An achromatic fiber optic coupler of the type wherein first and second single-mode optical fibers, each having a core and a cladding, are fused together along a portion of the lengths thereof to form a coupling region. A matrix glass of lower index than the fiber claddings surrounds the coupling region. The fiber diameters are smaller in the coupling region than in the remainder of the fibers. The refractive index n.sub.2 of the cladding of the first fiber is different from the refractive index n.sub.2 ' of the cladding of the second fiber, the difference between the refractive indices n.sub.2 and n.sub.2 ' being such that the coupler exhibits very little change in coupling ratio with wavelength over a band of wavelengths.