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41 Zamkotsian, F et al., MEMS-based slit generator for NGST-NIRMOS: modeling and characterization,Proceedings of SPIE 4850, 527, 2002
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48 Reimann, G et al., Compact adaptive optical compensation systems using continuous silicon deformable mirrors,Proceedings of SPIE 4493, 35, 2002
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69 Saint-Pe, O et al., Study of an uncooled focal plane array for thermal observation of the Earth,Proceedings of SPIE 3436, 593, 1998
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Microtechnologies for Science Instrumentation Applications 147
Trang 58 Microelectromechanical Systems for Spacecraft
Communications
Bradley Gilbert Boone and Samara Firebaugh
CONTENTS
8.1 Introduction 150
8.2 MEMS RF Switches for Spacecraft Communications Systems 150
8.2.1 MEMS Switch Design and Fabrication 151
8.2.1.1 Switch Configuration 151
8.2.1.2 Contacting Modes 153
8.2.1.3 Actuation Mechanism 154
8.2.1.4 Geometric Design 155
8.2.1.5 Fabrication Methods and Materials 155
8.2.2 RF MEMS Switch Performance and Reliability 156
8.2.2.1 Figures of Merit 156
8.2.2.2 Example Performance 157
8.2.2.3 Failure Modes 157
8.3 MEMS RF Phase Shifters 158
8.3.1 Switched-Line Phase Shifters 158
8.3.2 Loaded-Line Phase Shifters 159
8.3.3 Reflection Phase Shifters 159
8.4 Other RF MEMS Devices 161
8.5 RF MEMS in Antenna Designs 161
8.5.1 Electrically Steered Antennas 161
8.5.2 Fractal Antennas 162
8.6 MEMS Mirrors for Free-Space Optical Communication 163
8.6.1 Fabrication Issues 164
8.6.2 Performance Requirements 166
8.6.3 Performance Testing for Optical Beamsteering 168
8.7 Applications of MEMS to Spacecraft Optical Communications 169
8.7.1 Optical Beam Steering 169
8.7.2 Recent Progress 173
8.8 Conclusion 176
References 176
Trang 6configuration, the conducting bar sits between the signal line and ground The on state
is when the conducting bar is up, so that the signal can pass unimpeded
Researchers have pursued switches in series configurations15,16,26–30and shunt configurations.10,11,17,18 In series-configured switches, the insertion loss is deter-mined by the impedance of the switch in its closed state, which in turn depends on the intimacy of the contact achieved by the switch The isolation is set by the capacitance between the conducting bar and the signal line in the off state Series switches can be implemented with both microstrip and coplanar waveguide transmission lines.15,31–33 Figure 8.2 shows a series switch developed at RSC
In a shunt switch, the insertion loss is the result of any impedance mismatch that occurs because of the unactuated mechanical structure (with careful calculations, the unactuated switch can be sized to match the characteristic impedance of the line), and the isolation depends on ratio between the capacitance in the ‘‘down’’ state and the capacitance in the ‘‘up’’ state Shunt switches are only easily imple-mented with coplanar waveguide transmission lines.10,17Figure 8.3shows a scan-ning electron micrograph of a shunt switch
The impedance of a capacitor decreases with frequency Therefore, the isolation of
a series switch diminishes with frequency, while in a shunt switch that relies on a
V control
V control
Series Configuration
Shunt Configuration
FIGURE 8.1 Different configurations for microwave switches
Cross section through bridge Biased - ON
Unbiased - OFF Spring
Drive capacitor
RF line
RF line
Anchor Contact
shunt
FIGURE 8.2 Structure and operation of a MEM series switch developed by the Rockwell Science Center (Courtesy of the Rockwell Science Center and from Mihailovich, R E., et al.)
Trang 7element, easily implemented in microstrip, which separates the input signal into two
signals that are 908 out of phase The two switches are tied together If the switches
are closed, the signal is reflected back into the quadrature hybrid, where the two reflected waves will add constructively at one port and destructively at another port
If the switches are open, a total phase shift of Df will be added to the signal If the switches are perfectly matched and lossless, and the quadrature hybrid is lossless, these phase shifters should have little insertion loss Like the switched-line phase shifter, several bits with a binary sequence of phase delays can be combined for digital phase control
In a MEMS implementation of a reflection phase shifter, MEMS switches control the reflection stub length There are fewer MEM reflection phase shifters
FIGURE 8.5 Photograph of a 2-bit switched-line phase shifter developed by the University
of Michigan and Rockwell Scientific (Courtesy of Rockwell Scientific Company.)
Signal out Signal in
Trang 8deposition by low-pressure chemical vapor deposition (LPCVD), followed by patterning and etching, to create the desired structures on the silicon substrate Significant progress has been made in manufacturing commercial-quality mirrors using these methods
Stress-free optical thin film surfaces are critical for optical networking as well
as free-space beamsteering applications, but film stress is difficult to control in the fabrication process It can vary dramatically with a relatively small change in the number of atoms, and hence, the film’s chemical composition As a consequence, it
is difficult to make polysilicon mirrors very flat, particularly if they need to be relatively large (~few millimeters) After a surface is initially deposited and all the supporting layers are removed, it may not remain flat Even thin gold over-coatings can cause substantial deformation of an uncoated plate
Bulk micromachining is used to form MEMS microstructures by either wet or dry anisotropic etching In this case silicon on insulator (SOI) wafers are useful, especially in separating moving parts from the bulk silicon structure, and this was determined early When a plate-type structure is freed in the fabrication process, a mirror can be produced on either side, with that surface in contact with the oxide often being superior in terms of scattering properties The availability of both sides allows the deposition of perfectly stress-balanced gold reflection layers for en-hanced reflectivity, which makes manufacturing easier and more predictable Leading candidates for optical switches and cross-connects are free-space micromirror switch arrays, and a scheme to do this using conventional scanning mirrors was first proposed as early as 1982.80Arrays of collimators are positioned such that light from each collimator is directed toward a dual-gimbaled mirror The first mirror reflects the beam toward a corresponding mirror in the opposing array The latter mirrors adjust their angles to send their respective beams to each receiving fiber Light from each fiber can only be directed toward its corre-sponding mirror at a given instant Likewise, the receiving mirror can only send light
to its associated fiber, but both mirror arrays can be virtually infinitesimally adjusted,
so that any mirror that receives a beam can send it to any of the opposing mirrors, thereby making fully free connections The supporting parts of each mirror, such
as the hinges and drive structures, are kept small to maximize mirror area fill factors For low-loss transmission the mirrors must be very flat, with flatness better than one fifth the operating wavelength Mirrors with gold coatings can have reflectiv-ities over 98%, and mirror arrays can be several square millimeters in size, with square or rectangular aspect ratios Fiber-to-fiber losses through the cross-connect can be as low as 0.7 dB, and mirrors have been exercised over 60 billion cycles without any failures Cross-coupling between the various channels also turns out to
be negligible because even a small amount of angular offset between the input and output mirrors will cause a significant displacement of the inappropriate beam
at a given output fiber entrance Small-scale cross-connects with fewer optical switches have switching times as low as 50 ms or less, although larger N N switches, configured into 2-D crossbar arrays, have switching times on the order of
500 ms
Microelectromechanical Systems for Spacecraft Communications 165
Trang 9The two most basic requirements, FOR and angular accuracy, depend upon the required link range and terminal separation on the ground, as illustrated in Figure 8.11 For instance, for optical communication terminals down-linking to earth from GEO, beam widths on the order of 5 to 10 mrad are desired to support the link with reasonable laser transmitter powers (at hundreds of milliwatts), but their steered angular coverage will be limited to angles set by the dynamic limits of the MEMS mirrors and the optical transmitter beam expander design (assuming coarse steering via spacecraft attitude control) The laser beam reflecting from a given micromirror, however, must be significantly expanded to set the desired output (diffraction-limited) beamwidth to meet link margin requirements through the optical ‘‘antenna gain.’’ The mirrors need
to be physically steered to a greater angle than the output optical beam, given by the beam expansion ratio For example, a beam expansion ratio of 250 increases the transmitter beam waist (which is assumed to be 0.5 mm at the micromirror) up to 12.5
cm, which yields a diffraction-limited beamwidth of approximately 8 mrad Assuming
FOR
Coverage footprint
Ground
terminals
Multiple or sequential
beam positions
GEO S/C
Beam jitter
Beamwidth
FIGURE 8.11 GEO-to-ground scenario for applicability of MEMS micromirrors to multi-channel optical communications The same terminal could support intersatellite links