Like other gas lasers, the CO2 laser’s working principle and its stimulated emission process are also more complicated. Molecules have three different motions, that is, the motion of electrons in the molecule, and its motion determines the electronic energy state of the molecule; the second is the vibration of atoms in the molecule, that is, the atoms in the molecule constantly vibrate periodically around their equilibrium position-and It is determined by the vibrational energy state of the molecule; the third is the rotation of the molecule, that is, the molecule rotates continuously in space as a whole, and this movement of the molecule determines the rotational energy state of the molecule. The molecular motion is extremely complicated, so the energy levels are also very complicated.
The CO₂ molecule is a linear symmetrical molecule. Two oxygen atoms are on both sides of the carbon atom, which represents the equilibrium position of the atoms. The atoms in the molecule are always in motion and must vibrate constantly around their equilibrium position. According to the theory of molecular vibration, CO₂ has three different vibration modes: ①Two oxygen atoms vibrate in opposite directions along the molecular axis, that is, the two oxygens reach the maximum value and equilibrium value of vibration at the same time during the vibration, and the The carbon atom is stationary, so its vibration is called symmetrical vibration. ②Two oxygen atoms vibrate in the direction perpendicular to the molecular axis, and the vibration direction is the same, while the carbon atom vibrates in the opposite direction and perpendicular to the molecular axis. Since the vibrations of the three atoms are synchronized, it is also called deformation vibration. ③Three atoms vibrate along the axis of symmetry, and the vibration direction of the carbon atom is opposite to that of the two oxygen atoms, which is also called antisymmetric vibrational energy. In these three different vibration modes, different groups of energy levels are determined.
The carbon dioxide laser is a molecular laser. The main substance is the carbon dioxide molecule. It can express a variety of energy states, depending on the shape of its vibration and rotation. The mixed gas in carbon dioxide is a plasma (plasma) formed by a low-pressure gas (usually 30-50 Torr) caused by the release of electrons. As Maxwell-Boltzmann’s law of distribution says, in plasma, molecules exhibit multiple states of excitement. Some will present a high-energy state (00o1) which appears as an asymmetric swing state. When colliding with a hollow wall or naturally radiating, such molecules will accidentally lose energy. Through natural emission, this high-energy state will drop to a symmetrical swing form (10o0) and emit photons (a light beam with a wavelength of 10.6μm) that may travel in any direction. Occasionally, one of these photons will propagate down the cavity of the optical axis and will swing in the resonance mirror.
In the CO2 laser, the main working materials are composed of CO₂, nitrogen, and helium. Among them, CO₂ is the gas that produces laser radiation, and nitrogen and helium are auxiliary gases. The added helium can accelerate the thermal relaxation process of the 010 energy level, which is beneficial to the pumping down of the laser energy levels 100 and 020. The addition of nitrogen gas mainly plays a role in energy transfer in the CO₂ laser, and plays a powerful role in the accumulation of energy level particles on the CO₂ laser and the high-power and high-efficiency laser output.
Excitation conditions of CO₂ laser: In the discharge tube, a direct current of tens of mA or hundreds of mA is usually input. During discharge, the nitrogen molecules in the mixed gas in the discharge tube are excited by the impact of electrons. At this time, the excited nitrogen molecules collide with the CO₂ molecules, and the N2 molecules transfer their energy to the CO2 molecules, and the CO₂ molecules transition from a low energy level to a high energy level to form a particle number inversion and emit laser light.
It is the most critical part of the laser machine. It is usually made of hard glass and generally adopts a layered sleeve structure. The innermost layer is the discharge tube, the second layer is the water-cooled casing, and the outermost layer is the gas storage tube. The diameter of the discharge tube of the carbon dioxide laser is larger than that of the He-Ne laser tube. Generally speaking, the thickness of the discharge tube has no effect on the output power, mainly considering the diffraction effect caused by the size of the light spot, which should be determined according to the tube length. The longer tube is thicker, and the shorter tube is thinner. The length of the discharge tube is proportional to the output power. Within a certain length range, the output power per meter of discharge tube length increases with the total length. The purpose of adding a water cooling jacket is to cool the working gas and stabilize the output power. The discharge tube is connected to the gas storage tube at both ends, that is, one end of the gas storage tube is connected with the discharge tube with a small hole, and the other end is connected with the discharge tube through the spiral return tube, so that the gas can circulate in the discharge tube and the gas storage tube Flow, the gas in the discharge tube is exchanged at any time.
The resonant cavity of CO2 laser is usually flat and concave. The mirror is made of K8 optical glass or optical quartz, which is processed into a concave mirror with a large radius of curvature. The mirror surface is coated with a metal film with high reflectivity—a gold-plated film, at a wavelength of 10.6μm The reflectivity is 98.8%, and the chemical properties are stable. The light emitted by carbon dioxide is infrared light. Therefore, the reflector needs to use materials that transmit infrared light, because ordinary optical glass is not transparent to infrared light. It is required to make a small hole in the center of the total reflection mirror. Then seal a piece of infrared material that can transmit 10.6μm laser to seal the gas. This makes a part of the laser in the resonant cavity output from this small hole to form a laser beam.
3、Power supply and pump
The discharge current of the enclosed CO2 laser is relatively small, and cold electrodes are used, and the cathode is made of a cylindrical shape with molybdenum or nickel plates. The working current of 30～40mA, the area of the cathode cylinder is 500cm2, and the lens will not be polluted. A light barrier is added between the cathode and the lens. The pump is excited by a continuous DC power supply. The principle of DC power supply for exciting CO2 lasers. The DC voltage is to increase the AC voltage in the city with a transformer, and obtain high-voltage electricity through high-voltage rectification and high-voltage filtering to add to the laser tube.