Abstract

Video discs and compact discs are examples of modern optical storage techniques that use lasers to read from permanently recorded storage media, capable of storing large quantities (gigabytes) of digitised information. Current research is involved in the development of archival media ('Write-Once-Read-Many' times systems) and erasable optical media. Here locallised laser heating is used to create small pits (for WORM systems) or a locallised reversible change in the medium (in erasable systems) to form the basic 'bit' of information. The aim is to determine the optimum combination of materials for this process to produce the highest data density, minimum error rate, and maximum read and write rates. This will be achieved by accurately controlling pit size, increasing the sensitivity of the film to pit formation and providing a sharp threshold response to the laser.

Due to the extreme difficulties in observing the process of pit formation, which involves a micron-sized laser beam creating temperatures of over 1000°C in a time of less than 100 nsecs, finite element methods are required to model the process (a) to try and detect the process of pit formation (b) to optimise the combination of materials used. Currently, practical systems use either a tellurium alloy or an organic dye on a polycarbonate or PMMA substrate. Various complex layer structures have been tried to maximise energy absorption in the storage medium.

A finite element model will be described which has been used to solve the transient thermal conduction equation, with a time dependent heat source, to accurately model the temperature variation in a number of different storage media. The resultant temperatures are then used to determine stress distributions in the storage medium. The variation of temperature and stress is required to determine the mechanism of pit formation in the medium. It will be shown how a better understanding of the processes involved can lead to a significant change in the materials used by a close examination of the most important physical parameters.

© 1987 Optical Society of America