Patent GB-1022307-A (GB)
Title: Improvements in or relating to circuit arrangements employing photo-electric devices
IPC: H03C1/34
Filed: 1964-04-01
Abstract: 1,022,307. Photo-electric communication devices. SIEMENS &HALSKE A. G. April 1, 1964 [April 1, 1963; Dec. 20, 1963 (2); Jan. 27, 1964, No. 13349/64. Heading G1A. The invention relates to amplifying, modulating, and signal transmission devices employing a radiation link. In a simple device a photo-emissive diode polarized in the forward direction and modulated by the input signal is used as the radiation source. It may emit substantially monochromatic light. A photo-diode biased in the reverse direction and feeding the output load is used as the receiver. Power gains greater than unity are possible so the device may be used as an amplifier. To avoid bias current flow through the input source or output load it is preferred to separate the bias and signal circuits as is shown for the radiation source side only in Fig. 4. The bias is conveniently made adjustable. Cooling of the photo-emissive diode to the temperature of liquid nitrogen may be arranged to increase the amplifcation. A structural embodiment in which both diodes are positioned in co-axial lines is described with reference to Fig. 2, not shown. Radiation is passed through slots in the line walls arranged in the same direction as that taken by the wall currents. The slots may be filled with transparent material of high dielectric constant or high permeability if required. Identical diodes may be used in an arrangement in which the direction of signal transmission can be reversed by reversing the polarity of the biasing voltages. This arrangement may have lower power amplification, however, and it is preferred to use a series assembly of a galliumarsenide photo-emissive diode and a silicon photodiode in each circuit, the two diodes in the same circuit being biased in the same direction at any one time. In considering the efficiency of devices according to the invention it is pointed out that it is better to use a part of the characteristic of the photo-emissive diode where the differential of the quantum efficiency is a maximum rather than a part where the quantum efficiency itself is greatest. Further increases in efficiency are obtained if a plurality of photo-emissive diodes connected in series are used as a composite source and a plurality of photo-diodes in parallel used as the receiver. A push-pull arrangement utilizing two radiation links can be used as a D.C. amplifier and Fig. 7 shows such an arrangement in which each radiation path is divided into three sub-paths to increase efficiency as described above. U X  is the input signal source and R L  the output load. In a modulator the two input signals may be applied in series to the photo-emissive diode, which has non linear properties for mixing the signals. Means for filtering out the required signal are required in the load circuit. One mixed signal can also be supressed by using two photoemissive diodes fed in antiphase and each radiating to a common photo-diode. A push-pull mixer is described with reference to Fig. 10 (not shown) and an arrangement in which one signal source is in the radiation source circuit and the other in the radiation detector circuit with reference to Fig. 11 (also not shown). Two signal sources 81, 82 may also be connected to two photo-emissive elements using separate circuits (Fig. 12) and the radiation from the two sources allowed to fall on a common photo-electric detector 819 in series with a parallel resonant circuit 824 tuned to the frequency of source 82. Lenses, mirrors or fibre optic bundles may be included in the radiation path, as may a further modulating device dependent on the Kerr or Pockels effect to provide a means of adjusting the gain of the device. To measure the current flowing in a high voltage line a photo-emissive diode may be inserted in the line and its radiation received by a photo diode at a safe distance in a measuring circuit.