Tellurite Hollow-Core Antiresonant Fiber-Coupled Quantum Cascade Laser Absorption Spectroscopy

Mid-infrared absorption spectroscopy for gas detection is reported in this study using a quantum cascade laser (QCL) coupled with a custom-made tellurite hollow-core antiresonant fiber (HC-ARF). The HC-ARF is fabricated from tellurite glass by extrusion and subsequent fiber drawing. The QCL emitting at 5.26 μm is coupled into the 21-cm long HC-ARF. As a proof-of-concept, this spectroscopic system is demonstrated for nitric oxide (NO) detection by exploiting its strong absorption line at 1900.08 cm^-1. By quickly filling gas mixtures into the HC-ARF, we first conduct direct absorption spectroscopy of NO and achieve a noise equivalent absorption (NEA) of 2.1 × 10^-5 cm^-1. Besides, we also conduct wavelength modulation spectroscopy to improve sensing performance. A minimum detection limit of 6 ppb NO is achieved at the integration time of 30 s, corresponding to 1.0 × 10^-7 cm^-1 in NEA. The HC-ARF is tested to show a response time of only 0.3 s when applying a pressure difference of 11 kPa between the two fiber ends. Such a tellurite HC-ARF-coupled QCL spectroscopic system makes it attractive for developing optical gas sensors with compact size, fast response, and high sensitivity.

I. Introduction

Sensitive gas detection at ppm level or even lower is in high demand in many industrial, environmental and biomedical applications. Besides sensitivity, other features such as portable size and fast response are also important factors considered in practical applications. Conventional chemical analysis using gas chromatography–mass spectrometry (GC-MS) can accurately identify trace gas species but is not practical for real-time and in situ detection due to the bulky instrument size and long measurement time. Compact and low-cost electrochemical and semiconductor sensors use gas-sensitive substrates reacting with a gas analyte to conduct the relevant chemical analysis. However, these sensors suffer from several issues such as low selectivity and long response time. By contrast, laser-based gas sensors benefit from the species-specific nature of molecular spectroscopy, making it capable of highly quantitative and selective gas detection.