Measurement of Beacon Anisoplanatism Through a Two-Dimensional Weakly-Compressible Shear Layer

Abstract

An experimental investigation was conducted into the effectiveness with which aero-optic aberrations imposed on a collimated reference beam could be evaluated using a point-source beacon. The experiments were conducted in the University of Notre Dame's Compressible Shear-Layer Wind Tunnel which was used to create an optically-active shear-layer flow with high-speed Mach number of 0.8. Anisoplanatic effects included a difference in wavefront shape between the (spherical wavefront) beacon and the (planar wavefront) reference beam, and a difference in the regions of the flow sampled by the beacon and reference beams. A modal compensation approach was used to minimize the anisoplanatism between the beacon and reference wavefronts, which showed that the best compensation results were obtained when the shear layer was regularized using mechanical forcing.

Aero-Optic Measurements Using a Laser-Induced Air Breakdown Beacon

“…The figure shows that the plexiglas plate successfully produced aberration amplitudes in the range of Table 1. Furthermore, the aberrations measured by the spark (right side of each image) closely matched the aberration measured by the collimated reference beam, even without any corrections applied to the spark wavefronts to adjust for anisoplanatism effects [8]. The average cross-correlation value between the spark and collimated wavefronts, also without any corrections applied for anisoplanatism effects, was approximately 75%.…”

“…The plexiglas plate was placed in the optical path of the spark light and an aperture was placed on the collimating lens for the spark beam such that the diameter of the spark beam at the plexiglas aberration was approximately 17 mm. A 25 mm diameter reference beam was also passed through the plexiglas at the same location as the spark beam; the anisoplanatism [8] between the reference and spark measurements of the aberration therefore consisted of the difference in beam diameter and angle at the aberration, as well as the difference in wavefront shape at the aberration (spherical for the spark beam and planar for the reference beam).…”

“…The operational advantages offered by this kind of man-made guide star are significant since it can be placed at any point in space outside the aircraft. In [8], we described our experimental measurements of the aero-optic aberrations of a compressible shear layer using an artificial guide star that was simulated using the diverging light from an optical fiber. In these tests, anisoplanatism effects were evaluated by comparing wavefront aberrations from the fiberoptical point-source "beacon" to those from a collimated reference laser beam after passing through the compressible shear-layer flow; these measurements showed that it was possible to use the aberrations measured with the point-source beacon to accurately predict the aero-optic aberrations on the reference beam, despite the fact that the point-source beacon wavefronts had significant initial curvature and that the light from the point source sampled only a part of the aberrating flowfield through which the reference beam passed.…”

“…Furthermore, the size and effective location of laser-induced breakdown sparks can fluctuate between pulses of the driving laser, while the effective origin of the light from the optical fiber could be held stationary. The next step in our investigation of an aero-optic beacon was therefore to evaluate the effects of the non-point-source optical character of laser-induced breakdown sparks, and to demonstrate the viability of the aero-optic measurements of [8] using an actual laser-generated beacon; this paper documents the approach and results of this investigation.…”

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

“…1). One possible disadvantage of the approach is that the measured aberration can be distorted by anisoplanatism effects that originate, for example, from the diverging nature of the spark light; however, as shown in [12,13], anisoplanatism effects are typically minimized by the localized nature of aero-optic flows, or can be compensated [14]. A more serious problem is that sparkto-spark variations in the spark shape or position could result in variations in the outgoing spark wavefront that, if sufficiently large in magnitude, could obscure the aero-optic aberration that is the objective of the measurement.…”