Equilibrium Numerical Aperture  

• The light source has a short fiber flylead attached to it to facilitate coupling the source to a system
• The low coupling loss, this flylead should be connected to system fiber with identical NA and core At this junction certain amount of optical power approximately 0.1 to 1 dB is lost, the exact loss depends on method of connecting. Also excess power loss occurs due to non propagating modes scattering out of fiber.
• The excess power loss is to be analyzed carefully in designing optical fiber system. This excess loss is shown in terms of fiber numerical aperture(NA).

• Numerical aperture at input light acceptance side is denoted by NA_in. When light emitting area LED is less than fiber core cross-sectional area then power coupled to the fiber is NA = NA_in.
• If the optical powers of measured in long fiber lengths under equilibrium of modes, the effect of equilibrium numerical aperture NA_eq is significant. Optical power at this point is given by,

                                                                                                   … (4.5.1)

where,

P₅₀ is optical power in fiber at 50 m distance from launch NA.

• The degree of mode coupling is mainly decided by core – cladding index difference. Most optical fibers attain 80 – 90 % at their equilibrium NA after 50 m. Hence NA_eq is important while calculating launched optical power in telecommunication