Micro-Structured and Telecom Fibers for Acoustic and Ultrasound Sensing and Imaging

Ultrasound and acoustic sensors have been widely used in medical imaging, structural health monitoring (SHM) and non-destructive testing (NDT) in civil/mechanical structures. Detection of high frequency acoustic wave (tens of MHz to 1 GHz) enables high spatial resolution imaging, the acoustic wave can be generated by surface acoustic waves-electromechanical systems (MEMS) with high signal-to noise-ratio (SNR) are difficult, as the conversion from mechanical wave to electrical signal is low due to the larger dimension of the electrical device compared to the spatial period of the acoustic wave. This difficulty can be overcome by coupling acoustic/mechanical to optical wave within one SAW wavelength in a few micrometers via optomechanical interaction. The sensing signal is measured in optical domain at high sampling rate, instead of electrical domain as micro-wave. Novel micro-structured optical fiber from conventional SiO₂ to As₂Se₃ tapered fiber have brought major advancements in high frequency and high sensitivity detection by SAW generation and detection. This tutorial paper discusses basic principles of sound waves, acoustic waves and ultrasound, the photo-acoustic effect, the effect of electrical-strain and vice-versa. The acoustic generation by PZT, mechanical wave via a pencil break, laser ultrasound generation, and the SAW can be detected by micro-structured fibers and telecom fibers from kHz to 1 GHz are demonstrated with SNR of greater than 40 dB at ∼100 MHz and greater than 20 dB at ∼1 GHz. This sensing technology opens a new on chip sensing platform that combines electrical, mechanical and optical components on one chip for acoustic-optical sensing and imaging probe with high spatial resolution.