large size mems scanning mirror with vertical comb drive for tunable optical filter

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Large size MEMS scanning mirror with vertical comb drive for tunable optical filter Yingming Liu a,b,n , Jing Xu a , Shaolong Zhong a , Yaming Wu a a State Key Lab of Transducer Technology, Science and Technology on Microsystem Lab, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China article info Article history: Received 2 May 2012 Received in revised form 13 July 2012 Accepted 25 July 2012 Available online 23 August 2012 Keywords: MEMS scanning mirror Bulk micromachining technology Vertical comb drive Tuning optical filter abstract This paper presents the design, fabrication and characterization of a large size MEMS scanning mirror in application of tunable optical filter (TOF). In the design of scanning mirror, vertical comb drive is used for its large force density, low driving voltage and also no influence of pull-in effect. The mirror plate has a size of 2.7 mm 1.8 mm which is suitable for the optical requirement of TOF. In each side of the mirror, 186 combs are set to provide a suitable rotation angle. The length, width and thickness of the combs are 300 mm, 7 mm and 25 mm, respectively. The gap between the adjacent combs is 4 mm. The scanning mirror with vertical comb drive has been successfully realized using the bulk micromachining technologies. The measured static and dynamic characteristics show that the scanning mirror can achieve a maximal rotation angle of 2.31 with a direct current (DC) driving voltage of 95 V. The turn-on responding time of the scanning mirror is 1.887 ms and the turn-off responding time is 4.194 ms. The tuning function has also been demonstrated and a 1.507 ms tuning time between two adjacent channels is obtained. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. 1. Introduction Large size micro-electromechanical systems (MEMS) scanning mirrors have been key components in many optical systems, such as confocal laser scanning systems, optical fiber sensing systems and high performance tunable optical filters (TOF) [1]. These new applications usually require the scanners with better mechanical and optical performance, such as larger scanning angles, higher scanning frequencies, larger mirror sizes, lower driving voltage and better optical characteristics of mirror surface [2,3]. MEMS technology would be a promising way to make such devices, which use fabrication processes derived from single crystal silicon wafers. However, the design and fabrication of scanning mirror are big challenges to realize the desired performances, especially the implemented fabrication process. The structure of TOF which consists of MEMS scanning mirror and chromatic dispersion device like grating is called grating- MEMS optical system [4]. This kind of system is a good way to build TOF due to its advantages of simpler structure, lower optical insertion loss and crosstalk, faster tuning speed and polarization independence [5]. In this system, a polarization independent grating takes the role of light dispersion; while a MEMS scanning mirror takes functions of reflection, light choosing and tunning. As the grating lines per mm is fixed, a large mirror size is proposed in order to achieve high resolution. Besides, a suitable torsion angle is also needed to meet the tuning need. However, surface process was not often proposed. This is because when the mirror was given a torsion load, the surface of the mirror was prone to deformation, and it would cause aberration and astigma- tion of the light. This would be fatal for beam focus and filtering. Thus, bulk micromachining technology is more suitable for fabrication of high quality mechanical scanning mirror [6]. For MEMS scanning mirror, micro-actuation mechanism has priority to device structuring and fabrication. Electrostatic actuation has become the most popular approaches for scanning mirror, because of its faster speed with low power consumption and easy integration with electronic control systems [7]. In general, electro- static actuators contain comb drive actuator or parallel-plate actuator. The parallel-plate electrostatic actuator has a simpler structure and easier fabrication process, but it requests a higher driving voltage to obtain the required rotation angle. In the plate electrostatic actuator, a small overvoltage can cause the pull-in effect and results in device failure [8, 9]. On the contrary, comb drive actuator can offer the larger force density resulting in a larger torsion angle with a lower driving voltage and also no influence of pull-in effect [1013]. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2012.07.019 n Corresponding author at: State Key Lab of Transducer Technology, Science and Technology on Microsystem Lab, Shanghai Institute of Microsystem and Informa- tion Technology, Chinese Academy of Science, Shanghai 200050, China. E-mail address: [email protected] (Y. Liu). Optics and Lasers in Engineering 51 (2013) 54–60

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Page 1: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Optics and Lasers in Engineering 51 (2013) 54–60

Contents lists available at SciVerse ScienceDirect

Optics and Lasers in Engineering

0143-81

http://d

n Corr

Technol

tion Tec

E-m

journal homepage: www.elsevier.com/locate/optlaseng

Large size MEMS scanning mirror with vertical comb drive for tunableoptical filter

Yingming Liu a,b,n, Jing Xu a, Shaolong Zhong a, Yaming Wu a

a State Key Lab of Transducer Technology, Science and Technology on Microsystem Lab, Shanghai Institute of Microsystem and Information Technology,

Chinese Academy of Science, Shanghai 200050, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:

Received 2 May 2012

Received in revised form

13 July 2012

Accepted 25 July 2012Available online 23 August 2012

Keywords:

MEMS scanning mirror

Bulk micromachining technology

Vertical comb drive

Tuning optical filter

66/$ - see front matter Crown Copyright & 2

x.doi.org/10.1016/j.optlaseng.2012.07.019

esponding author at: State Key Lab of Transd

ogy on Microsystem Lab, Shanghai Institute o

hnology, Chinese Academy of Science, Shang

ail address: [email protected] (Y. Li

a b s t r a c t

This paper presents the design, fabrication and characterization of a large size MEMS scanning mirror in

application of tunable optical filter (TOF). In the design of scanning mirror, vertical comb drive is used

for its large force density, low driving voltage and also no influence of pull-in effect. The mirror plate

has a size of 2.7 mm�1.8 mm which is suitable for the optical requirement of TOF. In each side of

the mirror, 186 combs are set to provide a suitable rotation angle. The length, width and thickness

of the combs are 300 mm, 7 mm and 25 mm, respectively. The gap between the adjacent combs is 4 mm.

The scanning mirror with vertical comb drive has been successfully realized using the bulk

micromachining technologies. The measured static and dynamic characteristics show that the scanning

mirror can achieve a maximal rotation angle of 2.31 with a direct current (DC) driving voltage of 95 V.

The turn-on responding time of the scanning mirror is 1.887 ms and the turn-off responding time is

4.194 ms. The tuning function has also been demonstrated and a 1.507 ms tuning time between two

adjacent channels is obtained.

Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Large size micro-electromechanical systems (MEMS) scanningmirrors have been key components in many optical systems, suchas confocal laser scanning systems, optical fiber sensing systemsand high performance tunable optical filters (TOF) [1]. These newapplications usually require the scanners with better mechanicaland optical performance, such as larger scanning angles, higherscanning frequencies, larger mirror sizes, lower driving voltageand better optical characteristics of mirror surface [2,3]. MEMStechnology would be a promising way to make such devices,which use fabrication processes derived from single crystal siliconwafers. However, the design and fabrication of scanning mirrorare big challenges to realize the desired performances, especiallythe implemented fabrication process.

The structure of TOF which consists of MEMS scanning mirrorand chromatic dispersion device like grating is called grating-MEMS optical system [4]. This kind of system is a good way tobuild TOF due to its advantages of simpler structure, lower opticalinsertion loss and crosstalk, faster tuning speed and polarization

012 Published by Elsevier Ltd. All

ucer Technology, Science and

f Microsystem and Informa-

hai 200050, China.

u).

independence [5]. In this system, a polarization independentgrating takes the role of light dispersion; while a MEMS scanningmirror takes functions of reflection, light choosing and tunning.As the grating lines per mm is fixed, a large mirror size isproposed in order to achieve high resolution. Besides, a suitabletorsion angle is also needed to meet the tuning need. However,surface process was not often proposed. This is because when themirror was given a torsion load, the surface of the mirror wasprone to deformation, and it would cause aberration and astigma-tion of the light. This would be fatal for beam focus and filtering.Thus, bulk micromachining technology is more suitable forfabrication of high quality mechanical scanning mirror [6].

For MEMS scanning mirror, micro-actuation mechanism haspriority to device structuring and fabrication. Electrostatic actuationhas become the most popular approaches for scanning mirror,because of its faster speed with low power consumption and easyintegration with electronic control systems [7]. In general, electro-static actuators contain comb drive actuator or parallel-plate actuator.The parallel-plate electrostatic actuator has a simpler structure andeasier fabrication process, but it requests a higher driving voltage toobtain the required rotation angle. In the plate electrostatic actuator, asmall overvoltage can cause the pull-in effect and results in devicefailure [8,9]. On the contrary, comb drive actuator can offer the largerforce density resulting in a larger torsion angle with a lower drivingvoltage and also no influence of pull-in effect [10–13].

rights reserved.

Page 2: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–60 55

This paper presents the design, fabrication and characteriza-tion of a large size scanning mirror which is actuated by verticalcomb drive. The number of comb is optimized in order to offer alarger force density. The mirror is fabricated by using bulkmicromachining process technology, and it is characterized in aTOF optical system. The design, simulation and fabrication of thedevice are described in the following sections.

2. Design and structure simulation

As a key component of TOF, the mirror size and torsion angleshould meet the requirements of resolution and tuning, respec-tively. For example, if the grating lines per mm is 966/mm, for adense wavelength division multiplex (DWDM) system with100 GHz (0.8 nm) channel spacing in C-band, a 2.7 mm mirrorsize in the dispersion direction and a 721 torsion angle arerequired. By considering and fulfilling these requirements, avertical comb drive actuated scanning mirror has been proposed.

2.1. Structure design of scanning mirror

The schematic diagram of a scanning mirror is shown in Fig. 1.From the figure, vertical comb drive is selected as the actuator ofthe mirror plate for its larger force density and lower drivingvoltage [14,15]. The mirror plate is fastened to the fixed frame bya pair of torsion bars. On the surface of the mirror plate a layer ofsputtered gold is used as the reflecting layer. At both sides of thetorsion bar there are two cantilever beams with their one endconnected to the mirror plate. The rotor comb finger arrays areconnected to the cantilever beams and the stator comb fingerarrays are connected to the fixed frame. In order to excite atorsional motion, in the vertical direction a very small gap is setbetween stator comb finger arrays and rotor comb finger arrays.When the actuation voltage on one side is applied between thetwo parallel comb finger arrays, the torsion bars will rotate andresult in a controllable rotation angle of the mirror plate. Withdifferent driving voltages, the mirror can be rotated to desiredangles, therefore the desired light beams can be selected into theoutput channel.

For the scanning mirror of TOF application, the mirror size and thetorsion angle are decided by the optical system design of TOF, such asthe design of the grating’s angular dispersion and the working

Fig. 1. The schematic diagram of MEMS scanning mirror with vertical comb drive.

spectral range. When a static voltage is applied to rotor combs andstator combs, the electrical torque can be expressed as[16]

TðyÞ ¼ 2N1

2

dCðyÞdy

VðtÞ2 ¼Ne0VðtÞ2

g

dSðyÞdy

ð1Þ

where N is the number of the combs in one side of the mirror, C(y)is the capacitance, V(t) is the periodic excitation voltage, g is thegap between a rotor comb and an adjacent stator comb inhorizontal direction, S(y) is the overlapped area between rotorcombs and stator combs. From Eq.(1) we can see that the torque isproportional to the square of the applied voltage. As the number ofcombs increasing, the electrical torque will increase too. Reducingg can also improve the electrical torque, but the size of g is limitedby the process conditions.

The schematic diagram and operating principle of verticalcomb dive are shown in Fig. 2. The plan sketch of vertical combdrive is shown in Fig. 2(a). The state of no driving voltage ofvertical combs is shown in Fig. 2(b). Meanwhile Fig. 2(c) showswhen the combs are given a voltage, a rotation angle is obtained.In the figure, L is the distance from the torsion bar to the end ofthe rotor comb, l is the overlapped length of rotor comb and statorcomb, a, w and h are the length, width and thickness of the comb,respectively. From Eq.(1) above, the torsion angle is related to theoverlapped area. The relationship between torsion angle andoverlapped area is represented in Fig. 2(c). A formula about theangle and the area can be obtained by polynomial fitting as shownin the following equation:

SðyÞ ¼ k0þk1y1þk2y

2þ � � � þkny

nð2Þ

kn is the n-order coefficient of the polynomial. So if the structuralparameters are determined, there is a maximum torsion angle.When the torsion angle is at the maximum, S(y) is at themaximum point too. The scanning mirror can be operated witha torsion angle ranging from zero to the maximum angle.

When the thickness, length and overlapped length of thecombs are determined, the relationship of torsion angle andoverlapped area is shown in Fig. 3. In the figure, the three curvesreflect the relationship of torsion angle and overlapped area indifferent length of L. From Fig. 2(c), we know that L is the distancefrom torsion bar to the end of the rotor combs. As L becomingsmaller, the maximum torsion angle which the scanning mirrorcan achieve is becoming larger. But the force arm will becomesmaller too and this will increase the driving voltage. So adesirable value of L should be set to balance the requirementsof the maximum torsion angle and the driving voltage. As the

Fig. 2. The schematic diagram and operating principle of vertical comb drive.

Page 3: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Fig. 3. Overlapped area versus torsion angle under different lengths of the arm of

electrostatic force L.

Fig. 4. The FEM simulation of the scanning mirror under a 90 V DC bias driving

voltage.

Fig. 5. The calculated displacement and frequency in different thickness

and length of torsion bars.

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–6056

figure shows, when the length of L is 700 mm, the overlappedlength of combs is 250 mm, the thickness of the comb is 25 mm,can provide a 2.41 maximum tuning angle which is completelysuitable for the wavelength tuning requirement of TOF.

2.2. Structure parameters optimization based on finite element

analysis

The torsion angle is obtained by applying a voltage on thevertical comb drive. So by setting the parameters of vertical combdrive, a relationship between the angle and driving voltage will begained. In this finite element method (FEM) simulation, a typicalmirror plate in dimension of 25 mm in thickness, 1.8 mm in widthand 2.7 mm in length has been analyzed. The size of combs is25 mm in thickness, 7 mm in width, 300 mm in length and the gapg is 4 mm.

With the assistance of CoventorWare, the scanning mirrorwith vertical comb drive was simulated. The displacement ofmirror under 90 V DC bias is shown in Fig. 4. The maximumdisplacement was recorded as 57 mm, which was equivalent to

about 2.41 rotation angle of the mirror. By adjusting the thicknessand length of the cantilever beam, different displacements of thescanning mirror and different frequencies of fundamental vibra-tion mode were simulated as shown in Fig. 5. The results showthat the mirror performs a lower resonant frequency and largerdisplacement with a longer and thinner cantilever beams. Con-sidering all the factors mentioned above, the optimum width,thickness and the length of the torsion bar are 12 mm, 25 mm and120 mm, respectively; the optimum width, thickness and thelength of the comb are 7 mm, 25 mm and 300 mm, respectively.The width of the gap is 4 mm. And at the each side of the mirror,186 pairs of combs are set.

3. Fabrication

The dynamic deformation of the large mirror plate could be animportant issue, when the mirror operates in a high speed.However, surface micromachining process of micromirror giveslow optical quality when the mirror has a large size. One possiblemethod to make the mirror plate not easily distorted is toincrease the stiffness of the mirror plate [17]. Therefore, a siliconon insulator (SOI) wafer with a thicker device silicon layer hasbeen used to increase the stiffness of the mirror plate.

At the same time, the proposed scanning mirror is fabricatedby using a bulk micromachining process including Si–Si bondingtechnology. The Si–Si bonding has some obvious merits. First, itcan provide large mirror size with a high optical quality; second,offer more freedom in selecting mirror thickness and gap spacingbetween mirrors and electrodes; third, allow the independentfabrication of the mechanical and electrical components of adevice and simplify mechanical structures [18,19].

The proposed processing sequence to fabricate the scanningmirror can be found in Fig. 6. The starting substrate was a SOIwafer with a device layer of 25 mm in thickness and a buriedsilicon dioxide (BOX) layer in thickness of 2 mm (Fig. 6(a)). TwoSOI wafers were prepared and one of them was oxidized inthickness of 0.4 mm. Then the two device layers of the SOI waferswere bonded together as shown in Fig. 6(b). One side of thebonded wafer was etched with KOH solution until the buriedsilicon dioxide layer emerged as shown in Fig. 6(c). FromFig. 6(d) to Fig. 6(i), the emerged silicon dioxide layer was usedas the mask to fabricate the vertical combs by deep reactive ionetching (DRIE). First, using reactive ion etching (RIE), half

Page 4: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Fig. 6. The fabrication process of the scanning mirror.

Fig. 7. The surface roughness of the mirror plate.

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–60 57

thickness of the silicon dioxide layer covered on rotor combs wasetched out as shown in Fig. 6(d). Then the silicon dioxide layercovered on the gaps was etched clean as shown in Fig. 6(e). Thesilicon in gaps between combs was etched by using DRIE untilthe thin dioxide layer was emerged shown in Fig. 6(f). Next, theemerged thin dioxide layer and the remaining silicon dioxidelayer on rotor combs were all removed away as shown in Fig. 6(g).The exposed lower silicon device layer was completely etched asshown in Fig. 6(h). Finally, as Fig. 6(i) showed all the silicondioxide was removed. In Fig. 6(j), a silicon wafer was etched a pitwith the depth of 30 mm. Then the silicon wafer was bondedtogether with the SOI wafer. The substrate of the SOI wafer wasalso removed away as shown in Fig. 6(k), leaving the other buriedsilicon dioxide emerged. Silicon dioxide on the stator combs wascompletely etched as shown in Fig. 6(l). So the upper half of statorcombs was etched as shown in Fig. 6(m). Also the remainingsilicon dioxide was removed as shown in Fig. 6(n). At the last step,a hard mask was used to make the pads and reflection layer, whensputtering gold.

The scanning mirror was successfully fabricated by using thementioned fabrication process above as shown in Fig. 6. Thesurface roughness of the micromirror plate has been measured byusing a 3D interferometer (Veecos, WYKO) as shown in Fig. 7.The root mean square (RMS) of the surface profile is about8.64 nm which is adequate for the optical application.

Fig. 8 shows the scanning electron microscope (SEM) images ofthe fabricated scanning mirror. The mirror with vertical combdrives is shown in Fig. 8(a). The torsion bar is shown in Fig. 8(b).Fig. 8(c) presents the details of the vertical combs.

4. Performance testing

In order to verify its electromechanical performance andcompare the real measurements with the simulated results,experiments were conducted on the scanning mirror. Fig. 9 isthe picture of rotation angle test system, which consists of a 3Dinterferometer (Veecos, WYKO), DC power supply, control circuitand scanning mirror. The DC power supply generates a highvoltage in the range of 0–100 V. When a desired DC voltage was

applied to the scanning mirror by using a control circuit, themirror rotated to a desired angle. Then by using the interferom-eter, the rotation angle of the mirror was obtained and recorded.After a series of DC voltages were applied to the mirror, differentrotation angles of the mirror were obtained. The test results andthe theoretical curve are shown in Fig. 10. The test results indicatethat the maximal optical rotation angle is 2.31 with an appliedbias of 95 V and the rotation angle can fulfill the requirements ofTOF system. In comparison to the theoretical calculation, themeasured rotation angles are little lower than the theoreticalcurve. This slight discrepancy is probably caused by the fabrica-tion imperfections.

A TOF optical system tends to concern about the dynamiccharacteristics of the fabricated scanning mirror. The schematicconfiguration of the grating-MEMS optical systems is shown inFig. 11. It is composed of three major subassemblies,includingoptical collimators, a transmission grating and the fabricatedMEMS scanning mirror. The optical collimators are used tocollimate the input and output optical beams. The transmissiongrating is used to disperse the input optical beam, which consists

Page 5: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Fig. 8. The SEM image of fabricated scanning mirror with vertical comb drive.

Fig. 9. The rotation angle test system.

Fig. 10. Measurement results and theoretical curves of rotation angle versus

driving voltage.

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–6058

of multi-light channels. When the scanning mirror is rotated to adesired angle, which is determined by the driving voltage andcorresponds to a wavelength only, a chosen dispersed lightchannel will be reflected to the output collimator. By changingthe rotated angle, different channels will be chosen as an output,so the tuning function will be realized.

Comparing with the traditional optical system of TOF, in thisTOF system, the dispersed light channel was reflected and passedthrough the grating once again, so the system resolution wassignificantly improved. The benefits of this design is evident.Because of the improvement of resolution, the size of thecollimator and the scanning mirror can be designed smaller, sothe fabrication process also becomes simpler. And the whole TOFsystem becomes more compact. However, this also increases

Page 6: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Fig. 11. Optical system of TOF assembly scheme.

Fig. 12. Responding time of the scanning mirror: (a) turn-on responding time and

(b) turn-off responding time.

Fig. 13. Tuning spectrum of a 10-channel DWDM with a channel spacing of

100 GHz.

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–60 59

the difficulty of optical debugging. After the TOF systemwas assembled, some performance parameters were measured.The insert loss (IL) and polarization dependent loss (PDL) are2.3 dB and 0.4 dB, respectively.

The transient responses of the mirror for step input voltagesare measured by an oscilloscope (YOKOGAWA AQ6370B). When

no voltage is applied, the mirror is not rotated, so the output fiberhas no signal. The signal displayed on the oscilloscope is low level.When a voltage is applied on the vertical comb drive, the desiredwavelength will be selected, and the signal displayed on theoscilloscope will become high level. The conversion time whichmeans turn-on responding time of the mirror has been defined asthe time from mark a to mark b as shown in Fig. 12(a), recorded ofwhich is 1.887 ms. When the applied voltage is released, the turn-off responding time is recorded as 4.194 ms from mark a to markb as shown in Fig. 12(b). The reason why turn-off responding timeis longer than the turn-on responding time is that there is aremarkable difference between capacitor charging and dischar-ging time of the vertical comb drives.

In addition, the wavelength tuning function was successfullydemonstrated by using the fabricated TOF optical system.A DWDM system (IQS-12004B, EXFO) was used as the testinglight source. Ten adjacent channels were selected and the spacingof the two adjacent channels was 100 GHz (0.8 nm). The scanningmirror was operated by the control circuit which was used toprovide a fast conversion of the driving voltage. The obtained10-channels WDM tuning spectrum was shown in Fig. 13. Asshown in the figure, the measured transient responses of tuningtime between two adjacent channels are 1.507 ms which is littleshorter than the turn-on responding time. So if the C-band(40 channels) of the optical communication was completely

Page 7: Large size MEMS scanning mirror with vertical comb drive for tunable optical filter

Y. Liu et al. / Optics and Lasers in Engineering 51 (2013) 54–6060

tuned, the whole tuning time was about 70.9 ms. The small peaksin the edge of the figure were probably caused by stray light.From the figure, the base of the spectrum is uplifted, the mainproblem is that the adjacent channels are overlapped at the edges.But this effect can be reduced or eliminated by improving theoptical resolution of the system or extending the receiving opticalpath. Although the system was not operated at a high frequency, agood application prospects were suggested in the field of opticalperformance monitoring.

5. Conclusion

A scanning mirror with large mirror plate was designed andsuccessfully fabricated by using the bulk micromachining tech-nology. Vertical comb drive actuators were selected in order toprovide a larger electrostatic force and rotation angle. Due to thedevice silicon layer of SOI being used for the mirror plate, thefabricated reflector has a high quality optical surface with RMSroughness of 8.64 nm. The static and dynamic characteristics ofthe fabricated scanning mirror have been tested. A 2.3 rotationangle was obtained under a 95 V driving voltage which wascompletely suitable for the application of TOF. Meanwhile, themeasured turn-on responding time of the scanning mirror is1.887 ms and the turn-off responding time is 4.194 ms. In addi-tion, the tuning function was successfully demonstrated by usinga TOF optical system as well, and the tuning time of two adjacentchannels is 1.507 ms which is little shorter than the turn-onresponding time.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant no. 60877066).

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