Video: Introduction to Fiber optics Theory and Fiber Structure. W5LB

Historical Overview of Optical Communications

  • Prehistoric communication using fire and smoke signals.

  • In 1850s, Alexander Graham Bell invented the photophone for transmitting voices via sunlight.

  • Advances in optical communications using fiber optics developed from Bell's invention.

Current Applications of Fiber Optics

  • Optical fiber is present in:

    • Long haul telephone systems.

    • Interoffice networks.

    • Global fiber optic infrastructure reaching homes and desktops.

  • Additional applications in:

    • Industrial settings (machine vision, measurement).

    • Medical fields (high power laser delivery).

Understanding Light and Fiber Optics

The Electromagnetic Spectrum

  • Visible light is a small portion of the electromagnetic spectrum (radio waves to x-rays).

  • All electromagnetic waves travel at the speed of light (~86,000 miles/second).

Wavelengths and Light Colors

  • Wavelengths in nanometers determine light color:

    • Blue light ~400nm, Red light ~700nm.

    • Infrared wavelengths range from 800 to 100,000 nanometers.

Refraction and Refractive Index

  • Light slows down in denser materials (e.g., glass) causing refraction.

  • Refractive index (n) measures the ratio of light speed in vacuum to glass.

Structure of Optical Fibers

Components of Fiber Optics

  • Composed of:

    • Buffer coating (protection from mechanical stress).

    • Core (light transmitting part, often doped for higher refractive index).

    • Cladding (surrounds core and maintains internal reflection).

Total Internal Reflection

  • Light must enter within a specific angle (numerical aperture) to be transmitted effectively.

Types of Optical Fibers

Multimode vs. Single Mode Fibers

  • Single Mode Fiber:

    • Small core diameter, allows light to travel in one mode.

    • Preferred for long-distance, high bit-rate transmissions.

  • Multimode Fiber:

    • Larger core diameter, permits multiple pathways for light.

    • Prone to modal dispersion leading to overlapping pulses at high speeds.

Pulse Spreading and Dispersion Issues

Modal Dispersion

  • Occurs in multimode fiber where multiple light paths stretch optical pulses.

Chromatic Dispersion

  • Varies with light wavelengths; a major problem in single-mode fibers optimized for specific wavelengths.

Fiber Manufacturing and Specifications

Types of Construction

  • Step Index Fiber: Abrupt refractive index change, high dispersion.

  • Graded Index Fiber: Smooth variation in refractive index reduces modal dispersion.

Core and Cladding Dimensions

  • Single mode fibers: Core ~8-10 microns, cladding ~125 microns.

  • Multimode fibers: Specifications vary (e.g., 50/125 or 62.5/125 microns).

Quality Control in Fiber Manufacturing

  • High losses or weak fibers are rejected.

  • Intrinsic losses include absorption and scattering from impurities.

Signal Attenuation

  • Measured in dB/km; shorter wavelengths absorb more light.

  • Common operational wavelengths: 850nm, 1300nm, 1550nm.

Advantages of Fiber Optics

  • High bandwidth and information-carrying capacity due to light's frequency.

  • Low transmission losses compared to copper (enables longer spans).

  • Immunity to electromagnetic interference and enhanced security (difficult to tap).

  • Smaller and lighter than metallic cables, facilitating installation and storage.

  • Safe in hazardous environments (no electric current/radiation).

Future of Fiber Optics

  • Fiber optics essential in digital systems (telephone, TV) and industrial applications.

  • Continual evolution in technology for global information infrastructure.