also read about V no
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjU9Prbm8DMAhWQHY4KHV_BD5wQFggxMAM&url=http%3A%2F%2Fwww.pefiberoptics.com%2Fmeasurement-solutions%2Feffective-area-aeff&usg=AFQjCNHib1lPEF75v7W6rr4ml5S-Zjn9Lw&sig2=DG_CMRGKYHNINzSTPSZS0A
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&uact=8&ved=0ahUKEwjU9Prbm8DMAhWQHY4KHV_BD5wQFghEMAY&url=http%3A%2F%2Fwww.corning.com%2Fmedia%2Fworldwide%2Fglobal%2Fdocuments%2Fsfiber%2520WP7071_Mode%2520Field%2520Diam%2520and%2520Eff%2520Area_10-01-3.pdf&usg=AFQjCNHZG3fO4zRBx0Q_uwGuJVWcwdZhjQ&sig2=ys7EWdGZA7u2UQ14qkrItQ
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&cad=rja&uact=8&ved=0ahUKEwjU9Prbm8DMAhWQHY4KHV_BD5wQFghbMAk&url=http%3A%2F%2Ffobasics.blogspot.com%2F2012%2F07%2Feffective-area.html&usg=AFQjCNEUvLBecnl6bnE3ri-CQ2t-rp_ibQ&sig2=OWo-KPcyWTp6ntYeHJaRbQ