Patent GB-1493709-A (GB)
Title: Method for transmitting two-dimensional information and apparatus for effecting same
IPC: H04B10/00
Filed: 1975-02-25
Abstract: 1493709 Optical signal transmission R F AVRAMENKO V I NIKOLAEVA and L A ORLOV 25 Feb 1975 7903/75 Heading G2J A method of transmitting 2-D information at optical frequencies (e.g. for Video-telephone or 3-D T.V. networks) comprises transmitting in succession, through an optical communication line, a 2-D information signal formed by 2-D spatial modulation of a coherent light flux and a 2-D optical reference signal displaced by a time interval that does not exceed the time instability interval of the communication line and, at the output of the communication line, combining the information and reference signals so that the identical distortions due to the communication line are compensated. In the arrangement of Fig. 1, light from a laser 2 is pulsed by switching means 6 which may be a Kerr cell or an electro-optic modulator based on the Pockels effect. The pulses are then spatially separated at 7 by an electro-optic crystal deflector or ultrasonically into the information signal path 9, 10 and the reference signal path 8. Light in the information signal path passes through an information modulator 9 (which may be a device for the transillumination of slides or film; an electro-optical modulator matrix; an ultrasonic modulator or an eidophore modulator) and a unit 10 for optimum matching to the line 4. Light in the reference path passes through a unit 8 similar to unit 10. Pulses from units 8, 10 are applied to a spatial matching unit 11 (e.g. a mirror system) from which they travel to an input unit 12 which is controlled, in common with units, 6, 7 and 8 by synchrosignals from generator 38. The pulses travel successively along the line 4 which may be free space; a guide containing lenses or mirrors or a multi-mode light guide having polished internal walls (see below) to an output unit 13 also controlled by a synchrosignal generator 39. The pulses are discriminated in time by switch 14 and applied to the processing unit 15, Fig. 2. The processing unit 15 comprises a separation unit 16 which separates the information from the reference signals, one of which, depending on their sequence, is fed to a delay unit 17 before both are applied to the input of a spatial matching unit 18 where an interference pattern i.e. a hologram is formed which is recorded at 20 after amplification at 19. The hologram recorded at 20 is illuminated by source 22, under the control of synchrosignals from generator 39, and the hologram reconstruction is recorded by still or cine camera 21. The multi-mode light guide, Fig. 6, may be of square, circular or triangular cross-section and has polished internal faces. To eliminate reflections at angles in excess of the pre-selected beam divergence angle, the internal surface has evenly spaced black portions 45 whose cross-sectional dimensions are no less than those of the light guide and whose length equals the diameter of the guide divided by the sine of the beam divergence angle. The input 12 and output 13 units are deflectable semi-reflectors operated by selectors 46. Plates 48, 49 are installed in the light guide having pseudo-random phase transmission (48) or reflection (49) coefficients to reduce the effect of irregularities in the light guide. The light guide may be constructed as sealed pipe lengths each filled with an inert gas (e.g. argon).