Design of System for Rugged, Low-Noise Fiber-Optic Access to High-Temperature, High-Pressure Environments

This paper details a new design for fiber-optic access to harsh environments, with a particular focus on absorption spectroscopy in high-temperature, high-pressure combustion systems. A key component is a fiber-fused rod lens assembly which tolerates temperatures above 1100°C and enables simple optical access without interferences from etalons or ambient absorbers. The fibers, rod lenses, and supporting ports are designed to provide line-of-sight optical access without the need for adjustable optomechanics. Fiber modal noise was minimized by actively aligning a photodiode to the multimode fiber's output facet, resulting in a shot-noise-limited SNR of 53 000 for the system.