Partial unclad fibers with diameters ranging from d=121µm to d=125µm were fabricated using standard telecommunications optical fiber (SMF28) via low cost mechanical swipe-off technique. Graphene oxide (GO) was deposited using drop casting method on the outer side of the partial unclad SMF. IR laser with excitation wavelengths of λ=1310nm and λ=1550nm were launched along the graphene-coated SMF. The sensitivity of graphene based macrobend unclad SMFs were investigated by introducing two different pH of aqueous environment with values of 3.5 (acidic) and 12.5 (alkaline) that acted as sensing media. The optimum power loss was obtained as smallest diameter of partial unclad SMF with d=121µm was appointed. As uncoated SMF was replaced with the GO coated SMF which had been immersed into 3.5pH liquid solution, it was found that the optical power losses were increased about 6.79dBm and 5.15dBm using laser with λ1=1310nm and λ2=1550nm respectively. The uncoated SMFs experienced the increment of power losses about 2.11dBm and 5.15dBm as they were soaked into the solution with pH=12.5 using similar laser of λ1 and λ2. It is noteworthy to highlight the significant of graphene’s employment on macrobend unclad SMF by using λ1=1310nm in which better sensitivity and selectivity represented by maximum changes of power losses were apparently observed for both solutions. The usage of λ=1550nm exhibited poor selectivity where the partial unclad SMF unable to differentiate two contrasting pH solution. In conclusion, graphene based macrobend fiber optic sensor for pH detection was successfully developed by employing partial unclad SMF with cladding diameter of d=121µm and laser wavelength of λ=1310nm due to the enhancement of evanescent field’s strength. © 2020 Trans Tech Publications Ltd, Switzerland.

The presence of near-field indicates the existence of evanescent waves, which is one of the important requirements for the development of an optical sensor. This study was carried out to identify the presence of near-field and far-field as monochromatic light propagated through various structures and sizes of apertures. The confocal microscopy setup consisted of He-Ne laser with 633nm of excitation wavelength, apertures (slits and numerous sizes of pinholes), objective lens, projected screen and silicon photodetector. The near-field and far-field were determined by calculating the Fresnel number, FN as assorted size and design of apertures were added in the setup. The presence of far-field was identified with the usage of slits (single slits and double slits) represented by the value of FN, which was less than 1(FN?1). As the aperture was replaced with pinholes with diameters within 30?m and 1.2mm, the near-field was resulted (FN?1). Value of FN became greater as distance x between lens and pinholes increased. We also discovered a significant finding where the field intensities I were decreased with increment of distance x for the near-field, and vice versa (far-field). By using an extrapolating technique, it was found that the near-field can be created by using pinhole with a diameter of a < 1.46mm. In conclusion, the discovery of this work proves an excellence role of pinholes in confocal microscopy setup in creating the evanescent waves for optical sensing applications. � 2019 Akademi Sains Malaysia.

Surface plasmon resonance (SPR) can only be achieved if sufficient energy is provided at the boundary between metal and dielectric. An employment of prism as a light coupler by using Kretschmann configuration is one of the alternative for the production of adequate energy to be generated as surface plasmon polaritons (SPP). This work is carried out to investigate the effect of physical structure of the prism and its refractive index to the excitation of SPPs. A 50nm gold thin metal film with dielectric constant of ? =-12.45i+1.3 was deposited on the hypotenuse surface of the prisms. The physical structures of the prisms were varied such as triangular, conical, hemispherical and half cylindrical. These prisms were classified into two types of refractive indices (RI), namely n=1.51(type BK7) and n=1.77(type SF11). Based on SPR curve analyses, we discovered that strong SPR signals which consist of 82.98% photons were excited as SPPs can be obtained by using type-BK7 prism with physical structures of hemispherical or half cylindrical. From the view of selectivity ability as sensors, the usage of type-SF11 prisms (half cylindrical and hemispherical) able to enhance this impressive feature in which sharp SPR curves with small FWHM values were obtained. In conclusion, apart from properties of thin film materials, the physical structure of prisms and their RI values play crucial roles to obtain optimum SPR signal. High sensitivity SPR sensor can be established with the appointment of type-BK7 prisms (hemispherical or half cylindrical shape) as light couplers. � The Authors, published by EDP Sciences, 2017.

This study discussed the effect of nanocomposite film's thicknesses on the generation of SPR for detection of Pb2+ ions. Ag-GO nanocomposite film was deposited on the hypothenuse side of the triangular prism via Kretschmann configuration. The thicknesses of Ag-GO were varied between 50nm and 60nm by manipulating the Ag's thicknesses from 30nm to 50nm and GO's between 2nm and 10nm. The optimum SPR signal was obtained as thickness of Au-GO was fixed at 50nm with tAg=48nm and tGO=2nm resulting 75.77% of SPP excitation. As thicknesses of GO increased above 2nm and thicknesses of Ag were set less or more than 48nm, the SPP excitation became weaker. The sensitivity of SPR sensor shows a good agreement with the generation of SPP. The sensor exhibits the optimum sensitivity of s=0.452�/RIU with angle shifting of 0.905�as nanocomposite film under configuration of tAg=48nm: TGO=2nm and tAg=50nm: TGO=2nm were employed. In conclusion, the amplification of SPR sensor's sensitivity can be achieved by introducing 50nm nanocomposite films consist of �2nm thicknesses of GO. � 2018 IEEE.