Large dynamic range, high resolution optical heterodyne readout for high velocity slip events

Perry W F Forsyth; Kathryn S Hayward; Lyle E Roberts; Stephen F Cox; Daniel A Shaddock; Bram J J Slagmolen

doi:10.1063/5.0082970

Large dynamic range, high resolution optical heterodyne readout for high velocity slip events

Rev Sci Instrum. 2022 Jun 1;93(6):064503. doi: 10.1063/5.0082970.

Authors

Perry W F Forsyth¹, Kathryn S Hayward², Lyle E Roberts¹, Stephen F Cox², Daniel A Shaddock¹, Bram J J Slagmolen¹

Affiliations

¹ ARC Centre of Excellence for Gravitational Wave Discovery, Centre for Gravitational Astrophysics, Research School of Physics, and of Astronomy and Astrophysics, The Australian National University, Canberra, Acton 2601, Australia.
² Research School of Earth Sciences, The Australian National University, Canberra, Acton 2601, Australia.

PMID: 35778025
DOI: 10.1063/5.0082970

Abstract

We present a free-space optical displacement sensor for measuring geological slip event displacements within a laboratory setting. This sensor utilizes a fiberized Mach-Zehnder based optical heterodyne system coupled with a digital phase lock loop, providing a large dynamic range (multiple centimeters), high displacement resolution (with an amplitude spectral density of <10^-10 m/Hz for frequencies above 100 Hz), and high velocity tracking capabilities (up to 4.96 m/s). This displacement sensor is used to increase the displacement and the time sensitivity for measuring laboratory-scale earthquakes induced in geological samples by using a triaxial compression apparatus. The sensor architecture provides an improved displacement and time resolution for the millisecond-duration slip events, at high containment and loading pressure and high temperatures. Alternatively, the sensor implementation can be used for other non-contact displacement readouts that required high velocity tracking with low noise and large dynamic range sensing. We use 13 high-velocity slip events in Fontainebleau sandstone to show the large dynamic range displacement tracking ability and displacement amplitude spectral densities to demonstrate the optical readout's unique sensing capabilities.