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Fiber optic sensors based on surface plasmon resonance (SPR) have demonstrated outstanding performance in biomedical, environmental monitoring, public safety, and other aspects, which provide powerful platforms for qualitative detection and quantitative analysis of molecular interactions. Among them, multichannel fiber optic SPR sensor has become a core tool in parallel detection scenarios due to the superiorities of compensating for nonspecific binding and environmental fluctuations and implementing multianalyte determination. The realization methods of multichannel sensing revolve around fiber microstructures, enhancement by materials, and hybrid fiber. The applications of multichannel fiber optic SPR sensors are demonstrated in sensing of liquid refractive index (RI), RI and temperature, biochemical molecules, and physical parameters. This review thoroughly analyzes and compares the structure, excitation effect, sensing performance, and the advantages and disadvantages of each type of multichannel fiber optic SPR sensor. Even though there are still some challenges, such as insufficient sensing channels and difficulty in fabrication, in their advancement, efforts in multidiscipline including developing high‐performance sensitive films and innovating micro–nano fabrication processes will overcome these bottlenecks. Lastly, the future development directions of multichannel fiber optic SPR sensors from principle, structure, and material aspects are discussed. [ABSTRACT FROM AUTHOR]

This paper proposes a reflective fiber mercury ion sensor based on the surface plasmon resonance (SPR) principle and chitosan (CS)/polyacrylic acid (PAA) multilayer sensitive film. By optimizing the coating parameters of the gold film, the refractive index (RI) sensitivity of the reflective SPR sensor is demonstrated to be 2110.33 nm/RIU. Then, a multi-layer CS/PAA film is fixed on the surface of the gold film as a mercury ion sensitive film to form a reflective SPR fiber mercury ion sensor. Experimental results demonstrate that the sensor can be used to detect different concentrations of mercury ions with a high sensitivity of 0.5586 nm/μM and good specificity and repeatability. Therefore, the reflective SPR fiber mercury ion sensor shows great promise for future applications of environmental monitoring and drinking water safety. [ABSTRACT FROM AUTHOR]

Highlights • The sensing head is a non-adiabatic tapered fiber. • The sensor is encapsulated in a liquid-infiltrated glass microtube. • The sensor has low cost, high sensitivity and good mechanical strength. Abstract A non-adiabatic tapered fiber sensor was fabricated, and its applications in refractive index (RI) and temperature sensing were investigated in both theory and experiment. The experiment results demonstrated that the tapered fiber sensor with a diameter of 8 μm had RI sensitivity as high as 1800 nm/RIU and temperature sensitivity of about −300 pm/°C. Then, taking advantage of its high refractive index sensitivity property, an improved temperature sensor structure was proposed by encapsulating it in a glass microtube and filling the microtube with high thermo-optical coefficient liquid. This improved sensor structure showed a higher temperature sensitivity of around −900 pm/°C. The improved sensor structure is simple, low cost and easily fabricated. Besides, this temperature sensor has high sensitivity, good mechanical strength and strong anti-disturbance ability. [ABSTRACT FROM AUTHOR]

• The sensor is simply fabricated by a section of HCF with low cost. • A gourd-shaped structure is formed by using a fusion splicer. • The sensor structure behaves high refractive index sensitivity and low cross-sensitivity to temperature. • Encapsulation of this structure with glycerol can realize high sensitivity to temperature. A gourd-type hollow-core fiber (HCF) based Mach–Zehnder interferometer (MZI) is proposed, and its sensing properties are investigated and extended. The MZI splices both ends of a section of HCF with single-mode fibers and uses a fusion splicer to cone between the HCF-SMF fusion point and the HCF to form a gourd-shaped structure. The conical part and the collapsed region inside the HCF form the light splitting/combining part of the MZI. Studies have shown that the MZI has high refractive index (RI) sensitivity (up to 2705.7 nm/RIU) and low cross-sensitivity to temperature (1.21×10-5 °C/RIU), which is very suitable for biochemical sensing. Encapsulation of this structure using capillary glass tubes and glycerol can convert its high sensitivity to RI into a high sensitivity to temperature, increasing the temperature sensitivity (527.88 pm/ °C) by a factor of about 16. The temperature sensitivity and range of the sensor are adjustable. The MZI has the characteristics of easy preparation, low price, compact structure, and superior detecting performance, and has excellent application prospects in biochemical sensing and physical sensing such as RI and temperature. [ABSTRACT FROM AUTHOR]

Photonic crystal (PC) cavities, which possess the advantages of compactness, flexible design, and suitability for integration in a lab-on-a-chip system, are able to distinguish slight variations in refractive index with only a small amount of analyte. Combined with the newly proposed optofluidic technology, PC-cavity devices stimulate an emerging class of miniaturized and label-free biochemical sensors. In this review, an overview of optofluidic PC cavities based biochemical sensors is presented. First, the basic properties of the PC, as well as the sensing principle of the PC cavity, are discussed. Second, the applications of the sensors in detecting gas, liquid, and biomolecule concentrations are reviewed, with a focus on their structures, sensing principles, sensing properties, advantages, and disadvantages. Finally, the current challenges and future development directions of optofluidic PC-cavity-based biochemical sensors are discussed. [ABSTRACT FROM AUTHOR]

Ferrofluid is a novel kind of functional materials, which possesses both the magnetism of solid magnetic material and fluidity of liquid material. Due to its unique optical properties, especially the refractive index tunability under magnetic field, ferrofluid has been widely utilized for magnetic field and current sensing combined with various optical fiber structures. Hence, in this paper, the refractive index tunability of ferrofluid, which is mainly applied into the optical fiber sensors, is first reviewed and illustrated in detail. Then we mainly introduce our work on the applications of ferrofluid in optical fiber sensors based on the refractive index tunability of ferrofluid, including the whole physical configurations, the operating principles, and the performances of these optical fiber sensors. At last, the deficiencies of ferrofluid in the applications of optical fiber sensors as well as valuable potential prospects in further research are analyzed and presented. From all the above reviews and discussions, it can be foreseen that the characteristics of ferrofluid will be improved and the applications of ferrofluid-based optical fiber sensors will be developed rapidly and maturely. [ABSTRACT FROM PUBLISHER]

A miniaturized and high-sensitive electric field sensor based on liquid crystal (LC) infiltrated photonic crystal (PC) cavity is proposed, by combining the excellent resonant properties of PC cavity with the good demodulation properties of Mach–Zehnder interferometer (MZI). When external electric field is applied, the refractive index of LC will alter due to the electro-optic effect, and thus the corresponding resonant wavelength of PC cavity will also change. The wavelength shift of the transmitted light of PC cavity can be monitored by detecting the output intensity of MZI, and then enables the detection of the external electric field. With using finite difference time domain (FDTD) method, the wavelength sensitivity of the PC cavity is analyzed and optimized, and then the linear relationship between the output intensity of MZI and the external electric field is calculated. Results demonstrate that a quasilinear measurement of electric field can be achieved with high sensitivity of 7 nW per V/m and detection limit of 0.143 V/m. [ABSTRACT FROM PUBLISHER]

A new in-line mode coupling interferometer based on photonic crystal fiber (PCF) was designed for hydrogen sensing. The external surface of the PCF was coated with Pd-WO3 hydrogen-sensitive film using the sol–gel scheme. Pd-WO3 film refractive index and volume changing influenced the model of the PCF interferometer while the hydrogen concentration was changing, which caused the output spectrum to shift. Therefore, the relationship between the wavelength shift and the hydrogen concentration was established. In the experiment, the wavelength shift showed good linearity as the hydrogen concentration changed from 0% to 1%, while the sensitivity was 1.09 nm/% and the detection resolution was 0.3 perthousand. The sensor showed good repeatability and the wavelength error was less than 40 pm in 30 min. In addition, the sensor was not affected by external temperature fluctuations, which indicates good practical development prospects. [ABSTRACT FROM AUTHOR]

Broadly tunable and ultra-wideband slow light was theoretically demonstrated in a line defected photonic crystal waveguide (PCW) by infiltrating fluid with suitable refractive index into the first row of air holes adjacent to the waveguide. Simulation results showed that the working wavelength of the flat slow light in the optimized optofluidic PCW could be tuned from 1542.5 nm to 1569 nm with nearly constant group index of 80. Namely, broadly tunable slow light could be realized with the ultra-wide bandwidth over 26.5 nm and the group index up to 80. These achievements could not only open the possibility for achieving many excellent optical devices based on slow light in PCW, but also highlight the versatility, flexibility, and tunability offered by optofluidic infiltration schemes. [ABSTRACT FROM PUBLISHER]

Abstract: A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3×109 nm/RIU with the active region of only 1mm long. [Copyright &y& Elsevier]

Abstract: A high sensitive multi-component gas sensing system was proposed, in which the slotted photonic crystal waveguide (SPCW) was used as the gas cell and the fiber Fabry–Perot (FFP) was adopted as the wavelength filter. By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot respectively, the working wavelength of SPCW could be tuned to matches with one absorption line of the measured gas, thus, the system could detect different kinds of gases selectively. Meanwhile, a very high group index of 150 with flat bandwidth could be achieved and the electric field enhancement factor would reach to 12. Combined with the wavelength modulation techniques (WMS) for signal processing, the minimum detection limit of 1.56ppm was achieved. [Copyright &y& Elsevier]

A photonic crystal (PC) cavity with high quality factor (Q), wide measurement range, and high refractive index (RI) sensitivity is proposed and its possibility in tetrahydrofuran (THF) vapor sensing is theoretically demonstrated. The resonance characteristics of the PC cavity are simulated and optimized by using finite difference time domain (FDTD) method. Cholesteric liquid crystal (CLC) is selected as an example to infiltrate into local air holes of the PC cavity. Since the RI of the CLC and consequently the resonance wavelength of the PC cavity would change with the variation of the THF vapor concentration, this proposed PC cavity can be used for THF vapor sensing. By optimizing structural parameters of several air holes, the simulation results show that the RI sensitivity can be up to 194 nm RIU−1, along with a high Q of 2 × 105 and a detection limit (DL) as low as 4 × 10−5 RIU. For the THF sensing, the theoretical sensitivity and detection limit of THF vapor concentration can be 0.128 nm · L mmol−1 and 0.06 mmol L−1, respectively. Besides, it was theoretically verified that the PC cavity sensor has the ability to maintain RI sensitivity over a wide refractive index range of 1.33–1.53, enabling it to measure more other parameters. [ABSTRACT FROM AUTHOR]

A simple, highly sensitive, and all-fiber Mach–Zehnder interferometer (MZI) refractive index (RI) sensor was proposed and experimentally demonstrated, which was fabricated by welding a dual-waist photonic crystal fiber (DWPCF) with two single-mode fibers. The two waists in the PCF, which acted as splitter and combiner of the MZI, could be simply formed by fusion-tapering technique. The high-order cladding modes could be largely excited in the first waist region and propagated along the cladding until the second waist region, where it would be returned into the fiber core and interfered with the core mode. When the RI around the DWPCF changed, an effective RI variation of the cladding modes might generate, which resulted in the shift of interference spectrum. Therefore, the external RI variation could be obtained by monitoring the wavelength position of the interference pattern. Experimental results showed that the interference wavelength shifted with the increase of the external RI, and high measurement sensitivity of 263.5 nm/RIU was achieved with good linearity. [ABSTRACT FROM AUTHOR]

A high-sensitive gas sensing method is proposed, in which miniaturized photonic crystal (PC) cavity is utilized as the sensing element, and cryptophane E molecular is infiltrated in defected holes of PC cavity as the sensitive material to methane gas. Particularly, slow light in side-coupled PC waveguide is optimized to enhance refractive index sensitivity of the PC cavity and fiber loop ring-down technology is used to demodulate output resonant spectrum of the sensing system. Simulation results demonstrate that a refractive index sensitivity of 450 nm/RIU and a quality factor of 1105 can be obtained in the slow light engineered PC cavity. For measurement of methane concentration, the theoretical sensitivity can be up to 4.21 μs/% and the minimum detectable concentration can be as accurate as 2.37 ppm. [ABSTRACT FROM AUTHOR]

• A new C-type photonic crystal fiber array was first proposed for multi-channel sensing. • Surface Plasmon Resonance was chosen as the sensing principle to get high sensitivity. • A gradual changing sensitivity of 13091 nm/RIU ~ 45322 nm/RIU was achieved. A new C-type photonic crystal fiber array was first proposed for multi-channel sensing. This design utilized the high birefringence characteristic of C-type structure and the excellent of photonic crystal array. Surface Plasmon Resonance (SPR) was chosen as the sensing principle to get high sensitivity. Through a detailed optimization process, the planar array got an optimal structure model, whose inner layer pores and defect are the main sensing channels. If some analytes with different refractive indices were entered into these channels respectively, resonance -matching conditions would change, which would superimpose on the original birefringence effect and output multiple characteristic signals. Finally, in a multi-phase infiltration condition, the device showed 5 characteristic output peaks that corresponded to different analytes. A gradual changing sensitivity of 13,091–45,322 nm/RIU was achieved in a wide refractive index range (1.33–1.44). Such a new sensing device could be regarded as a multi-channel micro-laboratory, which had a great potential for simultaneous sensing of multiple component parameters. [ABSTRACT FROM AUTHOR]

• A physical stretching method for preparing PDMS-based fiber structure is reported. • The PDMS fiber is coupled with two multimode fibers by adhesion, forming an interferometer. • The PDMS fiber based interferometer sensor has good sensitivity to VOC gases. • The sensor has merits of simple preparation, high sensitivity, good tensile properties, and humidity resistance. In this paper, a method for preparing polydimethylsiloxane (PDMS)-based fiber structure was reported and its performance for volatile organic compound (VOC) gas sensing was demonstrated. The PDMS fiber prepared by the physical stretching method was coupled at the end faces of two multimode fibers by adhesion, forming an inter-mode interferometer. The PDMS was not easily dislodged after curing, which contributes to the strong robustness of the whole structure. When the PDMS optical fiber is exposed to VOC gas environment, PDMS will adsorb VOC gas molecules, resulting in changes in its refractive index and volume, which shifts the wavelength of inter-mode interference. Experiments show that the sensor has good sensitivity to VOC gases such as acetone, ethanol, and methanol. In the concentration range of 0–400 ppm, the sensitivity reaches 14.84 pm/ppm, 13 pm/ppm, 4.39 pm/ppm, 7.37 pm/ppm, and 5.99 pm/ppm for acetone, ethanol, methanol, toluene, and ether respectively. The proposed sensor has excellent characteristics such as simple preparation, high sensitivity, good tensile properties, and humidity resistance, which indicates that the sensor has some potential for VOC gas detection. [ABSTRACT FROM AUTHOR]

Ethanol gas has been widely used in biomedicine, transportation, and chemical industry. However, when ethanol gas reaches a certain concentration in a closed space, it is possible to cause explosion. Based on this, a self-assembled whispering gallery mode (WGM) microbottle resonator for ethanol gas sensing is proposed and demonstrated in theory and experiment. The microbottle can be simply fabricated by dipping and pulling a single-mode fiber repeatedly in polydimethylsiloxane (PDMS) by a dip coater. The refractive index and volume of the PDMS will change with the concentration variation of the ethanol gas, resulting in a shift of the resonance wavelength. Experimental results demonstrate that the maximum sensitivity of the sensor to ethanol gas is 36.24 pm/ppm with good stability. Besides, the proposed sensor can behave simple preparation process, low cost, and compact size. • The microbottle can be simply fabricated by dipping and pulling an SMF repeatedly in PDMS. • The microbottle can serve as sensitive film to ethanol gas. • The central radius of the microbottle can be controlled by adjusting the fabrication process. • The ethanol gas sensor based on WGM resonator avoids the fall-off problem of the sensitive material. [ABSTRACT FROM AUTHOR]