Lunar Optical Very Broad Band: a high-performance seismometer for Moon deep interior study

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Abstract
More than 50 years after Apollo, the Moon deep interior structure is still not well known. Several seismic experiments are expected on the Moon surface in the coming year (Chang’e 7, Chandrayan, Artemis III, FSS and SPSS). All of those seismometers are not expected to resolved the seismic background of the Moon and their performances are not meeting the International Lunar Network requirements (10-11 m.s-2/sqrt(Hz)).To meet this requirement, IPGP is developing an optical seismometer operated in open loop. Its mechanical oscillator is a 1Kg proof mass suspended by a 4 cross blades hinge and a leaf spring with extremely low damping. Its displacement sensor is a Michelson interferometer, associated to a narrow bandwidth laser source and an optical phase readout electronic inherited from fiber optics gyroscope. This instrument will be candidate for all flight opportunities around 2030 (launch date).The first prototypes performances tests demonstrated the potential of this technology. But it also revealed that stray light inside the interferometer is limiting its performance. Different techniques of characterization of the stray light are compared: in situ coherent detection, characterization using a delay line and short coherency length light source. Tests results are compared to simulation.Analysis of the stray light impact on the performances through the optical phase readout electronic modulation scheme shows the impact on performances. Expectation and performances potential of the next prototypes generation is discussed.