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166 MEMS and Microstructures in Aerospace Applications

Either electrostatic or electromagnetic drive mechanisms can be used to move
the mirrors, but electrostatic is preferred, since it takes up little room and needs
relatively low power. Large switch sizes, using relatively large mirrors (approxi-
mately few millimeters) with long focal lengths (tens of centimeters) are desired to
allow the use of larger light beams, which have less beam divergence and greater
useful relay distances. Larger angular deflections are also desirable.

8.6.2 PERFORMANCE REQUIREMENTS
Recent collaborative work between MEMX Corporation and the Johns Hopkins
University Applied Physics Laboratory (JHU/APL) 107 has focused on developing
MEMS micromirror technology for free-space multiaccess optical communications
between spacecraft. Key performance issues addressed in this effort for space-based
optical communications include micromirror heating due to input laser power,
achievable degree of mechanical damping at ambient and partial air pressures,
micromirror flatness, element size, angular field-of-regard (FOR), control-loop
bandwidth, and open-loop transfer function shape. For some parameters, these
devices already meet the ‘‘desired’’ application requirements, and for all cases,
experimental tests indicate that the application requirements can be met with some
redesign of existing devices. For example, the MEMX devices measured angular
field-of-regard (FOR) was approximately +7.98 optical, but would need to be +128
for a projected redesign for GEO-to-ground links, which is quite feasible with
slight micromirror redesign. Measured angular resolution was less than 360 mrad
(desired greater than 1000 mrad); bandwidth was approximately 1 kHz (desired 100
to 1000 Hz); and mirror radius of curvature was approximately 0.4 m (nominally
0.5 m approximately). These and other key device parameters (and their desired
range of values) are given in Table 8.1.

TABLE 8.1
Device Parameters of MEMX Micromirrors
Parameter Nominal Value

Angular field-of-regard +128 (+210 Vmrad) a
Angular resolution 1 mrad a
Closed-loop bandwidth 100 to 1000 Hz
Number of elements 4 4 (minimum)
Element size 0.5 mm
Element pitch ~2 mm
Element radius of curvature ~0.5 m
Angle or voltage scale factor 10 mrad/mV
a
Before beam expansion.

© 2006 by Taylor & Francis Group, LLC