WDM(Wavelength Division Multiplexing) systems are popular in fiber optic network because they allow to expand the capacity of the network without laying more fiber. Capacity of a given link can be expanded by simply upgrading the multiplexer and demultiplexer at each end. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network.
WDM wavelengths are positioned in a grid having exactly 100 GHz (about 0.8 nm) spacing in optical frequency, with a reference frequency fixed at 193.10 THz (1552.52 nm). The main grid is placed inside the optical fiber amplifier bandwidth, but can be extended to wider bandwidths. Today’s DWDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation.
Dense WDM (DWDM) uses the same 3rd transmission window (C-band) but with denser channel spacing. A typical DWDM system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. For example, FiberStore provides 50G DWDM Multiplexer Module.
Coarse WDM (CWDM) in contrast to conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs. To again provide 16 channels on a single fiber CWDM uses the entire frequency band between 2nd and 3rd transmission window including both windows but also the critical area. The channels 31, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used.
WDM, DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber, but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. DWDM systems have to maintain more stable wavelength or frequency than those needed for CWDM because of the closer spacing of the wavelengths. In addition, since DWDM provides greater maximum capacity it tends to be used at a higher level in the communications hierarchy than CWDM. These factors of smaller volume and higher performance result in DWDM systems typically being more expensive than CWDM.
DWDM transponders served originally to translate the transmit wavelength of a client-layer signal into one of the DWDM system’s internal wavelengths in the 1550 nm band. Signal regeneration in transponders quickly evolved through 1R to 2R to 3R and into overhead-monitoring multi-bitrate 3R regeneration. One dwdm transponder, with its tunable channel feature, can spare all DWDM channel 10G transceivers. It eliminates the need to purchasing individual transceivers (XFPs/Xenpaks) for each DWDM channel and greatly reduces sparing costs.
Transceivers versus Transponders
Transceivers – Since communication over a single wavelength is one-way (simplex communication), and most practical communication systems require two-way (duplex communication) communication, two wavelengths will be required (which might or might not be on the same fiber, but typically they will be each on a separate fiber in a so-called fiber pair). As a result, at each end both a transmitter (to send a signal over a first wavelength) and a receiver (to receive a signal over a second wavelength) will be required. A combination of a transmitter and a receiver is called a transceiver; it converts an electrical signal to and from an optical signal.There is usually types transceiver based on WDM technology, for example, there is CWDM XENPAK transceiver available.
Transponder – In practice, the signal inputs and outputs will not be electrical but optical instead (typically at 1550 nm). This means that in effect we need wavelength converters instead, which is exactly what a transponder is.
Transponders that don’t use an intermediate electrical signal (all-optical transponders) are in development.