ABSTRACT
A wavelength multiplexer is a device that can combine different wavelength signals from plural light sources and output them to a single optical fiber transmission line in a wavelength division multiplexing (WDM) system with no excess loss. A wavelength demultiplexer is a device that can divide a multiplexed signal incorporating different wavelengths, and output the result to plural optical receivers. WDM systems are categorized into three types based on their wavelength channel spacing. The simplest is the bi-directional type. The second type is the coarse WDM in which the channel spacing is 20 nm. The third category is dense WDM where the channel spacing is about 0.8 nm. A thin film filter has a multilayered structure composed of alternately deposited high and low refractive index films. Compared with other technologies, thin film filters have advantages such as a flat spectral response in the vicinity of the pass wavelength, a low insertion loss, and a low polarization dependent loss.
