Investigation of the influence of optical turbulence on DIRCM laser’s jamming effectiveness: A wave-optics approach

Abstract

Due to their exceptionally large jam-to-signal ratios (J/S), Directed Infrared Countermeasure (DIRCM) systems are the most efficient means for defeating heat seeking missiles. Although many studies have investigated DIRCM systems, the influence of optical turbulence on the jamming code effectiveness is mostly ignored. However, due to optical turbulence caused by the exhaust plume of the air platform and the atmosphere, a DIRCM laser’s beam may be exposed to time-varying intensity fluctuations, which may dramatically reduce the effective J/S at the seeker’s aperture compared to the one at the platform. In addition, previous studies have generally focused on the signal processing stages of seeker heads while ignoring diffractive and aberrative properties of the missile optics. In this study, we employ a wave-optics approach for estimating the jamming code effectiveness under turbulence. Specifically, we investigate time-varying influence of several degrees of optical turbulence on DIRCM laser beams with specified beam quality factors (M2) that are modulated with various jamming patterns. In our model, the DIRCM laser beam first passes through a highly turbulent region, which is caused by the rotor downwash of the exhaust plume of a rotary-wing platform. Next, the DIRCM laser beam propagates in an atmospheric turbulence region for much longer distances (on the order of few km’s) until it reaches the missile. Laser beam propagation in both turbulence regimes is simulated using the split-step method. Subsequently, using the ZEMAX software and its wave-optics-based Physical Optics Propagation (POP) package, we employ a generic model for the optical system of a first-generation spin-scan seeker and obtain the time-dependent intensity profiles of the laser beam at the focal point. Generic models for an uncooled lead-sulfide detector and the following signal processing stages have also been included in the model.