Hi Aasif sir,
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.
According to you “lambda= velocity of light/frequency=3000000000/193.1THz=1553.6nm (approx) but when changed laser frequency units from THz to nano meter(nm) …. to my surprise it shows 1552.524381149663nm”
The difference is nearly 0.8 nm.
It is due to channel spacing because by default channel spacing is 100 ghz(0.8 nm).
http://www.fiberoptic.com/adt_dwdm.htm