A Fiber Coupler is an Optical Fiber device with one or more input fibers and one or several output fibers. Light from an input fiber can appear at one or more outputs, with the power distribution potentially 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 intimate contact.

If all involved fibers are single-mode (i.e., support only a single Mode per Polarization direction for a given wavelength), 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 which can combine two inputs at different wavelengths into one output without exhibiting significant losses. Such couplers are used e.g. in fiber amplifiers to combine the signal input and the pump wave. Other wavelength-sensitive couplers are used as multiplexers in wavelength division Multiplexing (WDM) telecom systems to combine several input channels with different wavelengths, or to separate channels.