Optical fibers guide light because the core has a higher effective refractive index than the cladding. In step-index fiber, the core’s refractive index is essentially uniform and then drops abruptly at the cladding. In graded-index fiber, the refractive index gradually decreases from the centre of the core toward the cladding.
That graded profile changes how different rays travel and can greatly reduce modal dispersion in multimode fiber.
At a glance
| Point | Step-Index Fiber | Graded-Index Fiber |
|---|---|---|
| Index profile | Abrupt step | Smooth radial gradient |
| Ray path | Zig-zag total-internal-reflection model in simple ray picture | Curved/sinusoidal-like paths in ray model |
| Multimode dispersion | Relatively high | Reduced |
| Bandwidth-distance product | Lower for comparable multimode designs | Higher |
| Manufacturing/profile | Simpler conceptual profile | Precisely controlled graded dopant profile |
| Common teaching use | Single-mode and multimode step-index concepts | Multimode LAN/data links historically common |
Step-Index Fiber
Optical fiber with an approximately constant core refractive index and an abrupt step down to the cladding index.
Graded-Index Fiber
Fiber whose core refractive index varies gradually, typically highest at the centre and lower near the edge.
How grading reduces modal delay
In a step-index multimode fiber, high-angle rays travel a longer geometric path than near-axis rays. In a graded core, rays farther from the centre travel through lower-index material, where light propagates faster.
The speed compensation partially offsets the longer path, causing modes to arrive closer together in time.
Single-mode changes the story
Single-mode fiber largely avoids intermodal dispersion because only the fundamental mode propagates. Modern long-distance telecom therefore uses single-mode step-index-like designs rather than multimode graded-index fiber for the longest/highest-capacity links.
Real fibers are waveguides
Ray diagrams are useful for learning, but optical fibers are electromagnetic waveguides. Modes, dispersion and cutoff are more accurately treated using wave optics.
Frequently asked questions
Which fiber has higher bandwidth in multimode form?
Graded-index multimode fiber generally does.
Does graded-index eliminate dispersion completely?
No. It reduces intermodal dispersion; material and waveguide dispersion remain.
Is all single-mode fiber step-index?
Many standard fibers use step-index-like profiles, though engineered profiles can be more complex.
Why is the refractive index highest at the centre?
The gradient is designed so outer-path light travels faster and partially compensates for longer path length.
Sources and further reading
KnowDifferences Editorial Team
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