Development of acoustic input and output devices far MRI scanners and nuclear plants

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The guided-wave optical microphone can work even in a hazardous environment, such as a high magnetic field like the MRI, high RFI fields, and other environments that require EMI/RFI immunity.The optical microphone consists of a rectangular diaphragm and a straight waveguide on the diaphragm. The sensitivity of the microphone and the resonance frequency of the diaphragm are dependent on the diaphragm dimensions. In this study, the target values for phase sensitivity and resonance frequency were set at 2.5 mrad/Pa and 3.4 kHz, respectively. By design considerations, the diaphragm dimensions were determined to be 20 mm×20 mm×0.15 mm. Measured sensitivity of the fabricated microphone was L3 mrad/Pa, almost a half of the target value. The fabricated microphone detected a sound wave of 1 kHz and 2 Pa, corresponding to 100 dB-SPL (sound pressure level). Moreover from measured frequency characteristics of three fabricated microphones with a 20 mm×20 mm×0.15 mm diaphragm, resonance frequencies were 2.7, 3.3 and 3.7 kHz, similar to the theoretical one.Incidentally, silicon is an attractive substrate because of its excellent mechanical properties, which provide the optical microphone with minituarization feasibility. In the last two years, the silicon-based guided-optic microphone has also been developed, and the fabricated optical microphone with a 10 mm×10 mm×40μm diaphragm successfully detected a sound wave of 0.2 Pa (80 dB-SPL) and 1 kHz. Moreover from the frequency characteristic, resonance frequency was experimentally determined to be 5.3 kHz, quite similar to the theoretical one of 5.6 kHz.