Simulation of Time-Dependent Oscillations in a cw HF Chemical Laser Confocal Unstable Resonator

Abstract

To simulate the University of Illinois at Urbana-Champaign (UIUC) supersonic, cw HF chemical laser which extracts power with a confocal unstable resonator, the two-dimensional Bell Aerospace Textron Wave-Optics Model was coupled to the ORNECL fluid dynamic, chemical kinetic, rotational non-equilibrium, exact Ev,J model. The resulting code, denoted ORNECL-UR, was used to study the time-dependent oscillations which may occur in cw HF chemical lasers that use unstable resonators to extract power. ORNECL-UR accurately predicted outcoupled power as a function of the confocal unstable resonator magnification, size, and mirror spacing. ORNECL-UR accurately predicted the periods of the timedependent oscillations as a function of the resonator characteristics. The ORNECL-UR unstable resonator calculations for various resonator geometries support the proposed mechanism for the time-dependent oscillations in cw power that may occur in cw HF chemical lasers that use an unstable resonator to extract power. According to the proposed mechanism, the time-dependent oscillations are the result of a competition between chemical pumping and radiative deactivation of upper laser levels of HF. The oscillations occur only if the medium is not strongly coupled to the optical fields diffractively or geometrically. Calculations showed that either of two conditions may reduce or eliminate oscillations in a symmetric confocal unstable resonator. 1. A resonator Fresnel number less than 3. 2. The number of passes required for a wave to exit the Fresnel core of the resonator greater than 4. These new computations were made for a supersonic HF chemical laser whereas the previous computations were run for a subsonic device.