DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

An evanescent wave optical fiber biosensor based on titania–silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol–gel-based titania–silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol–gel based titania–silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol–gel based titania–silica-coated LPG is proposed as an effective ...

Sol–Gel-Based Titania–Silica Thin Film Overlay for Long Period Fiber Grating-Based Biosensors

A fiber-optic sensor for detecting hydrogen at various temperatures (30–200 °C) is reported in this paper. The sensor is composed of a long period fiber gratings coated with a nano-structured Pd thin layer as the sensing material. The sensing principle of the designed sensor is based on the change of the refractive index of the Pd thin layer upon exposure to hydrogen, which causes the resonance wavelength shift of the long period fiber gratings. The Pd layer prepared by sputter deposition has nanograins of 30–40 nm in size. The resonance wavelength of the long period fiber gratings coated with Pd decreases as hydrogen concentration increases from 0% to 16% at all temperatures. There is a significant wavelength shift of the sensor when the Pd film goes through the phase transition between α and β phases. The sensor has a short response time with a relatively slower recovery rate. The sensor maintains its functionality after several cycles of hydrogen detection and recovery.

Nano-structured Pd-long period fiber gratings integrated optical sensor for hydrogen detection

In this work a highly reflective long period fiber grating (RLPG) sensor is proposed. The sensor was fabricated by depositing the silver atom on both surfaces of the flat end-face and the tail profile of long period fiber grating (LPG), which makes the LPG work in high reflection mode without interferometric fringe and enhances the coupling at the resonant wavelength of original LPG. The RI-response characteristic of the RLPG was investigated. The reflection spectrum remains the resonant dip without interference infringe and the resonant wavelength has obvious blue shift with the increase of external RI. By utilizing the above characteristic, we propose an application for long distance RI measurement combining a standard optical time domain reflectometer (OTDR). Preliminary results of using the sensor to measure the refractive index of glycerol/water solutions in a distance of 100 km were successfully demonstrated.

Highly reflective long period fiber grating sensor and its application in refractive index sensing

This paper reports the development of a new zeolite thin film-coated long period fiber grating (LPFG) sensor for direct measurement of trace organic vapors. The sensor was fabricated by growing pure silica MFI-type zeolite thin film on the optical fiber grating by in situ hydrothermal crystallization. The sensor measures chemical vapor concentration by monitoring the molecular adsorption-induced shift of LPFG resonant wavelength (lambda(R)) in near infrared (IR) region. Upon loading analyte molecules, the zeolite's refractive index changes in the close vicinity of the fiber index where the LPFG has a large response to achieve high sensitivity.

Zeolite thin film-coated long period fiber grating sensor for measuring trace chemical.

The novel long-period fiber grating (LPFG) film sensor is composed of the long-period grating coated with solgel-derived sensitive films. The characteristics of the transmissivity of the LPFG film sensor are studied. By analyzing the relation among the sensitivity S-n, the thin film optical parameters, and the fiber grating parameters, the optimal design parameters of the LPFG film sensor are obtained. Data simulation shows that the resolution of the refractive index of this LPFG film sensor is predicted to be 10(-8). Experimentally, a LPFG film sensor for detection Of C2H5OH was fabricated, and a preliminary gas-sensing test was performed. (c) 2006 Optical Society of America.

Optical fiber long-period grating with solgel coating for gas sensor

A high sensitivity sensor based on novel CLPFG with wavelength and intensity modulation for simultaneous measurement of SRI and temperature

The response of a long-period grating coated with the sol-gel derived films on the cladding of the fiber grating region to surrounding medium (gases) was studied in this paper based on coupled-mode theory. A four-layered numerical model is also developed to determine the dependence of the central wavelengths of the attenuation bands on the thin film optical parameters ( thickness d 3 and refractive index n 3 ). By analyzing the relation between the sensitivity S n and the thin film optical parameters and the fiber grating parameters (the grating period, the core index change and the grating length ), the optimal optical parameters of thin film layer of the sensor is obtained. Data simulation shows that the sensitivity of this scheme to refractive index of the films is predicted to be more than 10 7 . In the end a gas-sensitive long-period grating sensor is fabricated according to the theoretical optimization results. The preliminary gas-sensing experiment was performed, and a novel LPFG gas sensor special to C 2 H 5 OH and CH 4 was established.

Kan Gao

Papers

Optical fiber long-period grating with solgel coating for gas sensor

A high sensitivity sensor based on novel CLPFG with wavelength and intensity modulation for simultaneous measurement of SRI and temperature

Structure optimization of long-period fiber grating gas-sensing sensors