Fiber Refractive Index Profile

International Zone (English) General Knowledge
, ,
1

What is the difference between Step-Index and Graded-Index fibers?

2

Step-Index and Graded-Index fibers differ primarily in their core’s refractive index profile, which affects how light propagates through them and, consequently, their transmission characteristics.

  1. Step-Index Fiber:

    • Refractive Index Profile: The core has a uniform refractive index throughout its diameter. At the boundary between the core and the cladding, the refractive index drops sharply, forming a distinct “step.”
    • Light Propagation: In multi-mode step-index fibers, light rays travel at various angles and follow different paths (modes) through the core. Rays traveling directly along the axis take a shorter path, while rays reflecting off the core-cladding boundary take longer, zig-zag paths.
    • Modal Dispersion: This difference in path length leads to modal dispersion, where different light modes arrive at the receiver at different times, causing signal broadening and limiting the fiber’s bandwidth, especially over long distances.
    • Single-Mode Fibers: Most single-mode fibers are also step-index, but with a very small core diameter (typically 8-10 µm). This small core allows only one or a few modes to propagate, effectively eliminating modal dispersion and enabling very high bandwidth transmission.

  2. Graded-Index Fiber:

    • Refractive Index Profile: The core’s refractive index is highest at the center and gradually decreases towards the cladding. There is no sharp “step” at the core-cladding boundary.
    • Light Propagation: Light rays traveling through the center of the core encounter a higher refractive index and thus travel slower. Rays traveling further from the center encounter a lower refractive index and travel faster.
    • Reduced Modal Dispersion: This graded index profile continuously refracts the light rays back towards the fiber axis. The faster speed of light in the outer regions compensates for the longer path length, causing different modes to arrive at the receiver more closely in time.
    • Bandwidth: Graded-index fibers significantly reduce modal dispersion compared to multi-mode step-index fibers, allowing for much higher bandwidths and longer transmission distances for multi-mode applications. They are commonly used for multi-mode fiber optic communication.

Here is a general image of an optical fiber:

85-fiber
85-fiber768×144 33.2 KB

OFSCN offers various types of optical fibers, including those typically used for single-mode (step-index) applications such as OFSCN® G.652D Optical Fiber, and multi-mode fibers like OFSCN® 300 MM Polyimide Optical Fiber, which often employ a graded-index profile for optimal performance.