Compact Photoacoustic Spectrometer Based on a Widely Tunable Mid-Infrared Pulsed Optical Parametric Oscillator

Abstract

For highly sensitive and selective photoacoustic (PA) trace-gas sensing it is desirable to use high power light sources with large wavelength tunability in the mid-infrared (MIR) region, where most molecules have strong vibrational transitions. The OPO technology is an old technology, but still an excellent choice as light source for PA spectroscopy. The advantages are their molecular specificity due to large wavelength tunability (1.5 to 5μώ), high energy output, cost-effectiveness and compactness [1]. The tunability of OPOs together with PA allow for multi-gas detection of several components with common signal processing and data analysis. MIR OPOs can operate in different configurations both pulsed and continuous wave (CW). The pulsed operation results in high peak power, potentially nonlinear absorption effects and a normalized noise equivalent absorption that is not extreme. We demonstrate a novel miniaturized PA trace gas analyzer platform integrated with a MIR OPO targeting the major market opportunity of environmental monitoring and breath analysis [2,3]. We demonstrate that a miniaturized photoacoustic spectroscopic (PAS) cell can be excited resonantly with the MIR OPO by adjusting the laser pulse repetition rate to match the acoustic resonance of the PAS cell. The application of the gas sensor for real time environmental measurements and breath analysis is demonstrated using three samples of gas concentration; 100 ppmV of methane (CH4), 100 ppmV of nitrogen dioxide (NO2), and approximately 1000 ppmV ammonia (NH3) in atmospheric air with a humidity of 40% [3]. A gas flow rate of 300 ml/min through the PAS cell was applied for the three samples. These gases are well-known environmental trace gases and biomarkers in exhaled breath. The gases cause environmental degradation through their effects on soil acidification, eutrophication, and stratospheric ozone depletion. The presence of ammonia in the environment is mainly due to the degradation of animal waste, industrial processes and diesel exhaust. NO2 is a toxic gas and a regulated air pollutant that possess a serious risk for human health. Monitoring them in human breath is also particularly relevant as they are potential cancer biomarkers. We acknowledge the financial support from EUREKA (Eurostars program: E10589 — PIRMAH) and the Danish Agency for Higher Education.