An on/off free space optical switch is described in this paper. The device consists of intersecting cantilevers and is electrostatically actuated. A fiber is put onto the cantilevers and by controlling the position of the flber

through actuation, switching operation can be achieved. The device is also suitable for inpackage fiber alignment application, where the cantilevers can be used as passive or active fiber alignment microstructure

Cheah, Wei Leow

Mohammad, A .B.

English

Universiti Teknologi Malaysia Institutional Repository

ICSE2004 Proc. 2004, Kuala Lumpur, MalaysiaMEMS On/off and x2 Optical Switch with Integrated FiberAlignment CapabilityCheah Wei Leow, Student member, IEEE, A.B. MohammadFakulti Kejuruteraan ElektrikUniversiti Teknologi Malaysia81310 Skudai, Johor, Malaysia.Email: weiwei2@pd.jaring.my, bakar@fke.utm.myAbstract An on/off free space optical switch isdescribed in this paper. The device consists ofintersecting cantilevers and is electrostaticallyactuated. A fiber is put onto the cantileversand by controlling the position of the flberthrough actuation, switching operation can beachieved. The device is also suitable for in-package fiber alignment application, wherethe cantilevers can be used as passive or activefiber alignment microstructure.I. INTRODUCTIONOptical switches are a next generation networkelement to enable all optical switching. Alloptical switching has a lot of advantages overconventional switching utilizing optical-electronic-optical (OEO) switches and arediscussed in details in [1]. Optical switches areneeded due to the increasing importance of theircapabilities of setting alternate routes andeffective operation of a network. Opticalswitches work at a much wider window of datatransmissions rate than OEO switches and canhandle optical signals directly.In this paper, a new microelectromechanicalsystem (MEMS) based free space optical switchis reported. The switch is fabricated using theMulti User MEMS Processes (MUMPS), asurface micromachining process. It is a movingfiber based design which works by mechanicallyaligning the transmitting and receiving fibersbetween on and off state. Compared with otherpublished works on moving fiber based switches[2]-[7], which have voltage up to 100 V, theswitch in this paper operates at a much lowervoltage of 20 V. This significantly makes thedriving electronic circuitry simpler, cheaper andsafer. The switch also features an integrated fiberalignment capability for optimizing the assemblyof fiber onto the device. Conventionally, externalsubmount or silicon v-groove fiber array is usedin alignment of fiber into and out of the switch.Other features of the switch are low insertionloss (0.97 dB at 1550 nm, 1mW input powersource), wavelength independent (as light travelsentirely in fiber and air, which has effect on allcommunication window laser source), low powerconsumption, compact size and low cost bulkfabrication through surface micromachiningprocess.The switch has applications in networkprotection switching and monitoring in whichswitching times in the ms range are commonlyused. In addition, the same design presented inthis paper can also be utilized as integratedpassive or active fiber alignment microstructure,whereby it can be integrated onto the same targetMUMPS optical MEMS chip or used as externalsubmount for in-package optical MEMSpackaging.II. DESIGNING AND MODELINGThe switch consists of an array of curledcantilevers arranged in an intersecting pattern asshown in Fig. 1. These cantilevers are made upof 2 layers of stacked polysilicon and coated withgold. The stacking of different material layercreates an out-of-plane cantilever due to thermalmismatch between polysilicon and gold duringfabrication. These cantilevers can be actuatedelectrostatically through application of a voltagedifference between the cantilevers and thesubstrate. Fig. 2 shows the schematic of theoptical switch. An input fiber is to be fixed ontoan electrostatic actuated cantilever beam. At thereceiving end, two fibers are assembled in such away that the input fiber is always preciselyaligned to any of the output fiber at theirassociated on or off position (i.e. pull-down orrest position).Chip on board (COB) packaging has beenchosen to be used in this work for testing of theprototype. The entire MUMPS die is attachedonto a gold plated printed circuit board and then0-7803-8658-2/04/$20.00(c)2004 IEEE 171ICSE2004 Proc. 2004, Kuala Lumpur, Malaysiawire bonded with gold wire. A completedprototype is shown in Fig. 3. The assembly offiber onto the device is made easier with theunique cantilevers formation in this design,which forms an integrated v-groove for easypassive alignment. In fact, a limited activealignment of the fiber can be performed in oneaxis for fine tuning any optical positioningmismatch caused by the crude passive alignment.The fiber is to be permanently fixed onto thefibers using UV curable epoxy.y-xThe intersecting cantilevers array forms a"V-groove" in which the fiber can be placed.actuating iptotucantilever inutotputrfiber actuat fibertdiiectionI substr.ate/L ~~sub.strate |"fFig. 2 Schematic of the optical switchChip on board packaging for the prototypeElectrostatic actuation is a highly nonlinearis still widely unknown in details in the researchworld, albeit a variety of reported work in themodeling of similar electrostatic actuateddevices. The modeling of electrostatic actuationcan be separated into numerical and analyticalmethod. For numerical method, the FiniteElement Method (FEM) is widely employed andis the engine behind all major MEMS simulationsoftwares. Analytical method, on the contrary,gives a clearer picture on such relationship.However, it is very difficult to derive an exactanalytical model which is applicable to differentMEMS devices. Electrostatic pull-in modeling isstill a continual work elsewhere in the researchcommunity [81-[12]. However, all theseanalytical models have their limitations and nottoo suitable for use in this work. [8] is too simpleand not suitable for most MEMS devices. Fullydistributed load condition is assumed in [9]. In[10], [11] and [12], curvature of the cantilever isnot modeled. As the cantilever used in this workhas a very large curvature, a new analytical hasbeen developed to catered for this need. Thisanalytical model, with closed expressiondescribing the pull in voltage of a singlecantilever, has been reported in [13]. A closedform analytical model comes in very handy forthe design of the switch in this work as it provideinsight at an early phase of designing of MEMSwithout the need for using computing intensivenumerical simulation tools. In this work, thecontrolled displacement of the fiber is enablesthe realization of an on/off switching function.Schematic of the cantilever design used inthis work is depicted in Fig. 4. The cantilever ismade of polysilicon. The entire structure sits on asilicon substrate. It consists on 3 separateregions. Region I and III are straight polysiliconregion. In region II, a thin layer of metal isdeposited onto the polysilicon layer. This makesregion II to be curling upward. This design isknown as the partially curled cantilever in thiswork. When a voltage is applied across thecantilever and the substrate, electrostatic forcepulls the cantilever down towards the substrate.A silicon nitride layer acts as an insulator,preventing voltage short between the cantileverand the grounded substrate.p Region 11LRegoeR _r~~~~~~~~~_., .,~~~~~~............... ^__0.,...........___.^^..Fig. 4 Schematic showing the partially curlcantilever model.0-7803-8658-2/04/$20.00(c)2004 IEEE 172ICSE2004 Proc. 2004, Kuala Lumpur, MalaysiaEquation (1) is the partially curled cantilevermodel developed for the modeling of thispartially curled cantilever and is reported indetails in [13]. In (1) the total tip deflection ofthe cantilever, T, is related to the applied voltageacross the cantilever and the silicon substrate.prototype. As can be seen in Fig. 7, thecantilevers of the switch pull in at 24 V.However, a voltage of 20 V is sufficient for theswitch to operate. The optical power is reducedto near zero as the cantilevers get actuated,thereby achieving an on/off switching operation.Insertion loss of the switch is measured to be2 0.97dB at 1550nm at 1mW power. This is usablefor optical network protection switching and"monitoring requirement.Due to limitation of our laboratoryequipment, the actual actuating range of thecantilever is not measured, but it is estimated tobe at 300jm based on observation onmicroscopy image. As such, the switch can beextended to become a 1x2 switch by arrangingtwo fibers at the receiving end, considering thatthe diameter of single mode fiber (SMF) is125,um. This is shown in II. Fig. 2. However, thishas not been tested yet.whereZm = Zn + ZaA typical plot of (1) is shown in Fig. 5. From(1) and from analysis of a typical graph as in Fig.5, a cantilever with desired pull in voltage anddimension can be designed.DefecIon Vs VoltaeoCCTV moniorngthrough microscope- Actuating endpowetmater140120*00iI1604020,130.73-4-Paooaltycodnood.I ta7I # PadhAvwfbdmodel %~~~~~~~11.39| 1n90ssu0redp7 In t02.3.3s4a47 ~ ~~~~/030__+<.377 ~ ~ ~ 49500 1 2 3 4 5 a 7 a 9 1O 11 12 13 14Vobg. I VFig. 5 Deflection of cantilever tip 8T increases asvoltage applied across the cantilever andsubstrate is increased.In this work, a prototype of the switch hasbeen successfully tested and the concept of thedesign has been verified.III. EXPERIMENT AND PRELIMINARYRESULTFig. 6 Experimental setup for insertion lossmeasurement0.90.8 -. 0.7 _E 0.6L.; 0.50X 0.4.2 0.30 0.20.1 \0 5 10 15 20 25Voltage /VFig. 7 Optical power observed at the receiving fiberversus actuation voltage.The setup as shown in Fig. 6 is used forinsertion loss measurement of the switch0-7803-8658-2/04/$20.00(c)2004 IEEE0173ICSE2004 Proc. 2004, Kuala Lumpur, MalaysiaIV. CONCLUSIONA novel and simple design of a MEMSoptical switch is presented. Although preliminaryresult is based on IXI, it should be possible toextend the switch to become a 1X2 switch. Infact, the switch can even be extended to becomea variable optical attenuator [14]-[16]. Theswitch design should makes the assemblystraight forward and can be achieved in lab.Modeling effort has covered area not previouslycovered by other reported works [8]-[12], inspecific the effect of cantilever curvature on thepull-in voltage. The analytical model developedshould can assists the author and future MEMSdesigner in designing the optimal dimension ofthe switch before anly time consuming numericalsimulation is applied.ACKNOWLEDGEMENTThe authors would like to thank theMalaysian Ministry of Science, Technology andEnvironment (MOSTE) for sponsoring this workunder National Photonics Top Down Project(Vot 74211) and also for the scholarship of theresearcher through National Science Foundation(NSF).REFERENCES[I ] E. Ionnone, and R. Sabella, "Optical PathTechnologies: A Comparison among DifferentCross-Connects Architectures," J LightwaveTechnol., vol. 14, pp. 2184-2196, 1996.[2] Hara, Hane, Kohl (1999). "Si MicromechanicalFiber-Optic Switch with Shape Memory AlloyActuator." Proceedings of Transducers 99, June7-10, 1999. Sendai, Japan: IEEE. 790-793.[3] Ollier, E. et al. "1x8 Micromechanical Switchesbase on Moving Waveguides," Proceedings ofJEEE/LEOS international conference on opticalmicrosystems and technologies, August 21-242000. Kauai, Hawaii: IEEE, 2000: 39-40.[4] Field, L. A., Burriesci, D. L., Robrish, P. R. andRuby, R. C. "Micromahined 1x2 optical fiberswitch," Sensors and Actuators A, 1996, 53(1):311-316[5] Yasseen, A.A., Mitchell, J., Streit, T., Smith,D.A. and Mehregany, M. "A Rotary ElectrostaticMicromotor 1x8 Optical Switches," J. ofSelectedTopics in Quantum Electronics, 1999, 5: 26-32.[6] Mark Herding, Franz Richardt, Peter Woias, "ANovel Approach to Low-Cost Optical FiberSwitches", Optical MEMS 2003, 141-142[7] Martin Hoffmann, Dirk Ntlsse and Edgar Voges,"Electrostatic Parallel-Plate Actuators with LargeDeflections for use in Optical Moving-FibreSwitches", J. Micromech. Microeng. 11, 323-328(2001)[8] Nathanson H.C., Newell W.E., Wickstrom R.A.and Davis Jr J.R., "The Resonant GateTransistor", IEEE Trans. Electron Devices, 14(1967), 117-133[91 Petersen K.E., "Dynamic Micromechanics on Si:Techniques and Devices", IEEE Trans. OnElectron Devices, 25 (1978), 1241-1250[1O] Osterberg P and Senturia S., "MTEST: a test chipfor MEMS Material Property Measurement UsingElectrostatically Actuated Test Structures", J.MEMS, 6 (1997), 107-118[1 I]Tilmans H.A.C and Legtenberg R,"Electrostatically Drivem Vacuum-EncapsulatedPolysilicon Resonators: Part II. Theory andPerformance", Sensors and actuators A, 45(1994), 67-8412] Sayanu Pamidighantam, Robert Puers, Kris Baertand Harrie A C Tilmans, "Pull-in VoltageAnalysis of Electrostatically Actuated BeamStructures With Fixed-fixed and Fixed-free EndConditions", J. Micromech. Microeng. 12 (2002),458464.[13]C. W. Leow, Mohammad A. B., N. M. Kassim,"Analytical Modeling for Determination of Pull-in Voltage for Electrostatic Actuated MEMSCantilever Beam". 2002 IEEE InternationalConference on Semiconductor Electronics (ICSE2002), Penang, Malaysia. 19-21 December 2002.[14]H. Toshiyoshi et al, "A 5-Volt Operated MEMSVariable Optical Attenuator", Proc. of the 12'hInt. ConfJ on Solid-State Sensors, Actuators andMicrosystems (TRANSDUCERS '03), Boston,USA, June 8-12, 2003, 1768-1771[15] Keiji Isamoto, Kazuya Kato, Atsushi Morosawa,Changho Chong, Hiroyuki Fujita, and HiroshiToshiyoshi, "Micromechanical VOA Design forHigh Shock-Tolerance and Low Temperature-Dependence", IEEE/LEOS Int. Conf on OpticalMEMS and Their Applications (MOEMS 03),Outrigger Waikoloa Beach, Hawaii, USA, Aug.18 - 21, 2003[16]R. R. A. Syms, H. Zou, J. Stagg, and D. F.Moore, "Multistate Latching MEMS VariableOptical Attenuator", IEEE Photonic TechnologyLetters, 2004, 16(1). 191-1930-7803-8658-2/04/$20.00(c)2004 IEEE 174

MEMS On/off and x2 Optical Switch with Integrated Fiber Alignment Capability

http://eprints.utm.my/2036/1/CheahMohammad2004__MEMSonoffAnd1splTimes2OpticalSwitch.pdf

MEMS On/off and x2 Optical Switch with Integrated Fiber Alignment Capability

Abstract

Similar works

Full text

Available Versions

Universiti Teknologi Malaysia Institutional Repository