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`

`
`TECHNICAL PAPERS
`
`= r
`
`Solid-F~State Sensor,
`Actuator and
`
`"‘
`
`Microsystems
`Workshop
`
`2004
`
`Hilton Head Island, South Carolina
`June 6-10
`
`xxiii
`
`

`
`This material may be protected by Copyright law (Title 17 U.S. Code)
`
`

`
`
`
`urusovnlnoflod
`run-way
`— ‘I. Ieuahdawn
`infilc
`lnhltirflly
`Ililblo
`
`
`
`Figure 4. Cantral ofelectrostatic mirrors through snapdown using
`opticalfeedback on the angular position [15, 16].
`
`for low
`Another area of technology development critical
`insertion loss optical MEMS devices is the accurate control of
`mirror and etalon) curvature. Some of these efforts are widely
`available as improved tolerances and process yields from MEMS
`foundries (e.g., minors with >1 in radius of curvature,
`l0 - 100x
`improved over
`initial
`efforts}. Also, basic work on thermal
`dependence of mirror curvatures has also been conducted at public
`universities including the University of Minnesota [17].
`
`TECHNOLOGY RE-DEPLOYMENT: AN EXAMPLE
`
`The basic technology development will have far-ranging
`impact on applications that have nothing to do with optical fiber
`networks. One example of this re—use is an on-going research
`project at UCSD involving the modification of a telecom-derived
`MEMS device for free-space (mobile) communications.
`Fast, electrically-controlled optical
`fiber attenuators were
`needed
`for
`transient
`suppression
`in
`long-haul
`fiber
`communications. In 1996, a Bell Labs research project involved a
`novel MEMS wavefront modulator for this application [19]. A
`membrane reflector was fon-ned over a hexagonal array of cavities
`so that electrostatic force could deform the normally flat reflector
`
`into a shape like the surface of a golf ball. A collimated beam
`illuminating the modulator is efficiently tnupled into the output
`fiber, but when voltage is applied. a controlled fraction of light is
`aberrated away from the optical fiber core and discarded. The
`primary advantage of this device was that the mechanical response
`time was determined not by the overall optical aperture, but by the
`diameter of the individual membrane aperture.
`This
`attenuator was successfully demonstrated using a
`variation on MARS etalon optical modulator process [23, where a
`silicon nitride membrane is held over the silicon substrate by a
`
`phosphor-silicate glass sacrificial layer. The only process change
`needed was to coat a uniform layer of gold over the top of the
`device. This device concept was independently conceived and
`demonstrated at the University of Delft in 1999 [20].
`The new application of this device involved orner-cube
`retroreflectors (CCR), which are basically three mirrors are right
`angles to each other to fonn a hollow cube that faithfully reflects an
`incident optical beam towards it's point of origin. CCRS are used
`
`arc .lDSU's latching 2x2 switch, which is based on a SOI comb-
`drive actuator [l l] and DiCon 1x2 switch with variable attenuation,
`which is based on an analog electrostatic tilt mirror. The DiCon
`switch pictured in Figure 1 has an insertion loss of 0.8 dB,
`switches in under 5 ms, requires only 15 V drive, and is guaranteed
`to operate over 100 million cycles [12].
`Optical MEMS companies started during the boom have also
`survived to offer high-performance products to the slowly
`recovering component market. Beam-steering optical
`cross-
`connects based on two-dimensional arrays of analog tilt mirrors are
`available from Glirnmerglass, which provides an 64x64 switch, and
`Calient Networks, which makes up to 256x256 switches [13].
`Chromux offers 1x2 protection switch arrays, as well as scanning
`Fabry-Perot wavelength monitors.
`Another
`important class of optical MEMS components
`
`integrates diffraction gatings to perform spectral demultiplexing
`onto the MEMS device to enable wavelengthseiective filtering and
`switching. Dynamic spectral equalizers and wavelength-blocking
`switches are offered by LightConnect and Polychromix, while more
`general wavelength switching [4,
`14]
`is offered by Capella
`Photonics. Figure 3 shows a representative sample.
`
`
`
`Figure 3. Examples ofcurrently available telecom products.
`clockwise from upper left: Lt'gh!Connect variable attenuator and
`dynamic gain equalizer. Glimmerglass 64x64‘ switch. Capella
`Phoronics wavelength selective switch. Polycltromix wavelength
`
`blocker, Chromto: protection switch array and wavelength monitor
`
`OPTICAL MEMS DEVICE PROGRES S
`
`These basic functions have been demonstrated with various
`
`device structures, some of which were never publicized and which
`remain ‘trade secrets’ of technology holding companies. Device
`packaging and processing techniques are particularly diffictilt to
`obtain. However, some of the device technology has since been
`published in the open literature.
`One good example is
`the feedback position control of
`electrostatic
`tilt mirrors used by Tellium in their modular
`crossconnect system [15]. Normally such mirrors are operated in a
`safe region, roughly half the full angular range, to avoid catastrophic
`snapdown as the voltage required to maintain a given angle quickly
`decreases. Tellium used a nonlinear controller with a novel
`
`technique for torque-to-voltage conversion [16] in combination
`with classic linear controller techniques with optical
`full-state
`feedback, state estimator, and reference input with fe-ed~forward.
`They
`achieved stable angular positioning accuracy of €120
`microradians over the full +;’- 8° mechanical range of the mirrors,
`right up to the point of edge touchdown (Figure 4).
`
`

`
`for example in a
`signals, as
`to self-align free-space optical
`surveyor‘s rangefinder. A retro modulator, a retroreflector with an
`
`electrically-controlled reflectivity, can communicate data signals
`back to a laser source without needing to align a separate laser
`transmitter at the remote node.
`
`Retro modulators can block the beam (amplitude modulation),
`but it is equally effective to phase—modt1late the back—propagating
`wavefront so that the return signal is dispersed and does not arrive
`at the remote detector. A MEMS retro modulator demonstrated by
`UC Berkeley [18] for their ‘Smart Dust“ project used two fixed
`micro-mirrors assembled over a tilting MEMS mirror, so that the
`CCR angle could be switched away from 90°. The resulting device
`had a 1 mm aperture and a 2 kHz response. However, our goal was
`fabricate a retro modulator with a substantially larger aperture (10
`to
`25 mm)
`suitable
`for
`long-range
`{multi-km)
`optical
`communications, and operate the device at much higher speeds
`(>100 KHZ) than possible moving a single large mirror. The Bell
`Labs attenuator device was ideal for this application.
`
`
`
`Figure 5. Simulation (top) and experimental re.ttn'tsfi'om the
`membrane nrodalator usedforfree-space optical communications.
`
`A theoretical calculation of the wavefront propagating from a
`membrane modulator with 1 mm pixels with a 100 cm radius of
`curvature is shown at the top of Figure 5 More than 20 dB
`contrast
`is achieved over a wide range of angles, distances, and
`operating wavelengths.
`'lhe lower three images are preliminary
`experimental
`results from a fabricated device. They show the
`optical far—fie1d signal reflected from an undeflectetl modulator, the
`surface profile from the actuated device, and the dispersed signal
`reflected from the actuated modulator.
`
`CONCLUSION
`
`The telecom boom yielded a wealth of optical MEMS
`technology which, in addition to serving it’s original purpose of
`telecom components,
`is available for new applications in sensing
`and communications. The next wave of research and development-
`hopefully less turbulent than the last — has already begun.
`
`ACKNOWLEDGEMENTS
`
`Research
`
`at UCSD's
`conducted
`supported by a grant
`Integration Laboratory is
`Corporation. The experimental results shown are
`graduate student researcher Trevor Chan.
`
`Photonics
`
`Systems
`from Cubic
`the work of
`
`REFERENCES
`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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`
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