Patent GB-1086926-A (GB)
Title: Improvements in or relating to apparatus for selectively deflecting a beam of plane polarized light
IPC: H03K17/78
Filed: 1965-06-14
Abstract: 1,086,926. Deflecting light. INTERNATIONAL BUSINESS MACHINES CORPORATION. June 14, 1965 [Sept. 25, 1964], No. 25039/65. Heading H4F. Each deflection stage of apparatus for selectively deflecting a beam of plane polarized monochromatic light to any one of a plurality of output positions comprises an electro-optic beam control element operable to control the plane of polarization of the light beam and a pair of birefringent elements so arranged that the light-beam passes therethrough along one of two paths dependent upon its plane of polarization, the optical path lengths of the two paths through the birefringent elements being equal. With the optical path lengths being equal a convergent beam of light may be focused at different points in a common plane at the output side of the light deflector and when a beam of light is passed through a limiting aperture a correct imaging of the light at such points may be obtained. As shown, Fig. 2, the two birefringent crystals 22 and 24 of a deflection stage are so arranged that the optic axis OA of crystal 22 lies in the diagonal plane BCDE and extends downwardly and the optic axis OA of crystal 24 lies in a plane spaced angularly 90 degrees from the plane BCDE and extends upwardly, and thus the extraordinary ray passes through crystal 22 upwardly in plane BCDE which is spaced angularly from a vertical plane by 45 degrees, and passes through crystal 24 downwardly in a plane spaced angularly 135 degrees from the vertical plane. When the light beam L is plane polarized in a plane perpendicular to the plane BCDE it passes through crystal 22 undeflected as an ordinary ray 22o and then through crystal 24 as an extraordinary ray 24eo and is deflected. If the direction of polarization of the incoming light is changed by 90 degrees by an electro-optic element (not shown in Fig. 2) then it passes through crystal 22 deflected as the extraordinary ray 22eo and through crystal 24 as the ordinary ray 24o. Since crystals 22 and 24 are of equal thickness the optical path lengths of the paths are equal. A three stage deflector is described with reference to Fig. 3 (not shown) in which each stage comprises a pair of crystals oriented in the manner of Fig. 2, each stage being preceded by an electrooptic device to which half-wave voltages may be applied to change the plane of polarization of the light beam by 90 degrees in order that the light beam may be deflected to any one of eight positions. To deflect the light beam both vertically and horizontally two deflection stages are used, one of which is oriented at 90 degrees to the other, Fig. 4 (not shown). In a modification, Fig. 8, each stage of a threestage deflector includes a pair of birefringent crystals with a half-wave plate between them. The first stage comprises electro-optic beam control device 46, birefringent crystals 40 and 41 and half-wave plate 42. The optic axis of crystal 40 is oriented in a direction as indicated by arrow 43 to effect an upward deflection of an extraordinary ray in a vertical plane. The other crystal 41 is oriented, as indicated by arrow 44, in a direction to effect a deflection of the extraordinary ray downwardly in a vertical plane. As shown the half-wave plate which changes the plane of polarization by 90 degrees causes the light to pass through one crystal of each pair as an ordinary undeflected ray and through the other crystal of each pair as an extraordinary deflected ray, thus ensuring equal optical path lengths. In the foregoing the hirefringent crystals have comprised a negative birefringent material such as calcite, but a positive birefringent material may be used. In a further embodiment, Fig. 10, a single stage of a deflector includes a positive birefringent element 70 and a negative birefringent element 71. The ordinary and extraordinary rays remain the same in both elements but the extraordinary ray has a larger optical path length than the ordinary ray in the positive birefringent material, the reverse being true in the negative birefringent material. Urea and calcite may be the positive and negative birefringent materials respectively. As an indication of the optical path length of the two rays in Fig. 10, divisions have been made showing that the extraordinary ray has seven length units in element 70 and only two in element 71 and the ordinary ray has four length units in element 70 and five in element 71. As before an electrooptic beam control element is located at the input side and is effective to change the plane of polarization by 90 degrees when switch 73 is closed.