Since Ted Maiman's creation of the first working laser nearly 50 years ago, the laser materials processing market has grown to become one of the largest industrial applications for lasers.
The CO2 laser, which will be 46 years old this year, has captured the largest portion of the market for laser materials processing. It has become the workhorse of the materials processing industry for a variety of reasons:
- Low cost (less than $100 per watt)
- Relatively high efficiency (greater than 10%)
- Small size per watt of output power
- Large variation in output power, ranging from a few watts up to more than 60kW
- Long sealed-off lifetime of greater than 20,000 hours
- Wide variety of output waveform formats
Adding to the attractiveness of the CO2 laser is the high absorption of its infrared radiation. Numerous discrete output wavelengths can be selected between 9 and 11 microns with the use of isotopes in the laser's gas mixture. Radiation in these wavelengths is strongly absorbed by paper, wood, cloth, ceramics, oxides, plastics, glass, stone, liquids, and most complex molecules. The use of different CO2 isotopes in the laser's gas mixture of CO2:N2:He can produce a selection of hundreds of discrete IR wavelengths.
Optically pumped solid-state lasers of the rod, disk, or slab variety; fiber lasers; excimer lasers; high-power laser-diode arrays; and CO2 lasers all play a role in the laser materials processing industry. Yet, of these commercially available lasers, the CO2 laser's beam quality, low cost, and other attributes have won it a 37% share in the $2.9 billion (Euro 2 billion) global market for lasers used in laser materials processing, according to 2008 figures from Optech Consulting.
The reason for the CO2 laser's dominance in the materials processing industry is that it has revolutionized many manufacturing processes.
Cutting and welding with multi kW lasers reduce cost and waste while speeding up processing. Automobile manufacturers have been major beneficiaries of this reduced production time. (See how remote laser welding systems are being used in the auto industry.)
The oldest and largest market for CO2 lasers systems is in cutting, welding, and drilling of sheet metal with high power CO2 lasers, with the largest revenues coming from laser systems that have output average powers in the 2 to 6 kW range. Even though the carbon dioxide laser is at a low-absorption disadvantage in processing metals over shorter wavelength lasers (such as Nd:YAG at 1.06 micron wavelengths and high-power laser diode arrays that operate between 0.808 and 0.975 microns), CO2 lasers still have the larger share of the market.
At the other end of the power spectrum, low-average-power CO2 lasers (below 100W) have made it easy to mark products with date of manufacture and expiration, lot and serial numbers, and other ID markers, as required by law in many places across the globe. Laser marking is a fast-growing market and is now cutting into market share of the ink-jet industry.
Other fast-expanding markets are the scribing of glass and cutting of plates for the display market, the cutting/drilling of plastics, and the drilling of holes in printed circuit boards. The introduction of miniature and relatively inexpensive electronic devices such as cell- phones, laptops, and hand-held computers has forced increased speed in the number of holes drilled per unit and the resulting reduction in cost per drilled hole in printed circuit boards. One popular approach for achieving the increased speed and lower cost is the use of pulsed laser drilling systems instead of mechanical drill machines.
Flexibility in the laser processing of materials tends to increase with the increased availability of laser output modulation formats. Various configurations of CO2 lasers have achieved continuous-wave (CW) output power ranging from a few watts up to 60kW. Operating efficiencies approaching 30% have been obtained in commercially available CO2 lasers, although 10% to 12% is more typical.
The CO2 laser offers the option of operating continuously or in a wide variety of pulse repetition frequencies, from a few hertz to well over 100kHz.
CO2 lasers available commercially operate in three modes:
- A super pulsed mode where the pulse peak power driving the laser discharge can be up to several times the average CW power
- A gain-switched mode yielding megawatts of peak power but at low pulsed recurrence frequency (PRF) from a few hertz to about a kilohertz maximum, such as in Traverse Excited Atmospheric (TEA) lasers
- A Q-switched mode where the pulse widths are tenths of microseconds wide at PRF up to 30 kHz and peak power is several hundred times the CW average power capability of the laser
Radio frequency (RF) excited, sealed-off CO2 lasers with operational lifetimes well in excess of 20,000 hours are also used commercially. The CO2 laser has been successfully energized electrically by RF and direct current (DC) energy as well as optically, gas-dynamically, chemically, and by electron-beam excitation (pumped). It has been excited by more different excitation approaches than any other laser. Electrical excitation is exclusively used in commercial applications.
The largest materials processing markets served by CO2 lasers are applications that require CW, pulsed, or super-pulsed outputs from convectively cooled DC- or RF-excited lasers with output powers from 1kW to 60kW (with the largest market in systems with outputs in the 5kW to 6 kW range) and sealed-off, diffusion-cooled, RF-excited lasers. The latter type can be either the slab or waveguide variety, with an average output power range from a few watts to approximately 2kW.
Heat removal is a primary concern in the design of a CO2 laser, as it is for all types of lasers, and a convenient way to name different types of CO2 lasers is by the means used to carry heat away from the discharge. Since only about 10% of the laser's input power generates laser energy, approximately 90% of the input electrical power needs to be carried away as heat.
Since the laser's output beam quality is dependent on the alignment stability of the resonator's mirrors, it is important to have symmetric and uniform heat extraction from the laser discharge to avoid bending and twisting of the mechanical structure holding the resonator's mirrors.
To obtain good beam output quality and good beam pointing stability in pulsed gas lasers, the laser designer has to minimize the optical distortions caused in the laser's gas mixture by acoustic shocks generated by the pulsing of the gas discharge.
This is difficult because the designer must use materials that are compatible with good high-vacuum technology so as not to contaminate the laser's gas mixtures. Unfortunately, such materials are not usually good acoustic absorbers, and the manufacturers usually specify that the laser not be operated at pulse rates that coincide with the acoustic resonances of the laser tube.
The two basic cooling techniques used in CO2 lasers are
- Convective cooling, where the heat is carried away by flowing the laser gas mixture that is at sub-atmospheric pressures within the laser tube
- Diffusion cooling, where the molecules within the laser discharge are cooled by making collisions with the cooled walls containing the gas discharge
The convectively cooled CO2 laser (also called flowing gas laser) is the oldest form of laser cooling and the most mature CO2 laser technology. It has been the workhorse of the high-power laser materials processing industry, and it has generated the highest average output power (about 60 kW) in a commercial laser to date because of the large discharge volume.
Diffusion cooling of CO2 lasers is the newer of the two cooling technologies and has found a commercial niche in CO2 lasers with average powers between a few watts to several kW. Waveguide and slab lasers are two examples and are favored for their compact size and smaller footprint on the factory floor.
Diffusion-cooled CO2 lasers utilize the collision of hot CO2 molecules with the electrodes or walls of the housing containing the discharge. These collisions de-excite the energy of the hot CO2 molecules down to the ground state, thereby cooling the CO2 molecules. The gas discharge container housing is, in turn, cooled externally by either liquid or air.
The output power capability of diffusion-cooled lasers is lower than for convectively-cooled lasers because of the small discharge volume resulting from the need to have the electrode cooling walls close together.
TRUMPF's TruFlow CO2 laser has a modular design and is used for sheet cutting, spot and seam welding, and other applications.
After RF-excited lasers, TEA lasers are the second-most mature of the CO2 laser types.
Intuition leads one to correctly conclude that to obtain more power from a given CO2 laser-tube diameter, simply increase the gas pressure. It is well known that CO2 lasers operate best with electric fields to pressure ratio (E/P) of 20 to 50 volts/Torr. This means that for a 1-meter discharge at a high gas pressure, say, of 800 Torr, voltages in the neighborhood of 800,000 to 4 million are needed to create the discharge. Steady-state discharges at such high gas pressures are difficult to maintain without having the discharge degenerate into an arc.
Consequently, CO2 lasers at such high pressures are operated in a fast-rise-time, pulsed mode.
TEA lasers have obtained single-pulse energies up to 10 Joules at low-pulse-per-second or less rates and up to pulse repetition rates of 800 Hz with approximately 1 Joule per pulse.
At such high pulse energies, acoustic shocks need to be minimized by the laser designer because the shocks cause gas turbulence in the laser medium, which deteriorates the beam quality and the pointing stability of the beam. Acoustic shocks are one of the reasons for the low-pulse-repetition frequency performance of TEA lasers.
TEA lasers have found niche applications requiring high peak power (up to megawatts), high peak energies (up to several joules per pulse), and relatively low-pulse-repetition frequencies (up to about 100 PRF, with 10 PRF being typical).
Despite this, TEA CO2 lasers have the smallest share of the laser materials processing market. In most cases, TEA lasers are manufactured to order according to individual users' unique specifications.
- This is an excerpt from a comprehensive review of the CO2 laser by Anthony DeMaria and Thomas Hennessey. The full text of their article on CO2 lasers is available as a PDF and includes a summary of state-of-the-art CO2 laser technology and a guide to choosing which laser is best for an application.
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Optech Consulting of Switzerland reports that the worldwide market for laser systems used in materials processing reached $9.14 billion (€6.4 billion) in 2008. Sales attributed to laser units were $2.9 billion (€2 billion). (Average currency conversion in 2008 was €70 to $100.)
Of those laser units sold for materials processing in 2008, CO2 lasers accounted for a 37% share, followed by solid-state rod and disk (36%), excimer (17%), fiber (7%) and direct diode lasers (3%).
While CO2 lasers are dominant in the materials processing market segment, they account for only 16% of all laser units sold in 2008, according to Laser Focus World. The lower-cost and much lower-power diode lasers had a 58% share of all laser units sold globally. All other laser technology revenues were below 10%, the magazine reported.
SPIE member C. Kumar N. Patel was a young physicist at Bell Labs in 1964 when he developed the first high-power gas laser using carbon dioxide. The CO2 laser would go on to become one of the most versatile and practical of all lasers. This laser cuts, welds, and drills on plastics and metals, aids in weather predictions and atmospheric analysis, and is used to perform intricate surgery without scalpels.
Patel, president and CEO of Pranalytica, a manufacturer of quantum cascade lasers and gas detection equipment, worked for Bell Labs for 32 years. He has earned 36 U.S. patents for lasers and their applications and was awarded the National Medal of Science in 1996. He was elected to the National Academy of Sciences in 1974 and the National Academy of Engineering in 1978.
Before founding Pranalytica, he served as the vice chancellor for research at UCLA from 1993 to 1999. His current work with QCLs and laser-based trace gas detection equipment is used mostly in industrial, environmental, military, and security applications.
He is a regular contributor and speaker at SPIE conferences, including SPIE Optics+Photonics, SPIE Photonics West and SPIE Security+Defence. Patel is one of many laser luminaries whose work will be featured in a laser display at SPIE Photonics West 2010. Pranalytica will be exhibiting at booth 4936 during SPIE Photonics West.
Materials processing was the top application for all 2008 laser revenues, according to Laser Focus World magazine. Materials processing accounted for 29% of all laser applications. Other popular applications for lasers were communications, at 27% of the market, information storage at 26%, and medical uses, 7%.
Researchers at the Fraunhofer Institute for Laser Technology ILT in Aachen (Germany) have developed a way of using lasers to automate the work of polishing metal surfaces such as molds for making plastic parts. Edgar Willenborg, group leader at the ILT, estimates that the new laser system will be ready for industrial use in one or two years.
SPIE Fellow and past SPIE President Anthony DeMaria is chief scientist at Coherent, Inc. and professor-in-residence at the Department of Electrical and Computer Engineering at the University of Connecticut (USA). He founded DeMaria ElectroOptics Systems which he later sold to Coherent.
Thomas V. Hennessey, Jr. is mechanical designer at Coherent, Inc. and has 21 years of experience working with CO2 laser technology.
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