A Fiber-optic Interferometric Seismometer

摘要

A fiber-optic interferometric sensor has been developedwhich consists of a seismic mass of 520 gm supported by two rubbermandrels, each wound with a single layer of single-mode optical fiber6.5 m long. One end of each fiber is cleaved to enhance reflection. Theother ends are interconnected via a fiber-to-fiber 3-dB coupler, forminga Michelson interferometer. When the case of the sensor is displaced,the fiber around one mandrel extends in length while the othercontracts. The resulting â push-pullâ mechanical operation of the sensorallows both legs of the interferometer to be active, providing goodcommon mode rejection of spurious effects, as a reference leg is notrequired. This, together with the fact that the light traverses each legof a Michelson interferometer twice due to reflection, provides the sensorwith four times the sensitivity of a conventionally constructed interferometricsensor. sensitivities of 8500 rad of optical phase shift permicrometer of case displacement have been measured above the massspringresonance, where the sensor operates as a seismometer. Belowresonance the sensor operates as an accelerometer with a measuredsensitivity of 10 500 rad/g, the highest reported to date. Including boththermodynamic and demodulator noise sources, belowresonance the sensor has a detection threshold of l ng/JHz, a 20-dB improvement over the best existing conventional noise vibrationsensors.