Beams carrying orbital angular momentum (OAM) are attractive as they offer a theoretically unbounded number of discrete states and have therefore been a subject of great interest for a variety of fundamental and applied research including optical communications, optical trapping, super-resolution microscopy, remote sensing and quantum information. In this work, we present a study on the use of fused silica capillary optical fibers for OAM beam propagation by means of antiresonant reflecting waveguiding. In particular, we propose the application of these simple and commercially available fibers for probing the light–matter interactions within the hollow core. We show that OAM beams (topological charge |L| = 1) of high mode purity (>90%) can be achieved in such capillary fibers. The stability of the OAM beam propagation was theoretically and experimentally demonstrated in the visible range. A numerical study based on full-vector finite-element method is conducted to characterize the change of OAM mode purity and loss as a function of the refractive index (RI) of the material filling the core. The propagation loss remains in the range of a few dB/m throughout the range of RI values varying from 1 to 1.39 that encompasses many analytes in the vapor or liquid phase. Finally, we propose that the simple capillary fiber can be used as a cost-effective optofluidic platform to study OAM light-matter interactions and new optical phenomena involving biochemical analytes.
In this work, we demonstrate a refractive index (RI) sensor employing an annular-core photonic crystal fiber (AC-PCF), which exhibits a large dynamic range and high sensitivity. The AC-PCF represents a very recent type of fiber tailored for the transmission of vector/vortex beams. The mode of operation of the proposed sensor is based on the simple and convenient intensity modulation scheme. A numerical study of the effect of varying the refractive index of the air holes (composing the photonic crystal cladding) on the propagation loss of the fundamental guided mode at 488, 980 and 1550 nm wavelength was performed via a full-vector finite-element method simulation with PML boundary conditions. Our results indicate that the fiber loss increases exponentially as the RI of the holey cladding approaches the value of that of the fiber material (i.e. fused silica in this case) due to outer radiation and scattering. Our simulation assumes that the holey cladding can be filled with analytes of RI varying between 1 to 1.4; thus, showing a large dynamic sensing range. In particular, we observed a strong propagation loss (spanning 2 orders of magnitude in dB/m scale) for analytes with RI values ranging from 1.31 to 1.39 that cover many biochemical solutions of interest. The theoretical results show that the sensitivity is as high as 2.65 × 104 (dB/m)/RIU at 1550 nm and 7.83 × 103 (dB/m/)RIU at 980 nm experimentally. The numerical results were validated with experimental demonstrations using a custom-fabricated AC-PCF.
We report a surface plasmon resonance based fiber optic hydrogen sulphide gas sensor using Cu/ZnO thin films. The
sensor works on wavelength modulation scheme. The fiber optic probe was fabricated by removing the cladding of
appropriate length from the fiber and depositing copper and ZnO thin films on the unclad core by thermal evaporation
technique. The presence of hydrogen sulphide gas around the probe changes the dielectric function of zinc oxide and a
dip is observed in transmitted spectrum. With the increase in the gas concentration shift in the resonance wavelength has
been observed. The proposed sensor can be used for online monitoring and remote sensing of hydrogen sulphide gas in
the environment.
We have experimentally demonstrated the role of the high index dielectric layer of zinc oxide over the metal layer on the performance of the surface plasmon resonance based fiber optic refractive index sensor. The configuration contains copper as a SPR active metallic layer covered by dielectric layer. The configuration of Cu/ZnO shows high sensitivity. Further increase in the thickness of ZnO layer increases the sensitivity of the sensor. A good agreement between experimental results and the simulated ones based on multilayer two dimensional approach is obtained. The additional advantages of ZnO layer, apart from sensitivity enhancement, are protection of metallic layer from oxidation, tunability of the resonance wavelength region, biocompatibility and capability of gas sensing.
We present an experimental study of fiber optic ammonia gas sensor based on the phenomena of surface plasmon resonance working on wavelength modulation scheme. The principle of the sensor is based on the change in dielectric constant of the bromocresol purple (BCP) in the presence of ammonia gas. The sensor works at room temperature. Two different kinds of coating configurations have been considered, namely copper + BCP and silver + BCP, on the unclad portion of the fiber. The experiments have been carried out at the low concentrations (1 ppm − 10 ppm) of ammonia gas around the probe. The sensor with copper and BCP layers has greater sensitivity than sensor with silver and BCP layers. The proposed sensor has small response and recovery times.
We present an experimental study of a fiber optic hydrogen gas sensor which works on the phenomenon of surface
plasmon resonance. The sensor operates in intensity modulation scheme. The fiber optic probe was fabricated by
removing a small section of the fiber cladding and symmetrically depositing a thin layer of indium tin oxide (ITO) by
thermal evaporation technique onto the fiber core. The presence of hydrogen in the air around the ITO changes the
dielectric function of ITO. The SPR spectra were obtained for 100% nitrogen as well as for a mixture of 4%
hydrogen and 96% nitrogen. A sharp dip in the transmittance spectrum was observed in the case of mixture of 4%
hydrogen and 96% nitrogen. The transmittance corresponding to the resonance wavelength was found to decrease
with the increase in the exposure time of the hydrogen gas to ITO. The present sensor can be used for the online
monitoring of hydrogen gas in various environments.
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