1917年，Einstein发表题为"论辐射量子理论 (On the quantum theory of radiation)" 的论文，提出激光的两条物理原理:
a.原子的发射和吸收机理 (Emmision and absorption of atoms)
b.原子能级占有数反转的实现 (Realizing the inversion of occupation number of atomic level)
即，论文中的三种条件原子的自发辐射 (Spontaneous radiation)、受激吸收 (Simulated absoption)、受激辐射 (Simulated radiation)。
"光和热辐射能量一样量子化"这一辐射理论观念在物理学中立即起到导火索的作用，Quantum Mechanics 这门新学科迅速发展起来，然而在非理论物理界的四十年后它才开始被广泛应用，人们制造出强"泵浦 (Pump)"和适当反馈的光学谐振腔发射可资利用的激光 (Light Amplification by Stimulated Emission of Radiation, ie. LASER)。
当然最不科幻的应用就是在通信中了，激光源的出现，再加上自然界送给光纤通信的礼物: "铒"元素，用它制造出的光纤"掺铒光纤"具备了奇迹性的巧合:
Er离子的增益谱与光纤传输最低损耗窗口重合。
这些因素直接促成了所谓"全光通信网"的产生，吊足胃口之后下面也该进入正题了。。。╮( •́ω•̀ )╭ •﹏•
The theses is simple at the first sight but not as easy as it seems, the topic as Optical Amplifier has rich and broad content in it, and, the most important thing it has, the votality.
The picture below just want to make you clear with the problem "What can Amplifier mainly help us to do?"
The Optical Amplifier, with the simple reason with "Easily breaking through the electronic bottle neck", has become a nova in Optical-fiber communication, which directly leads to the emergence of all-optical communication network, that means, provide us dozens of Tbits/s information capacity, however, in this active family, the most famous and has been put into use to get great benefits one is named "Erbium-doped fiber amplifiers (Abbreviation for EDFA)".
All the reason is made by the creature, ie. the miracle, that the erbium atom has its gain spectrum coincide with the energy level of the lowest minimum loss window (In optical-fiber telecommunitation).
There are two important portion of the Amplifier made by Erbium:
。。。。。。。。。。。。
I will explain what the Er-doped fiber（掺铒光纤EDF） in the following statement.
Easily speaking, EDF is a special form of optical fiber, and has a same structure as single mode fiber, the main difference is that it is made by .
As you can learn from the figurre of "EDF Structure", erbium ions are located at the center of the fiber, which allows themto absorb maximum pump and signal energy for better amplification. The glass cladding witn low refractive index is used to perfect the waveguide structure, besides, providess the properties to the fiber, and the coating is used to increase the diameter of the light.
Principle of EDFA function and course of EDFA work?
The pumped light source emits a laser () which excites erbium ions to transition from the ground state to the excited state (ie. the pumped energy band).
The excited state of the excited ion is very unstable and rapidly decays to metastabe states (Approximately to ), and release the excess energy as phonons.
Metastable particle accumulation will gradually forms the reversed particle number distribution, hence will have transition from the metastable state to the bottom ().
If can find the pump source formed of laser, electron will jump from ground energy level to the metastable state level, and then it will form the reversed particle number distribution again. (Maybe there will also exist the signal light to excites the electron)
The metastable state will soon jump back to the ground level with the corresponding photon of signal, and the laser it gives out will possess the same phase (That means, have good coherence proporties) of the signal.
Structure! Let's have a look of it!
The figures below gives you explamation of how these two components form the Amplifier as EDFA and how to install it into the Optical-fiber communication Net.
Advantages of it! (I'll be less rigorous to neglect the disadvantages)
1. It's oprating band is just at the lowest dissipation.
1525-1565nm
2. It's boardband can amplifier more WDM signal at the same time.
3. The sufficient interaction between light and matter greatly improves the efficiency of energy convertion.
4. High gain and low noise.
Of course, If can get multiple light source to excite fibers, the advantages will shine through better.
掺铒光纤放大器是大自然赠予光纤通信的礼物ヾ(´∀｀。ヾ)。。我带着营业式的表情这样复述到。。。