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Optical microphone sounds promising

12 Apr 2007

An optical microphone based on a fiber laser has many applications in defence.

An optical microphone developed for acoustic direction finding could be ideal for a number of security applications such as monitoring the movement of vehicles or persons over long perimeters or borders, according to researchers at QinetiQ (Meas. Sci. Technol. 18 884).

"The simplicity of the microphone design means that as the cost of fiber lasers falls it becomes increasingly viable as a disposable sensor" researcher Roger Crickmore told optics.org. "This design could also be made into a small, very lightweight device, opening up options for aerial deployment. The sensor was originally designed to locate artillery pieces on a battlefield."

Optical microphones are favoured over electronic systems in environments where electromagnetic interference is an issue, such as the intercom in MRI scanners. The multiplexing capability of this optical microphone opens up further applications for large-scale measurements desirable in a number of military situations.

Crickmore and his colleague John Wooler both modelled and tested the fiber-laser microphone revealing reasonable agreement.

Responsivity of this sensor was found to be significantly higher than other published sensors based on either fiber lasers or Bragg gratings. One end of the fiber sensor is bonded to a diaphragm and the other end is bonded to apparatus that enables the tension of the fiber laser to be altered without introducing twist in the fiber.

The model of the microphone was based on a mass-spring system with three spring elements, each with associated stiffness: the fiber laser; the air in the tube; and the diaphragm support. The responsivity of the device is the change in wavelength for a corresponding change in acoustic pressure. This model gives a theoretical responsivity of –58 dB re 1 nm Pa–1. The measured acoustic responsivity of the fiber-laser microphone is –62 dB re 1 nm Pa–1.

The fiber laser enables the signal to be obtained by passing the laser output through an interferometer with a path imbalance of several meters. This allowed a noise floor of –90 dB re 1 Pa Hz–1/2 to be obtained at 100 Hz. According to Crickmore a number of these sensors could be deployed on a single fiber by using a wavelength division multiplexing approach. This makes the device suitable for applications such as acoustic direction finding where arrays of sensors are required.

Thanks to the modelling performed, the effects of key parameters such as diaphragm diameter and fiber strain response could be established, enabling the team to predict that increased responsivity could be obtained by using a larger diaphragm, reducing the cladding diameter of the fiber or by shortening the fiber length.

"A key change would be to use 80 µm fiber diameter rather than 125 µm fiber, as this should give around an 8 dB increase in sensitivity," explained Wooler. "In future designs we would be looking to develop the package and components for a practical rugged microphone."

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