stepindex

Step-index refers primarily to a class of optical fibers in which the refractive index of the core is uniform and abruptly changes at the boundary with the cladding, forming a “step” in the index profile. This contrasts with graded‑index fibers, where the refractive index varies gradually from the center outward. Step‑index fibers are the simplest and most common form of multimode fiber and also serve as the basis for many single‑mode designs.

In a step‑index fiber the core index (n₁) is higher than the cladding index (n₂). Light launched into the core is confined by total internal reflection at the core‑cladding interface, allowing it to propagate over long distances with minimal loss. The numerical aperture (NA) of the fiber, defined as NA = √(n₁² – n₂²), determines the acceptance angle for incoming light and influences bandwidth and coupling efficiency.

Multimode step‑index fibers typically have core diameters of 50 µm, 62.5 µm, or 100 µm and support many propagation modes, leading to modal dispersion that limits bandwidth to a few hundred megahertz·kilometer. Single‑mode step‑index fibers have much smaller core diameters (≈8–10 µm) and support only the fundamental mode, reducing dispersion and enabling transmission rates in the tens of terabits per second over hundreds of kilometers.

Manufacturing of step‑index fibers involves drawing a preform composed of pure silica doped with germanium or other additives to achieve the desired index contrast. The abrupt index change simplifies analysis and routing of light, making step‑index fibers suitable for local area networks, data centers, and short‑reach communication links, while single‑mode versions are employed in long‑haul and submarine cable systems.