Evaluation of partial-response maximum-likelihood detection for phase-change optical data storage

We describe the application of partial-response (PR) maximum-likelihood (ML0 detection in rewritable phase-change optical data storage. The input to this detector, which is simulated in software, is the actual signal (without any equalization), reproduced from reading of the recorded sequence on an optical disk. The detection algorithm involves the extraction of the impulse response from the readout signal, PR equalization, the adjustment of gain and recovery of clock, ML sequence estimation with the Viterbi algorithm, and analysis of PRML performance. With a laser wavelength of 0.69 mm and an objective lens with a numerical aperture of 0.6, three linear densities are examined: 0.35 and 0.31 mmybit without modulation code and 0.2 μm/bit with the (1, 70 modulation code. The equalized signal exhibits good eye patterns, especially at the densities of 0.35 and 0.31 mmybit. Analyses of noise and bit-error rate indicate that jitter, rather than noise, is the main obstacle to realizing ultrahigh density in phase-change media with PRML detection. We also briefly discuss the problem of the inherent nonlinear effect in phase-change readout.