Flight laser module characterization for the ExoMars Raman Laser Spectrometer under a potential depressurization

The Raman Laser Spectrometer (RLS) is one of the Pasteur Payload instruments within ESA’s ExoMars mission to Mars. The RLS instrument scientific goal consists of performing Raman spectroscopy over different mineral powder samples of the Mars subsoil, been able to detect organic samples if they exist. The laser unit contained in RLS is one of the most critical parts of the instrument and must meet a series of requirements and functionalities that guarantee compliance with the scientific requirements. The flight unit was thoroughly characterized, obtaining a 2D mapping of its optical properties. Since the laser modules are hermetically sealed, this characterization was performed in ambient conditions. But what if the internal pressure conditions changed? How would performances be affected? Leak rate requirement was calculated in order to guarantee high internal pressure levels up to end-of-life but a sealing failure leading to a slow degradation or a depressurization event may still be feasible. Two batch of tests were carried out in order to know how the laser unit behaves if a depressurization occurs by chance. The first test phase consisted of characterizing unsealed laser units both in ambient pressure and in vacuum, so that we could see the contrast between both cases. In the second phase of tests, the pressure profile was varied while monitoring both the power and the spectral behavior, in order to study the evolution of performances of these units. This work summarizes the outcome of these test sessions.