An Optical Fiber Infrasound Sensor
SCRIPPS INSTITUTION OF OCEANOGRAPHY LA JOLLA CA INST OF GEOPHYSICS AND PLANETARY PHYSICS
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We have designed a new type of infrasonic sensor using optical fibers as distributed sensing elements. The design addresses the limitations of mechanical spatial filters used to average wind-generated, turbulent pressure fluctuations. We have built two styles of prototype. In the first design of an OFIS optical fiber infrasound sensor, we rely on the change in optical path that results from an optical fibers strain-optic coefficient i.e., the index of refraction is pressure dependent. In the second design, the optical fiber is attached to and strained by a compliant tube, which deforms under pressure. In both types, the optical fiber path length is monitored interferometrically. The second type has proven to have a better signal-to-noise ratio and is currently the primary focus of our developments. These devices lend themselves to deployment in long, continuous lines, effectively integrating the pressure variations along their lengths. The response over the length of the sensor is governed by the speed of light rather than the speed of sound as is the case for conventional infrasound filters. Since our sensors can readily be made kilometers long and deployed as spatial filters in arbitrary geometries, they potentially are a significant improvement over conventional systems.
- Fiber Optics and Integrated Optics