Abstract: The invention concerns an optical interferometer (1) with two waves defining two paths of different optical lengths and comprising at least a separator periscope (2, 3, 4) forming a Mach-Zender interferometer. The invention is characterised in that the interferometer (1) comprises, on the short path of the interferometer (1), a compensating plate (6) cut in glass of the same type as the separator periscopes (2, 3, 4) and having as thickness the difference of the glass thickness of the separator periscopes (2, 3, 4) traversed respectively by each of the waves. The invention is applicable to wavelength-multiplexed optical fibre transmission devices.