Molecular lasers, unlike most other lasers, operate on the vibrational energy states of the molecule. This places then in a different wavelength region than most lasers operating on electronic energy levels, since these are lower energy states.
| Type | Energy | Wavelength Region |
|---|---|---|
| Rotational | Er ~ 10^-3 eV | Submillimeter - microwave |
| Vibrational | Ev ~ 10^-1 eV | Infrared |
| Electronic | Ee ~ 10 eV | Visible |
While a great many substances will lase in some fashion or another, only so many are actually useful as lasers. Molecular laser from the most simple such as HF, to the very complex, CH3OH, I have even found laser emission spectra for water -- though no word on how it performed -- only a few are useful and fewer still enter the commercial market place. Carbon dioxide lasers are among the very few that make it to widespread commercial use. CO2 lasers are some of the most efficient lasers, relatively inexpensive, safe for their power, a fairly mature technology, and available commercially at high powers. All of these factors and more have made them the success they are.
By being triatomic carbon dioxide adds two more non-degenerate degrees of freedom. Stretch is expanded to symmetric stretch and asymmetric stretch. A completely new degree of freedom is added, bending. Due to its linear shape; however, rotation remains as simple as for the diatomic case and the case of bending is degenerate -- the same in any direction.
The carbon dioxide molecule at ground state; measurements are in picometers (Chang, pg. 372, 448)
The vibrational modes of carbon dioxide carry the primary responsibility for its lasing action. Stimulated emission occurs between the 00 ° 1 level and the 10 ° 0 level as well as between 00 ° 1 and the 02 ° 0 level, at wavelengths of 10.4 µm and 9.4 µm respectively.
Vibrational Modes of CO2
Above each vibrational level there is a manifold of states from the rotational modes of carbon dioxide. This creates a broad range of wavelengths that can lase, some 200 vibrational-rotational transitions between 8 µm and 18 µm (Verdeyen, pg. 278). The average of those that typically lase is 10.6 µm.
Rotational Modes of CO2
The standard carbon dioxide cycle starts by pumping nitrogen to an excited vibrational state (stretch, v=1, 2, 3, or 4). At v=1 nitrogen is only 18 1/cm below carbon dioxide's 00 ° 1 state, thermal energy bridges the gap. The energy is transferred to the carbon dioxide since the decay of a nitrogen, a homonuclear molecule, by radiative transition is forbidden. From the 00 ° 1 energy level carbon dioxide transitions by stimulated emission to either the 10 ° 0 or 02 ° 0 energy levels.
Energy Band Diagram of CO2 (Verdeyen, pg. 279)
A variety of gas mixtures have been used in CO2 lasers. Almost all use some mixture of carbon dioxide, nitrogen, and helium. The proportions of which also vary, typically around CO2 6%: N2 12%: He 82%, but quite a range have been used successfully.
The nitrogen molecule is used because it happens to have its first excited vibrational energy level almost perfectly aligned with carbon dioxide's pump state, the 00 ° 1 (asymmetrical stretch) vibrational energy level. The two energy levels are within 18 1/cm of each other, easily supplied by thermal energy. Nitrogen also has a much higher cross section for absorbing energy making much more efficiently pumped than carbon dioxide.
Helium's first excited state is at 20 eV, well above the 1 to 3 eV levels the laser is running at, so it doesn't effect discharge. If the temperature of the gas mixture becomes too high the thermal energy will slow down the removal of electrons from the lower lasing state or even start pumping electrons into it, ending population inversion and quenching lasing. Since helium has a high heat conduction rate, six times nitrogen or carbon dioxide, it is used help pull heat out of the system.
CO2 <=> CO = O
Carbon monoxide is formed by the dissociation of the carbon dioxide. During operation near fifty percent of the carbon dioxide will dissociate. Carbon monoxide will take on a role similar to that of nitrogen, pumping the CO2 laser. Carbon monoxide itself makes a good lasing agent, lasing around 5 µm, with the potential of being more efficient than even carbon dioxide lasers. Carbon monoxide however is not as efficient as nitrogen at pumping CO2 and along with the carbon monoxide the same volume of oxygen atoms are released into the laser -- a definite problem.
When the carbon dioxide dissociates to carbon monoxide and oxygen, hydrogen acts as a catalyst in the following reactions, reforming carbon dioxide.
O + H <=> OH
CO + OH <=> CO2 + H
Water can also be used in a similar fashion. with only a very small concentration the levels of dissociated compounds of carbon dioxide can be nearly eliminated.
If oxygen levels exceed 0.1 to 0.5%, lasing will usually cease. Oxygen also contributes to oxidation of the electrodes and other damage to the system. Even with the problems it causes oxygen is sometimes added to the lasing gasses. The primary purpose is provide oxygen to react with hydrogen to form water for limiting the dissociation of carbon dioxide. The reaction is quick and in short order the free oxygen is rapidly consumed, becoming water.
Xenon changes the energy distribution, increasing the number of electrons lower than 4 eV and decreasing the number over 4 eV. The electrons below 4 eV excite CO2 and N2 vibrationally better than high energy electrons. Xenon also has a lower ionization potential than the other gases, 12.1 eV about 2 to 3 eV lower, the ionized Xe atoms contribute electrons to maintain the discharge.
-Lower voltages: 100s of volts vs. 5 to 15 kV of DC discharge.
-Scalability: For lengths greater than 10 to 15 cm multiple DC discharges would be necessary to keep the overall voltages under 15 kV. An RF discharge can be scaled up without the necessity for multiple discharges or excessively high voltages.
-Higher efficiency: At radio-frequencies the discharge has a positive I-V characteristic eliminating the need for a ballast resistor. In a DC discharge the ballast resistor causes significant loss of power efficiency, about a half to a third of the power is dissipated in the resistor. The wider stable range of the RF discharge allows the laser to be operated in conditions more advantageous to lasing. Higher specific powers, near twice have been reported, as well as higher electrical to optical conversion efficiency.
-Longer sealed lifetime: The laser can be built "electrodeless", where the electrodes are covered by inert materials like ceramics or alumina, without effecting the discharge. By this method electrode erosion is eliminated as well as contamination by the eroded materials.
-Modulation: The discharge can be modulated at high-audio frequencies.
-Stable discharge between large-area parallel electrodes such as those found in TEA lasers.
The main disadvantages are the extra cost and design of the power supply, shielding it to meet FCC regulations (or using one of the FCC designated frequencies), and the limitations in how far the power supply can be from the laser. For the improved capabilities of the laser these are fairly minor concerns, at least for commercial lasers.
The advantages of TEA lasers also have to do with lower voltages and scalability. By having the discharge across only the bore of the laser the required voltage is much less and the laser can by scaled up with corresponding increases in current, without huge increases in voltage. The other advantage is that with the discharge only needing to reach across the bore the pressure of the gasses can be increased to atmospheric pressures, minimizing loads on the laser chamber as well as greatly increasing the gain of the lasing medium. Disadvantages mostly have to do with the large size of the electrodes if the discharge is run DC. This is due to the expense of using gold or platinum to prevent excessive erosion and sputtering. As long as RF excitation is used; however, the electrodes can be more common and just sealed by anodization or ceramic.
Lasers in many ways are ideal machine tools. They put almost no force on the workpiece, they cut quickly, do not require running cooling or 'cutting' fluid over the workpiece, can cut almost anything provided suitable power, and compared to other CNC machine tools are not necessarily that far out of line as far as price is concerned.
The areas of laser material processing I examined were laser welding, cutting and separation. The first two, welding and cutting, are closely related. In the first case the material is allowed to flow back together and resolidify, in the second the molten material is removed to prevent resolidification.
There are two types of laser welding depending on the intensity of the laser. A low power density will weld by conduction limited welding. A high power laser will form a deep penetration keyhole. Each of these methods have advantages and disadvantages.
The primary advantage to conduction limited welding is low power. The laser need only to be able to bring the material above its melting point, generally requiring considerably lower power than reaching the boiling point. It is also a good method for welding edges together where there is no hope of reaching across the entire area of the two contacting surfaces (i.e. two flat plates). The weld created is very similar to that fond in arc welding. As compared to the other method of laser welding it does have a large heat affected zone (HAZ).
To create a keyhole the laser must be able to bring the material above the boiling point, since the boiling gas is what forces the keyhole to start, and the plasma created is what absorbs much of the lasers energy and transmits it to the material. The weld created by this method is generally superior to conduction limited welding. The heat affected zone (HAZ) is very small, on the order of a few hundred microns beyond the weld. Once started the greater energy available can increase welding speed.
R.C. Crafer breaks the welding process up into three stages. The first, when the beam is initially shown on the material and the material heats to the melting point. The second when the material starts to melt. The third for higher power densities where vaporization takes place.
When the material is first exposed to the laser most of the beam is reflected. Most of the energy that is absorbed is deposited within a depth of few times of the skin depth. For common metals the skin depth is about 30 nm. As the metal heats falls with the increase of temperature, this along with surface contamination and other factors increase the absorbed energy.
Conduction limited welding occurs when the power density is high enough to melt the metal but not vaporize it. Since the power is still absorbed at the top surface the molten area spreads out to about twice as wide as it is deep. The shape of this weld is much like that from various types of arc welding.
The three stages of welding as described by R.C. Crafer
At power densities high enough to start the surface boiling, a three step process forms a penetration key hole. At first the pressure evaporating metal gasses push on the surface forming a depression. The depression acts as an absorbing cavity; further improving absorption, increasing pressure of the evaporating gas, and the depth and absorption of the cavity.
When the temperature climbs high enough the second step occurs. The metal vapor is ionized, absorbing more energy by 'Inverse Bremsstrahlung' absorption. The absorbed energy is then transmitted to the liquid metal.
Once the vapor is ionized and absorption by 'Inverse Bremsstrahlung' is occurring the depression can penetrate completely with the plasma absorbing the energy from the beam and transmitting it to the molten metal around the hole.
The process of keyhole formation
- Narrow kerf width: less material waste
- Square edges: Can be cut with square edges unlike the rounded edges from many thermal cutting methods.
- Finished edge: The edge can be cut smooth and clean not requiring further processing for many applications. There is no edge burr, found in mechanical cutting methods. The edge of the cut is clean enough to be directly re-welded.
- Minimal heat effects in the surrounding material. The resolidification layer has a thickness on the order of microns.
- Blind cuts: Can be made in those materials that vaporize well; such as wood and acrylic.
- Limited cutting depth: Depth is largely dependent on laser power, though 10 to 20 mm is about the limit for high quality cuts.
- Fast
- Clamping not necessary
- No tool wear
- Cuts in any direction
- Quiet
- Easily automated: Hard to do if not automated.
- Highly Flexible: Tool changes are mostly "soft".
- Stack cutting: Possible with some materials.
- Can cut almost all materials: Reflective materials can be problematic.
In this method of cutting a keyhole is formed as described in the section on previously for welding. In this case, however the power is high enough to not only vaporize the material, but to cause it to expand so rapidly it ejects the molten material so that the keyhole does not reclose behind it as in welding.
The penetration velocity can be roughly calculated using the following equation:
Equation 3.1 from (Steen) = Absorbed power density (W/m2) = Density of solid (kg/m3) = Latent heat of fusion and vaporization (J/kg) = Heat capacity of solid (J/kg/¡C) = Vaporization temperature (¡C) = Temperature of material at start (¡C)
W. M. Steen gives an example of using a 2 kW laser on iron with a 0.2 mm spot size. With a power density of 6.3x1010 the penetration velocity is approximately 1 m/s. The change in density from solid to vapor decreases the density by a factor of a thousand and increases the vapor escape velocity by a similar amount. At this exit velocity of around 1000 m/s pressure on the order of 40 atmospheres is formed. All of this is plenty to remove molten material, and cause a few side effects such as laser shock hardening.
In this method the molten material is removed by a strong jet of gas. By using a separate method to remove the material, it needs only to be heated above the melting point and over the point of vaporization. Approximately a tenth of the power is necessary for this method vs. vaporization cutting.
If a gas that will react exothermically with the material is used in place of the inert gas, then the cut speed can be increased or the laser power decreased. Hot steel reacts with oxygen to supply 60% of the energy or in the case of titanium about 90% of the energy (Steen). Generally the cutting speed is about doubled and with the increased speed, quality improves and heat penetration damage is reduced. The reaction of the material with the gas also causes chemical changes in the remaining material, in a material like titanium this can cause significant changes.
Brittle materials like glass can 'cut' using the laser to heat stress the material causing it to fracture along the stress lines. Once the crack is initiated it can be guided with the hot spot at speeds on the order of meters per second (Steen). The difficulty with this method is predicting and controlling the fracture near edges. Completing a cut of a closed shape is such a problem , as the leading edge of the crack approaches the starting point it may leap ahead to complete the cut along an unpredicted path. This problem has prevented this method from gaining widespread use since there are cheaper methods for cutting straight lines. One commercial use that was reported though was for cutting test tubes.
This method uses very little power, for thin glass as little as two watts. My own experiments with it have had success at ten watts. Using too high a power laser where the edge may melt would damage the edge.
Ed. Peter K. Cheo, Handbook of Molecular Lasers, New York: MARCEL DEKKER, INC., 1987.
William M. Steen, Laser Material Processing, London: Springer-Verlag, 1991.
O.D.D. Soares, M. Perez-Amor Editors, Applied Laser Tooling, Dordrecht, The Netherlands: Martinus Nijhoff Publishers, 1987.
Joseph T. Verdeyen, Laser Electronics Englewood Cliffs, NJ: Prentice-Hall, Inc., 1981.
Kenneth Smith and R. M. Thomson, Computer Modeling of Gas Lasers Leeds, England: Plenum Press, 1978.
W. J. Witteman, The CO2 Laser Berlin, Germany: Springer-Verlag, 1987.
P. W. Atkins, Molecular Quantum Mechanics 2nd ed. Oxford, England: Oxford University Press, 1983.
Raymond Chang, Chemistry 4th ed. US: McGraw-Hill, Inc., 1991.