Single and multiple optical switches that use freestanding silicon nanowire waveguide couplers

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1 (2012) 1, e16; doi: /ls ß 2012 CIOMP. All rights reserved /12 ORIGINAL ARTICLE Single nd multiple opticl switches tht use freestnding silicon nnowire wveguide couplers Yut Akihm nd Kzuhiro Hne Silicon photonic devices consisting of nnowire wveguides re promising technology for on-chip integrtion in future opticl telecommuniction nd interconnection systems sed on silicon-lrge scle integrtion friction. However, the ccommodtion of vrile opticl components on chip remins chllenging due to the smll size of microchips. In this study, we investigted the chrcteristics of microelectromechnicl silicon nnowire wveguide switch with gp-vrile coupler. Due to its cpcitive opertion, the proposed wveguide switch consumed negligile power reltive to switches tht use thermo-opticl effect nd crrier injection. The proposed switch ws chrcterized using nlyses sed on coupled-mode theory for rectngulr wveguides, s well s simultion using the finite difference time domin method. A 232 single switch with n improved configurtion nd 236 multiple switch composed of the 232 switches ws designed nd fricted y comintion of electron em lithogrphy, fst-tom em etching nd hydrofluoric cid vpor scrificil etching. The properties of the switches were mesured nd evluted t wvelength of 1.55 mm. (2012) 1, e16; doi: /ls ; pulished online 22 June 2012 Keywords: microelectromechnicl systems; opticl switch; silicon photonics INTRODUCTION Sumicron-scle silicon wveguide circuits re promising technology for opticl telecommuniction nd interconnection pplictions. 1 The monolithic friction of silicon wveguides nd silicon electronics is useful for future integrtion in opto-electronic systems. Due to the high refrctive index of silicon (,3.5 t wvelength of 1.5 mm), silicon wveguide circuits cn e miniturized to e severl orders of mgnitude smller thn silic wveguide circuits. Severl types of circuits tht employ sumicron-scle silicon wveguides, such s wveguide splitters/couplers nd micro-rings, 2 4 hve een reported. In ddition, wveguide switch tht uses thermo-opticl effect 5 nd n ultr-fst silicon wveguide light modultor sed on chnges in the refrctive index of silicon y crrier injection 6 hve een reported. Recently, sumicron-scle wveguide switches for opticl pth chnges using the nno-mechnicl motions of electrosttic ctutors hve een reported. 7 Due to the very low power consumption ssocited with cpcitive opertion, this technology suits the lrge-scle integrtion nd reduced energy consumption requirements of telecommuniction systems Among these wveguide circuits, coupler switches hve ttrcted interest due to the use of non-contct low-loss mechnisms. 13,15 A coupler switch using InP wveguides ws operted y pplying electrosttic forces etween the freestnding wveguides. 13 In the cse of prllel-electrode ctutor, coupler gp smller thn two-thirds of the initil gp is not consistently controllle due to force instility. Using n in-plne com-drive ctutor, stle low-voltge opertion hs een relized for silicon wveguides. 15 However, the chrcteristics of freestnding sumicron-wide silicon wveguide coupler switch hve not een theoreticlly or experimentlly nlyzed in detil. Moreover, the fesiility of mtrix switch tht employs coupler switches hs not een investigted. In this pper, the design nd friction of n improved sumicronscle silicon wveguide coupler switch tht uses n ultr-smll electrosttic com-drive ctutor re reported, together with theoreticl nlyses. The friction nd chrcteristics of multiple switches tht employ the coupler switch elements re lso reported. In the fricted single switch, periodic chnge in the coupling s function of the gp ws clerly oserved. The switching conditions were experimentlly demonstrted using the gp-vrile mechnism. The lrge-scle switch ws quntittively investigted to determine the fesiility of mtrix switch consisting of the proposed coupler switches. The switch chrcteristics, such s the port isoltion nd cross tlk, were experimentlly evluted. PRINCIPLE AND DESIGN Figure 1 presents the sic structure of the proposed wveguide coupler switch. The coupler consisted of two freestnding silicon wveguides, which were suspended in ir. Two wveguides were locted within their region of interction ( nm). The gp etween the wveguides ws vried in the horizontl plne, while the wveguides remin prllel. The shpe of the coupler ws symmetric with respect to the center line etween the wveguides to void symmetric electromgnetic distriution of light wves, which ws different from the previous design. 15 One of the wveguides of the coupler ws connected to n electrosttic com-drive ctutor with Deprtment of Nnomechnics, Tohoku University, Sendi , Jpn Correspondence: Dr K Hne, Deprtment of Nnomechnics, Tohoku University, Ao 6-6-1, Armki, Aoku, Sendi, Miygi , Jpn E-mil: hne2@hne.mech.tohoku.c.jp Received 29 Decemer 2011; revised 7 My 2012; ccepted 8 My 2012

2 2 Silicon nnowire wveguide coupler switches Gp chnge Actutor Coupler Suspension rm c 10 m Coupler Movle wveguide Fixed wveguide Output ports F E B A C D Switch 5 Switch 4 Switch 3 Switch 2 Switch 1 port 100 m Figure 1 () Generl structure of the silicon freestnding nnowire wveguide coupler switch with gp-vrile mechnism; () schemtic digrm of the coupler switch with n electrosttic com-drive ctutor; (c) schemtic digrm of 236 multiple switch consisting of 232 wveguide coupler switches. two suspension rms, s schemticlly illustrted in Figure 1. The movle wveguide ws trnslted y the ctutor. Although two ctutors were used in the previous design, 15 the ctutor structure ws improved to ssure the prllel motion nd void complex structure, s shown in Figure 1. The suspension rm nd the wveguide were connected y n ellipticl low-loss wveguide ridge. 17 The loss of the ellipticl wveguide ridge ws shown to e less thn 0.1 db. The other wveguide of the coupler ws lso freestnding in ir nd ws fixed to the sustrte vi the ellipticl wveguide ridges. By pplying voltge to the com-drive ctutor, the movle wveguide ws trnslted to decrese the gp, nd thus, the strength of the coupling etween the wveguides increses. We lso designed nd fricted lrge scle 236 switch using the wveguide coupler switch elements, s shown in Figure 1c. The wveguide coupler switches were connected in series nd in prllel. The individul wveguide coupler switch elements functioned s 232 switches. By pplying voltges to the respective coupler switches, different opticl pths were connected etween the inputs nd outputs. The switching rchitecture ws similr to switch consisting of micromirror 232 switches. 18,19 The re of the individul coupler switch 2d y n 1 n 1 2 G Wveguide 1 Wveguide 2 Figure 2 Cross-sectionl schemtic digrm of the wveguide coupler. n 0 x ws s smll s 100 mm3100 mm, nd thus, mtrix switch consisting of 100 switches cn e instlled in 1-mm31-mm re, for exmple. THEORETICAL APPROACH Wveguide couplers hve een studied for mny yers since the erly development of wveguide optics. A rectngulr wveguide coupler is sic coupler for studying the fundmentl chrcteristics of generl coupler nd for otining nlyticl equtions tht descrie wveguide coupling. 20,21 In the proposed coupler switch, the cross-sections of the silicon wveguides were ssumed to e rectngulr, s schemticlly illustrted in Figure 2. The two wveguides hd identicl dimensions with width of 2 nd thickness of 2d. The distnce etween the centers of the two wveguides ws D, nd the ir gp etween the wveguide wlls ws G, s shown in Figure 2. The refrctive indices of the wveguide nd the surrounding ir were n 1 nd n 0, respectively. In the proposed coupler switch, the gp G etween the two wveguides ws vried long the x xis y the electrosttic comdrive ctutor. Therefore, the coupling coefficient ws otined in mnner similr to tht for sic rectngulr wveguide coupler with the gp s prmeter. The coupling coefficient ws otined using the following eqution: 21 x~ k2 (n 2 1 {n2 0 ) ðk x Þ 2 ðc x Þ 2 ð1zc x Þu 4 exp ð {c xgþ ð1þ where k x nd c x re wve numer nd symmetric constnt, respectively, of the refrctive index distriutions long the x xis. The symols k nd represent the wve numer in vcuum nd propgtion constnt long the z xis, respectively. The normlized p frequency is given y u~kn 1 ffiffiffiffiffiffi 2D with the refrctive index difference given y D~ n 2 1 {n2 0 2n 2 1. Although silicon hs high refrctive index, this pproch ws used to otin simple pproximtion of the coupling coefficient.

3 Silicon nnowire wveguide coupler switches 3 Coupling coefficient ( m -1 ) Normlized intensity Gp G (nm) Gp G (nm) 1st switch point nd switch point Displcement of ctutor (nm) Figure 3 () Coupling coefficient clculted s function of gp G; () normlized light intensities t the through nd drop ports of the coupler switch clculted s function of the ctutor displcement nd gp G etween the wveguides. The output intensity I 1 of the trnsmitted light in wveguide 1 t the through port nd the output intensity I 2 of the trnsmitted light in wveguide 2 t the drop port were expressed, respectively, using the following eqution: I 1 ~I 0 cos 2 ðxl z Þ nd I 2 ~I 0 sin 2 ðxl z Þ ð2þ where I 0 is the input light intensity nd L z is the coupler length. The intensities chnged sinusoidlly with n increse in the coupler length or coupling coefficient. We selected wveguide width to llow for only the single lowest mode of the wveguide, nd we ssumed tht the mode profile ws tht of the lowest trnsverse electric (TE)-like mode. The width of the wveguide ws designed to e 400 nm. The refrctive index of silicon ws 3.47 t wvelength of 1.55 mm. The height of the wveguide ws 260 nm, which ws equl to the thickness of the top lyer of silicon on insultor (SOI) wfer. Under our experimentl conditions, the vlues of the prmeters were s follows: D50.46, 5200 nm, k x m 21, c x m 21, m 21 nd u52.7. The clculted coupling coefficient s function of gp G etween the wveguides is shown in Figure 3. The coupling coefficient decresed exponentilly. Therefore, the coupling ws chnged y vrying the coupler gp. The vlue of the coupling ws pproximtely mm 21 t G51000 nm, which ws sufficiently smll to e considered switched-off condition. We designed the gp of the coupler switch to e vried within rnge of less thn 1000 nm. Figure 3 shows the normlized light intensities t the through nd drop ports of the coupler switch clculted s function of G y ssuming tht the coupling length of the wveguide ws 10 mm in eqution (2). Becuse the gp ws vried y the displcement of the ctutor from n initil gp of 1000 nm, the displcement of the ctutor is lso shown in Figure 3. As the gp decreses, the intensity t the through port periodiclly chnges, wheres the intensity t the drop port periodiclly vried to e out of phse with the intensity t the through port. Two min switch points were found in the displcement region: one ws t pproximtely 805 nm (G<195 nm, the first switch point), nd the other ws t 865 nm (G<135 nm, the second switch point). Therefore, two switching conditions were pplied: one condition ws used to chnge the wveguide position from the zerodisplcement point (G51000 nm) to the first switch point with displcement of pproximtely 805 nm, nd the other condition ws used to shift from the first switch point to the second switch point with displcement of pproximtely 60 nm. A comprison of the two switch points indictes tht the tolernce of the ctutor displcement t the second switch point ws smller thn tht t the first point. The normlized light intensities t the through nd drop ports were lso clculted t the first switch point s function of wvelength. The mximum extinction rtio ws designed t wvelength of 1.55 mm. An extinction rtio of 30 db ws otined in the wvelength region of 20 nm t pproximtely 1.55 mm, nd 20 db ws otined in the region of 60 nm. When the gp of the coupler ws optimized t the respective wvelengths, the mximum extinction rtio ws otined, which ws similrly to the result t 1.55 mm. Although the pproximte chrcteristics of the proposed opticl switch were investigted using the nlyticl equtions shown ove, the properties of the switch were quntittively nd thoroughly simulted using electromgnetic softwre. We lso performed simultion using the finite difference time domin method (Crystl Wve) to confirm the mode profile nd coupling. Figure 4 shows exmples of the simultion results. Figure 4 shows the intensity distriution of G=260nm G=150nm 5 m 5 m Figure 4 Simulted light wve propgtion: () light wve intensity within the wveguides of the coupler switch t the first switch point with gp G vlue of 260 nm; () the light wve intensity for gp of 150 nm.

4 4 Silicon nnowire wveguide coupler switches Electrode 10 m Electrosttic Com-drive micro-ctutor R:6 m Filler for etching Ellipticl wveguide ridge Suspension rm Gp:20~1000nm Coupler region 100 m from the center port Movle wveguide 100 m from the center Coupler Fixed wveguide port port Figure 5 Msk pttern for the 232 wveguide coupler switch. light wve propgting in the switch region t the first switch point (G5260 nm). The input light wve ws clerly trnsmitted from the first wveguide to the second wveguide t wvelength of 1.55 mm with negligile loss. The vlue of the gp t the first switch point ws 65 nm lrger thn tht otined y the nlyticl eqution. As the gp decresed from the condition presented in Figure 4, the light trnsferred from the first wveguide to the second wveguide gin returned to the first wveguide t gp of 150 nm, s shown in Figure 4. Bsed on these simultions, the coupling length corresponding to the stright region of the wveguide ws designed to e 10 mm in the gp region from 100 nm to 300 nm for switching. The switching properties of the proposed switch were explined well y the simultion, lthough the chrcteristic vlues shifted. FABRICATION AND EXPERIMENTATION Figure 5 shows the msk pttern of the proposed single switch. The length of the wveguides tht were prllel ws designed to e 10 mm. The gp ws decresed from 1000 nm to 20 nm y the ctutor. The movle freestnding silicon wveguide ws suspended y 1.3-mmwide nd 8.0-mm-long ellipticl wveguide ridges, which connected the movle wveguide to the suspension rms of the ctutor with 0.2-mm-wide nd 1.6-mm-long silicon ems. The distnce etween the two wveguide ridges ws 52 mm. A mesh structure ws ptterned in the tringulr region etween the rms, which ws filler pttern used to prevent n excess dose of the electron em. The filler pttern fell down onto the silicon sustrte fter etching the SiO 2 lyer. The movle wveguide ws fixed to the sustrte t position 100 mm from the coupler on oth sides y the ellipticl wveguide ridges, s shown in Figure 5. Therefore, the movle wveguide ws supported y the two ellipticl wveguide ridges t oth ends, nd the coupler ws connected to the ctutor. The other freestnding wveguide of the coupler ws lso fixed to the sustrte y the ellipticl wveguide ridges, s shown in Figure 5. These wveguides were 260 nm in thickness nd 400 nm in width. The coupling prt of the coupler ws connected to two ent wveguides with rdius of 6 mm. The green region in Figure 5 ws etched on the upper silicon lyer of the SOI wfer. The width of the ir-cld wveguide ws 4.5 mm nd ws held constnt long the freestnding silicon wveguide. Therefore, the distnces from the silicon wveguide to the surrounding upper silicon lyer were 2.05 mm on oth sides of the wveguide. Certin minor modifictions were dded in series of frictions. It ws desirle to keep the ir-cld width constnt long the silicon wveguide. For connecting the single switches in series, one wveguide ridge ws needed to fix the movle wveguide etween the switches. The ctutor consists of com drive with 20 finger pirs. Ech finger ws 1.73 mm long nd 200 nm wide. The ctutor used doulefolded springs, nd ech em of the springs ws 200 nm in width nd 15 mm in length. The totl spring constnt of the ctutor ws clculted to e N m 21. The electrosttic force of the ctutor ws lso theoreticlly estimted to e pproximtely 0.32 mn t 23 V. The mximum displcement of the ctutor ws limited y stopper t displcement of 980 nm. Therefore, the minimum gp of the coupler ws designed to e 20 nm. Voltge ws pplied etween the two regions of the top silicon lyer tht were seprted y etching. The movle wveguide ws lterlly deformed during switch opertion. The spring constnt of the movle wveguide for horizontl ending ws less thn tht of the ctutor y fctor of 75. The verticl deformtion of the suspended structure due to grvity ws clculted to e less thn 0.1 nm. In the cse of virtion with 10 times the ccelertion of grvity, the displcement ws s smll s 1 nm, which ws lso similr to the lterl virtion. Moreover, ecuse the resonnt frequency of the device ws s high s 100 khz, the proposed switch ws not significntly ffected y environmentl virtions. A multiple switch ws lso designed nd fricted y connecting five 232 wveguide coupler switches in series nd in prllel to investigte the fesiility of mtrix switch, s shown in Figure 1c. Ech 232

5 Silicon nnowire wveguide coupler switches 5 5 m fcet on the input wveguide for light coupling. Finlly, the SiO 2 lyer ws etched using hydrofluoric cid vpor to otin the freestnding structure of the proposed device. The device chrcteristics were mesured under n infrred opticl microscope. A tunle lser (Agilent 81682A) ws used s light source t wvelength of pproximtely 1.55 mm. A lensed singlemode fier ws used to couple the lser light to the end surfce of the input port of the fricted device. The light intensities t the output ports were mesured sed on spot imges of scttered light t the ends of the wveguides using n infrred (IR) cmer (Goodrich SU32KTS-1.7RT). 10 Si Si c SiO 2 Outputs Wveguide 1 m Electrode 5 m 100 m 10 m Figure 6 Electron microgrphs of freestnding silicon wveguide coupler switches: () overview of the entire single switch with n inset showing the elongted output wveguide; () mgnified view of the coupler region with n inset showing cross-sectionl imge of wveguide prior to SiO 2 etching; (c)236 multiple switch switch ws connected with 200-mm-long stright wveguide to form single 236 multiple switch. In the friction process, n SOI wfer with 260-nm-thick upper silicon lyer nd 2-mm-thick uried oxide lyer on 625-mm-thick silicon sustrte ws used. First, 350-nm-thick positive resist polymer (ZEON ZEP-520A) ws coted onto the SOI wfer, which ws then exposed using n electron em ptterning mchine (JEOL JBX- 5000LS). After the development of the resist polymer, the top silicon lyer ws etched using fst tom em (Er FAB-60ML). The resist polymer ws removed using n H 2 SO 4 /H 2 O 2 solution. The SOI wfer ws cleved fter prtilly dicing the silicon sustrte to produce RESULTS AND DISCUSSION Figure 6 shows n electron microgrph of the fricted wveguide switch. The freestnding silicon wveguides were fricted, in close ccordnce with the designed specifiction. The movle wveguide ws connected to n electrosttic com-drive ctutor, nd the movle prt of the ctutor ws freely suspended y silicon springs. The electrode for the fixed com of the ctutor ppers white ecuse the electrode ws isolted nd chrged y the electron-em scnning. The filler of the ptterning ws lso chrged fter flling onto residul SiO 2 lyer, s shown in Figure 6. The inset of Figure 6 presents n olique view of n output wveguide, which confirms tht the wveguide ws suspended in ir. Figure 6 shows mgnified imge of the coupler region. Although portion of the residul SiO 2 lyer is pprent in Figure 6, the nrrow freestnding silicon wveguides were uniformly formed y the proposed friction processes. The gp etween the wveguides in the coupler region ws pproximtely 100 nm lrger thn the design vlue, nd the wveguide ws pproximtely 5% nrrower thn the designed vlue. The roughness of the sidewll of the wveguides, estimted sed n electron microgrph, ws less thn 15 nm. The inset of Figure 6 shows cross-sectionl imge of wveguide prior to SiO 2 etching, which confirms the rectngulr shpe of cross-section of the wveguide. Figure 6c shows the fricted 236 multiple switch comprising five 232 wveguide switches. The size of the multiple switch ws not minimized ecuse the electrode pds (250-mm squre) for the ctutors were enlrged for esy contct. The switch chrcteristics were mesured using single 232 wveguide switch designed s shown in Figure 5. The output intensities were mesured sed on IR imges otined for different voltges pplied to the ctutor. The power of the lser source ws 3 mw, nd the wvelength ws 1.55 mm. The ctutor voltge ws vried from 0 V to 30.5 V. Figure 7 d shows imges of the output region. Figure 7 shows n imge without the ppliction of voltge, which corresponds to gp equl to 1000 nm. Under this condition, the coupling etween the wveguides ws negligile, nd right spot ws oserved t the through port. At V (Figure 7), the intensity t the through port ws minimized, nd the intensity t the drop port ws mximized. This condition corresponds to the first switch point, t which the gp ws pproximtely 250 nm. As shown in Figure 7c, the second switch point occurred when the voltge ws incresed to V, t which the estimted gp ws 130 nm. A cler spot imge ws oserved t the second switch point. With further increse in the voltge, the output intensity t the drop port ws gin mximized, s shown in Figure 7d; the gp under this condition ws pproximtely 70 nm. The spot imges from the output ports were converted to intensity vlues, nd the output intensity vlues t the through nd drop ports were plotted s function of the displcement, s shown in Figure 7e. The gp otined sed on the voltge dependence of the ctutor

6 6 Silicon nnowire wveguide coupler switches port 0V, G=1000nm 25.30V, 250nm port c 27.75V, 130nm d 29.35V, 70nm e Gp G (nm) Normlized intensity Displcement (nm) Voltge (V) Displcement of ctutor (nm) Figure 7 Infrred imges of the output ports () t ctutor voltges of 0 V, () V, (c) V nd (d) V; (e) normlized output intensities t the through nd drop ports mesured s function of the ctutor displcement. The inset shows the voltge dependence of the ctutor displcement. displcement is lso shown (inset in Figure 7e). The ctutor displcement ws mesured y oserving the ctutor motion using scnning electron microscope. The displcement rte ws pproximtely 50 nm V 21 t voltges greter thn 20 V. Becuse the minimum voltge setting ws V, the resolution of the ctutor ws pproximtely 1 nm, which corresponds to signl fluctution of less thn 2%. As shown in Figure 7e, oscillting intensities with opposite phses were otined t the through nd drop ports. At the first switch point of the 250-nm gp, the mximum output intensity t the drop port ws 20% less thn tht t the originl intensity t the through port. However, the output intensity t the through port recovers to e slightly less thn the originl to the originl intensity under the coupling condition of the doule trnsition shown in Figure 4 t gp of 130 nm, which generlly greed with the gp of 150 nm otined in the simultion. Although the chrcteristic gps clculted using the nlyticl equtions were smller thn the experimentl vlues, the reltive dependence of the output intensities cn e explined y the theoreticl pproch descried in the section on Theoreticl pproch. The port extinction of the 232 switch ws defined s I 1mx /I 1min nd I 2mx /I 2min, where I 1mx, I 1min, I 2mx nd I 2min were the mximum nd minimum intensities t port 1 (through port in this experiment) nd port 2 (drop port), respectively. The port isoltion of the 232 switch ws the intensity rtio etween the ports under the worst-cse condition, which ws defined s (I 2 t I 1min )/I 1min t the first switch point in this experiment. The port extinction t the first switch point ws 7.2 db, nd the port isoltion ws 5.8 db. These low rtios were cused y the low intensity difference etween the two ports t the first switch point nd the position difference etween the mximum point t the drop port nd the minimum point t the through port. The reson for the differences is uncler. Becuse lrge difference in the intensity etween the ports ws otined t the second switch point, the ove differences t the first switch point my hve een cused y position-dependent chnge in the wveguide impednce. Certin minor modifictions in the wveguide rrngement generted light scttering effect within the device. The ends of the wveguides were elongted to emit light nd void reflection, s shown in Figure 5. The emission efficiency my hve een ffected y smll friction error due to the elongted structure, which my hve cused different losses for the respective switch sttes, s seen in Figure 7e. A circulr end of the wveguide my e etter suited for uniform scttering. To optimize the switch chrcteristics s function of the motion of

7 Silicon nnowire wveguide coupler switches 7 Output intensity (db) F E B A C D F Switch 5 Switch 4 Switch 3 Switch 2 E B Ports A C D Switch m Figure 8 () Spot imges t respective output ports of multiple switch (left column) with schemtic lyout of 235 switch; () opticl intensities t the respective ports when one of the outputs is in the on-stte. the ctutor, further opticl simultions of the entire structure of the switch, including the surrounding structures such s the cld nd sustrte, re needed. The multiple switch consisting of five 232 switches shown in Figure 6c ws lso tested in the experiment. A minor modifiction of the 232 switch ws introduced. The filler used for etching ws chnged to n islnd structure, nd the etched region for the ir-cld wveguide ws minimized in the coupler region. The wvelength nd power of the lser source used for the mesurement were 1.55 mm nd 3 mw, respectively. Figure 8 shows the results of switch test using the fricted multiple switch. In the test, input light ws incident t the input of the first switch (Switch 1), nd spot imges t the respective output ports A F were oserved using n IR cmer. Switches 1 5 were selectively turned on to construct n opticl pth to one of the output ports from A to F. Therefore, one of the six ports hd n onstte (right spot), nd the others hd n off-stte (no spot). In the left column of Figure 8, spot imges re shown t the circle centers for the respective ports with schemtic digrm of the multiple switch. In the cse of the output imge t port A, right spot ws oserved fter pssing vi two through ports of switches 1 nd 2. In the cse of port B, the light wve psses drop port of switch 1 nd through port of switch 3. When port B ws compred to port A, similr right spot ws oserved t port B, s shown in Figure 8. A spot imge similr to tht t port B ws oserved t port C, where the light wve pssed three switches, including one drop port nd two through ports. In the cses of ports D nd E, the light wve pssed three switches, including two drop ports nd one through port, nd spots with slightly weker intensities were oserved. In the cse of port F, the intensity ws weker thn tht of others. Figure 8 shows the mesured output intensity vlues t the respective ports under the conditions shown in Figure 8. The switch condition ws selected to e equl to the condition of the port isoltion mesurement of 232 switch t the first switch point, which corresponds to the worst cse for ll of the switching sttes of 236 switch. The output differences mong the respective ports during the on-sttes were within 2.3 db. The port isoltion rnged from 6.2 db to 27.3 db for the djcent ports nd for the ports other thn the djcent ports, nd the worst port isoltion ws 6.2 db t port D etween ports C nd D. The cross tlk rnged from 6.9 db to 28.1 db, nd the worst vlue ws otined t port D when port C ws t the on-stte. Bsed on the experiments descried ove, the insertion loss of the 232 switch ws lso roughly estimted to e pproximtely 1 db on verge. The switching time ws estimted to e less thn 50 ms ecuse the resonnt frequency of the single switch ws pproximtely 214 khz. These experiments demonstrte tht mtrix switch is fesile, lthough the opticl rrngement is not optimized. CONCLUSIONS We investigted single nd multiple silicon wveguide switches tht employ gp-vrile couplers with microelectromechnicl comdrive ctutors. The wveguide ws 400 nm in width nd 260 nm in thickness, nd the size of the coupler switch ws 50 mm wide nd 80 mm long. The proposed switch ws chrcterized using nlysis sed on the coupled-mode theory for rectngulr wveguides s well s simultion employing the finite difference time domin method. The two min switch points were found for gps smller thn 260 nm. A 232 single switch with n initil gp of 1000 nm nd 236 multiplex switch composed of five 232 switches were designed nd fricted using comintion of electron em lithogrphy, fst-tom em etching nd hydrofluoric cid vpor scrificil etching. In n experiment, the properties of the single switch were mesured in the gp region from 20 nm to 1000 nm. Two switch points were clerly oserved, nd the reltive gp dependence of the output intensities ws explined y theoreticl clcultions. The port extinction ws 7.2 db, nd the port isoltion ws 5.8 db. The opertion of the 236 switch ws lso confirmed in the experiment. The worst port isoltion vlue ws 6.2 db, nd the worst cross tlk vlue ws 6.9 db; these vlues were generted t djcent ports. The insertion loss of the 232 switch ws estimted to e pproximtely 1 db on verge. To improve these chrcteristic vlues, it is necessry to further minimize friction errors on nnometer scle nd to stilize the yield of the devices. It will lso e useful to design ctutors with low displcement rte per voltge round the switch point to more precisely control the switch position. In ddition, it will e useful to simulte the entire wveguide rrngement with ctutor motion using rigorous electromgnetic clcultions. Moreover, the ckground intensity is included in the output intensity mesurements sed on spot imges otined y n IR cmer, nd the dynmic rnge of the output vlues is smll. Opticl fier detection is preferle for future mesurements. The switching chrcteristics, such s the port isoltion nd extinction rtio, must e further improved (greter thn 40 db) to e comprle to commercilized silic mtrix switch. However, ecuse the insertion loss ws reltively low (,1 db) in the multiple switch experiment, series connections of few single switches re fesile for mtrix switch fter improving the chrcteristics of single switch. Due to its cpcitive opertion, the power consumption of the proposed wveguide switch ws negligile compred to switches tht use thermoopticl effects. Therefore, the proposed switches re suitle for use in integrted silicon photonic circuits.

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