Capacity of WDM Fiber Optic Communication Systems

A better utilization of the transmission capacity of an optical fiber can be obtained with the wavelength division multiplexing (WDM) technique. With this technique multiple optical channels, each operating at a different wavelength, are combined on a single fiber by an optical multiplexer and then they are transmitted on the same fiber. At the receive side the multiple signals are demultiplexed by an optical demultiplexer into separate optical channels.
 
 
Multichannel Wavelength Division Multiplexing (WDM) Systems

 

The ultimate capacity of a WDM fiber system depends on how closely optical channels can be packed in the wavelength domain. In the case of dense WDM (DWDM) applications optical channels are very densely spaced. For this case of DWDM applications the so-called “channel spacing” is expressed in the frequency domain. The maximum number of channels in a DWDM system operating in a given spectrum band (e.g. 1530-1565 nm for the C-band) is determined by the channel spacing.

Definition of Channel Spacing

The channel spacing is defined to be the nominal difference in frequency or wavelength between two adjacent optical channels. The minimum channel spacing is limited by interchannel crosstalk and it is related to many factors: the channel bit rate, the modulation format, the filter passband, and central wavelength variations (due to laser manufacturing and laser temperature variations).

Channel Spacing in DWDM, CWDM and WWDM Fiber Optic Systems

There are three categories of WDM systems:

Coarse WDM (CWDM)

CWDM have a channel wavelength spacing less than 50 nm, but greater than 1000 GHz (about 8 nm at 1550 nm and 5.7 nm at 1310 nm). The value of “c” (speed of light in vacuum) that should be used for converting between frequency and wavelength is 2.99792458 × 108 m/s; Devices within this class can cover several spectral bands.

Dense WDM (DWDM)

DWDM have a channel spacing less than or equal to 1000 GHz; Devices within this class can cover one or more spectral bands.

Wide WDM (WWDM)

WWDM have a channel wavelength spacing greater than or equal to 50 nm. This device class typically separates a channel in one conventional transmission window (e.g. 1310 nm) from another (e.g. 1550 nm).
 

 

 

Channel Spacing in DWDM, CWDM and WWDM Fiber Optic Systems

 

 

 
The “Conventional” C-band defined above has a spectrum range of 35 nm (1530-1565 nm, operating range of conventional optical fiber amplifiers), which exceeds 40 THz (0.8 nm = 100 GHz; 1 THz = 1000 GHz)
 
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