Fiber inline pressure and acoustic sensor fabricated with femtosecond laser

Pressure and acoustic measurements are required in many industrial applications such as down-hole oil well monitoring, structural heath monitoring, engine monitoring, study of aerodynamics, etc. Conventional sensors are difficult to apply due to the high temperature, electromagnetic-interference noise and limited space in such environments. Fiber optic sensors have been developed since the last century and have proved themselves good candidates in such harsh environment. This dissertation aims to design, develop and demonstrate miniaturized fiber pressure/acoustic sensors for harsh environment applications through femtosecond laser fabrication. Working towards this objective, the dissertation explored two types of fiber inline microsensors fabricated by femtosecond laser: an extrinsic Fabry-Perot interferometric (EFPI) sensor with silica diaphragm for pressure/acoustic sensing, and an intrinisic Fabry-Perot interferometer (IFPI) for temperature sensing. The scope of the dissertation work consists of device design, device modeling/simulation, laser fabrication system setups, signal processing method development and sensor performance evaluation and demonstration. This research work provides theoretical and experimental evidences that the femtosecond laser fabrication technique is a valid tool to fabricate miniaturized fiber optic pressure and temperature sensors which possess advantages over currently developed sensors --Abstract, page iii

Design of an optical microelectromechanical-system microphone with sub 15-dBA noise floor

This research work presents the modeling, fabrication, and characterization of the optical microphone. The optical microphone detects diaphragm displacement due to input sound pressure, using an interferometric-based displacement detection scheme instead of using capacitive readout technique, which is extensively used in commercial microelectromechanical-system microphones. The optical-based transduction mechanism enables a backplate design with an extremely high perforation density, which in-turn drastically reduces the backplate flow resistance, which is a dominant noise source in miniaturized microphones. Therefore, an accurate estimation of the backplate-induced flow resistance is a critical step to predict signal-to-noise ratio precisely. A flow resistance modeling technique via computational fluid dynamics is presented in this work. A prototype backplate is fabricated for a verification of the flow-resistance modeling technique. A 22.0-dBA noise floor is demonstrated using the prototype backplate, which is 6-dB better than state-of-the-art commercial capacitive MEMS microphones. Design of experiments were performed with the verified microphone model to illustrate design implications toward sub 15-dBA optical microphone. The design-of-experiments study focused on various microphone components including diaphragm compliance, acoustical low cut-off frequency, back-cavity volume, inlet port and vent to show how each parameter affect to the microphone signal-to-noise ratio and acoustic overload point. Finally, a force-feedback optical microphone concept is presented to achieve a higher acoustic overload pressure, which is defined by 10% total harmonic distortion, using a Si membrane with piezoelectric thin-film actuators. A feasibility study was performed to explore the concept of a force-feedback optical microphone, including a fabrication of the minimalistic backplate with high aspect-ratio spokes and Si membrane with piezoelectric-film actuators at Microelectronics Research Center at The University of Texas at Austin

Ferrule-top micromachined devices: A universal platform for optomechanical sensing

Iannuzzi, D. [Promotor

Ultra-sensitive graphene membranes for microphone applications

Microphones exploit the motion of suspended membranes to detect sound waves. Since the microphone performance can be improved by reducing the thickness and mass of its sensing membrane, graphene-based microphones are expected to outperform state-of-the-art microelectromechanical (MEMS) microphones and allow further miniaturization of the device. Here, we present a laser vibrometry study of the acoustic response of suspended multilayer graphene membranes for microphone applications. We address performance parameters relevant for acoustic sensing, including mechanical sensitivity, limit of detection and nonlinear distortion, and discuss the trade-offs and limitations in the design of graphene microphones. We demonstrate superior mechanical sensitivities of the graphene membranes, reaching more than 2 orders of magnitude higher compliances than commercial MEMS devices, and report a limit of detection as low as 15 dBSPL, which is 10 - 15 dB lower than that featured by current MEMS microphones.Comment: 34 pages, 6 figures, 7 supplementary figure

Cantilever-enhanced photoacoustic measurement of light-absorbing aerosols

Photoacoustic detection is a sensitive method for measurement of light-absorbing particles directly in the aerosol phase. In this article, we demonstrate a new sensitive technique for photoacoustic aerosol absorption measurements using a cantilever microphone for the detection of the photoacoustic signal. Compared to conventional diaphragm microphones, a cantilever offers increased sensitivity by up to two orders of magnitude. The measurement setup uses a photoacoustic cell from Gasera PA201 gas measurement system, which we have adapted for aerosol measurements. Here we reached a noise level of 0.013 Mm(-1) (one standard deviation) with a sampling time of 20 s, using a simple single-pass design without a need for a resonant acoustic cell. The sampling time includes 10 s signal averaging time and 10 s sample exchange, since the photoacoustic cell is designed for closed cell operation. We demonstrate the method in measurements of size-selected nigrosin particles and ambient black carbon. Due to the exceptional sensitivity, the technique shows great potential for applications where low detection limits are required, for example size-selected absorption measurements and black carbon detection in ultra clean environments.Peer reviewe

Advancements in microfabricated gas sensors and microanalytical tools for the sensitive and selective detection of odors

In recent years, advancements in micromachining techniques and nanomaterials have enabled the fabrication of highly sensitive devices for the detection of odorous species. Recent efforts done in the miniaturization of gas sensors have contributed to obtain increasingly compact and portable devices. Besides, the implementation of new nanomaterials in the active layer of these devices is helping to optimize their performance and increase their sensitivity close to humans’ olfactory system. Nonetheless, a common concern of general-purpose gas sensors is their lack of selectivity towards multiple analytes. In recent years, advancements in microfabrication techniques and microfluidics have contributed to create new microanalytical tools, which represent a very good alternative to conventional analytical devices and sensor-array systems for the selective detection of odors. Hence, this paper presents a general overview of the recent advancements in microfabricated gas sensors and microanalytical devices for the sensitive and selective detection of volatile organic compounds (VOCs). The working principle of these devices, design requirements, implementation techniques, and the key parameters to optimize their performance are evaluated in this paper. The authors of this work intend to show the potential of combining both solutions in the creation of highly compact, low-cost, and easy-to-deploy platforms for odor monitoringPostprint (published version

SIMULTANEOUS SENSING OF PRESSURE AND TEMPERATURE USING A SELF-TEMPERATURE-COMPENSATED FABRY–PÉROT MEMS MECHANISM

This thesis presents the design and development of a self-temperature-compensated sensor for measuring temperature and pressure in harsh environments using a combination of Fabry–Pérot interferometry and microelectromechanical systems (MEMS). A silicon-on-insulator (SOI) wafer is etched to form a dual mechanism consisting of a membrane and a solid block that is then coupled with two optical fibers contained in a unique and simple protective stainless-steel housing. The solid block uses the thermo-optical properties of silicon for temperature measurements, while the deflection of the membrane is used for pressure sensing. An empirically based model combines solid mechanics and optical theory and is in good agreement with experimental measurements. As part of this work, the thermo-optic coefficient (TOC) of the silicon was also investigated theoretically and experimentally. The results show a good agreement between the TOC extracted from the experimental data and such a coefficient in published literature. Furthermore, a novel optical model for the demodulation of the intensity-based pressure-sensing mechanism was developed. This model relates the whole sensor-response profile to the measured parameters and eliminates linear range limitations. By using this model, one can also obtain the initial cavity lengths of an FFPI sensor, which can be very challenging at the microscale. A series of experiments conducted to test the performance of this multi-functional sensor showed that it can easily withstand pressures up to 1,000 psi and temperatures of up to 120°C, where the range of the temperature measurements are restricted only by the fiber optic materials. The developed self-temperature-compensated multi-functional sensor therefore serves as a promising tool in the precise characterization of pressure and temperature in harsh and/or complex environments