Direct Overwrite in Magneto-Optical Recording Technology

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Abstract
Progress on implementing direct overwrite in magneto-optical recording is reviewed. Observations made with a 10 nsec exposure time magneto-optic camera system show that in single-layer media in which a constant bias field may be used for writing and erasing, circular domains erase by a collapse process. Observations made on stripe erasure show that erasure proceeds most easily from the end of the stripe. Experiments with high-speed scanning indicate that both circular domains and stripes may be overwritten at scanning speeds at least as high as 15 m/sec. Three techniques for achieving direct overwrite with these single-layer media are described. The single-beam technique utilizes only a single laser beam with a modulated power level to achieve direct overwrite. Because stripes are easier to erase from the ends, this technique may require restricted modulation codes in which two ones are separated by no more than one zero such as the (1, 7) code. The erase-before-write technique uses two beams through a single lens. The first beam erases any previously written data on the track and the second writes the new data. This technique requires no restriction on modulation codes and also prevents interactions between the previously existing data and the new data. The read-before-overwrite technique requires that the previously written data be read and selectively altered to produce the new data pattern. Because of the added complexity of the electronics, this is the least favored technique. The single-beam and erase-before-write techniques compare favorably with respect to approaches using field modulation or exchange-coupled multilayers, because both the head and media are relatively simple. On the other hand, data reported thus far in the literature indicate the best carrier-to-noise ratio is obtained with field modulation. Since research is still underway in a number of laboratories, the reported CNR, for all techniques are likely to improve, however.

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