ABSTRACT for the 2009 Celebration of Research and Creative Activity

Optical Engineering of CD/DVD Devices

Carolina Jakob and John Noe, Laser Teaching Center, Department of Physics
and Astronomy, Stony Brook University

Nearly everybody uses CD/DVD players and recording devices, but few people
understand or even appreciate the incredibly sophisticated optical
engineering that goes into these. 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-CD distance.

The basic idea of this project is to demonstrate and/or simulate some of
the optical "tricks" involved in these devices. We started off by creating
a setup which allowed the track spacing of various CD's to be accurately
measured by diffraction of a HeNe laser beam (wavelength 632.8 nm).  We
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. Our
result, 489 nm, was very close to one of the possible laser lines at 488 nm.

Currently we are studying how CD devices create an electronic "error
signal" proportional to the difference between the actual and desired
lens-to-CD distance. The essential optical element is a lens with
intentional astigmatism, that is, a different focal length in two
orthogonal planes. The focal spot created by such a lens can vary from an
ellipse oriented one way to an ellipse along the perpendicular direction,
depending on the convergence or divergence of the light entering the lens;
midway between these two extremes the spot is a blurred circle. We have
created a simple setup to demonstrate this behavior and are using various
ray-optic techniques to model the simulation. Future work will add
addtional optical elements such as a beam splitter to make our simulated
setup more closely resemble an actual CD player.