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A fiber-optic interferometric seismometer

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5 Author(s)
Gardner, D. ; Naval Postgraduate School, Monterey, CA, USA ; Hofler, T. ; Baker, S. ; Yarber, R.K.
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A fiber-optic interferometric sensor has been developed which consists of a seismic mass of 520 gm supported by two rubber mandrels, each wound with a single layer of single-mode optical fiber 6.5 m long. One end of each fiber is cleaved to enhance reflection. The other ends are interconnected via a fiber-to-fiber 3-dB coupler, forming a Michelson interferometer. When the case of the sensor is displaced, the fiber around one mandrel extends in length while the other contracts. The resulting "push-pull" mechanical operation of the sensor allows both legs of the interferometer to be active, providing good common mode rejection of spurious effects, as a reference leg is not required. This, together with the fact that the light traverses each leg of a Michelson interferometer twice due to reflection, provides the sensor with four times the sensitivity of a conventionally constructed interferometric sensor. Sensitivities of 8500 rad of optical phase shift per micrometer of case displacement have been measured above the mass-spring resonance, where the sensor operates as a seismometer. Below resonance the sensor operates as an accelerometer with a measured sensitivity of 10 500 rad/g, the highest reported to date. Including both thermodynamic and demodulator noise sources (approx 10 murad/sqrt{Hz}), below resonance the sensor has a detection threshold of 1 ng/sqrt{Hz}, a 20- dB improvement over the best existing conventional low noise vibration sensors.

Published in:

Lightwave Technology, Journal of  (Volume:5 ,  Issue: 7 )

Date of Publication:

Jul 1987

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