Laser Diode - Toshiba e-STUDIO200L Service Manual

8.3

Laser Diode

The laser used in this equipment is an AlGaAs type semiconductive laser. It is generated in the single-
horizontal mode, and its wavelength is approx. 785 nm. This semiconductive laser has 5 mW of rated
DC power output and the laser intensity is controlled by using the output from the PIN diode for monitor-
ing light output in the semiconductive laser.
The relation between the current and optical out-
put of the semiconductive laser is as shown at
right.
Light emission is started when the forward current
reaches or becomes larger than the threshold
current, and then outputs the monitor current
which is proportionate to the optical output. The
threshold current and monitor current vary
depending on each semiconductive laser. There-
fore, 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 (Auto Power Control) needs to be per-
formed to maintain a constant optical output.
Laser power
comparison
circuit
Fig. 8-8 shows a block diagram of the semiconductive laser control circuit. The semiconductive laser
performs a monitor efficiency regulation (the process to control monitor current for the light amount),
and laser itself is adjusted to initial output emission of approx. 1.31 mW (approx. 261 µW on the drum
surface).
The regulated voltage of the monitor output is fed back to the laser power comparison circuit. This mon-
itor output voltage is compared with the laser power voltage set for the control circuit for every scan-
ning.
The laser driver circuit increases the forward current when the laser power is insufficient, and
decreases the current when the power is excessive to maintain a constant optical output.
e-STUDIO200L/202L/203L/230/232/233/280/282/283
LASER OPTICAL UNIT
Power source
Laser driver
circuit
Constant
optical output
Monitor efficiency
regulation circuit
Fig. 8-8
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8 - 6
Regulation High temp.
Low temp.
Forward current
Monitor current
Current ( mA )
Threshold current
Fig. 8-7
Semiconductive laser
Monitor output