| 摘要 |
1,211,565. Photo-electric signalling systems . COMPAGNIE GENERALE D'ELECTRICITE. Feb. 19, 1968 [Feb. 28, 1967], No.8046/68. Heading G1A. [Also in Division H4] A monochromatic light beam is amplitude or phase modulated by passing it through a semiconductor or semi-metallic crystal disposed in a magnetic field at a constant preferably low temperature in an enclosed space, the absorption characteristic of the crystal which is associated with the interband transitions between the energy levels known as the Landau levels being varied by causing the free carriers of the crystal to perform absorbent transitions between the Landau levels, the crystal being transparent to the light beam in the absence of the magnetic field. The free carriers may be caused to perform absorbent transitions by varying the energy distribution curve of the free carriers or by varying the number of free carriers. To vary the energy distribution curve an electric field may be applied by means of capacitor plates on either side of the crystal or by means of induction created by a solenoid disposed around the crystal. To vary the number of free carriers the crystal may be irradiated with electro-magnetic radiation (photo-electric effect), or electrodes associated with the crystal may be connected to a modulating current, one of the electrodes forming an emitter which injects free carriers into the crystal, or free carriers may be injected into the crystal by means of a layer of a semi-conductor on the crystal connected to a modulating current, the semi-conductor layer being of opposite type to the crystal and thus forming a PN junction, or the number of free carriers may be effected by applying an electric field to the crystal which is sufficient to create an ionisation by impact from an appropriate impurity level. Alternatively the free carriers may be caused to perform absorbent transitions by modulating the magnetic field within the enclosed space. The magnetic field may be created by an electro-magnet or a super-conducting winding the cooling means of which being associated with means for maintaining the low constant temperature within the enclosed space. The crystal may comprise indium arsenide, cadmium telluride, antimony, bismuth, arsenic, indium antimonide having N-type conduction, or elements or compounds of elements in the II-VI groups of the Periodic Table. The windows of the enclosed space through which the light beam passes are associated with selective filtering means. A detector for detecting the amplitude modulation of the light beam may be identical to the modulating device. The absorption of a photon results in the excitation of a free carrier which passes from the fundamental level to a higher energy level at which its mobility is different i.e. the electrical conductivity (resistance) of the crystal is modulated. This conductivity may be measured by electrodes disposed on the crystal and connected to the input of a measuring bridge. |
