MEMS micro-mirror technology offers the opportunity to replace larger optical actuators with smaller, faster ones for
lidar, network switching, and other beam steering applications. Recent developments in modeling and simulation of
MEMS two-axis (tip-tilt) mirrors have resulted in closed-form solutions that are expressed in terms of physical, electrical
and environmental parameters related to the MEMS device. The closed-form analytical expressions enable dynamic
time-domain simulations without excessive computational overhead and are referred to as the Micro-mirror Pointing
Model (MPM). Additionally, these first-principle models have been experimentally validated with in-situ static,
dynamic, and stochastic measurements illustrating their reliability. These models have assumed that the mirror has a
rectangular shape. Because the corners can limit the dynamic operation of a rectangular mirror, it is desirable to shape
the mirror, e.g., mitering the corners. Presented in this paper is the formulation of a generalized electrostatic micromirror
(GEM) model with an arbitrary convex piecewise linear shape that is readily implemented in MATLAB and
SIMULINK for steady-state and dynamic simulations. Additionally, such a model permits an arbitrary shaped mirror to
be approximated as a series of linearly tapered segments. Previously, "effective area" arguments were used to model a
non-rectangular shaped mirror with an equivalent rectangular one. The GEM model shows the limitations of this
approach and provides a pre-fabrication tool for designing mirror shapes.
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.
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.
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.
The availability of recently developed microelectromechanical system (MEMS) micro-mirror technology provides an opportunity to replace macroscale actuators for free-space laser beam steering in light detection and ranging and communication systems. Precision modeling of mirror pointing and its dynamics are critical to the design and control of MEMS beam steerers. Beginning with Hornbeck's torque approach, we present a first-principles, analytically closed-form torque model for an electrostatically actuated two-axis (tip-tilt) MEMS mirror structure. The torque expression is a function of the mirror's physical parameters, such as angles, voltages, and size. An Euler dynamic equation formulation describes the gimballed motion as a pair of damped harmonic oscillators with a coupled torsion function. Static physical parameters such as MEMS mirror dimensions and voltages are inputs to the model as well as dynamic harmonic oscillator parameters, such as damping and restoring constants, which are calculated or fitted to measurements. A Taylor series expansion of the torque function provides insights into MEMS behavior, including operational sensitivities near "pull-in." MATLAB and SIMULINK simulations illustrate performance sensitivities, controllability, physical limitations, and other important considerations in the design of precise pointing systems. Commercial-off-the-shelf micromirror measurements confirm the model's validity in steady state and dynamic scanning operations.
The availability of recently developed MEMS micro-mirror technology provides an opportunity to replace macro-scale
actuators for free-space laser beamsteering in lidar and communication systems. Such an approach is under
investigation at the Johns Hopkins University Applied Physics Laboratory for use on space-based platforms.
Precision modeling of mirror pointing and its dynamics are critical to optimal design and control of MEMS
beamsteerers. Beginning with Hornbeck's torque approach, this paper presents a first-principle, analytically
closed-form torque model for an electro-statically actuated two-axis (tip-tilt) MEMS structure. An Euler dynamic
equation formulation describes the gimbaled motion as a coupled pair of damped harmonic oscillators with a
common forcing function. Static physical parameters such as MEMS mirror dimensions, facet mass, and height
are inputs to the model as well as dynamic harmonic oscillator parameters such as damping and restoring
constants fitted from measurements. A Taylor series expansion of the torque function provides valuable insights
into basic one dimensional as well as two dimensional MEMS behavior, including operational sensitivities near
"pull-in." The model also permits the natural inclusion and analysis of pointing noise sources such as electrical
drive noise, platform vibration, and molecular Brownian motion. MATLAB and SIMULINK simulations illustrate
performance sensitivities, controllability, and physical limitations, important considerations in the design of
optimal pointing systems.
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.