FIBRE OPTICS CO4 Dr Prabodh Sahai Saxena Prof

FIBRE OPTICS (CO-4 ) Dr. Prabodh Sahai Saxena Prof & Head Deptt of Physics LNCT BHOPAL

Contents Ø Ø Ø Ø Ø Introduction of Fibre Construction of Fibre Working principle of Fibre Light propagation in Fibre Terms related to Optical Fibre Classification of Optical Fibre Communication System Advantage of Optical Fibre Disadvantage of Optical Fibre Applications of Optical Fibre

Introduction of Fibre Ø An optical fibre cable is a CABLE containing one or more optical fibres that are used to carry light. Ø It is made of glass or plastic or transparent materials, as thin as human hair and transmits signals in the form of light. Contd. …

Ø There are different uses of fibres but mainly used in communication system. Ø A light-emitting diode (LED) or laser diode (LD) can be used as a source in communication system. Ø Advantages of optical fibre include greater bandwidth than copper wire cable & lower loss in signal. No electrical hazard.

Construction of fibre Fibre consist mainly three parts; Core - central region of very thin size made up of optically transparent dielectric medium and carries the light form transmitter to receiver. The core diameter can vary from about 5μm-100 μm. Thin glass center of the fiber where the light travels. Cladding - Outer optical material surrounding the core having reflecting index lower than core. It helps to keep the light within the core. Contd. …

Buffer Coating/Outer Jacket- Plastic coating that protects the fiber from damage and moisture. The typical diameter of fiber after coating is 250 -300μm The bundles are protected by the cable's outer covering, called a jacket. Ø If n 1 & n 2 be the refractive index of core & cladding material then it is necessary that n 1>n 2

Working principle of Fiber Ø It works on the principle of Total Internal Reflection(TIR). Ø In the optical fiber the rays undergo repeated total number of reflections until it emerges out of the other end of the fiber, even if fiber is bend. Due to this path of the rays inside fibre is Zig-Zag. Contd. ……

Ø Conditions for TIR: - When an ray of light travels from a denser to a rarer medium such that the angle of incidence is greater than the critical angle, the ray reflects back into the same medium. This phenomenon is known as Total Internal Reflection. Ø TIR occurs at core cladding interface.

Light propagation in Fibre i=angle of incident, r = angle of refraction Φ=critical angle, θm= max acceptance angle n 0=launching medium refractive index n 1=core refractive index n 2= cladding refractive index

Snell’s law at launching face, we get Sin i /Sin r = n 1 / n 0 …………………. . (1) now largest value of i occurs when Φ = Φc and r = 90 - Φ Therefore Sin i = n 1 / n 0 Cos Φc When Φ = Φc then i = θm , so Sin θm = n 1 / n 0 Cos Φc …………. (2) But at core cladding interface Sin Φc =n 2 / n 1 …………. (3) Therefore Cos Φc= (n 1 2 -n 22)1/2 / n 1 then from equation (2) Sin θm = = (n 1 2 -n 22)1/2 / n 0 ………… (4) Launching medium is generally air therefore n 0 =1 Therefore Sin θm = = (n 12 -n 22)1/2

Terms related to optical fibre �Relative Refractive Index Change (Δ): - it is defined as the ratio of the difference between refractive indices of the core and cladding to the refractive index of core. Δ =n 1 -n 2/n 1 Where n 1 & n 2 be the refractive index of core & cladding material. �Numerical Aperture(NA): - it is defined as the sin of the acceptance angle & measure of the light gathering capability of the fibre. NA= (n 1 2 -n 22)1/2 = n 1(2 Δ)1/2

ØAcceptance Angle (θm): - It may defined as the maximum angle that a light ray can have relative to the axis of the fibre and propagate down the fibre. θm =sin-1(NA)=sin-1 [(n 1 2 -n 22)1/2 ] = sin-1 [n 1(2 Δ)1/2] ØAcceptance Cone (2θm): - The twice of the acceptance angle is known as Acceptance Cone=2θm ØNormalized frequency/ V. No. : -It : is use to calculate the no. of modes supported by the fibers. V. no. =2πa/λ (NA) = 2πa/λ [(n 1 2 -n 22)1/2 ] = 2πa/λ [n 1(2 Δ)1/2] Where a=core radius, λ=Wavelength

Classification of Optical Fiber On basis of number of modes: Single mode fiber (SMF) - In single mode fiber only one mode can propagate through the fiber. Ø The light is passed through the single mode fiber through laser diode. Ø Difference between the refractive index of core and cladding is very small. Ø It has small core diameter (5 um) and high cladding diameter (70 um). Ø There is neither dispersion nor degradation therefore it is suitable for long distance communication.

Multimode fiber (MMF) - It allows a large number of modes for light ray travelling through it. Ø The core diameter is 40 um and that of cladding is 70 um. Ø The relative refractive index difference is also large than single mode fiber. Ø There is signal degradation due to multimode dispersion. Ø It is not suitable for long distance communication due to large dispersion and attenuation of signal.

There are two type of optical fiber: - Step-Index fiber (SIF): - The refractive index of core and cladding are constant. The light ray propagate through it in the form of meridional rays which cross the fiber axis during every reflection at the core cladding boundary. Graded-Index Fiber (GIF): - The cladding has a uniform refractive index. In this type of fiber core has a non uniform refractive index that gradually decrease from the center towards the core cladding interface. The light rays propagate through it in the form of helical rays. They never cross the fiber axis.

Optical Fiber Communication System

Advantage of Optical Fibre Ø Very low transmission loss. Ø High signal security. Ø Small size and light weight. Ø Immense Ø Very bandwidth to utilize. low power consumption and wide scope of system expansion etc. Ø Total electrical isolation in the transmission medium.

Disadvantage of Optical Fibre Ø System Ø Only installation is very costly. point-to-point communication is possible. Ø Precise and costly instruments would be required. Ø Splicing Ø It is time consuming. accept only unipolar codes.

Applications of Optical Fibre Optical fiber have wider range of application in almost all field, some are specified below. Ø In telecommunication field. Ø Broadband applications. Ø In military applications. Ø In decorations, etc. Ø Civil, consumer and industrial application. Ø The Endoscope(In medical field) There are two optical fibers in endoscope: (1) One for light, to illuminate the inside of patient. (2) Another for a camera to send the images back to doctor.