An Optical Diffraction Microphone with Active Grating Diaphragm
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J13.11.1
An Optical Diffraction Microphone with Active Grating Diaphragm
Kazuhiro Suzuki, Hideyuki Funaki and Yujiro Naruse Corporate Research & Development Center, Toshiba Corporation 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan
ABSTRACT
We present a new conceptual active optical microphone based on complementary metal-oxide semiconductor (CMOS) - micro electro mechanical systems (MEMS) micromachining techniques. The diaphragm of the microphone has a diffracting grating fabricated by the CMOS process with only a small number of post-processes. The active microphone actuates the diaphragm on an arbitrary frequency by electrostatic force, and detects sound waves by the light waves. From the verification experiment result, this active microphone was able to perform advanced functions, such as the amplifier effect and the detection of phase information of sound waves.
INTRODUCTION Recently, microphones have come to play an important role as universal input devices for digital equipment such as cell phones, personal computers, car navigation systems, and home-use robots. Generally microphones can be divided into three types, dynamic microphones, condenser microphones, and optical microphones. A dynamic microphone consists of a diaphragm equivalent to the tympanic membrane in the human’ ear, a voice coil attached to the center of the diaphragm, and a magnet inserts voice coil. As dynamic microphones have a very simple structure, there are few restrictions regarding climate conditions and they can be handled with ease. However, since dynamic microphones have a heavy voice coil structure attached to the diaphragm, they are difficult to create in small sizes. Moreover, they have low sensitivity and there is a tendency for an extreme fall in sensitivity at both ends of the frequency characteristics. On the other hand, condenser microphones conquer the size and sensitivity problems of dynamic microphones. Condenser microphones detect the change in capacitance between the diaphragm and the counter electrode. Since condenser microphones have an unnecessary voice coil, a diaphragm is formed with one light and thin membrane. This leads to excellent frequency characteristics. However, condenser microphones are susceptible to humidity, and there is room for improvement regarding directivity or noise tolerance. On the contrary, optical microphones, which optically detect the displacement of the diaphragm, require no back plate. This type of microphone is defined as a pressure gradient microphone, and has the characteristics of a bi-directional (figure-of-eight) polar pattern. Directional microphones by laser Doppler vibrometer have also been studied [1,2]. Furthermore, optical microphones of various systems to be used for highly efficient acoustic field measurement have been proposed. Although methods such as changing the reflective index with optical fibers,
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optical interferometry using diffraction grating, and Fabry-Perot resonators have previously been proposed, these complicate the system or requir
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