Abstract: An optical fibre interferometer particularly adapted to provide a light output that is modulated in relation to the intensity of an electromagnetic field includes a single-mode fibre of substantially non-electrooptic material (20, 22 and 24) to which has been spliced (at 26 and 28) a single-mode fibre of substantially electrooptic material (30). When the interferometer is placed in an electromagnetic field, the optical characteristics of the substantially electooptic fibre are varied, but the optical characteristics of the substantially non-electrooptic fibre are not. As a result, the light passing through the substantially non-electrooptic and electrooptic fibres undergoes a differential phase shift, causing interference upon light recombination that provides the desired light output modulation. Embodiments in which the fibre of substantially electooptic material is replaced by an optical waveguide of substantially electrooptic material, either formed separately (50, FIG. 3) or directly (70, FIG.