WDM systems I

Module 19 : WDM Components

Lecture : WDM Components - I


	$\includegraphics[width=\textwidth]{wdm3}$
	The figure shows a demultiplexer using a blazed reflection grating. Light consisting of a mixture of different wavelengths enters a GRIN lens which collimates the beam to fall on the grating. After reflection, the components are spatially separated and focussed by the lens as outputs to fibers carrying different wavelengths. Multiplexers may also be made using interference filters . Optical filters can be designed using various techniques, the cheapest being deposition of thin films of varying refractive indices on a substrate. When incident light falls on such a material, it encounters stacks of boundaries which produces constructive interference for some wavelengths and destructive interference for some others. As the number of sacks increases, the resolution becomes better and the band of selected wavelengths becomes narrower. The essential difference between filters based on reflection gratings and interference filters is that gratings selectively reflect a narrow range of wavelengths while the interference filters transmit a narrow range of wavelengths.
	In the figure, a demultiplexer using two optical filters $F_1$ and $F_2$ and a GRIN-lens-mirror assembly is shown. The Filter $F_2$ is attached at the end of the lens. The input fiber and the output fibers (shown only as light path) are at the focal plane of the lens. Filter $F_1$ is between the fibers and the lens.