10.2 Optical fibers: types, properties, and fabrication
3 min read•Last Updated on July 22, 2024
Optical fibers come in different types, each with unique properties. Single-mode fibers have small cores and carry one light mode, while multi-mode fibers have larger cores and carry multiple modes. Graded-index fibers offer a middle ground, reducing dispersion in multi-mode setups.
Fiber optics have revolutionized communication and data transmission. Single-mode fibers excel in long-distance, high-speed applications, while multi-mode fibers are cost-effective for shorter distances. The choice depends on the specific needs of the network or system being designed.
Types and Properties of Optical Fibers
Types of optical fibers
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Allow propagation of only one mode of light through the fiber
Exhibit lower modal dispersion enabling higher bandwidth and longer transmission distances (up to 100 km)
Commonly used in long-haul telecommunications (intercity networks) and high-speed data transmission (fiber-to-the-home)
Multi-mode fibers
Have a larger core diameter (typically 50-62.5 μm) compared to single-mode fibers
Allow propagation of multiple modes of light simultaneously through the fiber
Exhibit higher modal dispersion limiting bandwidth and transmission distances (up to 2 km)
Offer lower cost and easier light coupling compared to single-mode fibers
Commonly used in short-distance applications such as local area networks (LANs) and data centers
Graded-index fibers
A type of multi-mode fiber with a gradually changing refractive index profile from core to cladding
Reduce modal dispersion compared to step-index multi-mode fibers by allowing different modes to travel at similar velocities
Enable higher bandwidth (up to 10 Gbps) and longer transmission distances (up to 10 km) than step-index multi-mode fibers
Refractive index in fibers
Refractive index profile
Variation of the refractive index across the fiber's cross-section determining light guiding properties
Step-index profile exhibits an abrupt change in refractive index between the core and cladding found in both single-mode and multi-mode fibers and is simpler to manufacture compared to graded-index fibers
Graded-index profile exhibits a gradual change in refractive index from the center of the core to the cladding reducing modal dispersion in multi-mode fibers and enabling higher bandwidth and longer transmission distances compared to step-index multi-mode fibers
Numerical aperture (NA)
Measures the light-gathering ability of the fiber determined by the refractive index difference between the core and cladding
Higher NA allows more light to be coupled into the fiber but increases modal dispersion in multi-mode fibers (0.2-0.3 for multi-mode, 0.1-0.2 for single-mode)
Fabrication and Applications of Optical Fibers
Production of optical fibers
Materials
Silica (SiO2) most commonly used in optical fibers due to high purity and transparency in the infrared region
Dopants such as germanium and phosphorus used to modify the refractive index of the core and cladding
Fabrication processes
Preform fabrication using methods such as Modified Chemical Vapor Deposition (MCVD), Outside Vapor Deposition (OVD), or Vapor Axial Deposition (VAD) to create a large-scale version of the fiber
Fiber drawing by heating the preform and drawing it into a thin fiber (125 μm diameter) while applying protective coatings
Quality control
Ensuring high purity (99.999%) and uniformity of materials to minimize attenuation and scattering losses
Precise control of the refractive index profile to achieve desired optical properties
Minimizing defects (bubbles, inclusions) and impurities (OH-, transition metals) in the fiber to reduce absorption losses
Applications of fiber types
Single-mode fibers
Advantages: low modal dispersion enabling high bandwidth (up to 100 Gbps) and long transmission distances (up to 100 km), suitable for long-haul telecommunications and high-speed data transmission (5G networks)
Limitations: higher cost and more difficult light coupling compared to multi-mode fibers, requires precise alignment and specialized equipment (fusion splicers)
Multi-mode fibers
Advantages: lower cost and easier light coupling compared to single-mode fibers, suitable for short-distance applications such as LANs (Ethernet) and data centers
Limitations: higher modal dispersion limiting bandwidth (up to 10 Gbps) and transmission distances (up to 2 km), not suitable for long-haul telecommunications
Graded-index fibers
Advantages: reduced modal dispersion compared to step-index multi-mode fibers, enabling higher bandwidth (up to 10 Gbps) and longer transmission distances (up to 10 km)
Limitations: more complex and expensive to manufacture compared to step-index fibers, not as widely used as step-index fibers in commercial applications (Gigabit Ethernet)