GB1409712A - Optical systems - Google Patents

Abstract

1409712 Modulating light REDIFON Ltd 1 Sept 1972 20001/75 Divided out of 1409711 Heading H4F In an optical system a dispersive birefringent plate, of a thickness such that it acts as a whole-waveplate for one or more first wavelength components of an incident plane polarized beam and as a half wave plate for one or more second components, is followed by a polarization sensitive modulator whereby the first components are modulated independently of the second components without spatial separation. The system may be reversed to provide a single polychromatic beam with a single plane of polarization. The dispersive plate has no effect on the polarization of the first components, but serves to reflect the direction of polarization of the second components through its main optical (fast or slow) axis. In Fig. 1 the axis of dispersive plate 54 is at 22¢ degrees to the polarization direction of light from an argon laser 53 so that the polarization direction of the blue component is altered to 45 degrees with respect to the green component (0 degrees). Pockel cells 50 and 51 are set with their induced axes at 0 and 45 degrees respectively, whereby the blue light is polarization modulated only at cell 50, and green light only at cell 51. Phase modulation occurs at the other cells but is unimportant. The composite beam 52<111> is incident on a second dispersive plate 55 to provide blue and green light both with phase quadrature plane polarized components at 0 and 90 degrees (i.e. elliptically polarized with ellipse axes at 0 and 90 degrees) and passed to an analyser for transmitting the 90-degree components and absorbing or reflecting the 0-degree components. In Fig. 2 an argon krypton mixed gas laser provides R, band B light, and an arrangement of a Pockel cell 60 between two dispersive plates 65, 66 is followed by two similar arrangements, one for each colour component. The dispersive plates act as half wave plates only for their associated colour component and the Pockel cells are oriented with their induced axes parallel to the original plane of polarization for polarization modulating the associated colour component only. Again in the final beam incident an analyser 161 each colour component has plane polarization phase quadrature components at 0 and at 90 degrees. Plates 66 and 67, and 68 and 69, may be single plates of appropriate thickness. The polarization modulating cells may also be Ker cells. As described, the dispersive phase plate is of crystalline quartz with its optical axis in the plane of the plate. The optical systems are said to be particularly useful in laser television systems.