector regions S1 to S8. The band having the two kinds of, index marks 20 
and M1 to M8 is located inside the recording track region 7. The address 
region A comprises a separation or margin region GA and a track address 
data region DA, while each of the sector regions S1 to S8 comprises a 
separation or margin region G1, G2 . . . or G8, and a data recording 
region D1, D2 . . . or D8. The separation regions GA and G1 to G8 are not 
used for recording any information, and are provided for separating the 
data recording regions D1 to D8 from each other so that data is not 
erroneously written across two data recording regions. 
FIGS. 9A and 9B are enlarged views showing examples of the index mark 20 of 
FIG. 8. The example of FIG. 9A comprises a plurality of grooves 21 where 
each groove has a given depth and a length q1. A plurality of grooves 21 
are arranged at a pitch q2 in a direction parallel to the circumference, 
and a plurality of rows of like grooves 21 are coaxially arranged to form 
a sector groove portion. The other example of FIG. 9B differs from the 
index mark of FIG. 9A in that each of the grooves 21 is formed of grooves 
22 having a small pitch q3. 
Turning back to FIG. 8, each of the sector index marks M1 to M8 is formed 
in the same manner as the index mark 9 of the first embodiment as shown in 
FIG. 5A or FIG. 5B. The rotation index marks 20 having a structure of FIG. 
9A or FIG. 9B are arranged circularly in a direction parallel to the 
circumference together with the sector index marks M1 to M8 in such a 
manner that each rotation index mark 20 is interposed between two 
consecutive sector index marks. Since the structure of the rotation index 
marks 20 is different from that of the sector index marks M1 to M8, these 
two kinds of index marks can be readily distinguished from each other when 
reproducing. Namely, as will be described later in detail, a signal 
component indicative of the period q2 of the grooves 21 is selectively 
detected in a reading circuit. As an alternative, the angle .beta. of the 
sector-like groove portion of each of the sector index marks M1 to M8 (see 
FIGS. 5A and 5B) may be made different from that of the rotation index 
marks 20 for distinguishing them from each other. 
FIG. 10 shows a block diagram of an optical disk recording and reproducing 
apparatus which writes and reads information on and from respective 
sectors of the second embodiment disk. The disk 1 having the structure of 
FIG. 8 is arranged to be rotated by a disk drive motor 23 so that the disk 
1 spins stably. The apparatus of FIG. 10 comprises an index mark detector 
11 which is substantially the same as that of FIG. 6. The output signal of 
the index mark detector 11 is fed to a rotation index mark detecting 
circuit 24 and to a sector index mark detecting circuit 26. The rotation 
index mark detecting circuit 24 produces a pulse train signal, and each 
pulse will be referred to as a rotation starting point pulse "d". The 
rotation starting point pulse "d" is fed to a disk drive motor driving 
circuit 25 so as to control the phasic synchronism of the disk drive motor 
23. 
The sector index mark detecting circuit 26 is responsive to a signal 
component indicative of the sector index marks M1 to M8 on the optical 
disk 1, and produces a pulse train signal accordingly where each pulse is 
referred to as a sector pulse "e". The structure of the above-mentioned 
rotation index mark detecting circuit 24 and the sector index mark 
detecting circuit 26 will be described with reference to FIG. 12 in 
connection with a following third embodiment. The number of the sector 
pulses "e" from the sector index mark detecting circuit 26 is counted by a 
sector counter 27, and thus the output signal of the sector counter 27 
indicates the address of a sector of a track which is right above (or 
below) an optical head 28 through which reading and writing of information 
data are effected. The sector counter 27 is periodically reset by the 
rotation starting point pulse "d" corresponding to the rotation index mark 
20 so that a sector address output "f" from the sector counter 27 will be 
initialized. 
Writing operation in connection with a sector of a guide track on the 
optical disk 1 will be described with reference to FIG. 10. Data to be 
written is first transferred to a buffer memory 30 in accordance with the 
control by a CPU (central processing unit) 29. Then the CPU 29 sets a 
track address of a track, in which the data will be written, in a track 
address register 31, and also sets a sector address in a sector address 
register 32. When the CPU 29 emits a write instruction, a reproducing 
signal from the optical head 28 is equalized and shaped by an RF 
reproducing circuit 33 to be fed to a track address reproducing circuit 
34. Address information of a track which is being tracking-controlled at 
present time is read out, and the read out address will be stored in a 
present track address register 35. The present track address will be 
compared with a writing track address from the aforementioned track 
address register 31 in a track address coincidence control circuit 36 so 
that an actuator 37 is driven to move the optical head 28 at a high speed 
in the case of noncoincidence. The movement of the optical head 28 is 
performed with the tracking control of the optical head switched off, by 
counting the number of crossing tracks represented by a track crossing 
signal of a tracking error signal by means of a crossing track number 
counter 38. Namely, the track address conincidence control circuit 36 
determines the direction of the movement of the optical head 28 in 
accordance with the relationship between the present track address and the 
writing track address. When the difference between theses two addresses 
equals the number of crossing tracks represented by the output signal of 
the crossing track number counter 38, an actuator driving circuit 39 is 
controlled so as to stop the actuator 37. 
When the optical head 28 faces the writing track, tracking control is 
effected, and then comparison between sector addresses is performed by a 
sector address coincidence circuit 40. When the sector address indicated 
by the output signal of the sector counter 27 conincides with a writing 
sector address from a sector address register 32, the output signal of the 
buffer memory 30 is modulated by a data encoder 41 to be supplied to a 
semiconductor laser driving circuit 42. The semiconductor laser driving 
circuit 42 controls a semiconductor laser (not shown) of the optical head 
28 so that a laser light beam is applied to the optical disk 1 for 
effecting writing and reading. The intensity of the laser light beam is 
controlled so that optical modulation is effected in such a manner that 
the optical output whose power is sufficient for recording or writing is 
obtained for performing threshold writing. However, on reading, the output 
power of the semiconductor laser is set to a value which is much lower 
than that on writing so that the recording material of the optical disk 1 
is not optically changed. In other words, the optical output level is 
maintained low so as not to rewrite or erase the prerecorded information. 
In the present invention, the optical output of the semiconductor laser is 
several milliwatts on reading, while the optical output is 10 milliwatts 
on writing. 
A read/write control circuit 43 is provided to control the reading and 
writing operations of the apparatus. In detail, the read/write control 
circuit 43 is responsive to the output signals from the track address 
conincidence control circuit 36 and from the sector address conincidence 
circuit 40 for producing an output control signal when these two input 
signals coincide. The output control signal will be emitted after a given 
interval from the instant of conincidence and lasts for a predetermined 
interval so that the output control signal will be used as a gating signal 
in the data encoder 41, the buffer memory 30, the semiconductor laser 
driving circuit 42 and a data reproducing circuit 70 responsive to read 
out information from the RF reproducing circuit 33. After the data from 
the buffer memory 30 is written into a selected sector of the optical disk 
1 via the optical head 28, the semiconductor laser driving circuit 42 is 
controlled so that its optical output is set to a low level to be prepared 
for subsequent writing or reading. 
Data reading from a given sector will be performed in a similar manner to 
the above-described writing operation. Namely, the optical head 28 is 
moved and the rotation of the disk 1 is controlled so that the optical 
head 28 faces a selected track of a selected sector. After access of the 
optical head 28 is completed, the output signal of the RF reproducing 
circuit 33 is reproduced by the data reproducing circuit 70 and the 
reproduced data will be stored in the buffer memory 30. 
FIG. 11A shows a third embodiment of the optical disk according to the 
present invention. The third embodiment disk is used for recording mainly 
digital signals, and therefore, each track is divided into four sectors S1 
to S4. The third embodiment disk differs from the previous embodiments in 
that two address information regions 8 are provided; one between the first 
and fourth sector regions S1 and S4, and the other between the second and 
third sector regions S2 and S3. Namely, each of the coaxially arranged 
tracks has two address recording sectors which are located oppositely to 
each other with respect to the center of the disk 1. Inside the guide 
track region 7, an annular index mark region 47 is provided coaxially, and 
two address index marks 44 each indicative of a starting point of the 
address information region 8 are respectively positioned within an arc 
portion of the index mark region 47 where the angle of the arc equals that 
of the address information region 8. 
The reference 45 indicates sector index marks corresponding to the sector 
index marks M1 to M8 of FIG. 8, and the number of the sector index marks 
45 equals the number of the sectors S1 to S4 (in this embodiment, the 
number is four). The reference 46 is a rotation index mark corresponding 
to the rotation index marks 9 and 20 of the previous embodiments. 
FIG. 11B shows a cross-sectional view of the disk 1 of FIG. 11A taken along 
the track of the index mark region 47. The above-mentioned various index 
marks 44, 45 and 46 are formed in the form of concave or recessed portions 
as shown in the cross section. 
FIG. 12 is a block diagram of a reading circuit which picks up various 
information from the index mark region 47 of the third embodiment disk 1. 
The circuit of FIG. 12 also functions as the combination of the rotation 
index mark detecting circuit 24 and the sector index mark detecting 
circuit 26 of the second embodiment of FIG. 10. Namely, the circuit of 
FIG. 12 has three output terminals for delivering three output signals 
"m", "n" and "r" which respectively correspond to the signals "e", "d" and 
the other output signal of the sector index mark detecting circuit 26 of 
FIG. 10. 
In FIG. 12, incoherent light emitted from a light emitting diode 48 is 
applied to the index mark region 47 on the optical disk 1. When the light 
is applied to the index mark, the amount of light reflected thereat 
varies, and the variation will be detected by a phototransistor 49. A 
detection signal from the phototransistor 49 is fed to an amplifier 50, 
and is then supplied to a band pass filter 51 for removing from the 
received signal the variations in the amount of received light due to the 
vibrations of the surface of the optical disk 1. After this, the detection 
signal becomes a pulse train signal by means of a comparator 52. FIGS. 
13A-13I are timing charts showing the waveforms of various signals in the 
circuit of FIG. 12. In FIG. 13A, the same cross-sectional view as FIG. 11B 
is shown for easy understanding of the relationship between the waveform 
of the signals and various index marks. 
When the light from the light-emitting diode 48 is applied to the first 
index mark 45, the amplitude of the output signal of the phototransistor 
49 becomes small because the light is scattered (see signal "h" at FIG. 
13B). This signal "h" is processed in the above-described manner so that 
the comparator 52 produces an output pulse train signal "i" shown at FIG. 
13C. Each of the pulses of signal "i" will be referred to as an index 
pulse hereafter, and this index pulse "i" is fed to a retriggerable 
multivibrator 53 so that the multivibrator 53 produces a pulse "j" shown 
at FIG. 13D having a width "t" (T&lt;t&lt;2T wherein T is the period of the 
index pulses "i" which would be obtained in the case that index marks are 
successively provided. The output pulse "j" of the multivibrator 53 is fed 
to a D-flip-flop 56 to be latched therein by the above-mentioned index 
pulse "i" functioning as a clock pulse. As a result, a pulse "k" shown at 
FIG. 13E is developed at the output terminal Q1 of the D-flip-flop 56, and 
this pulse "k" is fed to another D-flip-flop 57 which is also responsive 
to the index pulse "i" functioning as a clock pulse. An output pulse "l" 
shown at FIG. 13F, is developed at the output terminal Q2 of the latter 
D-flip-flop 57. Two inverters (without reference numerals) are provided to 
produce negative logic signals "k" and "l". An "and" output of the signals 
"i", "k" and "l" is produced by an AND gate 58 to deliver a sector, 
starting point detection signal "r", shown at FIG. 13I. A second AND gate 
59 produces an "and" output of the signals "i", "k" and "l" as a rotation 
starting point detection signal "n", shown at FIG. 13H. A third AND gate 
60 produces an "and" output of the signals "i", "k" and "l" as an address 
starting point detection signal "m", shown at FIG. 13G. 
These index marks may be used not only for stably detecting the position of 
information on the disk 1 but also for controlling track-jumping operation 
during searching. For instance, in the case that information is 
prerecorded in the form of spiral tracks, it is sometimes necessary to 
repeatedly reproduce the data of a single track. In this case, it is 
required to perform track-jumping once a revolution. 
Jumping operation will be described with reference to FIGS. 14 and 15A-15E. 
The timing of jumping is basically determined by detecting the 
aforementioned rotation index mark 46. Since there is a time lag from the 
instant of detection of the rotation index mark 46 till the instant of 
performing track-jumping with the pickup of the optical head driven, the 
position of the optical detector 11 which detects the rotation index mark 
46 is shifted by an angle .theta.. With this arrangement, jumping is 
effected so that the information contained in a single track can be stably 
reproduced from the beginning of the address data. Since the optical head 
28 starts jumping earlier as much as a period of time corresponding to the 
angle 8 as shown in FIG. 14, jumping is effected within a predetermined 
jumping section 61, and thus reproduction from the beginning of 
information can be possible. Timing of jumping will be further described 
with reference to timing charts of FIGS. 15A-15E. 
In FIG. 15A, the reference 80 is a waveform of detection pulses of the 
index marks, 82 is reproduced address data, shown at FIG. 15B which is 
positioned at the beginning of one track. information. A waveform 84, 
shown at FIG. 15C, indicates the position of rotation starting point 
detection pulse which would be obtained if the index mark detector 11 is 
not shifted. Namely, actual timing of the rotation starting point 
detection pulse is shifted by the angle .theta. as shown by a pulse 86, 
shown at FIG. 15D. Track-jumping is triggered by the leading edge of the 
pulse 86. The reference 88 indicates the movement of the optical head 28 
in the radial direction or the disk 1, shown at FIG. 15E. With this 
operation, track-jumping will be completed within the jumping period or 
section 61 so that the optical head 28 is capable of reproducing 
information from its beginning. 
Although it has been described in the above that information is to be 
recorded on one side of the optical disk 1, it is also possible to record 
information on both sides thereof as is well known. In this case, it is 
necessary to detect the front or back of the disk 1. The index mark 
according to the present invention may be used to ascertain the side of 
the disk 1. Namely, index marks having different structures are 
respectively formed on the both sides of the disk 1. For instance, the 
number of index marks or the width of the index mark may be made different 
from the other index mark on the opposite side. The difference in 
structure will be detected in the same manner as described in the above to 
ascertain the side which faces the optical head. 
FIGS. 16A and 16B show a fourth embodiment of the optical disk according to 
the present invention, where the disk has index marks on its both sides 
for distinguishing the front and back sides from each other. The fourth 
embodiment optical disk 1 comprises two index marks 62 on its upper 
surface as shown in FIG. 16A, and one index mark 63 on its lower surface 
as shown in FIG. 16B. Each of the index marks 62 has the same groove, 
structure as the index mark described in the above with reference to FIGS. 
5A and 5B. However, if desired, the structure of the index marks shown in 
FIGS. 9A and 9B may also be used. 
In addition to, distinguishing between the sides of an optical disk, a 
similar index mark may be used to distinguish a disk from another. In 
other words, a plurality of kinds of disks may be distinguished from 
others by forming a specific index mark on disks of a given kind. Since 
the index mark can be used to ascertain the front or back side of a disk, 
and to ascertain the kind of a disk, there is an advantage that there is 
no need to scan a track carrying such information by means of a laser 
light beam. In the case that different intensity of reading laser beam and 
writing laser beam is required in accordance with the kind of the disk, 
suitable intensity of the laser light beam can be selected without 
energizing the laser. 
As described in detail in the above, according to the present invention 
synchronism between the address information on each track and recording 
signal can be readily obtained so that information can be recorded 
immediately after the address information when recording a real-time 
signal, such as a video signal on an optical disk on which address 
information is written in each track so as to perform high-speed searching 
which is required in an optical memory having large capacity. 
According to the present invention, since index mark(s) is used, rotation 
starting point, address information starting point and sector starting 
point can be readily and stably detected even in high-speed searching 
mode. 
The index mark(s) used in the present invention comprises a phase structure 
made of concave-convexo groove(s) in the same manner as guide tracks. 
Therefore, the index mark(s) can be formed simultaneously at the time of 
cutting the master disk. This means that the phasic relationship between 
the index mark(s) and the address information of each track can be 
accurately set. In addition, since there is no need to provide a process 
of adding (an) index mark(s) to the finished product of the disk as the 
aforementioned conventional disks, the productivity of the optical disks 
according to the present invention is very high. Furthermore, since the 
index mark(s) can be detected by using an incoherent light source, timing 
of track-jumping and various information indicative of the side as well as 
the kind of the disk can be detected without using a laser light beam. 
The above-described embodiments are just examples of the present invention, 
and therefore, it will be apparent for those skilled in the art that many 
modifications and variations may be made without departing from the spirit 
of the present invention.