Evanescent Wave Lab‐on‐Fiber for High Sensitivity Gas Spectroscopy with Wide Dynamic Range and Long‐Term Stability

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On‐chip waveguide spectroscopic sensors have attracted considerable attention due to its potential for large‐scale integration. However, existing waveguide gas sensors based on direct absorption spectroscopy (DAS) suffer from limited sensitivity and measurement range. Here waveguide‐based on‐chip photothermal spectroscopy (PTS) is demonstrated for gas detection with high sensitivity and large dynamic range. On‐chip photothermal field due to non‐radiation relaxation of gas molecules and the resulted photothermal phase modulation are analyzed. By selecting chalcogenide glass (ChG) as the core‐layer material and fabricating thermally‐isolated ChG‐on‐SU8 waveguide for thermal field accumulation, a twofold increase in photothermal phase modulation is achieved as compared to ChG‐on‐SiO2 waveguides. Different from the major concern of multi‐path etalon noise in DAS, piezoelectric transducer noise in the interferometer is identified as the main source in this PTS. For a fair comparison, acetylene (C2H2) detection experiments are conducted using PTS and DAS with a 2 cm‐long ChG‐on‐SU8 waveguide. A remarkable sensitivity of 4 parts‐per‐million (ppm) is achieved, which is 16 times better than that of DAS. The dynamic range extends over five orders of magnitude for PTS, ≈3 orders of magnitude larger than that of DAS. Such high performance opens the possibility of fully‐integrated chip‐level sensors for low‐power, light‐weight applications.

Section: Comparison On Pts Between On-chip Waveguide and Hcfmentioning

confidence: 99%

Waveguide‐Based On‐Chip Photothermal Spectroscopy for Gas Sensing

Zheng,

Pi,

Huang

et al. 2024

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Ultraminiature Optical Fiber‐Tip 3D‐Microprinted Photothermal Interferometric Gas Sensors

Zhao,

Krishnaiah,

Guo

et al. 2024

Optical fiber sensor emerges as a highly promising technology for trace gas detection due to their high sensitivity, remote capability, and immunity to electromagnetic interference. However, the state‐or‐the‐art fiber‐optic gas sensors typically use lengthy optical fibers as gas absorption cells or coatings with functional materials to achieve more sensitive gas detection, which poses challenges such as slow response and/or poor selectivity, as well as limitations on their use in confined spaces. Here, an ultraminiature optical fiber‐tip photothermal gas sensor via direct 3D micro‐printing of a Fabry‐Pérot cavity on the end face of a standard single‐mode optical fiber is reported. It enables not only direct interaction between light and gas molecules at the fiber output but also remote interrogation through an interferometric read‐out scheme. With a low‐finesse microcavity of 66 µm in length, a noise equivalent concentration of 160 parts‐per‐billion acetylene gas is demonstrated with an ultra‐fast response time of 0.5 s. Such a small high‐performance photothermal gas sensor offers an approach to remotely detecting trace gases for a myriad of applications ranging from in‐reactor monitoring to medical diagnosis.

Nanoliter‐Scale Light–Matter Interaction in a Fiber‐Tip Cavity Enables Sensitive Photothermal Gas Detection

Yan,

Xiao,

Nie

et al. 2024

Laser spectroscopy offers a significant tool for revealing specific molecular details with the desired accuracy and sensitivity. However, it poses challenges to maintain high sensitivity when targeting a micro‐region. Here, a dual‐enhanced photothermal approach is presented using a high‐finesse fiber Fabry–Pérot (F–P) cavity, tailored for highly sensitive chemical sensing with nanoliter‐scale light–matter interaction. A spheric surface (diameter: 50 µm, radius of curvature: 910 µm) is created on the fiber tip using focused ion beam milling. By adding a high‐reflectivity dielectric coating to the spheric surface, a fiber F–P cavity is obtained with a length of 473 µm and a finesse exceeding 4000. The intra‐cavity pump light within the gas‐filled fiber cavity generates a strong photothermal effect upon gas absorption. This effect induces phase modulation, which is amplified and detected by coupling a probe laser to the fiber cavity‐based interferometer. A minimum detection limit of 10 parts‐per‐billion (ppb) of C2H2 at 1530.37 nm is demonstrated using only 1 mW of pump power, corresponding to a normalized noise equivalent absorption coefficient of 9.1×10−11 cm−1∙W∙Hz−1/2. This platform breaks the bottleneck of ultrasensitive gas detection with a very short light–matter interaction length, promising significant advancements in microscale chemical analysis through optical investigations.