High-Sensitivity Diaphragm-Based Fiber Optic Acoustic Sensors and Applications

• Main Author: Jo, Wonuk
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2015
• Subjects: Acoustics; Materials science; Medical imaging; Optics; Remote sensing
• ISBN: 9798698512950

Acoustic sensors are used for applications in multiple disciplines, including oil and gas extraction, medical devices (magnetic resonance and acoustic/ultrasonic imaging), underwater communications, seismic research, monitoring large structures, and surveillance. Performance requirements vary greatly, but in general the sensors are expected to have a fairly low minimum detectable pressure (MDP), as low as 10 µPa/√Hz in some cases, a wide bandwidth (hundreds of Hz to a few kHz is typical), and a high dynamic range (60 dB in pressure is common). In the first half of this thesis, I present a highly sensitive fiber optic acoustic sensor that meets these requirements by utilizing a high-reflectivity photonic-crystal diaphragm 450 nm thick to convert the incident acoustic or ultrasonic pressure into a mechanical vibration. The diaphragm is placed ~30 µm from the reflective end of a single-mode fiber to form a miniature Fabry Perot (FP) interferometer, which converts this vibration into an optical readout. Thanks to the high compliance of the diaphragm, this sensor exhibits an extremely low MDP of ~2.6 µPa/√Hz over 1--30 kHz, the lowest value reported in the literature. This sensor has recently been used as a hydrophone to listen to the very weak sound generated by the beating of a small number of heart cells, of critical interest for cell sorting and stem-cell research. When used as a force sensor, this sensor was capable of detecting a force as low as 1.3 pN/√Hz in a laboratory environment. In a quieter environment, the minimum detectable force is predicted to be only ~76 fN/√Hz, which is lower than conventional room-temperature AFM-based force sensors. Additionally, a theoretical model based on a finite element analysis was developed to show that this sensor also operates at ultrasonic frequencies and predict its sensitivity dependence on frequency. Measurements of its sensitivity at 72 kHz and 1.025 MHz were perform to demonstrate this potential. In the second half of the thesis, I discuss a low-noise, compact fiber acoustic sensor that implements a novel transducer design, namely a microfabricated silicon diaphragm with a π/2 phase step combined to a single-mode fiber to form a simple two-wave interferometric sensor head. Compared to the FP sensor, this device has a similar strain resolution, it operates over a broad range of wavelengths (~±150 nm instead of ~±1 nm), and both its microfabrication and assembly are simpler. A prototype is reported with an average MDP as low as 5.4 µPa/√Hz between 1 kHz and 30 kHz, in agreement with a theoretical model.