December 2002 TOSHIBA TEC 8 - 5 e-STUDIO160/200/250 PRINTING
8.3 Laser Diode
The laser used in this copier is a GaAIAS type semiconductor laser. This semiconductor laser outputs in
the single-horizontal mode, and its oscillation wavelength is about 780 nm. This semiconductor laser is
powered by a + 5 V DC power supply, and its optical output is controlled to about 1mW. Output of a PIN
diode for monitoring light output built into the semiconductor laser is used for controlling this.
The semiconductor laser has the optical forward-
output current, and monitors the current character-
istics shown in the figure on the right.
Emission of light is started when the forward cur-
rent reaches or becomes greater than the thresh-
old current. A monitor current proportional to the
optical output is output. These threshold currents
and monitor currents are each disparate due to the
characteristics of each of the semiconductor lasers.
Optical output must be adjusted so that it is main-
tained at a constant value. Also, as the optical out-
put of the semiconductor laser drops when its tem-
perature increases, APC is also required to main-
tain the optical output at a constant value.
Fig. 8-3-1
Fig. 8-3-2 Block diagram
Fig. 8-3-2 shows a block diagram of the semiconductor laser control circuit. The monitor output from the
semiconductor laser is controlled for each of the semiconductor lasers beforehand so that a constant
voltage is maintained when the optical output is 1 mW. After conversion, the voltages are fed back to the
laser power comparison circuit. Here, comparison of the laser power voltage set to the control circuit is
carried out for every single scanning. As a result, when the laser power is insufficient, the forward current
is increased in the laser driver circuit, and alternatively when the laser power is excessive, the forward
current is decreased so that the optical current is constant.
Threshold current
Optical Output (mW)
Low temp.
High temp.
Forward current
Monitor current
Current (mA)
Monitor output
Monitor
output
conversion
Laser power
comparison
circuit
Laser driver
circuit
Semiconductor laser
Constant optical
output
08-03-01
08-03-02
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