GB1376158A - Pattern generating device and method of recording a generated pattern - Google Patents

Abstract

1376158 Modulating light HITACHI Ltd 9 Dec 1971 [9 Dec 1970 25 Dec 1970] 57237/71 Heading H4F [Also in Division G2] A light modulating element comprises an X, Y or Z cut plate of irregular ferroelectric material, i.e. one in which application of an electric field or mechanical stress above a threshold value results in a reversal of the electric polarization along the c-axis and an effective interchange between the a and b axes, and means for applying such an electric field along the c-axis, the plate having a thickness given by l=#/(#n)(p + ¢), where p is integral or zero, so that the phase of light incident normally thereon, with a plane of polarization parallel to the a or b axis, and having a wavelength # (#n being the difference in refractive index for light of wavelength # and having planes of polarization parallel to the a and b axes respectively, i.e. the plate thickness equals that of a #/2 Z cut plate of the material, irrespective of whether the plate is actually X, Y or Z cut) is changed by # when the plate is switched from one electric polarization state to the other. In a modification for use in a double pass system, the plate thickness is half that of a #/2 Z cut plate, Figs. 8 and 9 (not shown), The phase modulation is said to be less sensitive to deviations of the light beam from normal incidence than similar prior art intensity modulators comprising a fixed and a reversible #/4 plate in series between polarizers, the Y-cut plate being superior to the Z cut plate in maintaining the modulator depth. An array of such elements is therefore useful in holographic systems for example, and contours of light intensity, produced by such a system having a single element, are plotted as a function of deviation # from normal incidence in different planes (#) for a comparison between the present and pior art systems, Figs. 2a, 2b and 3 respectively (not shown). Apparatus for forming holograms and comprising such an array is described with respect to Figs. 7a, 8 and 9 (not shown), and also described is a method wherein a random pattern is superimposed on the information pattern generated by the matrix of elements whereby the light intensity distribution on the hologram recording medium is rendered more even. The electric field may be applied by electrodes on the c-faces of the crystal plate, or via an electron beam. The irregular ferroelectric material may be KDP Rochelle salt, ammonium cadmium sulphate, methyl aluminium sulphate dodecahydrate, but particularly Gd 2 (MoO 4 ) 3 and isomorphs of the formula R x R<SP>1</SP> 1-x ) 2 O 3 . 3Mo 1-e W e O 3 where R and R<SP>1</SP> are each rare earth elements 0#x#1 and 0#e#0À2.