CO2 laser principle

CO2 lasers have a relatively high photoelectric efficiency, do not cause damage to the working medium, and emit invisible lasers with a wavelength of 10.6μm. They are an ideal laser. According to the working form of the gas, they can be divided into closed and circulating types, and according to the excitation method, they can be divided into electrical excitation, chemical excitation, thermal excitation, optical excitation, and nuclear excitation. Almost 100% of CO2 lasers used in medical treatment are electrically excited.

Working principle of CO2 laser: Like other molecular lasers, the stimulated emission process of CO2 laser is also complicated. There are three different kinds of motions of molecules. The first is the motion of electrons in molecules, which determines the electronic energy state of molecules. The second is the atomic vibration in molecules, which means that atoms in molecules vibrate periodically around their equilibrium positions continuously, and it is determined by the vibration energy state of molecules. The third is the rotation of molecules, which means that molecules rotate continuously in space as a whole. This kind of motion of molecules determines the rotation energy state of molecules. Molecular motion is extremely complex, so the energy level is also very complex.

The CO2 molecule is a linear symmetrical molecule, with two oxygen atoms on either side of the carbon atom, which represents the equilibrium position of the atom. Each atom in the molecule is always moving and vibrates around its equilibrium position. According to the molecular vibration theory, CO2 has three different vibration modes: ① The two oxygen atoms vibrate in opposite directions along the molecular axis, that is, the two oxygen atoms reach the maximum value and equilibrium value of the vibration at the same time, while the carbon atom in the molecule is stationary, so its vibration is called symmetrical vibration. ② The two oxygen atoms vibrate in a 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. ③ The three atoms vibrate along the symmetry axis, and the vibration direction of the carbon atom is opposite to that of the two oxygen atoms, which is also called antisymmetric vibration energy. In these three different vibration modes, different groups of energy levels are determined.

Excitation process of CO2 laser: In CO2 laser, the main working substances are composed of three gases: CO2, nitrogen and helium. Among them, CO2 is the gas that produces laser radiation, and nitrogen and helium are auxiliary gases. The addition of helium can accelerate the 010 energy level thermal relaxation process, so it is beneficial to the evacuation of laser energy levels 100 and 020. The addition of nitrogen mainly plays the role of energy transfer in CO2 laser, and plays a strong role in the accumulation of energy level particles in CO2 laser and high-power and high-efficiency laser output.

Energy level diagram of CO2 molecule laser transition

Excitation conditions of CO2 laser: A direct current of tens or hundreds of mA is usually input into the discharge tube. 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. The CO2 molecules transition from a low energy level to a high energy level, forming a particle number inversion and emitting lasers.

Structure: ①Laser tube: It is the most critical component in the laser machine. It is usually made of hard glass and generally adopts a layer sleeve structure. The innermost layer is the discharge tube, the second layer is the water cooling sleeve, and the outermost layer is the gas storage tube. The diameter of the discharge tube of the carbon dioxide laser is thicker than that of the He-Ne laser tube. The thickness of the discharge tube generally has no effect on the output power. It mainly considers the diffraction effect caused by the spot size and should be determined according to the tube length. The longer the tube, the thicker it is, and the shorter the tube, the thinner it is. The length of the discharge tube is proportional to the output power. Within a certain length range, the power output per meter of the discharge tube length increases with the total length. The purpose of adding a water cooling sleeve 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 has a small hole connected to the discharge tube, and the other end is connected to the discharge tube through a spiral return gas pipe, so that the gas can circulate in the discharge tube and the gas storage tube, and the gas in the discharge tube can be exchanged at any time.

CO2 laser structure diagram

Optical resonant cavity: The resonant cavity of CO2 laser usually uses a flat concave cavity, and the reflector is made of K8 optical glass or optical quartz, which is processed into a concave mirror with a large radius of curvature. The mirror is coated with a high-reflectivity metal film - gold-plated film, with a reflectivity of 98.8% at a wavelength of 10.6μm, and the chemical properties are stable. The light emitted by carbon dioxide is infrared light. Therefore, the reflector needs to use infrared-transmitting materials, because ordinary optical glass is not transparent to infrared light. It is required to open a small hole in the center of the total reflector. Then seal it with a piece of infrared material that can transmit 10.6μm laser to seal the gas. This allows part of the laser in the resonant cavity to output from this small hole outside the cavity to form a laser beam.

Power supply and pump: The discharge current of the closed CO2 laser is relatively small, and a cold electrode is used. The cathode is made of molybdenum or nickel sheets into a cylindrical shape. The working current is 30-40mA, and the area of ​​the cathode cylinder is 500cm2. In order to avoid lens contamination, a light barrier is added between the cathode and the lens.

The pump is excited by continuous DC power supply. The principle of DC power supply for exciting CO2 laser is that the DC voltage is the AC voltage in the city, which is increased by a transformer, and then the high voltage is obtained by high-voltage rectification and high-voltage filtering and then added to the laser tube.