Remote optical interferometric displacement technology development for planetology

The Apollo seismic experiment yielded unique seismology data from the Moon. Howeverunderstanding of the Moon’s interior structure remains constrained by the limitations of theApollo’s sensors. Fifty years later, the Mars seismometer Insight/SEIS demonstrated reducedself-noise and enhanced resolution. A spare model will be deployed to the Moon in 2026 as partof the FSS (CLPS 12) mission. Nevertheless, it is still far from meeting the International LunarNetwork (ILN) requirements. This is the reason why a technological breakthrough is needed.By switching from electrostatic displacement sensors to optical interferometric ones, animprovement of several orders of magnitude is made in mitigating parasitic forces (electrostaticnoise to pressure radiation). Additionally, this approach allows us to minimize the electroniccomponents within the deployed sensor. This reduction is made possible by employing remoteoptical readout of the displacement via an optical link connecting the deployed sensor to thelander. Since the objective is to operate without force feedback, the primary challenge lies inmeeting two simultaneous requirements: accommodating proofmass rebalancing up to a fewmillimeters across a 100°C thermal variation and achieving an exceptionally fine resolution todetect proofmass displacements as small as 10-12 m @ 1 Hz induced by seismic activities.This challenge led to the use of a laser source within a phase-modulated Michelsoninterferometer. The critical objective is to isolate the interference between the two movingmirrors while minimizing the impact of all parasitic back reflections. Whereas, it is well-knownthat a -60dB parasitic reflection results in a -30dB variation of the interference pattern.Consequently, both experimental and theoretical work are conducted to characterize, model andquantify the effect of each parasitic reflection, depending of its position within the optical design.In this frame, the use of Rayleigh Optical Frequency Domain Reflectometry (OFDR) tocharacterize the interferometer will be described, in addition to the use of collected informationin the model to explain the observed fringe patterns.In conclusion, a comparative analysis of the performance of this optical readout technique inrelation to other published methods is performed, taking into account benefits and drawbacks ofeach of them. Notably, the capability of achieving remote readout of the signal is emphasized.Indeed, as the ability to minimize electronic components within the sensor is crucial for low-noise applications, we will explore the synergy with remote optical readout technology, such asthe one developed at ESEO (Engineering School in Angers, France). This opens the path tobroader applications across various type of geoscience sensors.