Abstract
We demonstrate mid-infrared photothermal interferometry (PTI) in a hollow-core fiber (HCF) for trace gas detection. Compared with free-space PTI, the optical intensity could be increased by orders of magnitude by confining the pump and probe lights in the hollow core. We coupled the pump light at 4.46μm from a quantum cascade laser and the 1555.14nm probe light into the HCF with a bore size of 200μm. The HCF was filled with nitrous oxide (N2O) which has strong absorption at the pump wavelength. The probe light detects the N2O absorption-induced phase change in the HCF via a fiber-optic Mach-Zehnder interferometer. With a 25cm fiber length and 6mW of pump power, we have achieved a minimum detection limit of 0.8ppm for N2O, corresponding to a normalized noise equivalent absorption coefficient of 4.9×10-7 cm-1 WHz-1/2.
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