Frishauf, Holtz, Goodman & Woodward
Optical memory device with means for detecting erroneous data writing
 An optical memory device comprises a modulator for modulating a laser beam in accordance with recording data, an objective lens for focusing the output light beam from the modulator on a recording medium, a sensor arranged in the far field of the objective lens, and a judging circuit for detecting an erroneous writing operation in accordance with an output from the sensor and the recording data. The sensor has a square detecting surface divided into four segments by lines which form angles of .+-.45.degree. with respect to the direction of relative movement between the recording medium and the focused light beam. The difference between the sums of outputs from two pairs of opposing segments is supplied to the judging circuit. 
Olympus Optical Co. Ltd.
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Kiichi
Kato