Question

In a step index fiber the refracitve indices of core and a cladding are $1.5$ and $1.431$ respectively at wavelength of $900nm$ is coupled to the optical fiber. The LED assumed to be Lambertian source, the efficiency of coupling LED light to the fiber is (The fibre is surrounded by air)

• A. $20.97\%$
• B. $15.97\%$
• C. $11.97\%$
• D. $44.97\%$

Answer 1

The correct option is D $44.97\%$

$N.A=\sqrt{n_1^2-n_2^2}$
$=\sqrt{(1.5{)}^2-(1.431{)}^2}$
$N.A=0.4497$
Now the N.A is nothing but the light gathering ability of the optical fiber i.e., efficiency
Coupling efficiency $\left(\eta \right)=0.4497\times 100$
$=44.97\%$