Laser mirror transmissivity optimization in high power optical cavities.
One of the problems encountered in building Fabry-Perot cavities for laser use is that of optimum output coupling through choice of appropriate output mirror reflection coefficients. This problem has been particularly acute in CO2 laser systems where the unusually high gain involved indicates that a wide range of reflectivities may be necessary for optimum coupling. Pre­ viously, owing to the typical low gain of He–Ne and argon laser, reflectivities of 97-99% or higher were necessary for laser action. This narrow range of permissible reflectivities reduced the pos­ sible miscoupling error. The theoretical analysis of the coupling problem in this range of reflectivities has been handled by Rigrod. CO2 lasers, however, have been operated with nonoptimized out­ put reflectors consisting merely of uncoated Irtran IV which has a first surface reflection of only 17% and even NaCl with a 4% reflection at 10.6 μ wavelength. The optimization problem lies in the fact that the gain of a lasing line G(ω) is a function of the radiation energy density in the laser cavity ω, which in turn is affected by the output coupling due to the mirror transmissivities. This problem may be solved if we know the appropriate form of G (ω). In this paper we shall assume that G(ω) is closely approximated by Rigrod's expression: