Fiber-optic microphones are EMI/RFI immune. Electrical, magnetic, electrostatic, or radioactive fields do not impact them. They do not react to these, either. Therefore, their design is ideal for settings where the use of traditional microphones is dangerous or ineffective. For instance, these scenarios include places like industrial turbines and MRI equipment setups. Additionally, Fiber-optic microphones tend to be strong and widely resistant to environmental changes regarding moisture and heat.
Most commonly, the music universe uses them for directionality or impedance matching. Industrial and surveillance acoustic monitoring need them too because the distance between the microphone’s light source and its photodetector does not require a preamplifier. It does not need any other electrical unit either.
Fiber-optic Microphones and Their Environments
Fiber-optic microphones can be appropriate in a variety of environments. Some harsh ones require sensors that can resist high heat and strong vibrations. For example, think of turbojet and rocket engines. Other scenarios require sensors that can reduce acoustic measurements under rough conditions. Cases of this are CFD code validation, structural-acoustic tests, and jet noise.
A single-mode temperature-insensitive Michelson interferometer and reflective plate diaphragm is another common example. Interferometers tend to monitor the plate deflection, which is related to the acoustic pressure. In this case, cooled-down water is the sensor, which offers thermal protection for the optical materials. It also helps to even out the mechanical properties of the diaphragm.
Fiber-optic Mics and their Interference
Fiber-optic microphones offer an interference effect between the incoming and the outgoing light beams. They achieve this by combining the two fibers and cleaving them at the minimum tapered point. First of all, a water-cooled stainless-steel tube joins the fibers with Epoxy. The end of this tube is polished until you can see the optical fibers. Then, aluminium is carefully inserted at one of the joint core ends to make its surface as reflective as a mirror. This fiber functions as a reference arm of the mic. It’s worth mentioning that the other core end is open and functions as a sensing arm. This mic achieves temperature insensitivity due to the proximity of both the reference and the sensing arms.