Single-Fiber CWDM Mux Demux Tutorial

In CWDM networks, bidirectional or dual-fiber CWDM Mux Demux is often used in dual-way transmission applications. It uses the same wavelengths for transmitting and receiving optical signals on both ends. The wavelengths of the two fibers are the same but running in different directions. However, in some cases, there is only one fiber or wavelength available for signal transmitting and receiving. Under such circumstances, single-fiber CWDM Mux and Demux is being used. This post will give details on single-fiber CWDM Mux and Demux tutorial.

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. Since it is a single-fiber CWDM Mux Demux, only one port of the duplex ports is in use, the other is usually marked with N/A and is unavailable. For instance, FS FMU single-fiber CWDM Mux Demux uses such a design.

Figure 1: FS FMU Single-Fiber CWDM Mux Demux

The single-fiber CWDM Mux Demux can achieve dual-way transmission like dual-fiber CWDM Mux Demux as well. In a bidirectional CWDM network, each wavelength runs in two opposite directions. While, in a single-fiber CWDM network, each wavelength just runs in one direction. In other words, if one want to build a dual-way transmission link between two sites, just use the same one wavelength over duplex fiber with dual-fiber CWDM Mux Demux, or use two wavelengths (one for TX and the other for RX) over simplex fiber with single-fiber CWDM Mux Demux.

Working Principle of Single-Fiber CWDM Mux Demux

The picture below shows how CWDM wavelengths are used in a single-fiber CWDM network. In this network, 16 wavelengths are used to support 8 pairs of dual-way transmission. On site A, an 8-channel single-fiber CWDM Mux Demux is deployed using 8 wavelengths for signal transmitting and the other 8 wavelengths for receiving. On the opposite site B, another single-fiber CWDM Mux Demux is deployed. However, the wavelengths for TX and RX are reversed. For instance, a pair of dual-way signals use 1270 nm for TX and 1290 nm for RX on site A, while use 1290 nm for TX and 1270 nm for RX on site B. This is how the single-fiber CWDM Mux Demux achieves dual-way transmission.

Figure 2: Working Principle of Single-Fiber CWDM Mux Demux

How to Select Fiber Optic Transceivers for Single-Fiber CWDM Mux Demux

Some might get confused about how to select the CWDM transceivers for single-fiber CWDM networks as there are two different wavelengths on a duplex channel port. The transceiver selection for single-fiber CWDM Mux Demux is mainly based on the wavelength for TX. The fiber optic transceivers used for single-fiber CWDM Mux and Demux are different on the two sites.

The following picture shows how to build a 10G single fiber CWDM network. As clearly showed, all the wavelengths just go in one direction. CWDM SFP transceivers working on 1470 nm, 1510 nm, 1550 nm and 1590 nm are linked with the CWDM Mux and Demux on one side of the network. The other CWDM Mux Demux deployed on the other end of the network is connected with CWDM SFP transceiver working on wavelengths of 1490 nm, 1530 nm, 1570 nm, 1610 nm. Thus, eight wavelengths are used for 4 pair dual-way transmission in this single-fiber CWDM network.

Figure 3: How to Build a 10G Single-Fiber CWDM Network

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.