Photothermal CO detection in a hollow-core negative curvature fiber

We demonstrate the first, to the best of our knowledge, photothermal carbon monoxide (CO) sensor using a hollow-core negative curvature fiber. The hollow-core fiber features a typical structure of one ring cladding containing eight nontouching capillaries to form a negative curvature core-surround. The photothermal effect in a 40-μm hollow core is induced by CO absorption at 2327 nm and detected by a Mach-Zehnder interferometer operating at 1533 nm. By using wavelength modulation spectroscopy, we achieve a normalized noise equivalent absorption coefficient of 4.4×10^-8 cm^-1 WHz^-1/2. As CO has a very slow vibrational-translational relaxation process, we enhance the photothermal signal by enhancing the relaxation with the water vapor additive.