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Optical Fibers

• Optical communication systems use optical fibers for high-frequency transmission in the optical frequency range.
• Optical wavelengths are extremely short, in the nanometer range.
• Optical frequencies range from approximately 10¹² Hz to 10¹⁵ Hz; wavelengths are used to express optical signals instead of frequencies.
• Optical frequencies are categorized into:
  • Visible light (380 nm to 800 nm)
  • Infrared (longer wavelengths than 800 nm)
  • Ultraviolet (shorter wavelengths than 380 nm)

Colors of Visible Light

• Visible light consists of several primary colors, forming the visible spectrum.
• A table provides the primary colors, corresponding frequencies, and wavelengths.
  • Example: Red has a wavelength around 750nm and a frequency around 4x10¹⁴ Hz.

Advantages of Optical Fibers

• Optical fibers are made of insulating materials (glass or plastic), making them immune to electromagnetic fields.
• The transmitted signal is optical, offering several advantages:
  • High transmission capacity: Optical systems have a significantly higher transmission capacity than other systems due to higher frequencies.
  • Immunity to electromagnetic interference: Optical fibers are not affected by electromagnetic fields, making them suitable for high electromagnetic field environments.
  • Low loss: Optical fibers have remarkably low signal loss, enabling transmission over long distances.
  • Small size and weight: Compared to copper cables, optical fibers are much smaller and lighter, increasing usability in various applications.
  • Higher security: The optical signal is confined to the fiber, making it extremely difficult to tap or intercept.
  • High flexibility: Optical fibers can be easily stored, transported and installed due to their high tensile strength in their flexibility.
  • Longer lifespan: Optical fibers have a significantly longer lifespan (20-30 years) than copper cables (12-15 years).

Ease of Expansion System Capability

• System capacity is easily increased by replacing components.

High Quality Transmission

• Optical communication offers superior quality with low loss, superior electromagnetic interference immunity, and minimal interference between adjacent lines—compared to other telecommunication systems.
• Optical systems have a much lower bit error rate (BER) compared to microwave or copper wire systems (e.g., BER ≤ 10^-9 vs. 10^-10)

Optical Fiber Disadvantages

• Higher cost of components compared to electrical systems.
• High cost of transmitters and receivers.
• Expensive installation process that requires specialized equipment and skilled labor.

Fiber Optic Applications

• Long-distance telecommunications
• Local telephone networks
• Closed-circuit television (CCTV)
• Internet and data communications
• Military communications

Fundamentals of Optical Fiber

• Optical fibers are the primary component of optical communication systems, using light as the information carrier (carrier wave).
• Three aspects explain light's behavior:
  • Wave theory: Light as an electromagnetic wave
  • Geometric optics: Light as a ray
  • Quantum theory: Light as photons

Light Ray Theory

• Light travels as a beam with a specific velocity, dependent on the medium.
• Refractive index (n) quantifies a medium's hindering effect on light propagation, calculated as the ratio between the speed of light in a vacuum (c) and the speed of light in the medium (v).
• The formula for light velocity in a different medium from a vacuum is c = c/n.

Snell's Law

• Snell's law relates incident, reflected, and refracted rays to the associated angles.

Critical Angle (Total Internal Reflection)

• Critical angle is the incident angle required to cause a refracted angle of 90 degrees when light propagates from a higher refractive index medium to a lower index medium.
• It results in complete internal reflection, wherein the light is reflected back into the fiber, if the angle of incident is greater than critical angle
• The main condition for total internal reflection is that the light must be propagating from a medium with a higher index to a medium with a lower index.

Fiber Structure

• Optical fiber consists of a core and cladding, coated by a protective layer.
• The core has a higher refractive index, and the cladding has a lower refractive index
• The protective layers safeguard the fiber from environmental factors.

Normalized Frequency (V)

• The V-number is a crucial parameter for analyzing propagation modes, calculated by =2πan/λ.

Optical Fiber Types

• Optical fibers are classified according to refractive index variation and the number of propagating modes.
• Types include step-index and graded-index fibers, depending on the refractive index profiles within the core.
• Multimode fibers allow multiple modes of propagation whereas, Single-mode fibers allow only one propagating mode.

Plastic Optical Fibers (POF)

• POF are made entirely of plastic.
• They are characterized by:
  • Low cost
  • Larger size
  • High NA
  • Flexibility

Practical Characteristics of Optical Fibers

• Fiber characteristics are summarized into groups based on core diameter, jacket diameter and other additional specifications.