PhD candidates Swapnil Daxini and Arthur Santos (with early contributions by Dr Deniz Aydin) succeeded in recording sound over 75 km of fiber optic cable. They have locked the wavelength of a laser diode to a sharp(ish) line of a pi-shifted Fiber Bragg Grating (FBG), which is our strain sensor.
Surprisingly, the laser remains locked even if the fiber sensor is at the end of a 75 km fiber tether, and sound recordings (up to a few kHz) were possible. The video above is the song of a humpback whale with some dolphin calls. The recording was originally made in 2013 by Ocean Networks Canada using a hydrophone and was played back using a piezo transducer to which the fiber sensor was attached.
Our sensor has applications to sound measurements and seismic recordings in hostile environments, from the ocean floor to volcanoes. Read more at Optics Express (https://doi.org/10.1364/OE.538786)