Fiber optic interferometric seismometer with phase feedback control.

A high-sensitivity ultralow-frequency fiber optic interferometric seismometer using phase feedback control is proposed and demonstrated. The principle of sensitivity improvement using feedback is described, and the characteristics of the seismometer, including the ultralow-frequency vibration sensing with Michelson interferometer with and without feedback control, are analyzed in terms of the amplitude response and phase response. The phase feedback control loop is designed and implemented, and higher sensitivity for very low frequency vibration is achieved. The efficacy of the new approach is demonstrated experimentally, showing that the weak vibration signal originally buried in noise can be observed unambiguously.