Short answer: Usually yes, you use them in pairs, but the “pair” can be a media converter on one end and a fiber switch (or SFP in a switch) on the other, as long as both sides speak the same speed, wavelength, and optical mode. Single fiber modules (BiDi) use one fiber for both transmitting and receiving data. They use a thin fiber. Should you use a single strand (BiDi) or two strands? Do converters need to be used in pairs? Can you mix brands? What wavelengths matter? This guide answers it all with clear diagrams, step-by-step checklists, and field-tested troubleshooting tips. It uses WDM technology to realize the bidirectional transmission of optical signals on one optical fiber. Understanding the compatibility constraints prevents costly downtime and troubleshooting.