Optical Engineering of CD/DVD Devices
Carolina Jacob and John Noé
Laser Teaching Center
Department of Physics & Astronomy
Stony Brook University
This project was motivated by my interest in CD/DVD players and recording
devices. Nearly everyone uses these, but few people understand or even
appreciate the incredibly sophisticated optical engineering that goes into
them. Information is stored in a sequence of short and long "pits," which
spiral outwards to form parallel tracks separated by only 1.6 micrometers
in a conventional CD. (The track spacing in a DVD is even smaller at 0.74
microns, and in a Blue-Ray disc it is only 0.32 microns.) The information
is recorded and read out by a laser beam focused to the smallest possible
size by a moveable lens. Precise and sophisticated opto-electrical
feedback mechanisms keep the laser spot centered on a single track and
maintain the optimal lens-to-surface distance. So this project basically
came about because the idea of having such sophisticated pieces of
engineering all together inside a CD/DVD player was very intriguing.
The goal of the project was to demonstrate and/or simulate some of the
optical "tricks" involved in these devices. We concentrated on the method
for controlling the focus of the lens, since this is so basic and so
clever. Before getting involved in this we created a setup which allowed
the track spacing of various CD's to be accurately measured by diffraction.
Measuring CD track spacings
Our setup allowed the CD being observed to be easily interchanged with
another one, and for the track spacing to be measured at different
distances from the center hole. We first attached a CD case to a stable
mount. We went on to placing a long piece of white cardboard directly in
front of that mount, and cut a tiny hole in the center of it through which
the laser beam would be pointed at the disc. By pointing the laser at the
disc, we obtained a diffraction pattern which allowed us to measure the
spacing between tracks. The first trial was done using a red helium-neon
laser with a wavelength of 0.632 microns. Using the measurements of the
first, second and third orders we were able to determine the wavelength of
the blue argon laser to be 0.489 microns. This result is very close to one
of the possible laser lines at 488 nm, so we can say for sure that the
light is 488 nm.
found that music and data CD's have the same track spacing and that this
spacing is quite uniform across the CD. We also used the measured spacing
of one CD to determine the wavelength of a blue-green argon-ion laser.
We measured the spacing for a data
CD, a music CD, and a DVD in this way.
Setup used to measure CD track spacings by diffraction. The laser is
behind the cardboard screen.
CD Player Optics
The drawing below shows how complex the CD player optical system really is.
Optics of a CD player. Drawing courtesy Laser Sam.
In the CD player collimated (parallel) light from a diode laser
passes through a beam splitter and then the movable objective lens, which
creates the tiny focal spot on the CD. Reflected light passes back through
the objective lens and then is diverted by the beam splitter through an
astigmatic lens and on to the face of a quadrant photodetector. The shape
of the spot on the photodetector changes as delta changes, and the
Results and Explanation
The optical engineering in a CD player is very precise and sophisticated,
yet these devices can be mass produced and sold relatively cheaply.
Future work will add
addtional optical elements such as a beam splitter to make our simulated
setup more closely resemble an actual CD player.