Optical Fiber Communications

Outline History Types of fiber Light propagation Losses in optical fiber Optical fiber classification Sources Detectors Optical fiber system link budget

Introduction EM waves are guided through media composed of transparent material Without using electrical current flow Uses glass or plastic cable to contain the light wave and guided them Infinite bandwidth – carry much more information

History Photophone Smoke signals and mirrors Uncoated fiber cables Alexander Graham Bell Mirrors and detectors transmit sound wave via beam light Awkward, unreliable, no practical application Smoke signals and mirrors Uncoated fiber cables 1930, J.L. Baird and C.W. Hansell scanning and transmitting TV image

History 1951 – light transmission via bundles of fibers – leads to fiberscope – medical field 1958 – light amplification – stimulated emission 1960 – laser invention 1967 – fiber cable with clad 1970 – low loss optical cable. < 2 dB/km 1980 – optical cable refined – affordable optical communication system 1990 – 0.16dB/km loss

History 1988 – long haul transmission system 1988 – SONET 1990 – optical voice and data network are common

Advantages Wider bandwidth Immunity to crosstalk Better than metallic cables Up to several thousand GHz Speed up to several Gbps Immunity to crosstalk glass fiber/plastic are non-conductor to electrical current immune to adjacent cables Immunity to static interference immune to static noise – EMI, lightning etc.

Advantages Environmental Immunity Safety and convenience more resistant to environment, weather variations wider temperature range operation less affected by corrosive liquids and gases Safety and convenience safer and easier to install and maintain no current and voltage associated no worry about explosion and fire caused lighter and compact, flexible, lesser space required

Advantages Lower transmission loss Security Durability and reliability lesser loss compared to metallic cables 0.19 dB/km loss @ 1550 nm amplifiers can be spaced more farther apart Security virtually impossible to tap into a fiber cable Durability and reliability last longer, higher tolerance to changes in environment and immune to corrosion Economics Approximately the same cost as metallic cables less loss between repeaters. Lower installation and overall system’s cost

Disadvantages Interfacing cost Strength Remote electrical power Optical cable – transmission medium Needs to be connected to standards electronics facilities – often to be expensive Strength lower tensile strength can be improved with kevlar and protective jacket glass – fragile – less required for portability Remote electrical power need to be include electrical line within fiber cable for interfacing and signal regeneration

Disadvantages Loss due to bending bending causes irregularities in cable dimension – the light escapes from fiber core – loss of signal power prone to manufacturing defect Specialized tools, equipment and training tools to splice, repair cable test equipment for measurements skilled technicians

Optical Spectrum

Optical Communication systems

Types of fiber Optical fiber construction

Types of fiber Optical fiber construction special lacquer, silicone, or acrylate coating – outside of cladding – to seal and preserve the fiber’s strength, protects from moisture Buffer jacket – additional cable strength against shocks Strength members – increase a tensile strength Outer polyurethane jacket

Types of fiber fiber cables – either glass, plastic or both Plastic core and cladding (PCP) Glass core – plastic cladding (PCS) Glass core – glass cladding (SCS) Plastic core – more flexible - easier to install but higher attenuation than glass fiber – not as good as glass Glass core – lesser attenuation – best propagation characteristics but least rugged Selection of fiber depends on its application – trade off between economics and logistics of particular application

Physics of light Physics of light Einstein and Planck – light behaves like EM wave and particles – photon – posses energy proportional to its frequency

light propagation the lowest energy state – grounds state energy level above ground state – excited state if energy level decays to a lower level – loss of energy is emitted as a photons of light The process of decaying from one level to another – spontaneous decay or spontaneous emission Atoms can absorbs light energy and change its level to higher level – absorption

light propagation Optical power flow of light energy past a given point in a specified time

light propagation Question Optical power generally stated in decibel to define power level (dBm) Question 10 mW in dBm?

light propagation Velocity of Propagation in vacuum – 3 x 108 m/s but slower in a more dense material than free space when it passes through different medium or from one medium to another denser material – the ray changes its direction due to the change of speed

light propagation from less dense to more denser material – the ray refracted closer to the normal from more denser material to less denser material – the ray refracted away from the normal

light propagation Refraction Refractive Index Occurs when the light travels between two different material density and changes it speed based on the light frequency Refractive Index the ratio of the velocity of propagation of a light ray in a given material

light propagation

light propagation Snell’s Law how a light ray reacts when it meets the interface of two transmissive materials that have different indexes of refraction

light propagation Snell’s Law angle of incidence angle of refraction angle at which the propagating ray strike the interface with respect to the normal angle of refraction the angle formed between the propagating ray and the normal after the ray entered the 2nd medium

light propagation Snell’s Law

light propagation Question medium 1 – glass = 1.5 medium 2 – ethyl alcohol = 1.36 angle of incident – 30o determine the angle of refraction

light propagation Critical Angle the angle of incident ray in which the refracted ray is 90o and refracted along the interface

light propagation Critical Angle the minimum angle of incident at which the refracted angle is 90o or greater the light must travel from higher refractive index to a lesser refractive index material

light propagation Critical Angle

light propagation Acceptance Angle the maximum angle in which external light rays may strike the air/glass interface and still propagate down the fiber

light propagation Acceptance Angle

light propagation Numerical Aperture - NA to measure the magnitude of the acceptance angle describe the light gathering or light-collecting ability of an optical fiber the larger the magnitude of NA, the greater the amount of external light the fiber will accept

light propagation Numerical Aperture - NA

Optical Fiber Configurations Mode of propagation single mode only one path for light rays down the fiber multimode many higher order path rays down the fiber

Optical Fiber Configurations Index Profile graphical presentation of the magnitude of the refractive index across the fiber refractive index – horizontal axis radial distance from core – vertical axis

Optical Fiber Configurations Index Profile step index – single mode step index – multimode graded index - multimode

optical fiber classification Single Mode Step Index dominant – widely used in telecommunications and data networking industries the core is significantly smaller in diameter than multimode cables

optical fiber classification Multimode Step Index similar to single mode – step index fiber but the core diameter is much larger light enters the fiber follows many paths as it propagate down the fiber results in different time arrival for each of the path

optical fiber classification Multimode Mode Graded Index non uniform refractive index – decreases toward the outer edge the light is guided back gradually to the center of the fiber

optical fiber classification Comparison Single mode step index (+) minimum dispersion – same path propagation – same time of arrival (+) wider bandwidth and higher information txn. rate (-) small core – hard to couple light into the fiber (-) small line width of laser required (-) expensive – difficult to manufacture

optical fiber classification Comparison Multimode step index (+) relatively inexpensive, simple to manufacture (+) easier to couple light into the fiber (-) different path of rays – different time arrival (-) less bandwidth and transfer rate Multimode graded index intermediate characteristic between step index single and multimode

losses in optical fiber Attenuation power loss – reduction in the power of light wave as it travels down the cable effect on system’s performance by reducing: system’s bandwidth information tx rate efficiency overall system’s capacity

losses in optical fiber Attenuation

losses in optical fiber Attenuation depends on signal’s wavelength generally expressed as decibel loss per km dB/km

losses in optical fiber Attenuation optical power in decibel units is P(dBm)= Pin(dBm)-A(dB) P= measured power level (dBm) Pin =transmit power (dBm) A= cable power loss, attenuation (dB)

losses in optical fiber Question Single-mode optical cable input power 0.1 mW light source 0.25 dB/km cable loss determine optical power 100 km from the transmitter side

losses in optical fiber Absorption Loss absorption due to impurities – absorb lights and convert it into heat contributors: Ultraviolet – ionized valence electron in the silica material. infrared – photons of light absorbed by glass’s atom – converted into random mechanical vibrations - heating ion resonance – caused by OH- in in the material. OH- trapped in the glass during manufacturing process

losses in optical fiber Absorption Loss

losses in optical fiber Material – Rayleigh, Scattering Losses permanent submicroscopic irregularities during fiber drawing process when the light propagates and strike one of the impurities, they are diffracted – causes the light to disperse and spread out some continues down the fiber, some escapes via cladding – power loss

losses in optical fiber

losses in optical fiber Chromatic – Wavelength, Dispersion Loss many wavelengths being txn. from LED each wavelength travels at different velocity arrives at end of fiber at different time resulting in chromatic distortion solution: using monochromatic light source

losses in optical fiber Radiation Losses loss due to small bends and kinks in the fiber two types of bend: microbend – difference in the thermal contraction rates between core and cladding. Geometric imperfection along the axis. constant radius bend – excessive pressure and tension during handling and installation

losses in optical fiber Modal Dispersion Losses pulse spreading difference in the propagation times of light rays that take different path occur only in multimode fiber solution: use graded index fiber or single mode step index fiber

losses in optical fiber Coupling Losses imperfect physical connection three types of optical junctions: Light source to fiber connection Fiber to fiber connection Fiber to photodetector connection Caused by: Lateral displacement Gap dispalcement Angular displacement Imperfect surface

losses in optical fiber Coupling Losses Lateral Displacement axis displacement between 2 pieces of adjoining fiber cable amount of loss – couple tenth to several decibels Gap displacement – miss alignment end separation the farther apart, the greater the light loss if the two fiber is spliced, no gap between fiber if the two fiber is joined with a connector, the ends should not touch each other

losses in optical fiber Coupling Losses angular displacement (misalignment) less than 2o, the loss will typically less than 0.5 dB imperfect surface finish end fiber should be polished and fit together squarely

losses in optical fiber coupling loss

Light Sources Light source for optical communication system efficiently propagated by optical fiber sufficient power to allow light to propagate constructed so that their output can be efficiently coupled into and out of optical fiber

Light Sources

Light Sources LED p-n junction diode made from a semiconductor (AlGaAs) emits light by spontaneous emission

Sources Homojunction LED Heterojunction LED p-n junction two different mixture of the same type of atoms Heterojunction LED made from p type semiconductor material from one set of atom and n type semiconductor material from another set Burrus Etched well surface emitting LED for higher data rate the well helps concentrate the emitted light ray allow more power to be coupled into the fiber ILD Injection Laser Diode

Sources

Sources

Sources

Light Detectors PIN diodes APD light doped material between two heavily doped n and p type semiconductor most common as light detector APD avalanche photo diode more sensitive than PIN diode require less additional amplification

Detectors Characteristic of Light detectors responsivity dark current a measure of conversion efficiency of photodetector ratio of output current to the input optical power dark current the leakage current that flows through photodiode when there is no light input transit time time of light induced carrier to travel across the depletion region of semiconductor spectral response the range of wavelength values that a given photodiode will respond light sensitivity the minimum optical power a light detector can receive and still produce a usable electrical output signal

Lasers LASER-Light amplification stimulated by the emission of radiation --laser technology deals with the concentration of light into a very small, powerful beam --there are 4 types of lasers 1)Gas lasers: Helium and Neon enclosed in a glass tube laser, CO2 lasers --Output is continuous mono chromatic (one colour) 2)Liquid lasers: organic dye enclosed in a glass tube for an active medium --A powerful pulse of light excites the organic dye 3)Solid lasers: solid, cylindrical crystal such as ruby, for the active medium. Ruby is excited by a tungsten lamp tied to an alternating-current power supply. --Output is continuous 4)Semiconductor lasers: Made from semiconductor p-n junctions and are commonly called Injection laser diodes (ILD’s). -- a direct-current power supply controls the amount of current to the active medium

Laser characteristics All lasers use an active material to convert energy into laser light a pumping source to provide power or energy optics to direct the beam through the active material to be amplified optics to direct the beam into a narrow powerful cone of divergence a feedback mechanism to provide continuous operation an output coupler to transmit power out of the laser