Laser activating system for laser printing

An apparatus for recording the information content of an electrical signal on the surface of a light sensitive medium by means of a plurality of scan traces across said medium comprises a source for providing a light beam of high intensity modulated in accordance with electrical information supplied thereto, an actuator for actuating the source means and generating sync signals, the frequency of the sync signals being altered in accordance with variations in scanning speeds of the light beam to impinge on the medium, a reflector having a plurality of contiguous reflective faces rotatable about a central axis, a rotation device for rotating said reflector about said central axis; and a lens, disposed between said source and said reflector, for receiving and passing through said light beam.

BACKGROUND OF THE INVENTION 
The present invention relates to a flying spot scanning system for 
communicating video information to a scanned medium, and more particularly 
to a system for activating a laser for a scanning system comprising a 
rotating polyhedron mirror for controlling a scanning laser beam. 
Recently, improved recording devices, the so-called "laser printers" have 
been gaining popularity and have been meeting with commercial success. The 
major performance of "laser printers" is that visual data such as letters 
and pictures etc. derived from a computer, a word processor, and a 
facsimile device etc. are imparted to a scanned medium in the form of an 
electrostatic charge pattern. A laser beam functions as scanning light. 
An example of the "laser printers" is disclosed in Starkweather, U.S. Pat. 
No. 4,034,408 issued July 5, 1977, entitled "Flying Spot Scanner". 
The conventional laser printer requires a collimator lens, a beam expander 
lens and an f-.theta. characteristics imaging lens, (f: focal length 
.theta.: inclination angle) which are very costly thereby making the laser 
printer expensive and impractical. 
Conventionally, the f-.theta. characteristics imaging lens is required to 
compensate for distortion aberration which is due to speedy beam scanning 
at the ends of the scanned medium. The f-.theta. characteristics imaging 
lens provides strong barrel aberration. With the help of the f-.theta. 
characteristics imaging lens, the laser beam scans on the scanned medium 
at a constant speed. 
Thus, it is desired to develop laser printers at a practical cost. U.S. 
patent application Ser. No. 352,151 was filed by T. Tagawa et al on Feb. 
25, 1982, entitled "FLYING SPOT SCANNER FOR LASER PRINTER", now U.S. Pat. 
No. 4,435,733, to propose a laser printer free of the f-.theta. 
characteristics imaging lens. This application is now U.S. Pat. No. 
4,435,733, issued Mar. 6, 1984. 
The laser printer of the present invention can be combined with the laser 
printer as disclosed in U.S. Pat. No. 4,435,733. The disclosure of that 
patent is further incorporated herein by reference. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved laser printer. 
It is another object of the present invention to provide an improved system 
for activating a laser for a flying spot scanning system suitable for a 
laser printer. 
It is a further object of the present invention to provide an improved 
system for activating a laser with a combined sync signal so that a laser 
beam scans on a medium at a varying speeds. 
Briefly described, in accordance with the present invention, an apparatus 
for recording the information content of an electrical signal on the 
surface of a light sensitive medium by means of a plurality of scan traces 
across said medium comprises source means for providing a light beam of 
high intensity modulated in accordance with electrical information 
supplied thereto, actuate means for actuating the source means and 
generating sync signals, the periods of the sync signals being altered in 
accordance with variations in scanning speeds of the light beam to impinge 
on the medium, reflector means having a plurality of contiguous reflective 
faces rotatable about a central axis, rotation means for rotating said 
reflector means about said central axis, and lens means, disposed between 
said source means and said reflector means, for receiving and passing 
through said light beam.

DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, a flying spot scanning system of the present 
invention comprises a semiconductor laser diode 21, a focus (convergence) 
lens 22, a polyhedron mirror 24, a motor 25, a scanned medium 28, and a 
control unit 30. 
The laser diode 21 can be replaced in the present invention by a gas laser 
such as a He-Ne laser accompanied with an acousto-optical modulator for 
modulating a laser beam in conformance with video signal information. 
A preferred embodiment of the present invention is described below in terms 
of the semiconductor laser diode. 
The scanned medium 28 may be a xerographic drum which rotates consecutively 
through a charging station depicted by a corona discharge device. The 
laser beam from the rotating polyhedron mirror 24 traverses a scan width 
on the drum 28. Usable images are provided in that the information content 
of the scanning spot is represented by the modulated or variant intensity 
of laser beam respective to its position within the scan width. The 
scanned spot dissipates the electrostatic charge in accordance with its 
laser intensity. 
When the laser diode 21 is turned on and off by the control unit 30 to 
modulate the laser beam according to the video signal information to be 
recorded, the presence and absence of the laser beam on the scanned spot 
forms a pattern in conformance with the video signal information. 
The electrostatic charge pattern thus produced is developed in a developing 
station and then transferred to the final copy paper. In this manner, the 
information content of the scanned spot is recorded on a more permanent 
and useful medium. Of course, alternative prior art techniques may be 
employed to cooperate with a scanned spot in order to utilize the 
information contained therein. 
The polyhedron mirror 24 is continuously driven by the motor 25 and 
synchronized in rotation to a synchronization signal representative of the 
scan rate used to obtain the original video signal. The rotation rate of 
the xerographic drum 28 determines the spacing of the scan lines. It also 
may be preferable to synchronize the drum 28 in some manner to the signal 
source to maintain image linearity. The source image is reproduced in 
accordance with the signal and is transferred to printout paper for use or 
storage. 
Thus, the flying spot scanning system can be adapted for the so-called 
laser printer. 
The semiconductor laser diode 21 may be selected to be a laser diode having 
a circular laser emission portion within about 2-3 .mu.m. Such a laser 
diode can be a double heterojunction GaAs-GaAlAs diode (DH type) having an 
emission portion of about 3 .mu.m or a buried heterojunction diode (BH 
type). 
The beam emitted from the laser diode 21 focuses with the focus lens 22 to 
form an impinging light beam 23. The beam 23 is reflected by the 
polyhedron mirror 24 rapidly driven by the motor 25 in a direction denoted 
as a around a central axis. A reflected beam 26 is applied to the scanned 
medium 28 to form a scanning line 29. 
The focus lens 22 comprises a combination of a pair of concave-convex 
lenses in axial symmetry, preferably, spherical lenses. The laser emission 
surface of the laser diode 21 is placed outside a focus of the focus lens 
22 and the emission point of the laser 21 focuses on the scanning medium. 
It may be possible that a cylindrical lens having a cross section of no 
curve in its longitudinal side is disposed between the polyhedron mirror 
24 and the scanned medium 28. 
When the cylindrical lens is disposed, the laser beam orthogonal to the 
junction surface of the laser diode 21 is incident upon the longitudinal 
side of the cylindrical lens. The cylindrical lens comprises a 
plano-convex lens. The flat surface of the cylindrical lens faces the 
scattered medium 28. The cylindrical lens is preferably separated from the 
medium 28 by a distance of about 15 mm, similar to the focus length of the 
cylindrical lens, also preferably about 15 mm. 
The laser diode 21 emits the laser beam having a cross section of an 
ellipse where the longitudinal axis extends in a direction parallel to the 
junction surface of the diode. When such a laser beam passes through the 
axially symmetrical focus lens 22, the focused spot also forms an ellipse. 
FIG. 2 shows a block diagram of the control unit 30 for activating the 
laser diode 21. The control unit 30 comprises a controller 11 and an 
actuator 13. The controller 11 receives print data information developed 
from a computer, a word processor, and a facsimile device etc. The 
controller 11 develops laser actuating signals P representative of the 
print data information. The actuator 13 is responsive to the laser 
actuating signals P developed from the controller 11 for causing the laser 
diode 21 to be turned on and off. 
FIG. 3 shows a conventional relationship between print data sync signals, 
print data, and printed data in accordance with a print principle for a 
conventional laser printer devoid of any print-width correction element. 
Conventionally, sync signals have a constant period (1/fo). A laser diode 
emits and stops a laser beam in synchronization with a rising of one cycle 
of the sync signals. One cycle of sync signals enable the laser beam to 
write one print dot, so that a desirable pattern can be printed in 
conformance with print data. 
As FIG. 3 shows, the sync signals are assumed to have a frequency 1/fo and 
a constant period fo. A print width on a scanned medium on which the laser 
beam reflected by a polyhedron mirror can impinge is represented by the 
product of a velocity V, at which the laser beam scans on the medium 
surface by rotating the polyhedron mirror, and a time when the laser beam 
is emitted by the laser diode. When one cycle of the sync signals enable 
the laser beam to write one print dot as shown in FIG. 3, the width of one 
print dot is different between the center and the ends of the scanned 
medium. Since the velocity V at the scanned medium ends is more rapid than 
at the center, the dot width at the ends is longer than that at the 
center. 
According to the present invention, the frequency of the sync signals are 
varied in conformance with the variations in the velocity of the laser 
beam impinging on the scanned medium to correct the print dot width 
variations. 
FIG. 4 shows a relation between laser beam scanning speed V and print 
positions T on the scanned medium 28 according to the present invention. 
In FIG. 4, t.sub.1 indicates a print starting time on the medium 28 and 
t.sub.2 indicates a print termination time. The laser beam can impinge in 
the period t.sub.1 -t.sub.2 on the medium 28. The speed V is one of the 
laser beam impinging on a position of the medium 28. The velocity graph 
shows a symmetrical relation around the center t.sub.0 (t=0). 
The speed V at t.sub.1-t.sub.3 and t.sub.2 -t.sub.6 is faster than the 
speed at t.sub.0 -t.sub.4 and t.sub.0 -t.sub.5. That is, the following 
inequality stands: 
EQU v.sub.1 &gt;v.sub.2 &gt;v.sub.3 
where 
v.sub.1 : the velocity at t.sub.1 -t.sub.3 and t.sub.2 -t.sub.6 
v.sub.2 : the velocity at t.sub.3 -t.sub.4 and t.sub.5 -t.sub.6 
v.sub.3 : the velocity at t.sub.0 -t.sub.4 and t.sub.0 -t.sub.5 
According to the present invention, when the laser beam scans on the ends 
of the medium 28, the period of the sync signals is made narrower to 
correct print dot width variations. 
FIG. 5 shows a time chart of combined sync signals according to the present 
invention. The periods of the sync signals are selected to be as follows: 
A first sync signal of a frequency f.sub.1 having the shortest period is 
used for the velocity v.sub.1. A second sync signal of a frequency f.sub.2 
having a medium period is used for the velocity v.sub.2. A third sync 
signal of a frequency f.sub.3 having the longest period is used for the 
velocity v.sub.3. 
Since the values of the velocities v.sub.1, v.sub.2 and v.sub.3 are not 
constant within the respective position intervals, it may be necessary to 
calculate a mean value or a medium between a maximum and a minimum. When 
the laser beam velocity is selected to be the mean value or the medium, an 
approximately correct amendment can be attained. 
To keep the width of one print dot constant over all the scanned positions 
of the medium 28, the frequencies of the three sync signals must satisfy 
the following equation. 
##EQU1## 
In FIG. 5, N.sub.1, N.sub.2 and N.sub.3 represent the number of cycles of 
the sync signals used for the respective print intervals, namely, the 
number of the print dots. 
Sync signals having the three different periods are combined by the 
controller 11. The combined sync signals are developed from the actuator 
13 toward the laser diode 21. The width of one print dot is constant 
between the center and the ends of the medium 28. At the ends, the laser 
beam scans faster, but a laser emission time is made shorter with the help 
of the shortest frequency f.sub.1, thereby compressing the width of one 
print dot. 
FIG. 6 shows a relation between print data widths per 1 dot and the print 
positions at t.sub.1 -t.sub.2 on the scanned medium 28. In FIG. 6, a solid 
line shows a case using the signals as shown in FIG. 5 and a dotted line 
shows a conventional case as shown in FIG. 3. 
The graph of FIG. 6 indicates that the linearity of the print dot widths is 
improved by the present invention, to provide well-qualified visibility. 
Ripples in the print dot width in FIG. 6 can be neglected in practice. 
The number of the sync signals having the different frequencies to be 
combined can be freely selected within the knowledge of the present 
invention. 
FIG. 7 shows a block diagram of the controller 11 as shown in FIG. 2. The 
controller 11 comprises three flip-flops 31 to 33, an f.sub.1 oscillator 
34, an f.sub.2 oscillator 35, an f.sub.3 oscillator 36, an OR gate 37, a 
D/T flip-flop 38, and a counter 39. 
A rotation sync signal is generated in synchronization with the rotation of 
the motor 25 in a sequence of a single pulse for each single-line 
scanning, as shown in FIG. 8. The rotation sync signal is applied to a 
flip-flop 31 connected to the f.sub.1 oscillator 34. The rotation sync 
signal acts as an oscillation ON signal. An oscillation OFF signal is 
entered to the flip-flop 31 from the counter 39. The counter 39 controls 
counting the number of cycles. The counter 39 provides oscillation ON/OFF 
signals to be applied to the flip-flops 32 and 33. 
The flip-flop 31 is turned on in response to the rotation sync signal, so 
that the f.sub.1 oscillator 34 oscillates to generate clocks. The f.sub.2 
and f.sub.3 oscillators 35 and 36 are prevented from oscillating. The 
clocks generated by the f.sub.1 oscillator 34 are entered to the counter 
39 through the OR gate 37, so that the counter 39 counts the number of the 
clocks. 
When the counter 39 counts the number of the clocks up to N.sub.1, the 
counter 39 causes the f.sub.1 oscillator 34 to be inoperative and, 
simultaneously, the f.sub.2 oscillator 35 to be operative. Then, the 
f.sub.2 oscillator 35 oscillates to generate clocks as shown in FIG. 9. In 
this manner, the f.sub.1 to f.sub.3 oscillators 34 to 36 oscillate to 
generate the combined sync signal as shown in FIG. 5. 
The D/T flip-flop 38 receives the clocks generated by the flip-flops 34 to 
36 at a T input terminal. The flip-flop 38 receives the print data 
information at a D input terminal. The flip-flop 38 generates the laser 
actuating signals P. 
FIG. 10 shows a block diagram of a gate signal generator, in one of the 
oscillators 34 to 36, for generating a gate signal for the oscillator. The 
oscillator itself is permitted to oscillate and prevented from 
oscillating. 
FIG. 11 shows a time chart of signals occurring within the gate signal 
generator of FIG. 10. 
The oscillator oscillates when it delays a moment, for example, about 500 
nsec as compared to an oscillation start signal. Therefore, it may be 
necessary to make the f.sub.2 oscillator 35 operative prior to the stop of 
the f.sub.1 oscillator 34 by this moment in order to continue to generate 
the clocks. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications are 
intended to be included within the scope of the following claims.