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Abstract

A fiber optic body motion monitor in which light is transmitted through an optical fiber waveguide physically coupled to a body being monitored, and modal noise at the output of the optical fiber waveguide is applied to a photodetector filtered, rectified and analyzed to detect breathing and heartbeats. Detection of breathing is performed by low-pass filtering the rectified and filtered signal, and adaptive thresholding of the low-pass filtered signal. A similar but separate technique is used to detect heartbeats. In one embodiment, reliability of detection is enhanced by reducing the number of speckles of light applied to the photodetector to approximately 50%, thereby to maximize modal noise current. In another embodiment the photodetector has two halves separately illuminated by approximately two-thirds of the light from the optical fiber waveguide. Outputs of the photodetector halves are applied to the positive and negative inputs of an operational amplifier, whereby common mode noise is eliminated and modal noise current and therefore sensitivity of modal noise detection increased.

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