Patent GB-1066612-A (GB)
Title: Electro-optical light modulation means
IPC: H01H71/20
Filed: 1965-01-20
Abstract: 1,066,612. Lasers. WESTINGHOUSE ELECTRIC CORPORATION. Jan. 20, 1965 [Feb. 7, 1964], No. 2486/65. Heading H1C. [Also in Division H4] An electro-optic light modulation means comprises two uniaxial birefringent electrooptic crystals 10 and 11 positioned between crossed polarizer 14 and analyser 15, each crystal being arranged as shown in Fig. 1 with its Z-axis (optic axis) perpendicular to the axis of the light beam 12 and to each other, and with the X-axis of crystal 10 and the Y axis of crystal 11 in coaxial alignment with each other and with the light beam. The plane of polarization of the light emerging from polarizer 14 is arranged to be at 45 degrees with respect to the Y and Z axes of crystal 10 and Z and X axes of crystal 11. To modulate the light beam an electric field is simultaneously applied across the crystals parallel to the Z-axes by means of the terminals 10a and 10b of the electrodes of crystal 10 and terminals 11a and 11b of the electrodes of crystal 11. The crystals are of the dihydrogen phosphate type, e.g. KDP, which without an electric field are uniaxial and with an electric field are biaxial. The Specification includes a brief outline of the theory of operation of the invention together with a mathematical analysis of the phase changes of the orthogonal components of plane polarized light propogating along the axis 12. With the present system a reduction in the half-wave voltage by a factor of L/d (see Fig. 1) is accomplished over the   system in which the light beam is propagated along the Z-axis. In a second embodiment, Fig. 2, the above electro-optic light modulation means is incorporated in a laser cavity and used in modulating the " Q " of the resonant cavity of the laser. The Fabry-Perot resonant cavity of the laser comprises totally reflective mirror 21 and partially reflective or apertured mirror 22 between which is positioned a suitably energized rod 23 of active laser material adapted to emit linearly polarized coherent light, its plane of polarization being at 45 degrees with respect to the respective Z axes of the crystals 10 and 11. Since the light beam makes two passes through the electro-optic modulator crystals before it re-enters the laser rod only half of the calculated half-wave voltage for the two crystal system is required to effectively rotate the plane of polarization 90 degrees in the cavity. If the voltage from the modulating driver 32 is zero, i.e. no change in polarization takes place, lasing will take place since the " Q " of the resonant cavity 19 is such that sufficient regenerative feedback is provided. To produce an extra large output spike from the laser the resonant cavity 19 is put into a non-resonant condition by arranging that there is a 90 degrees polarization charge between the original output of laser 23 and the emerging light beam from the left-hand side of the crystal 10 after reflection. If the laser rod 23 is then excited, e.g. by flash tube 27 energy will be stored in the energy states of the electrons of the laser rod and then when the modulating voltage is reduced coupling will take place as reflected light re-enters the laser rod 23 effectively raising the " Q " of the resonant cavity so that a large spike output pulse is produced. Pulse modulation or continuous modulation of the output from the laser cavity may be produced.