Patent GB-1181820-A (GB)
Title: Improvements in or relating to Apparatus for Modulating Electromagnetic Radiation
IPC: G02F1/03
Filed: 1967-11-01
Abstract: 1,181,820. Light modulators and deflectors. HONEYWELL Inc. 1 Nov., 1967 [14 Nov., 1966 (2)], No. 49665/67. Heading H4F. The invention relates to cumulatively modulating (change in direction or polarization) a beam of electromagnetic radiation by repeatedly passing the entire beam through the same volume of electro-optic material in an electrooptic modulating device to which an electric field is applied. In Fig. 1, a light beam from source 10 is polarized, 11, and directed via a beam splitter 12 to the modulating device 13 which in operation deflects the beam. The amount of deflection imparted to the beam is controlled by control signal means, such as deflection signal source 14. Light emerging from device 13 is reflected from mirror 15 back into the modulating device which returns the beam to beam splitter 12 where at least a portion of it is reflected to a utilization means 16. Device 13 comprises a prism 20 of electro-optic material, e.g. KTN, KDP or ADP, having electrodes 25 and 26, which intimately contacts a prism of compensating material 21 along an interface 22. The parallel end faces of the composite prism are coated with an antireflective coating 23 and a reflective coating 24. Reflective surface 24 may be spaced from the prism. Compensator 21, has an index of refraction matching that of the electro-optic crystal when no electric field is applied thereto, or it may be chosen to match the index of refraction for the ray (ordinary or extraordinary) to be deflected at some predetermined field-magnitude, alternatively it may comprise electrooptic material so orientated and supplied with a suitable field as to give additional deflection. Mirror 15 may be replaced by a pair of mirrors at right-angles. To modulate the polarization of a beam a rectangular block of electro-optic material, Fig. 1A (not shown), is substituted for modulating device 13 in Fig. 1, the beam being polarized so that it splits up into its ordinary and extraordinary components when it enters the electro-optic crystal. By inclusion of an analyzer in utilization means 16, beam intensity differences may be detected as the modulating electric field applied varies in magnitude. The deflecting arrangement may include two external mirrors (40, 41, Fig. 3, not shown) instead of mirror 15 in the arrangement of Fig. 1, so positioned that the beam passes six times (instead of four times) through the electro-optic prism thereby increasing the amount of total deflection available. A modification of the deflecting modulation device 13 comprises a pentaprism structure, Fig. 5, comprising an electro-optic prism 20 having anti-reflective coating 23 and reflective coating 24, in intimate contact (as shown) with a compensating prism 21 having reflective coatings 27 and 28. The crystal of electro-optic material is so cut that a plane polarized beam may pass through it in both horizontal and vertical directions as an ordinary ray in both directions or as an extraordinary ray in both directions. Thus by means of two external mirrors (240, 241, Fig. 4, not shown) the beam passes twelve times through the electro-optic prism 20. In a further modification, Fig. 6, the electro-optic prism 20 is equilateral and is in intimate contact with compensating prisms 621 and 670 having reflective coatings 627, 628 and 671, 672, so that the beam passes six times through the electro-optic prism 20. In a further deflecting arrangement, Fig. 7, a monochromatic light source 10 directs a vertically polarized beam of light through lens 711 which passes it through aperture 712 in an inverting means 713, comprising reflecting surfaces 721 and 722 intersecting at right-angles along a common edge 720, via lens 714 to modulating (deflecting) device 13. Lens 714 converts a diverging beam impinging from the direction of inverting means 713 to a parallel beam and converts a parallel beam from the opposite direction to a converging beam directed at inverting means 713. In operation, the diverging beam emerging from aperture 712 is converted into a parallel beam by lens 714, deflected in device 13 by an amount depending upon the electric field applied to the electro-optic crystal, reflected back through the deflector 13 by mirror 717 wherein it is further deflected, the resulting diverging beam being converted into a parallel beam by lens 714, and then reflected back through the arrangement by inverter 713 (which also inverts the beam in the direction perpendicular to the drawing but not vertically), the beam ultimately impinging upon the surface of utilization means 16. The beam may be reflected between inverter 713 and mirror 717 several times. Device 13 preferably comprises a prism of KDP or ADP with the crystal cut in the form of an isosceles triangular prism with an apex angle of 67 degrees and with the crystallographic X and Y directions at 45 degrees to the base of the triangle, and the Z direction perpendicular to the parallel triangular faces, Figs. 8 and 9 (not shown). A plane mirror and a spherical mirror, Fig. 10 (not shown), may be used to perform the same function as the lens 714 in Fig. 7. Reference has been directed by the Comptroller to Specification 1,060,954.