Improvements in or relating to Information Transmission Systems

1,199,457. Lasers. WESTERN ELECTRIC CO. Inc. 28 Nov., 1967 [8 Dec., 1966], No. 54026/67. Heading H1C. [Also in Division G1] In an optical information transmission system comprising a source of recurrent pulses of high frequency electromagnetic energy, the frequency spectrum of selected ones of the pulses is shifted in accordance with the signal parameter of an information signal and transmitted to a receiver, the receiver including means for beating selected pairs of the pulses to extract the information from the varying beat frequency. The output from a mode locked laser 12, Fig. 2 is pulsed by ultrasonic waves applied to a fused quartz block modulator 20 by a transducer 21 fed from an oscillator 22 oscillating at for example 20 M/C, and applied to a crystal 35 of, for example, lithium niobate, wherein successive pulses are upshifted and downshifted in frequency by an electric field derived from a second output from the laser through a photo-multiplier 25. The photomultiplier output is halved in frequency and modulated by the information signal input at 50, the frequency halving being necessary for the rise and fall in sinusoidal voltage to effect the respective upshift and downshift. At the receiver, adjacent pulses in pairs are applied simultaneously to a photo-detector 65 by means of mirrors 62, 63, 61, 64 the latter two of which are half silvered and the former two of which provide an additional path to introduce sufficient time delay to render some of the energy of a pulse, time coincident with the following pulse. A second photo-detector 67 responsive to energy reflected by mirror 64 may be provided. Frequency shift may be by a movable mirror and alternate pulses may be left unshifted.