Portland State University PDXScholar Electrical and Computer Engineering Faculty Publications and Presentations Electrical and Computer Engineering 5-1-1983 Radio-frequency-excited carbon dioxide metal waveguide laser Jonathan Gary Grossman Lee W. Casperson Portland State University O. M. Stafsudd Follow this and additional works at: https://pdxscholar.library.pdx.edu/ece_fac Part of the Electrical and Computer Engineering Commons Let us know how access to this document benefits ou.y Citation Details Jonathan G. Grossman, Lee W. Casperson, Oscar M. Stafsudd. Radio-frequency-excited carbon dioxide metal waveguide laser. Applied Optics, Volume 22, Number 9 (May 1983), pp. 1298-1305. This Article is brought to you for free and open access. It has been accepted for inclusion in Electrical and Computer Engineering Faculty Publications and Presentations by an authorized administrator of PDXScholar. Please contact us if we can make this document more accessible: [email protected]. Radio-frequency-excited carbon dioxide metal waveguide laser Jonathan G. Grossman, Lee W. Casperson, and Oscar M. Stafsudd A new type of waveguide laser is described in which a single-surface concave metal waveguide also doubles as the grounded electrode in a radio-frequency-excited gaseous discharge. The laser's output modes and spectrum have been studied theoretically and experimentally for cw CO2 operation of the laser. Overall op- erating efficiency and mode stability are comparable with those of conventional dielectric waveguide CO2 lasers, and advantages of the metal waveguide laser include convection cooling, intracavity mode control, and partial self-alignment. 1. Introduction CdS crystals4 and with optically pumped cylinders of The majority of laser oscillators are basically Rh6G dye.5 Whispering laser modes have been ob- Fabry-Perot interferometers containing an active me- served in ruby toroids,6-8 and whispering-gallery in- dium. For many applications, this design and its var- jection lasers have been fabricated. 9 10 Such whispering iants have the advantages of simplicity and good mode modes are also one of the dominant parasitic phenom- control. The first ruby laser, for example, was of this ena in very high-gain laser systems.1' type.' On the other hand, many more exotic types of In related developments, it has been noted that more laser resonator have also been developed. In this paper useful electromagneticmodes might be obtainable if the we report the results of a detailed study of a waveguide outer reflecting surface were curved slightly in the di- laser in which the cavity modes are basically the whis- rection of the axis of symmetry to reduce diffraction pering-gallery modes of a curved concave metal-strip losses in that direction. A sketch of such a resonator waveguide. is given in Fig. 1. Resonators of this type have been The first whispering-gallery laser was a sphere of studied theoretically, and it is found that the field dis- CaF2 :Sm++ in which the oscillating modes were con- tribution perpendicular to the laser plane is expressible fined by nearly total internal reflection at the interface in terms of Hermite-Gaussian functions, the field in the between the laser crystal and the surrounding liquid radial direction involves Bessel functions, and the field hydrogen.2 For reasons of potentially simpler fabri- in the azimuthal direction involves trigonometric cation, pumping, and radiation outcoupling,much more functions.12 -15 The low-order Bessel-function modes attention has been directed toward development of la- propagate primarily in the radial direction within the sers that are basically cylindrical in shape. An early resonator, while the high-order modes are essentially cylindrical laser consisted of a GaAs junction laser which whispering-gallery modes propagating along the mirror had been polished into the form of a cylinder 18 mils in surface. A CO2 laser of this type has been operated, and diameter.3 In this device, the radiation was believed a radial aperture has been introduced to discriminate to propagate primarily in the radial direction within the against the whispering-gallery modes.16 laser, and the output would consist of a 3600 disk- Independent of these developments concerning cy- shaped radiation pattern. Similar radiation patterns lindrical laser resonators came the realization that a have also been obtained with electron-beam-pumped curved metal strip might have some important advan- tages for the waveguiding of electromagnetic radiation. While there has been steady progress in the develop- ment of low-loss optical fibers, When this work was done both authors were with University of there remain important California, School of Engineering & Applied Science, Los Angeles, spectral regions where transparent materials are not yet California 90024. J. G. Grossman is now with Hughes Aircraft available. In these infrared and ultraviolet portions of Company, Electro-Optical and Data Systems Group, El Segundo, the spectrum, metal waveguides are still effective, and California 90245. such waveguides are also tolerant of very high power Received 21 September 1982. levels. The propagation of low-order modes in metal 0003-6935/83/091298-08$01.00/0. waveguides has been well-understood for many years, ©31983 Optical Society of America. and curved and twisted guides have also been consid- 1298 APPLIED OPTICS / Vol. 22, No. 9 / 1 May 1983 z A brief summary of the mode characteristics of the concave metal surface waveguide laser is presented in Sec. II, and special attention is given to the spectral 4, characteristics of the waveguide resonator. The rf properties of the laser are discussed in Sec. III, and our experimental results are presented in Secs. IV-VI. - r II. Theory The propagating modes of concave metal-strip waveguides have been the subject of detailed theoretical investigations.2'3 25' 26 Only the results that are directly Fig. 1. Low-loss cylindrical laser resonator. relevant to the present investigation will be summarized here. The coordinate system to be used in this dis- cussion is shown in Fig. 2. The major and minor radii 7 ered.1 More recently, flexible metal waveguides of of curvature are r and R0 , respectively, z indicates rectangular cross section have been developed for distance across the waveguide surface, r' is distance transmitting infrared wavelengths,18-20 and it has been perpendicular to the surface, and 0 measures angular shown that effective light guiding is possible using a distance along the waveguide. In this system of units single curved metallic surface rather than a fully en- the main Gaussian factor in the Airy-Hermite-Gaussian closed waveguide.21' 22 This concept has been extended mode solutions can be written 2 5 by making the curved waveguide concave and thus 2 G(0,z) = Go exp-i[Q(O)z /2 + S(0)z]}, (1) eliminating diffraction losses entirely.23-26 In the simplest cases, these waveguide surfaces are identical where the Q and S parameters are governed by the to the circularly curved mirror surface indicated in Fig. equations 1, and the Airy-Hermite-Gaussian modes are special Q2+ ° Q+ O =, (2) cases of the Bessel-Hermite-Gaussian modes of the r d r R radial-mode laser resonators. 0 0 The whispering-gallery waveguides can also be in- QS+ °-= . (3) corporated in Fabry-Perot-like gas laser resonators. ro do Thus, it has been found that a cw CO2 laser with a In these equations Q is the usual beam parameter slightly bent glass discharge tube supports modes which agree well with the theoretical Airy-Hermite-Gaussian R() W2() (4) functions. 2 7 We report here the results of various ex- perimental studies of a new rf-excited metal-waveguide where ko is essentially the free-space propagation con- laser oscillator. The resonator waveguide in these ex- stant, and R and w are the radius of curvature of the periments is a concave metal strip of the same type used phase fronts and the l/e amplitude spot size in the z in earlier waveguide studies, and this waveguide surface direction, respectively. The ratio da = -Si/Qi is the also serves as an electrode for the rf gas discharge. As displacement in the z direction of the amplitude center will be shown in later sections of this paper, the metal- of the beam, and the ratio dp = -Sr/Qr is the dis- placement in the z direction of the phase center of the waveguide CO2 laser has some potential advantages over conventional CO waveguide lasers. beam. The subscripts i and r denote the imaginary and 2 real parts of the parameters Q and S, respectively. There is substantial literature about rf-excited CO2 lasers. Many of the research efforts in this area involve Equations (2)-(4) can be solved analytically to obtain lasers that use magnetic coupling of the rf power to the the 0 dependence of the beam displacement, spot size, laser. While this is practical for dielectric waveguide and phase-front curvature for an arbitrary input beam. lasers, it is not feasible for metal-waveguide lasers. The However, only the matched-mode solutions are needed coupling of the rf power to the laser by impedance matching, however, is a practical approach, and it has been shown that the laser head can be accurately rep- resented by an electronic model.28 Standard rf- matching techniques are therefore applicable in this approach. Using Smith chart impedance-matching techniques, it has been possible to obtain near perfect rf matching of the laser to the rf source (zero rf power reflected from the laser head), with rf coupling ef- ficiencies as high as 85%-95%.29 Resistance in the network has been the main source of the rf coupling loss, with resistance as low as a tenth of an ohm leading to 10%loss. 30 Other experimental studies have included the confinement, stability, and efficiency of the rf dis- Fig.