CO2 lasers - functionality and application areas

1. Terminology

The CO2 laser, less commonly referred to as carbon dioxide laser, belongs to 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 forms the backbone of industrial laser technology. It can deliver very high average output powers up to 80 kW and pulse energies up to 100 kJ. As CO2 lasers have a relatively high efficiency for gas lasers of up to 15% and are inexpensive to buy. This laser source is used in industrial metalworking as well as for laser cutting and marking organic workpieces and other substrates.
Introduced in 1964 by Kumar N. Patel, the CO2 laser was designed and developed at Bell Laboratories (USA).

2. Functionality of a CO2 laser

The excitation is fundamental to the function of the laser source. For pulsed CO2 lasers this is done by irradiating electromagnetic waves with a frequency in the tens of megahertz range into the gas mixture of CO2, N2 (nitrogen) and He (helium) by means of antennas. Continuously emitting CO2 lasers can be excited by high voltage up to around 20,000 volts and the resulting glow discharge. 

The excitation ultimately ensures that CO2 molecules are brought 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 suitable wavelength (mid-infrared) now hits an excited CO2 molecule, stimulated emission occurs, for example the energy stored in rotation or vibration is emitted as a photon. The incident photon has thus produced a “twin photon” and reduced the energy of the CO2 molecule by the energy of the twin photon (as a direct consequence of energy conservation). If there are enough excited CO2 molecules, the number of photons generated by stimulated emission increases exponentially in an avalanche effect.

The generated laser beam is directed by mirrors towards the workpiece to be processed. The laser beam is focused in a cutter head positioned directly above the workpiece. If the laser strikes the cutter head with a diameter of about 20 mm, it is focused there by means of a converging lens, for example to a diameter of 0.1 mm in the focal plane. 
In the cutter head or near the point where the laser beam strikes the workpiece, gas is often supplied. Oxygen for flame cutting - such as the oxidation of the workpiece by the addition of oxygen supports the cutting process. Inert gas (often nitrogen or argon), however, should minimise oxidation processes as much as possible.

In the focal point where the laser beam is focused on the workpiece, temperatures arise during each (!) laser cutting or laser engraving process, which are above the vaporisation temperature of the respective material. The vaporisation then causes an engraving or the kerf. 

3. Designs of CO2 laser

There are several designs of CO2 laser available currently, which sometimes overlap in terms of construction. The most common types 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 design is comparatively simple and often used with high output lasers. In longitudinal- and transverse-flow lasers, a laser gas is continuously vacuumed through a discharge tube by means of a vacuum pump. Through a direct current discharge, a portion of the carbon dioxide contained in the gas mixture is split into carbon monoxide and oxygen. Through several pumps in the tube system, the gas mixture is continuously circulated, allowing more efficient removal of heat loss.

Sealed-off laser

In this design, the gas mixture is not replaced by a pump, but instead hydrogen, water vapour and oxygen are added to the gas mixture. These admixtures ensure that the resulting carbon monoxide reacts via an electrode made of platinum to carbon dioxide. CO2 is therefore catalytically regenerated.

Waveguide laser

The waveguide laser, also known as a slab laser, uses two electrodes as waveguides and has a resonator which is cuboidal. As the cross section has a high aspect ratio (for example height to width 10:1) the resonator has a relatively large surface area compared to the volume. This allows efficient removal of heat loss.

TEA laser

TEA stands for transversely excited atmospheric pressure laser. This type of laser is always used when high gas pressures up to one bar are required with pulse durations up to 100 ns. In this design the discharge voltage is applied in short pulses of under one microsecond across the gas flow, which prevents arcing.

4. What is the CO2 laser used for?

CO2 lasers are used in the power range of 10 to 400 watts for laser cutting, perforating or engraving thin, organic materials, including woods, textiles or plastics. Very high laser cutting quality can be achieved by cutting PMMA materials such as acrylic including Plexiglas. When laser cutting is correctly performed on this material, the laser cut edges are just as transparent as the rest of the material, known as a flame polished edge for cast acrylics. 

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 remains a 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. 

One future-proof market for the CO2 laser is the laser cutting of fiber-reinforced plastics such as GRP and CRP. Here, it is the automotive, aviation or wind energy industries, where fiber-reinforced plastics are used as part of the answer to important issues of the times, such as sustainability, resource efficiency or climate protection.

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