Optical Engineering of CD/DVD Devices

Introduction

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.

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. 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 detoctor

Our Simulation

Setup used to simulate range-finding in the CD player. The light source is a single-mode optical fiber.

Results and Explanation

Images formed as the position of lens-1 is changed from 7 to 13 cm in one cm steps.

These diagrams show how the different shapes of beam spots are created.

Conclusion

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

Future work will add addtional optical elements such as a beam splitter to make our simulated setup more closely resemble an actual CD player.