Mid-infrared fiber-optic photothermal interferometry

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 4.46 μm from a quantum cascade laser and the 1555.14 nm probe light into the HCF with a bore size of 200 μm. 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 25 cm fiber length and 6 mW of pump power, we have achieved a minimum detection limit of 0.8 ppm for N₂O, corresponding to a normalized noise equivalent absorption coefficient of 4.9×10^-7  cm^-1 WHz^-1/2.