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AR-651/810  LASER & OPTICAL UNIT  8 - 4

(2) Polygonal motor unit

This unit consists of the polygonal motor, polygonal

mirror and polygonal mirror cover.

a. Polygonal motor

This motor rotates the polygonal mirror at high

speed.

The DC motor controls the rotation speed of the

mirror polygon as follows:

During standby

:10,000.000 rpm (approx.)

During copying

:37,216.337 rpm (approx.)

b.

Polygonal mirror

Laser beams emitted from the laser diode are

reflected by this mirror. As the polygonal mirror

is rotated by the polygonal motor, the reflected

laser light moves in sync with the rotation. The

direction of the movement is the primary

scanning direction of the image. Two scans are

performed on one plane of the polygonal mirror.

As the polygonal mirror has eight planes, sixteen

scans are performed in one rotation of the

polygonal mirror.

c. Polygonal mirror cover

Polygonal mirror cover reduces the wind

and noise, prevents adhesion of foreign objects

onto the mirror surface and releases heat.

Laser light

Drum

Laser light

Drum

Laser light

Drum

(C)

(B)

(A)

2 scans are completed by completion of steps (A) to (C).
There are two lines of data are written on one plane of
the polygonal mirror. 16 lines of data can be written with
one rotation of the polygonal mirror.

AR-651/810  LASER & OPTICAL UNIT  8 - 5

(3) f

Ø lenses 1 and 2

These two lenses perform the following adjustment on the laser beams reflected by the polygonal mirror.

a. Uniform-velocity scanning

Since the polygonal mirror is rotating at a uni-

form velocity, the laser beam reflected from the

mirror scans over the drum surface at a uniform

angular velocity; namely, the pitch between the

dots on the drum is wider at both ends than at

the center of the scanning range. The f

Ø lenses

help to correct this difference, making all the

dot-to-dot pitches equal on the drum surface.

b.

Face tilt connection

The reflecting face of the polygonal mirror is tilted

slightly to one side against the perfect vertical.

Horizontal deviation of the laser light which is

caused by the tilt is corrected.

c. Sectional shape of laser beam

The shape of the laser beam spotted on the drum

is adjusted.

Wider

Narrower

Drum

Lens 1

Lens 2

Same intervals

Mirror plane is tilted.

(4) H-Sync detection PC board

The laser light which is started to be scanned from one of the reflected faces of the polygonal mirror is

reflected by the H-Sync detection mirror and goes into the PIN diode on the H-Sync detection PC

board. The primary scanning synchronizing signal is generated based on this reflected laser light tim-

ing.

The rudder sensor is attached to the H-Sync sensor to evenly space (42.3µm), by its detection value,

the intervals of the secondary scanning of the first and second beams.

Deviation

AR-651/810  LASER & OPTICAL UNIT  8 - 6

(5) Laser drive board

This control board has the following functions:

a. APC control function (adjusts disparity of the laser intensity caused by temperature)

b. Laser ON/OFF function

c. Function to generate the primary scanning synchronizing signals

(6) Slit glass

Slit glass is located where the laser beams are output from the laser optical unit, and it protects the unit

from dust.

AR-651/810  LASER & OPTICAL UNIT  8 - 7

Fig. 8.3-2 shows a block diagram of the semiconductive laser control circuit. The semiconductive laser

monitors the efficiency of the user drode and the laser itself is adjusted to initially output approx. 2.6mW

(395µW on the drum surface). The regulated voltage of the monitor output provides feedback to the laser

power comparison circuit. This monitor output voltage is compared with the laser power voltage set by the

control circuit for every scanning line. 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.

The laser used in this copier is a A1GaAs type semiconductive laser. It is generated in the single-horizontal

mode, and its wavelength is approx.780nm. This semiconductive laser outputs 15mW 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 greater than the threshold

current and outputs the monitor current which is pro-

portional to the optical output. The threshold current

and monitor current differs depending on the

semiconductive lasers due to their individual nature.

Optical output has to be adjusted to maintain a con-

stant value.

Since the optical output of the semiconductive la-

ser is decreased when the temperature of the laser

rises, APC control needs to be performed to maintain

a constant optical output.

Threshold current

Optical Output (mW)

Low temp.

High temp.

Forward current

Monitor current

Current (mA)

Monitor output

Monitor effi-
ciency regula-
tion circuit

Laser power
comparison
circuit

Laser driver
circuit

Semiconductive laser

Constant
optical output

Regulation

Power source