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What is an Optical Fiber? Optical fibers are long strands of thin and flexible cylindrical transparent materials made of plastic or glass designed to transmit visible or infra red light across great distance.

Main Parts of an Optical Fiber Cable:

  1. Core: innermost part of the fiber, made of glass or plastic or glass
  2. Cladding: layer surrounding the core, made of glass or plastic, refractive index is greater than that of core, constricts light within core using total internal reflection.
  3. Jacket: outermost layer protecting the cladding from environmental hazards, pollution and shocks, made of plastic or polymer

optical fiber cross section Dimension of fiber:
Core: 5 micrometer to 50 micrometer
Cladding: about 125 micrometer
Jacket: about 250 micrometer

Advantages of fibers over Cables and Wires:

  • Lighter in weight and occupy less space
  • More economical than cables and wires in the long run
  • No physical connection is required from sender to receiver
  • Transmission speed is very fast compared to wires and co axial cable.
  • The electrical noise does not interfere with propagated light signals. Thus in optical fiber, cross talking can be avoided.
  • The attenuation in a fiber is markedly low
  • The bandwidth of the fiber is higher
  • Raw materials required for production of optical fibers are available in plenty
  • The fibers last longer than copper wires as they are immune to pollution and environmental hazards
  • The information carrying capacity of an optical fiber is several (thousands) of times more than the copper cable or co-axial cables used for transferring radio waves for telephones.

Working Principle: The core is made of a material of refractive index n1 while the cladding is made of a material of refractive index n2 (n1>n2).

light-propagation through optical fiber

When light enters one end of the fiber, it travels from a denser medium towards a rarer medium with a finite angle. The light ray hits the core-cladding interface with an angle greater than the critical angle for the interface and thus gets reflected at the interface due to total internal reflection. It suffers multiple total internal reflections at the core-cladding interface along the whole length of the fiber and finally emerges out of the other end of the fiber. The transmission occurs without any loss of energy even when the fiber is bent.

The ability of light gathering of the fiber depends on:

  1. core diameter
  2. numerical aperture

Classification of optical Fiber:

Step-index optical Fiber: It has small core diameter (same order as that of the wavelength of light) along with a core and cladding of uniform refractive index n1 and n2 respectively (n1>n2). It is called so because a sudden change of refractive index occurs at the junction of the core-cladding interface in a single step to allow total internal reflection to take place.

Step-index optical Fiber

Advantage: Best for long distance communication as the pulse repetition can be high and maximum information can be send.

Disadvantage: use of thin core creates mechanical difficulties in manufacturing and handling of the fiber.

Utility: Used in under sea-level transmission of data where the expenditure is justified

Graded-index optical fiber: It is a multimode fiber with huge core diameter and with a core having non-uniform refractive index, the refractive index being a function of radial distance from the fiber axis. The refractive index of the cladding is uniform. Since the refractive index towards the core-cladding interface is lower than that at the center, the light rays traveling along the edge travels faster. Thus, all rays arrive the end of the fiber at approximately the same time.

Graded-index optical fiber

Advantage: The signal distortion is reduced as the light rays are continuously refocused within the core as they travel down the fiber.

Numerical Aperture and Acceptance Angle

Acceptance Angle: The maximum angle of incidence at the entrance aperture of the fiber for which the light ray suffers total internal reflection at the core-cladding interface and propagates through the fiber is called the acceptance angle.

Numerical Aperture and Acceptance Angle

Numerical Aperture: Sine of the acceptor angle of the optical fiber, measures the light gathering or accepting power of the fiber.

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