Corephy's 1x2 Single Fiber DWDM Splitter Red/Blue C Band Filter is a thin-film filter device. It is a three-port device. One port is called the "Common." The other two ports provide the conduit for the two wavelength "bands." 

The two bands are  the Red (λ>1547 nm) and the Blue (λ<1543 nm) . One band goes thru the Reflected leg, and the other band goes thru the Pass leg.

Specification

Red/Blue C Band Filter in DWDM Single Fiber Solution

The wavelengths traveling over the dark fiber are split into two spectral bands by using Red/Blue C Band Filter at both ends of the dark fiber. It makes it possible to utilize single fiber strand for both transmitting and receiving, thus significantly maximizing investment and decreasing costs.