《光通讯基础》PPT课件.ppt

1、Light :,1. Electromagnetic (EM) wave -In the wave view,2. A ray or beam -In the geometric optics view,3. A stream of photons - In the quantum view,1.l ELECTROMAGNETIC WAVES,1. Wavelength:,2. Velocity: c,3. Frequency: ,4. The period: T,5. Spectrum : Visible :=400 nm-700 nm,= 1/T =c (1.l),6.Interferen

2、ce 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.diffraction,1.2 BEAMS (RAYS) Refractive Index,n = c/v (1.2),What are the directions of the reflected and the refract

3、ed beams?,The refractive indexes for some media are given in Table 1. 1,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 critica

4、l 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 opti

5、cal fiber work as a communications link.,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 c

6、an change its energy only by jumping from one level to another; In level only discretely,A Photon What 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

7、 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 ph

8、otons frequency The higher the photons frequency, the more energy it carries. Light is a stream of photons.,Radiation and Absorption A 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 =

9、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 mater

10、ial to achieve different colors of radiated light.,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, radi

11、ating photons-that is, light. The process of making atoms jump to the higher levels by feeding them external energy is called pumping.,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 ato

12、m will jump to the appropriate higher level. If EP is not equal toE, the photon will pass by the material without interaction.,The Energy band theory of semiconductor,1. The value of Eg determines the conductive (resistive) properties of the material. Good conductors have no gap between the valence

13、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 mater