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The principle and Theory of Fiber Laser
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The principle and Theory of Fiber Laser
All types of lasers are methods of converting raw power, electrical; optical; chemical; nuclear; into a more controllable beam of power which can be focused down to a small spot to produce a localized high intensity area of power.
所有类型激光的转换方式都是由原始能量; 电;光学;化学; 原子转化成为一种更易控制的能量束，这种能量束能被聚焦成一个小的光点并可产生高密度的能量区域．
Diode lasers and LEDs (light emitting diodes) are one such method where the flow of electricity through small piece of semiconductor produces light. The light from an LED is emitted into large angle and can not be easily refocused to produce a useful spot. A diode laser is like an LED except only light that can bounce backwards and forwards inside the semiconductor can be generated, which produces a more parallel beam of light with a well defined ‘cooler’ or wavelength.
所有半导体激光和LEDs (半导体发光二极管)都是电子流通过半导体的一小部分而产生光.从LEDs产生的光其反射角度比较大，不易重复调焦而产生一个有用的光点. 和LED相同，半导体激光只是光线在半导体内来回跳跃而产生，所产生的是具有易于识别＂颜色＂的更加平行的光束或是波长.
Diode lasers are very efficient at converting electricity to light and can have incredible longevity – tens of years of maintenance free use – but high power laser diodes do not produce perfect laser beams. This means that although they are more focusable than a hand torch say, the size of spot they can be focused to is still relatively large, limiting their use.
半导体激光在把电力到光线的转化上很有效，可达到难以置信的使用寿命－十几年的免修．但高能量的激光二极管不能产生完美的激光束，这意味着他们虽然比手电筒更有聚焦能力，它们能聚焦的光斑尺寸仍然相对很大，从而限制它们的用途．
The increased focus ability and single colors nature of laser diodes can, however, be used to ‘pump’ or lower other types of lasers which can produce near perfect laser beams. Examples the diode pumped solid state lasers and more recently, fibre lasers.
激光二极管增加了它的聚焦能力和单色光性质，可用于 ‘泵’ 或其他类型激光时,可产生近乎于完美的激光束，比如二极管泵浦固态激光和最新的,光纤激光．
Fiber optic is one long cylindrical piece of glass or plastic contained within another piece of glass or plastic. The picture shows the green inner piece of glass; call the core, held inside a much larger outer piece of glass, called the cladding. The optical properties of the two glasses are different which means that if light starts to travel in the central core, when it reaches the interface between the glasses it will be reflected back into the core, trapping it inside the core. In this way light can be transmitted, essentially loss free, for very long distances. It is important to note however that only light that nearly parallel to the core will be reflected. Non-parallel light is not reflected at the interface and is lost. If the core is small enough. It will transmit only nearly parallel, high ’quality’ light, like that of from a laser. The core of these fibers is typically less than 10u, in diameter, less than the thickness of a cigarette paper. Lasers and optical communications have been transmitted in this way for many years, making very flexible systems for delivering both data and powerful lasers.
光纤激光的光体是一个长玻璃形柱体的或塑胶套着另一件同样形状的玻璃或塑胶体，如图所示中心是一块绿色玻璃体: 称为核, 外层安放一个更大的玻璃体,成为＂覆层＂.这两块玻璃体的光学性质不同，意味着如果有一光源到达两块玻璃交界点时会反射到中心并留到中心.在此情况下，光源可被传递很长的距离而没有能源损失，但重要的是只有接近平行核的光才能够被反射.不平行的光在交接处不被反射或损失掉.如果核有足够的小，就可传递一种接近平行的高质量光束.这些光纤的核每一光纤直径少于10um;就是说比香烟纸还薄，各种激光和光通信已被使用好多年了，正在使得数据传输方面更灵活,激光更有效．
The breakthrough was to produce the light inside the core instead of producing laser light and then using a fiber to conduct it. This is what happens in a fiber laser.
这就突破了在核的内部产生光,而不是产生光线,并使用光纤来传导,这就是光纤激光.
Diode lasers can be focused into the cores of large fibers but these produce large, unusable spots of light at the fiber output. In a fiber laser a second, much smaller core is produced inside the core transmitting the diode light. The diode light then occasionally passes into the smaller, ‘inner’ core. The inner core has been impregnated or ‘doped’ with a substance that will absorb the diode light, in the case shown the rare earth metal ytterbium (Yb).
半导体激光可聚焦到大的光纤核中，但会产生大的, 在光纤输出时是没用的光斑。在光纤激光中，片刻就会在传导二极管光的核中产生更小的核．然而二极管的光偶尔会进入这个更小的核心的内部。内核的内部被注入或掺杂一种能吸收二极管光线的物质，此物质为稀土金属 - 镱，（Yb）。
When the ytterbium absorbs the diode light it stores the energy from the light for a short time. In this state it is said to be ‘excited’. Excited states are how all lasers work. If a particular color or wavelength of light hits the ytterbium when it is in the excited state this light will be a aplined, generating a carbon copy of the original light. In this way a very lower perfect beam can be amplified many times to produce high power beam that are nearly perfect and are therefore very focusable. Because the fiber can be made as long as required, nearly all of the diode light can be captured by the inner core and converter to fiber laser light, making a very efficient system. The generated beam is contained in the inner core and so can be conducted to the point of use with virtually no loss and no reduction in its focus-ability.
当镱-Yb吸收这些二极管光线时，它能在短时间内贮存这些来自光线的能量，这种情形叫做＂激发”。激发状态是所有激光的工作的原理。如果一种特殊的颜色或光线的波长击中Yb，而此时正是激发状态时，这种特殊颜色的光线将被放大产生高能量的近乎完美而且有聚焦能力的光束．因为这些纤维不受长度的限制，几乎所有的光源可都被中心吞掉并转化成纤维激光光线，使之成为一个有效率的系统。这个再生的光束被包含在内核中，因此可被传导成一个光点，而没有损失和降低它的聚焦能力．
So good so far, how do you make a real, useable fiber laser?
The first problem is that the light will start traveling in both directions along the inner core, so there will be are output at both ends of the fiber. This is not ideal; we would like all the light to come out of one end. Therefore we have to redirect any backward traveling light in the forward direction. This is done by putting a 100% reflective mirror at the rear end of the fiber to redirect the light in the desired direction..
如此，如何制作一个真正有用的纤维激光呢？
首要的问题是光线可能开始顺着内核中心向两个方向传输，完美的是所有的光线都从一端出来。因为我们必须改变任一种向相反方向传输的光线使其都向同一方向传输，需要在纤维后面放一个100%反射镜来改变光线让其向我们想要的方向传输，
Next the diode pump light and the high quality fiber light are both having to enter and exit the fiber at the same place, the fiber ends. Therefore an optical system is required to separate the pump and output beams. And example of how this could be done is shown. In the diagram there is a diode laser injected at both ends of the fiber to increase the amount of ‘pumping’ or power source and so increase the fiber laser output.
第二个问题是二极管泵浦光线和高质量纤维光都从相同的地方，纤维端出入，因此需要一个光学系统来隔开泵浦和输出光线，如图所示操作例子，图表里面有一个二极管激光进入到纤维两端来增加‘泵浦’和能源数量，从而来增加光纤激光的输出．
However the optical systems at the ends of the fiber need to be aligned and have many optical surfaces all of which could become contaminated in use and therefore reduce the reliability of the system. It would be much better if all the optics could be contained inside the fiber itself, the only surface then being the final output of the fiber laser light. Also the above system can use only two pump laser diodes. For a high power laser many pump diodes may be required.
然而纤维两端的光学系统需要有序排列，而且有很多光学表面在使用过程中被污染而减少系统的可靠性，最好是所有的光学都放到纤维自身内部，表面只是纤维激光光线的输出口，以上系统只可用于两个泵浦激光二极管. 对于高能量的激光需要很多二极管．
It is possible to more reflectors inside the fiber using devices called ‘fiber bragg gratings’. In essence, these are an array of small disturbances to the optical properties inside the fiber which create lots of small deflections to reflections of the light in the fiber. In these disturbances are carefully arranged they can be made to combine their effects to produce partial or even complete reflection of the light. This reduces the number of out-of-fiber optics, or free-space optics as they are commonly known.
在纤维内放一个＂纤维布拉格光栅-Bragg gratings”装置作为一个反射器可得到更多的反射是可能的。 本质上说， 对在内部的光纤光学性质，这些光学装置是一小排的干扰因素，产生大量光线的小偏差或反射．如果这些干扰因素处置得当，它们能联合整体的效果来产生一部分甚至完整的光线反射，从而减少了通常所知的输出纤维的数量或活动空间。
More pump fibers can be added by fusing a number of fibers together in a ‘coupler’. This allows many pump fibers to be coupled to the main laser fiber so allowing more power to be pumped into the system without the use of free space optics, making a completely sealed and thereby highly reliable system, using these methods, almost any number of high lifetime diodes, each producing less than 10w of low quality diode light, can efficiently coupled into a fiber laser to produce hundreds of Watts of near perfect, focusable fiber laser light.
很多泵纤维可通过熔融许多纤维成为一个整合体．为产生更多的能量，制成一个完全密封的高可信度的系统，用这种方法几乎可制成任何数量的奈用的二极管，来产生小于10W的低质量二极管光线，还可能效地附加到纤维激光机上来产生几百瓦的几乎完美的，可聚焦的纤维激光光线．
So, we now have a sealed high efficiency, high quality laser but there are still a number of minore weaknesses to the system. Fusing large single core fibers is quite reliable but the double core laser is more delicate and each coupler onto this fiber is a potential weak spot that could fail in use. Also the very small fiber laser core is part of the structure to which all the pump fibers have to be attached but it then has to be separated at both ends to put on the fiber Bragg gratings and the output system. Being small this part of the fiber is delicate and reliably separating it from the rest of the fiber is difficulty.
尽管我们现有密封好，高质量的激光，但仍有很多小的不足。熔融大的单个核的纤维
束相当可靠，但对于双核的更精密的纤维激光，每个联接都是一个潜在的弱点，可导致操作失败，而且每个小纤维激光核都是结构的一部分，都要附在所有的泵纤维上，但它要被分开放在纤维Bragg grating的两端作为纤维机上小部分，很精密，所以把它从纤维上分开是困难的．
An ingenious solution to overcome these issues is to completely separate the pump and laser fibers and find another method of transferring the diode pumping light into the fiber laser. The pump fibers are completely separate but are held in intimate contact to the outer of the active fiber. This contact allows the pumping light to gradually transfer from pump fibers to the active fiber over its length. More than one pump fiber can be held in contact with the active fiber, typically two, which gives up to four ends into which the pumping light can be injected before having to resort to couplings. Even if couplings are required they are on the more robust pump fiber, completely divined from the delicate fiber laser. The fibers are held together by a soft plastic polymer which can be easily remove without risk of damaging the fibers, all of which add up to a highly reliable system.
解决这些问题的方案是完整的把泵和激光纤维分开，采取的另一种方法是把二极管放大的光源传入激光光纤里面。 泵与纤维分开, 但它们仍保持与外面活跃的光纤接触。此接触点可以使泵光源逐渐的把泵光纤传递到整个外面的活跃的激光上，不止一个泵纤维与活跃纤维接触，正常为2个，最多4个．泵源可输入而不用＂couple＂来归类，如果需要也只是用在非常强的泵光纤完全跟较精密的激光光纤分开．此纤维与一个软塑料在一起可以很轻易的移除而不损伤纤维．
Although a large length of fiber is required to create the powers required, the fiber can be coiled onto a small ?rum, making the overall footprint of the system very small. Also unlike previous generations on this power of laser the fiber generates very little heat so the metal?rum can be cooled very simply, low power consumption, low heat generation system, typically less than a tenth the volume of the same size diode pumped solid state laser. The photograph shows the thrum and fiber from a typical materials’ processing laser.
尽管需要较长的纤维来产生需要的能量，纤维可被卷成小卷，使此系统很小，这代纤维几乎不产生热所以金属卷可被一个风扇很快冷却，这样能耗就很小，低热系统，甚至低于相同体积的二极管半导体产生的1/10.，此图片展示了从材料出来激光中产生的鼓和纤维．
A recent development a fiber laser technology is the pulsed fiber laser. For some applications, such as marking and some cutting, very high powers are required but the power does not have to be continuous. In fact it is essential that it is not continuous in some applications such as removing the paint from the surface of plastic mobile phone key pads. This is because the power is required for just long enough to vaporize the paint
but not long enough to melt the plastic underneath. For this application the laser has to be on at a high power (1000W) for less than 50ns(1/20,000,000th of a second). Longer than this and the plastic cracks or turns black. Making a 1000W laser and turning it on and of in this time is neither practical nor cost effective. A 1000W laser would require more than 2000W of diode pump light, which would mean fitting more than 200 expensive laser diodes into the system. Fortunately there is an easier way. The fiber will store power for a limited time so it is possible to pump the fiber continuously but extract the power out in very short pulses at high frequencies. The total power is the same but it is now divided up into very short, very intense bursts. This is done using the fiber as a simple amplifier.
目前纤维光技术的发展是脉冲纤维激光，一些应用，如打标或切割，需要很高能量但这些能量不能持续，事实上一些应用不需要持续，比如从塑料手机键盘上移除表面的涂层，需求的能量正好而且足够去蒸发这些涂层但不能熔化下面的塑料，对此应用激光打标机必须低于50ns(1/20,000,000th 1秒)这就需要高功率（1000W），因为如果高于这个速度这些塑料就会裂或变黑，制造一个1000W激光机不可行并且也不经济，将需要于2000W的二极管泵光线，意味着需要多于200个激光二极管，幸运的是这里有一个很容易的办法：纤维将短时间贮存能量，所以用此为持续纤维是可能的，这些可用纤维作为一个简单放大器来完成．
（图片不能附贴呀）
