Optical Fiber Communications Objectives To discuss the key advantages of optical fiber communication...

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Transcript of Optical Fiber Communications Objectives To discuss the key advantages of optical fiber communication...

  • Optical Fiber CommunicationsObjectivesTo discuss the key advantages of optical fiber communicationTo introduce optical fiber communication systemTo discuss the light source in optical fiber communication systemsTo describe the principle of LED To describe the principle of laserTo illustrate light propagation in optical fibersTo explain total internal reflectionTo introduce the concept of numerical apertureTo study the transmission properties of optical fibersTo explain the working principle of photodetectorTo study optical fiber communication system design

  • Communication ChannelsThe major demand placed on telecommunication systems is for more information-carrying capacity because the volume of information produced (and required) increases rapidly.

    Especially for digital communication systems, they require more channel capacity than analog systems the need for higher information-carrying capacity.

    Information-carrying capacity is proportional to channel (transmission) bandwidth the channel bandwidth needs to be increased.

  • Communication ChannelsHowever, the channel bandwidth is limited by ----- the frequency of the signal carrier.

    The higher the carriers frequency, the greater the channel bandwidth and the higher the information-carrying capacity of the system.

    The rule-of-thumb for estimating the transmission bandwidth is that its maximum value is approximately 10% of the carrier frequency. Hence, if an electrical signal uses a 1 GHz carrier, then its maximum bandwidth is about 100 MHz.

  • Optical Fiber CommunicationsThe information bandwidth is limited to be equal tothe carrier frequency or some fraction of the carrierfrequency. The carrier wave with high frequency needs to be selected.

  • Communication ChannelsA copper wire can carry a signal up to several hundred kHz over several tens of kms of distance. A coaxial cable can propagate a signal up to several hundreds of MHz. Radio transmission is in the range of 500 kHz to 100 MHz. Microwaves, including satellite channels, operate up to 100 GHz. Optical communications uses light as the carrier, light frequency is between 100 and 1000 THz (T = 1012).

    Therefore, optical systems have the largest capacity for information transmission.

  • Optical Fiber CommunicationsOptical FibresThey are normally made of hair-thin high purity silica glass, covered with plastic.

    What are the key advantages of optical fiber communication system?1. Wide bandwidthThe amount of information transmitted is directly related to thebandwidth of the modulated carrier. The increasing of the carrierfrequency increases the available transmission bandwidth. The optical frequency range has a usable bandwidth of 105 times that of a carrier in RF range.2. Low lossOptical fibers have lower transmission losses than copper cables.In a copper cable, the attenuation increases with modulation frequency: the higher the frequency, the greater the loss.

  • Optical Fiber CommunicationsBandwidth is an indication of the transmission rate atwhich information can be sent. Loss indicates how far the information can be sent.

    The combination of high bandwidth and low loss ofoptical fiber communication system means more datacan be sent over longer distances, thereby decreasing thenumber of wires and the repeaters required, and hencedecreasing the system cost and complexity.

  • Communication ChannelsOther advantages of optical fibers:

    (1)are not susceptible to electromagnetic interference (because they are insulators) and therefore have small crosstalk;(2)high security (cannot be tapped, no sparks);(3)are cheaper (abundant raw material);(4)have lower weight, smaller size and are more flexible (thus are easier to install); and(5)are corrosion resistant (thus have longer operating lifetimes).

  • Communication ChannelsThe disadvantage of optical fibre:

    Coupling (for signal distribution) and connecting (to other fibres) cost is higher than coaxial copper cable.

  • Optical Fiber CommunicationsThe typical optical communications system essentiallyconsists of a transmitter with a diode laser, a receiverwith a photo-diode and an optical fibre serving as the transmitting medium.Block diagram of the optical fiber communication system

  • Optical Fiber CommunicationsLight sources

    The fundamental function of optical source in optical fiber communications is to convert electrical energy in the form of current into optical energy.

    A simple and low cost light source used in short haul optical communication is light emitting diode (LED)

  • Optical Fiber CommunicationsLight emitting diode (LED) An LED is a semiconductor p-n junction under forward bias. In forward bias condition, electrons cross the pn junction from the n-type material and recombine with holes in the p-type material. The recombining electrons release energy in the form of heat and light.


  • Optical Fiber CommunicationsLight emitting diode (LED)

    Energy diagram:


  • Optical Fiber CommunicationsLight emitting diode (LED) In LED, the dominant photon generation is spontaneous emission in which the electron drops to the lower energy level in an entirely random way. The output spectrum of an LED is relatively wide.


  • Optical Fiber CommunicationsSpontaneous emissionAn atom in an excited level can make a downward transition spontaneously (i.e., on its own) by emitting a photon corresponding to the energy difference between the two levels.

  • Optical Fiber CommunicationsDuring spontaneous emission, the amount of energy emitted is equal to the difference between the two energy levels, E2 and E1, which is also the photon energy.

    Photon energyh = E2 E1where h = 6.62610-34 Js is Plancks constant

  • Optical Fiber CommunicationsExampleSuppose you use an LED whose energy gap equals 2.5 eV. What is its radiating wavelength?Solution1 eV = 1.602 10-19 J Since the energy gap is the photon energy Ep, and Ep = h = hc /, then ashc = 6.62610-34 Js 3108 m/s 2010-26 mJ Ep = 2.5 eV = 2.5 1.60210-19 410-19 JWe have = hc / Ep = 2010-26 mJ / 410-19 J = 510-7 m = 500 nm

  • Optical Fiber CommunicationsThe most frequently used light source in optical communication systems is laser.

    Why we use a laser as the light source? monochromatic: suitable for elimination of white noise coherent: suitable for coherent detection(c) high power: improves signal to noise ratio(d) small divergence: improves efficiency of transmission(e) small source size: suitable for use with optical fibres

  • Optical Fiber CommunicationsLaserAn acronym for light amplification by stimulated emission of radiation.

    Stimulated emissionAn atom in an excited level can make a downward transition in the presence of external stimulation by emitting a photon corresponding to the energy difference between the two levels. The emitted photon is in phase with the incident photon.

  • Optical Fiber CommunicationsStimulated emission

  • Optical Fiber CommunicationsAbsorptionAn atom in a lower energy state can absorb photons and make an upward transition to the higher energy level.

  • Optical Fiber CommunicationsPopulation inversionIn laser operation, light amplification should be achieved, this needs that the population of the upper energy level is greater than that of the lower energy level, this condition is known as population inversion.

  • Optical Fiber CommunicationsHow to realize population inversion? --- By pumping techniques.

    Pumping is to excite atoms into the upper energy level and hence obtain a nonequilibrium distribution by using the external source, such as a current source, a light source etc.

  • Optical Fiber CommunicationsFurthermore, a resonant cavity is needed to build up stimulated emission by use of feedback.

    Optical feedback and laser oscillation Light amplification occurs when a photon colliding with an atom in the excited energy state causes the stimulated emission of a second photon, the two photons are in phase, and then both these photons release two more. A positive feedback mechanism has to be used to increase the net gain and achieve a laser light output.

  • Optical Fiber Communications

  • Optical Fiber CommunicationsLight propagation in optical fibersThe simplest way to view light in fiber optics is by ray theory. In this theory, the light is treated as a simple ray, shown by a line.An arrow on the line shows the direction of propagation.

    The speed of light in vacuum is: c = 300,000 km/s

    However, the speed of light in medium is more slowly, v = c / n.

    The ratio of the velocity of light, c, in vacuum, to the velocity of light in the medium, v, is the refractive index, n.n = c / v

  • Optical Fiber CommunicationsLight traveling from one material to another causes the change of speed, which results in the change of light traveling direction. This deflection of light is called refraction.


  • Optical Fiber CommunicationsThe relation between incident ray and reflection ray:r = i (Law of reflection)

    The relation between incident ray and refraction ray:n1sini = n2sint (Snells law)where n1 and n2 are refractive indices of the incident and transmission regions, respectively.

    An interesting phenomenon can be found in the light refraction.

  • Optical Fiber Communications

  • Optical Fiber CommunicationsTotal internal reflection

    From Snells law, n1sini = n2sint if n1 > n2, thensini = (n2/n1) sint < sint, which leads to i < t, i.e. the angle of refraction is always greater than the angle of inc