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🔬Modern Optics

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10.2 Optical fibers: types, properties, and fabrication

3 min readLast 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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  • Single-mode fibers
    • Have a smaller core diameter (typically 8-10 μm)
    • 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
    1. 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
    2. 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)


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.