Carbon dioxide laser

A carbon dioxide laser, CO2 laser or carbon dioxide laser denotes a laser class of different designs from the group of gas, molecular and infrared lasers in the mid-infrared. Be a laser medium is carbon dioxide, with a 4 -level system. He counts in addition to the solid-state lasers of the most powerful and most frequently employed lasers. It can output powers up to 80 kW and pulse energies up to 100 kJ achieved. CO2 lasers are relatively efficient and inexpensive, which is why they are used particularly in industrial material processing. The efficiency is about 15 to 20%. It was developed in 1964 by C. Kumar N. Patel at Bell Laboratories.

Function

The laser medium is usually made ​​of a CO2 -N2- He gas mixture. The N2 molecules excited by a DC or RF glow discharge in the resonator. The N2 molecules are particularly easy to oscillate. This is an actual kinetic molecular vibration (in this case a stretching vibration ), and no excitation of the electrons of the atoms, as with other solid state lasers. Electron excitation and ionization are also going on, but are not relevant for the excitation process of the CO2 molecules.

Are the N2 molecules excited, they can only with two discrete amplitudes oscillate ( ν 2 and ν ). Since the N2 molecule has no permanent dipole moment, the transitions between the vibrational levels with the emission of photons ( optical transitions ) banned and the N2 molecules can be very long (of the order of 1 ms ) remain in this excited state. Due to the long time in the excited state, there is a high probability that they stimulate CO2 molecules by collisions of the second kind to swing in one of its four normal modes (see molecular vibration ) - this makes the N2 molecules into a kind of energy storage. It should also be mentioned here that the CO2 molecules were excited to the 2ν3 level, only to fall through spontaneous loss of energy to an energy level before they can release a photon.

Have the CO2 molecules lost their kinetic energy to ν3, they are able to fall from this metastable state into the states ν1 and ν2 2 and to emit photons in the designated wavelengths. It is likely that the molecules select the transition ν3 → ν1. Therefore, only the wavelength emitted by 10.6 microns, although the gain bandwidth is greater. After this process, the CO2 molecules fall back into a metastable state. Due to the collision with helium atoms give their kinetic energy on it and fall back to the ground state. This is a major advantage of the CO2 laser with respect to the helium -neon laser, in which the excited molecules to collide with the wall to return to the initial state. Again, this is not the case, and therefore one can achieve large resonator diameter and thereby increases the efficiency of solid.

Designs

There are several possible types of carbon dioxide lasers, which overlap not only in terms of their structure:

• Longitudinal and transverse flow lasers Laser with slow flow
• Laser with faster flow

Longitudinal and transverse flow lasers

The basic structure of a slow axial flow laser is comparatively simple. The laser gas is a mixture of the three gases of nitrogen, carbon dioxide and helium is continuously sucked by a vacuum pump through the discharge tube. The optical pumping is in this design by a direct current discharge in the axial direction, which ensures that part of the carbon dioxide is dissociated to carbon monoxide and in the discharge of oxygen. For this reason, the aforementioned continuous feeding of the gas mixture is necessary because otherwise, after some time no more carbon dioxide was present. Cooling is achieved by heat conduction to the water-cooled tubes.

The filled in the pipe system fast axial flow laser gas mixture is the purpose of gas exchange and cooling is circulated with another pump ( rotary pump or turbo compressor). Thus, the excited Kohlenstoffdioxidmolekülen is given more time to regain the ground state. Quick flow lasers have a separate cooler ( heat exchanger ) in the gas stream, the discharge pipes are uncooled.

For very high power discharge and gas flow are arranged transverse to the beam direction, so that a very quick gas exchange is possible. However, this decrease efficiency and beam quality.

Enclosed laser

In a closed CO2 laser ( engl. sealed-off laser ), the gas mixture is not replaced by a mechanical pump. Instead, hydrogen, steam and oxygen are added to the gas mixture. The admixtures shall ensure that the resulting in the optical pumping carbon monoxide on a platinum electrode again reacts to carbon dioxide and thus regenerates the carbon dioxide content in the gas chamber again.

Also waveguides are used instead of a pipe system here.

Waveguide laser ( slab laser )

In this slab laser construction, referred to as two electrodes are used as a waveguide. Is pumped the gas mixture by means of high frequency. These lasers have an unstable resonator, high beam quality produced by beamforming. Slab lasers are mostly complete, but there are also variants in which the gas mixture must be replaced.

Transverse - excited atmospheric pressure laser ( TEA- Laser )

Axial-Flow laser can not be operated at a gas pressure above 1 mbar, otherwise arc would form. To circumvent this problem, the discharge voltage in pulses can be applied to the gas flow is shorter than a microsecond transversal. Corresponding carbon dioxide laser therefore be transverse excited atmospheric pressure laser, named TEA laser (TEA stands for English transversely excited atmospheric pressure, dt transversely excited atmospheric pressure). This will be possible gas pressures up to a bar. In this case, pulse durations in the order of 100 ns can be achieved.

Application Areas

In the range from 10 watts to 200 watts, they are mainly for cutting, engraving and perforating of thin organic material ( plastics, textiles, wood and so on. ) Are used. Pulsed CO2 lasers are used ( for hybrid circuits, for example, ceramic substrates ) for scoring and separating inorganic materials. In sheet metal machining ( laser cutting) typically beam power 1-6 kW are used. This steel can be cut to about 35 millimeters to about 25 millimeters and stainless steel. CO2 laser with more than 6 kilowatts are mainly used for welding, hardening and remelting and can also be increasingly used for oxide-free laser cutting up to 40 mm. CO2 lasers are the standard tool when sheet is individually cut into small batches, in large quantities punching is cheaper.

The wavelength of the CO2 laser is 10.6 microns clearly outside the transmission window high performance USEFULLY window materials such as fused silica. Therefore - unlike lasers for the visible or near infrared spectral range - the radiation of the CO2 laser can not be done in conventional optical waveguides on glass base. The light is therefore traditionally been performed with metallic mirrors to the workpiece. As an alternative, put more and more special optical fibers based on silver halide (PIR - fiber ) by. Focusing is done with parabolic mirrors made ​​of metal or lenses of single-crystal zinc selenide. The wavelength of the CO2 laser is strongly reflected by most metals - so it is not at first sight for their processing. However, as soon as a recess in the form of a capillary caused by the partial absorption of the laser and the subsequent removal of material (e.g. by evaporation ) on the surface of the metal workpiece, the laser beam is entirely absorbed by multiple reflections at the capillary walls. In addition, an interaction between the laser beam and the metal halide exist in the capillary due to the effect of the plasma resonance. This puncturing process initially required is technologically critical because of the high back reflection and possibly the focusing -reach metal splashes. Copper, gold and other non-ferrous metals can be edited only with difficulty with the CO2 laser.

The wavelength of the CO2 laser is absorbed by glass excellent, so CO2 lasers are also used in glass processing, for welding of tungsten halogen lamps, for engraving of drinking glasses or for scribing vials in the pharmaceutical industry.

It is also known a system based on laser-induced thermal stresses separation method for brittle materials (glass, ceramics). Here, the material with CO2 lasers is locally heated, but not melted.

There are attempts to use CO2 laser uranium enrichment. A uranium- containing gas is bombarded with the laser and reacts differently to certain laser frequencies. For example, uranium -235 and uranium -238 can be separated. Such technology has already been developed and is called SILEX process. The advantages of this technology compared to other enrichment methods are that it is much more energy -effective and can be made ​​more compact.

Postgraduate

• F. Kneubühl, M. Sigrist: Laser. 7th edition. Vieweg Teubner, Wiesbaden 2008, ISBN 978-3-8351-0145-6.
• Jürgen Eichler, Jurgen Eichler, Hans- Joachim Eichler: Laser: Designs, beam guidance applications. 6th edition. Springer, 2010, ISBN 9783642104619, pp. 96-110 (Chapter 6.2 CO2 laser).

• CO2 laser, UK Aachen, RWTH Aachen
• CO2 laser, DocCheck Flexikon