Wavelength Division Multiplexing (WDM) combines multiple optical signals on a single fiber by using different wavelengths (also referred to as “colors”) for each signal. This is particularly useful when minimizing the fiber count is advantageous e.g. when the fiber is being leased. Wavelengths used for optical multiplexing are specified in channel spacing grids issued by the International Telecommunication Union (ITU).
Type of multiplexing – CWDM and DWDM
The channel spacing between wavelengths determines the type of multiplexing. The narrower the channel spacing, the more signals that can be combined in a single fiber. A channel spacing of 20nm is known as Course Wavelength Division Multiplexing (CWDM). A channel spacing of 0.4 or 0.8nm allows many more signals to be combined in the same optical bandwidth. This is known as Dense Wavelength Division Multiplexing (DWDM).
Channel spacing versus laser performance
The narrower the channel spacing, the higher the performance requirement for the laser. (This is not usually true for the receiver as it is usually just a wideband receiver that covers the whole frequency band). Since temperature is a significant factor affecting the output of a laser, this means ensuring a very high degree of thermal stability to prevent drift. Therefore the design of a DWDM transmitter is significantly more complex than a single wavelength link.
Operating between 1270nm and 1610nm allows a theoretical maximum of 18 channels. However, high attenuation caused by water peaks mean that not all of these channels are practical. ViaLite equipment supports the combination of 16 signals on a single fiber. A ViaLite 1U CWDM rack unit uses a CWDM laser to combine up to 16 different colors and an identical 1U rack, at the other end of the link, de-multiplexes.
The ITU channel spacing grid for DWDM allows a theoretical limit of 300 channels between 1492.25nm and 1611.79nm. ViaLiteHD supports a practical limit of 80 channels per fiber simply because there is a limit to the power that can reasonably be sent down a fiber (roughly 13dBm as a rule of thumb) without causing non-linearity and signals modulating each other. Sharing this limited power across multiple signals reduces the noise performance of each channel. Therefore there is a point at which the noise performance of an individual channel is no longer acceptable.