Core Refractive Index Higher Than Cladding

Core Refractive Index Higher Than Cladding | Generated by AI

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Question: An optical fiber consists of a core and a cladding. The refractive index of the core is ____ the refractive index of the cladding.

Answer:

Answer: Greater than (大于)

The refractive index of the core is greater than the refractive index of the cladding.

Comprehensive Explanation

1. Basic Structure of Optical Fiber

An optical fiber has two main layers:

Layer	Role	Refractive Index
Core (纤芯)	Carries/guides the light signal	Higher (n₁)
Cladding (包层)	Confines light within the core	Lower (n₂)

So the relationship is: n₁ (core) > n₂ (cladding)

2. Why Must the Core Have a Higher Refractive Index?

The entire working principle of optical fiber is based on Total Internal Reflection (全内反射, TIR):

To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding.
The cladding causes light to be confined to the core of the fiber by total internal reflection at the boundary between the core and cladding.

Total Internal Reflection only occurs when light travels from a denser medium (higher n) to a less dense medium (lower n) and the angle of incidence exceeds the critical angle. This is exactly what happens at the core-cladding boundary.

3. How Total Internal Reflection Works in Fiber

Light enters the core (high refractive index n₁).
Light hits the core-cladding boundary at a steep angle (greater than the critical angle).
Light injected into the fiber optic core and striking the core-to-cladding interface at an angle greater than the critical angle is reflected back into the core. Since the angles of incidence and reflection are equal, the light ray continues to zigzag down the length of the fiber. The light is trapped within the core.
If the angle is too small (less than the critical angle), light passes into the cladding and is lost.

4. Real-World Example of Refractive Index Values

A typical single-mode fiber used for telecommunications has a cladding made of pure silica, with an index of 1.444 at 1500 nm, and a core of doped silica with an index around 1.4475.

This shows that the difference is small but critical — the core index is always slightly higher than the cladding.

5. Critical Angle Formula

The critical angle θ_c is determined by:

\[\sin\theta_c = \frac{n_2}{n_1}\]

where n₁ is the core index and n₂ is the cladding index. The refractive index of the core, n₁, is always greater than the index of the cladding, n₂. Light is guided through the core, and the fiber acts as an optical waveguide.

Summary

The core refractive index is greater than (大于) the cladding refractive index. This difference is the physical foundation that enables total internal reflection, which is how optical fibers guide light over long distances with minimal loss.

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