Carbon dioxide laser

by Paul Hilton

The carbon dioxide (CO₂ ) gas laser, is one of the most versatile for materials processing applications, and emits infra red radiation with a wavelength between 9 and 11µm, although emission at 10.6µm is the most widely used. Of the several types of CO₂ laser that are available, the waveguide, the low power sealed tube and the transversely excited atmospheric (TEA) lasers are used for small scale materials processing applications. The fast axial flow CO₂ laser and the less widely used slow flow laser, are used for thick section cutting 1-15mm and deep penetration welding. While these lasers share the same active medium, they have important functional characteristics, which contribute to the wide range of CW (continuous wave) powers, pulse powers and pulse durations available from the CO₂ laser.

The active medium in a CO₂ laser is a mixture of carbon dioxide, nitrogen and (generally) helium. It is the carbon dioxide which produces the laser light, while the nitrogen molecules help excite the CO₂ molecules and increase the efficiency of the light generation processes. The helium plays a dual role in assisting heat transfer from the gas caused by the electric discharge used to excite the gas, and also helps the CO₂ molecules to return to the ground state.

Sealed Tube CO₂ Lasers

These lasers are operated as conventional gas discharge lasers in the form of long narrow glass tubes, filled with the lasing gas mixture. Electrodes at either end of the tube provide the discharge current. A totally reflecting and partially transmitting mirror, usually made from polished metal and coated zinc selenide respectively, form the resonant cavity. The tube is sealed using Brewster angled windows. Fig.1, shows a schematic drawing of a sealed tube CO₂ laser. As the electric discharge in the tube breaks down the CO₂ , an ordinary gas mixture would stop working very quickly and so methods are provided to cause the CO₂ to regenerate, either by addition of hydrogen or water or by the use of catalytic action. Several thousand hours of operation are possible with sealed tube CO₂ lasers before the tube has to be cleaned and re-filled or replaced. DC and sometimes RF discharges are used with these lasers. CW power up to about 200W is available from these lasers with good beam quality. Pulsed power supplies can produce laser pulses lasting 0.1 - 1msecs with peak powers 5-10 times the CW power level.

Fig. 1 Sealed tube CO 2 laser schematic

Waveguide CO₂ Lasers

The waveguide laser is an efficient way to produce a compact CO₂ laser. It consists of (see Fig.2), two transverse RF electrodes separated by insulating sections that form a bore region. The lateral dimensions of the bore are a few millimetres, which propagates the beam in 'waveguide mode'. The tube is normally sealed with a gas reservoir separate from the tube itself. The small bore allows high pressure operation and provides rapid heat removal; both of which lead to high gain and high power output from a compact unit.

Fig. 2 Waveguide CO 2 laser schematic

TEA CO₂ Lasers

Discharge instabilities prevent operation of CW CO₂ lasers at pressures above about 100mbar. Pulses in the nanosecond to microsecond duration range can be produced by passing a pulsed current transversely through the lasing gas. Such TEA (transversely excited atmospheric) lasers operate at gas pressures of one atmosphere and above in order to obtain high energy output per unit volume of gas. A transverse discharge from two long electrodes is employed (see Fig.3). Prior to application of the pulsed discharge, a form of pre-ionisation is used to ionise the space between the electrodes uniformly, thus allowing the discharge to proceed in a uniform fashion over the entire electrode assembly. The prime attractions of TEA lasers are their ability to generate short intense pulses and the extraction of high power per unit volume of laser gas. Pulse duration as low as a few tens of nanoseconds up to a few microseconds are possible. Pulse energies range from the millijoule region to 500Joules at pulse repetition rates from about 300Hz down to single shot.

Fig. 3 TEA CO 2 laser schematic

Optics for CO₂ Lasers

Reflective mirrors	- silicon with high reflectivity coatings, gold coated copper.
Lenses and windows	- gallium arsenide and germanium (not transparent in visible region) and coated zinc selenide (orange in the visible region).
Wallplug Efficiency	between 5% and 20%

Beam Diameter and Divergence

The shape and length of the laser cavity and nature of the resonator optics determine the beam diameter and divergence of the CO₂ laser. Typical ranges are:

	beam diameter (mm)	beam divergence (mrads)
Sealed tube:	1 - 7	2 - 6
Waveguide:	1 - 2	3 - 10
TEA:	4 - 12	0.5 - 3

Copyright TWI Ltd, 2000

Tips for Tack Welding

A tack weld is a weld made to hold the parts of a weldment in proper alignment until the final welds aremade. A tack weld is generally a short weld made at intermittent points to hold abutting edges together. Tack welding is likely to be done lightly but tack welds should be subject to the same quality requirements as the final welds.
Here are tips for making sound tack welds.

(1) Specify the length of each tack weld and the measurement from center to center of the tack welds in advance. In addition, you should specify multiple-pass weld profiles and throat thickness of the tack weld for tacking thick section components.
The recommended minimum length of a tack weld bead, according to the Technical Recommendations for Steel Construction for Buildings of the Japanese Architectural Standard Specification (JASS 6) is :

Plate thickness <>6 mm Min Bead Length 40 mm

(2) According to the JASS 6 specification, do not do tack welding when the ambient temperature at a welding area is lower than 􀊵5􀋆. When it is in between 􀊵5􀋆 and 5􀋆, preheat the base metal at
an appropriate temperature for a distance up to 100 mm from the welding joint.

(3) In tacking high tensile strength steel and heatresistant low-alloy steel, a short tack weld bead causes faster cooling rates of the weld and thereby increases the hardness of the heat-affected zone of the base metal, which may cause cracking of the tack weld. In order to prevent this trouble, preheating temperature should be 40-50􀋆 higher than in the final welding.

(4) Use low hydrogen electrodes for tacking thick components of mild steel, high tensile strength steel and heat-resistant low-alloy steel to prevent cold cracking of tack welds.

(5) Avoid tack welding on sharp corners of the components where residual stress is apt to
concentrate. Figure 1 shows typical recommended locations for tack welds on a steel structure as per the Technical Recommendations for Steel Construction for Buildings.

(6) You should progress symmetrically when you carry out tack welding on strongly restrained thick section components as shown in Fig. 2.

(7) Whether they will be removed or left in place, tack welds should be made using a fillet weld or butt weld procedure qualified per the relevant code. Tack welds to be left in place should be made by welders qualified in accordance with the pertinent specification. They should be examined visually for defects and removed if found to be defective.