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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%.

At present high-density optical discs have narrow track pitch values, and hence it is necessary not only to verify the precision of the track pitch, but also to determine the degree of track pitch variation. Track pitch variation (TPV) is commonly measured using scanning electron, laser scanning and scanning tunneling microscopies. However, these methods are time consuming and it is difficult to take TPVreadings from large areas of the disc. We developed a TPVmeasurement system based on an optical disc player with two readout beams, and using this system we can rapidly measure the TPVover the whole disc surface. In this paper, we describe our TPVmeasurement system and present the results of TPVmeasurements taken from a disc containing continuous grooves with a track pitch of 0.84 µm. We found that the TPVof these grooved discs ranged from 30 nm to 100 nm peak-to-peak (pk-pk) with a typical value of 40 nm pk-pk. The measurement resolution of our instrument is about 10 nm pk-pk.