Toshiba e-studio550 Service Manual page 120

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
propostioned 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
constant 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 shows a block diagram of the semiconductive laser control circuit. The semiconductive laser
performs monitor efficiency regulation (a process to control monitor current for the light amount), and laser
itself is adjusted to initially output emission of approx. 2.6mW (395µW on the drum surface). The regulated
voltage of the monitor output is given feedback to the laser power comparison circuit. This monitor output
voltage is compared with the laser power voltage set for the control circuit for every scanning. Laser driver
circuit increases the forward current if the laser power is insufficent and drecreases the current when the
power is excessive to maintain a constant optical output.
JUNE 2002 © TOSHIBA TEC
Laser power
comparison
circuit
Fig. 8.3-2 Block diagram
Power source
Laser driver
circuit
Constant
optical output
Monitor effi-
ciency regula-
tion circuit
8 - 7 e-STUDIO550/650/810 LASER OPTICAL UNIT
High temp.
Regulation
Low temp.
Forward current
Monitor current
Current (mA)
Threshold current
Fig. 8.3-1
Semiconductive laser
Monitor output