Fiber Optic Fundamentals
Multimode and singlemode fiber types, OM ratings, core sizes, jacket colors, bend radius, and hybrid cables.
- Distinguish between multimode and singlemode fiber optic cable
- Identify OM ratings and their distance capabilities
- Explain core and cladding sizes for common fiber types
- Recognize fiber jacket color codes per TIA-598
- Describe bend radius requirements and hybrid cable applications
Lesson 1
Multimode vs Singlemode Fiber
How Fiber Optics Work
Fiber optic cables transmit data as pulses of light through a glass or plastic core. Light enters one end and bounces along the core due to total internal reflection - the cladding surrounding the core has a lower refractive index, keeping light confined within the core.
Multimode Fiber (MMF)
Core: 50 or 62.5 microns
Light source: LED or VCSEL
Distance: Up to 550 m (10G) or 2000 m (1G)
Cost: Lower transceivers, shorter distances
Use: Building backbone and horizontal
Singlemode Fiber (SMF)
Core: 8.3 microns
Light source: Laser
Distance: Up to 10+ km
Cost: Higher transceivers, unlimited distance
Use: Campus backbone and long haul
Multimode Fiber
Multimode fiber has a larger core (50 or 62.5 microns) that allows multiple light paths (modes) to travel simultaneously. The larger core makes alignment easier and allows less expensive LED or VCSEL light sources.
The downside is modal dispersion - different modes travel slightly different distances, causing the pulse to spread out over long runs. This limits both distance and bandwidth.
Singlemode Fiber
Singlemode fiber has a very small core (8.3 microns) that allows only one mode of light to propagate. With no modal dispersion, singlemode fiber supports much higher bandwidth over much longer distances.
Exam Tip
Multimode = short distance, larger core, cheaper transceivers. Singlemode = long distance, smaller core, laser source. The core size is the key differentiator - 50/62.5 um vs 8.3 um.
Multimode fiber (50/62.5 um core) uses LEDs or VCSELs for short-distance links. Singlemode fiber (8.3 um core) uses lasers for long-distance links. Modal dispersion limits multimode distance - singlemode has no such limitation.