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Optical fibre transmission

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  • 1. OPTICAL FIBER TRANSMISSION
  • 2. INTRODUCTION
    • Optical fiber is a coaxial cylindrical arrangement of two homogeneous dielectric material.
    • Fiber consist of a central core of refractive index n1 and cladding of refractive index n2.
    • 1. Step index fiber: the cross-sectional refractive index has a step function at the interface between the core and the cladding.
    • 2. Graded index fiber: refractive index profile varies as a function of the radial coordinate r in the core but is constant in the cladding.
  • 3. NEED FOR OPTICAL FIBER TRANSMISSION
    • Information carrying capacity is one of the most important criteria for communication.
    • According to Shannon theorem
    • C = BW log 2 (1+ SNR)
    • where C : information carrying capacity of the channel
    • BW : Bandwidth of the channel
    • With increase in bandwidth channel capacity increases.
    • Bandwidth is approximately 10 percent of the carrier frequency.
  • 4. CONTD.. TRANSMISSION CHANNEL FREQUENCE OF OPERATION Coaxial cable 1 MHz to 100 MHz Microwave 1 GHz to 100 GHz Optical fibers 100 THz to 1000 THz TRANSMISSION CHANNEL CARRYING CAPACITY Coaxial cable 13000 channels Microwave terrestrial link 20000 channels Satellite link 100,000 channels Optical fibers 300,000 channels
  • 5. TOTAL INTERNAL REFLECTION θ 1 θ 1 θ 1 θ 2 θ 1 θ 2 θ 1=incident angle /reflected angle θ 2=refracted angle θ 2
  • 6. HOW OPTICAL FIBER CONDUCTS LIGHT n1 core n2 cladding n2 cladding n1 core α C θ 1C
  • 7. BLOCK DIAGRAM
  • 8. OPTICAL SOURCES
    • 1. LED
    • <a> HOMOSTRUCTURED LED-
    • HERE BOTH p-TYPE AND n-TYPE SEMICONDUCTOR
    • HAVE SAME ENERGY GAP
    • <b> HETEROSTRUCTURED LED-
    • IT CONSISTS OF TWO ADJOINING SEMICONDUCTOR
    • MATERIALS WITH DIFFERENT BANDGAP ENERGY .
    • 2. LASER
  • 9. LIGHT EMMITING DIODE V FB LED R ELECTRONIC CIRCUIT OF A LED
    • At the p and n junction of a semiconductor material a depletion region is created because of the electron and hole recombination.
    • A depletion voltage is developed at the junction which prevents further recombination.
    • So an external voltage called forward bias voltage (V FB >V D ) is applied.
  • 10. BASIC MECHANISM IN OPTICAL SOURCES
    • W hen the pn junction of both LED and laser diode is forward biased, electrons and holes are injected into the p and n regions, respectively.
    • These injected minority carriers can recombine either radiatively,
    • causing a photon of energy hv to be emitted, or nonradiatively,
    • where recombination energy is released in the form of heat.
    • The nonradiative recombinations take excited electrons from useful, radiative recombinations and decrease the efficiency of the process.
  • 11.
    • INTERNAL QUANTUM EFFICIENCY:
    • It is the fraction of electron-hole pairs that combine radiatively .
    • η int :INTERNAL QUANTUM EFFICIENCY
    • η int = R r / (R r + R nr )
    • where R r : Radiative recombination
    • R nr : Nonradiative recombination
  • 12. LASERS
    • Laser is a device that amplifies light by stimulated emission of radiation
    • FEATURES OF STIMULATED RADIATION
    • 1. Narrow spectral width
    • 2. High intensity
    • 3. High degree of directivity
    • 4. Coherence
    E 2 E 1 ABSORPTION SPONTANEOUS EMISSION STIMULATED EMISSION h ν 12 h ν 12 h ν 12 h ν 12(in phase)
  • 13. LIGHT AMPLIFICATION AND POSITIVE FEEDBACK MIRROR MIRROR 2 MIRROR 1 ENERGY ENERGY
  • 14. POPULATION INVERSION VALENCE BAND CONDUCTION BAND ENERGY EXTERNAL ENERGY
  • 15. PHOTODETECTOR
    • The photodetector senses the luminescent power falling upon it and converts the variation of this optical power into a correspondingly varying electric current.
    • Photodiode is a type of semiconductor based photodetector used exclusively because of its small size, suitable material, high sensitivity, and fast response time.
    • 2 types of photodiodes used are—
    • <a> pin photodiode
    • <b> avalanche photodiode(APD)
  • 16. PIN PHOTODETECTOR
    • The device consists of a p and n region separated by a very lightly n-doped intrinsic (i) region.
    • A reverse bias voltage is applied across the device so that the intrinsic region is fully depleted of carriers.
    p i Hole electron R L LOAD REGISTER BIAS VOLTAGE n n PHOTODIODE h ν photon I
  • 17. PRINCIPLE OF PHOTODETECTOR
    • When light having photon energies greater than or equal to the band-gap energy of the semiconductor material is incident on a photodetector, the photons can give up their energy and excite electrons from the valence band to the conduction band.
    • This process generates electron-hole pairs, known as photocarriers.
    • These carriers are generated in the depletion region where most of the incident light is absorbed.
  • 18. CONTD..
    • The high electric field present in the depletion region causes the carrier to separate and be collected across the reverse-bias region.
    • This gives rise to a current flow in the external circuit, known as photocurrent.
    • Optical radiation is absorbed in the semiconductor material as
    • P(x) = P 0 (1- e^( α S ( λ )x))
    • where α S ( λ )=Absorption coefficient at a wavelength λ
    • P 0 : Incident optical power level
    • P(x) : Optical power absorbed in a distance x
  • 19. CHARACTERISTICS OF PHOTODIODE
    • QUANTUM EFFICIENCY ( η ) : It is the number of electron-hole carrier pair generated per incident photon of energy h ν .
    • η = (I /q) / (P 0 /h ν )
    • where I = average photocurrent generated
    • P 0 = optical power incident on the photodetector
    • RESPONSIVITY : It specifies the photocurrent generated per unit optical power.
    • R =I / P 0 =( η q) / (h ν )
  • 20. AVALANCHE PHOTODIODE
    • Avalanche photodiode (AVD) internally multiply the primary signal photocurrent.
    • This increases the receiver sensitivity, since the photocurrent is multiplied before encountering the thermal noise associated with the receiver circuitry.
    • The carrier multiplication M is a result of impact ionization.
    • R APD = ( η q /h ν )M = R 0 M
    • where R APD : Responsivity of AVD
  • 21. DIFFICULTIES FACED BY OPTICAL FIBERS
    • 1. ATTENUATION
    • 2. DISPERTION
  • 22. ATTENUATION
    • Losses in an optical fiber can be classified as
    • 1. Intrinsic losses : These are associated with a given fiber
    • material.
    • (a) Material resonance
    • (b) Raleigh scattering
    • 2. Extrinsic losses : These are associated with fabrication.
    • cabling and installation processes.
    • (a) Absorption losses
    • (b) Bending losses
  • 23. CONTD.. λ (nm) 500 1000 1500 2000 0.1 1 10 Attenuation(dB/km) RAYLEIGH SCATTERING INFRARED ABSORPTION ULTRAVIOLET ABSORPTION
  • 24. CONTD.. LOSS λ (nm) ABSORPTION LOSS PEAKS SCATTERING LOSS
  • 25. DISPERSION
    • 1. Chromatic dispersion
    • (a) Material dispersion
    • (b) Waveguide dispersion
    • 2. Polarization-mode dispersion
  • 26. CHROMATIC DISPERSION DISPERSION PARAMETER D( λ ) D mat ( λ ) D( λ ) D wg ( λ ) λ (nm) 1100 1200 1300 1400 1500 1600
  • 27. CONTD. 1310 1500 1 1 2 3
    • CONVENTIONAL FIBER
    • 2. DISPERSION SHIFTED
    • 3. DISPERSION FLATTENED
  • 28. COPING WITH CHROMATIC DISPERSION
    • There are two basis technique for dispersion compensation.
    • 1. DISPERSION COMPENSATION FIBER(DCF)
    • The positive dispersion of the conventional fiber is compensated with the negative dispersion characteristic so that the total dispersion of the link will be almost zero.
    • 2. DISPERSION COMPENSATION GRATING(DCG)
    • Chirped fiber bragg grating (FBG) is the most developed DCG.
    • FBG reflects a set of wavelength. The shorter wavelengths are reflected almost immediately and the longer wavelengths penetrate deeper into the grating before they will be reflected.
  • 29. PULSE SPREADING COMPENSATION BY USING DCF DISPERSION COMPENATING FIBER CONVENSIONAL FIBER LD PD
  • 30. POLARISATION MODE DISPERSION
    • Two modes travel along a singlemode fiber at different velocities because of fiber’s birefringence. This effect results in the form of pulse spread called polarization-mode dispersion.
    • PMD (polarization mode dispersion) is caused by the refractive indexes along x-axis and y-axis.
    • This difference in the refractive index is called birefringence (B).
    • B=n x – n y
    • In order to cope with PMD, we use special fibers and other components that allows to preserve and control the state of mode polarization.
  • 31. CONTD..
    • Polarization maintaining fibers have very low birefringence.
    • Low birefringence is achieved by having very high asymmetry in the core or cladding.
    • Besides using PM fibers we have to use all other fiber optic component to maintain the state of polarization. This set contains PM connectors, fiber optic polarizer and PM splitters.
  • 32. AREAS TO BE IMPROVED IN OPTICAL FIBER COMMUNICATI ON
    • 1. INTEGRATION OF TRANSCEIVERS INTO ONE-SINGLE CHIP
    • For full duplex communications, a transmitter and receiver
    • are combined in one unit called a transceiver.
    • 2. REPLACEMENT OF OPTO-ELECTRONIC COMPONENT WITH
    • OPTICAL COMPONENT.
    • The replacement of opto-electronic regenarators with optical
    • amplifiers.
  • 33. ADVANTAGES OF OPTICAL FIBER
    • 1. LOW TRANSMISSION LOSS AND
    • WIDE BANDWIDTH
    • 2. SMALL SIZE AND WEIGHT
    • 3. IMMUNITY TO INTERFERENCE
    • 4. ELECTRICAL ISOLATION
    • 5. SIGNAL SECURITY
    • 6. ABUNDANT RAW MATERIAL
    • 7. NO CROSSTALK
  • 34. REFERENCES
    • [1]Gerd Keiser, ”Optical Fiber Communication,” Tata McGraw-Hill, Second Edition, 2000.
    • [2]D.Myanbaev, and L.Scheiner,” Fiber-Optic Communications Technology,’ Pearson Education, Second Edition, 2006
  • 35. Thank You - Engineers Way
    • www.erway.in

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