Laser beam propagation in jet engine plume environments: a review

Abstract

Laser beam propagation through adverse turbulent environments such as the region close to a jet engine exhaust need to be studied in order to predict performance degradations on airborne laser systems. The turbulent plume region may introduce severe perturbations which accumulate and cause beam degradation in terms of beam wander, intensity scintillations and beam broadening at longer ranges. Applications of interest with respect to laser beam propagation in jet engine plume environments include e.g. directed infrared countermeasures (DIRCM) and active imaging. By characterising and evaluating the perturbation effects schemes for compensation or avoiding performance degradation can be devised. The turbulence effects in the plume region occur by mixing of hot exhaust flow from the jet engine with surrounding ambient air causing spatial and temporal fluctuations in the refractive index. In comparison to atmospheric turbulence considerably shorter outer- and inner scales have been observed. Typical values of the structure constant within the plume region range from 10^-10 to 10^-9 m^-2/3 making the turbulence several order in magnitude stronger in contrast to propagation through the atmosphere. Of importance in characterisation of the jet engine plume with respect to laser beam propagation are turbulent length scales, the extent of the turbid region, variation of the structure constant and temporal flow properties. In this paper reported experimental results and modelling approaches aimed for predicting laser beam propagation degradation in jet engine plume regions are reviewed. The results will be discussed in perspective of system performance.