光通讯基础.pptx

1Prof.Yang,The college of Optic and Electronic Information Engineering,USST Light:1.Electromagnetic(EM)wave-In the wave view 2.A ray or beam-In the geometric optics view3.A stream of photons-In the quantum view 1.l ELECTROMAGNETIC WAVES 1.Wavelength:2.Velocity:c 3.Frequency:4.The period:T 5.SpectrumVisible:=400 nm-700 nm=1/T =c (1.l)2Prof.Yang,The college of Optic and Electronic Information Engineering,USST 3Prof.Yang,The college of Optic and Electronic Information Engineering,USST 6.Interference There is interference between light waves of both arms.If light waves of both arms have the same phase,they will interfere constructively and destructively if they are reverse.7.diffraction4Prof.Yang,The college of Optic and Electronic Information Engineering,USST 1.2 BEAMS(RAYS)Refractive Index n=c/v (1.2)What are the directions of the reflected and the refracted beams?The refractive indexes for some media are given in Table 1.15Prof.Yang,The college of Optic and Electronic Information Engineering,USST Snells law:1=3 n1 sin1=n2 sin2 (1.3)Total internal reflection:When light travels from a medium with a higher refractive index to a medium with a lower refractive index and it strikes the boundary at more than the critical incident angle,(1 1C)all light will be reflected back to the incident medium,meaning it will not penetrate the second medium.The incident angle at which the angle of refraction equals 900 is called the critical incident angle,1C.Total internal reflection is a necessary condition to make optical fiber work as a communications link.6Prof.Yang,The college of Optic and Electronic Information Engineering,USST 7Prof.Yang,The college of Optic and Electronic Information Engineering,USST 1.3 A STREAM OF PHOTONS An energy-Level Diagram Bohrs model assumes that electrons rotate on stationary orbits and therefore possess a stationary value of energy.An atom can be at any of these levels,or other words,it can change its energy states,and it can change its energy only by jumping from one level to another;In level only discretely 8Prof.Yang,The college of Optic and Electronic Information Engineering,USST A PhotonWhat happens if an atom jumps from an upper level to a lower level,say,from level E3 to level E2?There is an energy gap between these two levels,E=E3 E2,and this difference will be released as a quantum of energy,which is called a photon.A photon as a particle,an elementary particle that carries a quantum of energy,EP,and that travels with the speed of light,c A photons energy,EP,is defined as follows:EP=h (1.4)Where h is Plancks constant(h=6.626 10-34 J.s)and is the photons frequency The higher the photons frequency,the more energy it carries.Light is a stream of photons.9Prof.Yang,The college of Optic and Electronic Information Engineering,USST Radiation and AbsorptionA photons energy,EP=h and a photon was created when an atom jumped from E3 to E2 and released energy(E3 E2).Therefore,EP=E=E3 E2 (1.5)But EP=h;hence,h=E3 E2 and =(E3 E2)/h.On the other hand,=c/,Therefore,=ch/(E3 E2)(1.6)Conclusion:the wavelength(the color)of radiated light is determined by the energy levels of the radiating material.These energy levels are given by nature and we can not control them.But we can choose another material to achieve different colors of radiated light.10Prof.Yang,The college of Optic and Electronic Information Engineering,USST Atoms want to exist at the lowest possible energy levels.To raise them to higher levels,we must energize them from an external source.When atoms absorb external energy,they jump to the higher energy levels and then drop to the lower levels,radiating photons-that is,light.The process of making atoms jump to the higher levels by feeding them external energy is called pumping.11Prof.Yang,The college of Optic and Electronic Information Engineering,USST What happens if an external photon(light)strikes a medium?If its energy,EP=h,is equal to the energy gap,E,the photon will be absorbed by an atom and the atom will jump to the appropriate higher level.If EP is not equal toE,the photon will pass by the material without interaction.12Prof.Yang,The college of Optic and Electronic Information Engineering,USST The Energy band theory of semiconductor 13Prof.Yang,The college of Optic and Electronic Information Engineering,USST 1.The value of Eg determines the conductive(resistive)properties of the material.Good conductors have no gap between the valence and conduction bands,good insulators have a big energy gap,and semiconductors have a gap somewhere in between.2.If an excited electron falls from a conduction band to a valence band,it releases a photon whose energy,is equal to or greater than the energy gap.3.To make a semiconductor radiate,it is necessary to excite a significant number of electrons at the conduction band.This can be done by providing external energy to the material.The most suitable form of this external energy is electric current flowing through a semiconductor.4.The wavelength(the color)of radiated light is determined by the energy gap of the radiating material.