A three-arm-Mach-Zehnder interferometer for splitting/combining a first and a second wavelength band, wherein the optical device includes: a first and second optical splitting/combining element; a differential optical delay device comprising a first, a second and a third optical path; each of the first and second optical splitting/combining elements, is of the (25-50-25%)λx(0-0-100%)λy type, wherein λx is a wavelength with the first optical band and λy is a wavelength with the second optical band, and the first, second and third optical paths of the differential optical delay device are configured to introduce, at a wavelength λz within the first optical band, a phase delay Δφ of 2πm.