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170 MEMS and Microstructures in Aerospace Applications

mirror-shape stability and fabrication tolerances are of key concern to a system
designer. To this end preliminary MEMX devices were evaluated in terms of
angular jitter, focal spot stability, and open and closed-loop response versus laser
transmitter power at both ambient air and lower partial pressures. The applicability
and scalability of this technology to multiaccess terminals was also considered and
appears to be readily transferable to a space-qualified design. For most spacecraft
platforms micromirrors should be compatible with direct body-mounting because of
their high intrinsic bandwidth and controllable damping. (Being able to body-mount
these devices is highly desirable to take advantage of their low mass, which implies
spacecraft attitude control would be used for overall coarse pointing.) Importantly,
these optical beamsteerers are highly miniaturized, very lightweight, require very
little prime electrical low power, and are scalable to 2-D multichannel (point-to-
multi-point) links.
Initially a key concern about the MEMS micromirror performance in a space
environment was the effect of partial vacuum on heat dissipation from the trans-
mitting laser beam and on the degree of mechanical damping of the mirror. It is
important that the beamsteering controller be critically damped under suitable
partial or full atmospheric vapor pressure. In addition, a trade-off between
the optical power required to support the link and the degree of thermal heat
loading experienced by the mirror elements under pulsed laser light must also be
determined. Furthermore, any micromirror curvature change induced by laser heat-
ing must be avoided. To this end preliminary optical, dynamic, and thermal
measurements of the MEMX micromirrors were made using the optical test bed
shown in Figure 8.13.
Using experimental measurements, physical optics modeling, and computer-
based ray tracing, the laser beam quality reflected off a micromirror was evaluated.
This included observing the beam waist, beam shape, and beam jitter. A quad cell
detector and CCD focal plane array were used as diagnostic sensors in conjunction
with the setup described in Figure 8.13, which included a vacuum chamber. The
laser spot (with a minor axis of approximately 300 mm) is shown on the micromirror
as well as at the CCD output focal plane in their respective insets. One concern was
how much would the radius of curvature of the micromirror vary under light flux,
but this was not initially evaluated because previous work had shown that a limit of
about 300 mW would be sufficient to support projected link margins (even from
GEO). The other concern, apart from beam jitter, is beam quality, which turned out
to be poor because of an artifact of mirror fabrication, that resulted in etch pits in the
mirror surface causing a diffraction pattern in the focal plane, rather than a nominal
Gaussian spot, as shown in Figure 8.13 inset. This can be readily corrected in flat,
smooth mirror designs specific to the application and through spatial filtering.
Significant degradation, however, of the far-field beam should not be a real concern
if the mirror is redesigned.
Micromirror frequency response measurements were made to establish basic
dynamic performance in ambient air, angle sensitivity to deflection voltage, and
dynamic response at lower pressures. The MEMX mirrors had very good frequency
response, out to almost 1 kHz (or more), as indicated in Figure 8.14(a), which is

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