GB1248752A - Apparatus for detecting the orientation of the plane of polarisation of a linearly polarized beam of radiation - Google Patents

Abstract

1,248,752. Light modulators. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 28 Nov., 1968 [1 Dec., 1967], No. 56573/68. Heading H4F. [Also in Division C1] Measurement of the fixed or slowly varying plane of polarization of a beam of radiation is facilitated by applying a periodic polarization variation, using at least three doubly refracting elements, at least one of which is electro-optic. A plane polarized beam, e.g. from a saccharimeter is collimated at 2, Fig. 1, and applied to a polarization modulator comprising #/4 plates 4, 6 and a KDP Pockels cell 5 fed from a sinusoidal voltage source 10. A KTN cell may be used instead of the KDP cell and arranged to have a substantially linear retardation against voltage characteristic by making the modulating voltage small compared with a simultaneously applied direct voltage. The oscillating plane polarized light emerging from #/4 plate 6 is split by a semi-transparent plate 20 and passes through analyzers 21, 22 mutually at 45 degrees to photo-detectors 33, 34 producing quadrature signals. These signals are multiplied in Hall generators by voltages synchronized with the sinusoidal modulating voltage and the modulating frequency is eliminated by filters to give signals proportional to sin 2a and cos 2a where a is the polarization angle of the radiation to be measured. Simpler detector circuitry may be used if a sawtooth voltage is used instead of the sinusoidal voltage 10. Another way of producing substantially sawtooth modulation is to use an odd number (e.g. three) of electro-optical KDP cells, Fig. 5 (not shown), the odd and even numbered cells being energized in quadrature. The electrooptical cells may be made up of a number of component cells with a proportionate amount of the operating voltage across each.