The fabrication of optical waveguides using MEMS technology usually leads to scallops on the waveguide sidewalls, causing scattering loss due to the resulting surface roughness. Since these waveguides are usually wide and multimode, we extend the model based on perturbation theory developed by Dietrich Marcuse to the case of multimode slab waveguides. We compare the resulting model to the use of the Generalized Harvey-Shack scattering model and the ray picture to model the scattering loss. The comparison is performed for a waveguide width between 20 μm and 500 μm, and length ranging from 1 mm to 5 mm.
Attenuated total reflection (ATR) spectroscopy is a powerful tool for the optical analysis of biomarkers in the mid-infrared spectral range. We study the signal-enhanced ATR spectroscopy of silicon ATR crystals using silicon micropillars etched on the active surface of the silicon crystal, where we use an effective-index approximation for the micropillar layer as the micropillars are much smaller than the optical wavelength. The reflectance, as well as the effective length due to using the micropillars are calculated as a function of wavelength using a model based on the Fresnel equations. The path length is shown to possibly increase by up to 10X due to using these pillars.
Analysis of microparticles in liquid samples is important in many settings, such as the analysis of bacteria in biofluids, or microplastics in water. In addition to their specific absorbance peaks, microparticles result in ripples in the ATR spectra measured especially in the shorter wavelength range due to resonance effects. By analyzing these ripples, in addition to the absorbance peaks, the microparticles material and size distribution can be simultaneously identified. We develop a method to extract the size distribution of particles of diameter 6 μm and 10 μm. In addition, we apply the model to ATR spectroscopic measurements of poly(methyl methacrylate) microspheres of different diameters in water to extract their size.
The conventional methods used for the diagnostics of viral infection are either expensive and time-consuming or not accurate enough and dependent on consumable reagents. In the presence of pandemics, a fast and reagent-free solution is needed for mass screening. Recently, the diagnosis of viral infections using infrared spectroscopy has been reported as a fast and low-cost method. In this work a fast and low-cost solution for corona viral detection using infrared spectroscopy based on a compact micro-electro-mechanical systems (MEMS) device and artificial intelligence (AI) suitable for mass deployment is presented. Among the different variants of the corona virus that can infect people, 229E is used in this study due to its low pathogeny. The MEMS ATR-FTIR device employs a 6 reflections ZnSe crystal interface working in the spectral range of 2200-7000 cm-1. The virus was propagated and maintained in a medium for long enough time then cell supernatant was collected and centrifuged. The supernatant was then transferred and titrated using plaque titration assay. Positive virus samples were prepared with a concentration of 105 PFU/mL. Positive and negative control samples were applied on the crystal surface, dried using a heating lamp and the spectrum was captured. Principal component analysis and logistic regression were used as simple AI techniques. A sensitivity of about 90 % and a specificity of about 80 % were obtained demonstrating the potential detection of the virus based on the MEMS FTIR device.
MEMS-based FTIR spectrometers are good candidates for handheld and IoT applications due to their high speed of operation, ultra-compact size and low cost. Light sources used for FTIR spectroscopy are usually limited to thermal light sources that continuously emit black body radiation. However, the use of pulsed sources has many advantages, such as reducing detector noise and enabling new kinds of spectroscopy measurements that depend on pulsed sources such as supercontinuum sources and non-linear infrared spectroscopy. The use of these pulsed sources with the high-speed MEMS is, thus, of great interest. In this work, we study the effect of using a pulsed IR source with a MEMS FTIR spectrometer on the obtained spectrum. The system is analyzed for different operation regimes from quasi-static to high-speed pulses for different duty cycles and repetition rates. Two measurement setups are used. The first involves using pulsed white light output from a thermal source with an optical chopper. The chopper frequency is changed from 20 Hz to 1 kHz at duty cycle values from 1% to 50%. The second setup uses an acousto-optic modulator to square-wave modulate the amplified spontaneous emission of a semiconductor optical amplifier with a repetition rate ranging from 20 Hz to 2 MHz and duty cycle values from 5% to 50%. Degradation in signal-to-noise ratio as well as spectral distortion are analyzed for different regimes of operation.
In this work, we report a novel notch optical filter based on the imaging properties of a MEMS-based Multimode Interference (MMI) waveguide. The concept is based on the dependence of the imaging lengths on the different wavelengths, where each wavelength exits the waveguide at a different lateral position. Thus, by properly choosing the output waveguide position, it is possible to have a good selective optical filter as well as a good notch optical filter (the complementary response). To validate this concept an MMI structure is fabricated using Deep Reactive Ion Etching (DRIE) technology on a silicon-on-insulator (SOI) wafer. The walls of the waveguide are metalized with Aluminum to decrease the insertion loss. The design makes use of the compactness of the parabolic butterfly shape to reduce the MMI length. The structure is fed by a 9/125 single-mode fiber and the Amplified Spontaneous Emission ASE out of a Semiconductor Optical Amplifier is used as a wideband source for the optical response characterization. The output is measured on an optical spectrum analyzer demonstrating a notch filter response around 1550 nm with about 20-dB rejection ratio. The reported results open the door for integrated, low-cost and fabrication insensitive optical MEMS notch filter.
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