Toshiba 4511, 3511 manual Laser power comparison circuit Laser driver Circuit Constant

Semiconductive laser

Monitor output

Power source

Fig. 8-301

Threshold current

Current (mA)

Forward current Monitor current

Low temp.

Regulation

Optical Output (mW)

High temp.

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 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 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 laser rises,

APC control needs to be performed to maintain a constant optical output.

8.3 Laser Diode

8

Laser power comparison circuit

Laser driver
circuit	Constant
	optical output
Monitor efficiency
regulation circuit
Fig. 8-302

Fig. 8-302 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 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.

November 2003 © TOSHIBA TEC 8 - 7 e-STUDIO3511/4511 LASER OPTICAL UNIT