Single strand fiber transmission use a single strand of glass (optical fiber) to send data in both directions, namely bidirectional (BiDi) transmission. In recent years, the mainstream single strand fiber transmission technology is based on two wavelengths traveling in opposite directions (also called TW BiDi transmission). This technology is achieved via wavelength division multiplexing (WDM) couplers, also known as diplexers, which combine and separate data transmitted over a single fiber based on the wavelengths of the light. Generally, this WDM coupler is integrated into a standard interface optical transceiver module.
Actually, in addition to the two wavelengths BiDi transmission, the single wavelength (SW) BiDi solution was hot when the fiber resource was rare and 1550nm DFB laser was expensive. It is based on single wavelength directional coupler technologies which allows the same wavelength (e.g., 1310 nm for up to 50 km or 1550 nm for longer distances) travels in Tx and Rx direction—two signals are coupled into a single fiber strand with a directional coupler (splitter-combiner). Then the coupler identifies the direction of the two signals (ingress or egress) and separates or combines them. This solution is normally very reliable and cost effective for gigabit applications since they need to deploy only one kind of transceivers at 1550nm (or 1310nm). However, the SW BiDi implementation could not support high bit rate because of the reflection noise.