Estimation of atmospheric optical turbulence strength in realistic airborne environments.

In this paper, atmospheric optical turbulence strength is estimated for realistic airborne environments using a modified phase-variance approach, as well as a modified slope-discrepancy approach. Realistic airborne environments are generated using wave-optics simulations of a plane wave propagating through increasing strengths of homogeneous atmospheric optical turbulence, both with and without aero-optical contamination (from in-flight wavefront sensor data) and additive-measurement noise. In comparison to the modified phase-variance approach, the results show that the modified slope-discrepancy approach more accurately estimates atmospheric optical turbulence strength over a wide range of conditions.

In-flight measurement of atmospheric-imposed tilt: experimental results and analysis.

The work presented here experimentally measures the tilt imposed on a laser beam by the atmosphere from Shack-Hartmann wavefront sensor measurements collected in-flight. Tip/tilt is imposed on the laser beam by propagating through optical turbulent structures larger than or of the order of the size of the beam diameter. This tip/tilt causes a dynamic, net deflection of the beam in the far field, referred to as jitter, which poses a serious problem for tracking in directed energy applications.

Turbulence scale effects and resolution requirements in aero-optics.

While optical aberrations caused by atmospheric turbulence have been extensively investigated and well characterized, recent research has identified structural differences in optical phase distortions caused by aircraft-induced, compressible turbulence. These so-called aero-optical distortions can be a critical obstacle in the development of airborne optical systems and reduce the fidelity and on-target intensity of optical beams. Using a model index-of-refraction spectrum that accounts for changes in density due to both temperature and pressure fluctuations in aero-optically active flow fields, expressions for the two-dimensional phase distortion over an aperture are developed.

The wavenumber spectra of aero-optical phase distortions by weakly compressible turbulence.

Optical distortions caused by turbulent airflow surrounding an aircraft, known as aero-optical phenomena, are a major impediment to applications of airborne laser systems. To better understand the spectral properties of aero-optical distortions, a general expression for the wavenumber spectrum of the refractive index is derived from the ideal-gas law and Gladstone-Dale relation. The derived index-of-refraction spectrum accounts for changes in air density due to both temperature and pressure fluctuations and is used to calculate the phase-distortion spectrum of an optical beam propagating through a weakly compressible, turbulent flow field.

Shock-related effects on aero-optical environment for hemisphere-on-cylinder turrets at transonic speeds.

The aero-optical environment around a hemisphere-on-cylinder turret with both flat and conformal windows was studied experimentally in flight using the Airborne Aero-Optical Laboratory-Transonic for a range of subsonic and transonic Mach numbers between 0.5 and 0.8.

Wavefront measurements of a laser-induced breakdown spark in still air.

Experimental measurements of the wavefronts of the light from a laser-induced breakdown (LIB) spark in non-moving air are presented and compared to spark dimensional data acquired from photographic measurements of the spark. The data show that the variation in the spark emitted wavefront between ignitions can be directly related to the motion of the spark volumetric centroid. The dominant modal components of the emitted wavefront variations are presented, as well as quantitative results for the magnitude of the wavefront variations.