Passive coarse wavelength division multiplexing (CWDM) is a method of multiplexing (mux) and de-multiplexing (demux) optical signals over fiber optic transmission cables. It is considered a cost effective method for scaling connections over existing fiber infrastructure for point-to-point (P2P) or point-to-multipoint (P2M) connections for add/drop or fiber ring applications. Both passive CWDM cassettes and barrel filters are useful in a wide range of applications that benefit from the aggregation and separation of fiber channels. These applications include telecommunications, R&D< test lab, research and mil/aero installations, among other applications that require extensions of existing fiber infrastructure.
Passive SWDM cassettes are commonly available for mux-demux of 4, 8, and 16 channels, with single channel barrel filters that can add to a composite signal or remove channel(s) from a composite signal. These cassettes and barrel filters can be made for insight plant (ISP) or outside plant (OSP) applications.
Compared to dense wavelength division multiplexing (DWDM) of fiber optical technology, CWDM is considered a lower cost, footprint, and power dissipation technology. This is why CWDM technology is being used to extend the life of legacy fiber optic network equipment and delay expensive upgrades to the infrastructure while still providing enhanced throughput. The main reason CWDM provides these benefits over DWDM is that CWDM channels are separated by 20 nanometer intervals, instead of the 0.4 nm to 1.6 nm spacing used with DWDM devices. However, CWDM signals aren’t amplified, so the range of CWDM technology is limited to tens of kilometers as opposed to the much longer distances achievable by DWDM. For many applications that aren’t as range sensitive, the cost savings and reduced footprint of CWDM compared to DWDM are appealing.
As the distributed feedback (DFB) lasers thermal cycle, DWDM systems require thermal compensation to account for the thermal drift. CWDM doesn’t require a thermoelectric cooler to provide compensation for thermal drift. Hence, CWDM devices can be realized with lower cost wideband optical filters, and the 20nm channel spacing is wide enough to allow for laser wavelengths to cycle over an operating temperature of 70 degrees Celsius.
Several advances in optical fiber technology are also likely to benefit future CWDM devices. These advances include full spectrum laser sources with acceptable dispersion and power while exhibiting minimal thermal drift. Enhancing backscatter tolerance using low-isolation techniques that rely on uncollimated optics, are another area of potential improvement. This technology is less expensive to implement. Lastly, enhancing coupling efficiency with laser sources with better divergence symmetry is another area of potential future improvement of CWDM technology.
