GB1437119A - Device for measuring the phase shift of radiation - Google Patents

Abstract

1437119 Measuring optical phase shift COMMISSARIAT A L'ENERGIE ATOMIQUE 17 Oct 1973 [23 Oct 1972] 48396/73 Heading G1A The phase shift produced by a system on an EM wave is measured by dividing the wave into two beams shifting the frequency of one of the beams by means of a rotating cylindrical diffraction grating, interacting one of the beams with the phase shifting system, recombining the beams and applying them to a non-linear detector which produces a beat signal the phase of which is measured to determine the phase shift of the system. As shown, Fig. 6, a laser 2 produces a wave 4 which is divided into two partial beams 8, 10. Beam 8 passes through the phase shifting system 16 while beam 10 is reflected from a rotating cylindrical diffraction grating 36 which produces a frequency shift by the Doppler effect. The phase shifted and frequency shifted beams are combined and applied to a non-linear detector 42 which produces a signal at the beat (difference) frequency the phase of which is related to the phase shift produced by system 16. Further beam splitters 108, 110 produce beams which are combined and applied to a second detector to produce a reference beat signal. The phase difference between the measuring and the reference beat signals is determined by circuits (70, 72, 74) which measure the respective zero crossing times of the two signals and subtract one from the other. A frequency or period measuring circuit receives one of the detector signals and produces an output which is combined with the zero crossing time difference to produce a measure of the phase difference which is equal to the phase shift introduced by the system 16. In a modification the components shown dotted in Fig. 6 are omitted. This arrangement can be used with systems 16 producing transient phase shifts the circuit being arranged to measure and store the zero crossing times when system 16 produces no phase shift. The zero crossing times of the signal when the system 16 produces the transient phase shift are then measured and compared with the stored values. The timing and processing circuitry may be digital. Alternatively the two detector outputs may be simultaneously displayed on a CRO and the phase shift estimated visually. The source may be a hydrocyamic acid HCN laser or a CO 2 laser, the IR radiation being detected by a pyroelectric detector e.g. of glycine sulphate or of TGS. A further embodiment producing two passes of one of the beams through the phase shifting system is also described Fig. 2 (not shown). The apparatus may be used for measuring the phase shift due to a plasma, the refractive index of a substance (which is related to the phase shift), the shape of a moving body e.g. a solid of revolution by reflecting the radiation from the surface and measuring the variations in phase as it is rotated, and displacements.