CO2 lasers - functionality and areas of application

1. Terminology

The CO2, or carbon dioxide, laser is a member of the gas laser group. The CO2 laser emits in mid-infrared between ~9µm to ~11µm wavelength, the strongest emission line is often 10.6µm (more rarely at 9.3µm or 10.2µm). Together with Nd: YAG/fiber lasers, the CO2 laser creates the backbone of industrial laser technology. It can produce great average output capabilities up to 80 kW and pulse energies up to 100 kJ. As CO2 lasers have relatively high efficiency for gas lasers of up to 15% and are economical to buy, they are used in industrial metalworking as well as for marking and cutting organic workpieces.
The CO2 laser has been around since 1964 and was devised and developed by Kumar N. Patel at Bell Laboratories (USA).

2. Functionality of a CO2 laser

Excitation is crucial to the operation of the laser source. This is created in pulsed CO2 lasers by irradiating electromagnetic waves with a frequency range in the tens of megahertz into the gas mixture of CO2, N2 (nitrogen), and He (helium) by means of antennas. Continuously emitting CO2 lasers can be excited by a high voltage up to about 20,000 volts and the resulting glow discharge.

The excitation makes it so CO2 molecules are taken to a higher energy level. The higher energy is stored in the resonator in the form of rotation or vibration of the CO2 molecules. If a photon of proper wavelength (mid-infrared) runs into an excited CO2 molecule, stimulated emission occurs, i.e. the energy stored in rotation or vibration is emitted as a photon. The incident photon has thus produced a “twin photon” and decreased the energy of the CO2 molecule by the energy of the twin photon (as a direct result of energy conservation). If there are enough excited CO2 molecules, the number of photons generated by stimulated emission will greatly increase (“avalanche effect”).

Mirrors direct the generated laser beam toward the workpiece to be processed. A cutter head positioned immediately above the workpiece focuses the laser beam. If the cutter head is struck by the laser with a diameter of about 20 mm, it is directed there by means of a converging lens e.g. to a diameter of 0.1 mm in the focal plane.
Gas is often provided in or near the cutter head close to where the laser beam contacts the workpiece. Oxygen may be used for flame cutting to support the cutting process through oxidation of the workpiece. Dormant gas (often nitrogen or argon), however, should reduce oxidation processes as much as possible.

At the focal point of the laser beam, temperatures climb during each (!) cutting or engraving process, which are over the vaporization temperature of the individual material. The vaporization then causes an engraving or the kerf.

3. Designs of CO2 laser

There is a wide array of CO2 laser systems, most of which seldom vary in the overall construction. The most well-known models include the longitudinal-flow and transverse-flow lasers, the sealed-off laser, the waveguide laser, and the TEA laser.

Longitudinal-flow and transverse-flow lasers

This type of laser system is relatively simple and frequently used with high production lasers. In longitudinal- and transverse-flow lasers, a laser gas is continuously vacuumed through a discharge tube using a vacuum pump. Through a direct current discharge, part of the carbon dioxide held in the gas compound is split into oxygen and carbon monoxide. Through several pumps in the tube system, the gas mixture is continuously distributed, providing more effective extraction of heat loss.

Sealed-off laser

In the sealed-off laser, oxygen, water vapor, and hydrogen are added to the gas mixture, rather than being replaced by a pump. These admixtures guarantee that the resulting carbon monoxide reacts via an electrode made of platinum to carbon dioxide. CO2 is consequently catalytically reformed.

Waveguide laser

The waveguide laser, or slab laser, uses two electrodes as waveguides and includes a resonator which is cuboidal. Due to the cross-section having a high aspect ratio (e.g. height to width 10:1) the resonator has a moderately large surface area compared to the volume. This allows for efficient removal of heat loss.

TEA laser

The TEA laser, short for “transversely excited atmospheric pressure,” is invariably used when high gas pressures up to one bar are needed with pulse durations up to 100 ns. The discharge voltage is utilized in short pulses of under one microsecond across the gas flow in this laser design. This prevents arcing.

4. What is the CO2 laser used for?

CO2 lasers are typically used with power ranging from 10 to 400 watts for marking, cutting, or engraving thin, organic materials including plastics, textiles, or wood. Very sharp cutting quality can be accomplished by cutting PMMA (“acrylic”, “Plexiglas”) - when processed correctly, the cutting edges are just as transparent as the surrounding exteriors of the workpiece.

CO2 lasers with increased power between 1 to 6 kilowatts are typical industrial lasers used for welding, hardening or remelting metals. In modern production, CO2 lasers are increasingly used for oxide-free laser cutting. In particular, the laser cutting machines are used for small batches in sheet metal processing. For larger quantities, however, punching is still the more economical option. 

The CO2 laser is used in a wide variety of industries. At the forefront is the automotive industry, where lasers are used to perforate the breaking point in the dashboard for airbags. Headliners or side panels are also manufactured using CO2 lasers. 
Even in the clothing industry, there is a variety of applications for the CO2 laser. From fabric blanks to the texturing of jeans, the laser is an environmentally-friendly alternative to chemical and abrasive processes. 

Cutting fiber-reinforced plastics like GRP and CRP in one application that isn't going away soon. The automotive, aviation, or wind energy industries, use these fiber-reinforced plastics as part of the solution to critical issues of the times, including sustainability, climate protection, and resource efficiency.

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