Carbon dioxide laser

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A carbon dioxide laser , CO 2 laser or colloquially also carbon dioxide laser refers to a laser class of different designs from the group of gas , molecular and infrared lasers in the mid- infrared . Its laser medium is carbon dioxide with a 4-level system. Alongside solid-state lasers, it is one of the most powerful and most commonly used lasers in industry. Output powers of up to 80 kW and pulse energies of up to 100 kJ can be achieved. A CO 2 laser produces a beam of infrared light with a wavelength in the 9.4 and 10.6 µm bands. CO 2 lasers are relatively efficient and inexpensive, which is why they are particularly used in industrial material processing. The efficiency is around 15 to 20%. It was developed in 1964 by C. Kumar N. Patel at Bell Laboratories .

function

Energy level diagram of a CO 2 laser with relevant degrees of freedom of the molecules

The laser medium usually consists of a CO 2 -N 2 -He gas mixture. The N 2 molecules are in the resonator by a DC or HF - glow discharge excited. The N 2 molecules can be particularly easily made to vibrate. This is an actual kinetic molecular oscillation (in the present case a stretching oscillation ) and no excitation of the electrons of the atoms, as in solid-state lasers . Electron excitation and ionization also take place, but are not relevant for the excitation process of the CO 2 molecules.

If the N 2 molecules are excited, they can only oscillate with two discrete amplitudes (ν and 2 ν). Since the N 2 molecule does not have a permanent dipole moment, transitions between the vibration levels with emission of photons (optical transitions) are forbidden and the N 2 molecules can remain in this excited state for a very long time (order of magnitude: 1 ms). Due to the long time in the excited state, there is a high probability that they will stimulate CO 2 molecules through collisions of the second type to oscillate in one of their four normal oscillations (cf. molecular oscillation ) - this makes the N 2 molecules a kind of energy store. CO 2 molecules that have been excited to the 2ν 3 level must first drop by one energy level through spontaneous energy loss before they can emit a photon.

If the CO 2 molecules have lost their kinetic energy up to ν 3, they are able to fall from this metastable state into the states 2 ν 2 and ν 1 and thereby emit photons in the designated wavelengths. It is more likely that the molecules choose the transition ν 3  → ν 1 . Therefore only the wavelength around 10.6 µm is emitted, although the gain bandwidth is larger. After this process, the CO 2 molecules return to a metastable state. When they collide with helium atoms, they transfer their kinetic energy to them and return to their basic state. This is the great advantage of the CO 2 laser compared to the helium-neon laser , in which the excited atoms have to collide with the wall in order to reach the ground state. This is not the case here, which is why one can achieve larger resonator diameters and thus massively increase the efficiency.

Designs

There are several possible designs of carbon dioxide lasers that overlap not only in terms of their structure:

  • longitudinal and cross-flow lasers
    • Slow current laser
    • Fast flow laser
  • locked laser
  • Waveguide laser (slab laser)
  • Transversely excited atmospheric pressure laser (TEA laser)
  • RF excited lasers
  • gas dynamic lasers
  • tunable high pressure laser

Longitudinal and cross-flow lasers

Functional principle of a carbon dioxide laser with a longitudinal flow

The basic structure of a slowly flowing laser is comparatively simple. The laser gas, a mixture of the three gases nitrogen , carbon dioxide and helium , is continuously sucked through the discharge tube by means of a vacuum pump . In this design, optical pumping takes place through a direct current discharge in the axial direction, which ensures that part of the carbon dioxide dissociates into carbon monoxide and oxygen during the discharge . For this reason, the aforementioned continuous supply of the gas mixture is necessary, since otherwise no more carbon dioxide would be present after some time. The cooling takes place by conduction of heat on the pipes cooled with water.

The pipe system in quickly filled axial flow laser gas mixture for the purpose of gas exchange and cooling with a further pump ( rotary pump or centrifugal compressor circulated). This gives the excited carbon dioxide molecules more time to return to their ground state . Fast-flowing lasers have a separate cooler ( heat exchanger ) in the gas flow, the discharge tubes are not cooled.

In the case of very high powers, the discharges and gas flow are arranged transversely to the direction of the beam, so that a particularly rapid gas exchange is possible. However, this reduces the efficiency and beam quality .

Completed laser

In a sealed CO 2 laser (engl. Sealed-off laser ) is not exchanged, the gas mixture by a mechanical pump. Instead, hydrogen , water vapor and oxygen are added to the gas mixture. The admixtures ensure that the carbon monoxide produced during optical pumping reacts again to form carbon dioxide on a platinum electrode and thus the carbon dioxide content in the gas space is regenerated.

Instead of a pipe system, waveguides are also used here.

Waveguide laser (slab laser)

In this design, known as a slab laser , two electrodes are used as waveguides. The gas mixture is pumped using high frequency . These lasers have an unstable resonator and a high beam quality is generated by beam shaping . Slab lasers are usually closed, but there are also variants in which the gas mixture has to be exchanged.

Transversely excited atmospheric pressure laser (TEA laser)

Longitudinally flowing lasers cannot be operated at a gas pressure higher than a few 10 mbar, as otherwise arcs would form. To circumvent this problem, the discharge voltage can be applied in pulses shorter than one microsecond transversely to the gas flow. Corresponding carbon dioxide laser are therefore called transverse excited atmospheric pressure laser, short TEA laser (TEA stands for English transversely excited atmospheric pressure , dt. Transversely excited atmospheric pressure ). This enables gas pressures of up to one bar. Pulse durations on the order of 100 ns are achieved.

Areas of use

In the range from 10 watts to 200 watts, they are mainly used for cutting , engraving and perforating thin, organic material ( plastics , textiles , wood and so on). Pulsed CO 2 lasers are used to scratch and separate inorganic materials ( e.g. ceramic substrates for hybrid circuits). In sheet metal processing (laser cutting), beam powers of 1 to 6 kilowatts are typically used. This means that unalloyed steels up to around 35 millimeters and high-alloy steels up to around 25 millimeters can be cut. CO 2 lasers with more than 6 kilowatts are mainly used for welding , hardening and remelting and can also increasingly be used for oxide-free laser cutting up to 40 millimeters. CO 2 lasers are the standard tool when sheet metal is individually cut in small batches ; for large quantities, punching is cheaper.

The wavelength of the CO 2 laser is 10.6 µm, well outside the transmission window of high-performance window materials such as B. Quartz glass . Therefore - unlike lasers for the visible or near-infrared spectral range - the radiation from the CO 2 laser cannot be guided in conventional glass-based optical waveguides . The light has traditionally been guided to the workpiece using metal mirrors. As an alternative, special optical fibers based on silver halide ( PIR fibers ) are becoming increasingly popular . Focusing is done with parabolic mirrors made of metal or lenses made of single-crystal zinc selenide . The wavelength of the CO 2 laser is strongly reflected by most metals - so at first glance it is not suitable for processing them. However, as soon as a recess in the form of a capillary is created on the surface of the metal workpiece due to the partial absorption of the laser and the subsequent material removal (for example by evaporation) , the laser beam is completely absorbed by multiple reflections on the capillary walls. In addition, there is an interaction between the laser beam and the metal vapor in the capillary due to the effect of plasma resonance . This piercing process, which is initially required, is technologically critical due to the high level of back reflection and metal splashes that may reach the focusing optics. Copper , gold and other non-ferrous metals can only be processed with difficulty with the CO 2 laser.

The wavelength of the CO 2 laser is excellently absorbed by glass, which is why CO 2 lasers are also used in glass processing, for example for welding halogen bulbs, for engraving drinking glasses or for scratching ampoules in the pharmaceutical industry.

A separation process for brittle materials (glass, ceramics) based on laser-induced thermal stresses is also known. The material is locally heated with CO 2 lasers, but not melted.

There are attempts to use CO 2 lasers for uranium enrichment. A uranium-containing gas is bombarded with the laser and reacts differently to certain laser frequencies. This is how uranium-235 and uranium-238 can be separated. Such a technology has already been developed and is called the SILEX process . The advantages of this technology over other enrichment processes are that it is much more energy efficient and can be built more compactly.

The CO 2 laser is also required for medical applications , for example for fractionated CO 2 laser treatment of the skin .

Further information

  • F. Kneubühl, M. Sigrist: Laser . 7th edition. Vieweg + Teubner, Wiesbaden 2008, ISBN 978-3-8351-0145-6 .
  • Jürgen Eichler, Jurgen Eichler, Hans-Joachim Eichler: Lasers: designs, beam guidance, applications . 6th edition. Springer, 2010, ISBN 978-3-642-10461-9 , pp. 96–110 (Chapter 6.2 CO 2 laser).

Individual evidence

  1. ^ A b F. Kneubühl, M. Sigrist: Laser . 7th edition. Vieweg + Teubner, Wiesbaden 2008, ISBN 978-3-8351-0145-6 , pp. 229 ff .
  2. ↑ Jürgen Eichler, Jurgen Eichler, Hans-Joachim Eichler: Lasers: Structures, beam guidance, applications . 6th edition. Springer, 2010, ISBN 978-3-642-10461-9 , pp. 96–110 (Chapter 6.2 CO 2 laser).
  3. ↑ VN Anisimov, AP Kozolupenko, A. Yu Sebrant: Plasma transparency in laser absorption waves in metal capillaries . In: Soviet Journal of Quantum Electronics . tape 18 , no. November 12 , 1988, pp. 1623-1624 , doi : 10.1070 / QE1988v018n12ABEH012779 .