High-quality optical fibers can be produced now at a low cost and large quantity, and this has further promoted the development of fiber optic (chemical) sensors. After over 30 years of innovation, fiber optic sensing technology has become mature because of acceptable costs, compact instrumentation, high accuracy and the capability of performing measurements at inaccessible sites, over large distances, in strong (electro)magnetic fields and in harsh environment. The technology is still proceeding quickly in terms of innovation, and respective applications have been found in highly diversified fields. This review covers work published in the time period between October 2015 and October 2019. It is written in continuation of previous reviews.

Fiber-Optic Chemical Sensors and Biosensors (2015-2019).

All-photonic integrated circuits are promising platforms for future systems beyond the limitation of Moore's law. Over the last several decades, one-dimensional (1D) nanowires have demonstrated great potential in photonic circuitry because of their unique 1D structure to effectively generate and tightly confine optical signals as well as easily tunable optical properties. In this Review, we categorize nanowires based on the optical properties (i.e., semiconducting, metallic, and dielectric nanowires) for their potential photonic applications (as light emitters or plasmonic and photonic waveguides). We further discuss the recent efforts in integration of nanowire-based photonic elements toward next-generation optical information processors. However, there are still several challenges remaining before the nanowires are fully utilized as photonic building blocks. The scientific and technical challenges and outlooks are provided to indicate the future directions.

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Nanowires for Photonics.

A plethora of research in recent years has been reported on biosensing in the surface plasmon resonant systems. However, very little research has reported a tunable and highly sensitive biosensor in a nanoscale platform. In this regard, we propose a nanoscale hyperbolic metamaterial (HMM)-based prism coupled waveguide sensor (PCWS) in the near-infrared range. The HMM layer makes up one of the constituents of the PCWS—comprised of a periodically arranged assembly of silver nanostrips. The structure is numerically simulated by the finite difference time domain method. It is demonstrated that the sensitivity of the reflected light can be tuned through the refractive index (RI) of the solution. Moreover, the effects of alteration of constituents of PCWS on the sensitivity have been analyzed. Results show that the sensitivity of PCWS can be harnessed by altering the thickness, slant angle of HMM layer, volume fraction (f) of metal in the HMM layer, and the incidence angle of light. For this purpose, the structure is numerically simulated by the finite difference time domain method. In the optimum design of the proposed sensor, the maximum value of sensitivity is achieved as high as S=3450  nm/refractive index unit with θ=10° and ϕ=10° and a metamaterial thickness of 250 nm. Moreover, the structure has a nanoscale footprint of 600  nm×400  nm×200  nm.

Nanoscale, tunable, and highly sensitive biosensor utilizing hyperbolic metamaterials in the near-infrared range.

Electrospinning, for the last few decades, has been extensively acknowledged for its ability to manufacture a macro/nanofibrous architecture from biopolymers, which is otherwise difficult to obtain, in a cost effective and user-friendly technique. Such biopolymer nanofibers can be tailored to meet applications such as drug delivery, tissue engineering, filtration, fuel cell, and food packaging etc. Due to their structural uniqueness, chemical and mechanical stability, functionality, super-high surface area-to-volume ratio, and one-dimensional orientation, electrospun biopolymer nanofibers have been proven to be extremely beneficial. A parallel method in nonwoven methodologies called “Solution Blowing” has also become a potential candidate to fabricate a similar type of architecture from biopolymer fibers, and is gaining popularity among researchers, despite its recent advent in early 2000’s. This review chiefly focuses on the fabrication of biopolymer macro/nanofibers via electrospinning and solution blowing, and several applications of such fiber architectures. Biopolymers include plant- and animal-derived biopolymers, such as cellulose, lignin, chitin, and chitosan, as well as proteins and their derivatives. The fabrication of biopolymer fibers from these biopolymers alone or as blends, predominantly with biodegradable polymers like Polyvinyl alcohol (PVA), Polyethylene Oxide (PEO), Polyethylene glycol (PEG), poly (lactide-co-glycolide) (PLGA) etc., or non-biodegradable polymers like polyamide, Polyacrylonitrile (PAN) etc., will be discussed in detail, along with the applications of several composites of such sort.

A Review on Biopolymer-Based Fibers via Electrospinning and Solution Blowing and Their Applications

A Comprehensive Review of Optical Fiber Refractometers: Toward a Standard Comparative Criterion

A highly sensitive refractive index sensor based on surface plasmon resonance in a side-polished low-index polymer optical fiber is proposed for biosensing. Benefitting from the low refractive index of the fiber core, the sensitivity of the device can reach ~44567 nm/RIU theoretically for aqueous solutions, at the expense of a lowered upper detection limit that is down to ~1.340. The sensor is fabricated by coating 55-nm-thick Au-film on the polished surface of a graded-index perfluorinated polymer optical fiber. Results show that the sensor exhibits a sensitivity of ~22779 nm/RIU at 1.335 with a figure of merit of 61.2. When employed for glucose sensing, the sensor presents an averaged sensitivity of 24.50 nm/wt%, or 0.46 nm/mM. This device is expected to have potential applications in cost-effective bio- and chemical-sensing.

Highly sensitive surface plasmon resonance biosensor based on a low-index polymer optical fiber

A liquid-crystal-filled photonic crystal fiber (PCF) is proposed for electro-optical modulation. The E7 liquid crystal is precisely filled in one of the innermost air holes of the PCF and forms an in-fiber optical coupler, and the resonance wavelength can be tuned with a sensitivity of 5.594 nm/ V rms when an external voltage is applied. The device can operate as an electro-optical switch/modulator and exhibits response and recovery times of approximately 47 and 24 ms, respectively from 1414 nm to more than 1700 nm. The proposed structure is expected to have potential applications in electric field sensing and wavelength-tunable electro-optical devices.

Tunable Electro-Optical Modulator Based on a Photonic Crystal Fiber Selectively Filled With Liquid Crystal

A surface plasmon resonance (SPR) sensor based on a side-polished single mode fiber coated with polyvinyl alcohol (PVA) is demonstrated for relative humidity (RH) sensing. The SPR sensor exhibits a resonant dip in the transmission spectrum in ambient air after PVA film coating, and the resonant wavelength shifts to longer wavelengths as the thickness of the PVA film increases. When RH changes, the resonant dip of the sensor with different film-thicknesses exhibits interesting characteristics for optical spectrum evolution. For sensors with initial wavelengths between 550 nm and 750 nm, the resonant dip shifts to longer wavelengths with increasing RH. The averaged sensitivity increases firstly and then drops, and shows a maximal sensitivity of 1.01 nm/RH%. Once the initial wavelength of the SPR sensor exceeds 850 nm, an inflection point of the resonant wavelength shift can be observed with RH increasing, and the resonant dip shifts to shorter wavelengths for RH values exceeding this point, and sensitivity as high as −4.97 nm/RH% can be obtained in the experiment. The sensor is expected to have potential applications in highly sensitive and cost effective humidity sensing.

Mechanism and Characteristics of Humidity Sensing with Polyvinyl Alcohol-Coated Fiber Surface Plasmon Resonance Sensor.

We propose and experimentally demonstrate a highly sensitive gas pressure sensor based on a near-balanced Mach-Zehnder interferometer (MZI) and constructed by hollow-core photonic bandgap fiber (HC-PBF) in this paper. The MZI is simply constructed by fusion splicing two HC-PBFs, which are of slightly different lengths, between two 3-dB couplers. The two output ends of each coupler are approximately equal in length, to ensure that the optical path variations of the MZI only result from the differences in the lengths between the two HC-PBFs. To apply the MZI for gas pressure sensing, a femtosecond laser is employed to drill a micro-channel in one of the two HC-PBF arms. The experiment result shows that the proposed MZI based gas pressure sensor achieves an ultrahigh sensitivity, up to 2.39 nm/kPa, which is two orders of magnitude higher than that of the previously reported MZI-based gas pressure sensors. Additionally, the effects resulting from the absolute length and relative length of the two HC-PBFs on gas pressure sensing performance are also investigated experimentally and theoretically, respectively. The ultra-high sensitivity and ease of fabrication make this device suitable for gas pressure sensing in the field of industrial and environmental safety monitoring.

High-sensitivity gas pressure sensor based on hollow-core photonic bandgap fiber Mach-Zehnder interferometer.

The temporal response of conventional ZnO nanowire-based photodetectors suffers from the influence of carrier mobility and high electrical resistance. As a result, these devices can be prohibitively slow for some applications, generally with a response time on the order of 1 second. This study presents a novel ZnO nanowire-based microfiber coupler structure for all-optical photodetection without the demand for photocurrent generation. In this design, two waveguides are directly adsorbed by van der Waals forces or electrostatic forces. Compared with the conventional electrical bridge structure, the optical coupling architecture makes the device both compact and stable. In this configuration, resonance dips form in the transmission spectrum when the phase-matching conditions of the two waveguides are satisfied. The measurements of the resulting wavelength shift, low noise levels, and repetition time confirmed that the proposed optical device could operate as a photodetector. The device exhibits superior performance with a sensitivity of 1.657 nm (W cm−2)−1 and a response time of 0.43 ms. In addition, the detector features a simple fabrication as there is no need for extra modification of ZnO nanowires. The resulting photodetection capabilities could provide a new functionality for novel all-optical applications.

Longfei Zhang

Papers

Highly sensitive surface plasmon resonance biosensor based on a low-index polymer optical fiber

Tunable Electro-Optical Modulator Based on a Photonic Crystal Fiber Selectively Filled With Liquid Crystal

Mechanism and Characteristics of Humidity Sensing with Polyvinyl Alcohol-Coated Fiber Surface Plasmon Resonance Sensor.

High-sensitivity gas pressure sensor based on hollow-core photonic bandgap fiber Mach-Zehnder interferometer.

A ZnO nanowire-based microfiber coupler for all-optical photodetection applications.