Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
When you are focussing the source on an optical fibre, you want to collect as much light as possible and you will need a high NA. I am not going to make a drawing but I hope this is clear. Fibre is optimum carrier for long distance communication.
When you are in open air, You will try to make a parallel beam for this purpose you will need a small NA; high NA will not focus well. This is useful only for a short distance like a remote control.