GB1422180A - Interferometric observation of small displacements - Google Patents

Abstract

1422180 Interferometry COMPAGNIE GENERALE D'ELECTRICITE 21 June 1973 [10 April 1973] 29625/73 Addition to 1339348 Heading G2J A method of interferometric observation of a small displacement of an optical object comprises (a) positioning a source of mono-chromatic coherent light; (b) positioning a beam-splitter to divide the initial beam into first and second beams; (c) placing first and second diffusing material objects so that when they are illuminated by beams respectively they produce first and second diffused beams to form first and second optical objects which are identical and appear to be illuminated by beams 8, 7 at the same angle of incidence; (d) positioning means for superposing the diffused beams so that the optical objects appear to be exactly superposed; (e) applying a small displacement to one of the two optical objects; (f) producing the initial beam and in turn the illuminating, diffused and superposed beams; (g) disposing in the path of the superposed beams a convergent optical system incorporating an astigmatic element so as to pro- vide a minimum focal distance in a plane containing the axis of the superposed beams and a maximum focal distance in another plane containing the same axis, means being provided for adjusting one of the focal distances and (h) adjusting the adjusting means to obtain interference fringes from the superposed beams. In one arrangement Fig. 1 one optical object is a material object 1 while the other is produced by a hologram 2. Both are illuminated by a laser 3 via a beam-splitter 6. The optical system 13 contains a convergent lens 13, two cylindrical lenses 15, 16 and a screen 12. In use, lenses 15, 16 are selected by trial and error from a set of varying powers until clear fringes appear on the screen 12. Alternatively, each lens 15, 16 may be replaced by a set of cylindrical lenses of the same fixed direction but of adjustable spacing to adjust the power of the set. In a modification of this arrangement Fig. 7, (not shown) lenses 15, 16 are replaced by a single cylindrical lens 38 which is mounted so as to be adjustably rotatable about the optical axis 11 as also shown in Fig. 8. Again, lens 38 may be selected from a set or be replaced by a set of parallel similar lenses. In another arrangement (Fig. 8) both optical objects are material objects 1, 820 illuminated by a laser 3. In setting up, objects 1, 820 are in identical states and orientations so that no fringes can be seen. The displacement to be observed is then applied to object 1 and lens 38 is adjusted until clear fringes are seen. Information is given on the interpretation of the observed fringe systems.