Plasma Adaptive Optics Evaluation Using Two-Wavelength Heterodyne Interferometry

Abstract

The third part of a research program that investigates the possibility of using low-temperature ionized air (plasma) for adaptive optics in an airborne laser directed-energy system is presented. It involves a novel optical measurement method that consists of a dual-wavelength Michelson interferometer that includes a microactuated mirror to allow heterodyne operation. The heterodyne interferometry is an important element in extracting the individual effects of electrons and heavy particles on the optical properties of the plasma. The plasma is generated in a hollow glass cylinder that is sealed at both ends by optical glass. It uses an ac voltage source for which the amplitude is modulated in order to produce sideband phase modulations for heterodyne analysis. A robust demodulation scheme is developed to extract the modulated interferometric phase shifts to reveal the temporal evolution of electron and heavy particle densities within the plasma. The contributions of the electron and heavy particle densities to the plasma optical path difference, which is a measure of optical control, are calculated. Based on an aero-optic standard wavelength, for the conditions examined in the experiment with air, the combined contributions yield an optical path difference of , which is two times larger than nominal optical path difference values in aero-optic applications.