Abstract
The fast and reliable analysis of the natural gas composition requires the simultaneous quantification of numerous gaseous components. To this end, fiber-enhanced Raman spectroscopy is a powerful tool to detect most components in a single measurement using a single laser source. However, practical issues such as detection limit, gas exchange time and background Raman signals from the fiber material still pose obstacles to utilizing the scheme in real-world settings. This paper compares the performance of two types of hollow-core photonic crystal fiber (PCF), namely photonic bandgap PCF and kagomé-style PCF, and assesses their potential for online determination of the Wobbe index. In contrast to bandgap PCF, kagomé-PCF allows for reliable detection of Raman-scattered photons even below 1200 cm−1, which in turn enables fast and comprehensive assessment of the natural gas quality of arbitrary mixtures.
Highlights
The controlled excitation of higher-order fiber modes has become an increasingly active area in photonics research with a range of interdisciplinary applications
Higher-order modes up to LP33 are controllably excited in water-filled kagomé- and bandgap-style hollow-core photonic crystal fibers (HC-PCF)
Mode indices are obtained by launching plane-waves at specific angles onto the fiber input-face and comparing the resulting intensity pattern to that of a particular mode. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation
Summary
The controlled excitation of higher-order fiber modes has become an increasingly active area in photonics research with a range of interdisciplinary applications. Mode-division multiplexing has been used to improve data transfer rates [7,8,9,10] All this previous work aims to control the light field at the end-face of glass-core fibers. We extend this work to liquid-filled hollow-core PCF, and demonstrate the controlled excitation of higher-order modes in bandgap and kagomé-style fibers whose core and cladding channels are infiltrated with water. Liquid-filled kagomé PCF [26] and simplified PCF [27, 28], on the other hand, are ideal for broadband chemical sensing applications In both cases, control over modal fields within these optofluidic waveguides would enable new fiber-based sensing and optical manipulation approaches
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