. The light source has a short fiber flylead attached to it to facilitate coupling the source to a system fiber.
. For low coupling loss, this flylead should be connected to system fiber with identical NA and core diameter. 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 power loss is shown interms of fiber numerical aperture (NA).
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| Fig 1 Change in NA as a function of fiber length |
. Numerical aperture at input light acceptance side denoted by NAin. when light emitting area of LED is less than fiber core cross-sectional area then power coupled to the fiber is NA = NAin.
. If the optical powers is measured in long fiber lengths under equilibrium of modes, the effect of equilibrium numerical aperture NAeq is significant.
Optical power at this point is given by,
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 NAeq is important while calculating launched optical power in telecommunication sytems.