design and performance of mems multifunction optical device using a combined in-plane and...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 24, DECEMBER 15, 2010 3589

Design and Performance of MEMS MultifunctionOptical Device Using a Combined In-Plane and

Out-of-Plane Motion of Dual-Slope MirrorQ. H. Chen, W. G. Wu, Member, IEEE, Z. Q. Wang, G. Z. Yan, Member, IEEE, and Y. L. Hao

Abstract—A reflection-type MEMS multifunction-integratedoptical device using the combined in-plane and out-of-plane mo-tion of a dual-slope mirror is proposed. The motion of the mirrorsresults in the corresponding optical axis offsets in the transmittingand receiving optical signals, which can enable the device withvariable optical power splitting, optical switching and variableoptical attenuating functions. The optical models for splitting andattenuating are presented, respectively. The electro-mechanicalcharacteristics of the device are also investigated. Measurementsof the fabricated devices show that the switching times is lessthan 9 ms. The excess loss of the device is less than 3 dB andthe controllable attenuation range is up to 39 dB, respectively.Moreover, polarization-dependent loss is less than 0.7 dB in thewhole attenuation and splitting range.

Index Terms—Microelectromechanical systems (MEMS), multi-functional device, optical power splitter, optical switch (OS), vari-able optical attenuator (VOA).

I. INTRODUCTION

T HE significant and rapid growth of optical fiber communi-cation networks has created a large demand for many op-

tical devices, including variable optical power splitters (VOPS),optical switches (OS) and variable optical attenuators (VOA)[1]–[3]. The issue of employing large quantities of these op-tical devices in optical networks for fiber-to-the-home applica-tions while maintaining low-cost and compact is particularly im-portant and beyond the demonstrated capabilities of current de-ployable technology. Conventional mechanical optical devicessuffer from large size and large element mass; on the other hand,guided-wave optical devices show the disadvantages of highloss and long device length. Recently, there has been a growinginterest in applying the MEMS technology to improve the per-formances and reduce the cost of the optical devices [4]–[13].However, up until now, despite excellent optical performance ofthe devices and low cost for each single device, most of theseMEMS-based optical devices are developed as single-function

Manuscript received December 17, 2007; revised June 09, 2008; acceptedJune 10, 2010. Date of publication October 07, 2010; date of current versionDecember 08, 2010. This work was supported in part by the National NaturalScience Foundation of China under Grant 60876084 and Grant 61006075 andby the Postdoctoral Foundation of China under Grant 20090460134.

The authors are with the National Key Laboratory of Science and Technologyon Micro/Nano Fabrication, Institute of Microelectronics, Peking University,100871 Beijing, China (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2010.2084290

Fig. 1. Schematic structure of the optical link protection module using themultifunctional device.

as the two type optical devices mentioned above. Therefore, theproblem of system complexity and total costs will still inten-sively come up when a large number of OS, VOPS and VOA areemployed together to construct an optical transport networks.

From a viewpoint of the advancement in multifunction in-tegration, developing MEMS-based multifunction optical de-vices might be the potential solution to the problem. By contrastto the single-function devices, besides the advantages derivedfrom MEMS type devices, MEMS-based multifunctional de-vice exhibits significantly higher functionality and lower costs.Additionally, choosing multifunctional device over individualsingle-function devices can also minimize the number of ex-ternal optical and electrical interconnections between individualdevices and maximize the system information capacity, opticalthroughput, and reliability, while minimizing the overall systemsize and weight, namely, compactness.

For the three kinds of devices, 1 2 type structures are thefundamental units for their applications. Therefore, a preferred1 2 multifunctional device would be developed for a widerange of potential applications. Fig. 1 shows a potential opticallink protection module in a medium and long distance point topoint transmission system. The device 1 and device 4 work as1 2 OS, while the device 2 and device 3 work as 1 2 VOPS.This protection scheme blends the advantages of the 1+1 pro-tection scheme [14] and the 1:1 protection scheme [15]. As aresult, it can simultaneously monitor two optical paths, and, dy-namically regulate the optical power in the protected path. Fur-thermore, with the VOA function in the multifunctional device,the required attenuators for switching the optical paths can beomitted. Moreover, the transmission and reception optical pathscan be mutually replaceable by using the multifunctional device.

In this paper, a 1 2 multifunctional device with OS, VOPSand VOA functions based on MEMS technology is proposed.The free-space reflective type solution has been applied to the

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3590 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 24, DECEMBER 15, 2010

Fig. 2. Schematic structure of the device. Parallel-plate actuators and comb-drive actuators connect with a reflective block supported by the unequal-heightsprings.

device by using a dual-slope sidewall as the reflector. In orderto reduce the insertion loss (IL), ball-lensed fibers which canachieve high coupling efficiency [16]–[18] have also been as-sembled with the device as input/output fibers.

This paper is organized as follows: In Section II, the devicefunctions are described. The optical and mechanical designs ofthe device are depicted in Section III. In Section IV, the mea-surements of the device are illustrated and analyzed. Finally, theconclusion of this paper is presented in Section V.

II. DESCRIPTION OF MULTIFUNCTIONAL DEVICE

The schematic drawing of the multifunctional device isshown in Fig. 2. The device mainly comprises an isosceles-tri-angular-shaped reflective block, parallel-plate actuators,comb-drive actuators and springs. At the front end of the block,two dual-slope areas (upper slopeI and lower slopeII) on eachsidewall of the bar are used as the reflective mirrors. The blockis supported by the unequal-height springs. The lower-heightsprings connect the block with parallel plates and can convertthe descending motion of the plates into out-of-plane tiltingmotion of the block efficiently. Also, the block is capable ofin-plane motion under the driving of the comb-drive actuatorthrough structural connection. By combination of the blockmotion types and the reflective slopes of the mirrors, the devicecan manipulate the light beam in different ways to perform theOS, VOA and VOPS functions, respectively.

A schematic drawing for describing the operation principleof the multifunctional device is shown in Fig. 3. The input andoutput fibers are placed in etched alignment grooves in siliconsubstrate. The angle between the input and each output fibersis designed as 90 , and the angle between the input fiber andthe sidewall mirrors is designed as 45 . Initially, the block iscoaxially positioned against the incident light beam and equallysplits the incident optical power into the two output fibers uti-lizing the slopeI mirrors. As the block moves in plane, the ef-fective reflective areas of the mirrors change in an opposite di-

Fig. 3. Schematic drawing of the operating principle of the device using dual-slope reflective mirrors. Several states are defined on the mirrors. (a) VOPS state(b) OS state (c) VOA state 1 (d) VOA state 2.

rection to each other. Accordingly, the variable power distribu-tion between the two output fibers is realized [VOPS state inFig. 3(a)]. Furthermore, the incident optical power can be en-tirely coupled into one output fiber when the block moves thedistance of about the spot radius [OS state in Fig. 3(b)]. More-over, as the block [simultaneous in OS state] tilts out-of-plane,it begins to replace the slopeI mirror with the slopeII mirror asactive reflective surface, thereby performs the attenuating func-tion by reflecting the transmission optical power out of receptionarea of the output fiber [VOA state 1 in Fig. 3(c)]. In addition, thedevice can realize not only individual attenuation for one outputport [VOA state 1 in Fig. 3(c)] but also simultaneous attenuationfor two output ports [simultaneous in VOPS state] [VOA state2in Fig. 2(d)]. Therefore, the VOPS function is expanded by pro-viding more options of splitting ratio.

The SEM (Scanning Electron Microscopy) pictures of thedevice are shown in Fig. 4. The device structures, includingthe mirror, springs and the electrostatic actuator are fabricatedon a single crystal silicon wafer using the DRIE (Deep Reac-tive Ion Etching) process. Fig. 4(a) is the comb actuator andfolded beam springs for in-plane motion. Fig. 4(b) shows theparallel-plate actuator, lower-height springs and the reflectiveblock. The height of lower-height springs is 5.5 m. By contractto the higher-height springs (80 m) used in this design, theselower-height springs have the advantage of occupying less spacewhile showing lower spring constant. Moreover, they ensurereliable mechanical isolation between the in-plane and out-of-plane motions because the vertical-direction flexibilities of thelower-height springs are much larger than those of the other

CHEN et al.: DESIGN AND PERFORMANCE OF MEMS MULTIFUNCTION OPTICAL DEVICE 3591

Fig. 4. SEM image of a fabricated multifunctional device: (a) comb drive ac-tuator and folded beam spring system for in-plane motion; (b) parallel-plateactuator; (c) close view and section view of the mirror.

higher-height springs. Because the sidewall profile can be tunedby changing etch/passivation time ratio [19], [20], two differentetch/passivation time ratios were orderly employed in the DRIEstep for achieving the slopeII reflective mirror along with slopeImirror on the sidewall. Typically, the two slopes on the crosssection of the sidewall include: one is about 89.6 for the slopeIreflective mirror while the other is about 87.7 for the slopeIIreflective mirror [in Fig. 4(c)].

The reflective sidewall mirror resulting from the DRIEprocess can be seen in SEM micrographs. An example is shownin Fig. 5. There is typical scalloping in the sidewall surface and

Fig. 5. SEM image of a fabricated dual-slope mirror surface.

the bottom 20–30 m of the sidewall on some of the slopeIIarea was also found to be degraded due to undercutting duringetching. However, this damage does not affect the opticalperformance of the device to a great extent since very limittedlight is transmitted to this portion of the slopeII area. Surfaceroughness measurements of the etched dual-slope sidewallreflectors were performed with an atomic force microscope(AFM). RMS (Root Mean Square) roughness on the upperslopeI part and the lower slopeI part was found to be 46 nm and55 nm, respectively. The 46 nm rms measured surface rough-ness is theoretically predicted to add an additional optical lossof 0.6 dB due to scattering. The profile quality of the sidewallcan be improved by placing a fallout guarding structure in frontof the mirror, which can minimize the gap in front of the wall toprevent from the lateral under-etching [21]. Both the two mirrorsurfaces are coated with a layer of gold (0.2 m thick) usingthe shadow mask technique. The self-aligned fiber grooves aremonolithically etched for passive alignment. The dimensionsof the device chip are 10 mm 4 mm.

III. DESIGN OF MULTIFUNCTIONAL DEVICE

A. Optical Design

As shown in Fig. 2, the basic idea of the optical design isthat the light beam from the input ball-lensed fiber propagatesto the reflective mirror and is partially or totally reflected tothe output fibers. The relationship between IL in splitting stateand lateral motion of mirrors and the relationship between ILin attenuating state and tilting motion of mirrors are theoreti-cally studied in this section, respectively. The studies use thefollowing assumptions:

1) The fundamental mode of the light beam from the ball-lenshas a Gaussian distribution.

2) The reflective sidewall mirror is with excellent reflectivityand sufficient reflective areas.

3) In the simplified model, the phase of the beam pattern doesnot vary significantly in the reflective sidewall mirror.

The model to calculate IL is an application of the Gaussianbeam propagation model and overlap coefficients. It is not theauthors’ intent to derive the model here but to give the appro-priate reference where the optical model and the approach canbe used to derive the model.


Fig. 6. The scheme of the block performing the proposed VOPS and OSfunctions.

Fig. 7. The equivalent model for the device performing the VOPS or OSfunctions.

Fig. 6 shows the scheme of the block performing the proposedthe VOPS and OS functions. The effect of lateral displacementof the mirror can be equally converted to a lateral misalign-ment between the two fibers. Fig. 7 shows the equivalent opticalmodel to analyze the IL resulting from the continuous lateralmoving of the mirror.

Based on this model, the insertion loss can be theoreti-cally calculated using the formula (1) derive from [16], [22]

(1)

where

and is the lateral displacement of the fiber, is the beamradius at the waist (9 m), the separation distance between thelens end surface and the mode field diameter (MFD) plane (150

m), is the refractive index of the medium between the fibersis the wavelength of the transmitted signal (1.55

m) and is the wave number , respectively.Using (1), the calculated IL versus the lateral displacement

of mirror for ideally aligned fibers with 150 m distance fromthe fiber to the mirror is shown in Fig. 8. Initially, the device isin original state, namely, no voltage is applied to the device andthe calculated IL is 12.3 dB. The IL gradually changes with themirror’s lateral displacement in different directions. When theright displacement is equal to the half of the ball-lensed fiber

Fig. 8. Variation of the insertion loss with the lateral displacement of mirror.

MFD, the IL will increase up to 49.2 dB. Note that even thoughthe left displacement reaches half of the MFD, other factors suchas the loss due to lateral or angular misalignment between thefiber end faces will inevitably contribute to the ultimate overallvalue of the IL in actual experiments.

For the attenuation function design, it is important to under-stand the influence of the tilting motion of the mirror on the fibercoupling. The downward motion of the parallel plates will re-sult in the upward and tilt motion of the reflective block throughbeam connections, which simultaneously causes a tilt motion inthe mirror [see Fig. 9(a)]. The tilt angle between later and orig-inal positions in reflective block is shown in Fig. 9(b). As pre-viously mentioned, the initial included angle between the inputoptical beam and reflective mirror slopeI is 45 and the slopeIIreflective mirror has a pre-tilted angle by fabrication. Accord-ingly, when the light beam spots on the dual-slope mirror, theincluded angle between the input optical beam and theslopeI mirror and the included angle between the inputoptical beam and the slopeII mirror can be found, respectively,as

(2)

and (3), shown at the bottom of the page.Therefore, the variation of included angle between the input

light beam and the dual-slope mirror compared to the includedangle in OS or VOPS state can be expressed as

(4)

(5)

where and are the variation value in slopeIand slopeII mirrors, respectively.

Fig. 10 shows a diagram of a free-space reflection-couplingVOA for the optical model. The basic idea of the optical model

(3)


Fig. 9. Scheme of the block performing the proposed VOA function: (a) Thetilted motion of the reflective block, (b) Schematic ‘A—B’ line lateral view ofa reflective block when the block is tilted by �.

is that the light beam from the input lensed fiber propagatesto the dual-slope mirror plane and is reflected by the slopeIand slopeII mirrors in two different directions: and

. It is then reflected and coupled to the accepting facetof the out lensed fiber. The light quantity on the upper slopeImirror and the lower slopeII mirror is decided by the liftingheight of the block resulting from the tilt motion. When thetwo reflected beams from the each mirror segment reach the ac-cepting facet of the output fiber, they will be partially overlappedand generate interference. In Fig. 10, and

denote the coordinate systems in the end facet of theinput lensed fiber, the equivalent dual-slope mirror plane andthe accepting facet of the output fiber, respectively.

Fig. 10. Equivalent model for the variation of insertion loss when the lightbeam is on both the slopeI and slopeII mirrors.

and denote the coordinate systems in the reflectiondirection for the partially reflected light from the slopeI andslopeII mirror, respectively.

At the , the complex amplitude of the Gaussianbeam is expressed as

(6)

where is a constant, is the beam radius at an axialdistance from the beam center where the dual-slope mirrorlocates, is the excess axial phase of the Gaussian beam,

.As previous mentioned, the input light at is

divided into two parts by the range and, respectively, and is reflected in two different

directions: and . By the method of coor-dinate transformation and Gaussian beam approximation, thecorresponding complex amplitudes of them in( or 2 for the light reflected by slopeI and slopeII mirrors,respectively) are expressed as (7) and (8), shown at the bottomof the page.

Based on the field overlap methods, the sum of the complexamplitude can be expressed as

(9)

(7)

(8)


Fig. 11. The equivalent model for the variation of insertion loss when the lightbeam is totally on the slopeII mirror.

On the other hand, the fundamental mode of the output fibercan be given by

(10)

where the term is used to normalize the light energyto 1.

Therefore, the power , which is coupled to the fundamentalmode of the output lensed fiber, is achieved by the mode-overlapintegral between the interfered beam and the fiber mode [13] ataccepting facet of the output fiber

(11)

where is the conjugate of . The (11) ex-presses the fractional power of the base of the fundamental fibermode .

The attenuation is given by

(12)

The model expressed in (6)–(12) uses the accurate interfer-ence formula, thus it is valid for various reflection type opticalattenuators. However, it cannot give an analytical expression be-tween the attenuation and the mirror position, which results inrequiring consuming numerical calculation.

When the slopeII mirror tilts up enough to reflect the wholelight spot, the IL can be theoretically calculated based on theequivalent optical model as shown in Fig. 11. The effect of theangular displacement can be equally converted to thelateral misalignment and the angular offset between the twofibers, where the can be approximately represented as

Accordingly, the insertion loss can be derive from [16],[22] as

(13)

Equation (13) reveals the direct relationship between themirror position and the attenuation. The attenuation can be es-timated without engaging time-consuming numerical integrals.

Fig. 12. Variation of the insertion loss vs the included angle variation of theslopeII mirror for the final regime of VOA function.

Fig. 13. Comparison between the insertion losses of the two optical models forthe initial and intermediate regime of VOA function.

In our design, when the is 1.57 degree, the inputoptical beam will totally spot on the slopeII mirror. Therefore,the final regime of VOA function can be obtained from therelationship between the IL and the included angle variation ofthe slopeII mirror as shown in Fig. 12 (set the as 2 degrees).

The equivalent model in Fig. 11 can be also used to give an in-formative sight into the IL for the initial and intermediate regimeof VOA function as shown in Fig. 10 by the utilization of the

instead of the . The comparison between thetwo optical models is illustrated in the IL versus the includedangle variation of the slopeI (Fig. 13). Both match well in lowattenuation levels (initial regime of VOA function) whereas inthe high and middle attenuation level (larger than 5 dB) thereis noticeable deviation (intermediate regime of VOA function).This might be due to the fact that in the high and middle level thepart of the optical beam spotting on the slopeII mirror occupiesa fairly large proportion, while which is considered to be still onthe slopeI mirror in the equivalent model to impose significantinfluence on the IL.

The IL is sensitive to angular offset of the reflective blockbecause of the small MFD and long distance from the ball-lensto the beam waist. The angular offset of 0.2 produces an ILof 1.1 dB whereas 2.5 results in 36.4 dB penalty. It is worth


Fig. 14. Schematic view of the electrostatic parallel-plate actuator for VOAfunction: (a) the designed parallel plate structure; (b) the approximate equivalentmodel.

mentioning that this 36.4 dB does not include the differencein IL due to different lateral offsets (in x-, y- and z-axis) androughness of mirror surface, therefore, the corresponding actualIL is even larger in the actual VOA state.

B. Device Modeling on VOA Function

The actuator of the multifunctional device for the VOA func-tion is designed to the parallel plate actuator. Fig. 14 illustratesthe structure of the actuator. As shown in Fig. 14(a), each ofthe parallel plates connects with the two lower-height serpen-tine springs and the spring is composed of meander beams. Theapproximate equivalent model of the actuator can be shown inFig. 14(b) since the flexibilities of the two lower-height springsare approximately equal to each other.

Neglecting the deformation of the movable parallel plate elec-trode during operation as well as the fringing effect of the elec-tric field around the edges of the electrode, the electrostatic forceacting on the upper plate electrode is given by

(14)

where is the permittivity of air, is the gap between the elec-trodes, is the displacement of the parallel plate in z axis direc-tion, is the applied voltage and is the area of the parallelplate, respectively.

On the other hand, the mechanical restoring force of the ac-tuator can be expressed as

(15)

where is the derived spring constant [23], [24], is theYoung’s modulus of the silicon, is the shear modulus of

TABLE IPHYSICAL PARAMETERS DESIGN OF THE PARALLEL PLATE ACTUATOR

the silicon, is the length of and is the length ofand are all defined in Table I. In addition, the

necessary dimensions and material constants of the springs andplate are also given in Table I.

Seen from (15), the restoring force is mainly affected by thelength and thickness of the beam than other parameters. There-fore, reducing the thickness of the beam is the most effectiveway to reduce the operating voltage without increasing the sizeof the device. Based on the compromise between performancesand fabrication considerations, the thickness of the beams areset to 5 m.

Numerical simulation of the actuator performance was per-formed to verify the theoretical analysis using ANSYS softwarewhich can solve electromechanical coupled field problems. Thestructure parameters used in these simulations are in accordancewith the actual design as listed in Table I. In the theoretical anal-ysis, the anchor point [point A in Fig. 14(b)] is assumed to befixed and the guided-end boundary conditions are applied to thefree-end point of the spring [point B in Fig. 14(b)]. Thus, theparallel plate which the spring attaches is assumed to descendvertically. However, actually there is a small tilted angle of par-allel plate to the lower horizontal substrate in its downward op-eration because of suffering non-uniform distribution of elec-trical field and asymmetry moments. As a result, not all nodaldisplacements of the parallel plate in the finite-element modelare equal. On the other hand, we are mostly interested in thenode with maximum displacement among all the nodes whereplate collapse takes place most easily. Thus, to some extent, thedisplacement of the node can represent the behavior of the par-allel plate. Fig. 15 shows a graph of the downward displace-ment of the parallel plate actuator as a function of the applied dcvoltage for both the analysis and simulation, and demonstratesa good agreement between them. For the driving voltage lowerthan the pull-in voltage in both lines, the actuator motion can becontrolled. While at the pull-in voltage, the upper parallel-plateelectrode collapses on the lower one. Moreover, this graph illus-trates that the analytic pull-in voltage (155.2 V) is slightly largerthan the simulated pull-in voltage (154.1 V), which is probably


Fig. 15. Simulated and theoretical characteristic of displacement versusvoltage for electrostatic parallel-plate actuation with pull-in effect.

Fig. 16. Schematic view of a folded beam spring system for in-plane motionof device.

due to the added electric force from the extended lower elec-trode and fringing effects in the actual design.

C. Device Modeling on VOPS and OS Function

The use of the large MFD ball-lensed fibers necessitates theuse of a large displacement actuator so that the reflector can bemoved completely to both splitting and switching positions. Theminimum required displacement is 10 m for input and outputfibers positioned 150 m from the sidewall reflector. Althougha large displacement is required, virtually the driving voltagecan be reduced by decreasing the constant of the springs andadjusting the parameters of the electrostatic comb drive actuatorusing in this design.

Since the in-plane comb drive actuator is a symmetrical struc-ture, only a half of the actuator need to be studied (see Fig. 16).The restoring force and the electrostatic force of thecomb drive actuator for the in-plane movement can be expressedas following equations:

(16)

(17)

where is the total spring constant of the folded beamspring structure [25], is the in-plane displacement of the actu-ator, is the number of the meanders in the spring, andare the length, width and thickness of the folded beam, respec-tively; is the number of the movable comb fingers, is

TABLE IIPHYSICAL PARAMETERS DESIGN OF THE IN-PLANE ACTUATOR

applied voltage to the comb drive actuator, and are thethickness of the fingers and the gap between the movable andfixed fingers, respectively.

In (16) and (17), there are mainly four controllable structureparameters to lower the driving voltage. One parameter is thenumber of comb fingers for the electrostatic force, and the otherthree parameters are the number of meanders, the width andlength of folded beam for the restoring force, respectively. Sincethe heights of the comb finger and spring are the same value inthis design, their affections on displacement can be omitted bycanceling each other in the driving voltage calculation. Basedon the compromise between performances and fabrication con-siderations, the gap between the comb fingers and the lengthof the folded beam spring are set to 4.5 m and 600 m, re-spectively. Assuming the designed detailed dimensions of thein-plane actuator and spring structure summarized in Table II,the maximum applied voltage to the comb actuator is estimatedto be 36 V.

IV. MEASUREMENT

In these experiments, three OptiFocusTM collimating lensedfibers are assembled and immobilized in the device chip. As iswell known, the alignment of optical fibers is a key target forthe optical device packaging because it significantly affects theoptical performance of the device. The fibers are firstly placedin the monolithic-integrated self-aligned grooves to achievecoarse adjustment. Then, accurate adjustment is performed byaligning the fibers actively with the five-dimension translationstage while monitoring the optical insertion losses of the twooutput ports. Once the optical fibers are best aligned for max-imum light coupling, they are immobilized using the Dymax™UV-curable glue.

The characterization measurements were performed on eightrepresentative samples randomly selected from several-tens ob-tained devices and also repeated several times for each sample.After assembling the OptiFocus™ lensed fibers into the groovesby passive alignment, the free-space fiber-to-fiber IL is between0.17 dB and 0.33 dB in a 1.55 m wavelength.

The VOPS function of the device is firstly characterized bysplitting the light power into two outputs ( and ).Under zero bias of the comb actuator, the typical initial IL atoutput1 and output2 are 16.0 dB and 16.3 dB (Fig. 17), respec-tively. The splitting ratio between them is 1:0.97.With the increase of the bias, the block approaches output2, andthus the IL of output1 gradually drops while that of output2 rises


Fig. 17. Measured insertion loss in the two output ports versus driving voltageof the comb-drive for power splitting performance of the device.

up. As the bias is then ramped up by 31 V, the device comes to anobvious switching point where the IL of output2 rises up to 43dB while that of output1 decreases to 2.5 dB. At this stage, thesplitting ratio is 57.26:1 and the devices thus serve as OS func-tion. The measured OS insertion losses of all the samples arewithin dB under the bias ranging from 29.3 to 36.9 V.There are similar behaviors in both IL curves when the blockmoves to output1. Moreover, the measured excess loss of thedevice is less than 3 dB in this VOPS state.

Then, to measure the switching speed, a square wave with20-Hz frequency and 30-V peak voltage is applied to the comb-drive actuators. The switched signal is coupled to the outputfiber, detected by a p-i-n photodetector and then captured by anoscilloscope. The measurement result, shown in Fig. 18, revealsa light switching response of 5 ms from OFF to ON states. Thedelay between the onset of the bias (mirror with 0 displacement)and the switching-on of light is measured to be 0.7 ms. The mir-rors are pulled back to the initial position due to restoring forceof silicon springs, resulting in an ON-OFF switching speed of 4ms. Since the envisioned application requires only modest OStime (ms), it is expected that this finite switching time associ-ated with tens-volts and tens-Hz control signals will not posea serious issue. Furthermore, the power required for the entireOS function is low and the OS operating principle is reliable.Additionally, the extinction ratio can be obtained by using themeasured output voltage since the voltage is proportional to theoptical power of the switched signal.

Also, the attenuation range of the VOA function versus theapplied voltage is tested (Fig. 19). Initially the device is in theOS state and has an attenuation loss of 2.5 dB at the output.As the applied voltage of the parallel-plate actuators increases,the attenuation accordingly increases and reaches its maximumvalue of (41.7 dB) at 139.3 V. As a result, a 39 dB attenuationrange is achieved. For all the measured samples, the attenuationranges vary from 36 to 39 dB with the corresponding drivingvoltage ranging from 127.3 to 144.1 V. Moreover, the back-reflection is low because neither the input and output fibers arecoaxial nor the reflected light and the input fiber are coplanar.

Fig. 18. Step response of the switching state of the device.

Fig. 19. Measured attenuation of the device versus applied voltage to the par-allel-plate actuator.

Besides small excess loss, fast switching time and large at-tenuation range, another substantial advantage of the multifunc-tional device is its low polarization dependent loss (PDL). ThePDL is less than 0.3 dB in the VOPS and OS states, and is lessthan 0.7 dB in the VOA state. This might be due to the largerscattering of the etched slopeII surface than the slopeI surface(higher roughness at larger depth position).

V. CONCLUSION

A reflection-type MEMS multifunction-integrated optical de-vice using the combined in-plane and out-of-plane motion of adual-slope mirror has been demonstrated. The device can oper-ates VOPS, OS and VOA functions based on the same structure.The device exhibits small excess loss, millisecond switchingtime, large attenuation range and low polarization-dependentloss. Therefore, it is expected to be conceivable for network de-ployment. This technology suggests great potentials in imple-menting advance optical device with high function integration,compactness and flexibility performances for optical communi-cation network.


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Q. H. Chen received the B.S. degree in electrical engineering from Nankai Uni-versity, Tianjin, China, in 2002, and the Ph.D. degree in microelectronics fromPeking University, Beijing, China, in 2008.

He is currently a Member of Postdoctoral Staff with the Institute of Micro-electronics, Peking University, Beijing, China. His research interests includedesign, fabrication, and characterization of optical MEMS devices.

W. G. Wu (M’08) received the B.S. degree in electronic engineering fromFudan University, Shanghai, China, in 1989, and the Ph.D. degree in micro-electronics from Xi’an Jiaotong University, Xi’an, China, in 1995.

He was a Member of Postdoctoral Staff with the University of California,Los Angeles, from 1997 to 2000. He is now a Professor with the Institute ofMicroelectronics, Peking University, Beijing, China. His current researches ofinterest are in the fields of optical and RF micro-electromechanical systems, andnano-electromechanical systems.

Z. Q. Wang received the B.S. degree in microelectronics from Peking Univer-sity, Beijing, China, in 2008.

His research interests include design and theoretical calculations of opticalMEMS devices.

G. Z. Yan (M’10) received the B.S. degree in microelectronics from PekingUniversity, Beijing, China, in 1974.

She is now a Professor with the Institute of Microelectronics, Peking Univer-sity, Beijing, China. Her research interests include VLSI fabrication technologyand the design and fabrication of MEMS sensors and actuators.

Y. L. Hao received the B.S. degree in physics, M.S. degree, and Ph.D. degreein microelectronics from Peking University, Beijing, China, in 1984, 1992, and2003, respectively.

He has been a Professor and the Director of the Institute of Microelectronics,Peking University, Beijing, China. His current researches cover a variety ofareas, including novel MEMS inertial, RF and optical devices, and MEMS CADtechnology.

design and performance of mems multifunction optical device using a combined in-plane and...

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