Optical Quality of Tilted Spherical Mirror Unstable Resonators

High power chemical lasers often have high aspect ratio gain regions, for which an unstable resonator with different magnifications in the two transverse dimensions is desirable. This paper considers a new class of unstable resonators which achieves the differential magnification property with spherical mirrors by suitably tilting the mirrors through large angles. Output optical quality has been investigated theoretically and experimentally. It was found that unstable resonators producing near uniphase laser beams can be fabricated using this new approach. A special property allows the transverse magnifications to be continuously varied, a significant aid for high power gas dynamic or chemical laser experimental development activities when the flow gain path length is varied. INTRODUCTION In various applications, including in particular high power gas dynamic and chemical lasers with relatively short gain lengths, compared to the gain height there is a need for unstable laser resonators which produce a collimated output beam but have asymmetric magnification, i.e. Mx f M . Such resonators have Y been achieved by others by use of mirrors with toroidal figuring, but there are substantial difficulties involved in fabrication and alignment of such mirrors. Special cases of the toroidal resonator include the cylindrical mirror system. Wave optics analyses of these special cases show that mode control difficulties exist. Fabrication of toroidal mirrors does not permit the control of the surface radii, Rx and R of Y each mirror to much better than about 2%. This does not always allow optimum mirror separations to provide a uniphase beam in both transverse dimensions simultaneously. Additionally some transverse mode control is sacrificed in the toroidal resonator concept. High energy laser research activities will require several sets of these complicated toroidal resonator designs, as the gain in the flow direction is varied. A need has evolved to develop a resonator that has the simultaneous properties of producing a nearly uniphase beam, of allowing a continuously variable magnification in the two orthogonal directions and of accommodating a continuously variable aspect ratio gain medium. We have previously [11 proposed a new class of resonators which achieves all these properties without the disArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980952