Optical Fiber

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Transcript of Optical Fiber

Need of Fiber Optic Communications

Basic Structure of an Optical FiberAn optical fiber is a flexible, transparent fiber made by glass (silica) or plastic to a diameter slightly thicker than that of a human hair.Optical fiber is a dielectric waveguide and ideally has a cylindrical shape.It consists of a core made up of a dielectric material which is surrounded by a cladding made up of a dielectric material of lower refractive index than core.

Principles of Light Transmission in a FiberFiber optics deals with the transmission of light energy through transparent fibers.How an optical fiber guides light depends on the nature of light and structure of the optical fiber.A light wave is a form of energy that is moved by wave motion.In fiber optics, wave motion is the movement of light energy through an optical fiber.

Properties of LightWhen light waves strike an object, some of the waves are absorbed by the object, some are reflected by it, and some might pass through it.When light strikes an object it is:ReflectedTransmitted Absorbed

Properties of LightWhat happens to the light depends on the materialTransparent (clear) materials- transmit light.Translucent (see through) materials- scatters the transmitted light.Opaque (not see through) materials- absorbs and reflects.

Properties of LightTransparent (clear) materials--Those materials that transmit almost all the light waves falling upon them are said to be transparent materials.you can clearly see other objects through materials such as glass and clear plastic that allow nearly all the light that strikes them to pass through.

Properties of LightTranslucent (see through) materials--The materials through which some light rays can pass but the objects can not seen clearly because the rays are diffused, are known as translucent materials.Although objects behind these materials are visible, they are not clear.A frosted glass or a piece of oiled paper are the examples of translucent materials.

Properties of LightOpaque (not see through) materials--Those materials that are unable to transmit light waves falling upon them are said Opaque materials.You cannot see other objects through opaque materials.Example: walls of a room etc.

Properties of LightReflection--Reflected waves are those waves that are neither transmitted nor absorbed but are reflected from the surface of the medium.When a wave approaches a reflecting surface such as a mirror, the wave that strikes the surface is called incident wave and the wave that bounces back is called the reflected wave.

The amount of incident energy that is reflected from a surface depend on: The nature of the surface The angle at which the wave strikes the surface

Properties of LightRefractionWhen a light wave passes from one medium to another medium having different velocity of propagation, a change in the direction of the wave will occur.This change of direction as the wave enters the second medium is called refraction. Refraction- bending of light waves due to a change in speed Lens, curved glass or transparent material that Refracts light

Properties of LightDiffusionWhen light wave is reflected from a piece of white paper, the reflected beam is scattered or diffused.Since the surface of paper is not smooth, the reflected light is broken up into many light beams that are reflected in all directions.

Properties of LightAbsorptionA light wave is reflected and diffused from a piece of white paper.But if the light beam falls upon a piece of black paper, the black paper absorbs most of the light and very small amount of light is reflected from the paper.If the surface upon which the light beam falls is perfectly black, then there is no reflection; the light is totally absorbed.

Ray Theory TransmissionRay Optics: basic lawsLight rays in homogeneous media are straight linesLaw of Reflection Reflection from a mirror or at the boundary betweem two media of different refractive index: the angle of reflection equals to the angle of incidence i.e. qr = qi Snells law of Refraction At the boundary between two media of different refractive index n the angle of refraction qt is related to the angle of incidence qi byni sin i = nt sin t

Ray Theory Transmission

Ray Theory TransmissionRefractive indexThe index of refraction of a material is the ratio of the speed of light in a vaccum to the speed of light in the materialn = c/vThe factor n is the index of refraction (or refractive index) of the medium.For air and gases n ~ 1. At optic frequencies, the refractive index of water is 1.33.Glass has many compositions, each with a slightly different n. An approximate refractive index of 1.5 is representative for the silica glasses used in fibers; more precise values for these glasses lie between ~1.45 and ~1.48.

Ray Theory TransmissionRefractive Index for Some MaterialsAir -----------------------------------------------------------------------1.0Water -------------------------------------------------------------------1.33Magnesium fluoride -------------------------------------------------1.38Fused silica (SiO2)----------------------------------------------------1.46Sapphire (Al2O3)------------------------------------------------------1.8Lithium niobate (LiNbO3)-------------------------------------------2.25Indium phosphide (InP)----------------------------------------------3.21Gallium arsenide (GaAs) ---------------------------------------------3.35Silicon (Si)---------------------------------------------------------------3.48Indium gallium arsenide phosphide (InGaAsP)-----------------3.51Aluminum gallium arsenide (AlGaAs)-----------------------------3.6Germanium (Ge) -------------------------------------------------------4.0The index varies with a number of parameters, such as wavelength and temperature.

Ray Theory TransmissionCritical Angle (qc)The angle at which total internal reflection occurs is called the critical angle of incidence.At any angle of incidence (q1) greater than the critical angle, light is totally reflected back to the glass medium.For n1 > n2, the angle of refraction q2 is always greater than the angle of incidence q1.When the angle of refraction q2 is 90o the refracted ray emerges parallel to the interface between the media.

The critical angle is determined by using Snells Law. The critical angle is given by :

Ray Theory TransmissionTotal internal reflectionAt angles of incidence q1 > qc the light is totally reflected back into the incidence higher refractive index medium. This is known as total internal reflection.

Ray Theory TransmissionAcceptance AngleThe acceptance angle of an optical fiber is defined as the maximum angle of a ray (against the fiber axis) hitting the fiber core which allows the incident light to be guided by the core.The sine of that acceptable angle is called the numerical aperture, and it is essentially determined by the refractive index contrast between core and cladding of the fiber, assuming that the incident beam comes from air or vacuum.

Ray Theory TransmissionNumerical Aperture The numerical aperture is a measurement of the ability of an optical fiber to capture light. The NA is also used to define the acceptance cone of an optical fiber. Mathematically it is defined as:

Where, n1 is the refractive index of core and n2 is refractive index of cladding.

Geometrical Optics DescriptionOptical fibers based on modes or mode types :Mode is the one which describes the nature of propagation of electromagnetic waves in a wave guide.it is the allowed direction whose associated angles satisfy the conditions for total internal reflection and constructive interference.Based on the number of modes that propagates through the optical fiber, they are classified as:Single Mode fibers can propagate only the fundamental mode.Multimode fibers can propagate hundreds of modes.

Geometrical Optics DescriptionSingle mode fibers:In a fiber, if only one mode is transmitted through it, then it is said to be a single mode fiber. A typical single mode fiber may have a core radius of 3 m and a numerical aperture of 0.1 at a wavelength of 0.8 m.

Characteristics of Single Mode Fiber The single mode fiber has the following characteristics:Only one path is available.Core diameter is small No dispersion Higher band width (1000 MHz)Used for long haul communicationFabrication is difficult and costly

Geometrical Optics DescriptionMulti mode fibers :If more than one mode is transmitted through optical fiber, then it is said to be a multimode fiber. The larger core radius of multimode fibers make it easier to launch optical power into the fiber and facilitate the end to end connection of similar powers.

Characteristics of Multimode Fiber The Multimode fibers has the following characteristics: More than one path is availableCore diameter is higherHigher dispersionLower bandwidth (50MHz)Used for short distance communicationFabrication is less difficult and not costly

Types of Optical FibersOptical fibers based on refractive index profile :Based on the refractive index profile of the core and cladding, the optical fibers are classified into two types:Step index fiberGraded index fiber

Geometrical Optics DescriptionStep index fiber :In a step index fiber, the refractive index changes in a step fashion, from the centre of the fiber, the core, to the outer shell, the cladding. It is high in the core and lower in the cladding. The light in the fiber propagates by bouncing back and forth from core-cladding interface. The step index fibers propagate both single and multimode signals within the fiber core. The light rays propagating through it are in the form of meridinal rays which will cross the fiber core axis during every reflection at the core cladding boundary and are propagating in a zig zag