Philips CDR880 - Laser Control and Optical Power Calibration

BASICS CDR/RW
13
MASTEF_PROCESSOR
CDR_DRIVE ia
DISCMOTOR
Figure 16
CDR drive blockdiagram
In the CDM mechanism the laser sends its output light to the
disc through the lightpath. A part of the source beam is catched
by the Forward Sense photodiode, placed just behind the gra-
ting. The resulting electrical FS photodiode signal is used to
control the laser output power by means of a laser control circuit.
The beam reflected by the disc is led to a photodiode system,
where signals are generated on behalf of servo control and
data extraction.
The CDM board contains the servo error extracting AEGER IC
(Analogue Error signal Generator for Erasable and Recordable
CD), the discrete laser control circuit and other functions.
The AEGER provides error signals for focus and radial servo
control, a push-pull signal for disc motor control and a central
aperture signal for disc motor contro! and data extraction.
The Main Board comprises most of the servo functions.
The DSICR IC incorporates the focus and radial servo transfer
functions. The DSICR output signals are power amplified by
the DSD driver IC and fed to the focus and radial actuator, as
well as the sledge motor on the CDM mechanism.
3.2.1.1 The CDM mechanism
The CDM mechanism consists of the Light Device Unit and a
2D actuator. Both units are mounted on a slide which is
driven by the slide motor.
Further on there is the Brushless Hall turntablemotor to spin
the CD-disc.
The radial tracking system is a 2-stage servo with a 3 spot
lightpath. Let us first compare the conventional lightpath with
the CD-R lightpath.
The 3 spot lightpath of the conventional CD system consists
of a laserdiode with the feedback information of the monitor-
diode, grating, plane parallel plate, collimator lens, objective
lens and photodiode. See figure 17.
disc disc
objactive_lens objective_lens
quarter_wave
plate
collimator_lens collimator_lens
laser
grating
cylindrical_lens
plane-parallel
plate
beamsplitter
Photodiode
grating
condenser_lens [ forward_sense
diode
photodiode laser
Figure 17 Figure 18
3 spot CD lightpath 3 spot CD-R lightpath
The 3 spot lightpath of the CD-R lightpath consists of the
following components:
- laserdiode, condenser lens, forward sense diode, grating,
polarizing beamsplitter, collimator lens, quarter wave plate,
objective lens, cylindrical lens, photodiode. See figure 18.
In the following a short description of this lightpath:
The condenser lens couples all laserlight to get a better result
of the output of the laserdiode. The internal monitor diode of
the laser is not in use in this path. Instead of that the forward
sense diode has been used to measure the intensity of the
output of the laser. Via the grating the lightbeam has been
divided into 3 spots. The polarizing beamsplitter conducts all
laserlight through the splitter. The collimator lens now forms
parallel beams of the 3 spots. In the quarter wave plate the
polarizing direction rotates for 45°. The moveable objective
lens has been focused on the reflecting layer on the disc.
The reflected beams return their way back to the quarter
wave plate where again 45° rotation has been made.
Now the total polarization rotation comes to 90°. This means
in the polarizing beamsplitter that all reflected lightbeams are
diffracted to the cylindrical lens. The effect of this lens consists
in the astigmatism of the spot on the photodiode. On figure 19
this effect has been demonstrated.
FOCUSING
focusing error
(A+C)-(B+D)
is_positive
DISC_TOO_CLOSE
ae focusing_erior_is_0
DISC_IN_FOCUS
focusing_enor
(A+C)-(B+D)
is_negative
DISC_TOO_FAR_AWAY
Figure 19
Focusing
Resuming: the differences between the 3 spot CD lightpath
and the 3 spot CD-R lightpath are the polarizing components
(quarter wave plate together with the beamsplitter) and the
forward sense diode.
3.2.1.2 The CDM requirements
The CDM requirements are:
- Good spot quality
- Accurate track following
- Eccentricity < 100um
- Low shocksensitivity
- Inner position: r = 22 mm
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