Laser Diode - Toshiba e-STUDIO281c Service Manual

8.3

Laser Diode

The laser used in this equipment is a A1GaAs type semiconductive laser. It is generated in the single-
horizontal mode, and its wavelength is approx. 785 nm. This semiconductive laser outputs 10mW with
standard DC power supply. Laser intensity is controlled by using the output from the PIN diode for mon-
itoring 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 larger than the threshold cur-
rent and outputs the monitor current which is proportionate to the optical output. The threshold current
and monitor current differs depending on each semiconductive laser. So optical output has to be
adjusted to maintain a constant value.
Since the optical output of the semiconductive laser is decreased as the temperature of the laser rises,
APC control needs to be performed to maintain a constant optical output.
The following diagram shows a block diagram of the semiconductive laser control circuit. The semicon-
ductive laser performs monitor efficiency regulation (a process to control monitor current for the light
amount), and laser itself is adjusted to initial output emission of approx. 3.0 mW (approx. 408 µW on the
drum surface).
The regulated voltage of the monitor output is 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 insufficient, and
decreases the current when the power is excessive to maintain a constant optical output.
Laser power
comparison
circuit
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Regulation
Low temp. High temp.
Forward current
Monitor current
Current (mA)
Threshold current
Fig. 8-7
Power source
Laser driver
circuit
Constant
optical output
Monitor efficiency
regulation circuit
Monitor output
Fig. 8-8
8 - 7
Semiconductive laser
e-STUDIO281c/351c/451c
LASER OPTICAL UNIT
8