Laboratory simulation of atmospheric turbulence induced optical wavefront distortion
Many creative approaches have been taken in the past for simulating the effect that atmospheric turbulence has on optical beams. Most of the experimental architectures have been complicated and consisted of many optical elements as well as moving components. These techniques have shown a modicum of success; however, they are not completely controllable or predictable. A benchtop technique for experimentally producing one important effect that atmospheric turbulence has on optical beams (phase distortion) is presented here. The system is completely controllable and predictable while accurately representing the statistical nature of the problem.
Previous experimentation in optical processing through turbulent media has demonstrated that optical wavefront distortions can be produced via spatial light modulating (SLM) devices, and most turbulence models and experimental results indicate that turbulence can be represented as a phase fluctuation. The amplitude distributions in the resulting far field are primarily due to propagation of the phase. Operating a liquid crystal television (LCTV) in the “phase-mostly” mode, a phase fluctuation type model for turbulence is utilized in the present investigation, and a real-time experiment for demonstrating the effects was constructed. For an optical system to simulate optical wavefront distortions due to atmospheric turbulence, the following are required: (1) An optical element that modulates the phasefront of an optical beam (2) A model and a technique for generating spatially correlated turbulence simulating distributions (3) Hardware and software for displaying and manipulating the information addressing the optical phase modulation device
The LCTV is ideal for this application. When operated in the “phase-mostly” mode some LCTVs can modulate the phasefront of an optical beam by as much as 2π and an algorithm for generating spatially correlated phase screens can be constructed via mathematical modeling software such as Mathcad. The phase screens can then be manipulated and displayed on the LCTV using a computer with an appropriate framegrabber and software.
The present system consists of an Epson liquid crystal television (which was optimized to modulate up to 2π of phase), a Macintosh IIci with a framegrabber card, a QuickTime movie consisting of multiple video frames of two dimensional arrays of spatially correlated grayscale images, and two polarizers. The movie is displayed on the television via the framegrabber, and the polarizers are used to operate the television in a mode that mostly modulates the spatial phase distribution of the optical wavefront. The frames of the movie are created using an accepted turbulence model for spatially correlated variations in index of refraction, and each subsequent frame of the movie is calculated following an accepted model for temporally varying turbulence. The model used for generating spatial functions or “phase screens” which simulate turbulence is the well known Kolmogorov model. These “phase screens” are then used, employing a Taylor's frozen flow model, to simulate temporally varying turbulence. A single “phase screen” is given a random velocity vector between 0 and .55 meters per second to simulate temporally varying turbulence. The system is used to distort optical beams as if the beams had propagated through a long pathlength of wavefront distorting medium, such as the atmosphere.
0544: Electrical engineering