CO2 lasers - functionality and areas of application
The carbon dioxide laser, more commonly known as the CO2 laser, belongs to the group of gas lasers. The CO2 laser emits in mid-infrared between ~9µm to ~11µm wavelength, with the strongest emission line being 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 far as gas lasers go, CO2 lasers have a relatively high efficiency of up to 15% and because they are inexpensive to purchase, they are commonly used in industrial metalworking and for cutting and marking organic items and workpieces.
The CO2 laser has been in existence since 1964 and was devised and developed by Kumar N. Patel at Bell Laboratories (USA).
2. Functionality of a CO2 laser
The so-called laser excitation is critical to the function of the laser source. For pulsed CO2 lasers this is carried out 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 laser excitation ensures that the CO2 molecules reach a higher energy level. This 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, then so-called “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 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 (“avalanche effect”).
The generated laser beam is directed by a series of mirrors in order to reach the workpiece that is to be processed. The laser beam is then focused at the cutting/engraving head, which is positioned directly above the workpiece. If the laser beam strikes the cutter/engraving head, which has a diameter of about 20mm, it is focused there by means of a converging lens e.g. to a diameter of 0.1 mm in the focal plane.
At the cutter/engraving head or close to the place where the laser beam will strike the workpiece, gas can also be supplied. Oxygen is used for flame cutting - i.e. the oxidisation of the workpiece by the addition of oxygen assists in supporting the cutting process. If an inert gas (often nitrogen or argon) is also added, this should minimise the oxidisation processes as much as possible.
At the focal point, i.e. at the precise point where the laser beam is focused on the workpiece, temperatures rise during each cutting or engraving process, which exceed the vapourisation temperature of the respective material. The vapourisation then results in an engraving or kerf.
3. Designs of CO2 laser
Currently, there are several design types of CO2 laser, which sometimes overlap in terms of their construction. The most common design types are 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 most often used with high output lasers. In longitudinal- and transverse-flow lasers, a laser gas is continuously vacuumed through a discharge tube by using a vacuum pump. By means of a direct current discharge, a portion of the carbon dioxide contained in the gas mixture is split into carbon monoxide and oxygen. By the means of several pumps in the tube system, the gas mixture is continuously circulated, enabling a more efficient removal of heat loss..
In this design, instead of the gas mixture being replaced by a circulating pump, hydrogen, water vapour and oxygen are added to the gas mixture. These added gas mixtures ensure that the resulting carbon monoxide reacts via an electrode made of platinum to carbon dioxide. CO2 is therefore catalytically regenerated.
The waveguide laser, also known as a slab laser, uses two electrodes as waveguides and has a cuboidal resonator. As the cross section has a high aspect ratio (e.g. height to width 10:1) the resonator has a relatively large surface area compared to the volume. This enables the efficient removal of heat loss.
The “transversely excited atmospheric pressure laser”, TEA for short, 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. This prevents arcing.
4. What is the CO2 laser used for?
CO2 lasers are used between the power range of 10 to 400 watts for cutting, perforating or engraving thin, organic materials such as wood, textiles or plastics. Very high cutting quality can be achieved by cutting PMMA (“acrylic”, “Plexiglas”) - if processed correctly, the cut edges will be just as transparent as all of the other workpiece surfaces.
CO2 lasers with a much higher power of between 1 to 6 kilowatts are typically industrial lasers used for welding or the hardening or remelting of metals. In modern production, CO2 lasers are increasingly being used for oxide-free laser cutting and in particular for manufacturing small batches in sheet metal processing. However, for much larger quantities, punching is still the more economical option.
The CO2 laser is now being used in a wide variety of different 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 being manufactured by using CO2 lasers.
Even in the clothing and textile industry, there is a wide variety of applications for the CO2 laser. From fabric blanks to the texturing of jeans, the laser is an environmentally-friendly alternative to the more traditional chemical and abrasive processes.
One future-proof market for the CO2 laser is the cutting of fiber-reinforced plastics such as GRP and CRP. These plastics are widely used in the automotive, aviation and wind energy industries, where fiber-reinforced plastics are increasingly used as a part of the solution to many of the important issues of today, such as sustainability, resource efficiency and climate protection.