4. Optical multiplexers/demultiplexers

The function of a fiber optic multiplexer is to combine, on the same output optical fiber, the optical signals carried by N different optical wavelengths (or frequencies), propagating in each of the N input optical fibers.

The function performed by a demultiplexer is reciprocal, and consists in distributing each of the N optical wavelengths (or frequencies) propagating in the input optical fiber over N distinct optical fibers.

The multiplexing function can be performed using optical couplers (in which case insertion losses are high). Demultiplexing involves optical filtering.