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United States Patent (19) (11 3,801,927 Allen (45) Apr United States Patent (19) (11 3,801,927 Allen (45) Apr. 2, 1974 54 LASER AND METHOD OF OPERATION Fluid-Mechanical Techniques, 'The Physics of Fluids, 75 Inventor: Richard A. Allen, Watertown, Mass. Vol. 8, No. 9, Sept. 1965, pp. 1601-1606. 73 Assignee: Avco Corporation, Cincinnati, Ohio Primary Examiner-Maynard R. Wilbur 22 Filed: May 24, 1967 Assistant Examiner-N. Moskowitz Attorney, Agent, or Firm-Charles M. Hogan; Melvin 21 Appl. No.: 643,296 E. Frederick 52 U.S. C. ................................. 331/94.5, 330/4.3 57 ABSTRACT (51) int. Cl............................................... HOs3/02 A laser and method of operating same wherein an op 58 Field of Search..................... 33 1194.5; 330/4.3; tical resonator which receives a gas from supersonic 219/121 nozzle includes a fully reflective mirror and an oppo sitely disposed mirror partially transmissive by reason 56 References Cited of hole coupling. The partially transmissive mirror UNITED STATES PATENTS does not include a permanently mounted transmitting 2,851,652 9, 1950 Dicke.......................... 331 f94.5 UX window during operation. During startup, the holes 3,292,102 12/1966 Byrne................................. 33 1194.5 are covered by a shutter or the like and after the laser 3,302,127 1 (1967 Lin................................... 330/4.3 X has been started aerodynamically, the holes are un OTHER PUBLICATIONS covered. Hurle et al., “Electronic Population Inversions by 4 Claims, 1 Drawing Figure 2 GASEOUS FLOW TO (2Ø DF USER ( g 2 27 HEAED GASEOUS 3O MIXTURE PATENTED APR 2 1974 3,801,927 7 7 -) RICHARD A. ALLEN INVENTOR. by (A.4a. 2. / ATTORNEYS 3,801,927 2 LASER AND METHOD OF OPERATION as a method of producing population inversion, is re ferred to as a gas dynamic laser. Optical masers or lasers, as the art has developed, The following references and materials cited therein generally involve the establishment of an artificial dis describe some of the background and physical princi tribution of bound electrons at energy levels other than ples involved in the gas dynamic laser under discussion the natural distribution in a host environment through and an insight, to some degree, of application of those the application of a source of energy known as the principles in the present state of the art: “pumping energy." This results in a greater number of 1. “Infrared and Optical Masers,' by A. L. Shawlow molecules or atoms in some high energy level than in and C. H. Townes in Physical Review, Vol. 1 12, a lower energy level to which it is optically connected. O No. 6, Dec. 15, 1958, pp 1940-1949. This is known as a population inversion. The electrons 2. “Attainment of Negative Temperatures by Heating present in the host environment in the artificial distri and Cooling of a System' by N. G. Basov and A. bution then give up their energy and undergo a transi N. Oraevskii, Soviet Physics, JETP, Vol. 17, No. 5, tion to the lower energy level. The released energy may November 1963, pp 1171-1 172. be in the form of electromagnetic radiation; which, in 5 3. “Population Inversion in Adiabatic Expansion of the majority of devices seen thus far in the art, has been a Gas Mixture' by V. K. Konyukhov and A. M. light, either in the visible or infrared. Prokhorov, JETP Letters, Vol. 3, No. 1 1 June 1, In laser devices currently available in the art, there 1966, pp. 286-288. may be employed a gas, such as a helium-neon mixture; 4. “Electronic Population Inversions by Fluid or a crystal, such as chromium doped aluminum oxide; 20 Mechanical Techniques' by I. R. Hurle and A. or a noncrystalline solid, such as neodymium glass; or Hertzberg, The Physics of Fluids, Vol. 8, No. 9, Sept. 1965, pp 1601-1607. a liquid, such as trivalent neodymium in selenium oxy 5. Polanyi, J.C., J. Chem. Phys. 34 347 (1961). chloride, as the environment which responds to the 6. Polyanyi, J.C., Applied Optics Supplement No. 2 pumping energy, permitting the population inversion of 25 on Chemical Lasers, 109 (1965). electrons between an excited state and a lower state. Broadly, operative gas dynamic lasers of the type The electrons in returning to the lower state give off here concerned comprise a gas containing chamber quanta of light energy or photons in what is known in having an exhaust outlet; means for heating a polya the art as a radiative transition. When the density of tomic gas to provide equilibrium vibrational excitation these photons becomes large, the radiative transition 30 in said gas, the polyatomic gas having an upper laser probability increases; and, in the presence of a popula level, lower laser level and a ground state, the upper tion inversion, electromagnetic modes into which the laser level having an effective relaxation time that is photons are emitted, in turn, become most readily able long compared to the effective relaxation time of the to induce further emission therein. This is known in the lower laser level; nozzle means for expanding the art as stimulated emission of radiation and results in a 35 heated gas into the chamber into a stream to provide narrowing of the emission line. In the currently avail a flow time in the nozzle means that is short compared able laser devices, electrical power is converted to opti to the effective relaxation time of said upper laser level cal power, pumping light or an electrical discharge or and long compared to the effective relaxation time of electric current; which, in turn, is used to establish the said lower laser level; and an optical resonator coupled population inversion. All known prior art lasers are of 40 to said stream of gas. For a more thorough and detailed relatively low power. A high power laser has been a discussion of operative gas dynamic lasers, reference is long sought need for a large number of potential appli made to patent application Ser. No. 626,357, filed Feb. cations, both military and commercial, and numerous 16, 1967, entitled "High Powered Laser' and assigned attempts have been made to provide a truly high power to the same Assignee as this application. laser. The gas laser is the general category into which 45 In accordance with the present invention, there is most of these efforts have fitted. provided in a gas dynamic laser a transmissive mirror In the Polanyi references identified hereinafter, it is having a passage or passages for coupling power out of suggested that total and partial inversions may be ob the optical resonator, the working medium within the tained as a direct result of chemical reaction. Without optical resonator being in direct communication with flow, such inversions are transient. Even if the gas is 50 the environment surrounding the laser via the transmis pulsed thermally and permitted to relax differentially, sive mirror. In use, the passages in the transmissive mir such disclosed devices are inherently low density de ror may be sealed as by a shutter or the like to prevent vices since the translational and rotational energy is re leakage into the optical resonator during startup. After moved by diffusion to the walls. The Hurle et al paper startup, the aforementioned seal is removed. also identified hereinafter suggests a gas dynamic laser 55 It is a principal object of the invention to provide an utilizing supersonic expansion as a method of produc improved gas dynamic laser and method of operation. ing population inversion between electronic states by differential radiation relaxation. While presumably in It is a further object of the invention to provide a theory (Hurleet al. admit that they were unable to ob laser which does not require a sealed optical system. serve an inversion) an inverted population can be pro 60 A further object of the invention is to reduce loss in duced in this fashion, the size of a gas dynamic laser the optical system of a gas dynamic laser. based solely on this principle is limited because of radi A still further object is a provision of an improved ative trapping and also the stagnation temperatures re method of operating gas dynamic lasers. quired to have a significant fraction of the energy in the Another object is to provide an improved gas dy desired electronic level at equilibrium are quite high. 65 namic laser having windowless output optics and a For the purpose of convenience, a laser, the principle method of operation which permits simple and efficient of operation of which is based on supersonic expansion aerodynamic starting of the laser. 3,801,927 3 4. The novel features that are considered characteristic ture level comparable to the usual heating encountered of the invention are set forth in the appended claims; in such devices. Accordingly, conventional means of the invention itself, however, both as to its organization cooling have not been shown for purposes of clarity and method of operation, together with additional ob and the discussion thereof is not deemed necessary. jects and advantages thereof, will best be understood The mirror metal preferably is as pure as possible and from the description of a specific embodiment when dead soft. In the case of copper, 99.999% purity and read in conjunction with the accompanying drawing annealing to make it dead soft is recommended. which is a pictorial view with parts broken away illus Metal mirror 8 is provided with hole coupling for trating an improved laser in accordance with the inven coupling light energy out of chamber 12. Mirror 18 tion. O may be formed of 99.999% pure, dead soft copper. The Referring now to the drawing, there is shown by way active or inner surface 19 is optically polished such that of illustration a portion of a gas dynamic laser for re the wavelength of the radiation striking it is greater ceiving from a high temperature gas source (not than the wavelength corresponding to the state of sur shown), such as for example a combustor or burner, a face 19.
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