Patent GB-1138985-A (GB)
Title: Multi-frequency light deflectors
IPC: H04J14/02
Filed: 1967-10-06
Abstract: 1,138,985. Multi-frequency light deflectors. INTERNATIONAL BUSINESS MACHINES CORP. 6 Oct., 1967 [16 Jan., 1967], No. 45641/67. Heading H4F. [Also in Division G2] A plane polarized polychromatic light beam is separated into two or more spatially separated and preferably parallel output beams each comprising one or more of the plurality of wavelengths of the polychromatic light beam by means of polarization control means for orienting the polarization plane of at least one of the wavelengths perpendicular to the polarization plane of the other wavelengths, and beam splitting means adapted to separate wavelengths having mutually perpendicular polarization planes. As shown, Fig. 1, a plane polarized light beam from a light source 10 having a plurality of wavelengths # 1 , # 2 , # 3  and # 4  is separated into two parallel spaced outputs beams comprising wavelengths # 1 , # 2  and # 3 , # 4  respectively by means of dispersion rotator 12 of optically active material such as quartz, which separates the four wavelengths into pairs polarized in two mutually perpendicular polarization directions, an electro-optic crystal assembly 14 which works as a polarization switch for a mutual exchange of the two polarization directions when the voltage is applied to those shown by the wavelength designations in the brackets, and a totally internal reflecting polarization dependent beam splitter 16. Beam splitter 16 includes a birefringent plate 22 e.g. calcite or sodium nitrate crystal, the optic axis of which is parallel to its reflecting surface and perpendicular to the plane of the drawing, the plate being inclined at an angle which is greater than the critical angle for a total reflection of light polarized perpendicular to the plane of the drawing. Beam splitter 16 also includes an alignment plate 24 with its reflecting surface parallel to plate 22. Plate 24 may be made of glass or sodium fluoride which has a refractive index less than or equal to the low index of refraction of the material of plate 22 so that external reflection occurs. The dispersion rotator may be replaced by a dispersive phase plate and the beam splitter by a pair of Wollaston prisms (Fig. 2, not shown). The two Wollaston prisms are deflection direction compensated i.e. the two beams which leave the first Wollaston prism divergently are made parallel by the action of the second Wollaston prism. The beam splitter may alternatively comprise a birefringent single crystal with special orientation. By using two such multi-frequency light deflectors four spatially separate output beams may be obtained (the second dispersion rotator being designed to make mutually perpendicular polarization pairs # 1 , # 3  and # 2 , # 4 ). Instead of using a dispersion rotator in combination with an electrooptic polarization switch, electric field controlled liquid crystal cells may be used, Fig. 5 (Figs. 6 and 7, not shown), each of which enables a single light frequency to be changed in its polarization direction such that it will be perpendicular to the remaining light frequencies in a polychromatic light beam. If a voltage is applied to a first electrode pair 76, 78 the molecules of the liquid crystal material will align in the direction of the electric field and birefringence will be exhibited. If a voltage is applied to electrodes 84 and 86 instead of electrodes 76 and 78 no birefringence occurs. For four wavelengths the number of combinations of polarization directions which can be achieved by three liquid crystal cells is eight, but by the addition of an achromatic electro-optic switch sixteen different combinations may be obtained (Fig. 6, not shown).