A Mach-Zehnder device includes a delay element in one arm which delays propagation of light at a first wavelength relative to propagation of light at a second wavelength. The delay element may be made up of a grating having a period which causes coupling of the first wavelength from a first mode into a second mode. In another aspect, the invention includes a second grating positioned downstream from the first grating and having a period which causes coupling of the portion of light from the second propagation mode back into the first propagation mode. Also disclosed is an optical waveguide in which light at a wavelength .lambda. can propagate in at least a first and a second mode. The waveguide has an effective index of refraction n.sub.1 with respect to the first propagation mode of .lambda., and an effective index of refraction n.sub.2 with respect to the second propagation mode. A grating formed in said waveguide and having a period approximately equal to(.lambda./2)(n.sub.1 -n.sub.2).