High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure

SARS-CoV-2 spike protein detection using slightly tapered no-core fiber-based optical transducer

The label-free detection of SARS-CoV-2 spike protein is demonstrated by using slightly tapered no-core fiber (ST-NCF) functionalized with ACE2. In the fabricated sensor head, abrupt changes in the mode-field diameter at the interfaces between single-mode fiber and no-core fiber excite multi-guided modes and facilitate multi-mode interference (MMI). Its slightly tapered region causes the MMI to be more sensitive to the refractive index (RI) modulation of the surrounding medium. The transmission minimum of the MMI spectrum was selected as a sensor indicator. The sensor surface was functionalized with ACE2 bioreceptors through the pretreatment process. The ACE2-immobilized ST-NCF sensor head was exposed to the samples of SARS-CoV-2 spike protein with concentrations ranging from 1 to 10⁴ ng/mL. With increasing sample concentration, we observed that the indicator dip moved towards a longer wavelength region. The observed spectral shifts are attributed to localized RI modulations at the sensor surface, which are induced by selective bioaffinity binding between ACE2 and SARS-CoV-2 spike protein. Also, we confirmed the capability of the sensor head as an effective and simple optical probe for detecting antigen protein samples by applying saliva solution used as a measurement buffer. Moreover, we compared its detection sensitivity to SARS-CoV-2 and MERS-CoV spike protein to examine its cross-reactivity. In particular, we proved the reproducibility of the bioassay protocol adopted here by employing the ST-NCF sensor head reconstructed with ACE2. Our ST-NCF transducer is expected to be beneficially utilized as a low-cost and portable biosensing platform for the rapid detection of SARS-CoV-2 spike protein.

Refractometer based on fiber Mach-Zehnder interferometer composed of two micro bending cores

In this paper, a novel refractometer based on Mach-Zehnder Interferometer (MZI) is proposed and experimentally investigated. The MZI is composed of 2 micro bending cores (MBCs), one of which excites the cladding modes and the other couples the modes back. This structure is formed by high-frequency CO₂ laser polishing and oxyhydrogen flame heating. With the unique deformation method, the interaction between the fiber core and the external status gets enhanced, moreover, higher modes in the cladding are excited, which leads to a high refractive index (RI) sensitivity. Due to the high temperature of the oxyhydrogen flame, the core of CO₂ polished fiber is modulated, furthermore, the cladding shape of MBC tends to be circular. Hence, relatively small modulating regions of 500 μm can form for interference. In the experiment, 2 transmission dips are chosen for RI measuring, which possesses the wavelength of 1530.4 nm and 1600.8 nm, respectively. The RI sensitivities of the 2 transmission dips are -271.7 nm/RIU and -333.8 nm/RIU with the RI range of 1.33-1.42. The temperature characteristic is also experimentally analyzed and the temperature sensitivities of which are 0.121 nm/℃ and 0.171 nm/℃ in the range of 34℃-154℃. By solving the matrix equation, the proposed sensor can be applied for simultaneous measurement of RI and temperature.

Nanostructured coating of graphene nanoparticles deposited onto a cladding etched no-core optical fiber for temperature measurement

We report on a temperature sensor based on an optical fiber with nanostructured cladding fabricated by graphene nanoparticles (G-NPs) deposited onto a chemically etched no-core fiber (NCF). The basic structure of this sensor comprises a single-mode-no-core-single-mode (SNCS) fiber section concatenation. The sensing head is the NCF segment where the modal interference is highly sensitive to temperature variations. The influence of the NCF diameter on the sensitivity of the sensor has been investigated. The obtained results revealed that the sensor with an etched NCF diameter of 60 µm coated with G-NPs embedded in polyvinyl alcohol exhibits a maximum sensitivity of -0.104nm/^∘C in the temperature range of 25-235°C, which is approximately six times higher than that of the basic structure without coating. To the best of our knowledge, this is the first demonstration of a temperature sensor based on an etched NCF coated with graphene-polyvinyl alcohol thin film.

Temperature-Insensitive Refractive Index Sensor with Etched Microstructure Fiber

A Mach–Zehnder interferometer (MZI) based on an etched all-solid microstructure fiber (MOF) has been demonstrated. The MZI works on the basis of interference between the vibrant core and cladding modes in the MOF. The all-solid MOF has a heterostructure cladding composed of Ge-doped rod arrays and pure silica, and thus can support and propagate a vibrant cladding mode with a large mode area. When the outermost cladding of MOF is etched, the cladding mode becomes sensitive to the ambient refractive index (RI). The etched MOF can work as a sensing head for RI sensing. By comparing the interference spectra, the extinction ratio has remained stable at around 20 dB after the MOF was etched. The RI sensing characteristics of the MZI with an etched MOF have also been investigated. The results show that the RI sensitivity can reach up to 2183.6 nm/RIU with a low-temperature coefficient (<10 pm/°C).