The ascorbic acid (AA) is a biomarker that can be used to detect the symptoms of severe disorders such as scurvy, Parkinson’s, Alzheimer’s, and cardiovascular diseases. In this work, a simple and effective sensor model is developed to diagnose the presence of AA samples. To develop the sensor, a tapered single-mode optical fiber has been used with the well-known phenomenon of localized surface plasmon resonance (LSPR). For LSPR, the tapered region is immobilized with synthesized gold nanoparticles (AuNPs) and zinc oxide nanoparticles (ZnO-NPs) whose absorbance peak wavelengths appear at 519nm and 370nm, respectively. On the basis of nanoparticles (NPs) configurations, two different biosensor probes are developed. In the first one, the sensing region is immobilized with AuNPs and named Probe I. In the second probe, the immobilized layer of AuNPs is further coated with a layer of ZnO-NPs, and a resultant probe is termed as Probe II. The characterizations of synthesized AuNPs and developed fiber probes are done by the ultraviolet-visible (UV-vis) spectrophotometer, high-resolution transmission electron microscope (HR-TEM), atomic force microscopy (AFM), and scanning electron microscope (SEM). To enhance the selectivity, a sensing region of probes is functionalized with ascorbate oxidase enzyme that oxidizes the AA in the presence of oxygen. The response of developed sensor probes is authenticated by sensing the samples of AA in the range from 500 nM to 1 mM, which covers the range of AA found in human bodies, i.e., 40µM–120µM. The performance analysis of the developed sensor probes has been done in terms of their stability, reproducibility, reusability, and selectivity. To observe the stability of AA, a pH-test has also been done that results in a better solubility of AA molecules in phosphate-buffered saline (PBS) solution.

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Tapered Optical Fiber-Based LSPR Biosensor for Ascorbic Acid Detection

We propose a compact silicon-on-insulator based device which is capable of arbitrarily converting optical modes TE $_\text{i}$ to TE $_\text{j}$ (i, j = 0, 1, 2, 3). The device consists of two symmetric 1 × 4 Y-junction couplers connected back-to-back with a 4 × 4 multimode interference coupler (MMI). While the two 1 × 4 Y-junction couplers are designed to divide and combine modes at the input and output of the device, the 4 × 4 MMI coupler is used to direct signals from its inputs to suitable outputs. The conversion of a pair of modes is decided by four phase shifters located at the input and output of the MMI coupler. Through beam propagation method (BPM) with effective index method (EIM) simulations, we show that by setting the phase shifters to 0° or 180°, 16 different conversion pairs of input-output modes (TE $_\text{i}$ –TE $_\text{j}$ ) are successfully realized with a mode conversion efficiency larger than 98% and a crosstalk smaller than −30 dB within a wide wavelength range from 1530 nm to 1565 nm.

Arbitrary TE $_0$ /TE $_1$ /TE $_2$ /TE $_3$ Mode Converter Using 1 × 4 Y-Junction and 4 × 4 MMI Couplers

In this work, fabrication techniques and optimization of single-mode fiber (SMF)- and multi-mode fiber (MMF)-based differently tapered optical fiber (TOF) structures are discussed. Further, taper characteristics such as diameter, transmitted intensity, and repeatability are analyzed. The results show that 25 µm TOF consists of lower transmitted intensity, meaning higher evanescent waves materialize at the tapered surface, which helps in suitable potential application of TOF as an optical fiber sensor.

Fabrication techniques and stability analysis of SMF-/MMF-based differently tapered optical fiber structures.

In mode-division multiplexing (MDM) networks, converting modes back and forth between a higher-order mode and a lower-order mode plays a vital role in improving network capacity and flexibility. This paper presents a spatial optical mode conversion supporting three modes based on two Ψ -junction couplers and two multi-mode interference couplers (MMIs). The proposed device can arbitrarily convert a TEi mode to a TEj mode (where i, j = 0, 1, 2) by setting three phase-shifters at 0 or 180 degrees, which are introduced at the Ψ -junction and MMI couplers. Through numerical simulations using the spatial three-dimensions beam propagation method (3D-BPM), the mode converter shows a high conversion efficiency with the insertion loss smaller than 0.2 dB and the crosstalk below −20 dB at the center wavelength of 1.55 μm. The device works effectively in C band, partially in S and L bands within a wideband up to 80 nm. We believe that the proposed device would be a potential platform for applications in MDM networks and photonic integrated circuit systems.

Optical mode conversion based on silicon-on-insulator material Ψ-junction coupler and multimode interferometer

Mode converters are one of the most fundamental and indispensable components in the mode division multiplexing system. We design a new mode converter that is capable of converting between mode TE0 to TE1/TE2 or TE1 to TE1/TE2 with the performance achieved in both cases over 99% at the wavelength 1550nm. Not only conversion efficiency is high but also the effect of the unwanted modes at the output is very small, it is always below 0.03dB. In addition, with outstanding advantages of the proposed structure such as large manufacturing tolerances and compact size will be good candidate for applications in on-chip MDM network.

Reconfigurable Mode Converter Using Two Silicon Y-Junction Couplers for Mode Division Multiplexing Network

We propose a novel mode converter (MC) based on multimode interference (MMI). This MC taking advantages of long-period fiber grating and multimode interference, it not only has high conversion efficiency, but also greatly reduces the interference between modes.

All-fiber mode converter based on fiber grating and multimode interference

A compact optical fiber reflective index sensor is proposed and experimentally demonstrated. It is based on the tilted fiber Bragg grating (TFBG) inscribed in the thin-core fiber that is downstream spliced to the conventional single mode fiber (SMF). Thanks to the core dismatch between the SMF and the thin-core fiber, more cladding modes can be recoupled back into the SMF .The sensor can be used to detect the change of surroundings refractive index (SRI). Besides, the sensor can solve the problem of cross-sensitivity between SRI and temperature.

Runhan Zhao

Papers

All-fiber mode converter based on fiber grating and multimode interference

Reflective index sensors based on tilted fiber Bragg grating inscribed in the thin-core fiber