|ENGINEERING TRIPOS PART II A and EIST I|
|EIET LABORATORY||ENGINEERING AREA ACTIVITY|
Areas 5,6,7 and 8
|INVESTIGATION OF THE DESIGN OF CD PLAYERS|
The biggest difference between compact discs and earlier systems such as magnetic tapes and vinyl records is the density of information stored, and the fact that this is digital. Optical methods have proved to be by far the most practical to read this high density information without causing wear to the recording medium. and furthermore this would not be possible without semiconductor lasers.
Since the "dots" of optical information on the disc are extremely small, it is essential to have:
These requirements are in fact extremely demanding, and approach physical limits.
The aim of your investigation should be to understand what the component parts of the optical module are, how they work individually, and how the signal recovery, focus and tracking systems work.
The semiconductor laser is essential to CD players since it has an extremely small spot (of the order of lmm across) and high power output compared with LEDs - around 1mW. Lasers are sources of coherent light, but in this application the coherence is a nuisance, and active steps are taken to reduce it. Since lasers are both temperature sensitive, and subject to aging processes, the light from the laser has to be monitored and applied to a feedback loop.
Various systems have been employed to detect a properly focussed spot on the disc, most of these needing several separate optical components. In the present unit the laser, monitor, signal detection diodes and focus/tracking detection diodes are built into a single small optical package; this also has a hologram affixed to its window. This system is particularly simple and elegant, and well worth understanding.
Although the tolerances are very small, the optical system is not particularly complicated, and much of its operation can be understood using normal laboratory tools such as meters and oscilloscopes -as well as visual examination.
The main sources of useful information are likely to be:
* Available to you when you start doing the activity.
A Laser Module from the Philips CD930 compact disc player has been mounted on a simple stand. It allows you to connect power supplies and meters to the laser and detector diode assembly. You can place a disc on the module and make small movements by hand. A separate detector diode is also included, for investigating the laser output.
A specially modified disc is provided which can be used to investigate the emitted optical light, rotation speed, focus condition etc.
Note, the tracking system cannot easily be investigated on the bench since it requires proper operation of the automatic focus as well as considerable feedback control circuitry to operate correctly.
SAFETY: The light levels emerging from the objective lens of the optical head are within safety limits, but as a safety precaution never stare directly at the emitting spot, especially with any lens. The spot is nominally in the infra-red, but is in fact just visible due to its high intensity.
3. SUGGESTED APPROACH
Read one of the general books on CD players to gain a general feel for the optics involved. If you have not met semiconductor lasers before, look these up as well. You may also find the handout for Experiment 3B6A helpful.
Then examine the optical module in detail, and read the documents supplied concerned with this.
With the equipment provided it is possible to characterise:
All the component of the optical module are highly developed for their individual performance, as well as how they work together; they have also been designed for extremely low cost manufacture and assembly. Try to understand why things have been done in the way they have, and the balance between cost/complexity and precision. In particular try to evaluate the advantages and disadvantages of the system under investigation compared with other systems described in books (or else where).
What are the most important things you have discovered ?
What is you audience likely to be interested in and able to understand in a short time?
You will not have time to describe all you have found, so you must select what you regard as the most important items. It will also be worth spending some time deciding what the best method of presenting you data will be.
Dr R G S Plumb September 1994, Dr P.R. Palmer, October 2002