Abstract: A laser seismometer may measure the change in a phase modulated optical signal. Unlike traditional interferometers, the laser phase is first modulated by a radio frequency (RF) source, which is then demodulated following detection to provide the signal of interest. The net result is a direct measurement of displacement with the effects of amplitude noise eliminated via limiting and the effects of 1/f phase noise (frequency drift, etc.) eliminated by self-interfering the signal. Because the signal-to-noise ratio (SNR) of the optical signal is strong, the technique provides a strong measure of the displacement and avoids the extremely low voltages and associated problems of traditional sensors.