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The most widely spread laser application is
unambiguously the CD player used for music playing or as high
density storage for computer data. So far, 650 million bytes can
be stored on a single CD-ROM and this is not yet the end of the
flagpole. The actual limitation of today is the wave-length of
the available diodelaser. The theoretical smallest spot size of
a focused Gaussian laser beam is directly related to its
wavelength. The smaller it is, the smaller this spot can be. The
data information is printed as so called pits onto the
reflective surface of the CD and can only be recognised if the
size of the pit is not smaller than the beam waist of the
focused laser beam. Since the beginning of the mass production
of CD-ROMs laserdiodes with a wavelength of 820 nm have been
used. Nowadays, laserdiodes with wavelength of 635 nm are
available, but recently some labs reported on the operation of
blue emitting laserdiodes. At this stage no one would change the
mass production when it can be foreseen that in a few years blue
emitting diodelaser will become available. These laserdiodes
emitting in the 400 nm regime allow the reduction of the pits to
half its actual size. This and the fact that the lateral
distance of the data tracks can be reduced, allow the
speculation for an increase in storage capacity of up to 10 Giga
bytes in a few years. However, the method of reading the
information will stay the same. The fine mechanical, optical and
electronically layout of the CD-ROM player or reader is a wonder
of engineering. Within this course the data detection, the
control loop for the reading laser head, the optical components
and more details will be demonstrated at an open frame CD player
with visual and electronical access to important sub assemblies.
Electrical signals will be visualised and interpreted by means
of an oscilloscope. The connection to a PC and with special
software, all operations of the CD player shall be performed
which are accessible via the regular software driver concept.
Besides the aspects of optics, laser technique and electronics
this work-shop also focuses on the interaction with PC hard and
software.
Principle of operation
The light emitted by
the laserdiode (LD) passes a diffraction grating and
subsequently the polarising beam splitter (BC). The orientation
of the laserdiode axis is chosen in such a way that almost the
entire light passes through the beam splitter. After leaving the
lens (L1), the beam is collimated and is focused down to app.
0.6 µm by the lens (L2) which has a short focal length of 3-4
mm. The focus of the laser beam hits the underside of the CD-ROM
where it is reflected back and the spot is imaged onto the
position sensitive detector (PSD). A quarter wave plate changes
the polarisation of the back travelling radiation in such a way
that it is reflected at the polarising beam splitter. In case
the beam does not hit the CD perpendicular the PSD notices a
change in the position of the imaged spot and a control loop
changes the orientation of the focusing lens. Once the laser
beam hits a pit, the back reflected light will be reduced. This
is in-terpreted as data information. The pit length or the time
duration within the light level decides whether the data is
interpreted as logically high or low. During the reading process
the control loop makes sure that the contrast of the read signal
is always maximised. This means the reading head follows the
data track on the CD. Full details of all aspects are given in
the comprehensive manual.
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