Question

The refractive index of the core of an optical fibre is $n_1$ and that of the cladding is $n_2$. If $(n_1-n_2)=\Delta n$, then the fibre can be made single mode with numerical aperture unchanged by

• A. Reducing both core diameter and $\Delta n$
• B. Reducing $\Delta n$ alone
• C. Reducing core diameter alone
• D. Reducing core diameter and increasing $\Delta n$

Answer 1

The correct option is C Reducing core diameter alone

A multimode fiber optic cable can be made single made by either varying the Numerical Aperture (NA) or by reducing the diameter of core, and if we vary $\Delta nNA$ will vary as $NA=n\sqrt{2\Delta }$
Here, since $NA$ is made fixed the fiber can then be made single mode fiber by reducing the core diameter alone, hence option (c) is correct.