Optical fiber infrasound sensor

A fiber optic infrasound detection system comprising:

• an equal-arm Michelson or equivalent type interferometer including
• a first optic fiber exposed to atmosphere and extending for greater than 100 meters, and
• a second optic fiber, hermetically sealed from the atmosphere, extending alongside the first optic fiber;
• a source of modulated light transmitted along both fibers in accordance with any changes in optical length of the fibers responsively to, inter alia, pressures and temperatures and strains to which each fiber is subject;
• and
• a detector, receiving the light transmitted along both optic fibers, of changes in the optical length of both optic fibers, the detector sensitive in a frequency band including from at least 0.2 to 4 Hz so that changes in optical lengths as result from, inter alia, (i) changes in temperature and (ii) changes in strain including as are due to earth movement, are all substantially canceled due to the interferometric arrangement of the two optic fibers while (iii) atmospheric pressure changes arising both from wind and from infrasound are sensed in the frequency band;
• wherein, because pressure changes due to the wind are not coherent over the greater than 100 meter extent of the optic fibers, detection of changes in optical length due to atmospheric pressure changes arising from the wind is a substantial integration of these wind-induced changes along the length of the fiber, and is much reduced from what would be detected by any sensing of pressure at but a single point in the atmosphere;
• wherein the detector serves to sense atmospheric pressure changes due to infrasound over the lengths of both optic fibers at a high signal to noise ratio over atmospheric pressure changes due to any of (i) temperature, (ii) strain and vibration, and (iii) wind.

A method of detecting infrasound comprising:

• providing modulated light illumination from a light source;
• extending co-parallel (i) a first optic fiber exposed to atmosphere, and (ii) a second optic fiber hermetically sealed from the atmosphere, each from the source of modulated light illumination to a mirrored end;
• wherein light illumination from the light source is transmitted and is reflected along both fibers in accordance with, inter alia, pressures and temperatures and strains to which each fiber is subject;
• and
• detecting the light reflected in both fibers in a frequency band including from at least 0.2 to 4 Hz so that changes in optical lengths of both fibers as are due to, inter alia, variations in (i) temperature, (ii) strain including as is due to earth movement and (iii) pressure as arises both from wind and from infrasound, are detected;
• wherein detected changes in optical length due to variations in (i) temperature and in (ii) strain are substantially canceled due to the interferometric arrangement of the two fibers;
• wherein, because changes in optical length due to wind-induced pressure changes are not coherent over the extent of the fiber, so that the detection of pressure changes arising from the wind are substantially integrated along the length of the fiber and are much reduced from what would be detected by any sensing of pressure at but a single point in the atmosphere;
• wherein the detecting primarily serves to sense atmospheric infrasound.

A fiber optic pressure sensor having a pressure sensor and a first optic fiber CHARACTERIZED IN THAT

• pressure is sensed by the pressure sensor in a band including from 0.2 to 4 Hz;
• in the first optic fiber which is both exposed to atmosphere and extends linearly for greater than 100 meters;
• wherein the first optic fiber is exposed to (i) atmospheric pressure changes arising both from wind and from infrasound as well as (ii) changes in temperature and (iii) changes in strain including as are due to earth movement;
• wherein, because pressure changes due to the wind are not coherent over the linear extent of the first optic fiber, the (i) atmospheric pressure changes arising from the wind are substantially integrated along the length of the first optic fiber, and are much reduced from what would be detected by any sensing of pressure at but a single point in the atmosphere.