Patent GB-1185970-A (GB)
Title: Device for converting circularly polarised electromagnetic radiation into plane-polarised electromagnetic radiation having a polarisation plane which rotates at a constant angular velocity
IPC: G02F1/09
Filed: 1968-10-07
Abstract: 1,185,970. Light modulators. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 7 Oct., 1968 [10 Oct., 1967; 11 June, 1968], No. 47455/68. Heading H4F. [Also in Division G2] Relates to apparatus for converting circularly polarized light into plane polarized light having a polarization plane which rotates at a constant angular velocity. In a first embodiment, Fig. 1, circularly polarizer light produced from source 1 by means of polarizer 3 and #/4 plate 4, is converted into plane polarized light having a polarization plane which rotates at a constant angular velocity by means of optically anisotropic electro-optic elements 5 and 6, e.g. KDP pockels crystals the main directions of which indicated by arrows 8 and 9 are at 45 degrees to each other. Alternatively KTN Kerr crystals may be utilized. In operation alternating voltage V 1  = V 0  sin wt. is applied to crystal 5 and alternating voltage V 2 =V 0  cos wt. is applied to crystal 6, these voltages producing fields which are parallel to the direction of light propagation and wherein amplitude V 0  is such that circularly polarized light incident upon a crystal when this voltage is applied to it, is converted into plane polarized light. With this arrangement the speed of rotation of the polarization plane is not completely constant and the light emerging from the series-combination of the crystals is slightly elliptically polarized except at times t = 0, T/4, T/2, 3T/4 and T where w = 2#/T. To improve the linearity a third electro-optic crystal is positioned between crystal 6 and detection system 7, Fig. 2 (not shown), with its main direction parallel to that of crystal 5 and having a voltage V 3  applied thereto where V 3 =V 1 =V 13  sin wt. and V 2 =V 22  cos wt., V 13  = 48/90 V 0  and V 22  = 82/90 V 0 . To improve the linearity still further, 2m + 1 crystals may be utilized where m is an integer. The angular velocity of the polarization plane may be varied by varying w. In a further embodiment the voltage from source 11, Fig. 1, is arranged to be equal to + V 0  during the first half of a period and - V 0  during the other half, while the voltage from source 10 increases linearity from - V 0  to + V 0  during the first half, and decreases from + V 0  to - V 0  during the other half (Fig. 5, not shown). Each crystal may be subdivided into a number of component crystals having correspondingly smaller voltages applied thereto. Optically active or doubly-refractive elements may be arranged between two successive crystals. In a further embodiment, Fig. 6, the circularly polarized light from source 61 via polarizer 63 and #/4 plate 64 is converted into plane polarized light having a polarization plane which rotates at a constant angular velocity by means of #/4 plates 65, 68 and 69, and magneto-optic crystals 66 and 67, e.g. YIG crystals, the main directions of #/4 plates 65 and 69 indicated by arrows 72 and 74 being at 45 degrees to that of plate 68 indicated by arrow 73. Crystals 66 and 67 are magnetized by means of alternating currents from sources 70 and 71 which produce magnetizations B 1 =B 0  sin wt. and B 2 =B 0  cos wt., the amplitude B 0  being such as to rotate incident plane polarized light through 45 degrees. Another #/4 plate may be interposed between #/4 plate 68 and crystal 66, the main direction of which is opposite to that of #/4 plate 65, the main direction of plate 69 being changed to be opposite to that of plate 68. To improve the linearity (as in the first embodiment) the circularly polarized light from #/4 plate 84, Fig. 8, is directed via #/4 plates 85, 89, 90, 91, 92 and 93 orientated as shown, and magneto-optic crystals 86, 87 and 88 energized by sources 95 and 96 as shown which produce magnetizations B 1 =¢B 0  sin wt. and B 2 =B 0  cos wt. respectively, amplitudes B 0  and ¢B 0  being such as to rotate incident plane polarized light through 45 and 22¢ degrees respectively. To improve the linearity still further or (2m+1) magneto-optic crystals may be utilized where m is an integer. The #/4 plates may comprise n#/4 plates where n is an odd integer.