Laser drum for use in recording and reproducing information on optical tapes

A laser drum which can optically record and reproduce information on optical tapes. The laser drum comprises an upper drum which has two projection holes, rotates and is fitted with semiconductor lasers, a total reflective prism, stoppers, diffraction gratings, beam splitters, object lenses, condensing lenses and photodetectors.

FIELD OF THE INVENTION 
The present invention relates to a laser drum for use in optically 
recording and/or reproducing information on optical tapes and more 
particularly to a laser drum which can optically record and/or reproduce 
video or audio signals on optical tapes. 
BACKGROUND OF THE INVENTION 
Conventionally, various means to optically reproduce video or audio signals 
on video discs or compact discs have already been proposed, but such discs 
are unable to optically record a large quantity of information because of 
the limited recording space, even though they can record various 
information in a high density (10.sup.8 bits/cm.sub.2). The recording 
density of magnetic tapes for use in video cassette recorder is too low 
(one hundredth to that of laser discs) to record a large quantity of 
information. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a laser drum which 
makes it possible to optically reproduce video or audio signals recorded 
on read-only-type optical tapes. 
It is another object of the present invention to provide a laser drum for 
optically recording and reproducing video or audio signals on DRAW (direct 
read after wire)-type optical tapes. 
In accordance with the present invention, a semiconductor laser (or a laser 
diode) is used in the laser drum and the advantages of high recording 
density of the laser discs and of wide recording space of the magnetic 
tape are realized. In fact, a magnetic tape with the surface area of 
0.5.times.7,870 inches has the recording space of 40 times larger than 
that of a laser disc with the diameter of 30 cm (about 11.8 inches).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of a laser drum according to the present invention 
is hereinbelow described in detail. In FIGS. 1, 2, 3 and 4, a fixing board 
28 is fitted with semiconductor lasers 5, 5', a total reflective prism 6, 
stoppers 7, 7', diffraction gratings 8, 8', beam splitters 9, 9', object 
lenses 10, 10', condensing lenses 11, 11' and photodetectors 12, 12' and 
the fixing board 28 is installed in an upper drum 1 formed with two 
projection holes 3, 3' at the opposite sides of its lower circumferential 
edge. 
Two semiconductor lasers 5, 5' are installed on the opposite side to one 
another at a certain distance and the total reflective prism 6 lies midway 
between the semiconductor lasers 5, 5'. On the left and right sides of the 
total reflective prism 6, stoppers 7, 7', diffraction gratings 8, 8', beam 
splitters 9, 9' and object lenses 10, 10' are installed in order to be 
kept at certain distances. The condensing lenses 11, 11' and 
photodetectors 12, 12' are set up by turns on the right of the beam 
splitters 9, 9' facing the diffraction gratings 8, 8' respectively. The 
upper drum 1 is combined with a lower drum 2 which has a slantig guide 
line 4 projected on its surface. 
In FIG. 4, the numeral 51 represents the cover of the upper drum 1. 
The upper and lower drums 1, 2 are fixed to be inclined at a certain angle 
to the direction of an optical tape's travelling course for helical 
scanning of tracks on the optical tape as show in FIG. 4. 
The optical tape 13 comes in contact closely with surfaces of the upper and 
lower drums 1, 2 by means of guide pins 14, 14'. As can be seen from FIG. 
5, when the upper drum 1 rotates on the motor shaft 50, the optical tape 
slides on the surfaces of the drum 1, 2 with its lower edge osculating to 
the guide line 4 of the lower drum 2. 
When the drums 1, 2 are operated as described in the above condition, laser 
beams, being radiated from the semiconductor lasers 5, 5', are reflected 
to the directions of the stoppers 7, 7' respectively by the total 
reflective prism 6. Though these laser beams have an elliptical section, 
they are made to have a circular section by passing through the stoppers 
7, 7'. The circular laser beams which have passed therethough are applied 
to the diffraction gratings 8, 8' and diffracted into three kinds of 
beams, e.g. zero-order, (+)first-order and (-)first-order beams. Then, the 
diffracted laser beams pass through the beam splitters 9, 9' and object 
lenses 10, 10' successively, reaching the surface of the optical tape 13 
respectively. The laser beams are focused on the surface of the optical 
tape 13 by the object lenses 10, 10' in order to record and/or reproduce 
the information in the optical tape 13. 
FIG. 8 illustrates the structure of a read-only-type optical tape 16 in 
which the video and audio signals are recognized by existence and/or 
nonexistence of the pits 17 and the length of the pits 17. A succession of 
spaced pits 17 coated with a transparent material 18 as a protective layer 
is formed on a reflective layer 19. The height of the pits 17 is 
1/4.lambda.', wherein .lambda.' is the wavelength which the laser drum has 
in the transparent material 18. Thus, a laser beam focused upon the pit 17 
and the reflected laser beam from the pit 17 come to have a phase 
difference of almost 1/2.lambda.' (180.degree.) and become extinct owing 
to the interference with each other. So, as shown in FIG. 10, each 
zero-order beam diffracted by the gratings 8, 8' is incident upon the 
optical tape and focused as A, so that the laser beam may become extinct 
where there is a pit on the surface of the optical tape 16 while 100% of 
the laser beam is reflected where there is not a pit 17. Also, the 
(+)first-order beams are focused as B and C respectively as shown in FIG. 
10. So, when the pit 17 leans to the right, (+)first-order beam incident 
upon B becomes extinct and (-)first-order beam incident upon C is 
reflected by 100%. The numeral 20 is a substrate of the tape 16. 
When the pit 17 leans to the left on the contrary, (-)first-order beam 
becomes extinct and (+)first-order beam is reflected by 100%. 
The zero-order and (.+-.)first-order beams reflected from the recording 
surface of the read-only-type optical tape 16 are respectively returned to 
the beam splitters 9, 9' via the object lenses 10. 10'. Then, the 
reflected beams returned by the beam splitters 9. 9' pass through the 
condensing lenses 11, 11' and reach the photodetectors 12, 12', which are 
set up at the focal distances of the condensing lenses 11, 11'. 
FIG. 6 shows the photodetecors 12, 12' which are represented by three 
separate photoelectric cells R, P, R'. The cell P can detect the change of 
the length of the pits 17 and the tracking signals for use in controlling 
the speed of the optical tape 16 can be obtained by the cells R, R'. 
FIG. 9 illustrates the structure of the DRAW-type optical tape 21 used for 
both recording and reproduction. The tracking signals recorded on the 
DRAW-type optical tape 21 can be read in the same way as the 
read-only-type optical tape 16. 
In optical recording operations, it is usual to focus the laser beam from 
the semiconductor lasers 5, 5' upon the surface of grooves 22, on which 
the transparent material 27 forming a protective layer is coated with 
sufficient thickness, to cause ablation of a recording material 24 (See 
the numeral 23 in FIG. 9). In optical reproducing operations, the laser 
beam is focused continuously upon the grooves 22 and the recorded signals 
can be reproduced by detecting the change of reflectance of the laser beam 
from the ablated regions of the grooves 22. 
The numerals 25, 26 are an adhesive layer and a substrate respectively. 
The laser drums 1, 2, which operate as described hereinabove, require high 
precision processing of micro-metric unit and the optical elements also 
require precise arrangement. 
As shown in FIG. 3, a square groove 29 for fixing the total reflective 
prism 6 is formed in the center of the fixing board 28. On the left and 
right sides thereof, grooves 30, 30' for the stoppers 7, 7' and 
deffraction gratings 8, 8', square grooves 31, 31' for the beam splitters 
9, 9' and grooves 32, 32' for the object lenses 10, 10' are formed 
respectively. In the opposite sides of the center of the fixing board 28, 
sleeves 33, 33' for fixing the semiconductor lasers 5, 5' are installed 
and on the outer sides thereof, sleeves 34, 34' for fixing photodetectors 
12, 12' are installed opposingly. On the external circumferences of lens 
casings 35, 35' and the internal circumferences of the holders 36, 36' of 
said lens casings 35, 35', micro-spirals are formed respectively for 
adjusting the focuses of the object lenses 10, 10' on the recording 
surface of the optical tape. After the adjustment of the focuses of the 
lenses 10, 10', the lens casings 35, 35' are fixed by tightening up screws 
37, 37'. 
Fixing plates 38, 38' and 39, 39' for the semiconductor lasers 5, 5' and 
the photodetectors 12, 12' are respectively fixed to laser casings 33, 33' 
and photodetector casings 34, 34' with screws shown in FIG. 3. The left 
and right sides of the fixing plates 38, 38' and 39, 39' are formed with 
two grooves so as to adjust their fixing positions. 
The numerals 40, 40' are jigs for fixing the stoppers 7, 7' and the 
diffraction gratings 8, 8'. 
FIG. 6 shows the control circuit of the laser drum, which transmits 
electric signals representative of the recorded information and receives 
control signals of the semiconductor lasers 5, 5' from an automatic power 
control circuit APC in FIG. 6 or a laser driving circuit (not 
illustrated). 
As described hitherto, the laser drum according to the present invention 
has two parts; One is installed within the upper drum 1 and rotates 
therewith and the other is fixed in the lower drum 2 which does not move. 
An upper transformer 43 provided with four coils on its bottom and an 
upper rounded-board 44 provided with two looped metal bands 41, 42 (See 
FIG. 2B) on its bottom are installed within the upper drum 1. The lower 
transformer 45 having four coils on its top and a lower rounded-board 46 
having two brushes 47 being in contact with the looped metal bands 41, 42 
of the upper rounded-board 44 are fixed in the lower drum 2. The motor 
shaft 50 is connected with the fixing board 28 installed in the upper drum 
1 and the upper drum 1 rotates by the torque of a rotor 49. The numeral 48 
represents a motor coil. 
The laser power of the semiconductor lasers 5, 5' is controlled by an 
automatic power control circuit APC in FIG. 6 in reproducing operations 
and is controlled by a laser driving circuit (not illustrated) in 
recording operations. The output signals of these circuits are transmitted 
to the semiconductor lasers 5, 5' via the brushes 47 and the looped metal 
bands 41, 42 of the rounded-boards 44, 46. The output signals of the photo 
detectors 12, 12' are transmitted to the upper transformer 43 and main 
video and audio signals A.sub.1, A.sub.2 and tracking signals B, C are 
induced by four induction coils of the lower transformer 45. Then, the 
tracking signals B, C are transmitted to a tracking servo circuit in FIG. 
7, which controls the operation of a servo motor 15 in FIG. 5. 
As noted hereinabove, the laser drum according to the invention has two 
projection holes 3, 3' formed on opposite sides to each other on the upper 
drum 1. For half a turn of the upper drum 1, the optical tape 13 travels 
the distance of one track-width on the optical tape 13 and the laser beams 
through the projection holes 3, 3' can precisely scan the tracks of the 
optical tape 13 successively. 
The laser drum according to the invention has the advantage of recording a 
great more information on the optical tape compared with the conventional 
magnetic recording type head drum or the compact disc. Further, the laser 
drum has the advantage of making less noises than the magnetic recording 
type head drum. Furthermore, it can have a simple structure because it is 
not influenced by the mechanical vibrations which can not be avoided when 
the compact disc is adopted. 
While the invention has been described with reference to specific 
embodiments, it will be understood by those skilled in the art that 
various changes may be made and equivalents may be substituted for 
elements thereof without departing from the spirit and scope of the 
invention. Also, various modifications may be made to adapt to a 
particular situation without departing from the essential characteristics 
of the invention.