Abstract: A single mode fiber optic multiplexer/demultiplexer has first and second optical fibers, each having an input and an output end. The first and second optical fibers are single mode fibers each having a core of a predetermined diameter and a cladding about the core of a predetermined diameter. Along a predetermined length two fibers are placed in juxtaposition and the claddings are fused together to form a fiber optic coupler. Previous to being fused, the claddings are reduced by etching. The coupling of the fiber optic device is such that a first light having a first predetermined wavelength and a second light having a second predetermined wavelength enter together on an input of one of the fibers and are substantially split such that substantially only the first light will appear on an output of one of the fibers and substantially only a second light will appear on an output of the other fiber.