Optical Reading From Videotapes Using Magnetic Garnet Film

In order to greatly increase the speed at which videotapes are read, we propose an optical reading method using a magnetic garnet film. This method allows several data tracks to be read in parallel. In order to realize the optical reading of data, the magnetic garnet films need to have not only high enough resolution for all data to be transferred from the videotape, but also a large Faraday coefficient for data to be reproduced with a high carrier-to-noise ratio (CNR). We have constructed trial films with a Faraday rotation angle of approximately 5deg per micrometer film thickness when a green laser is used. The data patterns transferred from a D-3 format videotape were clearly observed using a polarizing microscope. An experiment to reproduce videotape data was carried out at a tape speed of 0.367 m/s, and a CNR of over 30 dB was achieved for a bit-length of 0.385 mum. The film had a constant real permeability over a frequency range of 40 MHz, which corresponds to a tape speed of 353x with respect to the D-3 system.