Player; Opticalpick-Up - Sony LDP-1000 Manual

2-3. PLAYER,
Disc player is composed of an optical system to readout pits of
the disc, rotation mechanisms to rotate the disc and move the opti-
cal system, a video demodulator circuit, a sound demodulator cir-
cuit, servo circuits to con troll electromagnetic actuator of the
optical pickup, the disc rotation motor, and a control circuit for the
overall operation in accordance with operation commands.
The optical system and its servo system which are not found in
the conventional VTRs will be described here.
2-3-1. Optical Pick-up
Optical system of the videodisc player is the most important
section to readout information from the disc. The optical system is
the heart of the player, nucleus of the optical videodisc system.
The characteristics of light and its polarization will be described
here simply to understand the principle of the optical system. It is
considered that light is a kind of the electromagnetic waves and its
wavelength is visible to the human eye. The ordinary light includes
various lights having different wavelengthes and phases. It is conti-
nually changing its direction of polarization at random.
Since laser (Light Amplification by Stimulated Emission of Radia-
tion) is composed of extremely narrow bandwidth of wavelength,
aligned phases, and having a signal· color. The laser goes straight, it
can be converged into an extremely fine beam.
Light whose direction of polarization changes at random like the
ordinary light is called non-polarized light. The light shown in (a)
and (b) of Fig. 2-9 are called linearly polarized lights; (a) is called
vertically polarized light while (b) is horizontally polarized light.
When the lightwave rotates in only one direction, clockwise or
counterclockwise, itis called circularly polarized light.
PerJ:taps the most well known application of this phenomenon is
in polarized sunglass. The special lenses will not pass the horizon-
tally polarized light. Since most light from glare and reflections is
horizontally polarized, it can ""not pass through the lenses. The
videodisc player also uses optical system which are affected by the
polarity of light. '
The laser beam travels the optical path shown in Fig. 2-10 on the
optical sled. Fixed angle mirror merely changes the beam angle so
that player can be housed in a ·compact space. The reflection index
of the mirr.or is 99% to make the light loss minimum.
-The red light beam from the laser is vertically polarized. It is
diffracted by a grating that is the first optical component. This
optical device is a piece of glass with several fine horizontal lines
etched in it. Its effect is to divide the beam into three beams. The
three beams are the center bright beam, a secondary beam above
and below the main beam. The secondary beams are less bright than
the main beam. Actually, the raster grate creates more than three
beams, but the others are of such reduced brightness that they are
ignored. The center beam is used for detecting the FM signal on the
disc and a focus error detection. The two adjacent beams are used
for detecting a radial tracking error. The three beams are collective-.
ly referred to as a light bundle. For purposes of discussion, the light
bundle is treated as a signal beam.
13
i
The beam reaches a spot lens next. This lens focuses the beam
to the correct size so that the beam spot completely fills the aper-
ture of the objective lens.
The beam after the spot lens reaches a PBS (Polarizing Beam
Splitter) and a ¼ wavelength retardation plate. These two optical
components are used to separate the projected beam from the re-
flected beams. The vertically polarized beam can pass through the
PBS but the horizontally polarized beam is bent 90° as· passing
thtoi:tgh the PBS.
Since the laser beam is vertically polarized, it can pass through
the PBS and reaches the ¼;>,. retardation plate. This¼;,,. retardation
plate is also called as polarHy converter. This component provides a
¼ wavelength, i.e., a 90° phase difference between the electric field
wave and the magnetic field wave so that the vertically polarized
beam is converted to_circularly polarized beam (in counterclockwise
direction) while passing through the PBS.
This circularly polarized beam is reflected by the radial tracking
mirror and a . . fixed angle mirror and is sent to the objective lens.
The tracking mirror is servo-controlled so that the reflected beam is
kept correct tracking on the track of the videodisc. The objective
lens is similar to a microscope and is servo-controlled so that the
beam is kept focused info _a tiny spot on the surface of the videodisc
pits.
The theory · of the servo operation will be detailed later in the
description on the servo system.
The beam is reflected back from the surface of the disc and be-
comes luminance modulated by the. pits. (The light reflected from
the pit area is less bright than what is reflected from other area than
pit area.) The depth of the pit is equal to ¼ the wavelength (90° in
terms of phase) of the He-Ne laser as described before. The reflect-
ed light from pits is delayed ½ wavelength because ¼ of a wave-
length is delayed while going into the pit and another ¼ wavelength
is delayed while reflection. The intensity of reflected light from the
pit area is weakened by the interfere from the reflected light other
than pit area.
The reflected light takes the same path as the projected light all
the way back to the ¼ retardation pla.te. The reflected light is a
counterclockwise circularly polarized light and its phase is shifted
90° when passing through the ¼ retardation plate so that the beam
is changed to a horizontally polarized light. Fig. 2-11 illustrates the
polarity converter using a½ retardation plate.
(a)
Fig. 2-9. Polarization of light