CHEMICAL LASERS T. O. Poehler and R. E. Walker LITTLE OVER A DECADE HAS ELAPSED since the are reaching a limit set by the durability of the A demonstration of the first laser by Maiman. glass. The optical pumping process is inherently The intervening time period has seen many de­ inefficient and yields conversion efficiencies of in­ velopments in laser technology. Lasers whose put electrical energy to useful output radiant energy spectral outputs span the ultraviolet to far infrared of a few percent at best; a large part of the waste have been built with a variety of optically-active energy appears as heat. These difficulties can be mediums-gases, liquids, crystalline solids, and circumvented in a gas laser where the waste semiconductors. Power outputs have advanced energy which appears as heat can be convectively from a few microwatts cw produced by the first removed. Further, there are several gas laser He-Ne laser to over 60,000 watts cw produced molecules with radiative transitions between low by the CO2 gas dynamic laser. Energy pulses ex­ order vibrational levels; this allows the radiated ceeding 1000 joules have been extracted from photon energy to be an appreciable part of the modern glass lasers in contrast to the millijoule energy required to pump the system to the upper pulses delivered by Maiman's first ruby laser. laser level. Such a lasing system is potentially Novel techniques for pulse shortening (Q-switch­ capable of operating simultaneously at a high effi­ ing and mode locking) have been used to generate ciency and high power. transient peak powers of 101 3 watts. Many new Carbon Dioxide Laser Mechanisms interesting physical phenomena are observed at these intense electrical fields. The most promising of the molecular gas lasers Laser scientists have been particularly moti­ are those that use carbon dioxide as the optically­ vated to develop high-power lasers with an eye active molecule. 1 , 2 Since the CO2 lasing mecha­ toward efficiency and size. After all, no one gets nism forms an essential step in our chemical laser too concerned about the 100 watts used to power systems, it is worthwhile to review the physics of a 0.1 watt output He-Ne laser. But at the tens of this particular molecular laser. kilowatts laser output level, this inefficiency can­ Carbon dioxide is a simple poly atomic molecule not be ignored. 1 c. K. N. Patel, "Continuous-Wave Laser Action on Vibrational­ Extremely high powers have been obtained Rotational Transitions of C02," Ph),s. Rev. 136, Nov. 30, 1964, from large solid-state lasers, particularly glass, AI187-AI193. 2 C. K . N . Patel, " Selective Excitation through Vibrational Energy containing trivalent neodymium ions. However, Transfer and Optical Laser Action in N2-C02, " Ph ys. ReI'. the high-power densities obtained in these systems Letters 13, Nov. 23, 1964, 617-619. 2 APL Technical Digest Radiative transitions between these vibrational­ rotational levels give rise to a band spectrum. The strongest CO2 laser line arises from the vibrational-rotational transition from the V g = 1 level (00 l) to the Vl = 1 level (lOa). The radia­ Molecular gas lasers are the most promising sources tion emitted is approximately a wavelength of of high-efficiency, high-energy coherent radiation. 10.6 fLm in the infrared portion of the spectrum. The chemical laser, which can produce radiation In this wavelength range, the atmosphere has neg­ with little or no external excitation energy, has ligible attenuation; this is an important factor in begun to emerge as an important class of laser systems. both the construction of CO2 lasers and their pos­ sible applications. When thermal equilibrium exists, the popula­ tion of the upper and lower laser levels are gov­ erned by the Boltzmann factor so that N(OOl) _ g(OOl) - flE/ kT N(100) - g(100) e where N (Vb V2, Vg) is the population of a partic­ ular level, g( Vl, Vz, Vg) is a statistical weighting factor, t::.E is the energy difference between the energy states, k is the Boltzmann constant, and T is the system temperature. Obviously, at equilib­ and the details of its molecular structure are well rium N(OOl) < N(100). established. Polyatomic molecules are classified according to the relation between moments of inertia about the principal axes of symmetry of the molecule when considered as a rigid rotating body. If the three moments of inertia are different, the molecule is called an asymmetric top; if two of the principal moments are equal, it it a symmetric top. One of the special cases of the symmetric top is the so-called linear molecule in which the atoms lie along a straight line giving rise to two equal Vz moments and a third of approximate zero order. Carbon dioxide is a linear molecule, with the oxy­ gen atoms arranged symmetrically about the car­ bon atom as shown in Fig. 1. LINEAR MOLECULES (COz , HCN) The relative motions of the atom in the mole­ cule are characterized by rotation and vibration about its center of mass. Both of these motions Fig. I-Normal vibrations of linear laser molecules. are quantized. The vibrations of a poly atomic molecule can be described in terms of the normal When radiation interacts with this molecular vibrations of a simple harmonic oscillator. The system, photons at the resonant frequency linear CO2 molecule has symmetric stretching (V g - Vl) can induce radiative transitions to occur ( Vl), bending ( V2), and asymmetric stretching between these two energy levels. A photon can be ( Vg) modes as illustrated in Fig. 1. The vibra­ absorbed by a molecule in the lower energy state tional modes have a set of allowed energies which thereby "lifting" the molecule to the upper energy are represented in the energy level diagram of level; or it can induce a molecule in the upper Fig. 2. The allowed rotational energies are closely energy level to emit a second photon and fall to spaced compared to the vibrational energies and the lower energy level. The second process is can be described by superimposing a fine structure called "stimulated emission" and this emitted of rotational levels on each vibrational level. photon is coherent, i.e., in phase with the stimu- March -April 1972 3 Nt (v = 1) + CO2 (000) ~ v=1 N 2 (v = 0) + CO~(OOl). 001 ~ 0.3 VIBRATIONAL This latter process is very rapid and efficient be­ ENERGY TRANSFER cause of the close match (resonance) between the two vibrational levels as illustrated in Fig. 2. With f.4" a judicious choice of gas density and electric field 11 strength, one can funnel nearly 80 % of the elec­ >­ trical discharge power into the CO upper laser '-' 100 020 2 ~ \-J ~ level. This feature, coupled with the nearly 40% Z ~ 0.1 quantum efficiency (i.e., the fraction of the upper !",O v=O laser level energy that is radiated-see Fig. 2), GROUND provides a maximum electrical-to-laser efficiency N2 STATE of about 30%-one to two orders of magnitude o C02 000 GROUND STATE greater than in most other lasers. The maintenance of a population inversion in Fig. 2-Energy level diagram of CO 2 laser. CO2 depends not only on a source of energy to excite the upper laser level, but also on relaxation lating radiation. The absorption and stimulated processes to depopulate the lower level. In CO2 emission processes have equal probability as this is effectively accomplished in resonant colli­ shown by Einstein. It follows therefore that a sional processes beam of resonant radiation will increase in inten­ sity or be amplified only if N(OOl) > N(100), CO~ (100) + M ~ CO~ (020) + M that is the population ratio must be inverted-a CO2 (020) + CO2 (000) ~ laser amplifying medium is not in equilibrium. COz(010) + CO2 (010) In order to achieve laser action, external exci­ tation of some kind is necessary to promote the which provides an avenue to remove molecules inverted nonequilibrium population. This process from the (100) energy level. In the above reaction is called "pumping." Three useful techniques have M is any colliding partner. Rapid depopulation of been employed to achieve inversion in CO2 lasers: the (010) level by collisions with other gas mole­ electrical gas discharge, rapid adiabatic expansion, cules is essential for sustained efficient, high-power and chemical reaction. laser operation, i.e., CO2 (010) + M ~ CO2 (000) + M. A variety of gases including H 2 , H 2 0 , He, and CO are effective in relaxing the (010) level. Electrical Discharge CO 2 Lasers Many of these gas additives also rapidly depopu­ In conventional electrical discharge lasers, the late the upper laser level (001) and so cannot be upper laser level is excited by inelastic collisions used. Helium with a somewhat smaller rate con­ with free electrons accelerated by the externally­ stant is generally used for this purpose since it applied electric field does not seriously affect the upper level. Energy CO2 (000) + e ' ~ CO ~ (001) + e, finally deposited in the ground state appears as heat. Within a given vibrational band, all of the where the asterisk indicates that the CO molecule 2 rotational levels are very rapidly brought to is now excited to a higher energy state. However, thermal equilibrium because of extremely fast since the other vibrational levels (for ' example, rotational relaxation rates. the lower laser level) can be similarly excited, this process is not particularly efficient. To cir­ The above discussions have ignored three dele­ cumvent this problem, nitrogen is added to the terious effects that affect the overall laser efficiency. CO2 • In such a mixture, the N2 is vibrationally­ First, the upper laser level is also relaxed by col­ excited by electron impact lisions with all other molecules, representing a loss in potential laser radiation.