Optical fiber interferometry and Bragg gratings for measuring sound

Optical fiber interferometers are intrinsically differential path-length sensors that may be combined with transducers such as flexural disks and ellipsoids that produce push–pull response for wanted actions of sound and common mode rejection to unwanted actions such as vibration or thermal changes. Fiber Bragg gratings (FBG) can be “UV-written” through the sides of the optical fiber to realize internal wavelength selective mirrors. The FBG gratings can then be combined with the transducers to realize path-length matched interferometric sensors enabling interrogation of lengths of fiber as well as time and frequency division multiplexing. Steven Garrett and collaborators at the Naval Postgraduate School (Hofler, Gardner, Brown, Keolian et al.) demonstrated many of these interferometric devices to detect underwater sound and worked on push–pull sensors, thermal noise characterization, acoustic motion sensors, materials characterization, demodulation, and multiplexing four decades ago. See, e.g., J. Acoust. Soc. Am. 83, S19 (1988) and 88, 591 (1990) as well as many thesis and dissertations https://discover.dtic.mil/.