A MEMS-based resonant-scanning lamellar grating Fourier transform micro-spectrometer with laser reference system

Sensors and Actuators A 149 (2009) 221–228

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

A MEMS-based resonant-scanning lamellar grating Fourier transform micro-spectrometer with laser reference system Feiwen Lee, Guangya Zhou ∗ , Hongbin Yu, Fook Siong Chau Micro/Nano Systems Technology, Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore

a r t i c l e

i n f o

Article history: Received 4 August 2008 Received in revised form 1 December 2008 Accepted 1 December 2008 Available online 11 December 2008 Keywords: MEMS Lamellar gratings Spectrometer Fourier transform Resonant Reference sampling

a b s t r a c t A lamellar grating Fourier transform infra-red (FTIR) micro-spectrometer is presented in which the device is electromagnetically actuated in resonant mode so as to achieve larger displacements with a lower driving voltage. By actuating at resonance, we can also have a design with a higher spring stiffness design such that the micro-spectrometer will have little influence from external perturbation. A data acquisition electronic system is designed such that the interferogram of the IR source can still be acquired at a fixed optical path distance (OPD) intervals. This is achieved by using a reference laser source. Working at a resonant frequency of 330 Hz, a 100 ␮m (bi-directional) displacement is achieved by the device with an input voltage of 2.2 V. A tunable laser source is used to demonstrate the system performance. The peak of the recorded spectra is very close to the actual wavelength of the IR, with a maximum difference of less than 5 nm. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Minaturization of Fourier transform infra-red (FTIR) spectrometers using micro-electromechancical systems (MEMS) technology has been reported in the literature. There are two main categories of micro-spectrometers reported—those based on Michelson interferometers [1–4], and those utilizing lamellar grating interferometers. The latter increases light utilization efficiency and eliminates problems with the non-constant reflection-to-transmission ratio over the range of a broadband source. In addition, lamellar gratings can be fabricated relatively easily using current fabrication processes which also allow greater ease of integration, resulting in the further reduction of device size. Manzardo et al. [5] and Ataman et al. [6,7] have presented impressive results of lamellar grating-based micro-spectrometers which are actuated using electrostatic force, while Chau et al. [8] have implemented a stationary micro FTIR with one side of the grating tilted. It is desirable to design actuators with large travels as the resolution of the spectrometer improves with increasing lamellar grating displacement. However in MEMS, actuator designs are generally limited in the maximum travel displacement which can be produced. Relatively high input voltages are required to produce large actuation forces, especially in the case of electrostatic actuation, and this is coupled with the need for spring designs to have low stiffnesses making the device more

∗ Corresponding author. E-mail address: [email protected] (G. Zhou). 0924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2008.12.002

susceptible to external environmental perturbations. Hence there is always a motivation to actuate the device at resonant frequency, as a large displacement can be produced at relatively low voltages and the spring can generally be designed to be stiffer. Another advantage of actuating the device at resonant frequency is that the interferograms can be collected at higher speeds and therefore transient spectroscopy measurements is possible. A peak-to-peak displacement of 106 ␮m at 28 V using this method has been reported [6,7]. However, one major problem associated with actuating in the resonant mode is the difficulty of sampling the interferogram signal from the detector in uniform, discrete intervals. Ataman et al. [6,7] attempted to solve the problem by introducing an interpolation and resampling algorithm to the data before the Fourier transform operation. In this paper, we introduce a lamellar grating micro-spectrometer design which alleviates the sampling problem. This micro-spectrometer which is actuated in resonant mode by an electromagnet, uses a diode-pumped solid-state (DPSS) laser (533 nm wavelength) as an optical reference to measure precisely the displacement of the lamellar grating and also to trigger the sampling of the detector. The reference laser is commonly used in commercial spectrometer and was first used by Connes as a method of improving the frequency accuracy; hence it is also called the Connes Advantage [9]. The design and fabrication process of the spectrometer is briefly described, followed by the electronic design for the data acquisition system. The spectra measurement of a tunable laser source (ranging from 1520 to 1600 nm) performed with the micro-spectrometer is also presented.

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Fig. 1. Optical diagram of the lamellar grating with light of wavelength , incident normally on the grating surface and diffracted at an angle ˛. The width of the slit is a and the depth of the grating is l.

2. FTIR principle The lamellar grating interferometer can be treated as two sets of mirrors which are the front facets and the back facets [10] (Fig. 1). Each set of strip mirrors can be considered as a linear array of identical long, rectangular apertures. For the apertures, the diffracted wave amplitude is given by Bfront = Bs F

(1)

where Bs is the diffracted wave amplitude for a single facet and F expresses the interference between the diffracted wave from the array of mirrors or facets. For a long slit of width a/2 with normally incident waves of wavelength , the diffracted wave amplitude at an angle ˛ is given by



Bs ∝

sin[(a sin ˛)/2] (a sin ˛)/2



(2)

For an array of N facets at a distance of a apart, F=



sin[(Na sin ˛)/] (N − 1)a sin ˛ exp −i  sin[(Na sin ˛)/]



(4)

Since the front and back arrays are identical, the two reflected waves have only a phase difference ϕ between them. Thus we have, iϕ

(5)

The phase is given by ϕ = 2ı. The remaining problem is to calculate ı, the optical path difference (OPD). From Fig. 1, we observe that



ı =  (1 + cos ˛) + (a/2) sin ˛

(6)

The total amplitude of the wave diffracted from the lamellar gratings is given by



B∝



 

   1 + eiϕ e−iϕN

sin (a sin ˛) /2 sin (Na sin ˛) / (a sin ˛) /2

sin (a sin ˛) /

(7)

where ϕN ≡ [(N − 1)a sin ˛]/ So, the lamellar grating illumination is

 ∗

I ∝ BB ∝



4 cos2

[(sin a sin ˛)]/2 [(a sin ˛)]/2

ϕ  2

2 

sin[(N a sin ˛)]/ sin[(a sin ˛)]/



I(ı) ∝ 4 cos2

2

2ı 

(9)

where I(ı) is often referred to as the interferogram. Using the Wiener–Kintchine theorem [11], the light power spectrum B() and the interferogram is related to a Fourier transform by the following equation:



∞

B() =

B = Bfront + Bback



the front or back plane. The first and second terms are basically constants which account for the grating design and the incident angle of the light ray. The third term results from the phase difference between the waves diffracted from the front and back mirrors, respectively. Considering the zeroth order and normal incidence of the light, i.e. ˛ = 0 the intensity of the zeroth diffraction order beam is then proportional to the OPD through the following expression:

(3)

Considering the effect of both front and back facets, the total amplitude is

B = Bs F + Bs Fe

Fig. 2. Fabrication process flow: (a) wafer preparation, (b) front-side DRIE, (c) backside DRIE, (d) release and (e) attachment of magnet.

I(ı) exp(−i2ı) dı

(10)

−∞

where  = 1/ is the wavenumber. 3. Device design and fabrication The device is fabricated using the SOI MUMPs process offered by MEMSCAP® . A schematic of the whole fabrication process is shown in Fig. 2 [12]. The process starts with a silicon-on-insulator (SOI) wafer, which consists of a stack of handle wafer (400 ␮m), buried oxide (1 ␮m), and device wafer (25 ␮m). The design consists of a central platform which is suspended by four folded-beams at its four edges. Gratings are designed such that one set of the fingers is attached to the central platform while the alternate set is fixed to the main substrate. The design of the spectrometer is etched into the device wafer as shown in Fig. 3. Fig. 4 shows a close-up view of the various features. The dimensions of the various features are shown in Table 1. A gold layer is deposited onto the lamellar grating to improve the reflectivity of the grating. Considering a 100% reflectance of the coated gold thin film, and a duty cycle of 0.77, the light efficiency of the grating is estimated to be 0.77. A large hole is etched Table 1 Dimensions of structure.

(8)

The first term in Eq. (8) is the single facet or mirror term; the second term accounts for the array of N identical mirrors in either

Part

Size

Folded beam

Length: 2.43 mm; width: 25 ␮m Finger width: 10 ␮m; gap: 3 ␮m; Finger length: 500 ␮m 1 mm × 1mm

Grating Platform

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Table 2 Attributes of driving coil. Attribute types

Attribute values

Operating voltage Holding force Power consumption Magnet weight Magnet diameter

24 V dc 130 N 3.2 W 70 g 25 mm

central platform to actuate in an out-of-plane manner. Attributes of the driving coil can be found in Table 2. 4. Experimental setup

Fig. 3. SEM photograph showing the design of the device wafer.

The schematic of the optical setup is shown in Fig. 5. The output from the IR source and DPSS laser (which acts as the reference laser) are both coupled into an optical fibre and made incident on the device’s lamellar grating via a collimator and a focusing lens. An iris is used to filter out the unwanted diffraction orders emanating from the grating. The zeroth order beam is split into the IR and visible laser components via a cold mirror and each monitored by the respective detector. The driving coil is driven dynamically by a signal generator at the resonant frequency of the device. 5. Electronic data acquisition system architecture

Fig. 4. SEM photograph showing a close-up view of the central platform and the grating structures on both sides of the platform. The fourfold beams support the central platform from the four edges. The grating structure consists of movable fingers which are attached to the central platform while the alternate fingers are fixed stationary to the main substrate.

on the handle wafer just below the designs of the device wafer such that the suspended platform is released when the buried oxide layer is removed. A small permanent magnet (dimensions: 700 ␮m × 700 ␮m × 500 ␮m, mass: 2.7 mg) with the magnetization perpendicular to the device surface is then attached manually to the central platform, using UV epoxy. The actuation of the spectrometer is caused by a driving coil positioned under the device. The driving coil will attract and repel the small permanent magnet, causing the

In this section, a detailed description of the electronic data acquisition system (Fig. 6) is presented. A total of three signals are acquired from the optical setup. They are the driving signal generated by the signal generator, the IR signal whose spectrum is to be determined and the DPSS laser signal which is used for the internal wavelength reference. These signals have been modulated and conditioned into digital signals such that they can be used to trigger the respective devices and computer algorithms in the system. As mentioned earlier, the lamellar gratings of the spectrometer will be in resonant motion and this is controlled by the signal generator. The same signal also serves as a trigger to initiate the whole data acquisition process. It is desirable that any interferogram be recorded from the position where the lamellar grating has achieved the greatest displacement, corresponding to the time when the maximum voltage is supplied. This is done by connecting the signal from the signal generator to a comparator and to adjust the reference voltage of the comparator such that a digital high output signal – to initiate the data acquisition program – is generated when the input voltage is at its maximum voltage. The signal from the IR detector is amplified by operational amplifiers which are chosen for having high slew rates and relatively short

Fig. 5. Schematic of optical setup for dynamic data acquisition process.

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Fig. 6. Schematic of the data acquisition system.

response times. This is necessary so as not to introduce any undue delay in acquiring the IR interferogram after initiation. The resultant analogue signal is then digitized using an analogue–digital convertor (ADC). A conversion is initiated on the falling edge of the conversion signal that is derived from the DPSS laser signal. As the lamellar grating resonates, the OPD of the DPSS interferogram signal will vary in a sinusoidal fashion. Hence, the DPSS output signal obtained, when plotted in the time domain, will have a varying periodicity in accordance with the instantaneous velocity of the lamellar grating. The period of the signal increases as the movable grating slows down when it reaches the extremities. Conversely, the period is the shortest when the movable grating is at the time when it is in-plane position with the fixed grating. A theoretical plot of the interferogram of a monochromatic light source with the lamellar grating driven at 1 Hz is shown in Fig. 7. It should be noted that one cycle of the reference interferogram, although non-uniform in the time domain, corresponds to a constant OPD which is equivalent to the wavelength of the reference light source. The interferogram signal oscillates about an average value (the DC level). A high- precision electronic circuit is used to produce a trigger pulse each time the signal crosses the DC level. The circuitry

is designed such that one pulse is generated per cycle of the reference interferogram. Using this methodology, it is then possible to sample the IR interferogram at uniform discrete OPD intervals (which is basically the wavelength of the DPSS laser) regardless of the motion of the lamellar grating. Fig. 8 shows the interferogram signal of the DPSS laser obtained during the experiment. It shows the varying periodicity that is due to the resonant motion of the lamellar gratings as mentioned earlier. Furthermore, it can be observed that there is an inherent DC drift inside the interferogram as indicated by the dashed line in the figure. It can also be seen that there is a decaying trend in the amplitude of the signal as the displacement of the lamellar grating increases. This decay in amplitude is studied in detail and discussed in the next section. As mentioned earlier, the use of the DPSS reference signal as the trigger signal is aimed at ensuring that the IR interferogram is sampled at uniform discrete OPD intervals. However, the fluctuating DC level of the DPSS signal obtained (Fig. 8) will result into random sampling of the IR interferogram and hence introduce spectral noise in the resultant spectrum [13,14]. To counter this, the original DPSS laser signal is conditioned and modulated in four stages, namely amplification, high pass filtering, DC level shifting and generation

Fig. 7. Theoretical plot of interferogram of a monochromatic light source with the lamellar grating driven at 1 Hz.

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Fig. 8. Interferogram signal of the DPSS laser as the electromagnetic actuator moves at 1 Hz with an input voltage of ±24 V. A displacement of ∼50 ␮m (bi-directional) is achieved. Signal sample rate = 20 kHz. Dashed line indicates inherent DC drift.

of trigger pulses, in order to generate the correct trigger pulses for the ADC. The output signal from the (visible wavelength) photodetector is relatively small (∼10 mVpp) and needs to be amplified to a reasonable range of ∼1 Vpp. This is done at the first stage through an amplification circuit with an operational amplifier (AD825). At the next stage, the drifting DC component is removed using a high pass filter design that consists of a capacitor in series with the signal path in conjunction with a resistor in parallel with the signal path. After a constant DC oscillating voltage is achieved, the DC level of the signal has to be raised as the comparator (AD8561) only accepts positive signal input. A simple potential divider method is used to raise the input signal to the required value. A non-inverting amplifier with unity gain is placed before the potential divider. It is used as a voltage follower, acting as a buffer to provide good current drive. The conditioned signal is then sent to the comparator (AD8561) such that the sinusoidal input is transformed into a square wave by putting the reference voltage of the comparator at the DC level of the modulated signal. The square wave signal is then feed to the monostable multivibrator 74HCT4538 for the generation of trigger pulses. The function of the monostable multivibrator is to output a pulse by either the falling or rising edge of the input pulse. The duration and accuracy of the output pulse are determined by the external timing components. These trigger pulses are then used as the triggering signals for the ADC conversion. Effectively, the developed electronic circuit has conditioned the raw DPSS laser signal to allow it to be used by the comparator and subsequently for generating the trigger pulses for the ADC. The algorithms for controlling the data acquisition process are developed under the LabView Program FPGA system. Field programmable gate arrays (FPGA) are actually arrays of silicon chips with unconnected logic gates. The functionality of the FPGA can be defined by using software to configure the FPGA gates. Hence, the functionality of the software is effectively converted into hardwarebased (logic gates) algorithms. FPGA systems offer many advantages

over conventional software control, especially in terms of processing time. The fast response time offered by the FPGA system ensures that there is minimal delay in capturing the digital data from the ADC, thus reducing the probability that the processed spectra will be affected. The software algorithm developed is described in Fig. 9. 6. Experimental results In the experiment, the resonant frequency of the device is first identified by sweeping through the frequencies of the signal generator over the range 100–400 Hz at 10 Hz intervals. The peak-to-peak voltage of the signal is kept at 300 mV throughout the experiment. The amplitude of vibration of the lamellar grating is determined by observing the reference DPSS signal. Subsequently, the frequency response of the system can be obtained and the resonant frequency identified, as shown in Fig. 10. It shows that the device has a resonant peak at 330 Hz. By driving the device at this frequency, a total grating displacement of ∼100 ␮m (bi-directional) is achieved with an input voltage of 2.2 V. A tunable laser source is used in the experiment. IR radiations, with wavelength ranging from 1520 to 1590 nm are tested at 10 nm intervals. The interferograms are collected using the data acquisition system and the spectra calculated using Fourier transform as outlined earlier. The wavelength at which the peaks occurred in the various spectra are obtained and shown in Table 1. The full widths at half maximum (FWHM) spectral resolution are also measured and tabulated. The actual spectra for selected wavelengths are shown in Fig. 11. As can be seen from Table 3, there is very good agreement between the peak values obtained from the spectra and the actual wavelength of the IR radiations. 7. Analytical study and discussion of the DPSS signal The decaying trend of the DPSS signal, mentioned previously, whereby it decreases with increasing displacement of the lamellar

Fig. 9. Schematic of data acquisition software algorithm.

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grating, can be attributed to the lower efficiency of transmission of the diffracted wave from the back facets of the lamellar grating. There is a possibility that the diffracted waves from the back facets may have been absorbed or internally reflected by the sidewalls of the fixed grating, attenuating the resultant intensity of these beams. The magnitude of attenuation will increase with the depth of the grating; hence an attempt to explain this phenomenon theoretically can be done by revisiting the derivation of the diffraction equations for the lamellar grating as introduced by Strong and Vanasse [10]. The situation can be modeled by introducing an attenuation factor ˇ to the amplitude of the diffraction wave of the back facet, Bback in Eq. (4), i.e. Fig. 10. Frequency response of the device. The resonant frequency of the device is first identified by sweeping through the frequencies of the signal generator over the range 100–400 Hz at 10 Hz intervals. The peak-to-peak voltage of the signal is kept at 300 mV throughout the experiment. Table 3 Peak recorded and error factor at various wavelengths of IR radiation. Wavelength of IR radiation (nm)

Peak recorded in spectra (nm)

Full width at half maximum (FWHM) spectral resolution (nm)

1520 1530 1540 1550 1560 1570 1580 1590

1525 1535 1544 1555 1565 1575 1585 1595

19 19 20 20 20 20 20 20

B = Bs F + ˇBs Feiϕ

where 0 < ˇ ≤ 1

(11)

The total lamellar grating illumination, considering the amplitude loss of the back facets, can be rewritten from Eq. (8) as

 I ∝ BB∗ ∝



4ˇ cos2

[(sin a sin ˛)]/2 (a sin ˛)/2

ϕ 2

+ (1 − ˇ)

2

2 



sin[(Na sin ˛)/] sin[(a sin ˛) /]

2

(12)

Similarly, considering only the zeroth order and normal incidence of the radiation, ˛ = 0, Eq. (12) can be simplified as follows: 2

I(ı) ∝ {4ˇ cos2 (2ı/) + (1 − ˇ) }

(13)

where ı is the OPD.

Fig. 11. Spectrum of IR source at wavelength of (a) 1520 nm, (b) 1540 nm, (c) 1560 nm and (d) 1580 nm. A total grating displacement of ∼100 ␮m (bi-directional) is achieved with an input voltage of 2.2 V.

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Fig. 12. Plot of Eq. (15) is plotted with  = 0.005,  = 0.533 ␮m and ı ranging from 0 to 80 ␮m.

As mentioned earlier, it is expected that there will be loss of the diffracted wave intensity from the back facets as the displacement of the grating is increased. Hence the attenuation factor ˇ should be a function of the OPD ı and the function should decrease as ı increases. For simplicity, we assume that the attenuation factor ˇ is ˇ(ı) = 1 − ı

(14)

where  is some constant which describes the rate of amplitude loss with the displacement of the grating. Substituting Eq. (14) into Eq. (13), I(ı) ∝

 



4 1 − ı cos2 (2ı/) + (ı)

2



(15)

Eq. (15) is plotted in Fig. 12 with  = 0.005,  = 0.533 ␮m and ı ranging from 0 to 80 ␮m. The figure shows a gradual decay of the amplitude of oscillation and a non-constant DC level of the signal. It is observed that the actual signal from the photodetector as presented in Fig. 8 has close resemblance to the analytical plot of Eq. (15) (Fig. 12). The close resemblance of the two figures goes someway to reaffirm the hypothesis that the intensity of the diffracted waves from the back facets reduces as the displacement of the grating increases. The likely cause is the absorption or reflection by the sidewalls of the gratings. The attenuation of the signal due to the inability to focus as the lamellar grating is moved out of the zero position is not considered. Considering the parameters of the optical setup, such as beam width, lens focal length etc, standard Gaussian beam propagation calculating method can be used to estimate the depth of focus for the system. The depth of focus, z is calculated to be 0.729 mm which is much larger than the maximum travel of the device. Consequently, the lighting status can be treated as constant during the whole device operation and will not cause the intensity variation.

8. Conclusion In this paper, a working FTIR lamellar grating microspectrometer actuated in resonance mode at a frequency of 330 Hz has been presented. It has a maximum displacement of 100 ␮m (bidirectional) at a voltage input of 2.2 V. Electronic data acquisition system has been developed to acquire the interferogram at the resonant mode with the aid of a reference DPSS laser. The device has been used to measure the output of a tunable laser source at wavelengths ranging from1520 to 1590 nm and the results obtain show high accuracy; the maximum difference was about 5 nm or 0.4%.

Acknowledgement Financial support by the Ministry of Education (MOE) Singapore AcRF Tier 1 funding under Grant No. R-265-000-235-112 is gratefully acknowledged. References [1] Y. Kyoungik, L. Daesung, K. Uma, et al., Micromachines Fourier transform spectrometer on silicon optical bench platform, Sens. Actuators A 130 (131) (2006) 523–530. [2] O. Manzardo, H.P. Herzig, C.R. Marxer, et al., Miniaturized time-scanning Fourier transform spectrometer based on silicon technology, Opt. Lett. 24 (23) (1999) 1705–1707. [3] A. Kenda, C. Drabe, H. Schenk, et al., Application of a micromachined translatory actuator to an optical FTIR spectrometer, Proc. SPIE 6186 (2006) 618609.1–618609.7. [4] U. Wallrabe, C. Soft, J. Mohr, et al., Miniaturized Fourier transform spectrometer for the near infrared wavelength regime incorporating an electromagnetic linear actuator, Sens. Actuators A 123 (124) (2005) 459–467. [5] O. Manzardo, R. Michaely, F. Schadelin, et al., Miniature lamellar grating interferometer based on siliscon technology, Opt. Lett. 29 (13) (2004) 1437–1439. [6] C. Ataman, H. Urey, A. Wolter, A Fourier transform spectrometer using resonant vertical comb actuators, J. Micromech. Microeng. 16 (2006) 2517–2523. [7] C. Ataman, H. Urey, S.O. Isikman, et al., A MEMS based visible-NIR Fourier transform microspectrometer, Proc. SPIE 6186 (2006), 61860C.1–9. [8] F.S. Chau, G. Zhou, Y. Du, et al., A micromachined stationary lamellar grating interferometer for Fourier transform spectroscopy, J. Micromech. Microeng. 18 (2008) 025023.1–025023.7. [9] J. Connes, P. Connes, Near-infrared planetary spectra by Fourier spectroscopy. I. Instruments and results, J. Opt. Soc. Am. 56 (1966) 896–910. [10] J.D. Strong, G.A. Vanasse, Lamellar grating far infrared interferometer, J. Opt. Soc. Am. 50 (1960) 113–124. [11] O. Manzardo. Micro-sized Fourier Spectrometers, Neuchatel, University, Thesis (doctoral), 2002, pp. 105–107. [12] H. Yu, F.S. Chau, G. Zhou, et al., An electromagnetically driven lamellar grating based Fourier transform microspectrometer, J. Micromech. Microeng. 18 (2008) 055016.1–055016.6. [13] L. Palchetti, D. Lastrucci, Spectral noise due to sampling errors in Fouriertransform spectroscopy, Appl. Opt. 40 (19) (2001) 3235–3243. [14] D.L. Cohen, Performance degradation of a Michelson interferometer due to random sampling errors, Appl. Opt. 38 (1) (1999) 139–151.

Biographies Feiwen Lee received the BEng from the National University of Singapore, 2006. He is currently a Masters student at Micro/Nano Systems Initiative Technology with the Department of Mechanical Engineering, National University of Singapore. His research topic is on the design and fabrication of optical micro-spectrometers based on MEMS technology. Hongbin Yu received a BS in mechanical engineering, MS in electrical engineering and PhD in optical engineering from Huazhong University of Science and Technology, China, in 1999, 2002 and 2006, respectively. He is currently a research fellow at Micro/Nano Systems Initiative Technology with the Department of Mechanical Engineering, National University of Singapore. His research interests involve the

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design, simulation and fabrication technology of microelectromechanical devices and optofluidics. Guangya Zhou is an assistant professor with the Department of Mechanical Engineering, National University of Singapore. His main research interests include microoptics, diffractive optics, MEMS devices for optical applications, and nanophotonics.

Fook Siong Chau is an associate professor in the Department of Mechanical Engineering, National University of Singapore, where he heads the Applied Mechanics Academic Group. His main research interests are in the development and applications of optical techniques for nondestructive evaluation of components and the modeling, simulation, and characterization of microsystems, particularly bio-MEMS and MOEMS.