Abstract

We report the results of an experiment in which code words consisting of 0.1 μm lines are recorded on an optical storage medium and retrieved by using 633 nm wavelength illumination focused to a spot by a lens with numerical aperture equal to 0.7. The space-bandwidth product (the number of resolution elements) of this system is equal to unity. The information capacity of the system is determined by the space-bandwidth product and by the noise in the system. Using coding, we can increase the storage density up to the information capacity of the system. Sixty-three distinct code words are recorded in the memory, and the diffraction pattern for each code word is measured to form a template to classify future code words. To read from the memory, the diffraction pattern is measured repeatedly as the disk moves by 0.1 μm intervals. Since the illuminating spot is about 1 μm, each recorded bit is classified 10 times. The final assignment for each bit is by done by the majority rule or by Viterbi decoding. In our first experiment 90% of a test set of 560 bits were decoded correctly by using majority rule. We can use the measured error rate to obtain a lower bound of 2.5 bits per micrometer for the capacity.

© 1992 Optical Society of America