Fiber Optics Basics

Fiber Optics

• Fiber Optics deals with the transmission of light through glass or plastic as thin as a human hair, designed to guide the light waves along their length.
• The optical fiber works on the principle of total internal reflection.

Principle of Total Internal Reflection

• When a ray of light passes from a denser medium to a less dense medium (rarer), some part of the light is reflected back into the denser medium, and the rest is refracted into the less dense medium.
• The reflected ray is weak in intensity, but the refracted ray is bright with maximum intensity.
• As the angle of incidence increases, the angle of reflection also increases, and at some angle of incidence, the angle of refraction is 90°, and the refracted ray travels along the boundary.
• This angle of incidence is called the critical angle (𝜃𝑐).
• When the angle of incidence increases slightly, the reflected ray becomes bright, and all the incident light energy is reflected back into the denser medium.

Construction of Fiber Optics

• An optical fiber has three coaxial regions: the core, cladding, and sheath (protective jacket).
• The core is the innermost region, surrounded by the cladding, and the outermost region is the sheath.
• The core and cladding are made of glass or plastic, and the refractive index of the core is always greater than the refractive index of the cladding.
• The purpose of the cladding is to keep the light within the core.
• The sheath or buffer protects the cladding and core from moisture and abrasion.

Acceptance Angle and Acceptance Cone

• When a light ray enters a fiber, it makes an angle with the axis of the fiber, and if this angle is greater than the critical angle, the light is totally internally reflected and propagates through the fiber.
• The acceptance angle is the maximum angle that a light ray can have relative to the axis of the fiber and still propagate through the fiber.
• The acceptance cone is the light ray within the cone having a full angle of 2𝜃𝑚 that is accepted and transmitted through the fiber.

Fractional Refractive Index Change and Numerical Aperture

• The fractional refractive index change (Δ) is the ratio of the difference between the refractive indices of the core and the cladding to the refractive index of the core.
• The numerical aperture (N.A.) is the sine of the acceptance angle and signifies the light-gathering power of the optical fiber.

Classification of Optical Fiber

• There are two main types of optical fibers: single-mode and multimode fibers.
• Depending on the refractive index profile, there are three types of optical fibers:
  • Single-mode step index fiber
  • Multimode step index fiber
  • Multimode graded index fiber

Attenuation in Optical Fiber

• Attenuation is the reduction of signal strength over the length of the light-carrying medium.
• The main causes of attenuation in optical fibers are:
  • Absorption
  • Scattering
  • Bending
• Different mechanisms of attenuation include:
  • Intrinsic absorption
  • Extrinsic absorption
  • Rayleigh scattering
  • Microscopic and macroscopic bending

Advantages of Optical Fiber over Conventional Communication

• Higher bandwidth
• Smaller size and lower weight
• Absence of cross-talk
• Easy maintenance
• Signal security
• Low cost
• Longer lifespan
• Electrical isolation

Applications of Optical Fiber

• Communication applications (telephone, teleconferencing, cable TV, digital data transmission)
• Medical field (endoscopic applications, eye surgery, cardiology)
• Military applications (aircraft, ships, tank wiring, fiber-guided missiles)
• Industrial applications (inspection or control of operations in inaccessible areas)

LASER (Light Amplification of Stimulated Emission of Radiation)

• Laser light is monochromatic, directional, and coherent, with a focused beam and a high intensity.
• Laser light is produced by stimulated emission, which occurs when an excited atom is forced to emit a photon of the same energy and in phase with the incident photon.

Characteristics of Laser

• Monochromaticity
• Coherence
• Directionality
• Divergence
• Brightness

Ruby Laser

• Ruby is a crystal containing chromium ions, which gives it a pink or red color.
• The chromium ions have absorption bands in the blue and green regions of the visible spectrum.
• The ruby laser works on a three-level system, with the energy levels of the chromium ions in the crystal lattice.### Energy States and Laser Beam Generation
• The xenon discharge generates a beam of white light, which is absorbed by Cr3+ ions, exciting them from the ground state (E1) to the excited state (E3).
• The excited state (E3) is highly unstable, lasting only 10^(-8) seconds, and rapidly loses energy to transition to the metastable state (E2).
• The metastable state (E2) accumulates Cr3+ ions for a few milliseconds, allowing for population inversion between E2 and E1.
• Spontaneous emission of a photon by Cr3+ ions at E2 initiates stimulated emission by other ions in the metastable state, producing a strong laser beam.

Characteristics of Ruby Laser

• The laser beam is red in color, with a wavelength of 6943A0.
• The green light plays the role of pumping agent, not the amplified component.
• The effectiveness of the Ruby laser is low, as only the green component of the pumping light is utilized.

Applications of Laser

Scientific Applications

• Laser beam is used for precision measurement of distances and alignment in civil construction work due to its directionality and coherence.
• The distance between the Earth and the Moon has been precisely measured using laser beams.
• Laser can be used for determining the shape of objects and structure deformations.

Engineering and Industrial Applications

• Laser is used in communications due to its high directionality and energy density.
• Laser is used as a heat source for cutting, drilling, welding, etc.
• Laser can be used to investigate the structure of molecules.

Medical Applications

• Laser is used for diagnosis in endoscopy, allowing for the examination and photography of inner body parts using an optical fiber.
• Laser is used in eye surgery, fixing detached retinas and treating glaucoma and cataract.
• Laser is used in general surgery, making clean cuts and sealing blood vessels, minimizing blood loss.
• Laser is used in dentistry to destroy germs in tooth cavities without pain.

Holography

• Laser is used in holography to obtain a 3D view of an object.