Question

In a step-index optical fiber, the refractive indices of the core and the cladding are equal to 1.501 and 1.499, respectively, at the wavelength of $850\text{nm}$. The fiber is surrounded by air.

An LED emitting light around wavelength of $850\text{nm}$ is butt-coupled to the optical fiber. If the LED is assumed to be a Lambertial source, then efficiency of coupling LED light to the fiber is

• A. $1.02\%$
• B. $0.57\%$
• C. $0.60\%$
• D. $7.70\%$

Answer 1

The correct option is C $0.60\%$

Coupling Efficiency $=(NA{)}^2=(\sqrt{n_1^2-n_2^2}{)}^2$
Substituting values,
Coupling Efficiency $=({1.501}^2-{1.499}^2)=0.006$
$\Rightarrow$ Coupling efficiency $=0.006\times 100\%=0.60\%$