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Note for Optical Communications - OC By UPTU Risers

  • Optical Communications - OC
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  • uttar pradesh technical university - uptu
  • Electronics and Communication Engineering
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OPTICAL COMMUNICATION (EEC-701) UNIT – 1 Introduction: Block diagram of optical fiber communication system, Advantages of optical fiber communication. Optical fiber waveguides: structure of optical wave guide, light propagation in optical fiber using ray theory, acceptance angle, numerical aperture, skew rays, wave theory for optical propagation, modes in a planar and cylindrical guide, mode volume, single mode fibers, cutoff wavelength, mode field diameter, effective refractive index and group and mode delay factor for single mode fiber. Communication may be defined as the transfer of information from one point to another. Within the communication system the information transfer is frequently achieved by superimposing or modulating the information transfer on to an electromagnetic wave which acts as a carrier for the information signal. This modulated signal is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. The use of visible light to carry the information is called optical communication and the light travels through a optical fiber cable. Optical Fiber Communication: Wavelength → 1.7µm to 0.8µm Frequency → 1014 to 1015 Hz Attenuation → 0.2 dB/km 1.1 Block Diagram of Optical Communication System: The optical fiber communication system is similar in basic concepts, the block diagram is shown: The block diagram represents the information source provides an electrical signal to a transmitter comprising an electrical signal to a transmitter comprising an electrical signal which derives an optical source to give modulation of light wave carrier. The optical sources (LED or LASER) which provide the optical conversion used to convey the light travels through fiber cable in a particular manner. At the receiver end a optical detector exists, it can be a PIN or APD photodiode, it converts light energy to electrical signal. Electrical receiver receives the signal and converts it into a message format. This is the working of a optical fiber communication system. 1.1.1 Advantages: Optical fiber communication offers a number of advantages over other communication systems because it has very low attenuation: a) Enormous potential bandwidth: The optical fiber communication offers frequency in the range of 10 13 to 1016 Hz which yields a far greater potential transmission bandwidth. At present, the bandwidth available to fiber system is not fully utilized but modulation over three hundred kilometer without repeaters is possible. b) Small size and weight: Fiber cables have very small diameter, just like a hair, rather than when it cover with jackets for protection still they are very light and small diameter. c) Electrical Isolation: Optical fibers are fabricated from glass or sometimes a plastic polymer, they are electrical insulators and they do not exhibit earth loop. d) Immunity to interface and cross talk: Optical fiber form a dielectric waveguide and are therefore free from electromagnetic interference (EMI), radio frequency interference (RFI) or switching transients electromagnetic pulses. e) Signal Security: The light from optical fiber does not radiate significantly and therefore they provide a high degree of signal security. f) Low transmission loss: The attenuation in optical fiber cable is very low (around 0.2 dB/km) as compare to other communication channels. g) Ruggedness and flexibility: Optical fibers are manufactured with very high tensile strength. The fiber may bent to quite small radii or twisted without damage. By: SHUBHANSHI GUPTA email: er.shubhanshi@gmail.com Page 1

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h) System reliability and ease of maintenance: It reduces the requirement of intermediate repeaters or live amplifiers to boost the transmitted signal strength. The reliability is high due to predicted life time of 20 years to 30 years. 1.1.2 Disadvantages: a) Optical cables can be handle with a skilled hand, the perfect joining of cable is most important otherwise signal lost in between the cable. b) The detection of faulty area is very hard because these cable situated under Mud. c) The whole establishment of optical cables is very expensive. d) Bending loss occurs, so cable must be properly aligned. 1.2 Optical Fiber Waveguide: The light travels in cable, which have a transparent core with a refractive index η 1 surrounded by transparent cladding of slightly lower refractive index η2. The cladding supports the waveguide structure and reducing the radiation loss into the surrounding air. Refractive Index = ratio of speed of light in a vacuum to that in matter (speed ν) Typically the value of light in a vacuum of refractive indexes of η =1, for air η =1.33, for water η =1.5, for glass η =2.42, for diamond 1.2.1 Ray theory transmission: Reflection and refraction: When a light ray encounters a boundary separation of two different media, either the ray reflected back into the first medium; it‟s called Reflection, or it will bent towards second medium called Refraction. Refraction affects the refractive index of the medium. The relationship at the interface is known as Snell‟s Law. η1 sin i = η2 sin r or, η1 cos i = η2 cos r (1) Total Internal Reflection: As η1 is greater than η2, the angle of refraction is always greater than the angle of incidence. Thus when angle of refraction is 900 and refracted emerges parallel to axis, the angle is called critical angle. The critical angle is given by, sin ɸc = η2 η1 At angles of incidence greater than the critical angle the light is reflected back into the originating dielectric medium that is called total internal Reflection. This is the mechanism by which light at a sufficient shallow angle (less than 900 - ɸc) may be considered to propagate down an optical fiber with low loss. By: SHUBHANSHI GUPTA email: er.shubhanshi@gmail.com Page 2

Text from page-3 Numerical Aperture and Acceptance Angle: As per the diagram a meridional ray A enters at the critical angle ɸc within the fiber at the core cladding interface. The ray enters the fiber core at an angle θa to the fiber axis and refracted at the air – core interface at critical angle. Hence any ray which are incident into the fiber core at an angle greater than θ a will be transmitted to the core cladding interface at an angle less than ɸc and will not be totally reflected, This θa is called ‘Acceptance Angle’. ‘Numerical Aperture’ (NA) is relationship between acceptance angle and refractive indexes. Fig shows a light ray incident on the fiber core at an angle θa to the fiber axis which is less than the acceptance angle for the fiber. The ray enters to the fiber from medium (air) of refractive index number. As snell‟s Law, η0 sin θa = η1 sin θ (1) Consider the right angle triangle, then 𝜋 ɸ= − 𝜃 (2) 2 where ɸ is greater than the critical angle at core cladding interface. Hence, η0 sin θa = η1 cos ɸ (3) η0 sin θa = η1 (1-sin2 ɸ)1/2 When the limiting case for TIR is considered, ɸ becomes equal to critical angle, so sin ɸc = η2/ η1 so the limiting case will be, η0 sin θa = (η12 – η22)1/2 (4) This equation serves as a basic for the definition of Numerical Apertrure, so NA = η0 sin θa = (η12 – η22)1/2 The NA may also be given in the terms of relative refractive index difference between core and cladding. ∆= By: SHUBHANSHI GUPTA η12− η22 2 η12 ≈ η 1− η 2 η1 for ∆<<1 email: er.shubhanshi@gmail.com Page 3

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Hence, NA = η1 (2∆)1/2 Skew Rays: Skew rays are the rays which does not follow the fiber axis. These rays are not easy to visualize, only the direction can be predicted in helical path of direction change of 2γ at each reflection. γ is the angle between the projection of the ray in the two dimension and the radii of the fiber core at the point of reflection. When the light input to the fiber is non uniform, rays will therefore tend to have a smoothing effect on the distribution of light as it is transmitted, giving more information output. NA in case of skew rays, NA = η0 sin θas cos γ = (η12 – η22)1/2 1.2.2 Optical Fiber Modes: Modes: In a planar guide, TE(E2 = 0) and TM(H2 = 0) modes are obtained within the dielectric cylinder. Thus two integers, l and m are necessary in order to specify the modes, the single integer (m) required for the planar guide, for cylindrical waveguide we refer TE lm and TM ln modes. Modes in Fiber: There are two fiber modes exists. First is a) Single mode Fiber b) Multi mode Fiber The optical fiber is a dielectric waveguide that operates at optical frequencies. The fiber waveguide is normally cylindrical in form. Single mode fiber sustains only one mode of propagation, whereas multimode fibers contain many hundreds of modes. The diameter of core of SMF is comparatively very small from MMF. A disadvantage of MMf is that they suffer from intermodal dispersion but it can be reduced. SINGLE MODE FIBER MULTI MODE FIBER 1.2.3 Mode Theory for Circular Waveguide: In optical fibers, the core cladding boundary conditions lead to a coupling between the electric and magnetic field components. This gives rise to hybrid modes, which means optical waveguide analysis is more complex than metallic waveguide analysis. Fibers are constructed so that the difference in the core and cladding indexes of refraction is very small, i.e "η1− η 2 ≪ 1"". The field components are called linearly polarized (LP) modes and labeled as LP jm, where j & m are integers designating mode solutions. Figure shows a electric field distribution for several of the lower order guided modes in a symmetrical slab waveguide. By: SHUBHANSHI GUPTA email: er.shubhanshi@gmail.com Page 4

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