' Technical Digest
`
`Solidart-‘rotate Sensor,
`Actuator and
`r Microsystems
`Workshop
`
`Sponsored by the
`Transducers Research Foundation, Inc.
`
`2004
`
`Hilton Head Island, South Carolina
`June 6-10
`
`Additional support provided by the
`Defense Advanced Research Projects Agency
`
`1
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`Library of Congress Number 2004105683
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`2
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`
`TECHNICAL PAPERS
`
`Soliti-‘State Sensor,
`Actuator and
`
`Microsystems
`Workshop
`
`2004
`
`Hilton Head Island, South Carolina
`June 6-10
`
`3
`
`

`
`OPTICAL MEMS: LEGACY OF THE TELECOM BOOM
`
`Joseph E. Ford
`Electrical & Computer Engineering, University of California San Diego
`La Jolla, CA 92093-0407
`
`ABSTRACT
`
`From 1997 to 2002 tens of billions of dollars were invested in
`
`creating
`telecommunications,
`optical MEMS technology for
`products just in time for a nearly complete market collapse. With
`the subsequent closure of R&D efforts and many entire companies,
`has this new technology been lost? In fact, many MEMS-based
`components survived the collapse and are selling into a slowly
`recovering market. More importantly, the technology developed is
`available for new applications in communications and other areas.
`In this presentation I will
`review optical MEMS technology
`developed, provide a status report on surviving components, and
`describe one example of how a device created for a specific
`application (a broad wavelength variable attenuator) has been
`modified to a completely different application in free—space optical
`communications (a retro»reflecting data modulator).
`
`OPTICAL MEMS AND TELECOM
`
`Any optical effect demonstrated with bulk components can be
`implemented, typically on a reduced physical scale, using optical
`MEMS fabrication. These basic functions are illustrated in Figure
`1, which shows the four major categories of effects: movable first
`surface mirrors (analog or digital tilt mirrors on one or two axis),
`movable
`absorbing or
`refractive
`surfaces,
`tunable dielectric
`multilayer
`etalons
`{spectral
`phase
`and
`amplitude
`filters),
`translation stages (moving waveguides and shutters), as well as
`diffi-active structures (variable phase delay or grating pitch).
`
` 9
`
`
`Lll.£e&cJe
`(analog tilt mirror shown)
`
`no rive
`2 Absor tlv /
`
`(digital slmfrer shown)
`
`greatest driver for optical MEMS technology in terms of total
`investment has been telecommunications.
`
`telecom
`intended for
`Early optical MEMS components
`applications were viewed with considerable skepticism;
`the
`technology was considered unproven and far off the consensus
`path of waveguide integrated optics. A reflective data modulator
`demonstrated in 1994 for fiber-to-the-home networks {2} may have
`been the first practical MEM S device for single mode fiber. This
`device ‘was not developed into a successful product, bit it was
`followed by a series of optical MEMS switches and filters for
`wavelength-multiplexed signals which demonstrated the broad
`applicability
`of optical MEMS components
`for
`optical
`communications. References [3] — [9] provide some examples.
`By 1999, with increasing pressure to meet market demand for
`high-capacity networks, well-funded optical MEMS component
`efforts were under way at established and start-up companies.
`Many applications could be addressed with a range of technologies,
`but
`the large port-count provisioning switch {e.g.,
`the Lticent
`“Lambdakouter” [l0]) was a ‘must-have’
`telecommunications
`component that simply could not be constructed with any other
`available technology. Optical MEMS was perceived as critical-path
`technology for high-capacity optical networks.
`Demand for communications bandwidth had been greatly
`overestimated. The market peaked in mid 2000, and by 2002
`technology investment was substantially over. Relatively few of
`the companies founded during the investment boom (by some
`estimates more than 3000 in total) survive to date. But though this
`process, a wide range of optical MEMS components were taken
`from laboratory demonstrations to fully packaged components,
`proven to meet critical Telcordia reliability tests, and built into
`fielded systems carrying live communications traffic. Despite the
`considerable disruption, a wide range of MEMS-based components
`are still commercially available, and have make the transition to a
`“bread and butter" technology.
`
`C|:l::F-i:Fl:iD l:'i:l3:I:i:t:|
`
`I3
`rrrn
`
`(3! Interleromelric
`{MARS modulator shown)
`
`(ii Qiflroctive
`(grating light valve shown)
`
`Figure I. Fundamentally dlfierent types ofoptical MEMS devices.
`
`The initial driving application for Optical MEMS devices was
`Texas
`Instruments’ well-known DMD (digital mirror device)
`projection display,
`a 2D array of digital
`tilting microtnirrors
`fabricated on a silicon VLSI drive circuit [1]. DMD displays are
`becoming the dominant
`technology for commercial and even
`consumer projectors, and optical MEMS devices have now been
`developed
`for
`applications
`including remote pressure
`and
`acceleration sensors, biotechnology, and adaptive optics. Still, the
`
`Figure 2. A simple IX2 protect-ion switch is one example oftlie
`optical MEMSprc.-ducts now commonplace in telecom systems.
`
`TELECOM ‘SURVIVORS’
`
`indication of this acceptance is the use of
`The single best
`MEMS devices for protection switches, which are 1x2 and 2x2
`switches used to direct traffic around faulty transmission systems,
`as for example due to a line cut. MEMS switches were shown to be
`more reliable than the electromagnetically-actuated (bulk optic
`component) opto-mechanical switches previously used, in addition
`to being smaller and more power efficient. Two current products
`
`0-9640024-5-0
`
`Solid—Sta1e Sensor. Actuator and Microsystcms Workshop
`Hilton Head Island. South Carolina. June 6—|D. 2034
`
`4
`
`

`
`are IDSU‘s latching 2x2 switch, which is based on a SOI comb-
`drive actuator [1 1] 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-perforrnance products to the slowly
`recovering component market. Beam-steering optical
`cross-
`connects based on two-dimensional arrays of analog tilt mirrors are
`available from Glimmerglass, which provides an 64x64 switch, and
`Calient Networks, which makes up to 256x256 switches [13].
`Chromux offers lx2 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 wavelength-selective 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.
`917*
`
`
`
`Figure 3. Exampla ofcurrently available telecom products,
`cloclcwisefi-om upper left: LightCotmect variable attenuator and
`dynamic gain equalizer, Glimmerglass 64x64 switch, Capella
`Photonics wavelength selective switcli, Polycltromix wavelength
`blocker, Cltrotmrx 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 difficult 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 feed-forward.
`They achieved stable angular positioning accuracy of <120
`microradians over the full +t- 3° mechanical range of the mirrors,
`right up to the point of edge touchdown (Figune 4).
`
` ~ ‘zi Itflilehdflwfl
`
`IfI'II
`
`Figure 4. Control ofelectrostatic mirrors through snnpdown using
`opticalfeedback on the angular position [15, I6}.
`
`for low
`Another area of technology development critical
`insertion loss optical M]-EMS 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., mirrors with >1 in radius of curvature, 10 -100);
`improved over
`efforts). Also, basic work on thermal
`dependence of mirror curvatures has also been conducted at public
`universities including the University of Minnesota [17].
`
`'I'ECHN01JOGY 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
`MIEMS 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 fomied 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 coupled into the output
`fiber, but when voltage '5 applied, a controlled fraction of light is
`abet-rated 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
`membrane aperture.
`This attenuator was
`successfully demonstrated using a
`variation on MARS etalon optical modulator process [2], 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 unifonn layer of gold over the top of the
`device. This device concept was independently conceived and
`demonstrated at the University of Delfi in 1999 [20].
`The new application of this device involved omer-cube
`retroreflectors (CCR), which are basically three mirrors are right
`angles to each other to form a hollow cube that faithfully reflects an
`incident optical beam towards it’s point of origin. CCRS are used
`
`5
`
`

`
`to self-align free-space optical signals, as for example in a
`sin-veyor’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 efiective to phase-modulate 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 [13] for their ‘Smart Dust” project used two fixed
`micro-mirrors assembled over a tilting MEMS minor, so that the
`CCR angle could be switched away from 90°. The resulting device
`had a 1 mm aperture and a 2 1:112 response. However, our goal was
`fabricate a ret1'o modulator with a substantially larger aper:ture.(l0
`to
`25 mm)
`suitable
`for
`long-range
`(multi-ltnr)
`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 r~e.s-ultsfi-om the
`membrane modulator usedforflee-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.
`'1he lower three images are preliminary
`experimental results from a fabricated device. They show the
`optical far-field signal reflected from an undeflected 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.
`
`AGm0
`
`Systems
`Photonics
`at UCSD's
`conducted
`Research
`Integration Laboratory is supported by a grant
`from Cubic
`Corporation. The experimental results shown are
`the work of
`graduate student researcher Trevor Chan.
`
`
`
`REFERENCES
`
`1. P. Van Kessel et al, “A MEMS-based projection display,” IEEE
`Proceedings 86(8), pp. 1637-1704 (1998).
`2. l(.W. Goossen, J. A. Walker, S. C. Arney, “Silicon modulator
`based on mechanically-active anti-reflection layer with 1 mbittsec
`capability for fiber-in-the-loop applications,” IEEE Photonics
`Technology Letters 6(9), pp. 1119 — 1121 (1994).
`
`3. M. Wu, E. Vail, G. Li, and C. Chang—Hasna.in, Widely and
`continuously tunable micromachined resonant cavity detector with
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`4. L. Lin, B. Goldsteirr, R. Tkach, “Free-space micromachined
`optical switches with sub-ms switching time for large-scale optical
`crossconnects," IEEE Phot. Tech. Letters 10 pp. 525-527 (1998).
`
`4. J. Ford, J. Walker, “Dynamic spectral power equalization using
`micro-opto-mechanics,” IEEE Phat. Tech. Len. 10(l0), (1998).
`
`5. J. Ford et al, “Micromechanical fiber-optic attenuator with 3
`microsecond response,” IEEE J. Lightwove Tech. 16(9), (1998).
`6. P. Tayebati, P. Wang, D. Vakhshoori, and R. Sacks, “Widely
`tunable Fabry -Perot filter using Ga(Al)AslAl0x deformable
`mirrors” Optical Society ofAmerica Conference on Optical Fiber
`Communications 1993, pp. 22-27 (1998).
`
`7. J. Ford, J. Walker, V. Aksyuk and D. Bishop, “Wavelength
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`Tech. 17(5), pp. 904-911, (1999).
`
`8. C. Marxer and N. de Rooij, “Micro-opto-mech. 2 it 2 switch for
`single-mode fibers based on plasma-etched silicon nirror and
`electrostatic actuation” J’. Lightwave Tech. 17(1), pp. 2-6, (1999).
`
`9. P. Hagelin et al, “Scalable optical cross-connect switch using
`3i0c(r)p]r)11achined mirrors," IEEEPhat. Tech. Lett. 12(7), pp. 882-884
`10. D. T. Neilson et al, “Fully provisioned l12><ll2 micro-
`mechanical optical crossconnect with 35.8 This demonstrated
`capacity," OSA Conference on Optical Fiber Communications
`2000, postdeadline paper pp. 202 — 204, (2000).
`
`11. B. Hichwa et al, “A Unique latching 2x2 NEZMS Fiber Optics
`Switch
`IEEE International’ Conf on Optical MEALS‘, (2000).
`12. DiCon Fiber Optics product datasheet,
`'1'.’
`.diconfib
`'c . mi
`uc
`
`tin
`
`'
`
`13. J. Xuezhe et al, “Three-dimensional MEMS photonic cross-
`connect switch design and performance,” IEEE Journal ofSetected
`Topics in Quantum Electronics, 9(2), pp. 571-578 (2003).
`14. D. Marom et al, “Wavelength-selective 1x4 switch for 128
`WDM channels at 50 GHz spacing," Optical Society ofAmerica
`Conference an Optical Fiber Communications, Paper FB7, 2002
`
`15. J. Dadap et al, “Modular MEMS»based optical cross~connect
`with large port-count,” IEEE Photonics Technology Letters,
`15(12), pp. 1773 — 1775 (2003)..
`16. l. Brener er al, “Nonlinear servo control of MEMs mirrors and
`their performance in a large port-count optical switch,” OSA
`Conference an Optical Fiber Communications pp. 383-387 (2003).
`17. K. Cao, W. Liu and J. Talghader, “Curvature
`nsation in
`micromirrors with higr-reflectivity optical coatings,” Journal of
`MEMS 10(3), pp. 409-417 (2001).
`18. L. Zhou et al, “Corner-cube retroreflectors based on structure-
`assisted assembly for free-space optical communication, “ IBEE J.
`MEMS 12(3), pp. 233-242, 2003.
`
`19. J. Ford and J. Walker, “Technique for modulating optical
`signals in optical comrnunications,” United States Patent number
`5,796,880, filed November 1996.
`
`20. S. Sakarya, G. Vdovin and P. Sarto, “Micromachined SLM
`based on pixelated reflective membranes", SPIE Proc. 3760,
`November 1999.
`'
`
`6