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 = (2Ï€a / 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 × Î»] / (2Ï€ × 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] / [2Ï€ × 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 |