DWDM stands for dense wavelength division multiplexing.

WDM is a technique that multiple wavelengths are combined/multiplexed together and transmitted through a single optical fiber. Using multiple wavelengths, the overall bandwidth of the signal carried on a single optical fiber is multiplied.

With the improvement of wavelength-selective filter technologies, very narrow multiplexing thin-film filters (as well as other types of filters such as fiber Bragg gratings or arrayed waveguide gratings) are available on commercial markets. This enables the deployment of dense WDM systems. In DWDM systems, the channel/wavelength spacing is reduced to 200GHz, 100GHz or even 50GHz.

DWDM systems currently use C band (1525nm ~1565nm) or L band (1570nm~1610nm), because C and L bands are the operating bands of EDFA. EDFAs are capable of amplifying optical signals directly in optical domain regardless of the signal format/bit rate of each channel. DWDM+EDFA means optical transmission capable of achieving extremely high bandwidth and distance product.

ITU defines the exact wavelength/frequency of each wavelength at 100GHz channel spacing. There are 73 channels from 1519.48nm to 1577.03nm. For 50GHz channel spacing, the total number of channels is doubled.

A DWDM system has optical transmitters at ITU wavelength grid. The transmitters can have 2.5Gbps direct modulation DFB laser diode, or 10Gbps transmitters consisting of DFB laser + external modulator, etc. The wavelength of all transmitters are combined through DWDM multiplexers. After that, all wavelengths are transmitted inside one single fiber. The light may pass through modules such as optical amplifiers, dispersion compensation modules, couplers, isolators, circulators, attenuators, with which the conditions of the wavelengths are modified (amplified, attenuated, split, combined, isolated, redirected, etc., depending on the requirement in the system). At the receiver end, a DWDM demultiplexer is used to separate all wavelengths into individual channels, before each channel is detected by a receiver that demodulates the electrical signals from the optical carrier.

DWDM technology is the key enabling technology for modern high speed internet and telephony networks. DWDM makes it possible to transmit information across the globe at very low cost that is affordable by ordinary consumers.

DWDM has the potential to be able to reach the edge of the optical network and even into people\'s home directly. That will certainly deliver bandwidth that is unthinkable today to end users.