Core Diameter of Single-Mode and Multimode Fiber

The core diameter of an optical fiber determines how light travels through it and whether the fiber is single-mode or multimode.

1. Single-Mode Fiber (SMF)

A single-mode fiber allows only one propagation mode of light.

Typical Core Diameter

8 μm to 10 μm

(Common standard: 9 μm)

Condition for Single Mode

A fiber behaves as single-mode when the normalized frequency (V-number) satisfies:

V = (2πa / λ) × NA < 2.405

Where:

• a = core radius
• λ = wavelength
• NA = numerical aperture

Since core diameter d = 2a:

d = (2Vλ) / (2πNA)

For cutoff condition V = 2.405:

d = [2 × 2.405 × λ] / (2π × NA)

This formula helps calculate the maximum core diameter for single-mode operation.

2. Multimode Fiber (MMF)

A multimode fiber allows many light modes to propagate.

Typical Core Diameter

• 50 μm
• 62.5 μm
• Sometimes 100 μm

Condition

V > 2.405

Larger core diameter means more modes can travel through the fiber.

Example Calculation

Suppose:

• Wavelength λ = 1.3 μm
• Numerical Aperture NA = 0.12

For single-mode cutoff:

d = [2 × 2.405 × 1.3] / [2π × 0.12]

d ≈ 8.3 μm

Therefore, the fiber should have a core diameter around 8 μm to operate in single mode.

Comparison Table

Fiber Type	Core Diameter	Modes
Single-Mode	8–10 μm	One
Multimode	50–100 μm	Many