A fiber optic coupler is a device used in optical fiber systems with one or more input fibers and one or several output fibers. Lightentering an input fiber can appear at one or more outputs and its spectral power distributionpotentially depending on the wavelength and polarization. Such couplers can be fabricated in different ways, for example by thermally fusing fibers so that their cores get into intimatecontact. If all involved fibers are single-mode, there are certain physical restrictions on the performance of the coupler. In particular, it is not possible to combine two or more inputs of the same optical frequency into one single-polarization output without significant excess losses. However, such a restriction does not occur for different input wavelengths: there are couplers that can combine two inputs at different wavelengths into one output without exhibiting significant losses. Wavelength-sensitive couplers are used as multiplexers in wavelength-division multiplexing telecom systems to combine several input channels with different wavelengths, or to separate channels.