Power splitters and couplers have important military and commercial telecommunications and optical applications. The most common power splitters based on optical fiber are directional couplers. Fused-fiber directional couplers propagate two modes along multiple optical fibers. One of the two modes is an even spatial function. This mode is also the lowest order mode. The other mode is an odd spatial function. Although the light that enters the coupler may be single mode, two modes evolve as the light travels along the coupler, and the modes interfere in the coupling region. If the length or temperature of the coupling region of the coupler changes, or the polarization or wavelength of the light traveling the coupler changes, interference between the modes will change. Consequently, the spectral shape of the light input into the fiber and/or the power split ratio (the ratio of output intensity from a first channel to the output intensity of a second channel) will change as well, which inhibits the performance and reliability of the signal output from the coupler.
Polished fiber directional couplers utilize evanescent coupling between adjacent waveguides. In addition, three port Y-splitters are planar waveguides that use a single waveguide split into multiple paths. Planar Y-splitters suffer from power split ratios that are sensitive to the polarization state of the incoming light. Planar Y-splitters also do not preserve the spectral profile, nor the polarization state of the input light.