Optical recording system capable of changing the beam size

An optical shutter device comprises an electro-optic layer of a material that shows the electro-optic Kerr effect, an array of shutter regions defined on the electro-optic layer for controlling a passage of an optical beam through the layer, a plurality of electrodes provided on the electro-optic layer in correspondence to the shutter regions for applying a predetermined electric field upon energization, a driver circuit connected to each pair of the electrodes for energizing the same independently from each other, a biasing circuit connected to the plurality of electrodes for applying a predetermined d.c. bias voltage thereto, a switching circuit for inverting the polarity of the d.c. bias voltage, a first polarizing device provided above the electro-optic layer for causing a rotation of the polarizing plane of the optical beam in a first angular direction, and a second polarizing device provided below the electro-optic layer for causing a rotation of the polarizing plane of the optical beam in a second, opposite angular direction.

BACKGROUND OF THE INVENTION 
The present invention generally relates to the optical recording of images 
and more particularly to an optical recording system for recording an 
image on a recording medium by means of an optical beam. 
In the facsimiles and copiers, the optical recording system is used 
commonly. In the optical recording system, an image is recorded on the 
surface of a recording medium such as a recording sheet or photosensitive 
drum by means of a finely focused optical beam. 
In the recent optical recording systems, there is a demand of graded 
optical recording for forming half-tone images and pictures such as 
photographs with excellent quality. In order to achieve such a graded 
recording, it is necessary to change the size of the pixels and hence the 
size of the optical beam used for recording. 
Conventionally, an optical device is proposed for this purpose in the 
Japanese Laid-open Patent Application 3-75725, wherein the size of the 
beam can be changed. According to the foregoing prior art device, a 
plurality of minute apertures are formed on a crystal plate that shows the 
electro-optic Kerr effect. The apertures are arranged in the vertical 
scanning direction and activated independently from each other by applying 
an electric field. Upon activation, the plane of polarization of the 
optical beam passing through the aperture is caused to rotate as a result 
of the electro-optic Kerr effect. By providing polarizers at both sides of 
the crystal, one can control the passage of the optical beam through each 
of the apertures independently. In other words, each aperture acts as an 
optical shutter for allowing or prohibiting the passage of the optical 
beam therethrough. Thereby, the size of the beam of the optical beam can 
be changed during the horizontal scanning process. 
After intensive effort to develop the optical recording system that uses 
the foregoing optical device, it was discovered that the function of the 
optical switching is degraded with time when the device is used 
continuously. More specifically, the passage or interruption of the 
optical beam by the device becomes no longer complete after a continuous 
use of the device. This effect is called optical drift. Further, the 
foregoing conventional optical device has a problem in that the 
diffraction of the optical beam tends to occur at the edge of the 
apertures. When such a diffraction occurs, the quality of the recorded 
image is inevitably deteriorated due to the unwanted exposure. In 
addition, the conventional optical device has suffered from a problem that 
the characteristics of the device tend to be changed when the temperature 
of the crystal plate is changed. When such a fluctuation of the optical 
property has occurred, the optical recording system no longer operates 
properly. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
novel and useful optical recording system, wherein the foregoing problems 
are eliminated. 
Another and more specific object of the present invention is to provide a 
variable aperture device for an optical recording system for changing the 
size of an optical beam used for optical recording, wherein the operation 
of the variable aperture device is stabilized even after a continuous 
operation for a prolonged time period. 
Another object of the present invention is to provide an optical recording 
system for recording an image on a recording medium while changing the 
size of an optical beam used for optical recording, wherein the 
diffraction of the optical beam caused by the variable aperture device is 
eliminated. 
Another object of the present invention is to provide a process for 
controlling the passage of an optical beam through a variable aperture 
device that uses an electro-optic device for controlling the passage of 
the optical beam therethrough. 
Another object of the present invention is to provide an optical shutter 
device for controlling a passage of an optical beam, comprising: an 
electro-optic layer of a material that shows an electro-optic effect, said 
layer having upper and lower major surfaces and causing a rotation of the 
polarizing plane in response to an application of a voltage; an array of 
shutter regions defined on said electro-optic layer for controlling a 
passage of an optical beam through said layer, each of said shutter 
regions controlling the passage of the optical beam by inducing the 
electro-optic effect therein in response to an application of an electric 
field that acts across said shutter region in a predetermined direction; a 
plurality of electrodes provided on one of the upper and lower major 
surfaces of said electro-optic layer in correspondence to said plurality 
of shutter regions, said plurality of electrodes being arranged to form a 
number of electrode pairs each defining said passage region therebetween, 
said electrode pairs being separated from each other and applying said 
electric field independently from each other upon energization; driver 
means connected to each of the electrode pairs for energizing the 
electrode pair independently from other electrode pairs; biasing means 
connected to the plurality of electrode pairs for applying a predetermined 
d.c. bias voltage thereto with a predetermined polarity; switching means 
for inverting the polarity of the d.c. bias voltage that is applied to the 
electrode pairs; first polarizing means provided above the upper major 
surface of the electro-optic layer for setting the polarizing plane of the 
optical beam passing therethrough at a first angular direction; and second 
polarizing means provided below the lower major surface of said 
electro-optic layer for setting the polarizing plane of the optical beam 
passing therethrough at a second, opposite angular direction. According to 
the present invention, the formation of space charges in the crystal layer 
as a result of trapping and accumulation of photocarriers under the 
presence of the electric field is avoided by periodically inverting the 
polarity of the d.c. bias voltage. Thereby, the undesirable effect of 
canceling-out of the applied electric field by the space charges that are 
induced in the crystal layer, is effectively eliminated, and excellent 
control of the passage of the optical beam through the apertures is 
achieved even when the variable aperture device is operated continuously 
for a prolonged time period. By controlling the apertures individually, 
one can change the overall size of the optical beam that has passed 
through the variable aperture device. 
Another object of the present invention is to provide an optical recording 
system for recording an image on a recording medium by means of an optical 
beam that is deflected repeatedly in a horizontal scanning direction, 
comprising: beam source means for producing an optical beam with a 
polarizing plane that intersects with the horizontal scanning direction by 
an angle of 45 degrees in a first angular direction; a first optical 
system provided in a path of the optical beam that has been produced by 
the beam source for focusing the same at a first focal point to form an 
elongated optical beam having an elongated beam shape elongating in a 
vertical scanning direction that is perpendicular to said horizontal 
scanning direction; shutter array means provided in correspondence to a 
first focal point, said shutter array means comprising a plurality of 
apertures aligned in said vertical scanning direction and are activated 
independently for selectively passing the optical beam therethrough upon 
activation; a second optical system provided in a path of the optical beam 
that has passed through the shutter array means for focusing the same at a 
second location to form an elongated optical beam having an elongated beam 
stop elongating in said horizontal scanning direction; deflection means 
for deflecting the optical beam that has passed through the second optical 
system repeatedly in said horizontal scanning direction; a third optical 
system provided in a path of the optical beam that has been deflected by 
the deflection means for focusing the same on a recording surface of the 
recording medium; and a slit element provided on a path of the optical 
beam that has exited from the second optical system, said slit element 
carrying a slit extending in the horizontal scanning direction with a 
width set to eliminate diffraction beams that are formed when the optical 
beam has passed through the shutter array means for the diffraction higher 
than the first order. According to the present invention, one can 
eliminate the unwanted optical beam that has been produced as a result of 
the diffraction of the optical beam at the apertures of the variable 
aperture device. Thereby, the quality of recording on the recording medium 
is improved substantially. 
Other objects and further features of the present invention will become 
apparent from the following detailed description when read in conjunction 
with the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1(A) and 1(B) show a first embodiment of the optical recording system 
of the present invention, wherein FIG. 1(A) shows the side view and FIG. 
1(B) shows the plan view. In the side view of FIG. 1(A), the optical beam 
is deflected perpendicularly to the plane of the drawing in response to 
the horizontal scanning, while in the plan view of FIG. 1(B), the optical 
beam is moved within the plane of the drawing in response to the 
horizontal scanning of the optical beam. 
Referring to the drawings, an optical source 10 produces a polarized 
optical beam that is converted to a parallel optical beam by a coupling 
lens 11. The optical beam thus produced may have the plane of polarization 
that is substantially parallel to the horizontal scanning direction. The 
parallel optical beam is then passed through a half-lambda plate 12 
wherein the plane of polarization is rotated by the angle of 45 degrees 
with respect to the initial polarizing plane. Thereby, the optical source 
10, the lens 11 and the half-lambda plate 12 form an beam source. 
The optical beam thus produced with the predetermined polarization then 
enters to a cylindrical lens 13 that focuses the optical beam to form a 
flat optical beam having an elongating beam shape that extends in the 
vertical scanning direction. Thereby, the optical beam is focused at a 
focal point of the lens 13. See FIG. 1(B). At the focal point, there is 
provided a variable aperture device 100 carrying thereon a number of 
apertures that are arranged in the vertical scanning direction. As will be 
explained detail in later, the device 100 forms one of the essential 
elements of the present invention and used to change the size of the 
optical beam. 
The optical beam passed through the variable aperture device 100 enters to 
a lens 14 that forms a parallel optical beam when viewed in the plane 
parallel to the horizontal scanning direction as shown in FIG. 1(B). The 
optical beam passed through the lens 14 then enters to a cylindrical lens 
15 that focuses the optical beam incident thereto within the plane 
parallel to the vertical scanning direction as shown in FIG. 1(A). 
Thereby, the optical beam is focused at a position where a slit 16 is 
provided in the form of an elongated optical beam having an elongating 
beam shape extending in the horizontal scanning direction. The slit 16 
forms another essential part of the present invention and used to 
eliminate the diffraction of the optical beam higher than the first order. 
It should be noted that the diffraction of the optical beam is caused when 
the optical beam passes through the variable aperture device 100. 
The optical beam passed through the slit 16 then enters to a cylindrical 
lens 17 that focuses the optical beam in the plane parallel to the 
vertical scanning direction as shown in FIG. 1(A). As a result, an 
elongated image of optical beam extending in the horizontal scanning 
direction is formed on a reflection surface of a rotary polygonal mirror 
18. 
The rotary polygonal mirror 18 deflects the optical beam incident thereto 
as usual, and the optical beam thus deflected is focused on a recording 
surface 20 of a recording medium such as a photosensitive drum or 
photosensitive sheet after passing through a f.theta.-lens 19. As will be 
explained later, the size of the optical beam that is focused on the 
recording surface 20 is changed by the variable aperture device 100, and a 
graded recording of image is achieved thereby. 
Next, the variable aperture device 100 will be explained in detail. 
Referring to FIGS. 2(A)-2(D) showing an embodiment of the variable aperture 
device 100, the device 100 includes a PLZT layer 1 that shows the 
electro-optic Kerr effect, drive circuits 2A and 2B for inducing the 
electro-optic Kerr effect in the PLZT layer 1, a ceramic substrate 3 
provided with a slit 3A for passing an optical beam therethrough and for 
supporting the PLZT layer 1 and the drive circuits 2A and 2B thereon, an 
analyzer 5 provided on a lower major surface of the substrate 3, and a 
polarizer 4 provided above the PLZT layer 1. 
FIGS. 2(C) and 2(D) show the PLZT layer 1 in detail. Referring to the 
drawings, the PLZT layer 1 has a thickness of 400 .mu.m and has optically 
polished upper and lower major surfaces. Further, grooves 1B and 1C are 
formed on the upper major surface of the PLZT layer 1 to extend parallel 
with each other with a separation of about 100 .mu.m. The grooves 1B and 
1C have a depth of about 200 .mu.m and extend perpendicular to the plane 
of FIG. 2(C). 
The grooves 1B and 1C form an intervening region 1a therebetween for 
passing the optical beam. The region 1a has a width of about 100 .mu.m and 
extends vertically to the plane of FIG. 2(C). As the grooves 1B and 1C are 
formed at both sides of the region 1a, the region 1a projects in the 
upward direction. The grooves 1B and 1C may extend laterally with the 
width of 1 mm or more. Further, the grooves may reach the lateral edge of 
the PLZT layer 1. 
On the respective bottom surfaces of the grooves 1B and 1C, electrodes 1d 
and 1e of the NiCr:Al alloy are deposited by sputtering such that the 
electrodes 1d and 1e have portions 1a.sub.1 and 1a.sub.2 that cover the 
side walls of the projection 1a. 
After the projection 1a and the electrodes 1d and 1e are thus formed, a 
number of grooves if are formed to extend in the direction perpendicular 
to the elongating direction of the projection 1a with a pitch of about 100 
.mu.m. Each groove if may have a width of 20 .mu.m and a depth of 250 
.mu.m. As a result of the formation of the grooves 1f, the elongated 
projection 1a is divided into a number of generally square projections 1A 
that has a lateral dimension of 100 .mu.m.times.100 .mu.m. It will be 
understood that the projections 1A are aligned in the elongating direction 
and act as the aperture for passing the optical beam therethrough. 
Further, the electrodes 1d and 1e are also divided into a number of strip 
electrodes 1D and 1E as a result of the formation of the grooves 1f. 
There, the electrode strip 1D and the electrode strip 1E are located at 
both lateral sides of the projection 1A and side wall electrodes 1D.sub.1 
and 1E.sub.1 cover the side walls of the projection 1A in correspondence 
to the electrode portions 1a.sub.1 and 1a.sub.2. It should be noted that 
such electrodes 1D.sub.1 and 1E.sub.1 form an electric field in the 
intervening projected region 1A along the direction connecting these 
electrodes when an electric voltage is applied across the electrodes 1D 
and 1E. 
The PLZT layer 1 thus carrying the projections 1A and the corresponding 
electrodes 1D and 1E are provided on the ceramic substrate 3 such that the 
projections 1A are aligned with the slit 3A formed in the substrate 3. 
Further, the drive circuits 2A and 2B are mounted on the upper major 
surface of the substrate 3 and connected to the electrodes 1D and 1E by 
bonding wires 6A and 6B. See FIGS. 2(A) and 2(B). Further, the analyzer 5 
is attached to the lower major surface of the substrate 3 by an adhesive 
and the polarizer 4 is held above the PLZT layer 1 by suitable means. 
Thereby, the polarizer 4 and the analyzer 5 have respective polarizing 
planes that are perpendicular with each other. More specifically, the 
polarizer 4 may have the polarizing plane that is rotated by an angle of 
45 degrees with respect to the direction of the electric field applied 
across the projection 1A while the analyzer 5 may have the polarizing 
plane that is rotated by an angle of -45 degrees with respect to the 
direction of the electric field, or vice versa. 
When a laser beam having the intensity of I.sub.i is incident to the 
projected aperture 1A of the variable aperture device 100 described above 
with the plane of polarization coincident to the plane of polarization of 
the polarizer 4 under the presence of the electric field E acting across 
the aperture 1A, the intensity I.sub.o of the output optical beam is 
represented as a function of the phase difference .GAMMA. as 
EQU I.sub.0 =I.sub.i .multidot.sin.sup.2 (.GAMMA./2), (1) 
where .GAMMA. is represented as 
EQU .GAMMA.=(.pi./.lambda.).multidot.t.multidot.n.sub.0.sup.3 .multidot.R.sub.c 
.multidot.E.sup.2, 
in which .lambda. represents the wavelength of the optical beam, t 
represents the thickness of the PLZT layer 1 at the aperture 1A, n.sub.o 
represents the refractive index of the PLZT layer 1, and R.sub.c 
represents the second-order electro-optic constant. 
As shown in Eq. (1), the intensity I.sub.o of the output optical beam 
assumes the maximum or minimum when the phase difference .GAMMA. is equal 
to an integer multiple of .pi., or m.pi. where m is an integer. For the 
odd integers for the parameter m, the intensity I.sub.o becomes maximum 
while for the even integers for the parameter m, the intensity I.sub.o 
becomes minimum. Thus, the odd integer m corresponds to the state in which 
the aperture 1A is opened. On the other hand, the even integer m 
corresponds to the state in which the aperture 1A is closed. By 
controlling the electric field E such that the phase difference .GAMMA. is 
equal to the odd or even integer of .pi. for each of the apertures 1A 
independently, it is possible to control the beam size of the optical beam 
that has passed through the variable aperture device 100. 
Hereinafter, the change of the beam size achieved by the variable aperture 
device 100 will be described in detail. 
Referring to FIG. 3(A) showing the optical transmittance of the device 100 
along the axis .xi. that is taken coincident to the direction of alignment 
of the apertures 1A, the overall transmittance f(.xi.) is represented as 
EQU f(.xi.)=.SIGMA.F(.xi.-ml) (2) 
where F stands for the optical transmittance distribution of a single 
aperture 1A in the state that the aperture is opened and l stands for the 
pitch of the apertures. The summation is taken for the range of m from 0 
to N-1, wherein N represents the number of the apertures 1A that allow the 
passage of the optical beam. 
Referring to FIGS. 1(A) and 1(B) again, the laser beam that has passed 
through the variable aperture device 100 has the intensity distribution 
given by the foregoing Eq. (2) in the direction of the vertical scanning 
line. It should be noted that the variable aperture device 100 includes 
the apertures 1A that are aligned in the plane of FIG. 1(B) and hence in 
the vertical scanning direction of the optical beam. When the optical beam 
thus formed is focused on the location of the slit 16 by the lenses 14 and 
15, a Fraunhofer diffraction of optical beam, caused by the array of 
apertures 1A, appears in the vertical scanning direction. Such a 
diffraction pattern is represented analytically by the Fourier transform 
of the optical intensity distribution given by Eq. (2). 
More specifically, the amplitude distribution of the optical beam formed as 
a result of the Fraunhofer diffraction is represented as 
EQU g(X)=G(X).multidot..SIGMA.exp(-imlX) (3) 
where G(X) represents the Fourier transform of the function F(.xi.) and X 
represents the spatial frequency at the position of the slit 16. Again, 
the summation is taken for the apertures 1A that allow the passage of the 
optical beam. 
It should be noted that the summation of the exponential function in the 
above Eq. (3) is represented as 
EQU .SIGMA.exp(-imlX)=exp [-i(N-1)l/2X]{sin (NlX/2)}/}sin (lX/2)}. (4) 
Assuming the diameter of 2a for the aperture 1A as shown in FIG. 3(A), the 
Fourier transform G(X) is represented as 
EQU G(X)=sin (aX)/aX (5), 
and the amplitude distribution function g(X) as 
EQU g(X)=[sin (aX)/aX].multidot.[{sin (NlX/2)}/{sin 
(lX/2)}].multidot.exp[-i(N-1)l/2X]. (6) 
Thus, the optical intensity distribution is given as 
EQU .vertline.g(X).vertline..sup.2 =[sin (aX)/aX].sup.2 .multidot.[{sin 
(NlX/2}/{sin (lX/2){].sup.2. (7) 
It should be noted that Eq. (7) shows that the optical intensity 
distribution is given as a product of the Fraunhofer pattern of the 
individual apertures 1A (first term at the right hand side) and the 
interference pattern of the N apertures (second term at the right hand 
side). 
FIG. 3(B) shows the optical intensity distribution for the case where l is 
set equal to 4a in the vertical scanning direction. The horizontal axis 
shows the spatial frequency X. The broken line in FIG. 3(B) represents the 
first term of Eq. 7 while the continuous line represents the optical 
intensity distribution. The interval between the two adjacent peaks of the 
optical intensity is .pi./2a, while the half width value .DELTA.X of the 
principal peak becomes .pi./2Na. 
As can be seen in FIG. 3(B), the interval between the peaks and hence the 
diffraction peaks is determined equal to .pi./2a and the parameter a is 
determined by the radius of the aperture 1A. Thus, by setting the width of 
the slit 16 in the vertical scanning direction to be slightly smaller than 
the quantity .DELTA.f.pi./a (f stands for the synthetic focal length of 
the lenses 14 and 15), one can effectively eliminate the diffraction beams 
higher than the first order by the slit 16, even when the variable 
aperture device 100 causes the diffraction on the optical beam that passes 
therethrough. It should be noted that the actual width, measured in the 
vertical scanning direction, of the zeroth diffraction beam, i.e. the 
optical beam that passes through the slit 16 without interruption, is 
represented as 
EQU .DELTA.f.DELTA.X=.DELTA.f.pi./2Na. 
Referring to the side view of FIG. 1(A) again, the elongated image of the 
optical beam that is formed at the location of the slit 16 and extending 
in the horizontal scanning direction of the optical beam, is focused on 
the reflection surface of the rotary polygonal mirror 18 for deflection in 
the horizontal scanning direction. The optical beam thus deflected is then 
focused on the recording surface 20 of the recording medium after passing 
through the usual f.theta.-lens 19. By designating the magnification of 
the cylindrical lens 17 and the f.theta.-lens 19 in the vertical scanning 
direction as M.sub.1 and M.sub.2, respectively, the vertical size D of the 
beam spot on the recording surface 20 is given as 
EQU D=M.sub.1 .multidot.M.sub.2 .DELTA.f.pi./2Na. (8) 
As can be seen from Eq. (8), the size D of the optical beam in the vertical 
scanning direction changes inversely proportional to the number N of the 
apertures 1A. When the number of apertures 1A that passes the optical beam 
is increased, the size of the beam spot decreases to D.sub.1 as shown in 
FIG. 3(C), while when the number of the apertures 1A is decreased, the 
beam size increases to D.sub.2 as shown in FIG. 3(C). 
FIG. 4(A) shows an example of the actual change of beam spot on the 
recording surface 20 caused by controlling the number of apertures 1A that 
pass the optical beam. Referring to FIG. 4(A), the vertical beam size is 
changed while scanning the optical beam in the horizontal scanning 
direction. Further, one may change the beam size in both scanning 
directions by changing the width of the drive pulse LD supplied to the 
laser diode 10 for activating the same in synchronization to the change of 
the beam size in the vertical scanning direction as shown in FIG. 4(B). As 
a result of the change of the beam size achieved as described above, it is 
possible to achieve the graded recording of half-tone images such as 
photographs on the recording surface 20. By combining the usual technique 
of changing the driving power of the laser diode 20, a multi-level graded 
recording can be achieved without difficulty. 
In the recording media that show a threshold characteristic wherein no 
recording is made at all when the optical power is smaller than a 
predetermined threshold level, there can occur a case where the change of 
the optical beam size is not achieved even when the number of the 
apertures that pass the optical beam is changed. In such a case, the power 
of the laser diode 10 may be changed inversely proportional to the number 
of the opened apertures N such that the optical power is increased when 
the number N of the opened apertures 1A is decreased and such that the 
optical power is decreased when the number N of the opened apertures 1A is 
increased. 
Next, a second embodiment of the optical recording system of the present 
invention will be described with reference to FIGS. 5(A) and 5(B). 
Referring to FIG. 5(A) showing the plan view of the optical recording 
system of the second embodiment, the divergent optical beam produced by 
the optical source 10 is converted to a parallel optical beam by the lens 
11 and deflected to the left by a polarizing beam splitter 102. After 
passing through the beam splitter 102, the plane of polarization of the 
optical beam is rotated upon passage through the half-lambda plate 12 and 
focused by the cylindrical lens 13 to form a flat optical beam having an 
elongated beam shape that extends perpendicular to the plane of FIG. 5(A) 
at the position wherein a variable aperture device 101 is provided. 
FIG. 5(B) shows the construction of the variable aperture device 101 used 
in the system of FIG. 5(A). As shown in FIG. 5(B), the device 101 has a 
construction substantially identical with the device 100 except that there 
is provided a metal layer 70 acting as a mirror at the lower major surface 
of the PLZT layer 1. The mirror 70 reflects the optical beam that has 
passed through the activated apertures 1A. In the present device 101, too, 
the foregoing description of the principle of the activation of the 
apertures holds, except that the phase difference .GAMMA. is given as 
EQU .GAMMA.=(.pi./.lambda.).multidot.2t.multidot.n.sub.o.sup.3 
.multidot.R.sub.c .multidot.E.sup.2 
in correspondence to the optical path that is twice as large as the case of 
the device 100. 
The optical beam that is reflected by the device 101 passes the lens 13 and 
the plate 12 in the reversed direction and passes through the beam 
splitter 102. Thereby, the half-lambda plate 12 and the polarizing beam 
splitter 102 act as the polarizer 4 and the analyzer 5 of the first 
embodiment. 
The optical beam that has passed through the beam splitter 102 then enters 
to a cylindrical lens that focuses the optical beam within the plane that 
extends perpendicular to the plane of FIG. 5(A), and an elongated beam 
shape extending in the horizontal scanning direction is obtained at the 
position where the slit 16 is provided. The slit 16 eliminates the 
diffraction beam higher than the first order similarly to the first 
embodiment, and the optical beam obtained by the slit 16 is focused on the 
recording surface 20 by the cylindrical lens 17 and the f.theta.-lens 19. 
The deflection of the beam is achieved in the horizontal scanning 
direction by the rotary polygonal mirror 18 as usual. 
FIGS. 6(A) and 6(B) show a third embodiment of the optical recording system 
according to the present invention, wherein FIG. 6(A) shows the side view 
and FIG. 6(B) shows the plan view. As the construction of system is 
substantially identical with the first embodiment, only the difference 
that distinguishes the third embodiment from the first embodiment will be 
described. 
In the third embodiment, it should be noted that the slit 16 and the lens 
17 are eliminated and the lenses 14, 15 and 19 are used to focus the 
optical beam on the recording surface 20. Further, in order to eliminate 
the unwanted diffraction of the optical beam, a slit 160 is provided close 
to the recording surface 20 at a position between the f.theta.-lens 19 and 
the recording surface 20. It should be noted that the slit 160 has an 
elongated opening that extends in the horizontal scanning direction in 
coincidence with the plane in which the optical beam is deflected by the 
rotary polygonal mirror 18. In this construction, too, one can effectively 
eliminate the exposure of the recording surface 20 to the diffraction beam 
that is formed at the variable aperture device 100. 
FIGS. 8(A) and 8(B) show another embodiment of the variable aperture device 
that can be used in any of the foregoing optical recording systems. 
The variable aperture device of the present embodiment is developed to 
eliminate the problem of degradation of performance that appears in the 
variable aperture device 100 of FIGS. 2(A) and 2(B). More particularly, 
the variable aperture device 100 tends to interrupt the passage of the 
optical beam through the apertures 1A even when a drive voltage is applied 
across the electrodes 2A and 2B, particularly when the device is used 
continuously for a long time. 
It is believed that this undesirable degradation of the optical on/off 
function of the apertures 1A is caused by the formation of the space 
charges in the PLZT layer 1 caused as a result of continuous application 
of the drive voltage across the electrodes 2A and 2B. Such space charges 
are believed to be formed as a result of the trapping of the photocarriers 
that are migrated in the PLZT layer 1 by the electric field caused by the 
drive voltage. When the space charges are formed, the electric field that 
is produced by the space charges tends to cancel out the electric field by 
the drive voltage and the electro-optic Kerr effect no longer occurs 
satisfactorily. 
FIG. 7 shows the foregoing phenomenon in the form of optical transmittance 
characteristics. 
Referring to FIG. 7, the diagram shows the relationship between the 
intensity I.sub.o of the optical beam that has passed through an aperture 
1A of the variable aperture device 100 and the drive voltage V that is 
applied across the electrodes 2A and 2B provided at both sides of the 
aperture 1A. As shown in FIG. 7 by the continuous line, the optical 
intensity I.sub.o increases generally symmetrically with increasing 
magnitude of the drive voltage V. Thus by applying a positive bias voltage 
+V.sub.b or a negative bias voltage -V.sub.b and superposing positive or 
negative drive pulses thereon, one can control the passage of the optical 
beam through the aperture 1A as a result of the electro-optic Kerr effect 
that is induced in the aperture 1A. For example, one can control the 
aperture 1A to allow the passage of the optical beam by superposing a 
positive drive pulse to the positive bias voltage +V.sub.b. Alternatively, 
one may superpose a negative drive pulse to the negative bias voltage 
-V.sub.b. 
In the foregoing operation of the variable aperture device 100, it is 
observed that the characteristic curve of transmittance shifts laterally 
in the drawing as shown by the broken line, when the bias voltage of 
+V.sub.b is applied continuously across the electrodes 2A and 2B. Thereby, 
it will be understood that the intensity I.sub.o of the output optical 
beam is substantially reduced even when the drive pulse is applied, as 
shown by the output pulses represented by the broken line in FIG. 7. In 
other words, the activation of the apertures 1A of the variable aperture 
device 100 to pass the optical beam therethrough tends to become 
incomplete and the desired change of the beam size of the optical beam 
cannot be attained. 
In view of the evidence that the magnitude of shift of the characteristic 
curve increases with increasing intensity of the incident optical beam, it 
is believed that the foregoing effect is induced by the photocarriers that 
are created in response to the irradiation of the incident optical beam 
and trapped in the PLZT layer 1 to form the space charges. As already 
noted, such space charges tend to form the electric field that cancels out 
the electric field that is formed by the drive voltage V. 
FIGS. 8(A) and 8(B) show the variable aperture device 100 of another 
embodiment for eliminating the foregoing problem. 
Referring to the drawings, the variable aperture device includes an RF 
drive circuit 2 on the ceramic substrate 3 in place of the circuit 2A, and 
the drive circuit 2B is removed. Instead, there is provided a bonding pad 
3B on the substrate 3 and d.c. voltage source 8 supplies a d.c. bias 
voltage to the bonding pad 3B via a switch circuit 7. The switch circuit 7 
supplies the d.c. drive voltage to the bonding pad 3B with controlled 
polarities, and the bonding pad 3B supplies the drive voltage to each of 
the electrodes 1E simultaneously by respective bonding wires 6B. See the 
equivalent circuit diagram of FIG. 8(C). 
In the foregoing construction, one can change the polarity of the d.c. bias 
voltage periodically by the switch circuit 7. The polarity is changed with 
a period of several seconds or less, more preferably less than several 
seconds, and most preferably less than 3 seconds. For example, the 
polarity change may be achieved for every several horizontal scanning 
lines or every one page of optical recording. By switching the polarity of 
the d.c. bias voltage, one can eliminate the formation of space charges in 
the PLZT layer 1 and the variable aperture device shows a stable operation 
even after a continuous use. 
FIGS. 9(A) and 9(B) show another embodiment of the variable aperture device 
100. 
In this embodiment, the groove for holding the electrodes is provided at 
both upper and lower major surfaces of the PLZT layer 1. Thus, there are 
provided electrodes 1E' and 1D' at the lower major surface of the PLZT 
plate 1 with a projecting aperture region 1A' formed therebetween in 
alignment with the projecting aperture region 1A when viewed in the side 
view of FIG. 9(A). In the plan view of FIG. 9(B), on the other hand, it 
will be noted that the aperture 1A and the aperture 1A' are arranged 
alternately in the vertical direction of FIG. 9(B). Thus, the aperture 1A' 
is formed to project in the downward direction in correspondence to a 
space or gap formed between a pair of adjacent apertures 1A that project 
in the upward direction. 
Further, in correspondence to the aperture 1A', the electrodes 1D' and 1E' 
are provided such that the electrode 1D' is provided at the lower side of 
the plate 1 in correspondence to a space or gap formed between a pair of 
adjacent electrodes 1D and such that the electrode 1E' is provided also at 
the lower side of the plate 1 in correspondence to a space or gap formed 
between a pair of adjacent electrodes 1E. Thereby, the electrodes 1D and 
1D' are arranged alternately in the vertical direction of FIG. 9(B) at the 
right side of the array of the apertures 1A and 1A', and the electrodes 1E 
and 1E' are arranged alternately also in the vertical direction of FIG. 
9(B) at the left side of the array of the apertures 1A and 1A'. 
In the construction of the present embodiment, the electrodes 1D and 1D' or 
the electrodes 1E and 1E' are provided at different sides of the PLZT 
layer 1. Thereby, one can reduce the distance between adjacent electrodes 
on each side of the PLZT layer 1 and the apertures 1A and 1A' are formed 
on the PLZT layer 1 with a correspondingly reduced mutual separation. 
FIGS. 10(A) and 10(B) show another embodiment of the variable aperture 
device 100, wherein the electrodes 1D and 1E are provided on the 
substantially flat upper major surface of the PLZT layer 1. In other 
words, the groove is not provided in the present embodiment. In 
correspondence to the gap between the adjacent electrodes 1D and 1E, an 
aperture 1A.sub.1 is formed as illustrated. As can be seen in FIG. 10(A), 
there is no projection in correspondence to the aperture 1A.sub.1 in the 
present embodiment. 
FIGS. 11(A) and 11(B) show a still other embodiment of the variable 
aperture device 100, wherein the electrodes 1D and 1D' and the electrodes 
1E and 1E' are provided alternately on the upper and lower major surfaces 
of the PLZT layer 1. Similar to the embodiment of FIGS. 9(A) and 9(B), an 
aperture 1A.sub.1 ' is formed between the pair of electrodes 1D' and 1E', 
and the apertures 1A.sub.1 and 1A.sub.1 ' are aligned alternately in the 
vertical direction in the plan view of FIG. 11(B). In this embodiment, 
too, one can reduce the separation between the electrodes and hence the 
separation between the apertures. 
It should be noted that any of the variable aperture devices 100 that have 
been explained so far can be used for the optical recording system of the 
present invention. Further, by providing a mirror at one side of the 
aperture, one can use the variable aperture device 100 for the optical 
recording system described with reference to FIGS. 5(A)and 5(B). 
In the embodiment of FIGS. 8(A)-8(C), it will be noted that one can modify 
the device 100 to remove the RF driver unit 2 such that the variable 
apertures are driven directly by the RF pulses given externally. 
In the embodiments described so far, the variable aperture device lacks the 
means for controlling the temperature of the electro-optic layer. On the 
other hand, there arise cases wherein the control of temperature of the 
device is preferable as will be understood from the embodiments described 
below. 
FIG. 12 shows the characteristic curve of the electro-optic layer 1 for two 
different temperatures, wherein the continuous line represents the 
characteristics at a first, lower temperature, while the broken line 
represents the characteristics at a second, higher temperature. As will be 
obvious from FIG. 12, the intensity I.sub.0 of the optical beam 
transmitted through the aperture is decreased with increasing temperature, 
even when the aperture is in the opened state for transmitting the optical 
beam. 
FIG. 13(A) shows an embodiment of the variable aperture device that has a 
temperature control system. 
Referring to FIG. 13(A), a device having a construction substantially 
identical with the device of FIG. 8(A) is provided on a copper plate 40. 
More specifically, the ceramic substrate 3 of the device of FIG. 8(A) is 
provided on an upper major surface of the copper plate 40, and there is 
provided a slit S.sub.1 in the copper plate 40 in alignment with the slit 
3A formed in the ceramic substrate 3. Further, the analyzer 5 is provided 
on a lower major surface of the copper plate 40. 
On the PLZT layer 1, there is provided a temperature sensor 41 for 
detecting the temperature of the layer 1, and a temperature control device 
42 such as a heater is provided on the copper plate 40 such that the 
temperature control device 42 is driven by a controller 43 in response to 
the temperature detected at the temperature sensor 41. 
FIG. 13(B) shows a modification of the device of FIG. 13(A) in which the 
detection of the temperature is achieved by detecting the intensity of the 
optical beam passed through the device. As described with reference to 
FIG. 12, there exists a relationship between the temperature and the 
intensity of the optical beam that has passed through the aperture. Thus, 
the present embodiment employs an optical sensor 44 for the detection of 
the intensity of the optical beam, and the temperature of the PLZT layer 1 
is inferred based upon the intensity of the optical beam thus detected. 
The optical sensor 44 may be provided to detect the optical beam by 
deflecting the optical beam by a semi-transparent mirror provided at the 
lower side of the copper plate 40 or may be provided close to but offset 
from a range of scanning by the deflected optical beam. The output of the 
optical sensor 44 is supplied to a control unit 45 for amplification, and 
the control unit 45 drives the temperature control device 42. 
Typically, the PLZT layer 1 is set at a temperature higher than the 
temperature of the environment. For example, the temperature of the PLZT 
layer 1 may be set at 50.degree. C. When the temperature of the layer 1 
decreases below 50.degree. C., a device such as a heater that forms the 
temperature control device 42 is activated such that the temperature is 
maintained at 50.degree. C. Alternatively, one may use a Peltier device 
for the device 42 when it is desired to maintain the temperature of the 
PLZT layer 1 at a room temperature such as 20.degree. C. 
In the embodiments of FIGS. 13(A) ad 13(B), it should be noted that these 
devices use also the d.c. voltage source 8 and the switch circuit 7 
similarly to the previous embodiments, although the illustration of these 
parts are omitted for avoiding complexity. Further, it should be noted 
that there is provided a signal processor 2C in cooperation with the RF 
driver unit 2 such that the processor 2C is supplied with the image signal 
as well as the d.c. bias signal from the switch circuit 7 for producing 
the RF signal as will be described in detailed later. Further, it should 
be noted that one may cover the upper major surface of the copper layer 40 
by an insulating film. In this case, the ceramic substrate 3 may be 
eliminated and one can provide the PLZT layer 1 and the RF driver 2 
directly on the insulating film. Further, it should be noted that one may 
eliminate the RF driver 2 when the RF signal is supplied directly from an 
external device. 
FIGS. 14(A) and 14(B) are diagrams similar to FIGS. 6(A) and 6(B) showing 
the optical recoding system wherein the variable aperture device of FIGS. 
13(A) and 13(B) are used. In these drawings, those parts and elements that 
are described previously are designated by the same reference numerals and 
the description thereof are eliminated. 
Referring to the drawings, the system of the present embodiment employs a 
variable aperture device 1000 that corresponds to the device of FIG. 13(A) 
or FIG. 13(B) and includes a temperature regulation control as described 
previously. Further, the d.c. voltage source 8 and the switch circuit 7 
cooperates with the device such that the polarity of the d.c. bias is 
changed with a predetermined interval such that the optical drift is 
eliminated. As described previously with reference to other embodiments, 
such an inversion of the polarity may be achieved in each line or in each 
page. 
Further, there are provided slit members 200 and 300 respectively between 
the lenses 14 and 15 and between the lens 17 and the object 18 for 
eliminating the higher order diffraction caused by the variable aperture 
device 1000. There, the slit member 200 has an elongate opening extending 
in the direction perpendicular to the horizontal scanning direction for 
eliminating the diffraction components appearing along the horizontal 
scanning line, while the slit member 300 has an elongate opening extending 
in the direction of the horizontal scanning direction for eliminating the 
diffraction components appearing along the vertical scanning line. 
Next, the control process applicable to the optical recording system of 
FIGS. 14(A) and 14(B) for controlling the beam size of the optical beam 
will be described with reference to FIGS. 15(A)-15(F). It should be noted 
that these drawings show the control of passage of the optical beam 
occurring at a single aperture that forms an aperture array in the 
variable aperture device 1000 together with other apertures. In the 
present embodiment, the inversion of polarity of the d.c. bias is achieved 
in each one line of the image signal. 
Referring to the drawings, FIG. 15(A) shows a line synchronizing signal for 
providing the timing of the horizontal scanning line, while FIG. 15(B) 
shows a gradation signal provided externally for indicating the gradation 
of the image signal to be recorded. This gradation signal is given as a 
part of the image signal for representing the gradation of the image to be 
exposed. One may regard the gradation signal as the image signal 
representing the image to be exposed by the optical beam that passes 
through the single aperture in consideration. At the level 0, the 
gradation signal indicates the closure of the aperture for interrupting 
the optical beam. In response to this, the exposure level on the 
photosensitive body is reduced. When the gradation signal is high, on the 
other hand, the optical beam passed through the aperture in consideration 
is added to the optical beam used for recording, and the exposure level of 
the image is increased. Further, FIG. 15(C) shows the bias voltage that is 
applied commonly to all the apertures in the device 1000. As noted above, 
the polarity of the bias voltage is changed in each line. 
FIG. 15(D) shows the RF pulse for activating the aperture under 
consideration. As noted previously, the RF pulse is produced in the 
processor 2C (FIG. 13(A)) based upon the d.c. bias voltage of FIG. 15(C) 
and the gradation signal of FIG. 15(B). In the present embodiment, the RF 
pulse has a variable pulse width that may be changed in each aperture of 
the variable aperture device. The RF pulse changes between a level 
+V.sub.p and a level -V.sub.p in response to the level of the gradation 
signal of FIG. 15(B) as well as in response to the bias voltage of FIG. 
15(C), and a drive signal as shown in FIG. 15(E) is formed from the RF 
pulse of FIG. 15(D) by superposing the RF pulse to the bias voltage. It 
should be noted that the polarity of the RF pulse is set coincident to the 
polarity of the bias voltage in correspondence to the interval where the 
optical beam is to be passed, while the polarity is inverted in the 
interval where the optical beam is to be interrupted. Thereby, one obtains 
an output optical pulse as shown in FIG. 15(F). 
FIGS. 16(A)-16(F) show the time chart for controlling the size of the 
optical beam according to another embodiment of the present invention. 
Again, these drawings show the control of only one, single aperture that 
forms the array of apertures in the variable aperture device. 
Referring to the drawings, FIG. 16(A) shows the timing of an exposure clock 
pulse that specifies the timing for writing each picture element by the 
optical beam, while FIG. 16(B) shows the gradation signal corresponding to 
the signal of FIG. 15(B). 
In the present embodiment, the RF pulse for controlling the transparency of 
the aperture is provided in synchronization with the clock pulse of FIG. 
16(A), wherein the polarity of the RF pulse is changed, with respect to 
the bias voltage given in FIG. 16(D), in response to the gradation signal 
of FIG. 16(B). More specifically, the RF pulse has a polarity the same as 
the polarity of the bias voltage when there is a high level gradation 
signal indicating the exposure through the aperture in consideration. On 
the other hand, the RF pulse has an opposite polarity when the level of 
the gradation signal is zero. Thereby, one obtains a drive signal that is 
actually applied across the aperture as indicated by FIG. 16(E). In 
response to the drive signal of FIG. 16(E), one obtains an optical output 
at the aperture as indicated in FIG. 16(F). 
In the present embodiment, it should be noted that the RF pulse is produced 
continuously in response to the exposure clock, irrespective of whether 
the aperture is activated for transmitting the optical beam or not. 
Thereby, the generation of heat associated with the RF pulse becomes 
uniform for all the apertures in the variable aperture device, and a 
uniform temperature distribution is achieved. It should be noted that the 
heat generated in such a dielectric device is generally proportional to 
the square of the applied voltage and the frequency. As there are 
apertures that are activated frequently and also there are apertures that 
are activated seldom, there can occur a problem of non-uniform 
distribution of heat in the device when the RF pulse is applied only to 
the apertures that are activated. Such a non-uniform temperature 
distribution within the device is difficult to compensate for even when 
the temperature regulation system described previously is employed. In the 
present embodiment, the problem of non-uniform heat generation and hence 
the non-uniform temperature distribution is successfully eliminated. 
Next, another embodiment for controlling the size of the optical beam in 
the optical recording system of FIGS. 14(A) and 14(B) will be described 
with reference to FIGS. 17(A)-17(G), wherein FIG. 17(A) shows an exposure 
clock pulse corresponding to the clock pulse of FIG. 16(A), FIG.17(B) 
shows the line synchronizing signal corresponding to the synchronizing 
signal of FIG. 15(A), FIG. 17(C) shows the bias voltage described 
previously, and FIG. 17(D) shows the gradation signal also described 
previously. It will be noted that the polarity of the bias voltage is 
changed in response to the line synchronizing signal. 
FIG. 17(E) shows the RF pulse that is used in the present embodiment for 
activating the aperture, wherein it will be noted that the RF pulse is 
formed from two parts, the first part corresponding to the gradation 
signal with the same timing and the pulse width as the gradation signal, 
and the second part corresponding to the interval where the gradation 
signal is absent. Generally, the gradation signal has a pulse width much 
larger than the clock pulse of exposure. In the second part, on the other 
hand, the RF pulse has the pulse width and timing the same as the exposure 
clock pulse. Thereby, one obtains a drive pulse that is actually applied 
to the aperture as shown in FIG. 17(F), and an optical output as shown in 
FIG. 17(G) is obtained in response to the drive signal of FIG. 17(F). 
There, it should be noted that the optical output appears continuously 
during the interval of the gradation signal. 
Next, still another embodiment of the present invention for controlling the 
size of the optical beam in the optical recording system of FIGS. 14(A) 
and 14(B) will be described with reference to FIGS. 18(A)-18(G). 
In the previous three embodiments for the exposure process, there can occur 
a case wherein the duration of the gradation signal may be different in 
the first horizontal scanning interval and in the second horizontal 
scanning interval that follows the first scanning interval. In such a 
case, the duration in which the drive pulse for example shown in FIG. 
16(E) may be different in the positive drive pulse for the first interval 
and in the negative drive pulse for the second interval. When such a 
deviation in the polarity continues, there can occur a case that the PLZT 
layer 1 forming the variable aperture device may be charged up and the 
problem of the "optical drift" may occur in spite of the inversion of the 
polarity of the d.c. bias voltage. The present embodiment intends to 
eliminate the problem of charge up of the electro-optic layer. 
Referring to the drawings, FIG. 18(A) shows the exposure clock pulse 
corresponding to FIG. 17(A), FIG. 18(B) shows the line synchronizing 
signal corresponding to FIG. 17(B), FIG. 18(C) shows the bias voltage, and 
FIG. 18(D) shows the gradation signal. There, it will be noted that one 
horizontal scanning interval, defined in FIG. 18(B) as the region 
extending between a pair of line synchronizing signals, includes a 
scanning interval for exposing the image by the optical beam and a 
blanking interval for returning the scanning system to the initial state 
without causing exposure. In other words, the laser diode for producing 
the optical beam is deactivated in the blanking interval. 
There, the RF pulse is produced in synchronization with the exposure clock 
pulse of FIG. 18(A) as shown in FIG. 18(E), in correspondence to the 
gradation signal of FIG. 18(D) and with the polarity coincident to the 
polarity of the bias voltage of FIG. 18(C) as explained previously, except 
that the number of the RF pulses that appear in correspondence to the 
interval in which the gradation signal is in the non-zero state is counted 
in each horizontal scanning interval. In the illustrated example of FIG. 
18(E), there are five RF pulses in the first interval in correspondence to 
the gradation signal of FIG. 18(E), while in the next, second interval, 
the number of the RF pulse corresponding to the non-zero state of the 
gradation signal is only two. When the optical exposure is achieved 
according to the RF pulses thus obtained, there occurs a problem of the 
positive charge-up of the PLZT layer because of the unbalanced positive 
and negative RF pulses and hence the drive pulses. 
In order to eliminate this problem, the process of the present embodiment 
adds the RF pulses in the second interval in correspondence to the 
blanking interval such that the total number of the RF pulses becomes the 
same in the first horizontal scanning line and in the second horizontal 
scanning line. In the illustrated example, three RF pulses marked by 
arrows are added. Thereby, one obtains a drive pulse as shown in FIG. 
18(F) and the optical exposure is achieved as shown in FIG. 18(G). There, 
it will be noted that the exposure does not occur in correspondence to the 
RF pulses that are added artificially as these extra RF pulses are formed 
in the blanking interval where the laser diode is deactivated. Thereby, 
one can eliminate the problem of "optical drift" even when a pattern as 
shown in FIG. 18(G) is exposed. 
It should be noted that such a detection of the unbalanced exposure in the 
adjacent horizontal scanning intervals may be achieved by detecting the 
interval of the RF pulses that correspond to the gradation signal, instead 
of counting the number of the RF pulses. Further, one may compare the 
result of counting of the RF pulses with a reference number for activating 
the "compensation" in the blanking interval. 
Other objects and further features of the present invention will become 
apparent from the following detailed description when read in conjunction 
with the attached drawings.