Basic fiber optic system

Fiber Optics Basic Fiber-Optic System • Transmitter (laser diode or LED) • fiber-optic cable • Receiver (PIN diode or avalanche photodiode) • Most fiber systems are digital but analog is also used Fiber Structure • Core and cladding are both transparent, usually glass, sometimes plastic • Core has higher index of refraction • Light propagates down the core, reflecting from cladding Fiber Communication Fiber Advantages • • • • • • • Greater bandwidth than copper cable or radio link Lower loss than cable or radio Immunity to crosstalk and EMI Greater security from eavesdropping No spark hazard Less time delay than geostationary satellites Smaller size and weight than copper cables Optical Fiber Operation • Fiber works using total internal reflection • Core has higher index of refraction than cladding • If angle of incidence of light to the interface between core and cladding is great enough, light will be reflected back into core Index of Refraction c n= v = εr = vf Snell’s Law n1 sin θ1 = n2 sin θ n1 sin θ = sin θ1 n2 Critical Angle • If angle of incidence is such that angle of refraction is 90°, reflection takes place instead of refraction n2 sin θ c = n1 n2 θ c = arcsin n1 Numerical Aperture • derived from critical angle • sine of max angle a ray can have with the axis of the fiber and still propagate N A = n1 − n2 Optical Emitters • Only basic types in use: – Light-emitting diode (LED) • sometimes called IRED (infrared emitting diode) – Laser diode (LD) • power in optical communication systems is low (microwatts to a few milliwatts) Photons • Photon is a quantum of radiated energy • More energetic photons represent higher frequencies (shorter wavelengths) • For the same power level, shorter wavelength radiation has fewer photons per second than longer wavelengths • Photons can be ignored at radio frequencies Photon Energy • Energy of a photon is given by: E = hf E = energy in joules h = Planck’s constant = 6.626 × 10-34 joule-seconds f = frequency in hertz Electron Energy • When an electron drops through a potential of volt it gives off electron-volt (eV) of energy • eV = 1.6 ì 10-19 J ã This energy can be used to create a photon of radiation • A larger potential drop means shorter wavelength radiation Light-emitting Diodes • Heterojunction diodes use different materials for the junction – example: AlGaAs and GaAs • Wavelength of radiation depends on forward voltage, which depends on junction materials Laser Diodes • Laser: Light Amplification by Stimulated Emission of Radiation • Photons are produced as in LED • Laser forms an optical resonator which allows concentration of photons • Photons excite electrons to drop to lower levels in synchronism producing coherent radiation Laser Diodes • Produce narrower beam and smaller linewidth than LED • Operation is more critical – current too low operates as LED not laser – current too high device is damaged – often feedback of optical signal level is used for current control Laser-Diode Transmitters Typical Laser-diode TransmitterSpecifications • • • • • Junction materials: InGaAsP/InP Central Wavelength: 1310 nm Linewidth: nm Threshold current: mA Output power to fiber: 0.6 mW typ.with current 20 mA above threshold • Rise and fall time: 0.3 ns typ., 0.5 ns max • Voltage: 1.2 V typ., 1.6 V max Optical Detectors • PIN diode most common • Avalanche photodiode also used – more sensitive than PIN diode • Wavelength of greatest sensitivity depends on junction in a similar way to LED Detector Specifications • Dark current: – output with no light input – represents noise level • Responsivity: – output current per given light level – units A/W – specified at a particular wavelength .. .Basic Fiber- Optic System • Transmitter (laser diode or LED) • fiber- optic cable • Receiver (PIN diode or avalanche photodiode) • Most fiber systems are digital but analog is also used Fiber. .. Cable Construction • Fiber has little mechanical strength • Kinks in fiber cause losses • Protection from harsh environment often needed • Often several fibers are combined • Fibers usually manufactured... buffer coated right on to the fiber as a first step in protecting the fiber Basic Cable Types • Loose tube • Tight buffer Loose Tube Construction • Protects fiber from stresses due pulling