Multi-beam optical recording/reproducing apparatus for recording/reproducing on a medium having a recording flat part with a guide groove on each side

An optical recording/reproducing apparatus wherein a main beam is converged and projected onto a flat part between tracking guide groove parts adjacent to each other in an optical recording medium to record, reproduce or erase information, two sub-beams on both sides of the main beam are respectively converged and projected so as to cover the guide groove parts to form a tracking system for controlling tracking and one of the two sub-beams is utilized to discriminate the flat part.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
This invention relates to an optical recording/reproducing apparatus 
wherein information is recorded in a flat part between tracking guide 
grooves adjacent to each other. 
Recently, there has come to be noted an optical information 
recording/reproducing apparatus wherein information can be recorded at a 
high density by using light beams or information recorded at a high 
density can be reproduced at a high speed instead of an apparatus for 
recording/reproducing information by using a magnetic head. 
An optical apparatus of a photomagnetic system wherein both recording and 
reproducing can be made is being noted to be likely in the future to 
replace the floppy disc apparatus or hard disc apparatus. 
Now, in case information is to be recorded in an optical recording medium 
in the above mentioned photomagnetic system, concavoconvex groove parts 2 
for tracking and prepit parts 3 to be index signals are made in advance on 
a recording medium 1 as shown in FIG. 1. The above mentioned groove parts 
2 and prepit parts 3 are made by copying the signals of a stamper on the 
surface of a base plate of plastic or glass by an injection molding or 2-P 
method (wherein a liquid photosetting resin is applied to paint a stamper, 
is irradiated with a light to be set and is transferred). 
In the conventional apparatus, a beam is tracked on the groove or prepit 
part 3 by a push-pull method to read out index signals and signals are 
recorded, reproduced or erased in the groove part. 
As shown in the above mentioned FIG. 1, the prepit part normally has a pit 
depth of .lambda./4 so that the modulation degree may be maximum but, on 
the other hand, the groove part 2 is so formed as to be of a depth of 
.lambda./8 at which the strength of the push-pull signal will be maximum. 
Now, the stamper for forming the above mentioned groove part 2 or prepit 
part 3 is made by painting a glass original plate with a photoresistor and 
irradiating and exposing it with a laser light. 
When the recording medium in the above mentioned conventional apparatus is 
used, the width, depth or shape of the groove will become nonuniform under 
the influence of the sway or the like of the laser light. The groove of 
the groove part 2 will not be step-shaped, deep and square in the side 
wall as shown in FIG. 1 but will be a smoothly inclined deeply trapezoidal 
groove as shown in FIG. 2 and therefore, under the influence of the 
flatness of the inclined wall surface or the bottom part of the groove, 
the reflection factor will fluctuate, the polarizing surface will be also 
likely to fluctuate, noise components will increase and the S/N or C/N in 
the case of reproduction will be remarkably reduced. 
Also, the flat part of the groove is so narrow that the light amount of the 
reflected light refracted and returning to the objective lens will 
decrease and the signal component will become small. This fact also has a 
great influence on the deterioratio of the S/N or C/N of the reproduction. 
The difference between the depths of the groove part 2 and prepit part 3 
complicates the production process of the above mentioned recording 
medium. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical 
recording/reproducing apparatus which can make a recording wherein the S/N 
or C/N in the case of reproduction can be made large. 
Another object of the present invention is to provide an optical 
recording/reproducing apparatus which is easy to make and can be made low 
in the cost. 
According to the present invention, a recording medium wherein a tracking 
guide groove part is formed to trace a light beam, the flat part between 
the guide groove parts adjacent to each other is made an information 
recording region and an indexing part for discriminating the flat part is 
provided along the above mentioned guide groove part in the boundary part 
of each sectioned flat part is used so that a central main beam of three 
light beams may be converged and projected onto the above mentioned flat 
part to record, reproduce or erase information and the sub-beams on both 
sides of the main beam may be converged and projected onto the above 
mentioned guide groove part to control the tracking and to discriminate 
the flat part.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The (optical) recording medium 12 used in the optical recording/reproducing 
apparatus 11 of the first embodiment is of such formation as is shown in 
FIG. 3. 
The recording medium 12 is disc-shaped and has prepit parts 14 having 
recesses and projections according to index signals formed therein and a 
groove part 15 to be a tracking guide groove both formed along a 
concentric or spiral recording track on one or both recording surfaces. 
Both of the above mentioned prepit part 14 and groove part 15 are of a 
groove depth of .lambda./4 of the light to be used (See FIGS. 3b and 3c). 
As shown in FIG. 4, a flat part 16 is formed between the groove parts 15 
adjacent to each other and is to be an information data recording part 
onto which a main beam 18 will be projected in the form of a spot in case 
information is recorded or reproduced in a three-beam system. Such 
sub-beams 19, 19 will then be projected onto both groove parts 15, 15 on 
both sides of this flat part 16. By the way, each sub-beam 19 is to be 
projected so that the center of the beam may be located in the boundary 
between each groove part 15 and flat part 16. The prepit part 14 has 
discriminating codes respectively representing the indices of the flat 
part 16 on one adjacent side (for example, on the right or left side with 
respect to the light beam running direction). 
The optical system of the optical recording/reproducing apparatus 11 of the 
photomagnetic system of the first embodiment using the above mentioned 
recording medium 12 is of such formation as is shown in FIG. 5. 
A laser light of a single wavelength emitted from a laser diode LD is made 
a parallel light beam by a collimating lens 22 and is separated in the 
angle by a grating 23 into a main beam to be a zero-order refracted light 
beam and two sub-beams which are .+-.1-order refracted light beams and the 
beams then enter a beam splitter 24. 
The light beams having entered and passed through the above mentioned beam 
splitter 24 are converged onto the recording medium 12 by an objective 
lens 25 and are projected as spot light beams. In this case, of the spot 
light beams on the recording medium 12 of the photomagnetic system, as 
shown in FIG. 3a, the main beam 18 will be projected onto the flat part 16 
between the two groove parts 15, 15 and the two sub-beams 19, 19 will be 
projected so that about half of each beam will cover each of the two 
groove parts 15, 15 adjacent to the main beam 18. 
The light reflected by the above mentioned recording medium 12 and having 
passed through the objective lens enters the beam splitter 24. The light 
reflected by this beam splitter 24 is divided by the next beam splitter 
26. One of the divided light beams passes through the beam splitter 26, 
further passes through a light analyzer 27, is then converged by a lens 28 
and enters an avalanche photodiode 29. The above mentioned light analyzer 
27 has its arranging angle set so as to be able to detect the rotation of 
a minute angle of the polarizing surface of the reflected light in 
response to the magnetizing direction of the part in the recording medium 
onto which the main beam 18 is projected. Therefore, the light beam in 
response to the information recorded in the flat part 16 by the main beam 
18 enters the photodiode 29 and the output of this photodiode 29 becomes a 
signal reproducing the recorded information (in the case of a reproducing 
mode). By the way, the sub-beams 19, 19 having passed through the beam 
splitter 26 are converged at both side ends of the photodiode 29, 
therefore do not enter the photodiode 29 and have no influence on the 
photomagnetic recorded information. In the case of a recording mode, the 
flat part 16 in which a strong magnetic film is formed by the main beam 18 
is locally heated to warm the irradiated part to be substantially above 
the phase shifting point, the magnetization is turned in a bias magnetic 
field direction not illustrated and recording is made. 
On the other hand, the light beam reflected by the above mentioned beam 
splitter 26 passes through a light converging lens 31 and cylindrical lens 
32 and enters a six-part divided pin photodetector 33. In this case, the 
main beam will enter a central four-part divided photodetector 33A and a 
focus error signal F.sub.ER will be output from a differential amplifier 
34, for example, by an astigmatism method, the two sub-beams will be 
received respectively by photodetectors 33B and 33C on both sides and a 
tracking error signal T.sub.ER will be taken out by a differential output 
passed through a differential amplifier 35. Also, in this case, as about 
half of each sub-beam projected onto the recording medium 12 covers the 
groove part 15 as described above, in case the prepit part 14 is covered 
by the sub-beam, the sub-beam will be modulated in response to the 
recesses and projections of the prepit part 14. Therefore, the output of 
the photodetector (33C in the illustration) receiving one of the two 
sub-beams is passed through the amplifier 36 to read out an index signal 
(prepit signal). 
The above mentioned focus error signal F.sub.ER and tracking error signal 
T.sub.ER are impressed on a lens actuator respectively through a phase 
compensating circuit and driving circuit not illustrated and the objective 
lens 25 is finely moved to be held in a focus state and in a tracking 
state tracing the predetermined flat part. 
When this first embodiment is used, the information of the photomagnetic 
system will be able to be recorded in the flat part 16 high in the 
flatness between the adjacent groove parts instead of on the groove parts 
in the prior art, therefore the reflection factor will little fluctuate, 
the slope of the wall surface of the groove part 15 will have little 
influence, thus the noises will reduce, the substantial recording area 
will be able to be made large and the S/N in the case of reproduction will 
remarkably improve. Further, as compared with the apparatus of the 
conventional system wherein the groove part is traced with a one-beam 
system, it has been able to be confirmed that, though the modulation 
degree of the signal of the prepit part 14 reduces, if the pit depth of 
the prepit part 14 is made 1/4 the wavelength of the laser light and more 
than half the sub-beam is made to cover the groove part 15 at some 
sacrifice of the tracking error modulating degree, the S/N of the signal 
of the prepit part 14 will be able to be sufficiently taken. 
Also, there are advantages that, when the three-beam method is used for 
tracking, the depth of the groove of the groove part 15 will be able to be 
of a value of .lambda./4 from the conventional value of .lambda./8, 
therefore the depth of the groove will not be required to be changed in 
the prepit part 14 and groove part 15, the number of the production steps 
will be able to be reduced and, even if the precision of the depth of the 
groove is lower than in the same case of .lambda./8, it will be practical 
enough. 
Further, there will be no influence of the offset and inclination produced 
in tracking by the push-pull method and accurate tracking will be 
possible. 
The optical recording/reproducing apparatus of the second embodiment shown 
in FIG. 6 shows the case of a photodisc apparatus system forming pits in a 
recording medium 42. That is to say, as shown in FIG. 8, a prepit part 44 
and groove part 45 are formed on the recording medium 42 the same as in 
the recording medium 12 in the first embodiment and the information to be 
recorded in the case of the recording mode is formed as pits 47 in the 
flat part 46. In the case of the reproducing mode, the information by the 
pits 47 recorded in the flat part 46 will be read out by the three-beam 
method. 
In the apparatus for recording/reproducing information into/out of the 
above mentioned recording medium 42 shown in FIG. 6, for example, a 
P-polarized light beam having passed through the grating 23 is passed 
through a polarized beam splitter 51, is made a circularly polarized beam 
by a .lambda./4-plate 52 arranged between the polarized beam splitter 51 
and objective lens 25 and is projected onto the recording medium 42. In 
this case, the main beam 48 will be projected onto the flat part 46 and 
each sub-beam 49 will be projected to cover the flat part 46 and each 
groove part 45. 
The light reflected by the recording medium 42 passes through the objective 
lens 25, is S-polarized by the .lambda./4-plate 52 and enters the 
polarized beam splitter 51. The light beam reflected by this polarized 
beam splitter 51 is branced by the beam splitter 26 and the passing light 
beam is coverged by the lens 28 and is received by the photodiode 29. The 
light beam reflected by the beam splitter 26 is converged by a light 
converging lens 53 and enters a four-part divided photodetector 54. By the 
way, as shown in FIGS. 6 and 7, a knife-edged shielding plate 55 is 
arranged in front of the above mentioned light converging lens 53. The 
light beam shielded by this shielding plate 55 is received by a two-part 
divided detector 54A and its differential output is taken out by the 
differential amplifier 34 so that the focus error signal F.sub.ER by the 
knife-edge method may be obtained. 
Photodetectors 54B and 54C on both sides of the above mentioned two-part 
divided detector 54A receive the .+-.1-order refracted light and a 
tracking error signal T.sub.ER can be obtained by the differential 
amplifier 35. The signal having passed through one detector 54C is passed 
through the amplifier 36 so that a prepit signal may be obtained. 
The others are substantially the same as in the above mentioned first 
embodiment. As compared with the prior art wherein pits are formed within 
the groove part, even in this second embodiment, a reproduced signal high 
in S/N can be obtained by being little influenced by the slope form of the 
groove wall surface. 
FIGS. 9 and 10 show another embodiment of the recording medium of the 
photomagnetic system. 
In this recording medium 61, the groove part 62 is made wider and a flat 
part 63 is formed also within the groove part 62. As in the above 
mentioned first embodiment, information data are recorded by using the 
part of the flat part 64 before the formation of the groove part 62 and 
the flat part 63 within the groove part 62. The prepit part 65 is formed 
in the boundary part of the flat part 64 and groove part 62, for example, 
by incising the flat part 64. 
Therefore, the main beam is projected onto the flat part 63 or 64 and the 
sub-beams are projected onto the slope parts on both sides of the flat 
part so that information data may be recorded in the flat part 63 or 64 or 
recorded information data may be reproduced. 
When this recording medium 61 is used, the efficiency of utilizing the flat 
part will be able to be elevated and the recording medium will be adapted 
to high density recording. 
By the way, this recording medium 61 can be utilized not only for the 
photomagnetic system but also for the photodisc device and the like. 
In the present invention, for example, the knife-edge method is used in the 
first embodiment, the astignatism method is used in the second embodiment 
or any other known method can be also used. 
Also, for example, a known differential system is used for the 
photomagnetic system and thus the system and shape in the optical 
recording/reproducing apparatus are not limited. 
By the way, the present invention can be extensively utilized in optically 
recording, reproducing or erasing information by using light beams. 
In each of the above mentioned embodiments, the reflected light of the 
light beam projected onto the recording medium is received by the 
photodetector but the passing light may be led to the photodector. 
As described above, according to the present invention, information data 
can be recorded in the flat part between the grooves and therefore the S/N 
of recording/reproduction can be greatly improved. Further, by making the 
depths of the grooves of the prepit part and groove part equal to each 
other, the number of the production steps and the cost can be reduced. 
It is apparent that a working mode different in a wide range can be formed 
without departing from the spirit and scope of the present invention. The 
present invention is not restricted to any specific working mode except 
being limited in the appended claims.