CO2 laser tube composition structure
The basic structure of a typical sealed-off CO2 laser tube is shown in the figure above, which is mainly composed of three parts: hard glass , resonant cavity , and electrode .
1. Hard glass part
This part is composed of GG17 material fired into discharge tube, water-cooled tube, air storage tube and return air tube. The discharge tube is a key part of the CO2 laser. It basically determines the characteristics of the laser output. The length of the discharge tube is proportional to the output power. Proportional. The function of the water-cooled tube is to cool the working gas, maintain a stable output power, and prevent the discharge tube from bursting due to heat during the discharge pumping process. The function of the gas storage tube on the one hand increases the gas storage capacity of the gain medium, reduces the change of the working gas composition and pressure during the discharge process, and prolongs the operating life, on the other hand, it enhances the mechanical strength and stability of the discharge tube. The air return tube is a thin spiral tube connecting the two electrode spaces in the discharge tube, which can improve the unbalanced distribution of the voltage between the electrodes caused by the electrophoresis phenomenon. Discharge only occurs in the discharge tube.
2. Resonant cavity part
This part consists of a total mirror and an output mirror. The total mirror of the resonant cavity is generally based on optical glass, and the surface is gold-coated. The reflectivity of the gold-film mirror at 10.6um is above 98%; the output mirror of the resonant cavity is generally made of infrared materials that can transmit 10.6um radiation. Germanium (Ge) is the substrate, and a multilayer dielectric film is formed on it.
3. Electrode part
CO2 lasers generally use cold cathodes, which are cylindrical in shape. The selection of cathode materials has a great impact on the life of the laser. The basic requirements for cathode materials are: low sputtering rate and low gas absorption rate.
Working principle of CO2 laser tube
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 constantly vibrate around their equilibrium position. According to the molecular vibration theory, CO2 has three different vibration modes:
1. Two 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 perpendicular to the molecular axis. Since the vibrations of the three atoms are synchronized, it is also called deformation vibration
2. The two oxygen atoms vibrate in opposite directions along the molecular axis, that is, the two oxygens reach the maximum value and equilibrium value of the vibration at the same time in the vibration, and the carbon atoms in the molecule are still at this time, so the vibration is called symmetry vibration.
3. Three 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, it is determined that there are different groups of energy levels.
Energy level graph
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 basic energy network is shown in Figure 1. 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.
Excitation process
In the CO2 laser tube, 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. With helium, it can accelerate the thermal relaxation process of 010 energy level, which is more conducive to the pumping down of laser energy levels 100 and 020. The addition of nitrogen gas mainly plays a role in energy transfer in the CO2 laser tube, 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 tube: 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 CO2 molecules, N2 molecules transfer their energy to CO2 molecules, and CO2 molecules transition from a low energy level to a high energy level to form a population inversion and emit laser light.