What term describes the widening or spreading out of the modes in a light pulse as it travels through optical fiber?

The correct answer is dispersion. Dispersion in optical fiber refers to the phenomenon where different modes of light or different wavelengths of light travel at varying speeds, causing the light pulse to spread out over time. This effect can lead to signal degradation because as the light pulse widens, it can overlap with adjacent pulses, resulting in a potential data loss or reduced clarity in communication.

Each mode of light in a multi-mode fiber can be subject to different propagation speeds due to the refractive index profile of the fiber. In single-mode fibers, chromatic dispersion occurs when different wavelengths travel at different speeds, causing the signal to spread as it travels. Effective management of dispersion is crucial for maintaining high-speed communications in fiber optic networks and ensuring that the integrity of the signal is preserved over longer distances.

The other terms in the question describe different optical phenomena: refraction is the bending of light as it passes through different media, interference is a pattern formed when two or more waves overlap, and diffraction is the bending of light around obstacles or through apertures. While these terms relate to light behavior, they do not accurately describe the spreading out of light pulses in optical fibers.