Microresonators for compact optical sensors (μRCOS) for gas detection

This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of spherical micro-resonators, coupled to a symmetrically tapered optical fiber, as a gas sensor. We will discuss silica-based microspheres and optimized tapered fiber coupling systems to detect greenhouse gases, i.e. CO2 in this context. The coupling setup is designed to be portable and amenable to different controlled environments, from constrained and controlled geometries to open and flexible enclosures. 3D-printed spherical resonator and tapered-fiber holders were made to satisfy different requirements. We produced microspheres for absorption spectroscopy of targeted gas and fabricated tapers by HF etching, using an HF-resistant fixture for safer handling and reduced waste. Detection within loose enclosures was performed as a preliminary study, where we observed spectral shift and broadening in the cavity resonances induced by the gaseous environments. Optically coupled vacuum-tight vessels have been designed and built to understand environmental effects.