MATTHEW E. GODA, ANNA M. LINNENBERGER, JASON D. SCHMIDT, AND STEVE A. SERATI
X1 in X2 X3 X4 X1 in X2 in X3 in X4 Transparent out out out out Image electrode pixels Liquid
Cover glass Cover glass crystal
Common electrode
(0 volts)
Liquid crystal
Reflective pixels 0 volts 2. 5 volts 0 volts 5 volts Pin-grid-array package
VLSI die
VLSI die
LFIGURE 1. A cross-sectional view of a liquid-crystal-on-silicon spatial light modulator illustrates how the liquid-crystal layer causes a voltage-dependent phase shift iquid-crystal spatial upon entering light. Each pixel is fully programmable, offering a high-resolution pure phase-modulating array. light modulators are amazingly versatile devices with many ap- adversely affect many types of optical systems such as plications in optics. Their decreasing cost is those for optical communications and imaging as well as making them more widely for laser-based weaponry.
available to researchers. In recent years, The versatility of liquid-crystal During the last 100
students at the Air Force Institute of Tech- years, modeling the ef-
nology (AFIT) have been using spatial light spatial light modulators have fects of turbulence on
modulators (SLMs) to study atmospheric optical propagation has
turbulence—a problem that profoundly made them an important received much attention
impacts military laser systems. The large tool for research aimed at and a focus on statistical number of independently controllable el- modeling has produced
ements in SLMs has allowed students to emulation of atmospheric several useful theories.
study turbulence in ways that were previ- Statistical analyses are
ously impossible. turbulence and the mitigation necessary because it is
impossible to exactly de-
Atmospheric turbulence theory of its effects using advanced scribe the refractive in-
Turbulence in Earth’s atmosphere is caused dex for all positions in
by random variations in temperature and wavefront control. space and for all time—
pressure that alter the refractive index of there are too many ran-
air, both spatially and temporally. As optical waves propa- dom behaviors and variables to account for in a closed-
gate through the atmosphere, the waves are distorted by form solution. The most widely accepted theory, due to its
these refractive-index fluctuations. Such distortions can consistent agreement with observation, was first put for-
ward by A.N. Kolmogorov in 1941.1 His theory is the basis
MAT THE W E. GODA is an assistant professor in the Dept. of Electrical for all contemporary theories of turbulence. and Computer Engineering, Air Force Institute of Technology, Wright-
Patterson Air Force Base, Ohio; ANNA M. LINNENBERGER is a computer engineer and STEVE A. SERATI is president and director of R&D Impact on military laser systems at Boulder Nonlinear Systems, Lafayette, CO; and JASON D. SCHMIDT is a graduate student in the Electro-Optics Program at the University If the effects of atmospheric turbulence are not compen- of Dayton, Dayton, OH; e-mail matthew. goda@afit.edu. sated for, they would be catastrophic for many applications,
References:
Archives