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Airborne laser countermeasure applications (DIRCM) are hampered by the turbulence of jet engine exhaust. The
effects of this source of perturbation on optical propagation have still to be documented and analyzed in order to get a
better insight into the different mechanisms of the plume perturbations and also to validate CFD/LES codes. For that
purpose, wave front sensing has been used as a non-intrusive optical technique to provide unsteady and turbulent
optical measurements through a plume of a jet engine installed at a fixed point on the ground. The experiment has
been implemented in October 2007 along with other optical measuring techniques at Volvo Aero Corporation
(Trollhättan, Sweden). This study is part of a European research programme dealing with DIRCM issues. The Shack-
Hartmann (SH) wave front sensing technique was employed. It consisted of 64 x 64 lenslets coupled to a 1024x1024
pixel Dalsa CCD sensor working at a sampling rate of 40 Hz. A 15 ns pulsed laser synchronized with the SH sensor
enabled "freezing" turbulence in each SH image. The ability of the technique to substract a reference permitted a
simple calibration procedure to ensure accurate and reliable measurements despite vibration environment.
Instantaneous phases are reconstructed using Fourier techniques so as to obtain a better spatial resolution against
turbulent effects. Under any given plume condition, overall tilt aberration prevails. Phase power spectra derived from
phase statistics are drawn according to the plume main axis and to normal axis. They compare favorably well to the
decaying Kolmogorov power law on a useful high spatial frequency range. Averaged phases are also decomposed into
Zernike polynomials to analyze optical mode behavior according to engine status and to plume abscissa. With overall
tilt removed, turbulent DSP's amplitude drops by a factor of 30 to 40 and mean aberrations by a factor of 10 from an
abscissa 1 meter to another 3.5 meters away from the engine nozzle, due to quite different turbulent conditions.
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