Patent GB-1245760-A (GB)
Title: Apparatus for rotating the plane of polarisation of a linearly polarised radiation
IPC: G02F1/03
Filed: 1968-11-06
Abstract: 1,245,760. Light modulators. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 6 Nov., 1968 [9 Nov., 1967; 1Aug., 1968], No. 52552/68. Heading H4F. [Also in Division G2] Relates to apparatus for converting linearly polarized light having an arbitrary plane of polarization into linearly polarized radiation having a plane of polarization which rotates at a substantially constant angular velocity in a manner such that the orientation of the plane of polarization varies linearly as a function of time from the initial arbitrary orientation. In a first embodiment, Fig. 1, a linearly polarized light beam from polarizer 3 traverses three electro-optic crystals 4, 5 and 6, e.g. KDP or KTN crystals, the principal directions 11 and 13 of crystals 4 and 6 being parallel to one another and at 45 degrees to the principal direction 12 of crystal 5. An alternating voltage V 1  = V sin wt is applied to crystals 4 and 6 and an alternating voltage V 2  = V<SP>1</SP> cos wt is applied to crystal 5, the peak voltage amplitudes V and V<SP>1</SP> being such as to result in a quarter and a half wavelength phase difference respectively. At intermediate instants between the instants t = 0, T/8, T/4, 3T/8 and T/2 where T = 2#/w there are slight deviations from a linear velocity of rotation and the light is slightly elliptically polarized. To improve the system voltages V and V<SP>1</SP> may be more selectively chosen and alternatively or additionally a series sequence of (2n+1) crystals, where n  is an integer, may be utilized, the odd and even numbered crystals together exhibiting an anisotropy of #/2 at t = 0 and t = T/4 and the alternating voltages applied thereto being 90 degrees out of phase. Each electro-optic crystal may comprise a plurality of component crystals. A detector 17 may comprise an analyser succeeded by a photo-cell. In a second embodiment, Fig. 4, a linearly polarized light beam from polarizer 23 traverses a #/4 plate 25, a Pockels crystal 26 and a #/4 plate 27 having parincipl directions 28, 29 and 30 respectively, and a sawtooth voltage is applied to crystal 26, the difference between the maximum and minimum values of the voltage being such that a phase difference of unit wavelength is produced.By turning the principal axis of #/4 plate 27 through 90 degrees, the emergent phane of polarization rotates in the opposite direction. In a third embodiment, Fig. 6, a linearly polarized light beam from polarizer 63 traverses #/4 plates 65, 69, 70, 71, 72 and 73 having principal directions as indicated, and magnetooptic crystals 66, 67 and 68, e.g. YIG crystals. An alternating voltage source 75 produces a magnetization B 1  = ¢ B 0  sin wt in crystals 66 and 68 and an alternating source 76 produces a magnetization B2 = B o  cos wt in crystal 67, the amplitudes B o  and ¢B o  being such as to cause 90 degree and 45 degree rotations respectively in the crystals; (2m+1) magnetooptic crystals may be utilized where m is an integer, the #/4 plates may comprise n#/4 plates where n is an odd integer, and the n#/4 plates preceding and succeeding each magnetooptic crystal may have principal directions which are the same instead of inclined at 90 degrees to each other as shown.