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We have examined two unique data error causes for domain wall displacement detection (DWDD) media, dropout and bit shift, using 8-9 modulation and (1, 7)-RLL modulation. We have determined that dropout depends only on the mark length of recorded data, while the deviation of edge shifts, which causes bit shifts, maintains approximately the same value regardless of mark length. Using these characteristics of error causes, we have also shown a prospect of achieving several-tens-nm/bit recordings.

We detected bits sized at 0.237 µm bit length and 0.85 µm track pitch recorded by magnetic field modulation on a magneto-optical disk with a 690-nm laser diode and 0.60-numerical-aperture-objective using a partial-response maximum-likelihood detection method. We confirmed that the optimum partial-response for this high linear density had four coefficients of 1 by analyzing difference metrics of error critical paths. The recorded bits with areal density of 5.6 bit/µm2were also detected at a low error rate, which combination is 0.237-µm bit length and 0.75-µm track pitch. Its write power tolerance is limited to ±22%.

We realized magnetically induced super resolution (MSR) using magneto-optical (MO) disks with a thermally formed aperture. The aperture size and position affect the spatial frequency response of the readout signal. Using the principle of superresolution in laser scanning microscopy, we analyze the optical transfer function (OTF) for various apertures. Applying the analysis to MSR readout, OTF of MSR is calculated using scalar diffraction theory. The calculated results agree well with experimental values.

Premastered optical disk by superresolution (PSR) using rear aperture detection (RAD) has been realized. The PSR disk has a chalcogenide material. Using PSR by RAD with a current CD system, we demonstrated the feasibility of increasing both the linear density and the track density of read only memory (ROM) optical disks. By superresolution in the linear direction, a high C/Nof 47 dB was obtained for a pit length of 0.3 µm which is beyond the optical limit. Furthermore, by superresolution in the track direction, a low crosstalk of -38 dB and a wider margin of radial skew angle compared to conventional detection were obtained for a track pitch of 1.1 µm.

Premastered optical disk by superresolution (PSR) for practical use has been achieved for the first time. The PSR disk has a phase-change material Ge2Sb2Te5with stoichiometric composition. Superresolution results from a high contrast ratio of reflectance in the light spot between highly reflective crystal region and lowly reflective melted region. A high C/Nratio of 56 dB is obtained at a pit length of 0.4 µm using PSR. The C/Nratio is still as high as 50 dB for a pit length of 0.3 µm. The PSR disk with double linear densities of current CD is successfully read out with a clear eye pattern and a low error rate using current optics.