1. The coupling efficiency is defined as a measure of amount of power emitted from an optical source that can be coupled into a fiber.

   $\therefore$ Coupling efficiency

   $ɳ_c = \frac{P_{fiber}}{P_{source}}$

   where

   $P_{fiber}$ is the power coupled into the fiber

   $P_{source}$ is the power emitted from source

1. The coupling efficiency also depends on the type of fiber.

2. Many suppliers offer optical sources along with a short fiber already coupled to the source.

3. This small section of fiber that comes along with the source is called as “flylead” or “pigtail”.

4. case 1:

   If the source radius (rs)< fiber core radius (a) then

   $$ɳ_c = \frac{P_f}{P_s} = (NA)^2$$

   $$NA = sin θ_a$$

   $P_s$ = power emitted from the source

   $= π^2.r_s^2.B_0$

   $B_0$ is the Axial radiance

6. Case II:

$r_s\gta$

$P_f =(\frac{a}{r_s} )^2.P_s .(NA)^2$