Optical Fiber Optical Fiber l Communication system with

Optical Fiber

Optical Fiber l Communication system with light as the carrier and fiber as communication medium l Propagation of light in atmosphere impractical: water vapor, oxygen, particles. l Optical fiber is used, glass or plastic, to contain and guide light waves l Capacity § § Microwave at 10 GHz with 10% utilization ratio: 1 GHz BW Light at 100 Tera Hz (1014 ) with 10% utilization ratio: 100 THz (10, 000 GHz)

History l 1880 Alexander G. Bell, Photo phone, transmit sound waves over beam of light l 1930: TV image through uncoated fiber cables. l Few years later image through a single glass fiber l 1951: Flexible fiberscope: Medical applications l 1956: The term “fiber optics” used for the first time l 1958: Paper on Laser & Maser

History Cont’d l 1960: Laser invented l 1967: New Communications medium: cladded fiber l 1960 s: Extremely lossy fiber: more than 1000 d. B /km l 1970, Corning Glass Work NY, Fiber with loss of less than 2 d. B/km l 70 s & 80 s : High quality sources and detectors l Late 80 s : Loss as low as 0. 16 d. B/km

Optical Fiber: Advantages l Capacity: much wider bandwidth (10 GHz) l Crosstalk immunity l Immunity to static interference l Safety: Fiber is nonmetalic l Longer lasting (unproven) l Security: tapping is difficult l Economics: Fewer repeaters

Disadvantages l l l higher initial cost in installation Interfacing cost Strength: Lower tensile strength Remote electric power more expensive to repair/maintain § Tools: Specialized and sophisticated

Optical Fiber Link Input Signal Transmitter Coder or Light Converter Source-to-Fiber Interface Fiber-optic Cable Fiber-to-light Interface Light Detector Receiver Amplifier/Shaper Decoder Output

l Light source: LED or ILD (Injection Laser Diode): § amount of light emitted is proportional to the drive current l Source –to-fiber-coupler (similar to a lens): l A mechanical interface to couple the light emitted by the source into the optical fiber l Light detector: PIN (p-type-intrinsic-n-type) or APD (avalanche photo diode) both convert light energy into current

Fiber Types l Plastic core and cladding l Glass core with plastic cladding PCS (Plastic-Clad Silicon) l Glass core and glass cladding SCS: Silica-clad silica l Under research: non silicate: Zincchloride: § 1000 time as efficient as glass

Plastic Fiber l used for short run l Higher attenuation, but easy to install l Better withstand stress l Less expensive l 60% less weight

Types Of Optical Fiber Light ray Single-mode step-index Fiber Multimode step-index Fiber n 1 core n 2 cladding no air Variable n Multimode graded-index Fiber Index porfile

Single-mode step-index Fiber Advantages: l l l Minimum dispersion: all rays take same path, same time to travel down the cable. A pulse can be reproduced at the receiver very accurately. Less attenuation, can run over longer distance without repeaters. Larger bandwidth and higher information rate Disadvantages: l l l Difficult to couple light in and out of the tiny core Highly directive light source (laser) is required. Interfacing modules are more expensive

Multi Mode l Multimode step-index Fibers: § inexpensive; easy to couple light into Fiber § result in higher signal distortion; lower TX rate l Multimode graded-index Fiber: § intermediate between the other two types of Fibers

Acceptance Cone & Numerical Aperture Acceptance Cone q. C n 2 cladding n 1 core n 2 cladding Acceptance angle, qc, is the maximum angle in which external light rays may strike the air/Fiber interface and still propagate down the Fiber with <10 d. B loss. Numerical aperture: NA = sin qc = (n 12 - n 22)

Losses In Optical Fiber Cables l The predominant losses in optic Fibers are: § absorption losses due to impurities in the Fiber material § material or Rayleigh scattering losses due to microscopic irregularities in the Fiber § chromatic or wavelength dispersion because of the use of a non-monochromatic source § radiation losses caused by bends and kinks in the Fiber § modal dispersion or pulse spreading due to rays taking different paths down the Fiber § coupling losses caused by misalignment & imperfect surface finishes

Absorption Losses In Optic Fiber Loss (d. B/km) 6 5 4 3 2 1 0 Rayleigh scattering & ultraviolet absorption Peaks caused by OH- ions Infrared absorption 0. 7 0. 8 0. 9 1. 0 1. 1 1. 2 1. 3 1. 4 1. 5 1. 6 1. 7 Wavelength (mm)

Fiber Alignment Impairments Axial displacement Angular displacement Gap displacement Imperfect surface finish

Light Sources l Light-Emitting Diodes (LED) § made from material such as Al. Ga. As or Ga. As. P § light is emitted when electrons and holes recombine § either surface emitting or edge emitting l Injection Laser Diodes (ILD) § similar in construction as LED except ends are highly polished to reflect photons back & forth

ILD versus LED l Advantages: § § more focussed radiation pattern; smaller Fiber much higher radiant power; longer span faster ON, OFF time; higher bit rates possible monochromatic light; reduces dispersion l Disadvantages: § much more expensive § higher temperature; shorter lifespan

Light Detectors l PIN Diodes § photons are absorbed in the intrinsic layer § sufficient energy is added to generate carriers in the depletion layer for current to flow through the device l Avalanche Photodiodes (APD) § photogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electrons § avalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes

Bandwidth & Power Budget l The maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d (ms/km) is: R = 1/(5 d. D) l Power or loss margin, Lm (d. B) is: Lm = Pr - Ps = Pt - M - Lsf - (Dx. Lf) - Lc - Lfd - Ps 0 where Pr = received power (d. Bm), Ps = receiver sensitivity(d. Bm), Pt = Tx power (d. Bm), M = contingency loss allowance (d. B), Lsf = source-to. Fiber loss (d. B), Lf = Fiber loss (d. B/km), Lc = total connector/splice losses (d. B), Lfd = Fiber-to-detector loss (d. B).

Wavelength-Division Multiplexing WDM sends information through a single optical Fiber using lights of different wavelengths simultaneously. l 1 Multiplexer Demultiplexer l 2 l 3 ln-1 ln Laser Optical sources l 1 l 2 l 3 Optical amplifier ln-1 ln Laser Optical detectors

On WDM and D-WDM l WDM is generally accomplished at 1550 nm. l Each successive wavelength is spaced > 1. 6 nm or 200 GHz for WDM. l ITU adopted a spacing of 0. 8 nm or 100 GHz separation at 1550 nm for dense-wavedivision multiplexing (D-WDM). l WD couplers at the demultiplexer separate the optic signals according to their wavelength.