Abstract: At least three incoming light beams (10a-c) are combined partly combined into a null beam and partly into fringe tracking beams. The relative phase difference between the incoming beams (10a-c) is controlled so that a phase relationship between in the null beam corresponds to destructive interference of the main beam components in the null beam. In the fringe tracker beams this leads at least partly to constructive interference between the main beam components, with an intensity that depends on the phase differences. Measurements of the intensities of the fringe tracker beams are used to control the relative phase differences of the incoming beams (10a-c) to realize destructive interference of the main components in the null beam. To form the null beam successively more of the light beams are combined into the null beam in a series of beam combiners (14a, 14 b).

Abstract: At least three incoming light beams (10a-c) are combined partly combined into a null beam and partly into fringe tracking beams. The relative phase difference between the incoming beams (10a-c) is controlled so that a phase relationship between in the null beam corresponds to destructive interference of the main beam components in the null beam. In the fringe tracker beams this leads at least partly to constructive interference between the main beam components, with an intensity that depends on the phase differences. Measurements of the intensities of the fringe tracker beams are used to control the relative phase differences of the incoming beams (10a-c) to realize destructive interference of the main components in the null beam. To form the null beam successively more of the light beams are combined into the null beam in a series of beam combiners (14a, 14 b).

Abstract: The present invention relates to an achromatic phase shift device (1) for introducing a wavelength independent optical phase shift in a first optical beam (40) during operation, comprising at least one dispersive element (55k) formed by first refractive means (2k) and second refractive means (4k), the first refractive means (2k) having a first refractive means input plane (6) for receiving the first optical beam (40) and a first refractive means output plane (8) being at a predetermined angle (&bgr;) to each other, the second refractive means (4k) having a second refractive means input plane (10) and a second refractive means output plane (12) being positioned parallel to the first refractive means input plane (6). The device may comprise further pairs (55k) of respective first and second refractive means (2k, 4k). The present invention also relates to an interferometer comprising at least one achromatic phase shift device.