JPH0428016A - Method and device for optical recording and reproducting - Google Patents

Abstract

PURPOSE:To improve recording density by recording first information as a mark set to be encoded by a gap among plural pits formed in the width direction of a track, and encoding and recording second information in the length direction of the track by the gap among the mark sets. CONSTITUTION:The mark set 73 having the first information formed with the plural pits arranged at the prescribed gap in the width direction of the track are provided. Also, plural mark sets 73 are formed in the length direction of the track at the prescribed gap, which is set as the mark sets 73 having the second information encoded by the mutual gap of the mark sets 73. Diffracted light in which an interference pattern is changed by a laser beam irradiating those mark sets 73 sequentially is detected sequentially. In such a way, it is possible to stably perform multivalue recording and multivalue reproduction by the change of an optical interference pattern in the width and length directions of the track by a compact and simple means.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an optical recording/reproducing method and an apparatus thereof.

In particular, the present invention relates to a multilevel information recording and reproducing method and apparatus for improving recording density.

[Prior Art] Optical disks and optical cards are conventionally known as media for recording and reproducing information using light. In these media, read-only (ROM) CDs and laser discs, write-once (WROM) Te metal thin films and organic dye films, and reversible (E-DRAW) magneto-optical and chalcogenide phase change films are used to provide information. Recording and reproducing methods and devices have already been put into practical use. These methods and devices irradiate pits formed in the moving direction (track length direction) on the disk with a laser beam focused to about the wavelength, and detect reflected light (
or transmitted light) is optically detected through a lens. If pits are formed on the disk, the diffraction light reflected from the pits will spread out, causing a component that deviates from the receiving lens aperture. That is, when a laser beam hits a pit, the laser beam reflected from the bottom of the pit and the laser beam reflected from around the pit interfere with each other, and the intensity of the reflected light changes. Therefore, by detecting the reflected light intensity and converting it into an electrical signal, a reproduced signal can be obtained.

In order to improve the recording density of such discs,
The following methods have been adopted. That is, methods that change the reflectance at pits in stages, those that change the depth of the pits, those that change the size of the pits, those that change the pit spacing, those that reduce the track spacing, or those that change the disc spacing. Some devices improve recording density by recording information in each layer of a multilayer film. These are straightforward methods of changing the state of one pit step by step to create multiple values.

Another method, as shown in Japanese Patent Application Laid-open No. 61-129748, is to use a set of information as a pattern (optical interference or diffraction pattern) of a certain area with different surface shapes.
There is a method of converting the information into the original information and recording it, and then obtaining the optical interference/diffraction pattern from the pattern of the laser beam irradiated from the light source as the original set of information during reproduction.

[Problems to be Solved by the Invention] In the conventional optical recording/reproducing method and its device, it is difficult to determine the reflectance, depth, size, or interval of one pit, or the track interval, or the state of the multilayer film in the thickness direction. However, it is difficult to adjust the pit signals step by step to the extent that they can be accurately distinguished, and multi-value recording from 2 to 4 values is the limit. be. Further, a method of recording a set of information as a pattern in a fixed area can be used in a simple system that does not require tracking or focusing, but cannot be used in a high-density system because the recording density is low. If disks are made larger in order to promote multi-value technology, the size of the device must also be increased accordingly.

The present invention has been made in order to solve the above-mentioned problems, and its purpose is to achieve multilevel recording and optical recording in the track width direction and track length direction by compact and simple means. Multi-level reproduction by changing target interference pattern,
An object of the present invention is to provide an optical recording/reproducing method and a device thereof that can stably perform the following.

[Means for Solving the Problems] In order to solve the above-mentioned problems, an optical recording/reproducing method and an apparatus thereof according to the present invention irradiate a laser beam at predetermined intervals in the width direction of a track of an optical recording medium. forming a mark set consisting of a plurality of pits, and recording first information encoded by intervals of pits in the mark set in the width direction of the track on an optical recording medium.
and the first recording step in the longitudinal direction of the track to form the mark sets sequentially at predetermined intervals in the longitudinal direction of the track, a second recording medium for recording second information encoded by mutual spacing of the set on the optical recording medium;
a recording step of irradiating the mark set with the laser beam to record the first mark represented by the spacing between the plurality of pits of the mark set.
a first reproduction step of detecting and reproducing the information of the optical interference pattern of the laser beam irradiated on the mark set; a second reproduction step of sequentially irradiating the mark sets along the sets and detecting changes in the diffracted light to reproduce second information represented by the mutual spacing of the mark sets; In order to perform each of these steps, irradiating the laser beam at predetermined intervals in the width direction of the track on the optical recording medium to form a mark set consisting of a plurality of pits; a first recording means for recording first information encoded by intervals of pits in the width direction of the track in the mark set on the optical recording medium; a second recording means for sequentially forming a plurality of mark sets at predetermined intervals in the horizontal direction and recording second information encoded by the mutual intervals of the mark sets on the optical recording medium; An optical recording device and the first and second recording means direct the laser beam onto each of the mark sets on which the first and second information encoded and recorded are recorded, the length of the track. an irradiation means for sequentially irradiating the laser beam along a direction; and a first apparatus for detecting diffracted light of an optical interference pattern of the laser beam corresponding to each of the mark sets irradiated by the irradiation means and reproducing it as the first information. 1 reproduction means and the irradiation means sequentially detect changes in the diffracted light of the optical interference pattern of the laser beam corresponding to each of the mark sets irradiated by the irradiation means, and detect the changes in the diffracted light as the second information. an optical reproducing device having a second reproducing means for reproducing the image as a second reproducing means; The laser beam irradiated from the irradiation means to the mark set is 1. The length of the laser beam in the width direction of the track is longer than the maximum value of the interval between the mark sets formed in the width direction of the track, and the laser beam is irradiated from the irradiation means to the mark set. The length of the laser beam in the length direction of the track is shorter than the minimum value of the mutual spacing between the mark sets along the length direction of the track.

[Providing a mark set having first information encoded by a plurality of pits arranged at predetermined intervals in the track width direction, and also disposing the mark set at predetermined intervals along the track length direction By forming a plurality of mark sets and having second information encoded by the mutual spacing of the mark sets, and sequentially detecting diffracted light whose interference pattern has changed due to laser beams sequentially irradiated to these mark sets. Multi-level recording and playback becomes possible.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to the drawings.

With reference to FIGS. 1 to 3 and 7, a recording optical system 1o as first and second recording means responsible for the first and second recording steps will be described.

As shown in FIG. 1, a recording laser beam 11 having a predetermined light intensity emitted from a laser diode 1 is converted into a parallel beam via a collimator lens 2.

At this time, the cross section of the laser beam 11 has an elliptical shape, as shown in FIG. 2 as a cross section taken along the line ■-■.

Subsequently, the laser beam 11 is irradiated onto the AO deflection element 4 via the shaping prism 3. At this time, the cross section of the laser beam 11 has a circular shape, as shown in FIG. 3 as a cross section taken along the line ■-■. The beam 11 irradiated onto the AO deflection element 4 is frequency-modulated and optically deflected. The optically polarized laser beam 11 passes through the polarizing beam splitter 5, where only the P component is transmitted, and the polarization becomes 1/4.
It is rotated by 45 degrees through a wavelength plate, changing from linear polarization to circular polarization, and is irradiated onto a track (not shown) of an optical disk 8 through an objective lens 7 for exposure. At this time, since the laser beam 1 which was previously irradiated onto the disk 8 is optically deflected by the AO deflection element 4, it is deflected into a plurality of beams (in this embodiment, two beams) and is irradiated.

As a result, a pair of pits 71, 71 are formed in the track in parallel in the track width direction, and these are configured as a mark set 73 having first information encoded by the interval in the track width direction (seventh (see figure). The laser beam 11 reflected from the optical disk 8 passes through the 1/4 wavelength plate 6 again, so the polarization plane changes from the original linearly polarized light 90
'It is rotated linearly polarized light. Therefore, it is reflected by the polarizing beam splitter 5 and irradiated onto the focus/tracking servo detector 9. The focus/tracking servo detector 9 detects changes in the amount of light of the laser beam 11 irradiated to the detector 9 and detects pits 71 on the track due to surface vibration, eccentricity, etc. of the optical disk 8.
．． This prevents defective formation of pits 71 and 71 and controls stable formation of pits 71 and 71. In this embodiment, the depth of the pits 71 and 71 is 1/4 of the wavelength λ of the laser beam 11 in the case of a reflective type.

Using such a recording optical system 10, a plurality of mark sets 73 are sequentially formed at predetermined intervals in the running direction along the length of the track. As a result, the second information encoded by the change in the interval between these mark sets 73 is recorded in the track length direction.

Conventionally, only one pit was formed in the track width direction, so only binary data of whether there was a pit or not could be expressed, whereas the recording optical system 10 of this embodiment
According to , a pair of pits 71 parallel to each other in the track width direction
．． 71 are formed, and these are considered as one set as mark set 7.
3 are made up. Therefore, by appropriately setting the deflection timing in the AO deflection element 4, multivalued information corresponding to the interval between the pits 71, 71 is recorded as first information in the width direction of the optical disc 8, and the recording density is improved.

Furthermore, by changing the interval at which these pits are formed along the track length direction, second information corresponding to the change in this interval is recorded in the track length direction of the optical disc 8, improving the recording density. .

That is, mark set 7 consisting of a pair of pits 71 and 71
3, two pieces of information, the first information and the second information, are recorded, thereby improving the recording density.

Next, the reproduction optical system 40 as first and second reproduction means responsible for the first and second reproduction steps will be explained with reference to FIGS. 4 to 7. Note that the same components as those of the recording optical system 10 are given the same reference numerals, and detailed explanations thereof are omitted.

As shown in FIG. 4, a reproduction laser beam 11a having a predetermined light intensity weaker than the recording laser beam 11 emitted from the laser diode 1 is converted into a parallel beam via a collimator lens 2. . The cross section of the laser beam lla at this time has an elliptical shape, as shown in FIG. 5 as a cross section along the V-v line. Subsequently, the laser beam 11a is transmitted through a polarizing beam splitter 5.174.
The light is irradiated onto a track (not shown) of an optical disk 8 via a wavelength plate 6 and an objective lens 7. At this time, the irradiation area of the laser beam lla irradiated on the optical disc 8 is VI-
As shown in FIG. 6 as a cross section taken along line VI, the optical disk 8 has an oval shape that is long in the track width direction and short in the track length direction. The laser beam lla reflected from the optical disk 8 is again irradiated onto the beam splitter 42 via the 1/4 wavelength plate 6 and the polarizing beam splitter 5.

This beam splitter 42 functions as a half mirror, and splits the irradiated laser beam lla and irradiates it onto the focus/tracking servo detector 9 and the signal detection linear sensor 43. Then, the signal detection linear sensor 43 measures the diffracted light of the optical interference pattern from the detected mark set 73 and reproduces the information, as will be described later.

As shown in FIG. 6, the laser beam 11a irradiated onto the optical disc 8 has an elliptical shape because the emitted beam shape of the laser diode 1 is used as is. In addition,
At this time, the laser beam 11a is focused by the objective lens 7 and its long and short axes are reversed. As shown in FIG. 7, pits 71 and 71 formed by the recording optical system 10 are arranged in parallel in the track width direction. If a circular laser beam lla is used as the laser beam 11a for reproduction, the laser beam 11a will overlap the pits 71 of other mark sets 73 adjacent to each other in the track length direction, and this overlapping portion will generate noise during reproduction. Therefore, there is a possibility that the recorded information cannot be read out accurately. Therefore, the laser beam 11a irradiated by the reproduction optical system 40
As the irradiation area 74 of
The optical disk 8 is emitted with an elliptical laser beam lla that is longer in the track length direction and shorter than the minimum value 1 of the mutual spacing between the mark sets 73, that is, it is long in the track width direction of the optical disc 8 and short in the track length direction. It is. Note that even if the laser beam 11a has an elliptical shape, it is preferable to limit the irradiation area 74 by using polarizing means such as a slit or a polarizing plate.

In the optical recording and reproducing apparatus of this embodiment having the recording optical system 10 and the reproducing optical system 40 as described above, recording and reproducing of information is processed in units of a fixed amount of information called sectors. That is, FIG. 9 shows one sector of the sector format of the current ISO standardized continuous servo system applied to this embodiment. This sector consists of 1360 bytes, and the data section 90 in which information can be recorded is 1259 bytes. Various information is recorded in this data section 90. In the track width direction, 4-bit information is recorded at intervals of 71.71 pits. 8-bit information is recorded in two mark sets 73 and 73. The error correction code (E
CC, CRC), resynchronization pattern (RESYNC)
, recording information DATA (recording area, recording conditions), FAT (Pile) indicating the continuous state of sectors on the disk.
Predetermined pit position recording of 2-7 modulation is performed by combining information such as A11location Table). Since the preformat section 91 is a section that requires high reliability, it is not recorded and is only recorded in the track length direction.
A predetermined pit position recording of one pit is performed with -7 modulation. The ID area 92 in which the track number and sector number of the preformat section 91 are recorded is required to have high reliability. Furthermore, depending on the state of defect occurrence, it may become impossible to reproduce the information of an entire sector, which may reduce the usability rate for the user. Therefore, the ID area 92 contains a CRC (Cyclic Redundancy Code) for error detection.
dancy Code) is added and triple-written,
In front of each ID area 92, 92, 92, there is a mark for voltage controlled frequency oscillation, "'C(7) V F Or
VFO□ and VFO2 areas 93.93.93 are arranged. Even when SM (sector mark) 94 cannot be detected due to a defect, AM (address mark) 95
By making it an irregular pattern that is not in the modulation rules,
It is designed to detect the ID from AM. ID
Triple writing of the area 92.92.92 improves reliability.

The ODF area 96 is a 1-byte area without a guide groove, and is used to detect the amount of tracking offset. Furthermore, the 5-byte FRAG area within the ODF area 96 is an area that indicates whether information has already been recorded in the sector, and is used to prevent erroneous overwriting. In the ODF area 96, A L P C (Auto
matic La5er Power Control)
A 2-byte area (not shown) is provided as a trial recording area for adjusting the optimum recording laser power. A synchronization pattern (SYNC) area 97, a VFO3 area 93, and a PA area 98 are provided in front of this.

Next, the recording and reproducing method of this embodiment having the recording optical system 10 and the reproducing optical system 40 as described above will be explained with reference to FIGS. 1 and 4.
This will be explained with reference to the drawings and FIGS. 8A to 8C.

In the recording method, a preformat section 91 formed in advance in the track length direction is read by the reproducing optical system 40, and pits 7 and 7 are read.
Digital information is reproduced with brightness and darkness depending on the presence or absence of 1. next,
The data section 90 stores information to be recorded in the recording optical system 10.
Record simultaneously using That is, the information in the data section 90 is the first information about the pair of pits 71 .
The error correction and resynchronization patterns are recorded as second information as the intervals between mark sets 73 in the track length direction.

The reproduction method is performed using a reproduction optical system 40. A signal detection linear sensor 43 measures changes in the interference pattern (see FIGS. 8A to 8C) obtained by modulating the distance between a pair of pits 71 and 71 formed in the track width direction,
Play information.

On the other hand, in the track length direction, changes in the interference pattern are measured by the signal detection linear sensor 43 depending on the presence or absence of the mark set 73 at predetermined intervals (time), and information is reproduced.

Here, the above-mentioned reproduction method will be explained separately in the track width direction and the track length direction.

First, in the track width direction, when the reproduction laser beam 11a is irradiated to the position indicated by the symbol A in FIG. 7, an optical interference pattern corresponding to the distance between the pair of pits 71 and 71 is formed as shown in FIG. appears in Information is reproduced by measuring the peak positions D-1, Dl, and DO of the ±1st-order diffracted light and the 0th-order diffracted light of this pattern with the signal detection linear sensor 43. At the position indicated by the symbol B, since there is no mark set 73, no optical interference pattern is obtained, resulting in a pattern of only 0th-order diffracted light as shown in FIG. 8B. When the reproducing laser beam 11a is irradiated to the position indicated by symbol C, the optical interference pattern becomes as shown in FIG. 8th
This is different from the pattern in Figure A. In this case as well, the 8th C
Peak position D-1 of ±1st-order diffracted light and 0th-order diffracted light in the figure,
Information is reproduced by measuring Dl and DO with the signal detection linear sensor 43. As a measurement method, it is also preferable to measure either the change in D-1, Dl, or the change in DO using the signal detection linear sensor 43 and reproduce the information.

Next, in the track length direction, information is reproduced by sequentially detecting changes in the optical interference patterns (see FIGS. 8A to 8C) corresponding to symbols A1B and C in FIG. 7. That is, the information is reproduced as digital information depending on the presence or absence of the mark set 73. As a detection method, information is reproduced by changing the peak position DO of the 0th-order diffracted light corresponding to "0" and 1". Alternatively, the change in the peak position DO of the 0th-order diffracted light is reproduced. Alternatively, the peak position D-1, Dl of the ±1st-order diffracted light is
It is also preferable to reproduce information in correspondence with changes in "0" and "1".

FIG. 10 shows an example of a read-only optical disc. In the track width direction, pits 71.71 are shown as first information.
Image data having a recording capacity of 4 bits (16 levels) is recorded at intervals.

On the other hand, in the track length direction, EFM is used as second information.
By pit length recording, a digital audio signal with a changed pit length is recorded. That is, the audio signal and the image signal are combined and recorded simultaneously. This optical disc is
It is formed in the same manner as the CD manufacturing process. After the two signals are combined and recorded using a stamper and transferred to form a disk, a thin film with high reflectivity such as A1 is formed to form a disk. It is preferable that the depth of the jets 71 and 71 is approximately 1/4 of the wavelength λ of the laser beam.

This method of reproducing signals from the optical disc is performed using the reproducing optical system 40. That is, as the optical disk rotates, the reproducing laser beam 11a covering the pair of pits 71.71 is scanned in the track length direction, and the change in the optical interference pattern due to the change in the pitch of the pits 71.71 in the track width direction is ±1. Image data as first information is reproduced by measuring the second order diffracted light and the zeroth order diffracted light with the signal detection linear sensor 43. On the other hand, in the track length direction,
Changes in the ±1st-order diffraction light and 0th-order diffraction destination of the optical interference pattern are sequentially detected depending on the presence or absence of the mark set 73, and by EFM demodulating this, a CD audio signal as second information is obtained. Information in the track length direction is not limited to CD audio, but may also be recorded using image data or analog FM converted to pit length. Further, the track width direction is not limited to image data, but may be other data.

[Effects of the Invention] According to the optical recording/reproducing method and apparatus thereof of the present invention, a plurality of pits are formed in the width direction of the track, and recording is performed as a mark set in which the first information is encoded by the intervals between these pits. Furthermore, in the track length direction, the recording density is improved by encoding and recording the second information based on the mutual spacing between mark sets. Therefore, large-capacity recording can be made compact, and the device configuration is simple.
Moreover, it becomes compact. Furthermore, when reproducing information recorded in these mark sets, since the laser beam irradiation area is limited, adjacent mark sets are not irradiated with the laser beam, and noise due to diffracted light from adjacent mark sets is generated. This does not occur, and recorded information can be stably reproduced.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a recording optical system, which is one component of the present invention, used to record information on an optical disk. FIG. Figure 3 is a cross-sectional view of the laser beam along the line ■-■.
FIG. 1 is a sectional view taken along the line ■-■ of the laser beam for recording when emitted from the shaping prism of the recording optical system, and FIG. 4 is a configuration of the present invention used for reproducing information from the previous disk. FIG. 5 is a schematic diagram of the reproduction optical system shown in FIG. 4, and FIG. , a cross-sectional view along the line Vl-VI on the optical disk of the reproduction laser beam irradiated onto the optical disk by the reproduction optical system, FIG. 7 is a diagram showing pits formed on the optical disk by the recording optical system, Figures 8A and 8B are
FIG. 9 is a diagram showing the optical interference pattern of the reproduction laser beam irradiated onto the pit by the reproduction optical system, respectively.
FIG. 10 is a diagram showing the sector length when the sector format of the continuous servo system is applied to the present invention. As another example of reproducing information by applying the reproducing optical system of FIG. FIG. 3 is a diagram showing pits in which both are recorded in a composite manner at the same time. 4... AO deflection element, 10... Recording optical system, 11.
11a... Laser beam, 40... Reproduction optical system, 7
1...Pit, 73...Mark set.

Claims (1)

[Claims] 1. Form a mark set consisting of a plurality of pits by irradiating a laser beam at predetermined intervals in the width direction of a track of an optical recording medium, and form a mark set consisting of a plurality of pits in the width direction of the track. a first recording step of recording first information encoded at intervals of , on an optical recording medium, and performing the first recording step sequentially in the length direction of the track,
The mark sets are sequentially formed at predetermined intervals in the length direction of the track, and second information encoded by the mutual intervals of the mark sets in the length direction of the track is recorded on the optical recording medium. a second recording step of irradiating the mark set with the laser beam, and recording the first information represented by the intervals between the plurality of pits of the mark set with the laser beam irradiated onto the mark set; a first reproduction step of detecting and reproducing the diffracted light of the optical interference pattern of the beam; and sequentially irradiating the laser beam along the mark sets formed in the length direction of the track to reproduce the mark sets. An optical recording and reproducing method, comprising: a second reproducing step of detecting a change in the diffracted light and reproducing second information represented by a mutual interval of the diffracted light. 2. Forming a mark set consisting of a plurality of pits by irradiating a laser beam at predetermined intervals in the width direction of the track of the optical recording medium, and encoding by the interval of the pits in the mark set in the width direction of the track. The first
a first recording step of recording information on an optical recording medium;
The first recording step is performed sequentially in the length direction of the track to form the mark sets sequentially at predetermined intervals in the length direction of the track, and the mutual spacing of the mark sets in the length direction of the track is increased. a second recording step of encoding second information and recording it on the optical recording medium. 3. Irradiating the mark set with the laser beam,
A first reproduction step of reproducing the first information represented by the intervals between the plurality of pits of this mark set by detecting the diffracted light of the optical interference pattern of the laser beam irradiated to the mark set. Then, the laser beam is sequentially irradiated along the mark sets formed in the length direction of the track, and second information represented by the mutual spacing of the mark sets is obtained by changing the diffracted light. An optical reproducing method comprising: a second reproducing step of detecting and reproducing. 4. Form a mark set consisting of a plurality of pits by irradiating the laser beam at predetermined intervals in the width direction of the track on the optical recording medium, and determine the distance between the pits in the mark set in the width direction of the track. A first method for recording encoded first information on the optical recording medium.
recording means, and the first recording means sequentially forms a plurality of mark sets at predetermined intervals in the length direction of the track, and the second information encoded by the mutual intervals of the mark sets is stored in the optical disc. a second recording means for recording on a type recording medium; and a laser beam on each of the mark sets in which the first and second information encoded and recorded by the first and second recording means are recorded. Irradiation means for sequentially irradiating the beam along the length of the track;
a first reproduction means for detecting diffracted light of an optical interference pattern of the laser beam corresponding to each of the mark sets irradiated by the irradiation means and reproducing it as the first information; and irradiation by the irradiation means. second reproducing means for sequentially detecting changes in the diffracted light of the optical interference pattern of the laser beam corresponding to each of the mark sets, and reproducing the changes in the diffracted light as the second information; An optical recording/reproducing device comprising: 5. Irradiating the laser beam at predetermined intervals in the width direction of the track on the optical recording medium to form a mark set consisting of a plurality of pits, and determining the distance between the pits in the mark set in the width direction of the track. a first recording means for recording first information encoded by the optical recording medium on the optical recording medium; and a plurality of mark sets sequentially formed at predetermined intervals in the length direction of the track by the first recording means. and second recording means for recording second information encoded by the mutual spacing of the mark sets on the optical recording medium. 6. Directing the laser beam along the length direction of the track onto each of the mark sets in which the first and second information encoded and recorded by the first and second recording means are recorded. irradiation means for sequentially irradiating; and first reproduction means for detecting diffracted light of an optical interference pattern of the laser beam corresponding to each of the mark sets irradiated by the irradiation means and reproducing it as the first information. and a second step that sequentially detects changes in the diffracted light of the optical interference pattern of the laser beam corresponding to each of the mark sets irradiated by the irradiation means, and reproduces the changes in the diffracted light as the second information. 2. An optical reproducing device comprising: 2 reproducing means. 7. The laser beam irradiated from the irradiation means to the mark set has a length in the width direction of the track that is the maximum value of the interval between the mark sets formed in the width direction of the track. 7. The optical reproducing device according to claim 6, wherein the optical reproducing device is longer than . 8. The laser beam irradiated from the irradiation means to the mark set is such that the length of the laser beam in the longitudinal direction of the track is equal to the mutual spacing of the mark sets along the length direction of the track. 7. The optical reproducing device according to claim 6, wherein the length is shorter than the minimum value.