Comparative Study of Ultra-Precise Optical Displacement Sensing Techniques: From Planetary Seismology to Geosciences Applications

Optical sensing technologies are increasingly pivotal in geosciences and planetary exploration, where ultra-precise displacement measurements are essential for understanding seismic activity, subsurface dynamics, and environmental monitoring. This work presents a comparative analysis of two advanced optical measurement techniques developed in parallel by MAAGM: the LOKI optical interrogator — enabling all-optical, remote interrogation of electronics-free sensors based on a technology transfer from ESEO — and a planetary seismometer developed in collaboration with IPGP and funded by CNES. Both systems rely on optical distance measurements but employ distinct modulation schemes: phase modulation within an interferometer for the planetary seismometer, and optical frequency modulation upstream of the interferometer for LOKI. These approaches are positioned within the broader landscape of state-of-the-art optical displacement sensing, with emphasis on their respective interrogation architectures, sensitivity, robustness, and adaptability to harsh or remote environments — attributes directly relevant to next-generation fiber optic interrogator design.

The core of this study is an experimental campaign designed to cross-calibrate and compare the performance of these two optical methods. This work reflects a unique interdisciplinary collaboration between MAAGM, IPGP — a leader in planetary seismology — and ESEO, renowned for its expertise in optical sensing technologies. A single mechanical oscillator, originally developed by IPGP for planetary seismology, was instrumented with two phase-based optical sensors and one wavelength-based optical sensor, alongside a reference STS-2 seismometer. This setup enabled direct comparison of intrinsic noise levels and sensitivity, with particular focus on meeting the stringent requirements of lunar missions, where ambient noise is significantly lower than on Earth. The target sensitivity for such applications is 10⁻¹¹ m·s⁻²/√Hz at 0.1 Hz, necessitating exceptionally low self-noise instrumentation (cf. EGU26-13434).

Preliminary results demonstrate the noise performance of each interrogation method under controlled conditions, providing quantitative insights into their suitability for both planetary and terrestrial applications. Beyond planetology, these optical interrogation techniques show strong potential for instrumenting diverse geophysical transducers — including pressiometers, borehole seismometers, strainmeters, and rotational seismometers (cf. EGU26-18427) — thereby broadening the scope of retrievable observables in distributed and point optical sensing systems. The findings contribute to informing the design of next-generation optical interrogators optimized for deployment across a wide range of geoscience sensing contexts.