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Photonics West: Wednesday

27 Jan 2005

An optical microphone could soon be helping patients undergoing medical scans.

Patients inside magnetic resonance imaging (MRI) scanners could soon be able to speak to physicians controlling the scan thanks to an optical microphone being developed in Japan.

Conventional microphones, which convert sound waves into electrical signals, cannot be used inside a scanner because of its strong magnetic field. To make matters worse, they tend to contain metal parts that would corrupt the MRI image.

To get around the problem, Masashi Ohkawa and his colleagues from the Niigata University have built a metal-free microphone that converts sound information into a light signal.

At the heart of the design is a square (7 x 7 mm) Si-SiO2 diaphragm that is 27µm thick and contains a polymer optical waveguide embedded into its surface.

To turn the microphone on, the scientists focus a linearly polarized beam of red (633 nm) light from a He-Ne laser into one end of the waveguide. Light travels through the waveguide from one side of the diaphragm to the other before passing through a polarizer and striking a detector.

Incident sound waves cause the diaphragm to flex, distorting the waveguide and changing its refractive index via the elasto-optic effect. The index change modulates the phase of the light beam, effectively encoding it with the sound wave. This is then seen as a variation in light intensity at the detector.

Although the prototype microphone can pick up sound pressures ranging from 5 to 25 Pa (1 kHz), its sensitivity needs to be improved. Small output voltages (less than 1 microvolt) at the detector mean that the scientists currently have to use a lock-in amplifier to overcome the low signal-to-noise ratio, which is impractical for a commercial device.

Taking a deeper look, the researchers discovered that the diaphragm was buckling under thermal stress, making the microphone inefficient. However, Ohkawa says that there is a solution. "In the next step we will use a silicon nitride diaphragm layer," he told Optics.org at the Photonics West poster session. "This should help reduce the thermal stress and improve the sensitivity of the microphone."

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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