8 March 2013 A miniaturized laser-Doppler-system in the ear canal
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Abstract
Gathering vibrational data from the human middle ear is quite difficult. To this date the well-known acoustic probe is used to estimate audiometric parameters, e.g. otoacoustic emissions, wideband reflectance and the measurement of the stapedius reflex. An acoustic probe contains at least one microphone and one loudspeaker. The acoustic parameter determination of the ear canal is essential for the comparability of test-retest measurement situations. Compared to acoustic tubes, the ear canal wall cannot be described as a sound hard boundary. Sound energy is partly absorbed by the ear canal wall. In addition the ear canal features a complex geometric shape (Stinson and Lawton1). Those conditions are one reason for the inter individual variability in input impedance measurement data of the tympanic membrane. The method of Laser-Doppler-Vibrometry is well described in literature. Using this method, the surface velocity of vibrating bodies can be determined contact-free. Conventional Laser-Doppler-Systems (LDS) for auditory research are mounted on a surgical microscope. Assuming a free line of view to the ear drum, the handling of those laser-systems is complicated. We introduce the concept of a miniaturized vibrometer which is supposed to be applied directly in the ear canal for contact-free measurement of the tympanic membrane surface vibration. The proposed interferometer is based on a Fabry-Perot etalon with a DFB laser diode as light source. The fiber-based Fabry-Perot-interferometer is characterized by a reduced size, compared to e.g. Michelson-, or Mach-Zehnder-Systems. For the determination of the phase difference in the interferometer, a phase generated carrier was used. To fit the sensor head in the ear canal, the required shape of the probe was generated by means of the geometrical data of 70 ear molds. The suggested prototype is built up by a singlemode optical fiber with a GRIN-lens, acting as a fiber collimator. The probe has a diameter of 1.8 mm and a length of 5 mm.
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T. Schmidt, U. Gerhardt, C. Kupper, E. Manske, H. Witte, "A miniaturized laser-Doppler-system in the ear canal", Proc. SPIE 8565, Photonic Therapeutics and Diagnostics IX, 85651N (8 March 2013); doi: 10.1117/12.2006322; https://doi.org/10.1117/12.2006322
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