Analysis of laser cooling principle of fiber laser marking machine

Analysis of laser cooling principle of fiber laser marking machine

　　Temperature is a physical quantity that expresses the degree of heat and cold of an object. Microscopically speaking, it is the intensity of the thermal motion of the molecules of the object. As we all know, all the molecules and atoms around us are undergoing non-stop irregular thermal motion. The essence of our refrigeration is to reduce the intensity of the overall thermal motion of these molecules or atoms, fiber laser marking machine.

1. A very important technology in laser refrigeration is Doppler cooling technology. The principle of Doppler cooling technology is to block the thermal motion of atoms by emitting photons from the laser, and this blocking process is to reduce the momentum of the atoms. Achieved. So, how does laser reduce the momentum of these atoms?

　　 First of all, quantum mechanics proposes that atoms can only absorb photons of specific frequencies, thereby changing their momentum. The Doppler effect points out that the frequency of a wave becomes higher when the wave source moves towards the observer, and becomes lower when the wave source moves away from the observer. The same conclusion can be drawn when the observer moves.

　　 The same is true for atoms. When the motion direction of the atom is opposite to the photon motion, the photon frequency will increase, and when the atom motion direction is the same as the photon motion direction, the photon frequency will decrease. Then, another physical principle is that although light has no static mass, it has momentum. Then we can build a simple model of laser cooling by combining the above several physical characteristics.

　　2. The frequency of the laser is adjustable within a certain range, and when the frequency of the laser is adjusted to slightly lower than the frequency that an atom can absorb, there will be unexpected results. This happens when such a beam of light illuminates a specific atom. If the atom is moving toward the laser beam, due to the Doppler effect of light, the frequency of the photon increases, and the original laser photon frequency is just slightly less than the atom's absorbable frequency, then the Doppler effect is just right at this time Absorbed by atoms.

　　 And this absorption is manifested as a change in momentum. Because the movement direction of the photon is opposite to the movement direction of the atom, after the photon collides with the atom, the atom transitions to an excited state, and the momentum decreases, so the kinetic energy also decreases. For atoms in other moving directions, the frequency of the corresponding photons will not increase, so the photons in the laser beam cannot be absorbed, so there will be no increase in momentum, which is the same with respect to kinetic energy.

　　 When we use multiple laser beams to irradiate atoms from different angles, the momentum of the atoms in different moving directions will decrease, and the kinetic energy will decrease. Since the laser only reduces the momentum of the atoms, after this process continues for a period of time, the momentum of most of the atoms will reach a very low level, so as to achieve the purpose of cooling.

　　 But the scope of application of this technology is mostly used for atomic cooling, and for molecules, this method is difficult to cool it to ultra-low temperature. But ultra-cold molecules are more meaningful than ultra-cold atoms because their properties are more complex. At present, the method of cooling molecules is to combine super-cold base atoms together to produce double-base molecules. Not long ago, Yale University once cooled strontium fluoride (SrF) to a few hundred micro-Kelvin.

　　 Another type of laser refrigeration, also called anti-Stokes fluorescence refrigeration, is a new concept of refrigeration method under development. Its basic principle is the anti-Stokes effect, which uses the energy difference between scattered and incident photons to achieve refrigeration. The anti-Stokes effect is a special scattering effect in which the wavelength of scattered fluorescent photons is shorter than that of incident photons.

Therefore, the energy of scattered fluorescent photons is higher than the energy of incident photons. The process can be simply understood as: using low-energy laser photons to excite the luminescent medium, the luminescent medium scatters high-energy photons, and the original energy in the luminescent medium is taken out of the medium for cooling. . Compared with the traditional cooling method, the laser plays the role of providing cooling power, and the scattered anti-Stokes fluorescence is the heat carrier.