Laser Diode - Toshiba e-STUDIO453 Service Manual

8.3 Laser Diode

The laser used in this equipment is a AlGaAs type semiconductive laser. It is generated in the single-
horizontal mode, and its wavelength is approx. 785 nm. This semiconductive laser outputs 5mW 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 cur-
rent reaches or becomes larger than the threshold
current 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 la-
ser rises, APC control needs to be performed to
maintain a constant optical output.
Laser power
comparson
crcut
Fig. 8-302 shows a block diagram of the semiconductive laser control circuit. The semiconductive la-
ser 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. 2.11 mW (approx. 421 µW on the drum
surface). The regulated voltage of the monitor output is fed back to the laser power comparison cir-
cuit. 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.
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Power source
Laser drver
crcut
Constant
optcal output
Montor eff-
cency regula-
ton crcut
Fig. 8-302
8 - 7
Regulaton Hgh temp.
Low temp.
Forward current
Montor current
Current ( mA )
Threshold current
Fig. 8-301
Semconductve laser
Montor output
e-STUDIO350/352/353/450/452/453
LASER OPTICAL UNIT
8