Patent GB-1506966-A (GB)
Title: Method of and apparatus for optically producing a reproducible recording
IPC: G11B7/128
Filed: 1976-01-19
Abstract: 1506966 Optical recording SONY CORP 19 Jan 1976 [17 Jan 1975] 02023/76 Heading G5R In a method of optically recording a signal, the maximum and minimum intensities of light directed at photo-sensitive material 19, Fig. 1, on a record 18, and intensity-modulated by the signal to be recorded, are maintained within a predetermined range of light intensities within which range the photo-sensitive material 19 is photo-reacted to a degree which varies substantially linearly with light intensity, and the light is scanned over the record 18 to simultaneously form, upon development if required of the photo-sensitive material, a contoured tracking path portion unified with a recorded signal portion. The signal, e.g. from a TV camera 15, Fig. 1, causes modulator 13 to intensity modulate a laser beam 12, the modulated beam being focused by lens 17 to a spot which traces a spiral path on a disc record 18 rotated at a constant rate and translated linearly. A moving grey scale filter 16 is used to adjust the beam intensity to compensate for the fact that without filter 16 the light energy per unit area would be greater at the centre of disc 18 than at its periphery because of the decreasing linear speed. A circuit 14 is provided to compensate for non-linearity in the response of photo-sensitive material 19 and in the characteristics of the modulator 13. Modulator 13 may comprise a crossed polarizer (25), Fig. 3 (not shown), and analyser 28, a quartz wavelength plate (26) and an electro-optic crystal (27) or a device utilizing the light defraction effect produced by an ultrasonic wave. The intensity of beam 12 is modulated about an operating point set by applying D.C. to modulator 13 or by selecting plate (26) to provide the light passing therethrough with an appropriate phase. If material 19 is a positive photo-resist, a spiral groove is formed by development of the exposed material as the tracking path portion with the signal recorded as undulations in the groove bottom. If material 19 is a negative photo-resist, development yields a spiral ridge with a modulated height on record base 20. Conventional techniques are then used to mass-produce spiral-grooved copies of either form of master, the copies being playable by a transducer with a stylus, or optically or by sensing capacitance variations. The photo-sensitive material may alternatively be such that the beam 12 vaporizes it to a varying depth, or a photo-thermoplastic material may be used. The intensity distribution across the laser beam may be controlled so that the groove or ridge has a rectangular, parabolic or triangular cross-section. In alternative recording apparatus, Fig. 6B, the record is exposed to an unmodulated laser beam 12a which is combined with one or more further beams 12b 1 , 12b 2 , 12b 3  each modulated by a respective recording signal, beam 12a producing the tracking groove or ridge and the other beam(s) modulating the depth or height thereof. The sum of the intensities of beams 12a 1 , 12b 1 , 12b 2 , 12b 3  is maintained in the substantially linear region of the photo-sensitive material. A single laser may produce each beam, or separate sources, of the same or different wavelengths, may produce respective beams. All the beams may be focused on to the same spot, or they may impinge at respective locations spaced apart along the spiral track. The master record 18 may be rotated at 1800 r.p.m. during recording in real time so that one NTSC video frame occupies one spiral turn, or, by using a frequency converter, master 18 may be rotated at 3600 r.p.m. during recording of signals intended for replay at 1800 r.p.m.