A simple optical microphone

Abstract

In the past, a number of different working principles have been suggested for optical microphones, but many of these require a high degree of technical sophistication. Thus some suggestions are based on the interferometric principle, whilst others would utilize frequency modulation of laser light via a variable mirror (membrane) situated in the laser resonator. Light intensity modulation is a much more simple approach: A light beam emitted from an optical fiber is aimed at a reflecting membrane. The beam reflected from the membrane is captured by a second optical fiber and fed into a photodiode. Depending on the movement of the membrane, more or less light is launched into a second optical fiber. The photoelectric current of the photodiode thus expresses the movement of the membrane, and with it the sound field, as an electrical signal. As optical fibers transmit light with very few losses, the actual optoacoustic transducer can be positioned at a long distance from the light source and the photodiode. This allows problem-free use of the optoacoustic transducer in the presence of very strong electromagnetic fields (e.g., MRI). It is possible to improve upon this principle via simple modifications. The electroacoustic characteristics achieved are discussed and a model is presented.