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  1. 1. Course Code: EEE 409 Course Title: Optical Fiber Communication Department of Electrical and Electronic Engineering Hajee Mohammad Danesh Science and Technology University, Dinajpur-5200 Course Teacher Md. Sazedur Rahman Lecturer Dept. of Electrical and Electronic Engineering (EEE) Hajee Mohammad Danesh Science and Technology University (HSTU)
  2. 2. Course Contents: Introduction to optical communication. Guided and unguided optical communication system, Light propagation through guided medium, Optical Fibers: SMF and MMF, SI fibers and GI fibers. Fiber modes, mode theory for light propagation through fibers, single mode condition and multimode condition. Transmission impairments: fiber loss, chromatic dispersion in a fiber, polarization mode dispersion (PMD). Different types of fibers: DSF, DCF, Dispersion compensation schemes. Fiber cabling process, Fiber joints/connectors and couplers, Optical transmitter: LED and laser, Operating principles, Characteristics and driver circuits. Optical receivers: PN, PIN and APD detectors, Noise at the receiver, SNR and BER calculation, Receiver sensitivity calculation. IM/DD and Coherent communication systems. Nonlinear effects in optical fibers. Optical amplifiers, Optical modulators, Multichannel optical systems: Optical FDM, OTDM and WDM. Optical Access Network, Optical link design and Free space optical communication. Course Code: EEE 409 Credit Hour: 3.00 Course Title: Optical Fiber Communication
  3. 3. TEXT BOOKS: 1.Optical Fiber Communications – Gerd Keiser, Tata Mc Graw-Hill International edition. 2. Optical Fiber Communications – John M. Senior, PHI. REFERENCE BOOKS: 1. Fiber Optic Communications – D.K. Mynbaev , S.C. Gupta and Lowell L. Scheiner, Pearson Education. 2. Text Book on Optical Fibre Communication and its Applications – S.C.Gupta, PHI. 3. Fiber Optic Communication Systems – Govind P. Agarwal , John Wiley. 4. Fiber Optic Communications – Joseph C. Palais, Pearson Education.
  4. 4. Contents  Introduction to OFC  Brief history  Optical fiber communication system  Advantage and Limitation of OFC
  5. 5. Communication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance a communication system is usually required. Sophisticated techniques have been developed for this process using electromagnetic carrier waves operating at radio frequencies as well as microwave and millimeter wave frequencies. However, ‘communication’ may also be achieved using an electromagnetic carrier which is selected from the optical range of frequencies. Communication
  6. 6. Communication System
  7. 7. Block Diagram of Communication System
  8. 8. Background of Optical Communication Brief history of Optical Communication Technology:  Before 1792, fire beacons or smoke was used to send information  In 1792, Claude Chappe was invented optical telegraphy  He was succeeded to transmit information between Paris and Lille  By 1830, the network was extended In Europe (Bit/s < 1) Optical telegraphy system and its inventor Claude Chappe Relay Station
  9. 9.  The advent of electrical telegraphy in the 1830 replaced the use of optical telegraphy and began the era of electrical communication  The bit rate of electrical telegraphy was increased to ~ 10 bit/s by using Morse Code (dots and dashes)  The invention of telephone in 1876 enables to transmit electrical signals in analog form, which dominate comm. for a century or so.  The development of worldwide telephone networks led to many advances in the design of Electrical communication systems  Use of coaxial cable instead of wire pairs increased system capacity considerable Brief history of Optical Communication Technology
  10. 10.  The first coaxial cable put into service in 1940 with 3 MHz system capacity (300 voice channels or a single television channel)  The bandwidth was limited by frequency dependent cable losses(~10MHz)  This limitation was led to develop Micro-Wave communication (1~10GHz)  The first Microwave system was operated at 4 GHz  Most advanced coaxial cable put into service in 1975 (274 Mb/s, ~1Km)  Microwave communications generally allow larger repeater spacing, but bit rate is limited by the carrier frequency of such waves Bit rate- distance product, BL BL product 100 Mb/s-Km was achieved by 1970 and limited due to carrier frequency 1970 108 Brief history of Optical Communication Technology
  11. 11.  During 1950 it was realized that BL product can be further increased if optical waves were used as the carrier  During 1950 there was no coherent optical source nor a suitable transmission medium  In 1960 first LASER was developed (coherent light source)  After 1960 first idea was developed to use glass material as a transmission medium  In 1966 first optical fiber was made by Kao and Hockham but loss was 1000 dB/km  By reducing concentration of transition-metal ions and water ions (Fe, Cu, Cr, Ni, Mn, Cobalt and HO)  In 1970 Kapron et al. at Corning made a fiber with α < 20 dB/km in the wavelength region near 1 m  In the same time GaAs semiconductor lasers operating continuously at room temperature at 1 m were developed  Simultaneous availability of compact light source and low loss fiber led to a worldwide effort for developing FO Comm. systems Brief history of Optical Communication Technology
  12. 12. Electromagnetic Spectrum 106
  13. 13.  Purpose: Eliminate repeaters used in inter-office trunk lines  Technology: 0.8 µm GaAs semiconductor lasers, Multimode silica fibers  Repeater Spacing: 10 km  Limitations: Fiber attenuation 3 dB/km, Intermodal dispersion, bit rate 45 Mb/s  Deployed since 1974 First-generation Fiber optic Systems
  14. 14.  Opportunity: Development of low-attenuation fiber (removal of H2O and other impurities), eliminate repeaters in long-distance lines  Technology: 1.3 µm semiconductor lasers, Muti-mode fiber, low- attenuation silica fibers, bit rate: < 100 Mb/s due to dispersion 1.3 µm semiconductor lasers, Single-mode fiber, low- attenuation silica fibers, bit rate: 1.7 Gb/s  Limitation: Fiber attenuation 0.5 dB/km, repeater spacing ≈ 50 km  Deployed since 1978 Second-generation Fiber optic Systems
  15. 15.  Opportunity: Long-distance Communication  Technology: 1.55 µm single-mode semiconductor lasers, Single-mode fiber, low- attenuation silica fibers, bit rate: 2.5 Gb/s  Limitations: Fiber attenuation 0.2 dB/km, repeater spacing ≈ 60~70 km, Fiber dispersion, electronic repeaters  Deployed since 1982 Third-generation Fiber Optic Systems
  16. 16.  Opportunity: Development of erbium-doped fiber amplifiers and WDM technology Technology (deployment began in 1994): 1.55 µm single-mode, narrow-band semiconductor lasers, Single-mode, low-attenuation dispersion-shifted silica fibers, Wavelength-division multiplexing, with bit rate 2.5 Gb/s over 21000 km and 5 Gb/s over 14300 km by 1996. Using WDM technology bit rate was possible to increase 2.56 Tb/s by 2002 Nonlinear effects limit the following system parameters: Signal launch power, Propagation distance without regeneration, WDM channel separation, Maximum number of WDM channels per fiber Fourth-generation Fiber optic Systems
  17. 17. Fifth-Generation !!!!!!  Opportunity: Development of Raman amplifiers and WDM technology, dry fiber Technology (deployment began in 1994): Dry fiber with low loss over the wavelength region 1.3 to 1.65 µm lead to lightwave systems having 1000 WDM channels, Each channel 40 Gb/s, which can be extended to 160 Gb/s in future
  18. 18. BL product in several generations of lightwave systems
  19. 19. Optical fiber communication system Optical Transmitter Comm. Channel (Optical fiber) Optical Receiver Input Output Attenuation, Dispersion, crosstalk & noise
  20. 20. Greater bandwidth Low attenuation Electrical immunity (no RFI, EMI) Greater security Flexibility Falling cost Long repeater spacing Smaller size and weight than copper cables Advantages of OFC
  21. 21. Disadvantages of OFC  Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult.  Expensive over short distance  Requires highly skilled installers  Adding additional nodes is difficult
  22. 22. Disadvantages of OFC  Stimulated Raman Scattering (SRS): An interaction between light and vibrations of silica molecules, causes attenuation of short wavelength channels in WDM system  Stimulated Brillouin Scattering (SBS): An interaction between light and sound waves in the fiber, causes frequency conversion and reversal of propagation direction of light  Four Wave Mixing (FWM): Two or more optical waves at different wavelengths mix to produce new waves at other wavelengths  Self Phase Modulation (SPM): Change in signal phase due to change in intensity of the signal due to group velocity dispersion  Cross Phase Modulation (XPM): It is an interaction via the non-linear refractive index between the intensity of one light wave and optical phase of other light waves  Some other limitations: Dispersion, laser phase noise, relative intensity noise etc.
  23. 23. Application of OFC Fiber optic cables find many uses in a wide variety of industries and applications. Some uses of fiber optic cables include: • Medical: Used as light guides, imaging tools and also as lasers for surgeries • Defense/Government: Used as hydrophones for seismic waves and SONAR , as wiring in aircraft, submarines and other vehicles and also for field networking • Data Storage: Used for data transmission • Telecommunications: Fiber is laid and used for transmitting and receiving purposes • Networking: Used to connect users and servers in a variety of network settings and help increase the speed and accuracy of data transmission • Industrial/Commercial: Used for imaging in hard to reach areas, as wiring where EMI is an issue, as sensory devices to make temperature, pressure and other measurements, and as wiring in automobiles and in industrial settings. • Broadcast/CATV: Broadcast/cable companies are using fiber optic cables for wiring CATV, HDTV, internet, video ondemand and other applications.
  24. 24. Types of Optical Communication  Guided Optical Communication  In guided channel communication, optical fiber is used to transmit the light signal from transmitter to receiver. In this, the transmission of modulated signal is through glass fibers. Single Mode fiber is used for this transmission.  Unguided Optical Communication  Unguided channel communication means, that the transmission of modulated light signal through the atmosphere or vaccum to obtain optical communications. It can be termed as wireless optical communication (WOC).
  25. 25. Optical Fiber  An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length.  Light is kept in the "core" of the optical fiber by total internal reflection.
  26. 26. Fiber optic waveguide  An optical wave guide is a structure that "guides" a light wave by constraining it to travel along a certain desired path. If the transverse dimensions of the guide are much larger than the wavelength of the guided light, then we can explain how the optical waveguide works using geometrical optics and total internal reflection.
  27. 27. Construction of an Optical Fiber An optical fiber is a waveguide for light Consists of : Core: inner part where wave propagates Cladding: outer part used to keep wave in core Buffer: protective coating Jacket: outer protective shield SiO2 doped with GeO2 High n Low n
  28. 28. Types of Optical Fiber  According to the refractive index profile  Step index fiber  Graded index fiber  According to the mode of propagation  Single mode fiber (SM)  Multimode fiber (MM)
  29. 29. Single-mode fibers – used to transmit one signal per fiber (used in telephone and cable TV). They have small cores(9 microns in diameter) and transmit infra-red light from laser. Single-mode fiber’s smaller core (<10 micrometres) necessitates more expensive components and interconnection methods, but allows much longer, higher-performance links. Single Mode Fiber
  30. 30. Multi-mode fibers – used to transmit many signals per fiber (used in computer networks). They have larger cores(62.5 microns in diameter) and transmit infra-red light from LED. Multimode fiber has a larger core (≥ 50 micrometre), allowing less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. Multi Mode Fiber
  31. 31. Step index fiber is a fiber type of cylindrical waveguide core with inner core has a uniform refractive index of n1 and the core is surrounded by an outer cladding with a uniform refractive index of n2.. The refractive index of the core in graded-index fibers is not constant but decreases gradually from its maximum value n1 at the core center to its minimum value n2 at the core–cladding interface. Step index and Graded index fiber
  32. 32. Step index and Graded index fiber
  33. 33. Step index and Graded index fiber
  34. 34. 34 fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode
  35. 35. 35
  36. 36. The General Principle The classical understanding of fiber optics comes from Snell’s Law! • Step index fibers: by total internal reflection • GRIN (Graded Refractive Index) fibers: by layered changes in refractive index
  37. 37. Total Internal Reflection t i n n   sin sin 2 1  i Exit rays low index, n2 high index, n1 c i Incident rays t = 900 2 1 sin n n c   1 2 sin n n c   According to Snell’s Law
  38. 38. Step Index Fiber n2 n1 i i i i  c for total internal reflection Cladding Core Escapes core (freedom!) Stuck in core Escapes from core c = Critical Angle, c n1 > n2 1 2 sin n n c  
  39. 39. Graded Index Fiber n2 n2 n1 n varies Gradually
  40. 40. Propagation of Light Through Optical Fiber 40
  41. 41. Critical angle, θc  The minimum angle of incidence at which a light ray ay strike the interface of two media and result in an angle of refraction of 90° or greater.
  42. 42. Acceptance angle max   is the maximum acceptance angle to the axis of the fiber at which light may enter into the fiber in order to propagate max  A B   c for total internal reflection Lost by radiation Acceptance cone 42
  43. 43. Numerical aperture (NA) • Used to describe the light-gathering or light- collecting ability of an optical fiber. • In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light
  44. 44. Numerical aperture (NA) The NA defines a cone of acceptance for light that will be guided by the fiber 1 Air n0 n2 n1 A 2 B C  =90-2 > c At the air core interface 2 1 1 sin sin   n no  From the triangle ABC 2 2     
  45. 45. Numerical aperture (NA)   cos sin 1 1 n no  2 1 2 1 1 ) sin 1 ( sin     n no Using trigonometric relationship For total internal reflection, 1 = a, and   c 2 1 2 1 2 2 1 1 sin           n n n n a o  900 n2 n1 2 2 1 1 sin sin   n n  1 2 sin n n c     2 1 2 2 2 1 sin n n n a o      2 1 2 2 2 1 sin n n n NA a o     1 2 1 2 2 2 1 2n n n    1 2 1 n n n    2 1 1 ) 2 (   n NA Exmp. 2.1 and 2.2
  46. 46.  Class Test -01
  47. 47. Mode Theory for Light Propagation Through Optical Fiber
  48. 48. The mode theory uses electromagnetic wave behaviors to describe the propagation of light along a fiber. A set of guided electromagnetic waves is called the modes of the fiber. The mode theory is used to describe the properties of light that ray theory is unable to explain. An optical mode refers to a specific solution of the wave equation that satisfies the appropriate boundary conditions and has the property that its spatial distribution does not change with propagation. The fiber modes can be classified as guided modes, leaky modes, and radiation modes Fiber Modes
  49. 49. A set of guided electromagnetic waves is called the modes of an optical fiber. Maxwell's equations describe electromagnetic waves or modes as having two components. The two components are the electric field, E(x, y, z), and the magnetic field, H(x, y, z). The electric field, E, and the magnetic field, H, are at right angles to each other. Modes traveling in an optical fiber are said to be transverse. The transverse modes, shown in figure 2-17, propagate along the axis of the fiber. Fiber Modes
  50. 50. The mode field patterns shown in figure 2-17 are said to be transverse electric (TE). In TE modes, the electric field is perpendicular to the direction of propagation. The magnetic field is in the direction of propagation. Another type of transverse mode is the transverse magnetic (TM) mode. TM modes are opposite to TE modes. In TM modes, the magnetic field is perpendicular to the direction of propagation. The electric field is in the direction of propagation. Figure 2-17 shows only TE modes. he TE mode field patterns shown in figure 2- 17 indicate the order of each mode. The order of each mode is indicated by the number of field maxima within the core of the fiber. For example, TE0 has one field maxima. The electric field is a maximum at the center of the waveguide and decays toward the corecladding boundary. TE0 is considered the fundamental mode or the lowest order standing wave. As the number of field maxima increases, the order of the mode is higher. Fiber Modes
  51. 51. Like all electromagnetic phenomena, propagation of optical fields in fibers is governed by Maxwell’s equations. For a non-conducting medium without free charges, these equations take the form (in SI units) Faraday’s law : ∇×E = − ∂B ∂t (1) Ampere’s law: ∇×H = ∂D ∂t (2) Gauss’s law for static electric field / Coulomb’s law :∇·D = 0 (3) Gauss’s law for static magnetic field: ∇·B = 0 (4) Where, electric field E, magnetic field H, electric flux density D, magnetic flux density B and ∇ is a vector operator. Maxwell’s Equation
  52. 52. The flux densities are related to the field vectors by the constitutive relations D = 𝜀 E (5) B = µ H (6) Where, ε is the dielectric permittivity and μ is the magnetic permeability of the medium. Substituting for D and B and taking the curl of Eqs (1) and (2) gives: ∇ × (∇ × E) = − με 𝜕2𝑬 𝜕𝑡2 (7) ∇ × (∇ × H) = − με 𝜕2𝑯 𝜕𝑡2 (8) Maxwell’s Equation
  53. 53. Then using the divergence conditions of Eqs (4) and (5) with the vector identity: 𝛻 × (𝛻 × 𝑌) = 𝛻(𝛻 ⋅ 𝑌) − 𝛻2(𝑌) we obtain the nondispersive wave equations: 𝛻2𝑬 = με 𝜕2𝑬 𝜕𝑡2 (9) 𝛻2𝑯 = με 𝜕2𝑯 𝜕𝑡2 (10) where 𝛻2 is the Laplacian operator. And velocity of light is, c = 1 με 1 2 Maxwell’s Equation
  54. 54. If planar waveguides, described by rectangular Cartesian coordinates (x, y, z), or circular fibers, described by cylindrical polar coordinates (r, φ, z), are considered, then the Laplacian operator takes the form: 𝛻2𝜓 = 𝜕2𝜓 𝜕𝑥2 + 𝜕2𝜓 𝜕𝑦2 + 𝜕2𝜓 𝜕𝑡2 (11) or: 𝛻2𝜓 = 𝜕2𝜓 𝜕𝑟2 + 1 𝑟 𝜕𝜓 𝜕𝑟 + 1 𝑟2 𝜕2𝜓 𝜕∅2 + 𝜕2𝜓 𝜕𝑧2 (12) respectively. 𝜓 is a field (E or H) It is necessary to consider both these forms for a complete treatment of optical propagation in the fiber, although many of the properties of interest may be dealt with using Cartesian coordinates. Solution of Maxwell’s Equation
  55. 55. Basic solution of wave equation is, 𝜓 = 𝜓0 e𝑗 𝜔𝑡 − 𝒌 ⋅ 𝒓 where ω is the angular frequency of the field, t is the time, k is the propagation vector which gives the direction of propagation and the rate of change of phase with distance while the components of r specify the coordinate point at which the field is observed. When λ is the optical wavelength in a vacuum, the magnitude of the propagation vector or the vacuum phase propagation constant k, 𝑘 = 2𝜋 𝜆 k is also referred to as the free space wave number. Solution of Maxwell’s Equation
  56. 56. Assignment: Derive the equation of normalized frequency, V and normalized propagation constant, b for a cylindrical fiber. Also, show the LP modes of order l = 0, 1 against normalized frequency (V) for a circular optical waveguide for step index fiber. Ref: article 2.4.1, figure 2.15, 2.17. Ref. Book: Optical Fiber Communications – John M. Senior, PHI, 3rd Edition Solution of Maxwell’s Equation
  57. 57. Normalized Frequency, V: 𝑉 = 2𝜋 𝜆 𝑎 𝑁𝐴 = 2𝜋 𝜆 𝑎𝑛1 2∆ 1 2 Normalized propagation constant, b: b = 𝛽 𝑘 2 −𝑛2 2 2𝑛1 2∆ Normalized Frequency
  58. 58. Index profile for step index fiber: n(r) = n1 r < a (core) n2 r ≥ a (cladding) The total number of guided modes or mode volume 𝑀𝑠 for a step index fiber: 𝑀𝑠 = 𝑉2 2 Condition for step index fiber
  59. 59. Index profile for graded index fiber: n(r) = n1 √(1− 2∆ ( 𝑟 𝑎 ) 𝛼 ) r < a (core) n1 √(1− 2∆) = 𝑛2 r ≥ a (cladding) The total number of guided modes or mode volume 𝑀𝑠 for a graded index fiber: 𝑀𝑔 = ( 𝛼 𝛼+2 ) 𝑉2 2 For a parabolic refractive index profile core fiber (𝛼 = 2), 𝑀𝑔 = 𝑉2 4 Parabolic refractive index profile 𝛼 = 2 Trigonometric refractive index profile 𝛼 = 1 Step index refractive index profile 𝛼 = ∞ Condition for graded index fiber
  60. 60. Single-mode propagation of the LP01 mode in step index fibers is possible over the range: 0 ≤ V < 2.405 So, 𝑉𝑚𝑎𝑥 = 2.405 Normalized Frequency, 𝑉𝑚𝑎𝑥 = 2𝜋 𝜆𝑐𝑢𝑡 𝑜𝑓𝑓 𝑎 𝑁𝐴 = 2𝜋 𝜆 𝑎𝑛1 2∆ 1 2 Condition for single mode step index fiber
  61. 61. Example 2.1, 2.3 to 2.8 Ref. Book: Optical Fiber Communications – John M. Senior, PHI, 3rd Edition. Mathematical analysis
  62. 62.  Class Test - 02
  63. 63. Transmission impairments: fiber loss, chromatic dispersion in a fiber, polarization mode dispersion (PMD).
  64. 64. Source of Losses in Silica OF Losses in silica fiber are mainly occur due to two mechanisms:  Intrinsic absorption mechanism (due to characteristic of glass fiber)  Extrinsic absorption mechanism (due to impurities: such as OH bonds and transition metal ions (iron, cobalt, copper etc.))
  65. 65. Source of Losses in Silica OF Intrinsic absorption loss mechanisms are:  Material absorption: The atomic bonds associated with the core material absorb the longer wavelength light (Si-O; 9.2 m, Ge-O; 11.0 m; P-O; 8.1;m)  Electron absorption: In the ultraviolet region, light is absorbed in order to excite the electron in a core atoms to a higher energy state.  Rayleigh scattering: Due to small irregularities in the structure of the fiber core, which are caused by density fluctuations into the glass material at manufacture. This loss reduces with forth power of  (~ -4).
  66. 66. Attenuation wavelength Ch. Of Glass fiber
  67. 67. Attenuation wavelength Ch. Of Glass fiber
  68. 68. Fiber Transmission Windows (Bands)
  69. 69. Fiber bending and micro bending
  70. 70. Critical bending radius for MM and SM fiber Bending losses may reduce: - Large refractive index difference - Operating at shorter wavelength  2 3 2 2 1 1 4 3 n n n Rc                c cs n n R    996 . 0 748 . 2 ) ( 20 2 3 2 1 Large bending losses occur in SM fiber at critical bending radius Bending radius may reduce in SM fiber if operating wavelength becomes shorter for a fixed c and n The cutoff wavelength is the shortest wavelength at which the fiber will be single- moded. Wavelengths shorter than the cutoff will travel in multiple modes whereas wavelengths longer than the cutoff will travel in a single mode. For MM fiber
  71. 71. Other scattering losses Mie scattering: Due to imperfections such as irregularities in core-cladding interface, core-cladding refractive index differences along the fiber length, diameter fluctuations, strains, and bubbles Stimulated Brilloiun Scattering: Shift in incident light frequency in the acoustic range due to scattering process, which causes reversal of propagation direction Stimulated Raman Scattering: Shift in incident light frequency in the optical range causes attenuation
  72. 72. Problems:          out in P P dB 10 log 10 Attenuation in decibels (dB) In OFC attenuation is usually expressed in dB/Km          out in dB P P L 10 log 10 1  Example 3.3: When the mean optical power launched into an 8 Km length of fiber is 120 W, the mean optical power at the fiber out is 3 W. Determine: a) the overall signal attenuation in dB through the fiber assuming there are no connectors or splices; b) The signal attenuation/Km for the fiber c) The overall signal attenuation for 10 Km optical link using the same fiber with splices at 1 Km intervals, each giving an attenuation of 1 dB; d) The numerical input/out power ratio in (c). Example 3.1, 3.3 Senior, self study
  73. 73. Dispersion Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along optical fibers. Types of dispersion: • Chromatic or intramodal dispersion Material Dispersion Waveguide Dispersion • Intermodal dispersion or Modal Dispersion
  74. 74. • Chromatic or intramodal dispersion Chromatic or intramodal dispersion may occur in all types of optical fiber and results from the finite spectral linewidth of the optical source. Since optical sources do not emit just a single frequency but a band of frequencies then there may be propagation delay differences between the different spectral components of the transmitted signal. This causes broadening of each transmitted mode and hence intramodal dispersion. The delay differences may be caused by the dispersive properties of the waveguide material (material dispersion) and also guidance effects within the fiber structure (waveguide dispersion) .
  75. 75. Material Dispersion f1 f2 n1 n2 Changing optical path length due to a changing refractive index n1 Material Dispersion Velocity of electromagnetic wave in any medium = c/n For glass material n(), i.e, n changes with  or frequency Output wave t I Input wave
  76. 76. Mathematical Expression of Material Dispersion     d d  If  is the difference in propagation time between two sidebands then where  is the wavelength difference  = 1-2 Dmat is the material dispersion coefficient then           g mat v d d d d D 1    where  =1/vg for a unit fiber length  2 1 All optical signals consist of a range of wavelengths
  77. 77. Mathematical Expression of Material Dispersion                                       n d d c c n d d c d d c d d d d vg 2 2 2 2 2 1 c N d dn n c v g g            1 1 2 2 1    d n d c v d d D g mat                n 2  For free space c n vp      For any medium with n, 0 2     Again    c 2 
  78. 78. Single mode propagation The RI experienced by the wave is an average of the RI of core and cladding depending on the relative proportion of the wave that travels there. Thus the wave confined within the core, “see” a higher RI than that of the cladding. Therefore, wave in the core tend to travel more slowly than that of cladding. Thus signals are dispersed (because every signal consists of a range of wavelengths)
  79. 79. Waveguide Dispersion  Changing optical path length due to traveling the light wave in the core and cladding with different phase constant, which can be changed by θi θi θi t I Input light wave With different θi
  80. 80. Waveguide Dispersion Transit time per unit length of source linewidth                             i n d d c x d d c d x d cos 2 2 1 2 1 2 2   i i d d c n d dn n        cos cos           GVD=c= mat + wg
  81. 81. Fiber modes Core dia. large than  of light Core dia. large than  of light Core dia. comparable than  of light When light travels on a multimode fiber it is limited to a relatively small number of possible paths (called modes).
  82. 82. Modal Dispersion Time taken by the ray1 (axial) to travel fiber length L will be Tmin Ray2 will take maximum time to travel fiber length L, Tmax   cos cos tan 1 1 max c LN N c L velocity ce dis T g g    Using Snell’s law c LN N c L velocity ce dis T g g 1 1 min tan      cos sin 1 2   n n c ……… ….(i) ………… .(ii) ………… .(iii) t I Input light wave n1 n2 L θi =c c Ray2 θ θa Ray1(axial ) θi=900
  83. 83. Modal Dispersion Substituting Eq.(ii) into Eq.(iii) we have 2 2 1 max g g cN LN T  The difference in arrival time between the Ray1 and Ray2 is   2 1 2 1 1 2 2 1 min max g g g g g g g s N N cN LN c LN cN LN T T T        If we take Ng1/n1≈Ng2/n2 then dispersion per unit length n cn N n n cn N N N cN N L T g g g g g g s   2 1 2 1 2 1 2 1 2 1 ) ( ) (      Modal dispersion per unit length n cn Ng   2 1 mod  Modal dispersion is very sensitive for MM step index fiber
  84. 84. Effect of Dispersion on OFC n1 n2 1 0 1 1 L2 Long length fiber Input pulse n1 n2 1 0 1 1 L1 1 0 1 1 Short length fiber Output pulse Input pulse t t Output pulse No zero level Indistinguishable pulse Intersymbol Interference t
  85. 85. Wavelength dependence of Ng and n
  86. 86. Polarization mode dispersion (PMD) is a source of pulse broadening which results from fiber birefringence and it can become a limiting factor for optical fiber communications at high transmission rates. It is a random effect due to both intrinsic (caused by noncircular fiber core geometry and residual stresses in the glass material near the core region) and extrinsic (caused by stress from mechanical loading, bending or twisting of the fiber) factors which in actual manufactured fibers result in group velocity variation with polarization state. Polarization mode dispersion
  87. 87. Different types of fibers: DSF, DCF, Dispersion compensation schemes
  88. 88. Dispersion-shifted fiber (DSF) is a type of optical fiber made to optimize both low dispersion and low attenuation. Dispersion Shifted Fiber is a type of single-mode optical fiber with a core-clad index profile tailored to shift the zero-dispersion wavelength from the natural 1300 nm in silica- glass fibers to the minimum-loss window at 1550 nm. The group velocity or intramodal dispersion which dominates in single-mode fibers includes both material and waveguide dispersion. Waveguide dispersion can be made more negative by changing the index profile and thus be used to offset the fixed material dispersion, shifting or flattening the overall intramodal dispersion. This is advantageous because it allows a communication system to possess both low dispersion and low attenuation. However, when used in wavelength division multiplexing systems, dispersion-shifted fibers can suffer from four- wave mixing which causes intermodulation of the independent signals. As a result, nonzero dispersion shifted fiber is often used. DSF
  89. 89. A Dispersion Compensation Module (also called a Dispersion Compensation Fiber (DCF)) provides fixed chromatic dispersion compensation for diverse and disaster recovery DWDM networks. This can support medium to long distance fiber optic systems ranging from 30 km to 600 km. DCF
  90. 90. Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding. The three layers are made of materials with different refractive indices. DCF
  91. 91. Self Study: Dispersion compensation schemes
  92. 92. Fiber cabling process
  93. 93.  Liquid-phase technique  Vapour-phase deposition technique  Modified chemical vapour deposition (MCVD)  Plasma-activated chemical vapour deposition (PCVD) Preparation method of Optical Fiber
  94. 94. Fabrication of telecommunication-grade silica fibers involves two stages. In the first stage a vapor-deposition method is used to make a cylindrical preform with the desired refractive-index profile. The preform is typically 1 m long and 2 cm in diameter and contains core and cladding layers with correct relative dimensions. In the second stage, the by using a precision-feed mechanism that preform is drawn into a fiber feeds the preform into a furnace at the proper speed. To make the preform commonly used methods are modified chemical-vapor deposition (MCVD), outside-vapor deposition (OVD), and vapor-axial deposition (VAD). [Govind P. Agrawal] Fabrication Method
  95. 95. Modified chemical vapour deposition (MCVD) Figure 2.21: MCVD process for fiber fabrication [Govind P. Agrawal]
  96. 96. Modified chemical vapour deposition (MCVD) Figure 2.21 shows a schematic diagram of the MCVD process. In this process, successive layers of SiO2 are deposited on the inside of a fused silica tube by mixing the vapors of SiCl4 and O2 at a temperature of about 1800◦C. To ensure uniformity, a multiburner torch is moved back and forth across the tube length using an automatic translation stage. The refractive index of the cladding layers is controlled by adding fluorine to the tube. When a sufficient cladding thickness has been deposited, the core is formed by adding the vapors of GeCl4 or POCl3 (Phosphoryl Trichloride). These vapors react with oxygen to form the dopants GeO2 and P2O5: GeCl4 +O2 → GeO2 +2Cl2, 4POCl3 +3O2 → 2P2O5 +6Cl2. The flow rate of GeCl4 or POCl3 determines the amount of dopant and the corresponding increase in the refractive index of the core. A triangular-index core can be fabricated simply by varying the flow rate from layer to layer. When all layers forming the core have been deposited, the torch temperature is raised to collapse the tube into a solid rod of preform. The MCVD process is also known as the inner-vapor- deposition method, as the core and cladding layers are deposited inside a silica tube.
  97. 97. Fiber drawing Figure 2.22: Apparatus used for fiber drawing. [Govind P. Agrawal]
  98. 98. Fiber drawing The fiber drawing step is essentially the same irrespective of the process used to make the preform. Figure 2.22 shows the drawing apparatus schematically. The preform is fed into a furnace in a controlled manner where it is heated to a temperature of about 2000◦C. The melted preform is drawn into a fiber by using a precision-feed mechanism. The fiber diameter is monitored optically by diffracting light emitted by a laser from the fiber. A change in the diameter changes the diffraction pattern, which in turn changes the photodiode current. This current change acts as a signal for a servocontrol mechanism that adjusts the winding rate of the fiber. The fiber diameter can be kept constant to within 0.1% by this technique. A polymer coating is applied to the fiber during the drawing step. It serves a dual purpose, as it provides mechanical protection and preserves the transmission properties of the fiber. The diameter of the coated fiber is typically 250 µm, although it can be as large as 900 µm when multiple coatings are used. The tensile strength of the fiber is monitored during its winding on the drum. The winding rate is typically 0.2–0.5 m/s. Several hours are required to convert a single preform into a fiber of about 5 km length.
  99. 99. Fiber joints/connectors and couplers
  100. 100. Fiber Connectors Fiber connectors should have the following properties:  The fiber termination, which protects and locates the fiber ends  The fiber end alignment to provide optimum optical coupling  Protect the fiber ends from the environments and provide adequate strength at the joint
  101. 101. Why we need joints  Fiber can only be installed in lengths upto 2Km, for longer spans a joint is needed.  For the repair of damaged fiber.  For test purpose at terminal equipment.  All of the fiber cable in a building cannot be installed as one continues cable run.  joints are needed to complete network cabling.  Temporary access is needed for test purposes.
  102. 102. Type of fiber connectors  Cylindrical ferrule connectors  Bioconical ferrule connectors  Double eccentric connectors  Duplex and multiple fiber connectors  Expanded beam connectors
  103. 103. Cylindrical ferrule connector
  104. 104. Bioconical ferrule connector
  105. 105. Double eccentric connector
  106. 106. Duplex and multiple fiber connector
  107. 107. Expanded beam connector Lenses Optical Fiber Optical Fiber
  108. 108. Graded Index (GRIN) Rod Lens An alternative lens geometry to facilitate efficient beam expansion and collimation within expanded beam connectors is that of GRIN rod lens. Its diameter varies typically from 0.5 mm to 2 mm   10 to 50 GRIN Rod Lens
  109. 109. Graded Index (GRIN) Rod Lens   10 to 50 GRIN Rod Lens The GRIN-rod lens, which arose from developments on graded index fiber waveguides ,comprises a cylindrical glass rod typically 0.2 to 2 mm in diameter which exhibits a parabolic refractive index profile with a maximum at the axis similar to graded index fiber. Light propagation through the lens is determined by the lens dimensions and, because refractive index is a wavelength-dependent parameter, by the wavelength of the light. The GRIN-rod lens can produce a collimated output beam with a divergent angle of between 1° and 5° from a light source situated on, or near to, the opposite lens face.
  110. 110. The refractive index variation with radius therefore causes all the input rays to follow a sinusoidal path through the lens medium. The traversion of one sinusoidal period is termed one full pitch and GRIN-rod lenses are manufactured with several pitch lengths.Three major pitch lengths are as follows: 1. The quarter pitch (0.25 pitch) lens, which produces a perfectly collimated output beam when the input light emanates from a point source on the opposite lens face. Conversely, the lens focuses an incoming light beam to a point at the center of the opposite lens face.Thus the focal point of the quarter pitch GRIN-rod lens is coincident with the lens faces, thus providing efficient direct butted connection to optical fiber. 2. The 0.23 pitch lens is designed such that its focal point lies outside the lens when a collimated beam is projected on the opposite lens face. It is often employed to convert the diverging beam from a fiber or laser diode into a collimated beam, as illustrated in. 3. The 0.29 pitch lens is designed such that both focal points lie just outside the lens end faces. It is frequently used to convert a diverging beam from a laser diode into a converging beam. Hence, it proves useful for coupling the output from a laser diode into an optical fiber, or alternatively for coupling the output from an optical fiber into a photodetector. Graded Index (GRIN) Rod Lens
  111. 111. Quarter pitch lens 0.25P 0.25P
  112. 112. 0.23 and 0.29 pitch lens 0.23P 0.29P Source Fiber Fiber GRIN GRIN
  113. 113. Fiber joint loss  In fiber-fiber connection the optical loss encountered at interface.  The loss in optical power through a connection is defined as Po is the power emitted from the source fiber Pi is the power accepted by the connected fiber
  114. 114. Fiber joint loss  Intrinsic coupling losses are limited by reducing fiber mismatches between the connected fibers.  This is done by procuring only fibers that meet stringent geometrical and optical specifications  Extrinsic coupling losses are limited by proper connection procedures.
  115. 115. Reducing Fresnel Loss  To reduce the amount of loss from Fresnel reflection, the air gap can be filled with an index matching gel.  The refractive index of the index matching gel should match the refractive index of the fiber core.  Index matching gel reduces the step change in the refractive index at the fiber interface.
  116. 116. Misalignment losses These losses depends upon the fiber type , core diameter and the distribution of the optical power.
  117. 117. Fiber Splice A permanent joint formed between two individual optical fibers in the field or factory is known as a FIBER SPLICE. Used to establish long haul optical fiber links Two types of splicing: 1. Fusion splicing 2. Mechanical splicing
  118. 118. Fiber Coupler An optical fiber coupler is a device that distributes light from a main fiber into one or more branch fibers. The latter case is more normal and such devices are known as multiport fiber couplers. Optical fiber couplers are often passive devices in which the power transfer takes place either: • through the fiber core cross-section by butt jointing the fibers or by using some form of imaging optics between the fibers (core interaction type) • through the fiber surface and normal to its axis by converting the guided core modes to both cladding and refracted modes which then enable the power-sharing mechanism (surface interaction type)
  119. 119. Multiport Coupler Multiport optical fiber couplers can also be subdivided into the following: 1. Three- and four-port* couplers, which are used for signal splitting, distribution and combining. 2. Star couplers, which are generally used for distributing a single input signal to multiple outputs. 3. Wavelength division multiplexing (WDM) devices, which are a specialized form of coupler designed to permit a number of different peak wavelength optical signals to be transmitted in parallel on a single fiber. In this context WDM couplers either combine the different wavelength optical signal onto the fiber (i.e. multiplex) or separate the different wavelength optical signals output from the fiber (i.e. demultiplex).
  120. 120. Fiber Coupler
  121. 121. Mid Term Syllabus ( First slide to Previous slide)
  122. 122. Optical transmitter: LED and laser, Operating principles, Characteristics and driver circuits
  123. 123. Types of optical sources There are three main types of optical light source are available:  Wide band sources (incandescent lamps)  Incoherent sources (LEDs)  Coherent sources (LASERs)
  124. 124. Energy state basic The interaction of light with matter takes place in discrete packets of energy or quanta, called photons. Furthermore, the quantum theory suggests that atoms exist only in certain discrete energy states such that absorption and emission of light causes them to make a transition from one discrete energy state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the higher energy state E2 and the lower energy state E1 by the expression: E = E2 - E1 = hf where h = 6.626 × 10-34 J s is Planck’s constant.
  125. 125. Light Emission Absorption and Emission of Radiation
  126. 126. If the photon energy hν of the incident light of frequency ν is about the same as the energy difference Eg = E2 -E1, the photon is absorbed by the atom, which ends up in the excited state. Incident light is attenuated as a result of many such absorption events occurring inside the medium. The excited atoms eventually return to their normal “ground” state and emit light in the process. In the case of spontaneous emission, photons are emitted in random directions with no phase relationship among them. LEDs emit light through the incoherent process of spontaneous emission. Stimulated emission, by contrast, is initiated by an existing photon. The remarkable feature of stimulated emission is that the emitted photon matches the original photon not only in energy (or in frequency), but also in its other characteristics, such as the direction of propagation. All lasers, including semiconductor lasers, emit light through the process of stimulated emission and are said to emit coherent light. Absorption and Emission of Radiation
  127. 127. Absorption and Emission of Radiation Absorption Spontaneous emission Stimulated emission Initial state Final state
  128. 128. Population inversion Under the conditions of thermal equilibrium given by the Boltzmann distribution (Eq. (6.2)) the lower energy level E1 of the two-level atomic system contains more atoms than the upper energy level E2. This situation, which is normal for structures at room temperature, is illustrated in Figure 6.2(a). However, to achieve optical amplification it is necessary to create a nonequilibrium distribution of atoms such that the population of the upper energy level is greater than that of the lower energy level (i.e. N2 > N1). This condition, which is known as population inversion, is illustrated in Figure 6.2(b). In order to achieve population inversion it is necessary to excite atoms into the upper energy level E2 and hence obtain a nonequilibrium distribution. This process is achieved using an external energy source and is referred to as ‘pumping’. A common method used for pumping involves the application of intense radiation (e.g. from an optical flash tube or high-frequency radio field). In the former case atoms are excited into the higher energy state through stimulated absorption.
  129. 129. The LASERs Light Amplification by Stimulated Emission of Radiation (LASER) Type of lasers  Solid state lasers  Semiconductor lasers  Gas lasers  Dye lasers
  130. 130. Basic construction of Laser Mirror with 100% reflective Mirror with partially reflective Gain medium Laser pump Energy to create non equilibrium state Photon multiplication Initial state Amplified light !!
  131. 131. The light emitting diodes A forward-biased p–n junction emits light through spontaneous emission, a phenomenon referred to as electroluminescence. In its simplest form, an LED is a forward biased p–n homojunction. Radiative recombination of electron hole pairs in the depletion region generates light; some of it escapes from the device and can be coupled into an optical fiber. The emitted light is incoherent with a relatively wide spectral width (30–60 nm) and a relatively large angular spread.
  132. 132. The light emitting diodes Drawbacks of LEDs: • Lower optical power (microwatts) • Lower modulation bandwidth • Harmonic distortion
  133. 133. The light emitting diodes Merits of LEDS: • Simpler fabrication • Low cost • Reliability • Generally less temperature dependence • Simpler drive circuitry • Linearity
  134. 134. LED structures Ohmic contacts Light output n-type substrate p-type epitaxial layer Planar LED Ohmic contact p-type n-type Dome LED
  135. 135. Double Heterojunction LED
  136. 136. Surface emitting LED
  137. 137. The LED structures can be classified as surface-emitting or edge-emitting, depending on whether the LED emits light from a surface that is parallel to the junction plane or from the edge of the junction region. Both types can be made using either a p–n homojunction or a heterostructure design in which the active region is surrounded by p- and n-type cladding layers. The heterostructure design leads to superior performance, as it provides a control over the emissive area and eliminates internal absorption because of the transparent cladding layers. Figure 3.8 shows schematically a surface emitting LED design referred to as the urrus-type LED [22]. The emissive area of the device is limited to a small region whose lateral dimension is comparable to the fiber-core diameter. The use of a gold stud avoids power loss from the back surface. The coupling efficiency is improved by etching a well and bringing the fiber close to the emissive area. The power coupled into the fiber depends on many parameters, such as the numerical aperture of the fiber and the distance between fiber and LED. The addition of epoxy in the etched well tends to increase the external quantum efficiency as it reduces the refractive-index mismatch. Several variations of the basic design exist in the literature. In one variation, a truncated spherical microlens fabricated inside the etched well is used to couple light into the fiber [23]. In another variation, the fiber end is itself formed in the form of a spherical lens [24]. With a proper design, surface-emitting LEDs can couple up to 1% of the internally generated power into an optical fiber. Surface emitting LED
  138. 138. Edge emitting LED
  139. 139. LASER or LED as light source? Semiconductor lasers emit light through stimulated emission. As a result of the fundamental differences between spontaneous and stimulated emission, they are not only capable of emitting high powers (∼ 100 mW), but also have other advantages related to the coherent nature of emitted light. A relatively narrow angular spread of the output beam compared with LEDs permits high coupling efficiency (∼ 50%) into single-mode fibers. A relatively narrow spectral width of emitted light allows operation at high bit rates (∼ 10 Gb/s), since fiber dispersion becomes less critical for such an optical source. Furthermore, semiconductor lasers can be modulated directly at high frequencies (up to 25 GHz) because of a short recombination time associated with stimulated emission. Most fiber-optic communication systems use semiconductor lasers as an optical source because of their superior performance compared with LEDs.
  140. 140. Electrical bandwidth ) ( ) (det log 10 10 source input power electrical ector output power electrical REdB  Electrical bandwidth: The ratio of the electrical power output (at the detector) to the electrical input power in in out out dB R I R I RE 2 2 10 log 10  2 10 log 10        in out dB I I RE Electrical 3 dB point occur when 2 1 , 2 1 2               in out in out I I or I I
  141. 141. Optical bandwidth ) ( ) det ( log 10 10 source at d transmitte in power optical ector at received out power optical ROdB  Optical bandwidth: The ratio of the optical power output (received at the detector) to the electrical input power (transmitted at the source)        in out dB I I RO 10 log 10 Optical 3 dB point occur when 2 1        in out I I Example: 7.7 (self study)
  142. 142. Modulation bandwidth Electrical 3 dB point Optical 3 dB point Frequency Electrical BW Optical BW 0.50 0.707 Iout/Iin
  143. 143. Although an optical source is a major component of optical transmitters, it is not the only component. Other components include a modulator for converting electrical data into optical form (if direct modulation is not used) and an electrical driving circuit for supplying current to the optical source. An external modulator is often used in practice at bit rates of 10 Gb/s or more for avoiding the chirp that is invariably imposed on the directly modulated signal. Transmitter
  144. 144. Driving Circuit
  145. 145. The purpose of driving circuitry is to provide electrical power to the optical source and to modulate the light output in accordance with the signal that is to be transmitted. Driving circuits are relatively simple for LED transmitters but become increasingly complicated for high-bit-rate optical transmitters employing semiconductor lasers as an optical source Figure 3.26 shows a simple driving circuit that controls the average optical power through a feedback mechanism. A photodiode monitors the laser output and generates the control signal that is used to adjust the laser bias level. The rear facet of the laser is generally used for the monitoring purpose (see Fig. 3.25). In some transmitters a front-end tap is used to divert a small fraction of the output power to the detector. The bias-level control is essential, since the laser threshold is sensitive to the operating temperature. The threshold current also increases with aging of the transmitter because of gradual degradation of the semiconductor laser. The driving circuit shown in Fig. 3.26 adjusts the bias level dynamically but leaves the modulation current unchanged. Such an approach is acceptable if the slope efficiency of the laser does not change with aging. As discussed in Section 3.5.1 and seen in Fig. 3.20, the slope efficiency of the laser generally decreases with an increase in temperature. A thermoelectric cooler is often used to stabilize the laser temperature. An alternative approach consists of designing driving circuits that use dual- loop feedback circuits and adjust both the bias current and the modulation current automatically. Driving Circuit
  146. 146. Optical receivers: PN, PIN and APD detectors, Noise at the receiver, SNR and BER calculation, Receiver sensitivity calculation.
  147. 147. What is photodetector The detector is an essential component of an optical fiber communication system and is one of the crucial elements which dictate the overall system performance. Its function is to Photodetector is an important elements in OFC, which converts optical signal into electrical form.
  148. 148. Characteristics of photodetector  High sensitivity at the operating wavelength  High fidelity  Short response time to obtain a suitable bandwidth  Noise should be minimum  Stability of performance characteristics  Small size  Low cost
  149. 149. Photodetector types Photo- detectors Photomulti- plier tubes Vacuum Photo- diodes pn-PD P-i-N PD APD PD used in OFC
  150. 150. V-I characteristics of PD I V Region 2 Region 1 Region 3 Increasing optical power Photovoltaic mode Photoconductive mode
  151. 151. Photodetection principles Eg hf >Eg - + p n
  152. 152. Photon absorption in intrinsic material E2 - E1 hf >E2 – E1 E2 E1 To excite an electron incident photon should have energy E hc E E hc     1 2 0 
  153. 153. Absorption coefficient Absorption coefficient is a measure of how good the material is for absorbing light of a certain wavelength d   ) exp( 1 ) 1 ( 0 d hf r e P Ip      The photo current Ip produce by incident light of optical power P0 e : Electronic charge r : Fresnel reflection coefficient
  154. 154. Absorption coefficient of various materials
  155. 155. Quantum efficiency The quantum efficiency n is defined as the fraction of incident photons which are absorbed by the photodetector and generated electrons which are collected at the detector terminal p e r r   n = Number of electrons collected/ Number of incident photons rp: Incident photon rate re: Corresponding electron rate
  156. 156. Relationship between responsivity and n hf P r r p e 0     ) ( 1 0   AW P I R p hf P rp 0  where Ip: Photocurrent, P0: Incident optical power The incident photon rate rp in terms of optical power and photon energy can be written as The responsivity R of a photodetector is defined as Electron rate can be defined as Output photocurrent is: hf e P Ip 0   Thus hc e hf e R     
  157. 157. Wavelength dependence of responsivity Responsivity (A/W) 0.44 0.88 0.5 1.0 c Ideal Si PD Typical PD Exp. 8.1, 8.2 J. Senior
  158. 158. p-n photodiodes E-Field Depletion region Absorption region hf x p n Diffusion region Load
  159. 159. Output Ch. of a typical p-n photodiodes Reverse bias (V) Current A 10 20 30 40 200 400 600 800 High light level Low light level Dark current (no light)
  160. 160. p-i-n Photodiode E-Field Depletion region Absorption region hf x p i Load n
  161. 161. p-i-n photodiode structures Metal contact SiO2 Antireflection coating Depletion layer P+ n+ hf Front illuminated Si PD i Metal contact n+ p+ i Antireflection coating Reflection coating Side illuminated Si PD
  162. 162. Speed of response of PD There are three main factors that limit the speed of response of a PD  Drift time of carrier (depletion region)  Diffusion time of carriers (outside of depletion region)  Transition capacitance
  163. 163. Speed of response of PD Drift time of carriers through the depletion region: d drift v w t  w : width of depletion region vd : drift velocity For electric field 2x104 v/cm, vd=107cm/s, tdrift=0.1 ns when w=10 micron Diffusion time of carriers outside the depletion region: c difft D d t 2 2  d : carriers diffusion distance Dc : diffusion coefficient For 10 m diffusion distance, hole diffusion time 40 ns whereas electron diffusion time is only 8 ns
  164. 164. Speed of response of PD Time constant incurred by the capacitance of the PD with its load: w A Cj   To maximize the speed of response, the transit time need to minimize by Increasing bias voltage Decreasing layer thickness Increasing bias voltage resulting to increase drift speed, which lead to reduce drift time. Further depletion layer thickness may increase with bias voltage Quantum efficiency will fall with decreasing layer thickness, w. It also increase junction capacitance, which lead to rise RC time constant. Thus device speed will slowdown
  165. 165. PD response to a rectangular input pulse W W P n n n P P + - + + - - Large C Narrow W Low C and W>>1/s
  166. 166. Avalanche photodiodes hf Load n p p+ i Gain region Absorption region E-field
  167. 167. Silicon reach through APD p+ p  n+ 50m Absorption region Gain region E-field When reverse biased voltage is 10% less of the avalanche breakdown limit, the depletion layer reaches through to the  region
  168. 168. APD response time APD response time is limited by:  Transit time of the carrier across the absorption region  Time taken for avalanche multiplication  RC time constant
  169. 169. APD responsivity hc e hf e R      ) ( 1 0   AW P I R p Responsivity for p-i-n PD Responsivity for APD PD ) ( 1 0   AW P MI R p M: APD gain Responsivity for p-i-n PD hc e M R    Responsivity for APD PD
  170. 170. Basic structure of an optical receiver PD Preamplifier Post-amplifier Pre-detection filter Electrical signal Optical signal
  171. 171. Sources of noise in an optical receiver Photo- detection Avalanche gain Detector load bias Electronic gain Optical signal Photodetector Amplifier Electrical signal Noise • Quantum shot • Dark current • Surface leakage Noise Excess noise due to random gain mechanisms Noise Thermal Noise • Thermal noise • Device (active element) • Surface leakage currents
  172. 172. Photodetector noises DS n DB n Q n PD n i i i i 2 2 2 2    Q n i 2 DB n i 2 DS n i 2 : Due to quanta of light generating packets of electron-hole pairs : Due to thermally generated dark currents occurring in the PD bulk material : Due to surface leakage currents
  173. 173. Signal to noise ratio of p-i-n PD c n eq DS eq DB Q n s i B qI B qI i I N S 2 2 2 2 2     S/N for shot noise limited condition: eq s s Q n s B I q I i I N S 2 2 2 2 2   S/N for thermal noise limited condition: eq L s c th s KTB R I i I N S 4 2 2 2   Beq: Noise equivalent bandwidth IDB: Bulk leakage current IDS: Surface leakage current
  174. 174. Signal to noise ratio of APD PD c n eq DS eq DB eq s s i B qI B M F M qI B M F M I q M I N S 2 2 2 2 2 2 ) ( 2 ) ( 2     S/N for shot noise limited condition: eq s s Q n B M F M I q M I i Is N S ) ( 2 2 2 2 2 2 2   S/N for thermal noise limited condition: c th s i M I N S 2 2 2  M: Multiplication factor, F(M): Excess noise factor due to random fluctuation of APD gain
  175. 175. APD Noise k W k k M e      ) ) 1 ( exp( 1  x : is an empirical constant which is less than 1 F(M) can be approximated by: K:e/h e: Electron ionization coefficient h: Hole ionization coefficient x M M F  ) (
  176. 176. APD Noise e e e e M M K KM M F ) 1 2 )( 1 ( ) (     h h h h M M K KM M F ) 1 2 )( 1 1 ( ) (     F(M) depends on the value of K and type of carrier undergoing multiplication For Si APD with M=100 and K=0.02, Fe(M) ~ 4 For Ge APD with M=20 and K=0.5 gives Fe(M) ~ 11
  177. 177. S/N for shot noise limited condition: eq s s Q n B M F M I q M I i Is N S ) ( 2 2 2 2 2 2 2   S/N for thermal noise limited condition: c th s i M I N S 2 2 2  Signal to noise ratio of APD PD eq n in L eq n L B KTF P R R B KTF SR N S 4 4 2 2   S/N for thermal noise limited condition:

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