Abstract: A high stability interferometer is capable of continuous measurement of small changes in the refractive index of a sample. The interferometer has a diffraction grating placed to be movable sideways or radially, and diffract laser light into beams which include +1, -1 and zero order beams. The diffraction grating is in the input plane of a Fourier Transform lens. The beams are brought to a focus in the output plane of the lens and are reflected back towards the lens by a mirror placed in the transform plane of the lens. The sample whose refractive index is to be measured is placed in the path of the +1 or -1 order beam, between the diffraction grating and the mirror.

Abstract: An optical sensor with intermodal interferometry, using a bimodal optical fiber propagating a first mode and giving rise to a second mode coupled to the first in response to a disturbance cause by a physical or chemical quantity of which it is wished to determine the position along the optical fiber by compensating the difference in the delay time between modes at the input of the interferometer.

Abstract: An interferometric device for the detection of substances having a periodic or quasiperiodic absorption structure of their absorption spectrum includes a source of radiation, a chamber containing the material to be investigated and a downstream detector, a thermo-optically effective and temperature controlled interference filter means is interposed, its thickness determines the spacing of interference transmission lines and temperature control shifts these lines.