Hi jojo mathew
In the 1970s, manufactures glass had a loss profile which showed a minimum loss at around wavelength of 800 nm (0.8μm). By the time this study of glass was going on, LASERs also were invented which used a semiconductor material named Gallium Arsenide (GaAs) for emission of light. GaAs intrinsically is capable of emitting light of wavelength 800nm. So, coincidently we had glass which had minimum loss at the wavelength that was emitted by LASERs, and so it proved to be great combination. So initial optical communications were started at 800nm wavelength region and hence it is called as the “First Window” of optical communication.
As technology improved, glass was further purified and it showed a region of minimum loss at 1300nm and 1550nm regions.
For further reference refer to following link.
http://nptel.ac.in/courses/117101002/downloads/Lec02.pdf
Regards
Burhan