JNTU B.Tech III OPTOELECTRONICS AND LASER INSTRUMENTATION Previous Question Paper (August/September - 2021 - R18)

An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis through the principle of total internal reflection. It consists of a central core with a higher refractive index (n1) surrounded by a cladding with a lower refractive index (n2). Light is confined within the core as long as the angle of incidence at the core-cladding interface exceeds the critical angle.

The modes of operation refer to the number of paths or electromagnetic field distributions that light can take through the fiber, determined by the normalized frequency (V-number):

• Single-mode fiber: Characterized by a small core diameter (typically 8-10 μm) where V < 2.405. Only the fundamental mode (LP01) propagates, which eliminates modal dispersion and allows for high-bandwidth, long-distance communication.
• Multi-mode fiber: Features a larger core diameter (typically 50-62.5 μm) where V > 2.405. This allows many modes to propagate simultaneously, leading to modal dispersion, which limits the bandwidth and transmission distance.

Pulse broadening is the phenomenon where a light pulse spreads in time as it travels along an optical fiber. This effect limits the information-carrying capacity of the fiber, as overlapping pulses at the receiver cause inter-symbol interference (ISI).

In graded-index (GRIN) fiber, the refractive index decreases parabolically from the center of the core toward the cladding. This design significantly reduces pulse broadening compared to step-index fiber through the following mechanism:

1. Light rays traveling further from the axis (higher-order modes) move through regions of lower refractive index, where the speed of light is higher.
2. Light rays traveling near the axis (lower-order modes) move through the higher refractive index center, where the speed of light is lower.
3. Because higher-order modes travel a longer physical path but at a higher average velocity, and lower-order modes travel a shorter path at a lower velocity, all modes arrive at the receiver at nearly the same time.

This equalization of travel times for different modes minimizes modal dispersion, resulting in a much narrower pulse width at the output compared to multi-mode step-index fibers.