Hi Jyoti,
For simple Wdm system we could use EDFA amplifiers in addition to this we should use symmetric compensation technique to get better results as it compensate the losses in fiber caused by dispersion.
In long haul DWDM sytsem we should use hybrid (EDFA+ Raman) amplifier for better results.
We should use different amplifiers according to requirement as both amplifiers have advantage and disadvantages as
Advantages of EDFA
EDFA has high pump power utilization (>50%)
Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550nm region, with a relatively flat gain
Gain in excess of 50 dB
Low noise figure suitable for long haul applications
Disadvantages of EDFA
Size of EDFA is not small
It can not be integrated with other semiconductor deviecs
Advantages of Raman amplifier
Variable wavelength amplification possible
Compatible with installed SM fiber
Can result in a lower average power over a span, good for lower crosstalk and Very broadband operation may be possible
Disadvantages of Raman amplifiers
High pump power requirements, high pump power lasers have only recently arrived
Sophisticated gain control needed
Noise is also an issue