Hi Manoj,
A good quality transmission with WDM systems goes with larger channel spacing, to be careful to
not excessively increase to avoid nonlinear effects and There is a logical increase of the quality factor.
we increase the channel spacing more the link quality get better and the
BER decreases.
Conventional WDM and DWDM uses increased channel spacing to allow less sophisticated and thus cheaper transceiver designs.
The small channel spacing allows to transmit simultaneously much more information.
The channel spacing, in GHz, relates to the optical wavelength as follows: A spacing of 200 GHz corresponds to about 1.6 nm, 100 GHz corresponds to about 0.8 nm, and 50 GHz corresponds to about 0.4 nm channels spacing. Most commonly 50 GHz follows 100 GHz, although attempts at 75 GHz and 37.5 GHz show up in literature. While there is nothing magical about any of these numbers, it seems likely that 50 GHz will be the next logical step below 100 GHz. Using a channel spacing of 50 GHz (0.4 nm) allows 45 channels to occupy only 17.5 nm of optical bandwidth. This greatly simplifies the requirement for optical amplifiers in the system. Fiber increases in channels per fiber would likely lead to the use of 25 GHz spacing. Designing the optical demultiplexer to separate the signals at the receive end defines the greatest challenge in closely spaced optical channels.

Regards,
Rajguru