A MEMS Based Visible-NIR Fourier Transform Microspectrometer C. Ataman 1 , H. Urey 1 , S.O. Isikman 1 , and A. Wolter 2 1 Optical Microsystems Laboratory, Department of Electrical Engineering, Koc University Rumeli Feneri Yolu, 34450, Sariyer, Istanbul, TURKEY Phone: +90-212-338-1474, E-mail: cataman@ku.edu.tr, hurey@ku.edu.tr 2 Fraunhofer Institute of Photonic Microsystems, Germany ABSTRACT Design, fabrication and characterization of a novel out-of-plane vertical comb-drive actuator based Fourier transform microspectrometer (FTS) is presented. The spectrometer utilizes resonant mode vertical comb actuators as a variable-depth diffraction grating and a single photodetector to monitor the 0th order of the diffraction pattern. The spectrum of the source illuminating the gratings is computed by Fourier transforming the 0th order intensity as a function of the optical path difference. The vertical comb actuators have a travel range of 100µm under atmospheric pressure with 28V excitation, which yields a theoretical spectral resolution of 0.5nm in the visible and better than 5nm in the telecom wavelengths. Keywords: Comb actuation, Fourier transform spectroscopy, MEMS, diffraction grating. 1. INTRODUCTION A lamellar grating interferometer (LGI) is a special type of Fourier transform spectrometer based on a diffraction grating operating in the 0 th order [1]. If multiple periods of a resonating vertical comb actuator are illuminated by a light beam, the light diffracts into several different orders equally spaced by an angle determined by the period of the fingers. The intensity at the center of the 0 th diffraction order depends solely on the spectral content of the illuminating light. Fourier transform of the 0 th order intensity as the optical path difference of the grating changes gives the spectrum of the diffracted beam. For high resolution spectrometry, the travel range of the movable fingers should be as high as possible. Due to the lack of a beam-splitter in an LGI very compact spectrometers with good performance can be realized. A Lamellar Grating Interferometer based on out-of plane mode resonant comb actuators is presented. Out-of- plane resonant mode operation of the comb actuators allows for large deflections (high spectral resolution) while providing a large light collection mirror area compared to other microspectrometers reported in the literature [2- 3]. Fabrication and operation principles of the device are simple. This minimizes the size, complexity, cost and design effort of a fully functional system (an operational spectrometer using this device requires integration with a simple photodetector and readout circuit). Only moving part is the resonating MEMS structure. With slight modifications in the device dimensions, spectrometers for very wide range of wavelengths can be realized. Structural properties of the movable grating designed for this application is presented in section 2. Operation principle of the spectrometer system is described in section 3. Section 4 explores the theoretical and experimental performance analysis results for the system. 2. MOVABLE GRATING STRUCTURE 2.1 Device Layout The device is fabricated and packaged inside a LCC housing by Fraunhofer IPMS, Germany. In Figure 2, a picture of the packaged device can be seen. The core of the structure consists of two sets of electrostatic comb fingers that are simultaneously utilized as an actuator and a variable-depth diffraction grating. The comb fingers have a high fill-factor (70μm finger width and 5μm gap) and are covered with a thin aluminum layer to enhance reflectivity; thus a high light efficiency is obtained. Movable comb fingers are attached to a 250μm wide H- shaped backbone to increase the stability. This backbone is bonded to the fixed frame via four folded flexure