WDM filters provide access to all wavelength channels, where required. In Optical Termination Multiplexers (OTMs), all channels are terminated. Between two OTMs, use of a subset of channels are usually necesary. This leads to the concept of Optical Add/Drop Multiplexer (OADM). OADMs provide use of a subset of WDM channels; the other channels are transparently undergone. This avoids costly termination of WDM channels that aren’t required at intermediate sites.
OADMs derive from passive static WDM filter or Reconfigurable OADM (ROADM). ROADMs use wavelength-selective switching devices. Static OADM filters can be configured in a single stage or in up to three consecutive stages. The latter allows modular OADM design.
Different technology is used for passive WDM filters, depending on performance, port-count, and price requirements. Many modular DWDM Filters and just about all CWDM optics (e.g., CWDM MUX/DEMUX and CWDM Filter) derive from the Thin-Film Filter (TFF) technology. In a TFF, thin dielectric, wavelength-selective layers are put on a substrate. Light can traverse the substrate through the effect of total internal reflexion or it is coupled/decoupled in the corresponding thin-film areas. Wavelength decomposition is provided by spatial decomposition and individual add/drop wavelengths are available via dedicated fibers.
Alternatives to TFF include Fiber Bragg Grating (FBG) and Arrayed Waveguide Gratings (AWG). FBGs are periodic density variations inside fibers. These density variations are generated by UV laser radiation of the respective fiber sections, plus they can be chirped, that is, continuously changing from one wavelength to the next. In an OADM, the gratings are tuned to dedicated add/drop wavelengths; these wavelengths will be reflected rather than being transmitted. FBGs can be combined with fiber interferometers or circulators in order to separate the transmission directions.
AWGs are compact, single-stage filters that are implemented in planar waveguide technology. As OTM filters, they can provide use of a high quantity of wavelengths ranging from 32 to 96. They consist of Free Propagation Regions (FPRs) and waveguides interconnecting the FPRs. The waveguides have different path lengths leading to constructive or destructive interferences in the output FPR, and therefore multiplexing/demultiplexing. AWGs are cyclic filters. Different wavelengths, spaced at the free spectral range (FSR) from the filter, are accessible at the same output ports.
AWGs are utilized in static point-to-point configurations like high-capacity SAN and in certain ROADM configurations. The parameters of WDM filter technologies are shown in the table below.
Here are some pictures of the WDM Filters from Fiberstore.
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