Miniaturized all-optical Sound Pressure Sensor

The membrane-free optical microphone, an alternative concept to measure sound pressure, will be presented. A laser beam locked to a millimeter-sized Fabry-Pérot interferometer experiences a change in wavelength if the pressure inside the interferometer is altered by a sound wave. The resulting change in back reflection of the laser is measured by a photo diode, thus providing a direct measurement of the sound pressure. It is important to mention, that the system operates without any moving parts. Hence, the measured signal is not a result of a mechanically moving or deformable mirror, but a consequence of the change in the medium's density itself, and its related change of refractive index. Therefore, this sensor principle is free from mechanical resonances and allows for a linear frequency response expanding over 1MHz in air and over 25MHz in liquids. The talk compares operation in gaseous and liquid media and addresses potential contributions of internal structure-borne movements of the mirrors. It is shown that, even if Eigenmodes of the mechanical sensor structure are excited by sound pressure, their contribution is 3 orders of magnitude below the contribution of the refractive index change, and may therefore be neglected. Several applications are presented.