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Faculty Orientation WorkshopBroadband Communication Systems Prof Mrs. Tanuja S Khatavkar Department of Electronics & Telecommunication PVG’s COET, Pune 9/01/2016
Broadband Communication Systems (404190) Teaching Scheme Examination Scheme Lectures / Week: 4Hrs Paper: 30 (InSem) + 70 (End Sem) Practical /Week: 2Hrs. Lab Practise III* Oral: 50 Marks Term work: 50 Marks * BCS & MC
Introduction • This course is designed to provide engineers with an appropriate background in optical and satellite technology and link design. • Lightwave System study includes optical fibers, sources & detectors along with WDM component and • Satellite Communication combines diverse topics such as antennas, orbital mechanics and satellite link design.
A Fiber-Optic System fiber Optical Amplifier Transmitter Receiver Optional
Advantages of Optical Fiber • Enormous capacity • Low transmission loss • Cables and equipment have small size and weight • A large number of fibers fit easily into an optical cable • Applications in special environments as in aircrafts, satellites, ships • Immunity to interference • Nuclear power plants, hospitals, EMP (Electromagnetic pulse) resistive systems (installations for defense) • Electrical isolation • electrical hazardous environments • negligible crosstalk • Signal security • banking, computer networks, military systems • Silica fibers have abundant raw material Corning’s standard submarine cables can have up to 144 fibers in a single cable housing
Optical Fiber as a Communication Channel • Basic Laws and Definitions: Refractive Index, Reflection & Refraction
Optical Fiber Modes and Configurations • Fiber Types
Numerical Aperture • Definition & Interpretation • Numericals • Role of NA in optical link design • Experiment: To estimate the NA of given MMSI plastic optic fiber.
Mode Theory for Circular Waveguides • Overview of modes Electric field distribution in a symmetrical-slab waveguide • Summary of Key Modal Concepts • V number; Number of modes, Pclad/P ratio.
Single-mode Fibers, Graded-index Fiber Structure • Single –mode Fibers: MFD is an imp parameter as it is used to predict fiber properties such as splice loss, bending loss, cut-off wavelength and waveguide dispersion. • Graded-index Fiber Structure: The core R.I decreases continuously with increasing radial distance r from the centre of the fiber . • Power Law relationship: n(r) • NA of both fibers
Signal Degradation in Optical Fibers • ATTENUATION
Absorption • Scattering • Bending
Signal Degradation in the Optical Fiber Signal Attenuation It determines the maximum unamplified or repeaterless distance between transmitter and receiver. Has large influence on system cost. Signal Distortion • Causes optical pulses broaden. • Overlapping with neighboring pulses, creating errors in the receiver output. • It limits the information carrying capacity of a fiber.
Attenuation The Basic attenuation mechanisms in a fiber: • Absorption: • It is related to the fiber material(atomic defects, extrinsic absorption by impurity atoms in glass or intrinsic absorption by basic constituents atoms of fiber material). 2. Scattering: It is associated both with the fiber material and with the structural imperfections in the optical waveguide. 3. Radiative losses/ Bending losses: It originates from perturbation (both microscopic and macroscopic) of the fiber geometry.
Signal Distortion in Fibers Optical signal weakens from attenuation mechanisms and broadens due to distortion effects. Eventually these two factors will cause neighboring pulses to overlap. After a certain amount of overlap occurs, the receiver can no longer distinguish the individual adjacent pulses and error arise when interpreting the received signal.
Dispersion • Intermodal Dispersion • Intramodal or Chromatic Dispersion: • Material Dispersion • Waveguide Disperion
Intermodal delay/ modal delay The maximum pulse broadening arising from the modal delay is the difference between the travel time Tmax of the longest ray and the travel time Tmin of the shortest ray . This broadening is simply obtained from ray tracing for a fiber of length L: ∆T= Tmax – Tmin = n1/c ( L/sinøc –L) = (Ln12/cn2)∆ ∆T= Tmax – Tmin = (Ln12/cn2)∆
How to minimize the effect of modal dispersion? Answer is • Graded index fiber • Single mode fiber How to get one mode and solve the problem V = 2πa / λ x (n12 – n22)1/2 = 2πa / λ x (NA)
Step Index Multi-mode Graded Index Multi-mode
Intramodal Dispersion or Chromatic Dispersion Material Dispersion: This refractive index property causes a wavelength dependence of the group velocity of a given mode; that is, Pulse spreading occurs even when different wavelength follow the same path. • Material dispersion can be reduced: • Either by choosing sources with narrower spectral output widths OR • By operating at longer wavelengths.
LASER source will produce far less spectral dispersion or intramodal dispersion than an LED source since it is more nearly monochromatic
Signal Degradation in the Optical Fiber Attenuation Signal Distortion/ Dispersion Scattering Losses Absorption Radiative losses Intermodal Delay/ Modal Delay Intramodal Dispersion/ Chromatic Dispersion Polarization -mode Dispersion Intrinsic Absorption Extrinsic (Impurity atoms) Atomic Defects Material Dispersion Waveguide Dispersion Inhomogeneities or defects in fiber Compositional fluctuations in material Microscopic bends Macroscopic bends Absorption in Infrared region Absorption in Ultraviolet region
OPTICAL SOURCES • BASIC CONCEPTS: • E/O Convertor • Forward biased p-n junction emits light through SPONTANEOUS EMISSION. • RADIATIVE RECOMBINATION of electron-hole pairs in the depletion region regenerates LIGHT. • Emitted light is incoherent • Wide spectral width (20-80 nm) • Large angular spread
Wavelength and Spectral Width: • Radiated wavelength is determined by energy gap Eg. Spectral width is measured as full width at half maximum. • Spectral width determines the chromatic dispersion and hence BW of an OF. • Refractive index n = function(lambda). Therefore light of different wavelength travels with different velocities. So even if all beams travel the same path, they will arrive at the receiver end at different times. This results in spreading of output light pulse– chromatic dispersion. This limits the BW of SM fibers. • ∆t chroma = D(λ) L ∆λ • D(λ)—Chromatic dispersion parameter in ps/nm-km • L—length of fiber in km & ∆λ is the spectral width of light source in nm.
LED CHARACTERISTICSPower-Current Characteristics • The linear relationship(P-I) holds only over a limited current range. • Due to linear characteristics, LED is used as a source in analog modulation. • At higher currents the responsivity of the device decreases because of the increase in the active-region temperature. Internal quantum energy is temperature dependent. • At higher temperatures, there is an increase in nonradiative recombination rates.
LED CHARACTERISTICS (contd.) • Electrical Characteristics: • a) VF usually does not exceed 2V. • b) Capacitance C is inherent in an LED: C limits its practical modulation ability & thus restricts its BW. • Life time, Rise time/fall time & Bandwidth: • Life time of charge carriers is the time between the moment they are excited (injected into depletion region) and the moment they are recombined. • η int = rr/rt & 1/τ = 1/τr + 1/τnr • Rise & Fall Times: Depends on C, Ip, τ and temperature. • Reliability
Requirements from Optical Source • High Radiance Output: Required to couple sufficiently high optical power levels into a fiber • High Quantum Efficiency: So that more radiative recombinations occur. • Narrow spectral width: So as to reduce the dispersion in fiber. • Low Emission Response Time: Because delay between application of current pulse and the onset of optical emission limits the BW with which the source can be modulated directly by varying the injected current.
Requirements from Optical Source • Highly Directional Light Output: Size & Configuration must be compatible with launching light into an OF. • Source should be linear: It should accurately track the electrical input signal to minimize distortion and noise. • Should emit wavelengths where the fiber has low losses and low dispersion and also where the detectors are efficient. • Ease in modulating the source facility. • Stable Output: Immune to ambient condition (e.g. temp) • Cheap & Highly reliable
Source Materials • Shorter Wavelength: GaAs/AlGaAs • (Eg = 1.42/1.92) Wavengths: 876 nm/ 650 nm • Longer Wavelength: InGaAsP/InP • (0.75 - 1.35)Wavelengths: 1664 nm - 924 nm
LASER • POPULATION INVERSION: To get light amplification more electrons/atoms must be available in the higher energy level. So that when they are excited they drop to lower energy level, thus emitting more light. • To achieve N2>N1, an external energy source is used called pumping. • Optical Feedback • Stimulated Emission • Coherent Output • High Light Output • Highly Directed • Narrow Spectral Width.