Patent GB-736790-A (GB)
Title: Improvements relating to detecting arrangements for electrical signals
Inventor: LAGER FREDERICK JOHN WILKINS
IPC: H03D1/00
Filed: 1951-09-20
Abstract: 736,790. Light modulating controlling light beam by microwave electric signals. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Sept. 22, 1952 [Sept. 20, 1951], No. 22026/51. Class 40 (3). [Also in Group XL (c)] In a detecting and amplifying arrangement for microwave electrical signals, the signals are applied to vary the dipole moments of the molecules of a light-propagating medium and thereby to vary its light-propagating properties, means being provided sensitive to light propagated through the medium for deriving an output of electrical signals which are an amplified representation of the envelope shape of the applied signals. In one arrangement, Fig. 1, use is made of the change in the light scattering properties of a medium. Vertically polarized light, which may be monochromatic, is derived through a polarizing element 4 and traverses a light cell 5 to a second polarizing element 6 which is arranged to transmit only horizontally polarized light to a photo-cell 7. The light cell contains ammonia gas at a pressure of 1 mm. of mercury and may be constructed as shown in Fig. 2 in the form of a bent wave-guide 10 containing members 16 and 17 closing a chamber to hold the gas. Windows 13 and 14 permit the light beam to pass through the gas. Parallel plate electrodes (not shown) create a static electric polarizing field inclined at an angle to the vertical (preferably 45 degrees) in a plane normal to the optical axis 2. Microwave signals to be detected are propagated in the wave-guide and cause of change in the dipole moments of the ammonia molecules. Light which is normally scattered without change of polarization is thereby subjected to a change of polarization and passes through element 6 in proportion to the intensity of the signals. In a modification, Fig. 3 (not shown), polarizing element 4 is dispensed with, and polarizing element 6 and photo-cell 7 are disposed along an axis at right-angles to the incident light axis 2. The polarizing field is inclined in a vertical plane parallel to the optical axis 2. Another arrangement, Fig. 4, makes use of the change in the dielectric constant and therefore the velocity of propagation of a medium when the dipole moments of the molecules forming the medium are changed. Monochromatic light from a source 20 is split into two beams 31, 34 by means of parallel-sided block 21 and transmitted to a further block 25 where they recombine and reflect to photo-cell 7. Light of one beam 31 passes through a cell 41 containing ammonia gas at a predetermined pressure whilst light of the other beam passes through a tube 40 containing air at a predetermined pressure. By means, not shown, microwave signals to be detected are applied to set up a field in the gas in cell 41. In the absence of signals the path lengths of the two light beams are arranged to differ by an odd integral number of half wavelengths whereby a minimum of light is transmitted to the photo-cell. The microwave field controls the dielectric constant of the light path in cell 41 and hence the relative phase of the two beams where they impinge the photocell. In consequence, the intensity of photocell illumination is made dependent on the intensity of the microwave signals. Cell 41 may be formed within a wave-guide, cavity resonator or concentric line.