Light scanning apparatus

In a light scanning apparatus, such as in a laser printer, an optical switch is provided between a light source and an acousto-optic light-modulating element for modulating the light according to data to be printed. The optical switch may be an acousto-optic light-modulating element as well. The optical switch blocks or passes the continuously input light from the light source according to a predetermined schedule, such as eight bits prior to the start of black dots and eight bits after the end of the block dots, or during a stand-by period. The time during which a fine light spot is focused on the acousto-optic light-modulating element is thus shortened. Accordingly, the life of the acousto-optic light-modulating element is increased because of the shortened duration of the focusing of the input light. Achievement of the fine spot size allows finer print-out resolution without sacrificing scan speed or the life of the element. The light spot focused on the optical switch is broad enough in size for the element to obtain to also achieve a long life.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a light scanning apparatus for reading or 
developing an image, and more particularly to a light scanning apparatus 
in which a light source of the light scanning apparatus is continuously 
lit during operation of the apparatus. 
2. Description of the Related Art 
As is well known, when an ultrasonic wave of a particular frequency is 
applied to an acousto-optic modulating element, light input thereto in a 
specific direction with respect to the propagation direction of the 
ultrasonic wave is deflected, due to an effect called Bragg Reflection. 
Thus, in a high speed printer employing a gas laser as its light source, 
for example, an acousto-optic modulating element has been employed to 
modulate, i.e. switch, the continuously output laser light. Light emitting 
diodes (LEDs), which can be directly modulated, have also been employed as 
a light source for high speed printers. However, it is difficult to match 
the light spectrum of LEDs with that of a photosensitive drum of the 
printer and LEDs have a relatively short life. Gas lasers are therefore 
still employed as light sources. 
A block diagram of a prior art laser printer system using an acousto-optic 
light-modulating element is shown in FIG. 1. A laser light produced by a 
light source 1 (e.g., gas laser) is focused by a focusing lens system 2 
onto a crystal of an acousto-optic light-modulating element 4 to form a 
fine light spot thereon. On receiving an enabling signal from a control 
circuit 10, a modulator driver 3 outputs a modulating signal of 
approximately 200 MHz to the acousto-optic light-modulating element 4. The 
acousto-optic light-modulating element 4 deflects the light input thereto 
by an angle of 23 m radians, through a slit 20 in a plate 19 and onto an 
expander lens system 5. 
When no modulating signal is applied to the acousto-optic light-modulating 
element 4 from the modulator driver 3, the acousto-optic light-modulating 
element 4 does not deflect the light through slit 20. Instead, the light 
is blocked by plate 19 and thus the light from the focusing system 2 never 
reaches the expanding lens system 5. 
In FIG. 1, the deflected light is designated by arrow D and the 
not-deflected light is designated by arrow ND. The deflected light D 
reaches the expanding lens 5 through slit 20; however, the not-deflected 
light ND is directed onto the plate 19 and is blocked from reaching 
expander lens system 5. 
The deflected light D reaching the expander lens system 5 is expanded to 
form a parallel light beam which is directed to rotating polygonal mirrors 
6. Before reaching a photosensitive drum 8, the light is deflected by the 
rotating polygonal mirrors 6 so as to scan the beam detector 9. The light 
deflected by the mirrors 6 is focused by an f..theta.lens 7. The lens 7 
directs a finely focused light spot onto the photosensitive drum 8. Beam 
detector 9 located optically next to the photo-sensitive drum 8 detects 
the light scanning a single frame (i.e., a single light scan from one end 
of drum 8 to the other end of drum 8) on the drum 8, prior to the scanning 
light beginning to actually scan drum 8. Upon detecting the light, the 
beam detector 9 transmits a detection signal BD (beam detection) to 
control circuit 10. In response to receiving the detection signal BD, the 
control circuit 10 allows a VIDEO signal to be transmitted sequentially to 
the modulator driver 3. The VIDEO signal acts as a modulator driver 
signal. When the signal is a "1" level, the modulator driver 3 causes the 
acousto-optic modulating element 4 to deflect the input light through the 
slit 20. 
Referring to FIGS. 2 and 3, the structure and operation of the 
acousto-optic light-modulating element 4 are explained below. FIG. 2 
schematically illustrates the structure of the acousto-optic 
light-modulating element 4. The acousto-optic light-modulating element 4 
includes a crystal portion 4' comprising, for example, lead molybdate 
(PbMo0.sub.4) or tellurium dioxide (Te0.sub.2) and an electro-acoustic 
transducer 4" comprising, for example, lithium niobate (LiNb0.sub.3). FIG. 
3 illustrates waveforms of the modulating signal "a" provided by the 
modulator driver 3 and the deflected light "c" output from the 
acousto-optic light-modulating element 4. When no modulating signal "a" is 
applied to the transducer 4", the laser light beam "b" travels straight 
through the crystal portion 4' of the element 4 and is output as shown by 
dotted lines L5 and L6. When a modulating signal "a" is applied to the 
transducer 4", an ultrasonic wave L7 of approximately 200 MHz generated 
therein propagates through the element 4 approximately orthogonal to the 
direction of the input light beam "b". As soon as the front end of the 
ultrasonic wave reaches the upper edge L1 of the input light beam "b", the 
upper edge of the light is deflected along line L3 in the direction "c". 
It takes a time period t.sub.r (FIG. 3) for the ultrasonic wave L7 to 
reach the lower edge L2. Therefore, the rise and fall of the waveform of 
the deflected light pulse, i.e., the output of the acousto-optic 
light-modulating element 4 is delayed and deformed as shown by "c" in FIG. 
3. The delay time period, i.e., the rise time period t.sub.r is determined 
by: 
t.sub.r =0.66d/v 
where d is the diameter of the input light beam "b", and v is the 
propagation velocity of the ultrasonic wave L7 in the crystal portion 4' 
of the acousto-optic modulating element. 
In order to achieve a less deformed modulated light output, i.e. one having 
fast rising and falling edges, the diameter of the light spot is required 
to be as small as possible. This is because the delay time period t.sub.r 
is shorter for a smaller diameter d of the input light beam b. If the 
light output pulse is deformed, a dot to be printed may be missed. The 
recent trend for speeding up the light scanning apparatus and achieving 
higher resolution requires that the light spot size be reduced from about 
100 .mu.m in diameter, to as fine as 30 .mu.m. The smaller spot size means 
that there is a larger energy density of the focused light; the energy 
density varies inversely with the square of the spot diameter. However, 
the allowable light input to the acousto-optic light-modulating element is 
limited by the energy density of the light input thereto as well as by the 
accumulated duration of the light input. As a solution to this problem, a 
mechanical shutter may be provided between the light focusing system 2 and 
the acousto-optic light-modulating element 4; however, the shutter speed 
is too slow and the durability of the shutter is too short to be 
efficient. Moreover, a solenoid used to drive the mechanical shutter 
generates electrical noise which disturbs electronic circuits in the 
vicinity of the solenoid. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a high speed, high resolution, 
light scanning apparatus employing a long-life acousto-optic 
light-modulating element and a gas laser as a light source. 
It is another object of the invention to provide a high speed, high 
resolution light scanning apparatus which narrows the focus of a light 
beam from a light source when a narrowly focused light beam is needed for 
imaging. According to the present invention, there is provided a light 
scanning apparatus operatively connectable to receive light and image 
data, comprising first optical switch means for optically passing the 
light in accordance with a first control signal; focusing means, 
operatively connected to the first optical switch means, for focusing the 
light passed by the first optical switch means; and second optical switch 
means, operatively connected to the focusing means for optically passing, 
in accordance with a second control signal, the light focused by the 
focusing means. 
The above-mentioned features and advantages of the present invention, 
together with other objects and advantages, which will become apparent, 
and will be more fully described hereinafter, with reference being made to 
the accompanying drawings which form a part hereof, wherein like numerals 
refer to like parts throughout.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 is a block diagram of a system embodying the present invention. In 
FIG. 4, an optical switch 18 (which may be an acousto-optic 
light-modulating element) is provided between a light source 1 and a light 
focusing means 2. A gate circuit 15 provides a control signal "a.sub.1 " 
to control the optical switch 18. The switch 18 optically passes to or 
blocks the light input from light source 1 in accordance with the control 
signal. The control is also in accordance with a pre-programmed schedule, 
and depends on the length of blank video signals continuously being input 
for printing or display. In FIG. 4, gate circuit 15 and control circuit 10 
comprise a control circuit means for providing control signals to the 
optical switch 18 and the acousto-optic light-modulating means 4. With the 
present invention, the light density of the light spot projected onto 
acousto-optic light-modulating element 4 is increased. However, the 
duration of the light spot projected onto the acousto-optic 
light-modulating element 4 is reduced, resulting in increased life of the 
acousto-optic light-modulating element 4. Thus, the reduced light spot 
size increases the output resolution; e.g., the resolution of a print-out 
without sacrificing the working life of an acousto-optic light modulating 
element such as element 4. The light density of the light spot projected 
onto the optical switch 18 is low enough to allow a long working-life for 
optical switch 18 even though light is continuously applied thereto. 
A block diagram of a laser printer embodying the present invention is shown 
in FIG. 5. Waveforms of the signals in FIG. 5 are shown in FIG. 6. A light 
source 1 can be a gas laser (for example, an argon (Ar.sup.+), helium-neon 
(HeNe) or helium-cadmium (HeCd) gas laser). The light beam output from the 
light source 1 is continuously applied to a first acousto-optic 
light-modulating element 14. The light beam is not finely focused (a beam 
of approximately 600 .mu.m in diameter). 
As is well known, prior to reflecting light by the rotating polygonal 
mirror 6 to scan a photo-sensitive drum 8, an enabling portion of a VIDEO 
signal (discussed below) enables the light to be deflected to first scan 
beam detector 9 located optically next (typically just before) the 
photosensitive drum 8. Upon detecting light, the beam detector 9 outputs a 
detection signal BD. This signal indicates that the light is to begin 
scanning a single frame on the drum 8. In the control circuit 10, the BD 
signal enables VIDEO signals (e.g., image data) to be output to the 
modulator driver 3 through an OR gate 12, which comprises a local gate 
circuit together with an inverter 11. Details and varieties of the 
programming to control the printer signal LPRT are described below. 
The VIDEO signal is typically a sequence of pulses, each having 
approximately a 28.5 ns width for single dot printing. The enabling 
portion e of the VIDEO signal shown in FIG. 6 is of a duration which is 
just long enough to be detected by beam detector 9. The VIDEO signal 
enables the modulator driver 3 to output an approximately 200 MHz 
modulating signal "a" when the VIDEO signal is, for example, a logic 1 as 
shown in FIG. 6. A switch circuit 13 is arranged so as to deliver the 
modulating signal "a" to the acousto-optic light-modulating element 4 as 
the signal (i.e., a control signal) "a.sub.2 38 when the switch 13 
receives a logic "1" level of the print signal LPRT. No modulating signal 
is delivered to the first acousto-optic light-modulating element 14 during 
this time. 
When receiving no modulating signal, the first acousto-optic 
light-modulating element 14 allows the light input from the light source 1 
to pass through slit 21 of plate 22, without being deflected. This light, 
indicated by the arrow ND', is applied to a focusing lens system 2. The 
light input to the focusing lens system 2 is focused onto the second 
acousto-optic light-modulating element 4 to form a light spot as fine as 
30 .mu.m. When receiving the modulating signal "a.sub.2 ", the second 
acousto-optic light modulating element 4 deflects the input light from the 
focusing lens system 2 as indicated by the arrow D". The deflected light 
D", i.e. modulated or optically passed light, passes through slit 20 of 
plate 19 to an expander lens 5. 
When no modulating signal is applied to the second acousto-optic 
light-modulating element 4, the light input to the second acousto-optic 
light-modulating element 4 passes straight through it without being 
deflected (as indicated by the arrow ND") onto plate 19; in other words, 
the light ND" is blocked. 
The light input to the expander lens 5 is expanded to form a parallel beam 
and is projected on to the rotating polygonal mirrors 6. The light beam 
projected onto the polygonal mirrors 6 is reflected to scan the 
f..theta.lens 7. The light beam input onto the polygonal mirrors 6 is 
uniformly focused onto a photo-sensitive drum 8, generally formed of an 
alloy of selenium (Se) and tellurium (Te), and the focused light spot 
scans along a line of the photo-sensitive drum 8 as shown in FIG. 8. This 
produces an electrostatic latent image on the drum 8, in accordance with 
the input video signals. 
When a "0" level of the print signal LPRT is inverted by the inverter 11 to 
enable the modulator driver 3 via the 0R gate 12, switch circuit 13 
delivers the modulation signal "a" from the modulator driver 3 to the 
first acousto-optic light-modulating element 14 as the modulating signal 
"a.sub.1 " without delivering a modulating signal "a.sub.2 " to the 
acousto-optic light modulating element 4. On receiving the modulation 
signal "a.sub.1," the first acousto-optic light-modulating element 14 
deflects the light applied thereto by an angle of approximately 23 m 
radian as indicated by the arrow D'. The deflected light D' is directed 
onto plate 22 so as to block the light D". Accordingly, no light is input 
to the second acousto-optic light-modulating element 4. 
The duration of the "0" level of the print signal LPRT may be programmed as 
desired. For example, the duration may equal the stand-by or waiting 
period before the start of a printing operation; the fly-back period; or a 
period during which blank signals, i.e. a "0" level of the video signals 
continues for more than eight bits. In other words, the print signal LPRT 
becomes a "1" level eight bits prior to the start of VIDEO signals of a 
level "1". The print signal LPRT becomes a "0" level eight bits after the 
end of VIDEO signals of a level "1". This number (eight bits) is chosen to 
meet the slow rise and fall time of the acousto-optic light-modulating 
modulating element 14 that has a large spot size applied thereto. The 
print signal LPRT, as well as the VIDEO signal, can also be made "1" as a 
routine by a timer which measures the time elapsing after the previous 
frame and indicates the timing that the circuit must be ready to start 
scanning a new frame, i.e. so that the paths are open for the light to 
reach the beam detector 9. 
FIGS. 7 and 8 show a perspective view and a schematic drawing, 
respectively, of the present invention used in a laser printer 
application. The light paths in FIG. 7 are schematically illustrated in 
FIG. 8. In the strict sense of the word, the light input to the 
acousto-optic light-modulating elements is slanted for a predetermined 
degree as shown in FIG. 7. Mirrors 36-40 reflect the various light beams 
to allow the components of the laser printer to be compactly packaged. For 
simplifying the explanation, the location of plate 19 and slit 20 in FIG. 
4 and 5 is drawn just after the second acousto-optic light-modulating 
element 4, though in FIG. 7 and 8 it is located after the expander lens 5. 
For the case where the print signal LPRT is programmed depending on the 
VIDEO signals, the circuit shown in FIG. 10 is additionally included in 
the control circuit 10. A graph of the print signal LPRT compared to the 
VIDEO signal pulses of the present invention is shown in FIG. 9. The data 
to be printed is stored in a memory device 31, typically a semiconductor 
RAM (random access memory). The print data (VIDEO signals) for a single 
frame is read out by a read circuit 32 from the memory device 31. The read 
circuit 32 also checks the logic level of the data (VIDEO signals) 
sequentially in eight-bit advance. When all of the eight bits in advance 
are a "0" level, the print signal LPRT becomes "0", as shown in FIG. 9. 
When a "1" level of the data (the VIDEO signal) is read out in eight-bit 
advance, the print signal LPRT becomes "1". Furthermore, the read circuit 
32 counts continuous "0" VIDEO signal bits currently printed. When it 
counts eight, the print signal LPRT becomes "0". 
Supposing that the scan width of the light is 16 inches, and the length of 
a printed line of characters is 15 inches wide and 8 inches wide, 
respectively. The blank area at both sides total 7% and 50% of the scan 
width, respectively. Even for the 15 inch width, considering the line 
spaces, the black portion, i.e. portion to be printed, is as low as 
several percent of the entire light scanning. 
Consequently, the duration of the light input to the second acousto-optic 
light-modulating element 4 can be reduced to several percent since there 
is no need for input light to the second acousto-optic light-modulating 
element unless printing is occurring on the paper. This means that the 
life of the acousto-optic light-modulating element 4 is extended more than 
ten times that of the prior art systems. This compensates for the 
shortened life caused by the increased light energy density needed for 
reducing the spot size focused on the prior art acousto-optic 
light-modulating element. The light beam input to the first acousto-optic 
light-modulating element 14 is broad, i.e. the light energy density is 
sufficiently low for the element so that it does not have a shortened 
life, even though the light is always input thereto. 
The achievement of the 30 .mu.m diameter spot projected on the second 
acousto-optic light-modulating element 4 without sacrificing its working 
life accomplishes printing resolution as much as 2.6 times better than 
that of the prior art 100 .mu.m diameter spot, without sacrificing scan 
speed. 
Switching of the first acousto-optic light-modulating element 14 is not 
required to be as fast as that of the second acousto-optic 
light-modulating element 4; therefore, the first acousto-optic 
light-modulating element 14 can be of lower specification level, i.e. of a 
lower cost, as long as the same modulation frequency is useable. 
The location of the slits 20 and 21 in the figures are only for example; 
therefore, the slits may be located at places other than those 
above-described, or may be omitted, as long as the function of the optical 
switch is fully performed. 
Though in the description of the preferred embodiment the first 
acousto-optic light-modulating element 14 is on before and off after eight 
continuous bits of blank signals, this bit number may be chosen other than 
eight as long as the response of the first acousto-optic light-modulating 
element 14 does not disturb the switching of the second acousto-optic 
light-modulating element 4. 
Though the essentially same kind of acousto-optic light-modulating element 
is used for both acousto-optic light-modulating elements 4 and 14 and both 
of them are driven by the same modulator driver, a different kind of 
optical switch, such as a liquid crystal switch, may be used, driven by an 
independent driver, in place of the first acousto-optic light-modulating 
element 14. 
Though in the above-described preferred embodiments, polygonal mirrors are 
employed for scanning the light, it is apparent that other types of 
scanning devices, such as galvano mirrors or holograms, may also be 
employed. 
Though only a laser printer is referred to in the above-description as a 
preferred embodiment, the light scanning apparatus of the present 
invention may be applied to an apparatus for writing data on photo film, 
or to an optical character/image reader, or to a display device of a 
high-resolution television receiver. In other words, though the light 
switched by the second acousto-optic light-modulating element 4 scans a 
light-sensitive drum 8, this light-sensitive drum 8 may be replaced by a 
photo micro film on which an image or character is to be recorded, or a 
character or an image which is to be optically read out, or an optical 
system which projects the scanned light on to a screen such as a 
television screen. When this light scanning apparatus is used for a 
character/image reader or television projection, the light must be scanned 
not only along the horizontal direction (as along the ridgeline of the 
light-sensitive drum) but also along the vertical direction by the 
rotating polygonal mirrors, or the character/image to be read out is fed 
vertically. 
The many features and advantages of the invention are apparent from the 
detailed specification and thus, it is intended by the appended claims to 
cover all such features and advantages of the system which fall within the 
true spirit and scope of the invention. Further, numerous modifications 
and changes may readily occur to those skilled in the art. It is not 
desired to limit the invention to the exact construction and operation 
shown and described, and accordingly, all suitable modifications and 
equivalents may be resorted to, falling within the scope of the invention.