Abstract

A high-performance broadband seismograph system is described that uses a laser-stabilized confocal interferometer as a transducer to measure seismic vibrations. Major goals are to improve sensitivity and the signal-to-noise of the sensor beyond currently available siesmometers. A reference cavity is arranged so that seismic motion is translated into a change in the cavity length. This high-finesse cavity has a laser locked to it. When the cavity length changes the laser frequency follows this change. The output beam of the laser is mixed on a photodiode with the beam of a stabilized laser. The seismic motion is indicated by a change in the beat frequency from the photodiode. The system can operate in either velocity or displacement modes. The design goals are a minimum detectable signal of ${10}^{-9}\text{m}/{\text{s}}^2\sqrt{\text{Hz}}$, a dynamic range of the signal amplitude of 140 dB, good linearity, and a system passband with a frequency range of 0.0025–20 Hz.

© 1988 Optical Society of America