Optical recording and reproducing disc

An optical recording and reproducing disc for recording and/or reproducing information by the application of laser beams having a wavelength .lambda. in the range of 0.7 to 1.0 .mu.m, the disc being constituted by a disc-shaped base having at least one spiral guide track thereon with top surfaces parallel to the surface of the base at a height above the base different from the top surfaces of the portions of the base between the spires of the track, which height is in the range of .lambda./12 to .lambda./6, the guide track having a width approximately equal to the effective diameter of the laser beams; and a uniform thickness coating of a light recording material on the top surface of the guide track and the portions of the base between the spires of the guide track, the light recording material being a material the transmittance and/or the reflectance of which is varied by irradiation by the laser beams.

The present invention relates to a disc for optical recording reproduction 
and more particularly, a disc for optical recording and reproduction 
wherein a beam modulated by the information signal such as an image signal 
or the like is applied to a recording medium in the form of a recording 
film which changes in reflectance or transmittance due to temperature 
change caused by the beam application. 
In recent years, in addition to an optical video disc used exclusively for 
reproduction, a disc for optical recording and reproduction wherein the 
reproduction can be performed immediately by a semiconductor laser, etc. 
has been in development. Most of the developments are based on a system of 
drilling a hole in the recording film by laser generated heat to form the 
recorded bits, as disclosed in Japanese Patent Laid Open Publication No. 
11430/1977. In addition, a group to which the present inventors belong 
developed other recording films, for instance, a method of using a 
sub-oxide film as a recording medium, such as thin film composed of a 
sub-oxide such as tellurium. In this method the film composed of the 
sub-oxide of the tellurium on the base plate absorbs the laser beam so as 
to change the refractive index and the optical density due to the heat 
thus generated, and the quality of the beam reflected from the surface of 
such a film substantially changes. 
According to the drilling system of the former method, the film has holes 
drilled therein due to the evaporation or melting of the film by the laser 
beam. Thus, a great amount of laser power is required and a hollow disc 
construction for providing a shelter for evaporated material is required 
to be provided. The thin film of the latter method has advantages in that 
the recording operation and the erasing operation can be performed with 
relatively lower power than is needed for the former method. However, the 
latter method has many problems yet to be overcome. Cracks may be caused 
in the recording film due to a decrease in the recording power and the 
heat during the recording operation, or incomplete erasing may occur 
during the erasing operation. These problems of the latter method are 
required to be overcome. 
Accordingly, an object of the present invention is to provide a new disc 
which is free from the above-described problems, which is superior in 
terms of recording efficiency and recording bit positional accuracy. 
Another object of the present invention is to provide an optical disc 
having recording film thereon, which changes in beam reflectance or beam 
transmittance without any accompanying geometrical deformation, due to the 
raised temperature which occur at the time of laser beam application, the 
film being formed on a disc-shaped base having geometrically rugged guide 
tracks formed thereon in advance. 
A further object of the present invention is to provide an optical 
recording disc to which focused laser beams are applied to record 
information, and in which concentrically or spirally convex or concave 
guide grooves are formed in advance on a disc-shaped resin base material 
of acryl or the like, and on which a recording film the reflectance or 
transmittance of which is changed by the application of the light the film 
being provided on the entire surface of the guide grooves. 
A still further object of the present invention is to provide an optical 
recording disc to which focussed laser beams are applied to record the 
information, and in which the guide grooves comprising geometrically 
rugged guide tracks are formed in advance on a disc-shaped resin base 
material of acryl or the like and on which a recording film is formed, the 
film thickness of the film on the different parts being such as to 
considerably improve the recording sensitivity and to eliminate the 
cracking during the recording operation, and the relationship between the 
groove width and the laser beam diameter being such as to increase the 
recording efficiency and avoid unerased portions being left after erasing.

Before proceeding with the description of the present invention, it is to 
be noted that the like parts are designated by like reference numerals 
throughout the accompanying drawings. 
FIG. 1(a) shows one example of a disc base material 1, here shown with the 
rear side up, which is provided with concave, with respect to the rear 
side, optical guide grooves B. (FIG. 1(a) shows, in a partial enlarged 
section, the disc base material, and a convex recording track A shown in 
FIG. 1(a ) is formed, in concentric or spiral shape, almost uniformly on 
the entire face of at least an area one side of the medium 1, the rear in 
FIG. 1(a), for recording signals the grooves B constituting portions of 
the base between the tracks A. The width W of the track A is different 
depending upon the wavelength of the light source to be used or the degree 
of coherency thereof. When a semiconductor laser of approximately 8000 A 
in wavelength is used as a light source, a width of approximately 0.7 to 1 
.mu.m is preferable. Reference character T shows the track pitch of the 
tracks A which is prescribed by the content of the signal to be recorded 
and reproduced and the amount of the crosstalk of the signals which can be 
tolerated between the tracks. The track pitch T is approximately 1.4 to 
2.0 .mu.m. The height difference d between a track A and an adjacent 
groove B, which is located between adjacent tracks, is 1/6 to 1/12 of the 
wavelength .lambda. of the incident light so that an asymmetrical 
diffraction effect is provided with respect to light, which is applied to 
the track A. The disc base material 1, which is provided with such 
concave-shaped guide grooves B can be made by the art of making a 
conventional video disc. 
FIG. 1(b) shows a layer 2 of a light sensitive recording material 
(hereinafter referred to as the light recording material) of uniform 
thickness on the track and groove surfaces of the disc base material 1 
shown in the FIG. 1(a). The light recording material 2 has a thickness in 
the range of several hundreds A to two thousands A, although the thickness 
can differ depending upon the types of the recording materials. Referring 
to FIG. 1(b), the recording light L which is incident from the front side 
of the base material is a focused light beam having a diameter at the 
recording track equal to 0.7 to 1.0 .mu.m (the width of the recording 
track A) or more. When the dimension d between the surface of the material 
2 on track A and the material 2 in the groove B on the disc is kept in the 
above-described range of .lambda./6 through .lambda./12 the edge faces p 
and q of the track A will cause an asymmetrical diffraction effect in a 
direction normal to the track A with respect to the axis of the incident 
light, and the far field pattern of the light reflected from the face of 
the light recording material provides a better representation of the 
positional shift normal to the track between the recording light and the 
track A so that tracking error signals can be obtained. Accordingly, the 
track of FIG. 1(b) can be followed with a tracking mirror. As the base 
material 1, a material, which is optically homogeneous and transparent, 
such as polymethyl (PMMA) resin, polyvinyl chloride resin or the like can 
be used. 
The method of forming the guide grooves B from these resins can be a 
conventional method of making a video disc. Namely, grooves are formed by 
a hot press method or an injection method or the like. The groove layer 
can be provided on the base material 1 using these resins with the other 
resins such as ultraviolet hardening resin. The recording 2 can be a 
sub-oxide light absorbing film such as film composed of TeOX 
(x.apprxeq.1). The film can be formed by a vacuum evaporated method to a 
given thickness independently of the ruggedness of the base material. 
The effects to be produced from the construction and the construction for 
producing the effects to a maximum will be described hereinafter in 
detail. 
FIG. 2(a) shows a construction corresponding to that of FIG. 1(b). 
Referring to FIG. 2a, a laser beam is applied to the recording film 2a 
disposed on the convex, relative the rear side, portion A, which 
constitutes the recording track of the base material 1. The recording film 
2a absorbs the laser light which causes an increase in the temperature. 
The quantity of heat produced at this time moves to the other portion of 
the base material and the recording film 2b to escape. The difference in 
the levels of corresponding surfaces between the portion 2a and a portion 
2b reduces, or in the case of FIG. 3(a) eliminates, the cross section of 
the recording film, and reduces or substantially blocks this escape of 
heat. If the disc merely has a recording film formed on a flat base having 
no grooves therein, the heat generated in a film when struck by laser 
light escapes in all the directions through the recording film, with the 
result that the efficiency of temperature increase from application the 
laser beam is inferior. The thermal conductivity of the recording film is 
10 times or 100 times that of the resin base, so little heat escapes 
through the base. Since the difference in height between portions of the 
film is caused by the grooves, it is correspondingly difficult for the 
heat to escape, with the result that the temperature is easily raised at 
the position of application of the laser light. Thus operation can be 
carried out with less laser power. According to the experiments of the 
inventors, cracks were caused in the film due to the raised temperature of 
the film during the recording operation in a flat base having no grooves. 
However, no cracks were caused at all in the grooved disc of the present 
invention. The recording film with surfaces at different levels has the 
effect of reducing the thermal distortion of the recorded information. 
The track width W which is most effective for raising the temperature due 
to the action of the laser beams, will be described hereinafter. A convex 
with respect to the rear side, shape having a constant width and 
sufficiently long in length and constituting a track A formed in 
concentric circle or spiral, will be described. The width W must be 
sufficiently greater than the spot of the beam to be applied. Since the 
amount of film material in the track A per unit length becomes smaller as 
the width W becomes narrower, the thermal capacity per unit length is also 
reduced, so that the temperature is more easily raised. Also, since the 
contact area between the recording film and the base plate becomes smaller 
as the information track width W becomes narrower, the thermal transfer to 
the base plate becomes smaller. The temperature rise is better when the 
guide track width W is the same as the thickness .omega. of the light beam 
or is somewhat smaller than that. Namely, under the condition 
(.omega..gtoreq.W), the effect of the beam is greater. The thickness 
.omega. of the light beam means effective beam diameter which has 
sufficient strength to change the characteristics of a recording material 
within the area beam application during the time on signals are impressed 
on the beam. The effect is the same even when the guide track A is concave 
relative to the rear side, as shown in FIG. 2(b) i.e. is a groove with the 
raised ridges between the grooves being portions of the base between the 
tracks. Particularly, when the thermal conductivity of the base material 
is sufficiently smaller than that of the recording film, there is no 
difference between the convex shape shown in FIG. 2(a) and the concave 
shape shown in FIG. 2(b). When the width W of the track becomes narrower 
than the effective beam diameter W, difficulty is caused in reproduction. 
Namely, the amount of reflected or transmitted light caused by signals 
recorded on the track is reduced, thus resulting in a reduced S/N ratio. 
Generally, speaking, the width of guide track A is optimum when it is 
approximately as wide as or slightly narrower than the effective beam 
diameter. To achieve an desired object, the width of the guide track A is 
required to be in the range of .+-.20% of the effective beam diameter and 
is preferable in the range of .+-.10% of the effective beam. 
The relationship between the thickness t of the recording film and the 
depth (height) d between the surfaces of the base material 1 on the track 
and the groove will be described hereinafter. When the relationship of 
t.ltoreq.d exists as shown in FIG. 3(a), the heat conduction within the 
recording film is cut off at the gap between the portions of the light 
recording material, and thus the above-described effect is greatest. Even 
when the relation of t&gt;d exists as shown in FIG. 3(b), the adiabatic 
effect can be expected except when t is sufficiently larger than d. 
Particularly, in the case of t.ltoreq.2d, the heat conduction of the 
recording film at the point of the difference in height becomes half or 
less of the heat conduction of a flat uniform thickness film, so that the 
effect is large. Thus the recording film is preferably at least two times 
as thick as the difference in height between the bottom of the groove and 
the top of the track. 
In one embodiment of the new disc construction described hereinabove, PMMA 
1 mm thick was used as a grooved base material. The difference in height 
was 700 A, the width of the concave portion was 8000 A, the evaporated 
recording film was TeOX (x.apprxeq.1) and was 1400 A in thick. When a 
recording operation was performed with a semiconductor laser, which was 
focused to a spot of approximately 8000 A diameter by an optical system of 
Na=0.5, the recording operation could be effected by a laser power of 
approximately three fifths that needed for a non-grooved base material. 
An example where a protective layer has been coated on the recording film 
for the construction of a practical disc will be described hereinafter. 
FIG. 4 shows such embodiment wherein the recording film 2 is covered with 
the protective layer 3 for the purpose of mechanical protection. Even in 
this case, geometrical convexities and concavities are provided on the 
base material 1 as shown in the drawing, and the thermal conductivity of 
the protective member 3 is sufficiently low to improve the recording 
sensitivity. An optically homogeneous resin such as polystyrene having a 
thermal conductivity of from 0.8 to 1.2.times.10.sup.-3 J/cm sec..degree. 
K. is dissolved in a aromatic solvent or the like such as xylene and is 
coated on the recording film 2 and dried to form the protective layer 3. 
When such an adhering protective layer is to be provided, deformation and 
evaporation can not be used in recording material on the recording film by 
laser beams. Only the method of varying the optical condition, such as the 
reflection factor, without change in the shape of the recording film 2 can 
be used. Generally, the recording film is extremely thin, for example 
approximately 1000 A thick. Since the film is heated during the recording 
operation, it is likely to be oxidized if it is exposed to air. Once it is 
oxidized, the transmission factor becomes lower in that portion. 
Transparency is distributed in accordance with the distribution of the 
temperature near the recorded bit. Thus blurring is caused in the vicinity 
of the recorded bit. This causes the S/N ratio to decrease FM type 
recording. However, the adherent protective layer can prevent such 
oxidization. 
Since a certain composition of the low oxide film of tellurium described 
hereinabove as the recording film has a property which permits it to be 
erased under certain conditions, as disclosed in the Japanese Pat. No. 
3725/1979. The member is whitened at a portion where a slightly strong 
beam from the semiconductor laser is applied and is blackened at a portion 
where a beam weaker than the slightly strong beam is applied. Also, the 
whitening and blackening can be obtained by variation of the laser pulse 
width. Namely, the member is whitened by a short time application of a 
beam of 100 nsec. or less and is blackened by a long time application of a 
beam of 200 nsec or more. This whitening operation and blackening 
operation can be repeatedly performed. 
FIG. 5 is a view illustrating an embodiment of record erasure in such 
erasing disc as described hereinabove. Referring to FIG. 5, the base 
material is generally indicated by reference numeral 1. A film for 
recording and erasing is generally designated by reference number 2. A 
laser beam generally designated by D.sub.1 is provided for the writing-in 
operation and has a diameter less than the width of the convex-shaped 
guide track. A laser beam for erasure and which is narrower than the guide 
track is generally represented at D.sub.2. Also, reference character g 
designates a white bit signal written in by beam D.sub.1. Reference 
character h designates a portion of the white bit signal not erased. When 
the laser beam diameter is less than the track width, portions of recorded 
bits are left unerased due to some shifting of the erasing laser beam 
D.sub.2 during relative movement of the track and the erasing beam 
D.sub.2. FIG. 5 also shows beams D.sub.1 ' and D.sub.2 ' and recorded bits 
g' corresponding to the above-described beams D.sub.1 and D.sub.2 and bits 
g. The diameter of the latter laser beam is almost the same as the width W 
of the guide track or is slightly greater. In this case, unerased portions 
are not left, since the laser beams cover the entire width of the track. 
The beam diameter .omega. is greater than the groove width W as described 
hereinabove to achieve complete erasing. 
Accordingly, the disc according to the present invention can, due to the 
presence of the grooves and the difference in level of the surfaces of the 
recording material film, considerably improve the recording sensitivity 
and eliminate the cracking during the recording operation. In addition, 
when the relationship between the groove width and the laser beam diameter 
is properly, the recording efficiency can be increased and unerased 
portions of the recorded bits eliminated. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only and is not to be taken by way of limitation, 
the spirit and scope of the present invention being limited only by the 
terms of the appended claims.