Fiber-optic interferometric geophone with hydrophone applications

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 mandrels act as the springs as shown schematically in Fig. 1. The two mandrel-wound lengths of optical fiber, each 6.5 m long, with reflecting ends are attached to a coupler to form the two legs in 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 4 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 7500 rad/μm have been measured at frequencies above the mass-spring resonance.