FIBER OPTICS Light propagation through thin glass fibers

FIBER OPTICS Advantages Low Attenuation (less signal degradation) Very High Bandwidth over long distances Small Size and Low Weight No Electromagnetic Interference Non-Conductive Cables Easy installation Eliminates Spark Hazards (non-flammable) Data Security

PRINCIPLE OF FIBER OPTICS Optical fibers are used as dielectric waveguides for guiding the electromagnetic waves at optical frequencies Light is guided through transparent glass fibers by total internal reflection

TOTAL INTERNAL REFLECTION Snell’s law When the transmitted (refraction angle) reaches 90⁰, the incidence angle is called as the critical angle

TOTAL INTERNAL REFLECTION Angle of incidence less than the critical angle Angle of incidence equal to the critical angle Angle of incidence greater than the critical angle Total internal refection takes place only when the light travels from denser to rarer medium and angle of incidence should be greater than the critical angle of the denser medium

TOTAL INTERNAL REFLECTION

Structure Fiber Optic Cable Glass fiber consists of a central core glass (≈ 50 μm) surrounded by a cladding Refractive index of cladding is slightly lower than the refractive index of core Core + Cladding ≈ 125 – 200 μm Fiber is made of silica (SiO2) glass Cladding is also made of silica but the outer layer is added with small amount of B, Ge or P to decrease the refractive index

Structure Fiber Optic Cable Cladding provides Proper light guidance (to retain the light wave within the core) High mechanical strength High safety to the core from the scratch Buffer Jacket Made of plastic and protects the fiber from moisture and abrasion

Structure 0f Fiber Optic Cable Strength Members Provides necessary toughness and tensile strength Fiber optic cable withstands during hard pulling, bending, stretching or rolling without fracture even though the fiber is made from brittle glass Black Polyurethane Outer Jacket Avoids mechanical crushing

Propagation of Light through Optical Fibers Consider the light propagation through a step index fiber Fiber is placed in a medium of refractive index “no” Let ɸmax is the incident angle at the entrance end of the fiber with respect to the axis of the fiber Let θr be the corresponding angle of refraction

Propagation of Light through Optical Fibers In the limit of total internal reflection By Snell’s law

Propagation of Light through Optical Fibers Since Angle of incidence at the core-cladding interface (or)

Propagation of Light through Optical Fibers

Propagation of Light through Optical Fibers If the fiber is placed in air (n0 = 1)

Acceptance Angle Maximum value of the angle of incidence at the entrance end of the fiber, at which the angle of incidence at the core-cladding surface is equal to the critical angle of the core medium

Acceptance Angle Acceptance angle of the fiber is the maximum angle of incidence up to which a light ray can enter into the fiber and still be totally internally reflected

Numerical Aperture Numerical Aperture (N.A.) is the light collecting efficiency of the fiber and is the measure of amount of light rays that can be accepted by the fiber If n1 ≈ n2, then (n1+n2) = 2n1 Relative refractive index difference of the fiber (Δ)

Step Index Fiber The refractive index of the core of the step index fiber is constant through out the core Graded Index fiber The refractive index of the core of the graded index fiber is maximum at the center of the core and then it decreases towards core-cladding interface