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In this paper, we report the effects of a side-polished fiber (SPF) coated with titanium (Ti) films in different thicknesses, namely 5 nm, 13 nm, and 36 nm, protected by a thin layer of transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), which provide ultra-sensitive sensor-based surface plasmon resonance (SPR) covering from the visible to mid-infrared region. The SPF deposited with Ti exhibits strong evanescent field interaction with the MoS2 and WS2, and good optical absorption, hence resulting in high-sensitivity performance. Incremental increases in the thickness of the Ti layer contribute to the enhancement of the intensity of transmission with redshift and broad spectra. The findings show that the optimum thickness of Ti with 36 nm combined with MoS2 causes weak redshifts of the longitudinal localized surface plasmon resonance (LSPR) mode, while the same thickness of Ti with WS2 causes large blueshifts. The redshifts are possibly due to a reduced plasmon-coupling effect with the excitonic region of MoS2. The observed blueshifts of the LSPR peak position are possibly due to surface modification between WS2 and Ti. Changing the relative humidity from 58% to 88% only elicited a response in Ti/MoS2. Thus, MoS2 shows more sensitivity on 36-nm thickness of Ti compared with WS2. Therefore, the proposed fiber-optic sensor with integration of 2D materials is capable of measuring humidity in any environment.

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A single-mode optical fiber sensor uses surface plasmon resonance (SPR) with a bimetallic silver–copper (Ag–Cu) coating compared to a single layer of Ag and Cu itself. Bimetallic Ag–Cu sensors are constructed by simple fabrication on a side-polished optical fiber, followed by an electron beam evaporation of Ag and Cu films. For this investigation, the thickness of the single Ag layer was set to 30 nm and the single Cu layer was set to 30 nm; whereas for the bimetallic combined Ag–Cu layer the thickness of Ag was 7 nm and Cu 23 nm. The sensor performance was analyzed and compared experimentally and numerically using the COMSOL Multiphysics. A white light source was used with a broad optical bandwidth to provide a range of wavelengths to the optical fiber. The characteristics of the thin layers of Ag, Cu, and Ag–Cu as alcohol sensors were evaluated. We found that Cu was the most sensitive metallic layer compared to the Ag and the bimetallic Ag–Cu layers. For a 100% alcohol concentration, Cu showed a sensitivity of 425 nm/RIU followed by the bimetallic Ag–Cu layer with 108.33 nm/RIU, whereas the Ag layer was not detected. Interestingly, sensitivity reached saturation beyond the 20 nm thick layer of Ag. This shows that the Cu and the bimetallic Ag–Cu layers are suitable for an alcohol-based optical sensor.

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This research focuses on the side-polished Single-Mode Optical Fibers (SMOF) as refractive index (RI) sensor utilizing properties of Surface Plasmon Resonance. The SMOF with cladding stripped off shows a D-shaped optical fiber with high sensitivity functionalization. Here we show silver protected by graphene oxide (GO) as viable candidates. A few layers of GO on top of silver thin layer were applied as a material to overcome silver coating degradation process. This silver/GO sensor was characterized using various RIs of analyte likely deionized water and alcohol solutions. Sensor with GO has shown fast response and high sensitivity in respect of RI, where sensitivity increases in range from 1.30 to 1.34 RIU (refractive index unit). This device shows highest sensitivity of up to 833.33 nm/ RIU. The polishing process using SMOF stops at few microns above the core of the fiber with the drop of output power fixed to be 1 dB as an indication to standardize the amount of cladding been removed. The thickness of 30 nm Ag was deposited using electron beam evaporation method on D-shaped optical fiber with subsequent protected layer of GO in solutions form. The use of COMSOL Multiphysics has also been carried out to observe numerical findings. The response of the GO has shown a sensitivity improvement which proves new promising approaches for the newly developed sensors.

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