Single-Fiber CWDM Mux Demux Tutorial

In CWDM networks, dual-fiber CWDM Mux Demux is commonly used with duplex fiber patch cable to achieve bidirectional transmission applications. The wavelengths transmitted on these two optical fibers are the same but operate in different directions. However, what would you do when you need bidirectional transmission but only have one optical fiber available? In such a scenario, a single-fiber CWDM Mux Demux can be a good choice. The following will provide a detailed introduction to what a single-fiber CWDM Mux Demux is and how it achieves bidirectional transmission.

Understand Single-Fiber CWDM Mux Demux

Normally, the single-fiber CWDM Mux Demux has only one simplex line port, which is the biggest difference from the dual-fiber CWDM Mux Demux on the appearance. However, there are also some single-fiber CWDM Mux Demux made with duplex ports. As to this kind of CWDM fiber optic multiplexers, only one port is in use, and the other is usually marked with N/A and is unavailable. For instance, FS FMU single-fiber CWDM Mux Demux uses such a design.

Working Principle of Single-Fiber CWDM Mux Demux

So why can a single-fiber CWDM MUX/DEMUX achieve bidirectional transmission? The reason lies in the fact that it utilizes CWDM wavelengths differently compared to the dual-fiber CWDM MUX/DEMUX. As shown in the diagram below, in a dual-fiber CWDM network, the wavelengths at both ends of the MUX are the same and transmitted in two opposite directions. In a single-fiber CWDM network, however, signals are transmitted on both ends through a single optical fiber. To avoid interference between signals, the two ends of the single-fiber MUX must use different wavelengths, typically adopting adjacent wavelengths. This is the fundamental difference between single-fiber MUX and dual-fiber MUX in achieving bidirectional transmission.

The picture below shows how CWDM wavelengths are used in a single-fiber CWDM network. In this network, 8 wavelengths are used to support 4 pairs of bidirectional transmission. On site A, an 4 channel CWDM mux demux is deployed using 4 wavelengths for signal transmitting and the other 4 wavelengths for receiving. On the opposite site B, another single-fiber CWDM Mux Demux is deployed. However, for the CWDM MUX DEMUX at both sites, the wavelengths for TX (transmitting) and RX (receiving) are reversed. For instance, a pair of bidirectional transmission signals use 1270nm for TX and 1290nm for RX on site A, while use 1290nm for TX and 1270nm for RX on site B. This is how the single-fiber CWDM Mux Demux achieves bidirectional transmission.

How to Choose Optical Transceivers for Single-Fiber CWDM Mux Demux

Since the channel ports on a single-fiber CWDM Mux Demux support two different wavelengths, some people may be confused about how to select a CWDM transceiver for a single-fiber CWDM network. In fact, the transceiver selection of single-fiber CWDM Mux Demux is mainly based on the TX wavelength. For example, if one duplex port uses 1270nm for TX and the other uses 1290nm for RX, then that port should use a 1270nm CWDM transceiver. At the other end of the link, a 1290nm CWDM transceiver is required.

In addition, wavelength compatibility should also be considered. Ensure that the optical transceivers' wavelengths match the CWDM channels used by the Single-Fiber CWDM Mux Demux. Each channel corresponds to a specific wavelength, so the transceivers at each end of the link should use wavelengths that align with the Mux Demux configuration.

If you would like to learn more detailed information about module selection, you can refer to "Decoding CWDM and DWDM SFP+: A Comprehensive Buying Guide".

Conclusion

From all the above, single-fiber CWDM Mux Demux should be used in pairs and it transmits and receives the integrated signals through the same fiber. In addition to CWDM Mux Demux, there are various CWDM transceivers including CWDM SFP/SFP+/XFP available in the market and they can be customized to support the whole network running. One should choose proper single-fiber Mux Demux and its accessories like optical transceivers according to the wavelengths and specifications needed.