We demonstrated the first photothermal CO sensor using a hollow-core negative curvature fiber (HC-NCF). 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 achieved a normalized noise equivalent absorption (NNEA) coefficient of 4.4×10 -8 cm -1 WHz -1/2 . As CO has a very slow vibrational-translational (V-T) relaxation process, we enhanced the photothermal signal by enhancing the relaxation with the water vapor additive.
Ultrasensitive mid-infrared absorption spectroscopy was demonstrated by the use of a novel silica-based hollow-core negative curvature fiber (HC-NCF). The HC-NCF used in this work consists of a single ring of six nontouching cladding capillaries around the hollow core, thus forming a unique core boundary with a negative curvature. Such a silica HC-NCF enables the broadband single-mode transmission in the mid-infrared. By using the HC-NCF as a compact gas cell, a proof-of-principle experiment was conducted to detect the N2O line at 2778.37 cm -1 with a distributed-feedback interband cascade laser emitting at 3.6 μm. A minimum detectable absorbance of 3×10 -5 was achieved for a fiber length of 120 cm, corresponding to a noise equivalent absorption (NEA) coefficient of 2.5×10 -7 cm -1 . Silica HC-NCFs offer a new opportunity of developing sensitive and compact gas sensors using mid-infrared absorption spectroscopy.Index Term-Mid-infrared absorption spectroscopy, Gas sensor, Optical fiber sensor, Photonic crystal fiber, Hollow core fiber
I. INTRODUCTIONollow-core microstructured optical fibers (HC-MOFs) typically comprise a glass microstructure with a tiny hollow core in the central region where light and gas/liquid samples could interact perfectly. Flexible HC-MOFs with a low optical transmission loss provide opportunities for developing sensitive and compact optical sensors that would otherwise be difficult for free-space optics. The use of HC-MOFs for gas sensing has been investigated by several spectroscopic methods such as direct absorption spectroscopy (DAS) [1-5], wavelength modulation spectroscopy (WMS) [6-8] and photothermal spectroscopy (PTS) [9,10].
We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS-1f) detection of formaldehyde, one of the most common volatile organic compounds (VOCs), in a silica hollow-core negative curvature fiber (HC-NCF). The photothermal gas sensor adopts a mid-infrared interband cascade pump laser at 3.6 µm and a near-infrared fiber probe laser at 1.56 µm. At the optimal modulation frequency (8 kHz) and modulation index (1.8) of the pump laser, we obtained a normalized noise equivalent absorption (NNEA) coefficient of 4×10 -9 cm -1 WHz -1/2 . The use of HC-NCF with an inner diameter of 65 µm enables the sensitive photothermal detection even for a very low pump power of microwatts. The background-free PTS-1f detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic (2f) detection. A theoretical model was established in this work to interpret the experimental results.
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