GB1474049A - Arrangement for the modulation of light - Google Patents

Abstract

1474049 Measuring displacement photoelectrically WETZLAR GmbH 19 Dec 1974 [12 Jan 1974 1 July 1974] 54879/74 Heading G1A Apparatus responsive to motion or displacement of an object for modulating light comprises: a plurality of diffracting means so disposed to act on light from a source that the light is subjected to at least three successive diffraction processes; the first process causing light to diverge from a point on the first diffracting means; the penultimate process causing the diffracted rays to converge towards the region of the last diffracting means; the last diffracting means directing the light components in the same direction and in the same phase. Divergent rays 10'(-2) and 10'(+2), Fig. 1 diffracted from a point on a first grating 11 strike a second grating 12, from which parts of the respective rays, diffracted as rays 10"(+4) and 10"(-4), recombine at a point on a third grating 13, whence components of the converging rays are further diffracted in at least one common (zero-order) direction. The ratio between the spacings n, m between adjacent gratings is a function of the relative grating constants of the first two gratings 11, 12. The blaze-angles of the gratings are chosen to favour the respective orders of the diffractions which they are intended to perform. By the use of an extended source 14 Fig la the light emerging from the grating 13 can be made to form an interference pattern in detectors a-d placed at the focal plane 17 of a lens 16, the detectors supplying phase-related signals indicative of the magnitude and sense of any displacement of the grating 12 in the plane of the diffracted rays and transverse to the optical axis. Four stages of diffraction may be provided by four gratings Fig. 2 (not shown), or by two gratings, one of which is arranged on the hypotenuse face of a reflex prism Fig. 3 (not shown). Three diffractions Fig. 4 (not shown) may be provided by a wheel having concentric inner transmissive and outer reflective gratings, in co-operation with reflex mirrors. Gratings of different spacings and correspondingly different grating constants may be effectively provided by a transmissive grating 11 Fig. 8 and a reflective grating 12 tilted relative to the grating 11 so that the return light path is of a different length from the outward path. A spaced-phase modulation pattern may be provided by causing convergent rays, Fig. 7 to strike the last grating 13 at two points, the recombined light due to different diffraction orders forming a pattern at an image plane 14. A displaceable diffracting element may comprise a grating, a standing or travelling acoustic wave, a light-scattering surface, or a beam-splitting polarizing device.