| This version reflects the comments of the core participants as reviewed and incorporated in accordance with CORD's FIPSE-supported Curriculum Morphing Project. | |||||||||||||||||||||||||||||||||||||
MODULE 3-8 A great many molecular gases are capable of laser action. They usually operate in the infrared region of the spectrum because of the relatively low energies involved in most molecular laser transitions. The most important molecular laser by far is the carbon dioxide laser, but other triatomic molecules (CS2, HCN, H20) also laser, as do several diatomic molecules (CO, HF, DF). This module discusses energy transfer mechanisms that occur in molecular lasers and explains how the energy flow process affects design and output characteristics of the laser system. Molecular energy states of diatomic and triatomic molecules and the transitions between states are described in detail. Basic characteristics of diatomic molecular lasers are described using the carbon monoxide laser as an example. Most of the module is devoted to study of the carbon dioxide laser as an example of a triatomic molecular laser. Effects of several parameters on typical CO2 laser output will be described and correlated to the energy transfer processes occurring in the laser gas. A greater discussion of design and construction of CO2 laser systems is contained in Module 3-9, "CO2 Laser Systems." A small CO2 laser will be operated in the Laboratory section of this module, and output power will be examined as several parameters are varied.
Upon completion of this module, the student should be able to: 1. Draw and label an energy-level diagram representing the first few rotational energy levels in two adjacent vibrational states of a diatomic molecule. On this diagram draw and label the following:
2. Draw and label an energy-level diagram showing the lasing cascade in a carbon monoxide (CO) molecular laser. Include at least four vibrational levels, several rotational levels within each vibrational level (not necessarily to scale), and one P-branch transition between three successive vibrational-rotational bands. Use this diagram and explain the concept of a lasing cascade. 3. Draw and label diagrams showing symmetric stretch, bending, and asymmetric stretch vibrational modes in a carbon dioxide (CO2) molecule. 4. Draw and label a simplified energy-level diagram of a CO2 laser. The drawing should include upper and lower laser levels for two infrared lines, nonradiative transitions to lower vibrational states, and the ground state. Vibrational energy transfer from nitrogen (N2) molecules should be indicated, along with the two lowest-energy vibrational levels of N2. All vibrational levels should be labeled with appropriate quantum numbers. 5. Write symbolic expressions for the four energy transfer mechanisms that excite CO2 molecules to the upper lasing level (001) in a typical CO2 laser and the two energy transfer mechanisms necessary to move CO2 molecules from the lower lasing level (100) to the vibrational ground state (000) in a typical CO2 laser. 6. Explain the concept of lasing due to a partial inversion in a CO2 laser, and explain why lasing usually occurs on a single P-branch transition in a CW CO2 laser. 7. For a coaxial, CW CO2 laser, make a brief statement explaining how and why the output power depends upon each of the following parameters.
8. Explain how gain and loss in a CO2 laser are affected by varying the diameter of the laser tube. 9. Operate a low-power CW CO2 laser in the laboratory. Measure output power as a function of tube current, tube pressure, and gas flow rate. Examine effects of the focused and unfocused CO2 laser beam on various materials.
Before the design, operating parameters, and output characteristics of molecular gas lasers can be understood, the energy transfer processes at work within the gas itself must be understood. This module will encourage this understanding through discussions of the following topics:
Molecular Energy Levels Gas lasers may operate due to energy transitions between excited states in neutral atoms (HeNe), ions (argon), or molecules. Energy transitions involved in neutral atom and ion lasers are electronic transitions in which an electron in an atom or ion moves from one orbit to another (in the simplified Bohr model) by absorbing or emitting a photon. The energy-level structure of a molecule is far more complex than that of an atom (or ion) because there are more transition possibilities. Molecules not only have atoms with electronic energy levels, but have their own characteristic vibrational and rotational energy levels as well. Excited electronic states of a molecule result when an electron in one of its component atoms is in an orbit corresponding to an excited atomic state. While electronic transitions (mainly in the visible and near UV) have been observed in some molecular lasers, they are of lesser importance and will not be considered here. All important molecular lasers operate with the atoms of the molecule in the electronic ground state (except nitrogen lasers which will be discussed briefly in Module 3-10, "Liquid Dye Lasers"). Atoms comprising a molecule contain energy in the form of vibrational motion due to molecular binding forces between the constituent atoms. Simply speaking, atoms of the molecules are bound together by their shared electronic cloud like balls interconnected by springs. They have natural vibrational frequencies (called normal or resonant modes) that depend upon the masses of the particles and the stiffness of the springs. Molecules can move from one vibrational state to another by absorbing or emitting a photon of the proper energy. Each vibrational possibility (mode) for the molecule involves a particular configuration of its total electronic cloud and, thus, is associated with a specific energy. This gives rise to a discrete set of vibrational energy levels, as is the case with electronic energy levels. The effect is to split each of the electronic energy levels for the atoms of the molecule into a series of "almost" equally separated vibrational levels. In addition, a molecule may undergo quantized rotations about various axes in space. Again, the molecule changes from one energy state to another by absorbing or emitting only certain discrete or quantized amounts of E-M radiation. Rotational energy of the molecule produces further line splitting by subdividing each vibrational energy level into a series of finely spaced rotational levels. Unlike vibrational levels, rotational energy levels are not as equally spaced. The energy difference between two adjacent rotational levels becomes larger as higher rotational energy levels are reached. Table 1 is a comparison of energies involved in electronic, vibrational, and rotational transitions. Also given are spectral regions of the photons exchanged in each type of transition. The total energy of a molecule is the sum of its electronic, vibrational, and rotational energies. The molecule may move from one energy state to another by emitting or absorbing a photon or through a collision with another atom or molecule or with a free electron. Collision processes also involve the energy of motion or kinetic energy. Many collisional energy transfer processes involve an increase or decrease in the total kinetic energy of the two particles during a collision. Table 1. Molecular Spectra.
Energy Levels and Transitions in Diatomic Molecules Diatomic molecules (CO, HF, N2) are composed of two atoms bound together. Such molecules have only one normal or fundamental vibrational mode. This mode, shown in Figure 1, consists of a stretching of the molecular bond as the two atoms move away from one another, and a shortening of the bond as they are pulled back toward one another. An increase of the amplitude of this motion corresponds to an increased vibrational energy content of the molecule.
Fig. 1 Diatomic molecules also rotate around an axis perpendicular to the molecular bond and passing through the center of mass of the molecule. An increase in rotational rate about such an axis corresponds to an increase in rotational energy. Quantization of both vibrational and rotational energies leads to the molecular energy-level diagram shown in Figure 2.
Fig. 2 The quantum number V indicates the vibrational state of the molecule, and the quantum number J indicates the rotational state. Vibrational states are nearly evenly spaced and each vibrational state is composed of a large number of rotational states of which the energy spacings increase as J increases. Figure 2 shows the rotational levels of the V = 2 vibrational state only. Rotational levels of the other vibrational states have a similar form. The energy spacing of rotational levels is much less than for vibrational levels. Four types of energy transitions between molecular states are important in diatomic molecules. These are illustrated in Figure 3 and described below.
Fig. 3
Pure Rotational Transitions Pure rotational transitions involve a change in rotational energy only. Such a transition is indicated in Figure 3 by the letter R followed by the number 3 in parentheses, i.e., R(3). The number in parentheses is always the J quantum number of the lower rotational state involved, whether molecular energy is increasing (absorption) or decreasing (emission). The only pure transitions that can occur in most diatomic molecules are those in which the J value changes by one (D J = ± 1). Such a transition, R(3), is shown in Figure 3. This transition is from the J = 4 rotational level to the J = 3 rotational level, within the single vibrational state V = 2. It involves photons in the microwave region.
P-Branch Transitions These transitions involve (during emission) a decrease in vibrational energy (D V = –1) accompanied by an increase in rotational energy (D J = +1). They are the "shortest" transitions (lowest energy difference) between two vibrational states and, therefore, the most likely vibrational-rotational transitions. They are identified by the letter P followed by the J (rotational) quantum number of the lower vibrational energy state. Figure 3 shows a P(4) transition from the V = 2, J = 3 state to the V = 1, J = 4 state.
Q-Branch Transitions These are pure vibrational transitions in which there is a change in vibrational energy (D V = +1) but no change in rotational energy (D J = 0). A Q (4) transition is shown in Figure 3, from the V = 2, J = 4 state to the V = 1, J = 4 state.
R-Branch Transitions These vibrational-rotational transitions involve a decrease in both vibrational and rotational energies during emission (D V = –1, D J = –1). They are the "longest" (highest-energy) vibrational-rotational transitions. Once again, the number of the transition is the J quantum number of the lower vibrational energy state. The R(2) transition in Figure 3 is from the V = 2, J = 3 state to the V = 1, J = 2 state. Notice that the notation "R(2)" might mean either a pure rotational transition (J = 3 to J = 2 in V = 2, for example) or an R-branch transition (V = 2, J = 3 to V = 2, J = 2, as in Figure 3). Since the longer transition is of greater interest, the term R(4) is assumed to refer to an R-branch transition unless it is specified as a pure rotational transition. To further complicate the situation, the vibrational states are not exactly evenly spaced. A P(4) transition from the V = 4 state to the V = 3 state is not of the same energy (or wavelength) as a P(4) transition from the V = 3 state to the V = 2 state. These two transitions fall in different vibrational bands. The bands commonly are designated by the vibrational transitions involved. Thus, the 5-4 band contains all transitions (P-, Q-, and R-branch) occurring between the V = 5 vibrational state and the V = 4 state. A P5 – 4(6) transition denotes the P-branch transition between the V = 5, J = 5 and V = 4, J = 6 vibrational-rotational energy levels.
Diatomic Molecular Lasers—CO A number of diatomic molecules are capable of lasing action, and several are of scientific and industrial importance. Mechanisms in all of these lasers are essentially the same. While lasing can be achieved on all four types of transitions previously discussed, the following general rules apply in most cases: · Lasing on pure rotational transitions can be achieved in a number of molecules in the far infrared and short microwave regions. This, however, is of relatively less importance and will not be considered further in this module. · Q-branch transitions produce lasing in a few molecules but usually are not present if the P-branch will lase. · Most diatomic molecular lasers will operate on either the P-branch or R-branch. In these lasers, P transitions are always the strongest. R-branch transitions will lase only in pulsed lasers or, if the P-branch transition is suppressed, in CW lasers. The carbon monoxide (CO) laser is chosen as a representative diatomic molecular laser for the remainder of this section. Figure 4 is a simplified energy-level diagram of a CO laser. The spacings of the rotational levels are not shown to scale in this diagram. In a laser, the CO molecule is excited to a high-energy vibrational state by a collision with an electron in the discharge. Average lifetime of the vibrational state is on the order of 10–3 seconds. Lifetime of a rotational state within the vibrational level, however, is only about 10–7 seconds. This means that the CO molecule will change rotational levels thousands of times before a vibrational transition occurs. The energies absorbed or released during these rotational transitions are in the form of photons in the far IR or microwave region of the spectrum, or in the form of increased or decreased kinetic energy of the molecules.
Fig. 4 At some time, the molecule will be in a rotational state from which a downward P transition has sufficient gain for lasing to occur. Stimulated emission takes place, and the molecule changes energy states by V = –1, J = +1 (P-branch transition). Figure 4 shows lasing transitions in a pulsed CO laser in which a large number of molecules have been raised to the V = 9 state and in which all lower vibrational states except the ground state have low populations. Several P transitions in the 9-8 band probably will have sufficient gain for lasing to occur. This lasing action quickly populates the V = 8 vibrational level. As V = 7 has a relatively low population, a new population inversion is created and lasing begins in the 8-7 band. This leads to an inversion between V = 7 and V = 6, and so on. This process, called a lasing cascade or ladder, may produce laser output on a hundred distinct transitions during a single laser pulse. The higher-number bands begin lasing first. As more molecules move downward in energy, these higher band transitions die out and are replaced by transitions in the lower bands. Lasing has been achieved in CO gas in 20 bands containing a total of more than 250 separate P transitions. Most of these transitions lie in the V-range of 5 to 25. Table 2 shows a few of these transitions.
Table 2. Laser Transitions in CO Wavelengths in Micrometers.
The lasing cascade is a unique characteristic of most pulsed diatomic molecular lasers and is responsible for the high efficiencies, pulse energies, and peak powers available from these types of systems. In CW diatomic systems, the pumping rate is too low to produce the abundance of wavelengths observed in the pulsed models. Only that transition with the greatest gain within each band will lase. (This phenomenon is discussed in detail in the following section on CO2 lasers.) The number of bands that have population inversions is also greatly reduced in CW lasers. Often, only a single output wavelength is present at any one time. Other transitions may be made to lase by using a tuning mechanism.
Energy Levels and Transitions in Triatomic Molecules Triatomic molecules are composed of three atoms bound together (CO2, H20, HCN). They exhibit the same types of energy-level diagrams and transitions as do diatomic molecules and obey the same general rules:
The difference between diatomic and triatomic systems arise from the additional fundamental vibrational modes present in triatomic molecules. Because the CO2 laser is the most important of the triatomic molecular lasers, it is chosen as an example. There are three normal or fundamental modes of vibration in the CO2 molecule. Figure 5 shows these modes and gives the energy of the first excited vibrational state for each mode in reciprocal centimeters (1 eV = 8065 cm–1). They are:
Fig. 5 The vibrational energy state of a CO2 molecule is described by three quantum numbers and is written as follows: CO2(V1V2V3)
Thus, CO2 (000) indicates a molecule in the vibrational ground state; CO2 (100) indicates the first excited symmetric stretch state; CO2 (020) indicates two quanta of the excited bending state; and so on. Figure 6 is an energy-level diagram showing the first few vibrational energy levels of CO2 molecules.
Fig. 6 CO2 molecules also rotate in the same manner as diatomic molecules and with the same result; each vibrational state is split into a number of rotational levels. One exception worth noting is the absence of some rotational states due to symmetry conditions of the molecule. The asymmetric stretching mode of coo contains only odd rotational levels (J-1, 3, 5, …). Symmetric modes (bending and symmetric stretch) contain only even rotational levels (J-0, 2, 4, …). This occurs because CO2 is a symmetric molecule (0 = C = 0). It does not occur for H – C º N or other asymmetric molecules.
Triatomic Molecular Lasers–—CO2 Any of a variety of energy transfer mechanisms may be employed in triatomic molecular lasers. The CO2 laser is used as an example here for two reasons: First, it is the most frequently used triatomic molecular laser medium; second, it employs, to a large extent, all of the energy transfer mechanisms found in other triatomic molecular lasers. Popularity of the CO2 laser stems as well from its relative inexpensiveness, great efficiency, and the fact that it emits in an atmospheric window, i.e., there is little absorption at its lasing wavelength by the atmosphere. Figure 7 is a simplified energy-level diagram of a CO2 laser. Only the important vibrational levels are shown. Other vibrational levels and all rotational levels are excluded from the diagram for simplicity.
Fig. 7 The two strongest lasing transitions in CO2 are (001) è (100) centered at 10.6 m m, and (001) è (020) centered at 9.6 m m. The 10.6-m m line is the strongest line and will be the only one considered during the remainder of the discussion. All energy transfer mechanisms are essentially the same for both the 10.6-m m and 9.6-m m laser transitions. The excitation processes of the CO2 laser are shown in Figure 8. These are the general processes used to establish population inversions in most triatomic molecular lasers.
Fig. 8 Figure 8a shows the collision of a CO2 molecule in the ground state with an energetic free electron. Some of the kinetic energy of the electron is absorbed by the CO2 molecule, raising the molecule to the upper lasing level (001) (V3 = 1). While this energy transfer mechanism is very effective in diatomic systems and some triatomic systems, it is not very effective in CO2. Figure 8b shows a much more efficient population mechanism. It involves the addition of nitrogen (N2) to the lasing medium. The nitrogen molecule is excited to its V = 1 state by an electron collision. The nitrogen V = 1 state is very close in energy to the CO2 (001) state, making a resonant transfer of energy from the excited N2 molecule to the ground state CO2 molecule highly probable. Thus, the excited N2 molecule collides with a CO2 molecule in the ground state (000) and transfers its energy to the CO2 molecule, raising the CO2 molecule to the (001) state. The N2 molecule returns to its ground state (V = 0) and soon is excited by another electron collision, repeating the process. The energy difference of 18 cm–1 between the two states is compensated for by a decrease in kinetic energy after the collision. In a low-pressure nitrogen discharge, approximately 30 percent of the N2 molecules are in the V = 1 state at any given time, thus providing constant excitation for the CO2 molecules. Most collisions between N2 molecules and electrons will result in placing the N2 molecule in an energy state above the V = 1 level. The extra energy is not wasted, however, since the N2 vibrational levels are roughly equally spaced, as are those of the CO2 asymmetric stretch mode (001). Figure 8c shows the collision between an N2 molecule in a higher vibrational state and a CO2 molecule in the ground state. In such a collision, the CO2 molecule usually is raised to an asymmetric stretch state with V3>1. That is, (002), (003), (004), … This excited CO2 molecule later will collide with a CO2 molecule in the ground state as shown in Figure 8d, raising it to a higher level. This process continues until each vibrational quantum of the original N2 molecule results in a CO2 molecule in the upper lasing level, that is, CO2 (001). The above energy transfer mechanisms can achieve efficiencies of 75 percent. That is, for every four vibrational quanta imparted to the N2 molecules, three photons actually emerge in the laser beam. This is not to be confused with overall efficiency of the CO2 laser (10% to 30%), although it is one of the primary reasons for this high operating efficiency. From the (001) state lasing occurs on the 10.6-m m line. This results in an increased population of the CO2 (100) (V1 = 1) lower laser state. If the population of this state rises greatly, the population inversion is destroyed and lasing stops. Figure 9a shows the depopulation mechanism for removing CO2 molecules from the (100) state. CO2 (100) collides with ground state CO2 (000) to result in two CO2 (010). The slight energy difference appears as increased kinetic energy of the molecules. This process is very efficient as long as the population of the (010) level is low. Unfortunately, a CO2 molecule in the (010) state tends to stay there for a relatively long period of time (10–2 sec) and the population of this state increases rapidly. This problem may be solved by the addition of a third gas. Figure 9b shows the depopulation of CO2 (010) by helium atoms. When CO2 (010) collides with a low-speed He atom, the CO2 molecule drops to the ground state and the He atom carries away the excess energy in the form of increased kinetic energy (heat). Addition of helium to the gas mixture increases the depopulation rate of the CO2 (010) level by as much as 40 times.
Fig. 9 The only remaining problem is that of removing the kinetic energy from the helium. This usually is done by maintaining the laser tube walls at a relatively low temperature with water or other fluid cooling. The helium collides with the walls and releases most of its kinetic energy. Other mechanisms include fast flow of the gas mixture to remove it from the lasing volume before excessive heat buildup. While other gases sometimes are used, helium usually is chosen because it has good thermal transfer properties and also is believed to aid in excitation of both N2 and CO2. Following is a review of the energy flow through a CO2 laser:
Effects of Rotational Levels on Co2 Laser Output Thus far discussion has centered on vibrational energy levels of the CO2 molecule. Actual laser output is affected strongly by the rotational levels as well. Both P- and R-branches will lase in CO2. Figure 10 shows the absorption spectrum of CO2 gas in the 10.6-m m region. In the figure, numbers above each peak indicate the number of the vibrational-rotational transition. Only even-numbered transitions are possible because the (100) state has only even values for J, and there is no Q-branch because the (001) state has only odd values for J. This is due to the allowed rotational states mentioned earlier, as CO2 is a symmetric molecule. Lasing can occur on any of the transitions shown.
Fig. 10 It might seem that a large number of transitions would produce lasing simultaneously in CO2 lasers. This is true in high-gain pulsed systems only (Note that there is no lasing cascade in CO2.) In CW CO2 lasers, only one transition lases at a time. The reason for this is revealed by an examination of the rotational level population curve shown in Figure 11. The length of each horizontal line in this figure represents the population of the indicated rotational level in the (001) vibrational level. Even-numbered rotational states in the (100) vibrational level have a similar population curve. This means that different transitions will have different amounts of gain.
Fig. 11 Lasing will begin on the P transition that has the highest gain (the greatest population inversion). It would appear that the gain for that transition would be quickly depleted and that it would die out, to be replaced with another transition with sufficient gain. This is not, however, the case. A CO2 molecule remains in the (001) state for an average of about 2×10–3 seconds in a typical CO2 laser. During that time, it changes rotational levels approximately 20,000 times. Thus, as lasing depletes the population of one rotational level, molecules enter from adjacent levels (D J = ± 2 in this case). The result is an overall decrease in the population of all rotational levels with no appreciable change in the relative population distribution within the vibrational state. Thus, lasing continues on a single P-branch transition. Table 3 is a list of P-branch transitions in CO2. The output wavelength of CO2 lasers is commonly stated as 10.6 m m because most operate on the P(20) transition with a wavelength of 10.5912 m m. Many other transitions are, however, possible in the wavelength range of 9 to 11 m m. Table 3. Measured Co2 Laser Wavelengths of the P-Branch
Figure 12 is a diagram of laser gain versus upper rotational level (001) for a CO2 laser. Solid lines are for the P-branch, and broken lines are the R-branch. Numbers beside each curve give the relative population of the (001) vibrational state to the (100) vibrational state for that curve. Notice that some P transitions have significant gain when relative populations of the upper and lower lasing states are only 0.95. This means that lasing can occur on these transitions without a population inversion between the vibrational levels. This is due to differences in the rotational population distribution curves of the (001) level and (100) level of CO2 (see Figure 11. The result is that the population of the (001), J = 31 state is greater than that of the (100), J = 32 level, even though total population of the upper vibrational level is less than that of the lower level. The laser can then lase on the P(32) transition.
Fig. 12 The above condition is called a partial inversion and is a characteristic of all molecular lasers. Pumping mechanisms affect the vibrational states only. Increased gain due to the rotational population distribution is a bonus that is built into the molecule itself. Additional gain available depends upon the type of molecule used and the temperature of the gas. Lasing has been observed in CO lasers at a temperature of 3000 K (0° C) with a relative population of only 0.80.
Other Molecular Lasers Lasing has been produced in more than 25 molecular species. Most are diatomic or triatomic, but several more complicated molecules will lase. Table 4 is a partial list of molecular lasers. In several cases, specific isotopes have been used to shift the available wavelengths of the laser output. Optical and chemical excitation have been used with some molecules, but most are excited by current flow through the gas. Molecular laser output wavelengths vary from the vacuum UV (H2) to the far IR (HCN), but the most important ones have outputs between 2 and 20 m m. While all molecular lasers have some common characteristics, they are by far the most diverse laser class.
Table 4. Molecular Lasers.
Operating Parameters for Low-Power Co2 Lasers A wide variety of CO2 laser configurations are employed for both CW and pulsed operation. Module 3-9, "CO2 Laser Systems," contains a discussion of most important types. In the following discussion, only the more common coaxial CW CO2 laser is considered as an example of a molecular laser. This discussion will emphasize the correlation between the theory already presented and an actual laser. Figure 13 shows a simple configuration for a coaxial flowing CO2 laser excited by a dc discharge. While this is the most common design for low-power CO2 lasers, several variations may be present.
Fig. 13
As would be expected from the previous discussion, CO2 lasers commonly contain carbon dioxide, nitrogen, and helium. Table 5 lists ratios suggested by several laser manufacturers, along with power ratings of the laser using specific gas mixtures. As can be seen, ratios vary greatly from one laser to another. The only really constant characteristic is that He comprises most of the gas, followed in order by N2 and CO2. The reasons for this ratio are fairly simple:
Table 5. Typical Co2:N2:He Gas Ratios Recommended by Laser Manufacturers
The best ratio for any particular laser should be determined experimentally. That ratio depends on total pressure, gas temperature, current, tube diameter, gas flow rate, mirror reflectivity, etc. In general, a CO2:N2:He ratio of 1:2:10 is a good point from which to start. Figure 14 shows the dependence of the optimum gain on the diameter of the laser tube. Because the gain is determined by the efficiency with which waste heat can be removed from the CO2 gas and transported to the tube walls by helium atoms, gain decreases as diameter of the bore increases. This reduction in gain is a result of the increased distance traveled by helium atoms in removing waste heat from the center of larger tubes and the corresponding reduction in cooling efficiency. Thus, the greater gain occurs at the smaller tube diameters.
Fig. 14 Reducing the tube diameter also increases diffraction losses in the laser cavity. As the diameter is reduced below a certain point, diffraction loss rises rapidly. Optimum design for a CO2 laser is usually a tube bore that is as small as possible to increase the gain but not small enough to introduce large losses through diffraction. Design equations for CO2 laser cavities are contained in Module 3-9, "CO2 Laser Systems." Figure 15 shows changes in the output power of a CO2 laser as four tube parameters are changed. Figure 15a shows that power increases with a current increase at low currents, but that beyond a certain point greater currents result in lower output powers. This is due to increased heating of the gas by current flow. Each CO2 laser has its own optimum current, and the exact value of this current depends upon other tube parameters. This figure shows output power versus current curves for two different gas flow rates as an example of this variation. Most CO2 laser tubes operate at currents in the range of 30 to 60 mA, although higher or lower currents sometimes are used, depending on system design.
Fig. 15 Figure 15b shows variation in power as a function of tube wall temperature. Because energy transfer from helium atoms to the wall is more efficient at lower wall temperatures, laser output power drops as well temperature rises. Power decrease with increasing wall temperature is lessened as gas flow rate is increased. Figure 15c shows the output power of a CO2 laser as a function of CO2 gas pressure. Like the current curve, the pressure curve increases to a peak and then drops off. Optimum is usually around a CO2 pressure of about 1.5 torr. Increasing cooling efficiency by increasing the gas flow rate or lowering the temperature will give a higher optimum pressure. Increasing the bore diameter reduces cooling efficiency and gives a lower optimum pressure. A good rule of thumb is that the product of the CO2 pressure in torr and the tube diameter in centimeters should be about 3 torr-cm for optimum performance for normal flow rates. Figure 15d shows a variation in output power as the gas flow rate is increased. In a sealed-off tube or one with a very low gas flow rate little mixing of the gas occurs, and the only way for helium atoms to reach the wall is by diffusion. Fairly low gas flow rates cause the gas to be somewhat turbulent, carrying heat to the walls more rapidly. An increase in gas flow rate further increases this effect and also sweeps heated gas out of the laser tube more rapidly. Figure 16 shows tube voltage and output power of two carbon dioxide lasers as functions of tube current. Voltage per unit length is greater for smaller tubes and drops as current increases. This negative dynamic resistance requires the use of a ballast resistor or current-regulated power supply. Also shown is the laser output power which tends to level off and then begins to drop at higher currents as would be expected from previous discussion.
Fig. 16 Summary Molecular gas lasers operate on energy transitions between vibrational energy states of molecules that produce photons in the infrared portion of the spectrum. Several molecular lasers are in fairly common use, but the carbon dioxide laser is the most popular by far because of its high power, high efficiency, and simplicity. Energy transfer mechanisms in a CO2 laser include the excitation of nitrogen molecules by collisions with electrons, transfer of this energy to CO2 molecules through collision, lasing in CO2 molecules, and collisions between CO2 molecules and helium atoms to remove waste heat. Rate of heat removal is the most important factor limiting output power of a CO2 laser. Output power of a CO2 laser depends on the tube current, tube pressure, gas mixture, wall temperature, gas flow rate, and tube diameter. Obtaining optimum output from any CO2 laser requires optimization of each of these parameters.
1. Draw and label a diagram showing the first five rotational levels in the V = 2 and V = 3 vibrational states of a diatomic molecule. Draw and label the following transitions on the diagram.
2. Explain with an energy-level diagram lasing cascade in a CO laser. 3. Draw, label, and explain a simplified energy-level diagram of a CO2 laser. The explanation should include a description of each energy transfer mechanism involved. 4. Explain how and why the output power of a CO2 laser depends on each of the following parameters:
5. Explain the effect on small signal gain and loss in the optical cavity of a CO2 laser as tube diameter is reduced. 6. Use data presented in Figure 16 for tube voltage and current and laser output power to determine the efficiency of each laser as a function of tube current and as a function of output power. Draw the following graphs:
7. Refer to Figure 15 and its explanation in the text to answer the following questions about CO2 lasers. In all cases, assume that all parameters are fixed unless otherwise specified:
8. Explain the concept of a partial inversion and how it is an advantage in CO2 lasers. 9. Explain how the energy transfer processes in CO2 molecules lead to lasing on one P transition only in CW CO2 lasers.
CW CO2 laser system with flowing gas, variable dc current, and liquid cooling Instruction and operation manual for laser system Bottle of premixed gas as specified by laser manufacturer Gas regulator Pressure gage for measurement of total tube pressure (thermocouple, McLeod, or manometer) Gas flowmeter capable of measuring maximum gas flow rate of system Two needle valves Ac wattmeter capable of measuring maximum ac electrical input of system Optical power meter capable of measuring maximum ac electrical input of system (Coherent Radiation Model 201 or equivalent) Dc milliammeter capable of measuring maximum tube current (unless contained in laser power supply) Safety goggles for CO2 laser Focusing lens for CO2 beam (f.l. » 1.5") Materials for irradiation (wood, plastic, etc.) Beam block (fire brick) Beam display screen for CO2 laser (optional)
Operation of a Co2 Laser In this laboratory, students will operate a Class I CO2 laser and analyze its operational characteristics. Students should read all procedures before beginning. Be sure to have all personnel in area wear goggles or glasses. 1. Read the instruction manual for the CO2 laser. Locate all parts referred to in the manual. Observe all appropriate safety precautions during laser operation. 2. Connect the gas flow system as shown in Figure 17 if it is not already assembled in the configuration.
Fig. 17 3. Connect the ac wattmeter to the input of the laser power supply. Place the dc milliammeter in series with the cathode with its positive terminal connected to the tube cathode. 4. Place the beam block in a position to intercept the beam near the laser output aperture. 5. Follow instructions in the laser operation manual to establish proper gas pressure and flow rate for optimum lasing. 6. While observing all safety precautions turn on the laser power supply and verify electrical operation of the laser tube. 7. Turn off the laser discharge. 8. Place the beam block in position to intercept the laser beam. Turn on the laser and verify laser operation. (A CO2 beam display screen may be used if available. Follow instructions provided with the screen.) 9. Turn off the laser discharge. 10. Place the detector head of the power meter in position to intercept the laser beam. Turn on the laser, measure and record maximum output power of the laser. 11. Turn off the laser discharge. 12. Place the focusing lens to focus the laser beam to a minimum spot two or three inches in front of the beam block. Turn on the laser. Be sure everyone is wearing protective eyewear. 13. Pass a variety of nonreflective materials through the focal spot, and observe interaction with the beam. Note that metal surfaces will reflect much of the power, creating a hazard, and that many plastics result in toxic combustion products that should be exhausted from the area. 14. Write a laboratory procedure for taking necessary measurements to accomplish each of the following tasks (as assigned by your instructor). Follow your procedures to produce a graph for each set of data.
15. Prepare a report of your experiment. Include specifications for all major equipment items and conclusions that can be drawn from each set of data.
LABORATORY REPORT Each student should prepare a report of the experiment. This report should include all procedures, pertinent data, problems encountered, and experimental methods used to overcome those problems. Descriptions and model numbers of major equipment items should be included. 
Duley, W.W. CO2 Lasers - Effects and Applications. New York: Academic Press, 1976. O’Shea, Donald C.; Callen, Russell, W.; and Rhodes, William T. Introduction to Lasers and Their Applications. Reading, MA: Addison-Wesley Publishing Co., 1977. Pollack, M.A. "Molecular Gas Lasers," Handbook of Lasers. Pressly, R.J. ed. Chemical Rubber Co., 1971. Ready, John F. Industrial Applications of Lasers. New York: Academic Press, 1978.
|
