fiber optics
fiber optics is long,thin strands of very pure glass about the diameter of a human hair.It is used to transmit light signals over long distances.
a single fiber optics has the following tree parts:
1 core:thin glass center of the fiber where the light travels.
2 cladding:optical material surrounding the cire that reflects the light back into the core.
3 buffer coating:plastic cover that protects the fiber from damage and moisture.Fiber optics comes in two types,single-mode and multi-mode;
1 single:mode fiber optics has small cores(about 9 microns in diameter)and transmits infrared laser light,waverlength from 1300 to 1550 nanometers.
2 Multi-mod fiber optics has larger cores(about 62.5 microns in diameter) and transmits infrared light from LEDs(light-emitting diodes),wavelength from 850 to 1300 nm.
how does fiber optics transmit light?
suppose you want to shine a light beam down a long,straight tunnel.Just point the beam straight down the tunnel-light travels in straight lines,so it is no problem.What if the tunnel has a bend in it?You could place a mirror at the bend to reflect the light beam around the corner.What if the tunnel is very winding with multiple bends?You bight line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the tunnel.This is exactly what happens in an fiber optics.
The light in a fiber optics travel through the core(tunnel)by constantly bouncing from the cladding(mirror-lined walls),Based on a principle called total internal reflection,the cladding with higher index of refraction does not absorb any light from the core,the light wave can travel great distances,However,some of the light signal degrades within the fiber,mostly due to impurities in the glass.The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted ligth.for example,850 nm is 60 to 75 percent/km;1300 nm is 50 to 60 percent/km;1550 nm is less than 50 percent/km.some premium fiber optics show much less signal degradation-less than 10 percent/dm at 1550 nm.
a fiber optics relay system
1 optical transmitter:
It receives the electrical signal and directs the optical device to turn the light "on" and "off" in the correct sequence,thereby generating a light signal.
The transmitter is physically close to the fiber optics and has a lens to focus the light into the fiber.lasers have more power than LEDs,but are more expensive and more sensitive to the changes in temperature.The most common wavelengths of light signals are 850 nm,1300 nm,and 1550 nm(infrared,non-visible portions of the spectrum).
Optical regenerator
as mentioned above,some signal loss occurs when the light is transmitted through the fiber optics,especially over long distances such as with undersea cables.Therefore,one or more optical regenerators are spliced along the cable to boost the degraded light signals.
Basically,the optical regenerator is a laser amplifier.It consists of fiber optics with a special coating(erbium-doping).The doped portion is "pumped" with a laser.When the degraded signal comes into the doped coating,the energy from the laser allows the doped molecules to become lasers themselves.The doped molecules then emit a new,stronger light signal with the same characteristics as the incoming weak light signal.
Optical receiver
the optical receiver uses a photocell or photodiode to detect the light,and convert the light signals to the electric signals.
advantages of fiber optics
1 less expensive:optical cables can be made cheaper than copper wire.
2 thinner:
3 less signal degradation
4 No interference
5 Low power:signals in fiber optics degrade less,lower-power transmitters can be used.
6 non-flammable
7 lightweight
how is fiber optics made?
fiber optics is made of extremely pure optical glass.One vivid description of the quality of glass is as follows:if you were on top of an ocean that is miles of solid core fiber optics glass,you could see the bottom clearly.making fiber optics requires the following three steps:
1 making the preform blank
the glass for the preform is made by a process called modified chemical vapor deposition(MCVD).In MCVD,oxygen is bubbled through solutions of silicon chloride(SiCl4),germanium chloride(Gecl4)and/or other chemicals.The precise mixture governs the various physical and optical properties(index of refraction,co-efficient of expansion,melting point,etc.).The gas vapors are then conducted to the inside of a synthetic silica or quartz tube(cladding) in a special lathe.As the lathe turns,a torch is moved up and down the outside of the tube.The extreme heat from the torch causes two thins to happen:
粗加工的成品玻璃是采用一种称为修正化学气相淀积法（MCVD）的过程制作的。在MCVD中，氧气通过四氯化硅、四氯化锗和/或其他化学物质的溶液冒泡。这些物质的混合比决定不同的物理特性与光学特性（折射率、膨胀系数、熔点等等）。然后这些蒸气被引导到在一台特别的机床上的一个合成的硅或者石英管（覆盖层）里面。当机床转动的时候，一束火焰沿着管子的外部上上下下地移动。极热的火焰使得两件事情发生。
1 the silicon and germanium react with oxygen,forming silicon dioxide and germanium dioxide.
硅和锗起氧化反应，形成二氧化硅和二氧化锗
2 the SiO2 and GeO2 deposit on the inside of the tube and fuse together to form glass.
二氧化硅和二氧化锗党淀积在管子内部并且熔化在一起形成玻璃。
The lathe turns continuously to make an even coating and consistent blank.The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system(valve blocks,pipes,seals)and by precisely controlling the flow and composition of the mixture.The process of making the preform blank is highly automated and takes several hours.After the preform blank cools,it is tested for quality control(index of refraction).
机床连续旋转做出均匀的外壳和一致的内芯。由于在气体传送系统（阀门、管道、密封等）使用抗腐蚀的塑料，并且精确控制混合物的流动和合成来维持玻璃的纯度。这个制作粗加工成品的过程是高度自动化的，需耗费几个小时。在粗加工的成品冷却以后，进行管理检查（折射率）。
drawing the fibers from the preform blank
once the preform blank has been tested,it gets loaded into a fiber drawing tower.The blank gets lowered into a graphite furance(1900 to 2200 degrees Celsius) and the tip gets melted until a molten glob falls down by gravity.As it drops,it cools and forms a thread.
粗加工的成品检查以后，被装进一座光纤抽丝塔。它被降到一座石墨坩埚中。它的顶尖被熔化并且随重力落下。评测在落下过程中冷却，形成一股细丝。
The operator threads the strand through a series of coating cups(buffer coatings)and ultraviolet light curing ovens onto a tractor-controlled spool.The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber and feed the information back to the tractor mecvhanism.Fibers are pulled from the blank at a rate of 10 tp 20 meters/s and the finished product is wound onto the spool.It is not un-common for spools to contain more than 2.2 km of fiber optics.
操作员把细丝拉过一系列保护层杯（缓冲保护层）和紫外线烘烤炉，挂在一个受辊卷机控制的卷轴上。辊卷机把炽热的粗加工成品缓慢地制成光纤细丝，用一具激光测微计测量光纤的直径，并且把信息回馈到辊卷机的控制机制。光纤以10-20 m/s的速率拉出，成品卷到卷轴上。一个卷轴上常常能拉出2.2km以上的光纤。
Testing the fibers
1 tensile strength
2 fiber geometry
3 attenuation
4 information carrying capacity(bandwidth)
5 chromatic dispersion
6 operating temperature/humidity range
7 temperature dependence of attenuation
8 ability to conduct light underwater