CO2 laser working principle and classification

                                                                CO2 laser working principle and classification

The carbon dioxide laser first used its wavelength of 10.6μm in 1964. Because this is a very efficient laser with a conversion efficiency of 10% as a commercial model, carbon dioxide lasers are widely used in laser cutting, welding, drilling and surface treatment. As a commercial laser, it can reach several kilowatts, which is currently the strongest material processing laser.

1. Principle of operation

Carbon dioxide laser is a kind of 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 exhibits an asymmetric swing state. When colliding with a hollow wall or radiating naturally, 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 travel down the cavity of the optical axis and will swing in the resonance mirror.

Figure 1 Simple carbon dioxide molecular energy level diagram

Generally, the working substance of carbon dioxide laser is a mixture of carbon dioxide, helium, and nitrogen. Nitrogen acts as a buffer gas and its molecules resonantly transfer stimulating energy to carbon dioxide molecules. Because the relaxation level is the bottleneck, the role of helium is to transfer energy to helium atoms as a heat sink.

2, the type of carbon dioxide laser

The way the waste heat is rejected has a great influence on the laser system design. In principle, there are two possible ways. The first method is based on the automatic processing of naturally diffused heat to the wall of the tube. The principle of operation is sealing and slow axial flow laser. The second is based on forced convection of gas, and its operating principle is fast axial flow laser. In general, there are five types of CO2 lasers:

sealed or no flow

slow axial flow

fast axial flow

fast lateral flow

Lateral Excitation Atmosphere (TEA)

sealed or flowless carbon dioxide lasers are usually marked by lasers used for beam deflection. Its discharge tube is completely sealed. The quality of this laser beam is very good. And in most cases, the entire discharge tube can be replaced with a new one and can be refilled so it is easy to maintain. This eliminates the need for a separate gas supply system. Only need to make fewer connections in the laser head. So it is compact and lightweight. But its energy output is low (usually less than 200 watts).

TEA carbon dioxide laser is usually used in the production of protective covers. It can only be operated under pulse conditions. The air flow is low and the air pressure is high. The excitation voltage is about 10,000 volts. The energy distribution of this laser beam is uniform in a relatively large area. Its maximum energy can reach 1012 watts and its pulse width is very small. Nevertheless, due to the multi-state operation, this type of laser is difficult to concentrate on a small spot.

3, the power supply of the pump

For the CW CO2 laser, there are generally three main ways to power the pump. For example: direct current (DC), high frequency (HF), radio frequency (RF). The design of DC power supply is the simplest. In the high-frequency power supply mode, the electrons alternate between 20-50 kilohertz. Compared with direct current, high frequency power supply is compact in size and more efficient. In radio frequency power supply, the electrons alternate between 2 and 100 MHz. Compared with direct current, its voltage and efficiency are lower.