This paper presents the design, fabrication and characterization of a linear-variable optical-filter (LVOF) that will be used in a micro-spectrometer operating in infrared (IR) for natural gas composition measurement. An LVOF is placed on top of an array of detectors and transforms the optical spectrum into a lateral intensity profile, which is recorded by the detectors. The IR LVOF was fabricated in an IC-compatible process using a photoresist reflow technique, followed by transfer etching of the photoresist into the optical resonator layer. The spectral range between 3 to 5 μm contains the absorption peaks for hydrocarbons, carbon-monoxide and carbon-dioxide. The resulting optical absorption is utilized to measure the gas concentrations in a sample volume. Two LVOF structures were designed and fabricated on silicon wafers using alternate layers of sputtered silicon and silicon-dioxide as the high-and low-refractive index materials. These filters consist of a Fabry-Pérot resonator combined with a band-pass filter designed to block out-of-band transmissions. Finally, the filters were fully characterized with an FTIR spectrometer and showed satisfactory agreement with the optical thin-film simulations. The characterization showed a spectral resolution of 100 nm, which can be further improved with signal processing algorithms. This method makes it possible to fabricate small and robust LVOFs with high resolving power in the IR spectral range directly on the detector array chip.

A miniaturized methane (CH 4 ) sensor based on nondispersive infrared absorption is realized in MEMS technology. A high level of functional integration is achieved by using the resonance cavity of a linear variable optical filter (LVOF) also as a gas absorption cell. For effective detection of methane at λ = 3.39 µm, an absorption path length of at least 5 mm is required. Miniaturization therefore necessitates the use of highly reflective mirrors and operation at the 15th-order mode with a resonator cavity length of 25.4 µm. The conventional description of the LVOF in terms of the Fabry-Perot resonator is inadequate for analyzing the optical performance at such demanding boundary conditions. We demonstrate that an approach employing the Fizeau resonator is more appropriate. Furthermore, the design and fabrication in a CMOS-compatible microfabrication technology are described and operation as a methane sensor is demonstrated. 1-3), 191-197 (2000). 604-606 (1990). 42. T. T. Kajava, H. M. Lauranto, and R. R. E. Salomaa, "Fizeau interferometer in spectral measurements," J. Opt. ©2016 Optical Society of AmericaSoc. Am. B 10(11), 1980-1989 (1993). 43. P. Langenbeck, "Fizeau interferometer-fringe sharpening," Appl. Opt. 9(9), 2053-2058 (1970) Characteristics," Appl. Opt. 6(8), 1343-1351 (1967). 49. J. Altmann, R. Baumgart, and C. Weitkamp, "Two-mirror multipass absorption cell," Appl. Opt. 20(6), 995-999 (1981

Current Fourier transform infrared spectroscopy (FTIR) systems have very good spectral resolution, but are bulky, sensitive to vibrations, and slow. We developed a new FTIR system using a microelectromechanical system (MEMS)-based lamellar grating interferometer that is fast, compact, and achromatic (i.e., does not require a beam splitter). The MEMS device has >10  mm² active surface area, up to ±325  μm mechanical displacement, and a 343 Hz resonant operation frequency. The system uses a 5 MHz bandwidth custom infrared (IR) detector and a small emission area custom blackbody source to achieve fast interferogram acquisition and compact form factor. Effects of lamellar grating period, detector size, laser reference, apodization, and averaging of data on the spectral resolution are discussed. The measurement time ranges from 1.5 to 100 ms depending on the averaging time. In the target range of 2.5-16 μm (625-4000  cm^-1) a spectral resolution of 15-20  cm^-1 is demonstrated. The measurements are shown to be stable over a long time.

No abstract

Design, fabrication, and characterization of a highperformance micromachined lamellar-grating-interferometerbased Fourier transform spectrometer are presented. The device is designed to give high deflections with very low dynamic deformation and good mode separation. Mechanical self-stoppers are introduced to withstand accelerations larger than 500 g due to shock. The clear aperture area of the grating is about 10 mm 2 . The maximum deflection while electrostatically actuated at ambient conditions is ±356 µm at 71.2 V and 340 Hz, setting a record for comparable devices. At a pressure of 8.6 Pa, the same deflection is reached at 4.3 V. Six hundred eighty spectra per second can be recorded with a resolution of 14 cm −1 . With a HeNe laser at 633 nm, a spectral resolution of 0.54 nm (22 cm −1 ) is reached using electrostatic actuation. The microelectromechanical systems device is integrated into a compact Fourier transform spectrometer setup including a blackbody source, an infrared (IR) detector, and a visible laser using the device back side for reference. Early results with IR interferograms are also reported. In addition, the devices are actuated with pressure waves in the ambient air to reach deflections up to ±700 µm. With this setup, the spectrum of a red laser is measured with a resolution of 0.3 nm (12.4 cm −1 ).[2011-0252] Index Terms-Fourier transform spectroscopy (FTS), infrared (IR) spectroscopy, interferometer, lamellar grating, microoptoelectromechanical systems. I. INTRODUCTIONF OURIER transform spectroscopy (FTS) has evolved into a standard analytical technique in the fields of physical, biological, and chemical sciences, as well as a tool for quality and process control, bomb detection, mining industry, and gas detection. Compared to other spectroscopy methods, it provides high accuracy, high throughput, a compact form factor, and low cost. The conventional FTS devices make use of a Michelson Manuscript