1, spontaneous radiation and excited radiation
Spontaneous radiation is in the absence of any external action, the excited state of the atom spontaneously from the high-energy level to the lower energy level, while radiating a photon. hn = E2-E1.
Set the luminous material in the unit volume of the number of atoms in the energy level E1, E2, respectively, N1, N2, then per unit of time from E2 to E1 spontaneous radiation number is
A21 is the probability of spontaneous radiation (spontaneous leap rate): it indicates that an atom from E2 to E1 within a unit of time is the number of atoms in the energy level. The number of atoms spontaneously radiating from E2 to E1 in a unit of time is
A21 is the probability of spontaneous radiation (spontaneous leap rate): it indicates the probability of an atom spontaneously radiating from E2 to E1 in a unit of time.
An atom at a high energy level E2, excited by an external photon with energy hn= E2- E1, is forced to leap from the high energy level E2 to the low energy level E1, while radiating a photon that is all the same as the excited photon. It is called excited radiation.
W21 is the probability of representing an atom that leaps from E2 excited radiation to E1 in a unit of time.
2, particle number reversal
Excited absorption is proportional to the number of atoms N1 in E1, and excited radiation is proportional to the number of atoms N2 in E2. When N2 "N1 when the occurrence of stimulated radiation is much less than the occurrence of stimulated absorption, it is impossible to realize the light amplification. To realize the light amplification, must take special measures to break the number of atoms in thermal equilibrium under the Boltzmann distribution, so that N2> N1. we call this state of the system for the number of particles inversion (or "negative temperature" system). Therefore, the first condition for generating lasers is the realization of particle number inversion.
The medium in which particle number inversion can be achieved is called the activation medium. To create a particle number inversion distribution, firstly, the medium is required to have an appropriate energy level structure, and secondly, it is necessary to have the necessary energy input system. The process of supplying energy to the atoms in the lower energy states to induce them to jump to the higher energy states is called pumping process.
3, optical resonant cavity
In the laser using optical resonant cavity to form the required strong radiation field, so that the radiation field energy density is much greater than the value of thermal equilibrium, so that the probability of excited radiation is much greater than the probability of spontaneous radiation.
The main part of the optical resonant cavity is two mirrors that are parallel to each other and perpendicular to the axis of the activation medium, with one being a total reflector and the other being a partial reflector. Under external excitation by various means, such as light, heat, electricity, chemistry or nuclear energy, the activation medium inside the resonant cavity will invert the number of particles between two energy levels. At this time to produce stimulated radiation, in the production of stimulated radiation light, along the axial propagation of light in the two mirrors between the back and forth reflection, back and forth through the activation of the number of particles has been realized inversion of the medium, constantly giving rise to new stimulated radiation, so that the axial travel of the frequency of the light to be amplified, the process is called optical oscillation. This is an avalanche of amplification process, so that the resonant cavity along the axial light suddenly enhanced, so the radiation field energy density is greatly enhanced, the excited radiation is far more than spontaneous radiation. This excited radiation light output from part of the mirror, it is laser. The light propagating along the other direction soon escapes from the side of the resonant cavity, can not be continued to be amplified. And the frequencies generated by spontaneous radiation are not amplified. Therefore, the laser light output from the resonant cavity has good directionality and monochromaticity.
Properties of laser light
1, good monochromaticity
2, good directionality
3, good coherence
4, energy concentration
Laser applications
1, laser ranging
2, laser processing and laser medical
3, optical information processing and laser communication
4, laser applications in controlled fusion
5, laser nonlinear effects
Lasers are an application of the principles of optics, but what exactly does it take to go from ordinary light to laser? It is necessary to first understand the principle of atomic luminescence. When an atom drops from a high energy order to a low energy order, it emits a photon of light called self-emitted light. When an atom is hit by a photon at a high energy level, it will be excited to emit another identical photon, which will become two photons, called stimulated emission. If the process of stimulated emission continues, more and more photons will be emitted. As long as we control the number of atoms at higher energy levels than the number of atoms at lower energy levels, the process of stimulated emission of light will continue to be generated, and this device for controlling the stimulated emission of light from atoms is called a "light amplifier".
We also know that light travels at the speed of light in all directions when it is emitted, and in order for the light to be collected and continuously amplified and utilized, it is necessary to utilize a device called a "*** vibration chamber" to confine the light produced by the optical amplifier to a specific range with mirrors, so that the light can be reflected back and forth and, because the photons produced by the optical amplifier are the same, the direction of travel is also the same, so the direction of travel is also the same. Since the photons produced by the optical amplifier are the same, they travel in the same direction. Through the role of the **** vibration cavity, can make the light travel in the same direction, that is to say, have with the **** vibration cavity in the same direction of the light will be amplified, the rest of the different direction of the light will not be amplified, this is the primary conditions for the generation of laser.
The ****-cavity also serves another purpose, which is to limit the frequency of the laser. Light to produce *** vibration in the *** vibration cavity must comply with the relationship L = nλ / 2 (L is the *** vibration cavity length, λ is the wavelength, n is a fixed number of times), so not all frequencies of light can be produced in the *** vibration cavity in the *** vibration, but only in line with the rules of this *** vibration will be produced. However, the length of the **** cavity (L) can be several meters long, while the wavelength of light (λ) is measured in microns, which is a difference of 1 million times, that is to say, the eligible n range is quite large, rather than only a single frequency. The ability to emit light at so many frequencies at once gives us the conditions to build a pulsed laser.