I. INTRODUCTION. Conventional Fourier transform infrared (FTIR) systems use a Michelson interferometer configuration with a movable mirror in addition to a ...
MEMS Fourier Transform IR Spectrometer 1
N. Pelin Ayerden, 1Sven Holmstrom, 1Huseyin R. Seren, 1Selim Olcer, 1Jaibir Sharma, 2Stephan Luettjohann, 3Thilo Sandner, 1Hakan Urey 1 Koç University Istanbul, Turkey Bruker Optics Ettlingen, Germany 3 Fraunhofer IPMS Dresden, Germany 2
thoroughly treated by our group in [8]. The mechanical characterization of the device is made with a laser Doppler vibrometer (LDV). The movable grating is driven by eight comb-finger sets, which are driven with an offset square wave at a frequency of twice the mechanical resonance frequency. Driven at 65 V at ambient pressure, the grating deflects 652 m peak to peak at a resonance frequency of 335 Hz. Voltage response is linear, while the frequency response follows a typical electrostatic spring softening hysteretic behavior (Figure 2).
Abstract—A comb-actuated MEMS lamellar grating FTIR spectrometer with maximum OPD of 652m and clear aperture area of 9.6mm2 is developed. Laser and IR interferograms in 2.516m wavelength band are acquired at ambient pressure. Keywords-MEMS;Fourier transform spectroscopy;IR;lamellar grating
I.
INTRODUCTION
Conventional Fourier transform infrared (FTIR) systems use a Michelson interferometer configuration with a movable mirror in addition to a beam splitter and a stationary reference mirror. When the lamellar grating interferometer (LGI) was demonstrated in 1960 the machining necessary to commercialize such systems was not available [1]. With the rapid development of microfabrication techniques in recent years this technology has become possible to realize. In an LGI device the 0th order of the diffraction pattern from a dynamic diffraction grating is measured with a single photodetector. This configuration essentially eliminates the reference mirror and the need for beam splitter and dispersion compensation plates, enabling more compact and simpler systems. The present device is a micromechanical systems (MEMS) stage carrying a lamellar grating. It is electrostatically actuated using comb fingers and driven at resonance. It exhibits a large out-ofplane stroke with a small dynamic deformation (less than /10 for the mid-infrared range) and a large aperture, making it the highest performing LGI yet published including our previous work [2,3,4,5]. II.
IRSource MEMSdevice Offaxisparabolic mirror,f=20mm
(a)
(b)
Offaxisparabolic mirror,f=5.8mm IRDetector
Figure 1.(a) MEMS device (b) IR-setup including the source, detector and the MEMS device.
DEVICE DESIGN AND PROPERTIES
As in Fig.1.a, the grating consists of moving and fixed fingers. The platform holding the moving fingers are hinged by four beams, each ending in a spring structure. To achieve the large stroke needed for the LGI, pantograph type of suspensions, originally introduced by Fraunhofer IPMS [6,7], are used and modified. Pantograph suspensions have a highly efficient torsional motion conversion mechanism. The optical design of an LGI spectrometer is a compromise between several factors to reach the highest performance. The two most important factors to consider for an LGI device are the following: A large grating period leads to problems with mixing of the diffraction orders, reducing the signal-to-bias ratio. A small period, on the other hand, has a short Talbot image distance limiting the maximum usable mechanical deflection range. The resulting compromise is dependent on the desired wavelengths. These considerations have earlier been
Figure 2. Frequency response of a MEMS device measured with laser Doppler vibrometry.
III.
A. Laser Interferogram In order to test the functionality of the device, a HeNe red laser with 632.8 nm wavelength and 0.8 mm beam diameter
This project is sponsored by MEMFIS project, supported by EC FP7 program grant no: 224151.
c 978-1-4577-0336-2/11/$26.00 2011 IEEE
INTERFEROGRAM ACQUISITION
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was used. The signal from the photodetector, i.e. the interferogram, is recorded by an oscilloscope. Since the device is operated at resonance, the speed of the device varies sinusoidally. This results in an interferogram that is composed of fringes with varying fringe frequency, a chirped sine wave. The interferogram is resampled in MATLAB so that the sinusoids are equidistant and then its Fourier transform is calculated, which gives the spectrum. The recorded interferogram is shown in Figure 3 together with a zoomed in detail to show the chirped nature of the signal.
interferogram data show that the static and dynamic flatness are below the critical limits Ultimately, whole system design will be completed with IR source and detector towards the project goal of achieving a resolution of 10 cm-1 in the infrared region.
zoomedin
Figure 4. Interferogram captured from the IR-source.
ACKNOWLEDGMENT This project is sponsored by MEMFIS project, supported by EC FP7 program grant no: 224151. The authors would like to thank all MEMFIS partners especially Fraunhofer IPMS and Bruker Optics. All microfabrication was carried out at CMI at École Polytechnique Fédérale de Lausanne, Lausanne Switzerland. We are grateful to the CMI staff for all their help.
Figure 3. Interferogram from a 632.8 nm laser. The graph contains unprocecessed data from the photodetector, so the output sinus is chirped.
B. IR Interferogram In the setup given in Figure 1.b, the core of FTIR spectrometer is demonstrated. The IR source and detector are developed for the MEMFIS project by Bruker Optics and Vigo Systems, respectively. The source is a blackbody radiator at 1069 K temperature with an emission diameter of only 0.5 mm. Then, it is collimated by an off-axis parabolic mirror with 5.8 mm focal length. After the beam is reflected from the slightly inclined MEMS device, the second off-axis parabolic mirror with 20 mm focal length collects and focuses it onto the detector. The signal obtained from the detector, IR interferogram, then goes through a similar procedure as the laser interferogram does. In the complete spectrometer setup, a reference laser is used for the optical position feedback. The backside of the device is used for this purpose. The laser is sent with an angle and the 0th order is collected. The IR interferogram can be sampled at every zero crossing of the reference laser signal to obtain spatially equidistant samples. Then, simple FFT gives the spectrum without any post processing. Quality of IR and laser fringes show that the static and dynamic flatness is at acceptable levels and the device does not have any significant mode coupling issues. Figure 4 shows an IR interferogram of a device with 56 m peak to peak deflection recorded with the IR detector. IV.
REFERENCES [1] [2]
[3]
[4]
[5]
[6]
[7]
CONCLUSIONS
FTIR spectrometer with out-of-plane resonant mode is implemented and characterized. Maximum peak to peak deflection of 652 m is obtained with 65 V peak to peak input voltage at a resonance frequency of 335 Hz. The obtained
[8]
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