A fiber-optic interferometric geophone

Abstract

A fiber-optic interferometric geophone has been developed which consists of a seismic mass (520 g) supported by two rubber mandrels wound with a single layer of single mode optical fiber. The two mandrel-wound lengths of optical fiber, each 6.5 m long with reflecting ends, are interconnected via a fiber-to-fiber 3-dB coupler to form a Michelson interferometer. When the case of the sensor is displaced at frequencies above the mass-spring resonance frequency (i.e., in the mass-controlled frequency regime), the mass remains approximately at rest while the fiber around one mandrel is compressed and the other is expanded. This geometry has the advantage of not requiring a “reference leg” and providing four times the sensitivity of a single sensor by its “push-pull” operation and the fact that the light traverses each leg twice due to reflection. Sensitivities of 3500 rad/μ have been measured at frequencies above the mass-spring resonance, corresponding to an optical leverage factor of 700. [Work supported by the Naval Research Laboratory and the Office of Naval Research.]