This requires an understanding of the construction and specifications of the WDMs. WDM basics WDMs are made by partially melting two parallel fibers, which are brought together and stretched until their cores have the right amount of taper to couple the desired wavelength into and out of the common fiber. At the crucial point for OOB WDMs, all of the 1,625 nm light is coupled into the 1,550 nm fiber. Similarly, all of the 1,625 nm light entering through the common port is coupled to the 1,625 nm port. In reviewing WDM specifications, there are three key parameters that must be considered, as shown in Figure 2 .
Insertion loss is the attenuation in decibels for a particular path through the device.
Generally, WDMs will specify the insertion loss between each wavelength-specific port and the common port. If the WDMs will be connectorized and patchcords used to place them into the link, the connector and patch cord insertion losses must be added to the overall loss budget. Measured in decibels, it is expressed as an attenuation value of the undesired wavelength. Typical values for high-isolation WDMs, which must be used for OOB testing on live fibers, are around 50 dB, indicating that 1,550 nm light entering the common port will be attenuated by 50 dB before exiting the 1,625 nm port. Usually, each path has its own isolation value.