3optical fiber1

1、contents,1) reviewing “physics of light” from the point view of electromagnetic waves、beams and a stream of photons (2)learning chapter 3: optical fibers-basics,key points review,conception of wavelength/period refractive index (conception (2)making a second measurement by placing a short piece of f

2、iber between an led and a power meter, writing down this power as pin; (3)estimating the na number by using the formula: na=pin/p0,3.2 attenuation,question: what is attenuation? in fiber-optic communications technology, attenuation is the decrease in light power during light propagation along an opt

3、ical fiber. bending losses : macrobending loss and microbending loss macrobending loss: figure 3.5 shows two conflicting situations,1)the beam forms a critical propagation angle with the fibers central axis at the flat part of the fiber. (2)but the same beam forms a propagation angle that is more th

4、an critical when it strikes the boundary of the bent fiber,figure 3.5 bending loss,in a word, bending an optical fiber introduces a loss in light power, or attenuation. can this lossmacrobending loss be overcome? there is no straightforward way to eliminate this cause of attenuation. although the ma

5、nufacturers of optical fiber have learned that by designing the refractive index profile, the bending sensitivity can be changed, but improvement in bending sensitivity can be achieved only at the expense of the degradation of a fibers other parameters. manufacturers inform users what bending loss c

6、an be induced at a certain bending radius. the only thing we can do about it is to be cautious when bending an optical fiber,microbending loss: like the macrobending loss, the microbending loss is also caused by failure to achieve the condition of total internal reflection. from figure 3.7, we can s

7、ee the mechanism of microbending loss,figure 3.6 microbending loss,mechanism : imperfection of the geometry of the core-cladding interface might result in micro-convexity(微凸起），or micro-dent（微凹陷,to summarize: macrobending is loss caused by the curvature of the entire fiber axis microbending is loss c

8、aused by microdeformations of the fiber axis, which is really an imaginary line , (in reality, this line is determined by core-cladding geometry) microdeformation of the core-cladding boundary. attention: we users should always handle the fiber with care,scattering loss: an imperfection in the core

9、material will lead to scattering loss. figure 3.7 shows the mechanism of scattering loss,figure 3.7 scattering loss,the modern technology to make sure that the fiber is so fine and clean that it is impossible to find any mechanical particles in the core, the reason to cause scattering is the slight

10、variation of the refractive index. even very small changes in the value of the cores refractive index will be seen by a traveling beam as an optical obstacle and this obstacle will change the direction of the original beam. how to overcome the scattering loss? only by making better optical fibers. t

11、his type of loss is simply included in the total attenuation reported. incidentally ,this type of scattering is called rayleigh scattering.(瑞利散射,absorption reviewing the phenomenon of pumping and radiation. imagine that light ( a stream of photons)travels down an optical fiber and encounters a mater

12、ial whose energy level gap is exactly equal to the energy of these photons. this impact will lead to light absorption, resulting in a loss of light power.-basic mechanism of absorption. absorption properties that still remain are caused not by silica atoms but by some molecules of the hydroxide anio

13、n oh,these molecules are incorporated in silica during the fabrication process and hard to be eliminated. molecules have major peaks of absorption at 945,1240 and 1380nm. how to reduce or eliminate absorption? we can change the light frequency or work with another material. transparent window: since

14、 we can not eliminate the molecules, we can change the operating wavelength. from figure 3.8, we can see there are three major transparent windows, where absorption is low. near 850nm, 1300nm and 1550nm.and the 1550nm is the most widely used wavelength today in long-distance communications,figure 3.

15、8 typical spectral attenuation,calculations of total attenuation,fiber loss is the ratio of power at the output of the fiber,pout to the power at the input of the fiber,pin: loss= pout/ pin where power is measured by watts. in communications technology, we measure loss(attenuation)in db: loss(db)=-1

16、0log10 (pout/ pin) a(db/km)=loss(db)/fiber length(km,measuring attenuation,a power meter (光功率計）measuring the power of light. the result is displayed in dbm, which is a specific unit of power in decibels when the reference power is 1mw: 1dbm=-10log10(pout/1mw) figure 3.9 shows the procedure of the at

17、tenuation measurement,figure 3.9 experimental arrangement for measuring attenuation,the precision of this method(cut method) is mainly determined by two factors: how accurately you can reproduce connection loss and how negligible is the attenuation introduced by a short piece of fiber. to calculate loss in db when obtaining readings in dbm, use