Recording in Hostile Environments


Fiber optic microphones do not react to or influence any electrical, magnetic, electrostatic or radioactive fields. In other words, this is EMI/RFI immunity. The fiber optic microphone design is therefore ideal for use in areas where conventional microphones are ineffective or dangerous, such as inside industrial turbines or in magnetic resonance imaging (MRI) equipment environments.

Fiber optic microphones are robust, resistant to environmental changes in heat and moisture. Sound engineers can use them for any directionality or impedance matching. The distance between the microphone’s light source and its photodetector may be up to several kilometers without the need for any preamplifier or another electrical device, making fiber optic microphones suitable for industrial and surveillance acoustic monitoring.


Extreme measurements


Direct acoustic measurements in hostile environments, such as in turbojets or rocket engines, require sensors which can withstand high heat and strong vibrations. Consequently, computational fluid dynamics (CFD) code validation, structural acoustic tests, and jet noise abatement require acoustic measurements under such hard conditions. 


Michelson interferometer


For such applications, a fiber-optic interferometric microphone can be quite suitable. A single-mode temperature insensitive Michelson interferometer and reflective plate diaphragm compose one such design. So, the interferometer monitors the plate deflection, which is directly related to the acoustic pressure. Subsequently, the sensor is water cooled to provide thermal protection for the optical materials and to stabilize the mechanical properties of the diaphragm.


Fiber-Optic Microphones Components


By fusing together the two fibers and cleaving them at the minimum tapered region, they provide an effect of interference between the incoming and outgoing light beams. Firstly, a water-cooled, stainless steel tube incorporates the fibers. Epoxy resides within the internal space of the tube. The end of the tube is polished until the optical fibers are observed. Next, aluminum is selectively deposited at one of the fused fiber core ends to make its surface mirror reflective. Precisely, this fiber serves as a reference arm of the microphone. The other fiber core is open and serves as a sensing arm. The close proximity of the reference and sensing arms of the assembly provide the temperature insensitivity.






People use fiber optic microphones in very specific application areas such as infrasound monitoring and noise-canceling. They have proven especially useful in medical applications. They allow radiologists, staff, and patients within the powerful and noisy magnetic field to converse normally, inside the MRI suites as well as in remote control rooms. Other uses include industrial equipment monitoring and audio calibration and measurement, high-fidelity recording and law enforcement.


Additional Resources and Source Texts