What is Step Index Fiber?

Definition of Step-Index Fiber

Definition: A step-index fiber is a type of optical fiber classified based on its refractive index distribution. As an optical waveguide, it features a constant refractive index within the core and another constant refractive index within the cladding. Notably, the refractive index of the core is slightly higher than that of the cladding, with a sudden change occurring at the core-cladding interface—hence the term "step-index."

The refractive index profile of a step-index fiber is illustrated in the figure below:

Propagation in Step-Index Fibers

When a light ray propagates through a step-index optical fiber, it follows a zigzag path composed of straight lines, a phenomenon enabled by total internal reflection at the core-cladding interface.

Mathematically, the refractive index profile of a step-index fiber is expressed as:

a is the core radius ；r is the radial distance

Modes of Step index fiber

Step-Index Single-Mode Fiber

In a step-index single-mode fiber, the core diameter is so extremely small that it permits only one propagation mode, meaning only a single light ray travels through the fiber. This unique characteristic eliminates distortion caused by delay differences between multiple rays.

The propagation of a light ray through a step-index single-mode optical fiber is illustrated in the figure below:

Step-Index Single-Mode Fiber Characteristics

The core diameter here is remarkably narrow, enabling only one propagation mode to travel through the fiber. Typically, the core size ranges from 2 to 15 micrometers.

Step-Index Multimode Fiber

In step-index multimode fibers, the core diameter is large enough to allow multiple propagation modes, meaning several light rays can travel through the fiber simultaneously. However, this simultaneous propagation of multiple rays causes distortion due to differences in their propagation delays.

The propagation of light rays through a step-index multimode optical fiber is shown in the figure below:

Multimode Fiber Core Characteristics

The figure above clearly illustrates that the core diameter is large enough to permit multiple propagation paths. Typically, the core size ranges from 50 to 1000 micrometers.

Refractive Index Variation in Step-Index Fibers

It should be noted that the refractive index profile of step-index fibers is characterized by:

Light Source and Characteristics of Step-Index Fibers

Light-emitting diodes (LEDs) are the primary light sources used in these fibers.

Advantages of Step-Index Fibers

• Simple manufacturing process

• Cost-effective production

• Propagation via total internal reflection

Disadvantages of Step-Index Fibers

• Single-mode propagation limits information-carrying capacity due to the restriction of only one light ray transmitting at a time.

• Difficulties in light coupling caused by the small core diameter.

Applications of Step-Index Fibers

Step-index fibers are mainly applied in local area network (LAN) connections. This is because their information transmission capacity is lower than that of graded-index fibers.