Open-Path Methane Sensing via Backscattered Light in a Nonlinear Interferometer

TL;DR

This paper introduces a novel nonlinear interferometry technique for remote methane detection using backscattered light, enabling outdoor sensing with high sensitivity despite significant transmission loss and surface variability.

Contribution

It demonstrates a new methane sensing method based on stimulated parametric down conversion that works effectively with diffusely backscattered light in outdoor environments.

Findings

Detects methane at 4.6 meters with 60 dB loss

Works on various real-world surfaces like glass and metal

Uses CMOS camera for near-infrared detection

Abstract

Nonlinear interferometry has widespread applications in sensing, spectroscopy, and imaging. However, most implementations require highly reflective mirrors and precise optical alignment, drastically reducing their versatility and usability in outdoor applications. This work is based on stimulated parametric down conversion (ST-PDC), demonstrating methane absorption spectroscopy in the mid-infrared (MIR) region by detecting near-infrared (NIR) photons using a silicon-based CMOS camera. The MIR light, used to probe methane, is diffusely backscattered from a Lambertian surface, experiencing significant transmission loss. We implement a single-mode confocal illumination and collection scheme, using a two-lens system to mode-match the interfering beams to achieve background methane detection at a distance of 4.6 meters under a 60 dB loss. Our method is also extended to real-world surfaces, such as glass, brushed metal, and a leaf, showing robust background methane sensing with various target materials.