Patent GB-823801-A (GB)
Title: Light modulation system
IPC: H04B10/532
Filed: 1956-11-06
Abstract: 823,801. Polarizing apparatus. MUELLER, H., and RINES, R. H Nov. 6, 1956, No. 3819/56. Class 97(1) [Also in Group XL(b)]  Vibrations set up in an optically inactive medium 13, such as a block of glass, a block of non-crystalline fused quartz, or a liquid, by means, such as a piezo-electric crystal vibrator assembly 15 in contact with the lower surface, render the medium birefringent whereby plane polarized light from a polarizer 7 impinging on one face '9 of the block, at right angles to the wave front of the vibrations, emerges from the other face 11 as elliptically polarized light and passes, in dependence upon the degree of ellipticity, through a second polarizer 25 crossed with the first. In accordance with the invention, vibrator 15 comprises a plurality of elements as shown in Fig. 6 with the axes of principal vibration substantially parallel to one another and normal to the surface of the medium. In the case of quartz crystal vibrators, the X or polar axes, along which maximum dimensional changes take place, are oriented normal to the surface of block 13. The Z- axes of adjacent crystals are disposed alternately orthogonally to one another so as to compensate for the effects of transverse vibration. The crystals may be metallized and soldered to a silver-chromium or copper layer 17 evaporated on to block 13. Reference is made to the use alternatively of magneto-strictive and magneto-active vibrators. The vibrations are produced at a high frequency, between several hundred kilocycles and ten megacycles are suggested, and may result in standing waves being set up in the medium. Reference is made however to arrangements in which the dimensions of the medium are not related to the wavelength of the vibration. The standing wave pattern may be displayed on a screen 27 and comprises bands of light 2 corresponding to the regions of compression and dilation which give rise to birefringence separated by dark bands 4 corresponding to the nodal regions. The bands are produced periodically at twice the frequency of the vibrations, each region changing back and forth between compression and dilution. The difference in the index of refraction along the vertical and horizontal polarization directions changes in sign from compression to dilation with the result that the direction of the resulting elliptical polarization also changes periodically. As an alternative to crossing polarizer 25 with 7, it may be oriented so as to pass light due to the nodal regions and suppress light due to the compressed and dilated regions. Reference is also .made to the use of circularly polarized light by the association of quarter-wave plates with polarizers 7 and 25. The vibrator is energized from an oscillator 21 and variation in the magnitude of transmitted light is effected by modulating the amplitude of the oscillations. A modulator in accordance with the invention may operate with visible, infra-red or ultraviolet light and may be applied to television image projection, a stroboscopic display arrangement, a single or multichannel system for communicating information over a light beam, Figs. 3 and 4 (not shown) and a secret communication system, Fig. 5 (not shown). Although the vibrations set up in the medium are principally in the vertical direction, transverse vibration does occur. By measuring the wavelength of the vertical and transverse wave patterns as projected on to a viewing screen Youngs modulus and Poisson's ratio for the medium may be determined.