DWDM tutorial | Tutorial on DWDM basics
This DWDM tutorial covers DWDM basics including DWDM transmitter and receiver. The tutorial on DWDM also describes optical fiber basics, optical amplifier(EDFA) and other system components.
DWDM stands for Dense Wavelegth Division Multiplexing. It is basically optical multiplex technique. This helps combine multiple discrete transport channels of different wavelengths and transmit them on a single optic fibre. For example, if we multiplex 32 signals of STM-1 level, then optic fibre capacity will be increased from 2.5Gbps to 80 Gbps.
Figure depicts transmitter and receiver of a DWDM system. Information signals in the form of binary data are converted into corresponding wavelengths of light. These are later multiplexed using coupler and multiplexer devices as shown. Optical isolator is used to minimise back reflection. This information is amplified using EDFA before being transmitted over single optic fibre cable. As signal need to be travelled 50-100 km and in order to withstand path loss amplification to the signal is a must.
Optical circulator along with FBG is employed to add or to drop some channels. WDM is used if we need to combine 1550nm signals with 1310nm signal. At the receiver demultiplexing is carried out and 1310nm signal is extracted from the combined signals.
At the receiver DWDM demultiplexer is employed to seperate out all os thr 1550nm signals.
Optic fibre
Due to availability of optical amplifiers at 1550nm and less attenuation at 1550nm DWDM systems operates at C band. The C band ranges from 1530 to 1565 nm. Due to higher dispersion conventional fibres are not used at these wavelengths. Hence dispersion compensating fibres or zero dispersion fibres as per ITU G.653 are used. ITU G.655 compliant fibre has been used due to lower dispersion and high speed of transmission over long distance.This dispersion is such that it can suppress nonlinearity(four wave multiplexing). Optic cables as per ITU-G.652 is used for access network while ITU G.655 is use for backbone network. Most of the systems use combination of both.
Optical amplifier(EDFA)
In DWDM system, post amplifier is used at the transmitter to boost the signal while pre-amplifier is used to boost the signals at the receiver side. EDFA is used as in-line amplifier before signal gets completely degraded. EDFA is mainly used to boost the signal in the 1530nm to 1570nm spectrum region.
With EDFA signal need not to be converted back to electrical form before being regenerated. EDFA uses laser source of either 980 nm or 1480 nm wavelength and amplification of about 30dB can be achieved.
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