Optical recording and reproducing apparatus having erasing beam spot with asymmetrical intensity distribution

Disclosed is an optical recording and reproducing apparatus having an erasing function. The invention aims at providing a practical apparatus which can effect recording, reproducing and erasing of signals in an erasable recording film by applying laser beams thereto. A substantially circular first fine beam spot is formed by a beam source such as a laser, and an elliptic second beam spot for erasing purposes is formed by another beam source such as a laser, with both beam spots are disposed in close proximity of each other on the same guide track. The erasing beam spot is elongated and has an axis which is tangent to the guide track on the recording medium. The power of intensity profile of the erasing beam spot is so controlled that the highest intensity is obtained at the leading end portion of the erasing beam spot as viewed in the direction of the scan, so that the signal can be erased stably and reliably with minimal erasing power. The portion of the recording medium scanned by the erasing beam spot is scanned by the recording beam spot so that the erasing and recording are carried out substantially simultaneously.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical recording and reproducing 
apparatus. More particularly, the invention concerns an optical recording 
and reproducing apparatus in which a laser beam is converged into a fine 
beam spot of about 1 .mu.m dia. and applied to an optical recording medium 
to record signals at a high density and to reproduce the recorded signal, 
and the recorded signal is erased as the medium is irradiated with another 
laser beam spot. 
In a typical example of an optical recording apparatus of the type 
described, the laser beam spot of small diameter is applied to a rotating 
optical recording disk. The recording of signals is made at a high density 
by making use of the energy of the laser beam, the intensity of which is 
modulated by the signals to be recorded. On the other hand, the 
reproduction of the recorded signals is carried out by applying a laser 
beam of a constant intensity on the signal recording portions of the 
optical recording disk and detecting any change in the laser beam 
reflected or transmitted by the optical recording disk. 
This type of optical recording and reproducing apparatus offers various 
advantages such as a high recording density, a low memory cost per bit, a 
high access speed and stable recording and reproduction without requiring 
direct contact between the optical head and the optical recording medium. 
Because of these advantages, this type of optical recording and 
reproducing apparatus has been expected to provide novel memory media in 
the future information society. 
Two types of optical recording and reproducing methods are available: 
namely, the write-once type and the erasing type. 
The write-once type method is further sorted into several types of methods 
such as a method in which the optical recording film is locally evaporated 
by the heat energy of the laser beam to form pits by means of which the 
signals are recorded and reproduced, a method in which the optical density 
of the recording film is locally changed by the energy of the applied beam 
to record and reproduce the signals, and so forth. 
The erasing type method also can be sorted into several methods such as a 
method in which signals are recorded and reproduced by a cooperation 
between the heat effect of the laser beam and an external magnetic field, 
and a method which is a modification of the write-once type method making 
use of the optical density change wherein the optical density is 
reversibly changed by making use of only the heat energy of the laser 
beam. 
The reversible change in the optical density can be effected by various 
methods by making a repeated use of a change of state of the recording 
film between the amorphous state and the crystalline state, between one 
amorphous state and another amorphous state which is stable, or a change 
in the size of crystal grains in an amorphous matrix. 
The optical recording and reproducing apparatus of the invention makes use 
of the above-described reversible change in the optical density of the 
optical recording film. The principle of the invention will be explained 
briefly hereinunder, before turning to the description of the invention. 
For an easier understanding, it is assumed here that the change in the 
optical density is attained by making use of the change of state between 
an amorphous state and the crystalline state of the medium. 
Referring to FIG. 1, illustrating, a model of the transition between an 
amorphous state and the crystalline state of the medium, the recording 
film in the amorphous state represented by A exhibits a small reflection 
factor and a large light transmittance. Conversely, the reflection factor 
is large and the light transmittance is small when the recording film is 
in the crystalline state represented by C. 
When a portion of the recording film in the amorphous state A shown in FIG. 
1 is locally heated up to near or above the melting temperature and then 
gradually cooled, the state of this portion is changed from the amorphous 
state A into crystalline state C. Conversely, when the temperature of a 
portion of the recording film in the crystalline state is locally heated 
to near or above the melting point and then quenched, the state of this 
portion is changed from crystalline state C into amorphous state A. 
A practical method of realizing the heating/quenching cycle and 
heating/slow cooling cycle will be explained hereinunder. 
Referring to FIG. 2a, a substantially circular minute spot L of, for 
example, a laser beam is applied to a recording medium which moves in the 
direction of the arrow relatively to the beam spot. If the intensity of 
this beam spot L is increased momentarily to locally heat up the thin 
film, the temperature rise in this local portion is promptly diffused to 
the recording film and the substrate so as to realize the 
heating/quenching process. 
On the other hand, when a beam spot M, elongated in the direction of 
movement of the recording medium indicated by the arrow, is applied as 
shown in FIG. 2B to the recording medium while its intensity is increased 
progressively or intermittently, the irradiated portion of the recording 
medium is heated and then cooled at a cooling rate much smaller than that 
in the case of FIG. 2A, thus realizing the heating/slow cooling process. 
Thus, the heating/quenching process is attained by applying the fine beam 
spot to the recording film in the form of pulse and modulating the 
intensity of the beam as a function of time, whereas the heating/slow 
cooling process is obtained by applying, continuously or discontinuously, 
a beam spot elongated in the direction of movement of the recording 
medium. 
FIG. 3 shows an example of an erasable optical recording and reproducing 
apparatus which operates in accordance with the principle explained 
hereinabove. 
The apparatus shown in FIG. 3 is designed such that two beams are applied 
to a guide track 51 on an optical disk. As is well known, an optical 
recording thin film is applied to the optical recording disk. An arrow A 
represents the direction of movement of the optical recording medium 
relatively to the beam spots M and L, while X represents a single point on 
the optical recording medium. The signal which has been recorded on the 
point X is erased when scanned by the elongated beam spot M or a new 
signal is recorded and reproduced when the same portion is scanned by the 
circular beam spot L. FIG. 4 shows examples of intensity distribution 
profiles of the beam spots M and L shown in FIG. 3. In this Figure, 
r.sub.m represents the optical axis of the beam spot M. The beam intensity 
is distributed around the optical axis r.sub.m substantially in the form 
of Gaussian beam such as to form an elongaged beam spot along the guide 
track 51. Similarly, the beam intensity for forming the beam spot L is 
distributed in the form of Gaussian beam about the optical axis r.sub.1 
such as to form the circular beam on the guide track 51. In consequence, 
the signal recorded on the point X is erased when the point X is heated 
and then slowly cooled by the application of the beam spot M. Then, as the 
point X is heated and then quenched by the application of the beam spot L, 
a new signal is recorded. The recording and erasing of the signal are thus 
performed. This method is advantageous in that it permits recording and 
erasing in real time with a simple arrangement but encounters a problem in 
that the laser beam has to be elongaged in order to hold the medium at the 
temperature necessary for the crystallization during the erasing. 
Therefore, for the purpose of obtaining a beam power density sufficient 
for the temperature rise, it is necessary to employ a laser of large 
power. 
From FIG. 4, it will be clear also that the temperature of the point X 
approaches the melting point only after it has been accessed by the 
optical axis r.sub.m of the erasing beam spot M. Therefore, only the left 
half part of the optical spot M is utilized for the slow cooling of the 
heated portion of the recording medium. Thus, the length of the beam spot 
M along the guide track has to be further increased, in order to attain a 
time long enough for allowing the crystallization, requiring a further 
increase in the laser power. Thus, the described method encounters a 
problem in that the independent control of the power level and the length 
of the erasing spot M is often prohibited. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide an erasable optical 
recording and reproducing apparatus in which the length N (distance 
between the energy peaks of two beam spots) shown in FIG. 4 is materially 
increased by changing the power distribution or power profile of the beam 
spot M in FIG. 4 along the guide track, without varying the distance or 
length of the beams as a whole, thereby allowing an optimum erasure. 
Another object of the invention is to materially reduce the laser power 
used in the erasing by varying the power profile as mentioned above. 
Still another object of the invention is to attain a stable erasing 
operation by forming, on the recording film, a longer region for slow 
cooling. 
A further object of the invention is to reduce the overall length D (see 
FIG. 3) of the beam train along the guide track, thereby simplifying the 
arrangement and adjustment of the beam spots and ensuring high positional 
stability of these beams. 
A still further object of the invention is to provide an optical recording 
and reproducing apparatus which can reduce the influence of the erasing 
energy on the recording and reproducing energy thereby affording an 
optical recording and reproducing apparatus which can simultaneously and 
stably conduct both the erasing and the recording without thermal 
interference between two beam spots. 
To this end, according to one aspect of the invention, there is provided an 
erasable optical recording and reproducing apparatus in which the profile 
of the intensity distribution of the beam spot for effecting the 
heating/slow cooling process of the recording film is modified in the 
direction of relative movement of the spot such that the leading portion 
of the beam spot raises the temperature of the recording film to a 
temperature near or above the melting point and the trailing portion of 
the beam spot effects a slow cooling. 
According to another aspect of the invention, there is provided an erasable 
optical recording and reproducing apparatus in which the portion of the 
recording medium scanned by a beam spot having a modified intensity 
distribution profile is scanned by a substantially circular writing or 
reproducing beam spot, whereby erasure and recording can be conducted 
substantially simultaneously.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will be fully understood from the following description of 
the preferred embodiments of the invention when the same is read in 
conjunction with the accompanying drawings. 
Referring to FIG. 5, an embodiment of the optical recording and reproducing 
apparatus of the invention has a recording semiconductor laser 101 adapted 
to oscillate a laser beam of a wavelength .lambda..sub.1. The output beam 
from this laser is represented by l. A collimating lens 102 is adapted to 
turn the diverging output beam l into a parallel beam. 
An optical beam composer 105 is adapted to allow a laser beam of wavelength 
.lambda..sub.1 to pass therethrough while reflecting a later-mentioned 
beam of wavelength .mu..sub.2. Numerals 106 and 107 denote, respectively, 
a beam splitter and a reflecting mirror. The output beam l from the 
semiconductor laser 101 comes into a focussing lens 108 through the 
above-mentioned optical elements. 
The focussing lens 108 is adapted to converge the beam l such as to form a 
substantially circular beam spot L on the guide track 51 formed in an 
optical recording disk. A reference numeral 109 designates an actuator for 
driving the focussing lens 108 in the direction of the optical axis in 
response to any oscillation of the disk surface thereby to effect a 
focussing control which is known per se. The actuator drives the 
converging lens 108 also in the radial direction of the disk, thus 
performing a tracking control which also is known per se. 
The apparatus further has a semiconductor laser 103 for generating an 
optical beam m of the wavelength .lambda..sub.2 mentioned before, while a 
numeral 104 designates a focussing lens associated with the semiconductor 
laser 103. The collimating lens 104 is adapted to convert the output beam 
m from the semiconductor laser 103 into a substantially parallel beam 
having an elliptic cross-sectional shape. The beam m is reflected by the 
beam composer 105 and is applied to the focussing lens 108 along a path 
which is substantially the same as that for the beam l. The beam m forms a 
beam spot M on the same track as that 51 on which the spot L is formed. As 
will be seen from FIGS. 3 and 4, the beam spot M has an elliptic form with 
its longitudinal axis extending in the longitudinal direction of the guide 
track 51. 
Referring again to FIG. 5, the beam reflected by the optical recording disk 
comes into a beam splitter 106 through the focussing lens 108 and the 
mirror 107. After alteration of the path by the beam splitter 106, the 
beam comes into a filter plate 111. In the illustrated embodiment, the 
filter plate 111 allows only the beam l of the wavelength .lambda..sub.1 
to pass therethrough and does not transmit the beam m of the wavelength 
.lambda..sub.2. A single lens 112 is adapted to convert the reflected beam 
l into a converging beam. Reference numeral 113 denotes a reflecting 
mirror which is adapted to interrupt almost half of the converged beam 
from the signal lens 112 and to reflect this half of the converging beam 
towards a photodetector 115. 
Reference, numeral 114 designates a split-type photodiode for detecting the 
focussing error signal. The photodiode 114 is disposed at the focal point 
of the single lens 112 such as to detect a focus error signal in response 
to the movement of the splitted beam l.sub.1 in a manner known per se. The 
photodetector 115 is a splitted photodiode adapted for detecting a 
tracking error signal. This photodiode detects the guide tracking error 
signal by means of the beam l.sub.2 reflected by the mirror 113 in a 
manner known per se. 
The signal recorded in the guide track 51 on the optical disk is reproduced 
by the photodetector 114 or 115. 
Reference numeral 116 denotes a laser driving circuit which is adapted to 
vary the intensity of the elliptic beam spot M on the guide track 51 as a 
signal applied to its terminal Q is controlled. 
Another laser driving circuit 117 is adapted to vary the intensity of the 
substantially circular beam spot L on the guide track 51 as a signal 
applied to its terminal P is controlled. 
FIG. 5 shows, by way of example, a diffraction element used in the 
invention for changing the beam intensity distribution. The diffraction 
element 118 is adapted to impart a diffraction effect to the incident beam 
m in the direction of the guide track 51, mainly in the one-dimensional 
direction, and is used for altering the intensity distribution of the 
erasing beam spot on the guide track 51. A description will be made 
hereinunder as to the function and construction of this diffraction 
element. 
FIG. 6A shows how the diffraction element 118 is related to the converging 
lens 108 and the guide track 51 on the optical recording disk. An arrow A 
represents the direction of movement of the track 51. 
The parallel beam m coming into the diffraction element 118 has an 
intensity distribution resembling a Gaussian distribution as will be seen 
from FIG. 6B(1). The beam portion coming into the point X.sub.1 of the 
diffraction element 118 runs straight without diffraction and is applied 
to a point x.sub.1 on the focussing lens 108. The beam portion coming 
along the optical axis of the incident beam m comes into a point X.sub.2 
on the diffraction element and is diffracted such that the beam fraction 
of "0" order reaches a point x.sub.2 on the focussing lens 108, while the 
beam fraction X.sub.2.sup.+1 of primary order is diffracted by an angle 
.theta..sub.1 towards the point x.sub.1 on the focussing lens 108. Another 
beam fraction X.sub.3.sup.+1 of primary order applied to the point X.sub.3 
is also diffracted towards the point x.sub.1 on the lens by an angle 
.theta..sub.2. If the diffraction element is constructed to meet the 
condition of .theta..sub.2 &gt;.theta..sub.1, a beam intensity distribution 
as shown in a larger scale in FIG. 6B(2) is obtained at the focal point of 
the focussing lens. This beam intensity distribution has a profile which 
is different from that of the beam coming into the diffraction element 
resembling the Gaussian distribution. It is thus possible to obtain an 
erasing beam spot which has such an intensity distribution that the 
portion of the new recording region of the disk coming into this beam spot 
is first irradiated with a beam portion of high intensity, i.e., with a 
beam spot which has a higher intensity in its seemingly leading end than 
in its trailing end. 
For instance, the diffraction element 118 may be such a one-dimensional 
diffraction element that the diffraction angle linearly varies from the 
point X.sub.1 to the point X.sub.3 or a one-dimensional diffraction 
element in which the direction and the amount of diffraction are 
controlled. 
An explanation will be made hereinunder with specific reference to FIGS. 7A 
to 7C as to the difference in the effect between the erasing beam M (see 
FIG. 7B) with intensity distribution modified along the guide track 51 and 
a conventionally used erasing beam (see FIG. 7C). 
FIG. 7A shows the shapes and positions of two beam spots L and M formed on 
the guide track 51. An arrow A represents the direction of movement of the 
recording medium relative to the beam spot. A point on the recording 
medium is indicated by X. 
FIG. 7B shows an example of the intensity distribution of the beam spot 
used in the invention along the guide track, while FIG. 7C shows an 
example of the intensity distribution of a conventionally used beam spot. 
The effect of the beam spot of the invention, having a modified intensity 
distribution as shown in FIG. 7B, will be compared with that produced by 
the conventionally used beam spot shown in FIG. 7C. In FIGS. 7B and 7C, 
m.sub.1 represents a point at which the recording film is heated nearly to 
the melting point, while m.sub.2 represents the point at which the 
temperature of the recording film is lowered from the melting point. 
Referring to FIG. 7B, when the point X comes into the area of the erasing 
beam spot M, the point X is first heated by the leading end portion of the 
beam spot M where the intensity is specifically high so that the 
temperature is raised drastically and reaches a level around the melting 
temperature at the point m.sub.1. Then, as the recording medium is further 
moved, the temperature comes down below the melting temperature at a point 
m.sub.2 where the power of the beam starts to fall. Then, the point X is 
slowly cooled down as it is moved through a region d.sub.1 over a time 
duration long enough to crystallize the structure of the recording medium, 
so that the signal recorded in the recording medium is erased. 
Subsequently, the point X is irradiated by the recording beam while it 
passes the region W.sub.1 so that a new signal is recorded. 
The effects of the conventional beam intensity distribution shown in FIG. 
7C are substantially the same as those explained in connection with FIG. 
7B. The conventional beam intensity distribution shown in FIG. 7C, 
however, suffers from the following disadvantages as compared with that 
explained in connection with FIG. 7B. Namely, a considerably large beam 
power is exerted in the region between the point p and the point m.sub.1 
shown in FIG. 7C. This power contributes to a slow rise of the temperature 
but does not substantially contribute to the melting. Thus, the power is 
consumed wastefully as compared with the case shown in FIG. 7B. In case of 
FIG. 2C, the slow cooling of the recording medium is effected in the 
region d.sub.2 between the point m.sub.2 and a point q. Assuming that the 
total light quantity of the beam spot M shown in FIG. 7B equals to the 
total light quantity of the beam spot M shown in FIG. 7C, the beam 
intensity distribution in FIG. 7C provides only a poor slow cooling effect 
as compared with the beam intensity distribution shown in FIG. 7B because 
the slow cooling region d.sub.2 is smaller than the slow cooling region 
d.sub.1 both in the light quantity and the length which represents the 
time length in which the point X passes this region, i.e., the slow 
cooling period. In order to attain a longer slow cooling region d.sub.2 in 
FIG. 7C, it is necessary to prolong the length of the beam spot M along 
the guide track and to apply greater quantity of light, which essentially 
requires a semiconductor laser of high power as the beam source. 
Thus, the beam spot arrangement in accordance with the invention shown in 
FIG. 7B, constituted by two beam spots having the illustrated intensity 
distribution profiles, offers the following advantageous features when 
this arrangement is used in an optical recording and reproducing appartus. 
(1) The medium can be heated to a level around the melting temperature only 
by the leading end portion of the erasing beam spot, i.e., the portion of 
the beam spot at which any desired point on the recording medium comes 
into the area of the erasing beam spot. Consequently, the heating of the 
recording medium can be made with minimal power loss and the surplus power 
can be effectively used in the subsequent slow cooling of the medium. 
(2) The beam spot arrangement shown in FIG. 7B, in which the length D (see 
FIG. 7A) of the beam spot cycle is maintained constant, can provide a much 
longer cooling region d.sub.1 than in the conventional beam spot 
arrangement shown in FIG. 7C. 
(3) The length or time interval between the region M.sub.1 -m.sub.2 in 
which the temperature around the melting is maintained by the erasing beam 
spot and the recording or reproducing beam spot L is large in the case of 
FIG. 7B as compared with the case of FIG. 7C. Therefore, with the beam 
spot intensity distribution shown in FIG. 7B, it is possible to stably 
record the new signal by the recording beam spot L after a sufficient 
stabilization of the thermal condition of the erasing beam M. 
As a practical example of the diffraction element explained in connection 
with FIGS. 5, 6A and 6B, it is possible to use an element having a 
transparent glass substrate and stripes of a certain density formed on the 
transparent substrate such that the width and the pitch of the stripes 
vary linearly, thereby diffracting the light beam orthogonally to the 
stripes. An equivalent effect is obtained by using a diffraction element 
having a transparent glass substrate and fine grooves formed in the 
substrate with the groove width and pitch varying linearly such as to 
diffract the beam orthogonally to the grooves. 
The design of the apparatus will be facilitated if the diffraction element 
is positioned at a portion of the apparatus where the beam is parallel. 
Such an arrangement facilitates also the assembly and adjustment of the 
apparatus. The diffraction element, therefore, is placed at a portion 
where the beam runs as a parallel beam. 
In the foregoing description, particularly in the description in connection 
with FIG. 5, the erasing beam source has been explained as being a 
semiconductor laser having a single laser beam emitting surface. This, 
however, is not exclusive and an equivalent effect is produced by a 
semiconductor laser having a plurality of laser beam emitting surfaces 
arranged in the direction of the guide track. Namely, an erasing beam spot 
elongated in the direction of the guide track and having a leading end 
portion of high intensity can be obtained with such a semiconductor laser. 
FIG. 8 shows another example of the arrangement for modifying the power 
profile. In FIG. 8, the same reference numerals are used to denote the 
same parts or members as those used in FIG. 5, and portions which do not 
constitute any critical feature are omitted for the simplification of the 
drawing. 
A laser beam emitted from an erasing laser 103 is changed into a parallel 
beam by a second collimating lens 104. The parallel beam is then diverged 
one-dimensionally by, for example, a concave cylindrical lens 119. The 
concaved cylindrical lens 119 may be substituted by a convexed cylindrical 
lens. The laser beam is then applied to a multiple reflection plate 120 
which has two parallel surfaces A and B. The surface A has a multi-layered 
coat having a reflectivity R.sub.1 and a transmittance (1-R.sub.1), while 
the surface B has a multilayered coat of a reflectivity R.sub.2 which is 
about 100%. Consequently, reflection and transmission take place both on 
the surfaces A and B such as to produce an infinite number of beams, only 
four of them (P.sub.1 to P.sub.4) being shown; for purposes of 
illustration. Representing the total light quantity of the beam coming 
into the multiple reflection plate by I, the light quantities of the beams 
P.sub.1 to P.sub.4 are given as follows, respectively. 
EQU P.sub.1 =R.sub.1 .times.I 
EQU P.sub.2 =(1-R.sub.1).sup.2 .times.R.sub.2 .times.I 
EQU P.sub.3 =(1-R.sub.1).sup.2 .times.R.sub.2.sup.2 .times.R.sub.1 .times.I 
EQU P.sub.4 =(1-R.sub.1).sup.2 .times.R.sub.2.sup.3 .times.R.sub.1.sup.2 
.times.I 
Assuming here that the reflectivities R.sub.1 and R.sub.2 are 0.5 and 1.0, 
respectively, the light quantities of the beams P.sub.1, P.sub.2, P.sub.3 
and P.sub.4 are calculated to be 0.5I, 0.25I, 0.125I and 0.063I, 
respectively. 
The incident beam I has been diverged one-dimensionally by the concaved 
cylindrical lens 119, so that the beams P.sub.1, P.sub.2, P.sub.3 and 
P.sub.4 have to travel different distances between the cylindrical lens 
119 and the focussing lens 108 so that these beams are focussed by the 
focussing lens 108 at different points disposed along the optical axis 
such as to form an image at a point between the focussing lens 108 and the 
focal point of this lens. Thus, the beams are focussed at a position which 
is spaced by .DELTA.X from the recording surface of the optical recording 
disk 121. In consequence, the beam spots p.sub.1, p.sub.2, p.sub.3 and 
p.sub.4 are formed on the disk such as to be disposed in an elongated form 
along a straight line. The beam spot P.sub.1 has the highest intensity and 
the intensity level is progressively decreased such that the beam spot 
P.sub.4 exhibits the lowest intensity. In consequence, as will be seen 
from a graph in FIG. 8 which shows the relationship between the light 
intensity and the distance, it is possible to obtain a modified power 
profile equivalent to that explained in connection with FIG. 6. 
FIG. 9A shows the beam spot intensity distribution obtained in the 
arrangement shown in FIG. 8. It will be seen that the beam spot having the 
highest intensity comes first followed by the beam spots of progressively 
decreased intensity. Consequently, a heat distribution as shown in FIG. 9B 
is obtained on the recording disk. As a result, the time duration required 
for heating the disk is considerably shortened as compared with the time 
length which is represented by the length between points p and ml in FIG. 
7C and the laser power necessary for the erasing is decreased 
correspondingly. For the same reason, the length of the erasing beam spot 
can be reduced advantageously. 
In this embodiment, it is essential that the beam spots P.sub.1 to P.sub.4 
be arrayed correctly on a guide track 51 as shown in FIG. 9A. More 
practically, when the focal distance f of the focussing lens is 4.5 mm 
while the width of the track 51 on the disk is 0.6 .mu.m it is necessary 
to arrange the beams within a tolerance of less than 0.1 .mu.m in the 
widthwise direction of the track. In order to meet this condition, the 
relative offset of the angles of incidence of the beams to the converging 
lens have to be maintained less than Tan.sup.-1 (0.0001/4.5) 
=0.0013.degree.. In the case of the described embodiment, however, such a 
high precision of beams can be attained and, hence, the undesirable offset 
of the beam spots can be avoided without substantial difficulty by virture 
of the use of a single plate having parallel surfaces. Obviously, the 
concaved cylindrical lens 19, which is disposed between the lens 104 and 
the reflection plate 120 in the described embodiment, may be disposed 
between the reflection plate 120 and the focussing lens 108.