Through understanding the structure of substance and principle of laser radiation and laser absorption, we can know the generation of laser.

Diagram 1 Carbon atom diagram
　　Substance is composed of atoms. The Diagram 1 is a schematic diagram of a carbon atom. The atoms The center of the atom is atomic nucleus which consists of protons and neutrons. Proton has a positive charge, while neutrons are not charged. The atomic nucleus is surrounded by negatively charged electrons, which is circling around it. Interestingly, the energy of the electrons in the atom is not arbitrary. Quantum mechanics which describes microscopic world, tell us that these electrons will be in fixed energy level, different energy level matches different electrons energy. For the sake of simplicity, as shown in figure 1, imagine these energy level to be the track which circles around atomic nucleus. The further from nucleus, the higher technology. Moreover, different track has different capacity to accommodate electrons. For example, the lowest orbit can only accommodate two electrons, while the highest orbit can accommodate 8 electrons. In fact, this over-simplified model is not complete right, but it can help us know the basic principle of laser.
　　Electrons can move from one energy level to another level by absorbing or releasing energy. For example, when an electron absorbs a photon, it possibly will move from its low energy level to higher one. (Figure 2 a )Meanwhile, an electron which is located at high energy level, can move to low energy level by launching a photon(Figure 2 b ). In these course, the energy which electron absorbs or releases equals the difference between two energy levels. As the photon energy will determine the wavelength of light. Thus, the light which is absorbed or released has a certain color.

Diagram 2 The electron transition inside the atomic
　　 When all the electrons are located in the possible lowest energy level, the entire energy of the atomic is the lowest, we call that this atom is in ground state. The diagram 1 indicates the arrangement of electrons when atom is in ground state. When one or more electrons at higher energy level, we call that this atom is in excited state. As mentioned above, the electrons can transmit by absorbing or releasing photons. The transition can be divided into three forms:
1.Spontaneous absorption- electron will move from low energy level to high energy level by absorbing photon (figure 2 a ).
2.Spontaneous radiation- electron will move from high energy level to low one by releasing photon (figure 2 b ).
3.Stimulated emission- when inject the material with photon, it will cause that electron will transmit from high energy level to low one, it will release photon. The input photon and the released photon have the same wavelength and looks. This wavelength matches the difference between two energy level. One photon will stimulate one atom to release one photon, thus it will occur two same photons (figure 2 c).

Diagram 3 Schematic diagram of ruby laser
　　Basically the laser is generated by the third form. The figure 3 indicates the principle of ruby laser. It consists of one flashing lamp, one laser medium, and one double sides mirror. Laser medium is ruby crystal which contains a few chromium atoms. In the beginning, the incident light from the flashing lamp enters laser medium, it will stimulate chromium in the atoms. The electron in the highest energy level will move to excited states. At this time, some electrons will move back to the lower energy level. The released photon will be reflected by the two mirrors which is fixed at the end of the laser medium, it will stimulate more photons to be radiated. It will enhance laser’s strength. One of the mirror will reflect all photons, the other mirror will reflect majority of photons, let a minority of photons pass, thus these photons is just laser seen by us.

Diagram 4 The state of population inversion
　　The another trick of generating laser is to achieve the status of so-called population inversion. Take ruby laser for example (figure 4), firstly atoms absorb energy, it will transit to excited state. Atoms stay in excited state for a very short period, it will fall to medium state called metastable state. Atoms stay in metastable state for a long period. Electron long staying in metastable state will cause that the electrons in metastable status is more than electrons in ground state. This phenomenon is called population inversion. It is the key of laser’s creation, because it makes that the stimulated photons from metastable state to ground state, is more than the photons from ground state to metastable state. It ensures that the quantity of photons in medium will increase, so that it will output laser.