make sure that MFD of one fiber is near about equal to another one
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFggjMAI&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMode_field_diameter&usg=AFQjCNHFbTkxqUDzTKyQlAdjO24QN3zInw&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFggqMAM&url=http%3A%2F%2Foptiwave.com%2Foptifiber-manuals%2Foptical-fiber-mode-field-diameter-and-mode-area-definitions%2F&usg=AFQjCNGnvUOWi2jbxDbGtrrJRDInDl4CAg&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFgg_MAg&url=http%3A%2F%2Ffibercore.com%2Ffiberpaedia%2Fmode-field-diameter-mfd&usg=AFQjCNGGQiR3u5e2a_683IDEhXRsbrv9XA&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFghLMAo&url=https%3A%2F%2Fwww.ophiropt.com%2Fuser_files%2Flaser%2Fbeam_profilers%2FMFDandSpotSize.pdf&usg=AFQjCNEgL1pGXN7vh3wdiWfSZ5bit7eJyA&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFghaMAw&url=http%3A%2F%2Ffobasics.blogspot.com%2F2012%2F07%2Fmode-field-diameter.html&usg=AFQjCNFMU8VoyGFsBfZm-MRp3tXf7TmXqg&bvm=bv.121421273,d.c2E