C. L. Edwards

A simplified analytic CAD model for linearly tapered microstrip lines including losses

Quasi-TEM propagation, appropriate for a linearly tapered microstrip line is modeled, and known microstrip impedance behavior is approximated directly. The telegraphists equations and the resulting ABCD matrix are solved in terms of Airy functions. The theoretical model, which includes conductor and substrate losses, has been verified experimentally.

Adaptive compensation of atmospheric effects with a high-resolution micro-machined deformable mirror

We present experimental results for the adaptive compensation of atmospheric turbulence effects on a free-space laser communication links at near horizontal propagation paths over 2.5 km and 5 km lengths. A high-resolution micro-machined piston type mirror array (12x12 elements) and a fast beam steering mirror were used in an adaptive optics laser communication system based on the model-free stochastic parallel gradient descent (SPGD) optimization wavefront control technique. Control of the mirror was performed by a VLSI SPGD micro-controller. The experimental results demonstrate the improvement of the receiver performance (fiber coupling efficiency) on a summer day with a refractive index structure constant in the order of Cn2≈10-14 m-2/3.

A simplified analytic CAD model for linearly tapered microstrip lines

Quasi-TEM propagation, appropriate for a linearly tapered microstrip line (LTML), is modeled and known microstrip impedance behavior is approximated directly. The telegraphers equation and the resulting ABCD matrix is solved in terms of Airy functions, commonly available in scientific programming libraries. The theoretical model has been verified with measurements.

Free-space high data rate communications technologies for near terrestrial space

Recent progress at the Applied Physics Laboratory in high data rate communications technology development is described in this paper. System issues for developing and implementing high data rate downlinks from geosynchronous earth orbit to the ground, either for CONUS or in-theater users is considered. Technology is described that supports a viable dual-band multi-channel system concept. Modeling and simulation of micro-electro-mechanical systems (MEMS) beamsteering mirrors has been accomplished to evaluate the potential for this technology to support multi-channel optical links with pointing accuracies approaching 10 microradians. These models were validated experimentally down to levels in which Brownian motion was detected and characterized for single mirror devices only 500 microns across. This multi-channel beamsteering technology can be designed to address environmental compromises to free-space optical links, which derive from turbulence, clouds, as well as spacecraft vibration. Another technology concept is being pursued that is designed to mitigate the adverse effects of weather. It consists of a dual-band (RF/optical) antenna that is optimally designed in both bands simultaneously (e.g., Ku-band and near infrared). This technology would enable optical communications hardware to be seamlessly integrated with existing RF communications hardware on spacecraft platforms, while saving on mass and power, and improving overall system performance. These technology initiatives have been pursued principally because of potential sponsor interest in upgrading existing systems to accommodate quick data recovery and decision support, particularly for the warfighter in future conflicts where the exchange of large data sets such as high resolution imagery would have significant tactical benefits.

Free-space optical communication through a forest canopy

We model the effects of the leaves of mature broadleaf (deciduous) trees on air-to-ground free-space optical communication systems operating through the leaf canopy. The concept of leaf area index (LAI) is reviewed and related to a probabilistic model of foliage consisting of obscuring leaves randomly distributed throughout a treetop layer. Individual leaves are opaque. The expected fractional unobscured area statistic is derived as well as the variance around the expected value. Monte Carlo simulation results confirm the predictions of this probabilistic model. To verify the predictions of the statistical model experimentally, a passive optical technique has been used to make measurements of observed sky illumination in a mature broadleaf environment. The results of the measurements, as a function of zenith angle, provide strong evidence for the applicability of the model, and a single parameter fit to the data reinforces a natural connection to LAI. Specific simulations of signal-to-noise ratio degradation as a function of zenith angle in a specific ground-to-unmanned aerial vehicle communication situation have demonstrated the effect of obscuration on performance.

Design and analysis of dual-camera dual-band infrared imaging system

Emerging dual-camera dual-band (DCDB) infrared camera systems are playing an increasing role in temperature estimation and range measurement. This paper discusses the optimal design of a DCDB imaging system that makes use of contemporary filter fabrication technologies and improving detector performance. A two-color stereographic system allows for the temperatures of the objects to be measured without assuming a priori knowledge of emissivities, as well as providing a basis for estimating the distances to the objects. Multiple system design approaches are compared and key elements of the design trade space are described, including the selection of camera separation distance and specific infrared passbands. Analytical support for the methodology is provided by analyzing data from simulated infrared scenes. Finally, data from a laboratory-based DCDB system are analyzed and compared with model predictions.

First principles jitter characterization of two-axis MEMS mirrors

Recently developed MEMS micromirror technology provides an opportunity to replace macroscale actuators for laser beamsteering in lidar and free-space optical communication systems. Precision modeling of mirror pointing and its dynamics are critical to the design of MEMS beamsteerers. Beam jitter ultimately limits MEMS mirror pointing, with consequences for bit error rate and overall optical system performance. Sources of jitter are platform vibration, control voltage noise, and Brownian motion noise. This work relates the random jitter of the mirror facet to its originating sources via a multidimensional first-order Taylor expansion of a first-principles-derived analytic expression for the actuating torque. The input torque, consisting of deterministic and stochastic components, is related to the 2-D jitter through a pair of coupled damped harmonic oscillator differential equations. The linearized 2-D jitter model for the mirror is simulated using Matlab, while the full nonlinear torque model was simulated using Simulink. The work describes an experimental setup and methodology that is used to make precise micromirror measurements. Experimental measurements are in agreement with the jitter model, i.e., the linearized model is able to predict mirror facet jitter based on the measured power spectral densities for the sources of jitter.