Optical thin-film filters are essential components of current optical communications systems because they transmit specific wavelengths while rejecting others. These components consist of a multilayer stack of thin films placed on a glass substrate. Optical interference theory underpins the operation of an optical filter coating.
The diverse filter functions of transmitting, reflecting, or blocking light in certain wavelength bands depend on the design and manufacturing of the coatings. The filter coating function follows the wavelength grid standards defined by the ITU or IEEE. It also fulfills other requirements in some applications, such as wavelength division multiplexing (WDM), optical add-drop multiplexing (OADM), and passive optical networks (PON). Specific requirements drive the customization of the filter performance.
A filter for optical communications systems is typically in the range of 1 to 3 mm in size. Before processing to the final size, coating technicians deposit the filter on a massive substrate or wafer.