Laser diode drive system for multione image forming apparatus

A laser diode drive system suited to a multitone image forming apparatus. For stopping laser emission from a laser diode the laser diode drive system superimposes a signal having a level corresponding to a maximum level of a video signal applied to the laser diode on a detection signal corresponding to emission intensity of the laser diode. A signal formed by the superimposition is used for negative feedback to a laser diode drive circuit.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a system for driving a laser diode used as 
a light source in a laser beam printer or the like, and more particularly 
to a system for stopping emission from the laser diode in action. 
(2) Description of the Prior Art 
With a laser beam printer, it is necessary to stop laser emission when the 
printer is switched on and a video signal is input, for safety's sake and 
to avoid unnecessary laser emission outside an image forming time. 
For this purpose, a known laser beam printer employs an emission stopping 
circuit system as shown in FIG. 1. This system comprises a laser diode 31, 
a differential amplifier 32, and a DC supply circuit 33. The differential 
amplifier 32 and DC supply circuit 33 constitute a laser diode drive 
circuitry 34. The DC supply circuit 33 supplies direct current above the 
threshold level to the laser diode 31. 
The differential amplifier 32 receives a video signal at one input terminal 
32a and a feedback signal at the other input terminal 32b. The video 
signal is applied to the input terminal 32a of the differential amplifier 
32 after undergoing level adjustment through a DC bias voltage circuit 35 
for assuring that the laser diode 31 is used only in a laser emission 
range (above "Do" in FIG. 4). The feedback signal is produced by detecting 
emission from the laser diode 31 with a PIN photodiode 36 and amplifying 
the resulting signal with an output monitor circuit 37 comprising a video 
buffer amplifier. The output monitor circuit 37 amplifies the feedback 
signal to a level corresponding to one to one relationship with the video 
signal. Consequently, the feedback signal applied to the input terminal 
32b of the differential amplifier 32 causes the laser diode 31 to increase 
and decrease laser emission following the level of the video signal. 
The circuit system further comprises an ON/OFF circuit 38 including a 
photocoupler 39 and an output transistor 40 for stopping the laser 
emission. The photocoupler 39 has a light emitting diode 39a for receiving 
a signal from a CPU, not shown, for controlling the emission from the 
laser diode 31. 
When forward current is applied to the light emitting diode 39a, the diode 
39a emits light whereby a phototransistor 39b becomes conductive. With the 
operation of phototransistor 39b, the base voltage of the transistor 40 
drops to zero volts and the transistor 40 becomes nonconductive, whereby 
the video signal is applied to the input terminal 32a of the differential 
amplifier 32 in a normal way. When forward current is not applied to the 
light emitting diode 39a, the diode 39a stops emitting light whereby the 
phototransistor 39b becomes nonconductive. As a result, the transistor 40 
becomes conductive and the input terminal of the differential amplifier 32 
is applied to ground through the collector and emitter of transistor 40, 
whereby the laser diode 31 stopes the emission. 
With the above known systems, it is difficult to cause the laser diode 31 
to stop the laser emission completely since the laser diode 31 is 
controlled through the output transistor 40 of the ON/OFF circuit 38. More 
particularly, even when the output transistor 40 is conductive, its 
internal resistance maintains the voltage between the collector and the 
emitter at several millivolts instead of allowing the voltage to fall to 
zero. Since the collector of the output transistor 40 is connected to the 
input terminal 32a of the differential amplifier 32 through a point P2, 
the several millivolts are applied to the differential amplifier 32 as a 
leakage input. As a result, an output from the differential amplifier 32 
is applied to the laser diode 31 even when the video signal is zero. This 
leakage input causes the laser diode 31 to emit light since the diode 31 
is operable by the DC supply circuit 33 even with a slight voltage applied 
thereto through the differential amplifier 32. The amount of emission in 
this instance is dependent on the level of leakage input, namely the 
performance and resistance of the output transistor 40. The amount of 
emission is also influenced by the maximum input level of the video signal 
controlled by the DC bias voltage circuit 35. That is, while the video 
signal level at a point P1 varies from 0V to 2V by reason of the system 
construction, the maximum level at point P2 may be reduced to half of the 
level at point P1 or less in order to secure responsivity of the laser 
diode drive circuitry 34. In such a case, the level of the leakage input 
applied through the output transistor 40 rises relative to the video 
signal level at point P2, thereby increasing the amount of leakage 
emission. 
When the above laser diode drive system is applied to a printer, the 
leakage emission does not always give rise to a problem. This is true 
where, for example, a sensitive material irradiated by a laser beam to 
form an image thereon has low sensitivity, or where the sensitive material 
has high sensitivity but is exposed outside an image forming area to the 
leakage emission. With the laser beam printer, however, scanning often is 
repeated numerous times along one line on the sensitive material; the 
first scan for recording an image and subsequent scans without the laser 
emission. If leakage emission takes place during the second and subsequent 
scans, the image area of the sensitive material becomes exposed thereby 
damaging image quality. 
SUMMARY OF THE INVENTION 
A primary object of the present invention, therefore, is to provide a laser 
diode drive system capable of stopping laser emission without causing 
leakage emission. 
Another object of the invention is for use in provide a laser diode drive 
system suitable to a laser beam printer. 
These objects are fulfilled, according to the present invention, by a laser 
diode drive system for use in an image-forming apparatus for forming a 
multitone image on a sensitive material by means of a laser beam emitted 
from a laser diode, comprising detecting means for detecting intensity of 
the laser beam; drive means for driving the laser diode in response to a 
multitone analog image signal corresponding to a multitone image to be 
formed, and modulating the intensity of the laser beam, the drive means 
including comparing means for comparing the intensity detected by the 
detecting means with the multitone analog image signal and supplying the 
laser diode with a current having a level corresponding to a comparison 
result; stopping signal generating means for generating a stopping signal 
having a level exceeding a maximum level of the multitone analog image 
signal; and stopping means for stopping the emission from the laser diode 
by effecting negative feedback of the stopping signal to the comparing 
means. 
The detecting means may include photoelectric converting means for 
outputting a detection signal corresponding to the intensity of the laser 
beam, the detection signal being negatively fed back to the comparing 
means. 
The stopping means may be operable to superimpose the stopping signal on 
the detection signal provided by the photoelectric converting means for 
negative feedback to the comparing means. 
Further, the stopping means may include an amplifying circuit for 
amplifying a feedback signal formed by superimposing the stopping signal 
on the detection signal output from the photoelectric converting means to 
a level corresponding to the level of the multitone analog image signal. 
Preferably, the stopping signal generating means includes a circuit having 
a transistor and a level adjusting resistor connected in series, the 
transistor being driven to become conductive for causing the laser diode 
to stop the laser emission. 
The laser diode drive system according to the present invention may further 
comprise command means for outputting a laser beam emission stop command. 
Then the stopping means is responsive to the laser beam emission stop 
command for superimposing the stopping signal on the detection signal to 
stop the emission from the laser diode. 
According to the present invention, the emission from the laser diode is 
constantly monitored by the detecting means when the emission stopping 
signal is not generated. Negative feedback of the detecting means output 
to the comparing means is effected to adjust the amount of laser emission. 
Consequently, the laser diode varies the amount of emission by following 
the input signal level. 
On the other hand, when the emission stopping signal is generated during 
the light emission by the laser diode and the signal is superimposed on 
the monitor output for negative feedback, the feedback portion of the 
drive system causes the comparing means to stop its output, since the 
emission stopping signal has a level exceeding a maximum level of the 
input signal. As a result, the laser diode stops the laser emission. The 
emission stoppage is effected reliably without leakage emission as 
experienced in the prior art, as long as the feedback is carried out in a 
normal way.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 shows an example of laser diode drive system embodying the present 
invention. This system comprises a laser diode 1, a differential amplifier 
2, and a DC supply circuit 3. The differential amplifier 2 and DC supply 
circuit 3 constitute a laser diode drive circuitry 4. The system further 
comprises a DC bias voltage circuit 5 including two resistors R1 and R2, a 
light receiving element 6 comprising a PIN photodiode, and an output 
monitor circuit 7. The DC supply circuit 3 supplies direct current above 
the threshold level to the laser diode 1. 
The laser diode 1 takes a spontaneous emission state when the current 
applied to the laser diode 1 is below the threshold level, and a driven 
emission state when the current exceeds the threshold level. Consequently, 
the relationship between the current applied to the laser diode 1 and 
light intensity is non-linear as shown in FIG. 4. In order to effect 
high-speed analog modulation for the laser diode 1 having such a 
non-linear characteristic while maintaining linearity between the input 
signal and light intensity, it is necessary to constantly apply a current 
exceeding the threshold level Th to the laser diode 1. The DC supply 
circuit 3 is provided to satisfy this requirement. 
The differential amplifier 2 receives a video signal at one input terminal 
2a and a feedback signal at the other input terminal 2b. The video signal 
comprises a multi-level analog signal, and is applied to the input 
terminal 2a of the differential amplifier 2 after undergoing level 
adjustment through the DC bias voltage circuit 5 for assuring that the 
laser diode 1 is used only in a laser emission range. The feedback signal 
is produced by detecting emission from the laser diode 1 with the PIN 
photodiode 6 and amplifying the resulting signal with the output monitor 
circuit 7 comprising a video buffer amplifier. The output monitor circuit 
7 amplifies the feedback signal to a level corresponding to a one to one 
relationship with the video signal. Consequently, the feedback signal 
applied to the input terminal 2b of the differential amplifier 2 causes 
the laser diode 1 to modulate the intensity of the laser beam in response 
to the level of the video signal (analog signal), thereby forming a 
multitone image on a sensitive material. 
The circuit system further includes an emission stop signal generating 
circuit 8 which, for example, comprises a photocoupler as shown in detail 
in FIG. 3. The photocoupler 8 has a light emitting diode 9 connected to an 
output port of a CPU not shown. The photocoupler 8 also includes a 
phototransistor 10 having a collector connected to a DC source through a 
level adjusting resistor R5 and an emitter connected to an input end of 
the video buffer amplifier or output monitor circuit 7. 
For stopping the laser emission, the unillustrated CPU causes forward 
current to flow to the light emitting diode 9 for making the 
phototransistor 10 conductive. Then the resistor R5 is connected in series 
to a resistor R4, and the voltage resulting from the current flowing 
through the resistor R5 is superimposed on the voltage generated across 
the resistor R4 (see FIG. 5a) by the current flowing through the PIN 
photodiode 6 when the phototransistor 10 is nonconductive. This voltage to 
be superimposed is adjustable by selecting the resistance of the resistor 
R5. In the present invention, the resistance of resistor R5 is selected so 
that the voltage to be superimposed (emission stopping signal) is above a 
maximum level of the video signal input to the laser diode LD drive 
circuitry 4 (see FIG. 5b). Consequently, when the phototransistor 10 
becomes conductive, the laser diode drive circuitry 4 provides zero output 
whereby the laser emission is positively stopped. 
If, in FIG. 3, the photocoupler 8 comprises a high speed type and clock 
pulses of the printer are applied to the light emitting diode 9, a laser 
drive mode is realized wherein the laser emission is stopped with pixel 
clock cycles of the printer. This is the laser drive mode disclosed in 
U.S. patent application Ser. No., 38,367 filed earlier in the name of 
present Applicant, which is effective as a practical method of checking 
image quality deterioration due to mode hopping noise peculiar to the 
laser diode. It is also possible, by adjusting the resistance of resistor 
R5, to reduce the emission level within a range not impairing the effect 
with respect to the mode hopping noise without stopping the laser 
emission. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.