How does the refractive index profile differ between graded-index and step-index multi-mode fiber, and what is the impact on signal propagation?

The key difference between graded-index and step-index multi-mode fiber lies in their refractive index profile, which significantly impacts signal propagation. The refractive index is a measure of how much light bends when it passes from one medium to another. Step-index multi-mode fiber has a core with a uniform refractive index. This means the refractive index is constant throughout the entire core and then abruptly changes at the boundary between the core and the cladding, creating a "step." Light traveling through step-index fiber reflects off the core-cladding boundary at different angles, resulting in varying path lengths and travel times for different modes. This difference in travel times leads to significant modal dispersion, which limits the bandwidth and distance capabilities of step-index fiber. Graded-index multi-mode fiber, in contrast, has a refractive index that gradually decreases from the center of the core towards the cladding. The highest refractive index is at the center, and it continuously decreases as you move outward. This graded profile causes light rays traveling closer to the edge of the core to experience a lower refractive index and therefore travel faster than rays traveling near the center. This helps to equalize the travel times of different modes. While modes traveling near the center take a shorter, straighter path, they travel slower due to the higher refractive index. Modes traveling closer to the edge take a longer, more curved path, but they travel faster due to the lower refractive index. This compensation reduces modal dispersion, allowing graded-index multi-mode fiber to achieve higher bandwidth and longer distances compared to step-index multi-mode fiber. For example, a signal traveling through graded-index fiber will experience less pulse spreading than if it traveled the same distance through step-index fiber, enabling faster data rates over longer links.