Abstract
In most fiber-optic gas sensing applications where the interested refractive index (RI) is ~1.0, the sensitivities are greatly constrained by the large mismatch between the effective RI of the guided mode and the RI of the surrounding gaseous medium. This fundamental challenge necessitates the development of a promising fiber-optic sensing mechanism with the outstanding RI sensitivity to achieve reliable remote gas sensors. In this work, we report a highly sensitive gas refractometer based on a tapered optical microfiber modal interferometer working at the dispersion turning point (DTP). First, we theoretically analyze the essential conditions to achieve the DTP, the spectral characteristics, and the sensing performance at the DTP. Results show that nonadiabatic tapered optical microfibers with diameters of 1.8-2.4 µm possess the DTPs in the near-infrared range and the RI sensitivities can be improved significantly around the DTPs. Second, we experimentally verify the ultrahigh RI sensitivity around the DTP using a nonadiabatic tapered optical microfiber with a waist diameter of ~2 μm. The experimental observations match well with the simulation results and our proposed gas refractometer provides an exceptional sensitivity as high as -69984.3 ± 2363.3 nm/RIU.
Highlights
Reliable gas sensors with high sensitivity, accuracy, portability and cost-effectiveness are widely demanded in real-world applications, including pressure or leak detections in industrial surveillance [1], discrimination of toxic and biological threats in environment monitoring [2], clinical diagnosis from exhaled breath [3], etc
Numerous fiber-optic sensing mechanisms have been proposed for gas detections, of which the great majority can be classified into two categories, interferometry [5] and absorption spectroscopy [6]
Compared with absorption spectroscopy where the fingerprints of most gases fall within the mid-infrared range, interferometry based gas sensors are accessible for the well-developed near-infrared communication network with merits of high integration, cost effectiveness and remote monitoring
Summary
Reliable gas sensors with high sensitivity, accuracy, portability and cost-effectiveness are widely demanded in real-world applications, including pressure or leak detections in industrial surveillance [1], discrimination of toxic and biological threats in environment monitoring [2], clinical diagnosis from exhaled breath [3], etc. In this study, we mainly focus on fiber-optic interferometry based gas sensors, which are realized by measuring the refractive index (RI) of the gaseous sample [6,7,8,9]. Researchers observed the similar points in long-period fiber gratings [23,24] and few-mode optical fibers [25,26,27], and these sensors with DTPs can offer superior sensing performance, compared to their counterparts without the DTPs. A recent study numerically proved that tapered multimode optical microfiber with the DTP could provide ultrahigh RI sensitivity [28]. Guided by theoretical and numerical analysis, we properly design the structural parameters of tapered optical microfiber and experimentally realize the exceptional gas RI sensing performance of −69984.3 ± 2363.3 nm/RIU
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