Mid-infrared photothermal gas sensor enabled by core-cladding mode interference in a hollow-core fiber

Abstract

Photothermal spectroscopy provides an attractive way of sensitive gas detection with a large dynamic range. Previous photothermal gas sensors normally use an optical interferometer for measuring the slight phase change caused by gas absorption. However, an active opto-mechanical stabilization system is required in the photothermal interferometry, making the sensor complex, bulky and hard for field applications. Here, we report a new mid-infrared photothermal gas sensor based on core-cladding mode interference in a tellurite hollow-core antiresonant fiber (HC-ARF). This method relies on the co-propagation of the HE₁₁ mode of a mid-infrared pump laser and the core-cladding modes of a probe laser in the tellurite HC-ARF. As a proof-of-principle demonstration, we detect nitric oxide in the 35 cm long fiber by using a quantum cascade laser (5.2 µm, 21.7 mW) as the pump laser. We achieve a noise equivalent concentration of 50 ppbv, corresponding to a normalized noise equivalent absorption coefficient of 8.6 × 10^-9 cm^-1WHz^-1/2. Our technique has long-term stability without using any active stabilization devices and holds promise for fast, sensitive and background-free gas detection with a simplified configuration.