Basic Optical Fiber Working


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Here you can understand how optical fiber works and what are the applications and future possibilities........

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Basic Optical Fiber Working

  1. 1. What is lightwave technology? <ul><li>Lightwave technology uses light as the primary medium to carry information . </li></ul><ul><li>The light often is guided through optical fibers (fiber optic technology). </li></ul><ul><li>Most applications use invisible (infrared) light . </li></ul>
  2. 2. Why lightwave technology? <ul><li>Economics </li></ul><ul><li>Speed </li></ul><ul><li>Distance </li></ul><ul><li>Weight/size </li></ul><ul><li>Freedom from interference </li></ul><ul><li>Electrical isolation </li></ul><ul><li>Security </li></ul>(HP)
  3. 3. Basic Fiber-Optic System <ul><li>Transmitter (laser diode or LED). </li></ul><ul><li>Fiber-optic cable. </li></ul><ul><li>Receiver (PIN diode or avalanche photodiode). </li></ul><ul><li>Most fiber systems are digital but analog is also used. </li></ul>
  4. 4. How “fast” is fiber optics? <ul><li>Copper wire (twisted pair) up to ~ 100 Mb/sec (short distances) </li></ul><ul><ul><li>1,500 phone calls </li></ul></ul><ul><ul><li>2 TV channels </li></ul></ul><ul><ul><li>2 Bibles/sec </li></ul></ul><ul><li>Coaxial cable (also copper) Up to ~1 Gb/sec (short distances) </li></ul><ul><ul><li>15,000 phone calls </li></ul></ul><ul><ul><li>20 TV channels (> 200 with “data compression”) </li></ul></ul><ul><ul><li>20 bibles/second </li></ul></ul><ul><li>Optical Fiber up to 50 Tb/s (50,000 Gb/s) (long distances) </li></ul><ul><ul><li>0.78 billion phone calls </li></ul></ul><ul><ul><li>1 million TV channels </li></ul></ul><ul><ul><li>1 million Bibles/second </li></ul></ul>(Light travels in fibers at about 2/3 the speed of light, but so do electrical signals in wire!)
  5. 5. The Carrier - Light Rays Waves Particles Absorption Emission Interference Refraction Reflection n 0 n 1 n 0 Bandgap Conduction band Valence band
  6. 6. Light Properties - Wavelength Distance Field Strength 1000 pm (picometer) = 1 nm (nanometer) 1000  m = 1 mm (millimeter) 1000 nm (nanometer) = 1  m (micrometer) 1000 mm = 1 m (meter) Wavelength  : distance to complete one sine wave 
  7. 7. Electromagnetic Spectrum Frequency Sonic Ultrasonic AM Broadcast Shortwave Radio FM Radio/TV Radar Infrared Light Visible Light Ultraviolet X-Rays Wavelength 1 kHz 1 MHz 1 GHz 1 THz 1 ZHz 1 YHz c = f •  • n c: Speed of light ( 2.9979 m/µs ) f: Frequency  Wavelength n: Refractive index (vacuum: 1.0000; standard air: 1.0003; silica fiber: 1.44 to 1.48) 1 Mm 1 km 1 m 1 mm 1 pm 1 nm
  8. 8. Light Wave Transmission Bands Near Infrared Frequency Wavelength 1.6 229 1.0 0.8 µm 0.6 0.4 1.8 1.4 UV (vacuum) 1.2 THz 193 461 0.2 353 Longhaul Telecom Regional Telecom Local Area Networks 850 nm 1550 nm 1310 nm CD Players 780 nm HeNe Lasers 633 nm
  9. 9. Wavelength and “Color” Names <ul><li>Wavelength (and “color”) can be controlled by type and amount of “dopants” (alloy materials) used to make the P and N sides of the light emitting diode. </li></ul><ul><ul><ul><li>Light emitting diodes (LEDs) with visible light output are used for indicator lights, etc. </li></ul></ul></ul><ul><ul><ul><li>LEDs with infra-red output used as electro-optic (EO) converters for step or graded index fibers </li></ul></ul></ul><ul><ul><li>Construction of two parallel semi-reflecting surfaces on the diode with proper spacing relative to desired wavelength produces enhancement of one wavelength, yielding almost monochromatic LASER radiation (laser diode -- LD), used for single-mode fiber </li></ul></ul><ul><ul><li>Proper efficient coupling of light into the fiber core is a major design consideration as well (not discussed here) </li></ul></ul>400nm Ultra-violet* blue 500nm 600nm 700nm 850nm 1300nm 1550nm green red Infra-red* *not visible to human eyes 850, 1300 and 1550 nm are local minima in the fiber transmission spectrum, wavelengths often used for fiber systems.
  10. 10. Optical Power <ul><li>Power (P): </li></ul><ul><ul><li>Transmitter: typ. -6 to +17 dBm (0.25 to 50 mW) </li></ul></ul><ul><ul><li>Receiver: typ. -3 to -35 dBm (500 down to 0.3 µW) </li></ul></ul><ul><ul><li>Optical Amplifier: typ. +3 to +20 dBm (2 to 100 mW) </li></ul></ul><ul><li>Laser safety </li></ul><ul><ul><li>International standard: IEC 825-1 </li></ul></ul><ul><ul><li>United States (FDA): 21 CFR 1040.10 </li></ul></ul><ul><ul><li>Both standards consider class I safe under reasonable forseeable conditions of operation (e.g., without using optical instruments, such as lenses or microscopes) </li></ul></ul>
  11. 11. Refraction and reflection
  12. 12. Meaning of refractive index <ul><li>Refractive index, n defined by: </li></ul>n 1 n 2  1  1  2 Here n 1 < n 2
  13. 13. Snell’s “Law” <ul><li>Demonstration with glass of water </li></ul>Material with higher dielectric constant   , slower wave speed, c 2 , larger index n 2 . Line perpendicular to interface at point where ray intersects interface. Angle of Refraction F Angle of Incident Ray D Angle of Reflected Ray R R=D and Sin(R)=Sin(D) Material with lower dielectric constant   , faster wave speed, c 1 , smaller index n 1 . n o =1/c o =  o  o :vacuum (or air) n 1 =1/c 1 =  1  o :lower index medium n 2 =1/c 2 =  2  o :higher index medium Snell’s “law”: n 2 • Sin(D) = n 1 • Sin(F) Incident ray power is partly in reflected ray, partly in refracted ray.
  14. 14. Total Internal Reflection <ul><li>When angle of incidence is beyond B, ~100% of optical power is reflected internally </li></ul><ul><ul><li>some sources measure angle from the perpendicular line rather than from the interface, so inequality is stated differently </li></ul></ul><ul><li>When you (or a fish) go under a smooth water surface (e.g., a swimming pool), you can see up to the air only inside of a circle. Outside that circle, you see only reflections from the surface. </li></ul>B Location of your (underwater) eye
  15. 15. Total Internal Reflection <ul><li>There is a critical angle at which no light can be refracted at all, so 100% of the light is reflected </li></ul><ul><ul><li>Light is trapped in the water and cannot escape into the air </li></ul></ul><ul><ul><li>This works with any dense medium, such as plastic or glass, the same way it works with water </li></ul></ul>
  17. 17. The Logarithmic Scale 0 dBm = 1 mW 3 dBm = 2 mW 5 dBm = 3 mW 10 dBm = 10 mW 20 dBm = 100 mW -3 dBm = 0.5 mW -10 dBm = 100  W -30 dBm = 1  W -60 dBm = 1 nW 0 dB = 1 + 0.1 dB = 1.023 (+2.3%) + 3 dB = 2 + 5 dB = 3 + 10 dB = 10 -3 dB = 0.5 -10 dB = 0.1 -20 dB = 0.01 -30 dB = 0.001 dB = 10 • log 10 (P 1 / P 0 ) dBm = 10 • log 10 (P / 1 mW)
  18. 18. Interference <ul><li>Incoherent light adds up optical power </li></ul><ul><li>Coherent light adds electromagnetic fields </li></ul><ul><li>Zero phase shift: constructive interference </li></ul><ul><li>180º phase shift: </li></ul><ul><li>destructive interference </li></ul>+ = + =
  19. 19. Coherence <ul><li>Coherent light Photons have fixed phase relationship (laser light) </li></ul><ul><li>Incoherent light Photons with random phase (sun, light bulb) </li></ul><ul><li>Coherence length (CL) Average distance over which photons lose their phase relationship </li></ul>1/e 1 CL
  20. 20. Reflections <ul><li>Reflections: root cause for many problems Return loss definition: </li></ul><ul><li>RL = 10 * log </li></ul>P reflected P incident P r P i
  21. 21. Polarization <ul><li>Most lasers are highly polarized </li></ul><ul><li>Degree of polarization (DOP): DOP = P polarized / P total </li></ul><ul><li>State of polarization (SOP): describes the orientation and rotation of the polarized light </li></ul>y x z SOP: linear horizontal SOP: linear vertical
  22. 22. Brief quantum description of gain process
  23. 23. Optical Resonator
  24. 24. Focusing to overcome diffraction
  25. 25. Principles of Operation - Refraction <ul><li>Light entering an optical fiber bends in towards the center of the fiber – refraction </li></ul>Refraction LED or LASER Source
  26. 26. Principles of Operation - Reflection <ul><li>Light inside an optical fiber bounces off the cladding - reflection </li></ul>Reflection LED or LASER Source
  27. 27. Principles of Operation - Critical Angle <ul><li>If light inside an optical fiber strikes the cladding too steeply, the light refracts into the cladding - determined by the critical angle </li></ul>Critical Angle
  28. 28. Principles of Operation - Angle of Incidence <ul><li>Also incident angle </li></ul><ul><li>Measured from perpendicular </li></ul>Incident Angles
  29. 29. Principles of Operation - Angle of Reflection <ul><li>Also reflection angle </li></ul><ul><li>Measured from perpendicular </li></ul>Reflection Angle
  30. 30. Principles of Operation - Angle of Refraction <ul><li>Also refraction angle </li></ul><ul><li>Measured from perpendicular </li></ul>Refraction Angle
  31. 31. Principles of Operation - Angle Summary <ul><li>Three important angles </li></ul><ul><li>The reflection angle always equals the incident angle </li></ul>Refraction Angle Reflection Angle Incident Angles
  32. 32. Meridional Rays Skew Rays
  33. 33. Refraction (Bending) of Light <ul><li>Ray A comes from straight up and does not bend much </li></ul><ul><li>Ray B comes at a shallow angle and bends a lot more </li></ul>
  34. 34. Principles of Operation - Index of Refraction <ul><li>n = c/v </li></ul><ul><ul><li>c = velocity of light in a vacuum </li></ul></ul><ul><ul><li>v = velocity of light in a specific medium </li></ul></ul><ul><li>light bends as it passes from one medium to another with a different index of refraction </li></ul><ul><ul><li>air, n is about 1 </li></ul></ul><ul><ul><li>glass, n is about 1.4 </li></ul></ul>Light bends in towards normal - lower n to higher n Light bends away from normal - higher n to lower n
  35. 35. Principles of Operation - Snell’s Law <ul><li>The amount light is bent by refraction is given by Snell’s Law: n 1 sin  1 = n 2 sin  2 </li></ul><ul><li>Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection) </li></ul><ul><li>Light can either bounce off the cladding or refract into the cladding </li></ul>
  36. 36. Principles of Operation - Snell’s Law Example 1 <ul><li>Calculate the angle of refraction at the air/core interface </li></ul><ul><li>Solution - use Snell’s law: n 1 sin  1 = n 2 sin  2 </li></ul><ul><ul><li>1 × sin(30°) = 1.47 × sin(  refraction ) </li></ul></ul><ul><ul><li> refraction = sin -1 (sin(30°)/1.47) </li></ul></ul><ul><ul><li> refraction = 19.89° </li></ul></ul>n air = 1 n core = 1.47 n cladding = 1.45  incident = 30°
  37. 37. Principles of Operation - Snell’s Law Example 2 <ul><li>Calculate the angle of refraction at the core/cladding interface </li></ul><ul><li>Solution - use Snell’s law and the refraction angle from Example 1 </li></ul><ul><ul><li>1.47sin(90° - 19.89°) = 1.45sin(  refraction ) </li></ul></ul><ul><ul><li> refraction = sin -1 (1.47sin(70.11°)/1.45) </li></ul></ul><ul><ul><li> refraction = 72.42° </li></ul></ul>n air = 1 n core = 1.47 n cladding = 1.45  incident = 30°
  38. 38. Principles of Operation - Critical Angle Calculation <ul><li>The angle of incidence that produces an angle of refraction of 90° is the critical angle </li></ul><ul><ul><li>n 1 sin(  c ) = n 2 sin(  °) </li></ul></ul><ul><ul><li>n 1 sin(  c ) = n 2 </li></ul></ul><ul><ul><li> c = sin -1 (n 2 /n 1 ) </li></ul></ul><ul><li>Light at incident angles greater than the critical angle will reflect back into the core </li></ul>Critical Angle,  c n 1 = Refractive index of the core n 2 = Refractive index of the cladding
  39. 39. Principles of Operation - Acceptance Angle and NA <ul><li>The angle of light entering a fiber which follows the critical angle is called the acceptance angle ,   = sin -1 [(n 1 2 -n 2 2 ) 1/2 ] </li></ul><ul><li>Numerical Aperture (NA) describes the light- gathering </li></ul><ul><li>ability of a fiber NA = sin  </li></ul>Critical Angle,  c n 1 = Refractive index of the core n 2 = Refractive index of the cladding Acceptance Angle, 
  40. 40. Optics-Hecht & Zajac The acceptance angle (  i ) is the largest incident angle ray that can be coupled into a guided ray within the fiber The N umerical Aperture (NA) is the sin(  i ) this is defined analogously to that for a lens Acceptance Angle
  41. 41. From Snell’s Law, For total internal reflection, θ 2 =90º What value of φ 1 corresponds to θ c ? That is the maximum acceptance angle for the fiber. φ 2 = 90º-θ c sinφ 2 = cos θ c , so Again from Snell’s Law, (= NA), so Numerical Aperture (NA)
  42. 42. Numerical Aperture (NA) Acceptance / Emission Cone NA = sin  = n 2 core - n 2 cladding 
  43. 43. Principles of Operation - Acceptance Cone <ul><li>There is an imaginary cone of acceptance with an angle  </li></ul><ul><li>The light that enters the fiber at angles within the acceptance cone are guided down the fiber core </li></ul>Acceptance Cone Acceptance Angle, 
  44. 44. <ul><li>For example, a typical silica fibre has n 1 =1.48 and n 2 =1.45 giving an NA of 0.3. </li></ul><ul><li>For a ‘large’ (extended) source, such as an LED, which also emit light over a wide range of angles, the product of the NA and the fibre entrance aperture area determines the fraction of the LED output light that can be coupled into the LED. Normally this fraction is small. </li></ul><ul><li>A laser is effectively a very small source (it is said to be spatially coherent) and can be matched to the fibre to give high power coupling efficiency </li></ul>LED LASER
  45. 45. Principles of Operation - Formula Summary <ul><li>Index of Refraction Snell’s Law Critical Angle Acceptance Angle Numerical Aperture </li></ul>
  46. 46. Basic Step-Index (SI) Fiber <ul><li>Most common designs: 100/140 or 200/280  m </li></ul><ul><li>Plastic optical fiber (POF): 0.1 - 3 mm  , core 80 to 99% </li></ul>Refractive Index (n) Diameter (r) Cladding Primary coating (e.g., soft plastic) Core 1.480 1.460 SiO 2 Glass 140  m 100  m
  47. 47. What is an optical fiber? <ul><li>It’s basically, a highly transparent “light pipe” </li></ul>Input Light Low index cladding High index Core “ Total internal reflection” up to many kilometers
  48. 48. Fiber Optic Components - Fiber <ul><li>Extremely thin strands of ultra-pure glass </li></ul><ul><li>Three main regions </li></ul><ul><ul><li>center: core (9 to 100 microns) </li></ul></ul><ul><ul><li>middle: cladding (125 or 140 microns) </li></ul></ul><ul><ul><li>outside: coating or buffer (250, 500 and 900 microns) </li></ul></ul>
  49. 49. Fiber With Cladding <ul><li>Developed in 1954 by van Heel, Hopkins & Kapany </li></ul><ul><li>Cladding is a glass or plastic cover around the core </li></ul><ul><li>Protects the total-reflection surface contamination </li></ul><ul><li>Reduces cross-talk from fibers in bundles </li></ul>
  50. 50. How Light Travels in Fiber
  51. 51. Fiber Structure <ul><li>Core and cladding are both transparent, usually glass, sometimes plastic. </li></ul><ul><li>Core has higher index of refraction. </li></ul><ul><li>Light propagates down the core, reflecting from cladding. </li></ul>
  52. 52. Typical Fiber Construction Core - Thin glass center of the fiber where the light travels Cladding - Outer optical material surrounding the core that reflects the light back into the core Buffer coating - Plastic coating that protects the fiber from damage and moisture Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket .
  53. 53. Typical Fiber Structure <ul><li>Many fibers may be gathered in a protective covered cable, with steel or kevlar plastic “rope” (not shown) incorporated for pulling strength. </li></ul>High index glass core Lower index glass cladding typical light ray Plastic protective jacket, prevents mechanical damage to outside surface of fiber. Can be removed for splicing by cutting or dissolving. Typically color coded for identification of each fiber.
  54. 54. Why use Guided Waves?
  55. 55. Optical Waveguides
  56. 56. Optical Waveguide Properties
  57. 57. Waveguide Principles <ul><li>➤ Waves propagating in a waveguide are called MODES </li></ul><ul><li>➤ Perpendicular Polarised Wave </li></ul><ul><li>➤ Electric Field Transverse to the direction of Propagation (TE MODE) </li></ul><ul><li>➤ Parallel Polarised Wave </li></ul><ul><li>➤ Electric Field Parallel to the direction of Propagation (TM MODE) </li></ul>
  58. 58. Fiber Types SM step index MM step index MM graded index
  61. 61. Multi-mode (Step-index), Graded Index, Single Mode <ul><li>Cross sectional views ( should be circles*) </li></ul><ul><li>Multi-mode Graded Index Single Mode </li></ul>125  m ~10  m ~80  m Accurate alignment less needed for splicing. Higher loss. Major time dispersion of short optical pulses due to different geometric paths. Less used today, but historically important. Accurate alignment less needed for splicing. Higher loss. Reduced dispersion due to lower wave speed in central rays, higher wave speed (lower index) in outer part of core. Used for “last mile” and service drops with single mode for long runs. Accurate alignment needed for splicing. Best low loss. Most widely used fiber type for long spans. *non-circularity is an artifact of computer artwork software .
  62. 62. Mechanical structure of single-mode and multimode step/graded index fibers
  63. 63. Single mode and Multimode Fiber <ul><li>Single mode fiber has a core diameter of 8 to 9 microns </li></ul><ul><li>Multimode fiber has a core diameter of 50 or 62.5 microns </li></ul><ul><li>Both have a cladding diameter of 125 microns </li></ul>
  64. 64. Representative Fiber Parameter Values
  65. 65. Fiber Optics Communication Technology-Mynbaev & Scheiner No intermodal time shift for single Mode Fiber BABDWIDTH FOR VARIOUS FIBER TYPES
  66. 66. Single mode Fiber <ul><li>Singlemode fiber has a core diameter of 8 to 9 microns, which only allows one light path or mode </li></ul>Index of refraction
  67. 67. Multimode Step-Index Fiber <ul><li>Multimode fiber has a core diameter of 50 or 62.5 microns (sometimes even larger) </li></ul><ul><ul><li>Allows several light paths or modes </li></ul></ul><ul><ul><li>This causes modal dispersion – some modes take longer to pass through the fiber than others because they travel a longer distance </li></ul></ul>Index of refraction
  68. 68. Multimode Graded-Index Fiber <ul><li>The index of refraction gradually changes across the core </li></ul><ul><ul><li>Modes that travel further also move faster </li></ul></ul><ul><ul><li>This reduces modal dispersion so the bandwidth is greatly increased </li></ul></ul>Index of refraction
  69. 69. Step-index and Graded-index <ul><li>Step index multimode was developed first, but rare today because it has a low bandwidth (50 MHz-km) </li></ul><ul><li>It has been replaced by graded-index multimode with a bandwidth up to 2 GHz-km </li></ul>
  70. 70. Plastic Optical Fiber <ul><li>Large core (1 mm) step-index multimode fiber </li></ul><ul><li>Easy to cut and work with, but high attenuation (1 dB / meter) makes it useless for long distances </li></ul>