Three beam optical memory system

摘要

A three beam laser optical system comprising a writing beam and two reading beams all incident on the same track of a rotating disk optical recording medium. One of the reading beams is incident on the track ahead of the writing beam so as to serve as a read-before-write beam, while the other reading beam is incident on the track behind the recording beam so as to serve as a read-after-write beam. The read-before-write beam is used during the recording of data to provide an advance indication as to whether there is previously recorded data ahead of the writing beam, in which case the recording operation is interrupted to prevent an overwrite error. The read-after-write beam is used during recording to check the accuracy of the recording operation.