GB1335197A - Apparatus for determining the position of an object in a beam of radiation - Google Patents

Abstract

1335197 Position finding PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 2 Dec 1970 [5 Dec 1969] 57239/70 Heading H4D General.-In a position determining system, a beam of radiation is produced from at least two virtual or real spatially separated sources of coherent radiation, polarized at right angles to one another and having a time dependent complex amplitude difference, and a polarization sensitive detection system is disposed at the position to be determined. The radiation may be light or radio. Light embodiments.-The light source 1, Fig. 1, produces linearly polarized light, the plane of polarization of which rotates at an angular velocity Q. It may comprise an origonal light source emitting plane polarized light, a quarterwave plate and three electro-optical crystals. A light ray from source 1 passed through a Savart plate 2 and is thereby split into two beams P1 and P2, Fig. 2, plane polarized at right angles to each other and at Œ45 degrees to the instant polarization plane of the source 1 (dotted line, Fig. 2) and positioned in crosssection with respect to the incident 1 as above. (The planes of polarization of beams P1 and P2 are # also and rotate at an angular velocity #. At the position 3 to be determined, the beams P2 and P1 appear to emanate from virtual sources 7 and 8. On the axis CD, the path lengths to the virtual sources 7 and 8 are equal and the beams P1 and P2 will sum at detector 3 when placed there to give a resultant beam of constant amplitude and plane polarized in the same rotating plane as the source 1. At any point in plane 13, off the axis CD, however, a path length difference exists causing the light waves of the beams to be out of phase and the resultant beam to have elliptical or possibly circular polarization. In plane 13 a case of elliptical polarization may be illustrated as in Fig. 3, angle # varying at rate Q. Such polarization is represented on a PoincarÚ sphere by a point P having the spherical co-ordinates 2# and 2# as shown. Pole points A1 and A2 (#=45 degrees) represent circular polarization, and points on the equator such as those representing the beams P1 and P2 (#=0 degrees) representing plane polarization at various angles. The polarization at point 3, subtending an angle with the axis CD is thus represented by a point which moves at a rate 2 # round a great circle inclined at an angle 2 # to the equator proportional to the path difference and indicative of angle α. A detection system, able to determine # and α in one co-ordinate plane is shown in Fig. 6. An isotropic beam splitter 20 divides the resultant beam into two. One half passes through a quarter wave plate 21 which converts the left-hand and right-hand circular polarization components into respective orthogonally linearly polarized beams which are detected for amplitude at 23 and 24. The detector outputs are fed to difference amplifier 25 which gives an output corresponding to sin 2 #. sin 2 #t. Polarization beam splitting prism 26, detectors 27, 28 and difference amplifier similarly produce a signal corresponding to cos 2 #. sin 2 #t. The angle # can thus be obtained. The source 1 emits a collimated beam, Fig. 7 (still with a rotating plane of polarization), a Wollaston prism 30 may be substituted for the Savart plate 2 to give two orthogonally polarized collimated beams P1, P2 diverging from the axis of the prism with angles Œ#. The same effect as far as path difference &c. obtains as in the embodiment of Fig. 1 and the detection system of Fig. 6 may be used. In Fig. 9 is shown how a beam splitting polarization-separating prism and mirrors 42, 43 may be used instead of the Savart plate 2. Fig. 11 shows how the same apparatus can be used instead of the Wollaston prism. A Kosters prism may similarly be substituted, Figs. 10, 12 (not shown). To detect the position 3 in two dimensions, two sources may be used, the beams therefrom being combined for direction to the position 3, by a semi-mirror. Different colours or different modulation frequencies (KDP or KDDP crystals) may be used to distinguish the beams at the detection system. Radio embodiment, Figs. 17, 18.-A short wave oscillator 100 produces a signal at frequency # which is amplitude modulated at 103 and 104 by the output of oscillator 101 at frequency #, the modulation being in phase quadrature (phase shifter 102). The modulated signals are transmitted, after amplification, by aerials 107, 108 with mutually orthogonal planes of polarization. In the receiver at point 110 giving a path difference delay of #, the beam is received and separated into its polarization components, by aerial 120. The two components, respectively indicative of cos #t. sin (#t + #) and sin #t. sin (#t-#) are fed to magic T's 111 and 112 directly and via a 90 degrees phase shifter 119 in the cosine branch, respectively. Detectors 113, 114 receive the anti-outputs of magic T 111 and feed a difference amplifier 117 to give a signal indicative of cos 2#. sin 2#t. Diodes 115, 116 and difference amplifier 118 similarly give a signal indicative of sin 2 #.sin 2 #t. The arrangements of Figs. 17 and 18 are analogous to those of Figs. 1 and 6.