Stray light simulation in device for gas pressure/density measurement through Rayleigh scattering

This technical report is aimed to give some insights on the estimation of stray light impact occurring in the realisation of a compact device for gas density/pressure measurements based on Rayleigh scattering. The performed optical simulations fit into the context of the framework of WP2 of JRP “18SIB04 QuantumPascal”: the purpose of this work package (‘Alternative non Fabry Pérot based approaches for the realisation of absolute and partial pressure standards’) is to evaluate the metrological performance of several innovative non-FP based methods and techniques for the realisation of absolute and partial pressure standards; in particular, in coherence with Task 2.1 of this WP a novel system for gas/density pressure measurements (in the range 10 ÷ 1 MPa) was developed. Stray light phenomenon is one of the main factors affecting the performance and limiting the resolution of any device based on Rayleigh scattering, because of the spurious light scattered by particles which are different by the working gas; this results in an unwanted contribution in the intensity collected by the detector, so, identifying the stray light sources, properly designing the geometry of the device and taking care of the arrangement of mechanical countermeasures for breaking down its influence is key to improve the overall accuracy and sensitivity of the system: the density and the pressure of the gas species are the quantities to be inferred from the direct measurement of the light scattering. To fulfill such objectives, the monolithic chamber with the features shown in Figures 1 and 2 was designed and realised.