We discuss the use of nematic liquid-crystal phase modulators (LCPMs) as repeatable, programmable optical disturbance test sources to simulate propagation through deep atmospheric turbulence in a laboratory setting. LCPMs can introduce controlled dynamic aberrations into optical systems at low cost, low complexity and high flexibility. Because they are small they can easily be inserted into the optical path of optics system. The programmed sequence can be modified to simulate changing atmospheric conditions and engagement scenarios. In this paper we describe phase screens generated with multiple LPCMs set up at different conjugate points in an optical path to simulate multiple atmospheric turbulence layers. The goal is to simulate deep turbulence optical propagations to test subsequent wave-front sensors and control algorithms. We investigate phenomena related to deep turbulence propagation such as phase-front branch points and optical field intensity fluctuations with medium to high Rytov number.
A continuous-face-sheet Xinetics deformable mirror (DM) is characterized and analyzed using Fizeau interferometer measurements. The influence of each individual actuator on the DM surface is measured. The measured data is then processed to be used as influence functions that define the influence of each actuator on the DM surface. Using the measured data as a spanning set, an orthonormal basis is computed for the span of the set using singular value decomposition. Real surface measurements are taken of the orthonormal basis influences to test for both linear independence of the actuators and actuator linearity with respect to voltage. The efficacy of using the measured basis function data to converge toward a more accurate basis in an iterative process is evaluated. Performance of lower rank (compressed) bases are also evaluated.
Laser illuminated imaging has a number of applications in the areas of night time air-to-ground target surveillance, ID, and pointing and tracking. Using a laser illuminator, the illumination intensity and thus the signal to noise ratio can be controlled. With the advent of high performance range gated cameras in the short-wave infra-red band, higher spatial resolution can be achieved over passive thermal night imaging cameras in the mid-wave infra-red due to the shorter wave-length. If a coherent illuminator is used the resulting imagery often suffers from speckle noise due to the scattering off of a rough target surface, which gives it a grainy “salt and pepper” appearance. The probability density function for the intensity of focal plane speckle is well understood to follow a negative exponential distribution. This can be exploited to develop a Bayesian speckle noise filter. The filter has the advantage over simple frame averaging approaches in that it preserves target features and motion while reducing speckle noise without smearing or blurring the images. The resulting filtered images have the appearance of passive imagery and so are more amenable to sensor fusion with simultaneous mid-wave infra-red thermal images for enhanced target ID. The noise filter improvement is demonstrated using examples from real world laser imaging tests on tactical targets.