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AR-651/810 LASER & OPTICAL UNIT 8 - 7
Fig. 8.3-2 shows a block diagram of the semiconductive laser control circuit. The semiconductive laser
monitors the efficiency of the user drode and the laser itself is adjusted to initially output approx. 2.6mW
(395µW on the drum surface). The regulated voltage of the monitor output provides feedback to the laser
power comparison circuit. This monitor output voltage is compared with the laser power voltage set by the
control circuit for every scanning line. Laser driver circuit increases the forward current if the laser power is
insufficient and decreases the current when the power is excessive to maintain a constant optical output.
8. 3. Laser Diode
The laser used in this copier is a A1GaAs type semiconductive laser. It is generated in the single-horizontal
mode, and its wavelength is approx.780nm. This semiconductive laser outputs 15mW with standard DC
power supply. Laser intensity is controlled by using the output from the PIN diode for monitoring light output
in the semiconductive laser.
The relation between the current and optical output
of the semiconductive laser is as shown at right.
Light emission is started when the forward current
reaches or becomes greater than the threshold
current and outputs the monitor current which is pro-
portional to the optical output. The threshold current
and monitor current differs depending on the
semiconductive lasers due to their individual nature.
Optical output has to be adjusted to maintain a con-
stant value.
Since the optical output of the semiconductive la-
ser is decreased when the temperature of the laser
rises, APC control needs to be performed to maintain
a constant optical output.
Fig. 8.3-2 Block diagram
Threshold current
Optical Output (mW)
Low temp.
High temp.
Forward current
Monitor current
Current (mA)
Monitor output
Monitor effi-
ciency regula-
tion circuit
ciency regula-
tion circuit
Laser power
comparison
circuit
comparison
circuit
Laser driver
circuit
circuit
Semiconductive laser
Constant
optical output
optical output
Fig. 8.3-1
Regulation
Power source
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