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In this paper, a novel platform for simultaneous measurement of relative humidity (RH) and temperature using dual Fabry–Perot interferometers (FPIs) based on C-shaped fiber was demonstrated. The sensor was composed by splicing two sections of C-shaped fiber between single mode fiber (SMF). Polydimethylsiloxane (PDMS) and polyvinyl alcohol (PVA) are filled in the two sections of C-shaped fibers to increase sensitivity to temperature and RH. This is the first demonstration that solid polymer materials can be added to the C-shaped fiber interferometers for sensing. In our experiment, RH sensitivities of -0.128 nm/%RH and 0.038 nm/%RH in the range of 20%RH to 45%RH, and temperature sensitivities of 0.022 nm/°C and -0.722 nm/°C in the temperature range of 15 °C to 45°C, were acquired for dual FPIs, respectively. We verified that it is possible to use the sensitivity matrix method to measure two parameters simultaneously. In addition, it has the benefits of simple structure, excellent stability and high sensitivity, and has a broad application prospect in agriculture, food processing and environmental measurement. [ABSTRACT FROM AUTHOR]

A fiber-optic pH sensor, based on layer-by-layer (LBL) self-assembly and surface plasmon resonance (SPR) technology, is proposed and experimentally confirmed. This article proposes poly (diallyldimethylammonium chloride) (PDDA) as the pretreatment solution for the surface of the gold film and the highly sensitive polyacrylic acid/chitosan (PAA/CS) self-assembled nanomembrane as the pH-sensitive film. It has been experimentally verified that the sensor can detect pH from 3.18 to 11.84 and it exhibits a high average sensitivity of 32.31 nm/pH in this range. In addition, experiments have confirmed that the sensor has attractive stability, repeatability, and hysteresis characteristics. [ABSTRACT FROM AUTHOR]

A reflective optical fiber nickel ion (Ni2+) sensor based on surface plasmon resonance (SPR) and surface ion imprinting was proposed and demonstrated. The sensor is made of multimode optical fiber with a core diameter of 600 $\mu \text{m}$ and coated with a gold film for an exciting SPR effect. To realize highly sensitive and highly selective detection of Ni2+, graphene oxide (GO) and Ni2+-imprinted Chitosan (IP-CS) are subsequently combined on the sensor surface. The GO can not only act as the intermediate binder between the Au film and the CS but also increase the sensitivity of the sensor. The Ni2+ holes on the IP-CS can realize the specific capture of Ni2+. The experimental results show that the sensitivity of the sensor to Ni2+ is 5.220 nm/[lg(mg/L)] with a limit of detection of $2.393\times 10^{-3}$ mg/L, and the selectivity of the sensor is significantly improved after imprinting. Besides, the sensor has good stability and repeatability, and fast responsiveness. [ABSTRACT FROM AUTHOR]

A new type of two-channel surface plasmon resonance (SPR) sensor based on hollow-core fiber (HCF) for simultaneous measurement of seawater salinity and temperature was presented in this article. One of the SPR channels that are excited on the outer surface of the HCF with Au film coating was used to measure the seawater salinity, while another SPR channel that is excited on the inner surface of the HCF with Ag film coating and polydimethylsiloxane (PDMS) infiltration was used for seawater temperature measurement at the same time. The experimental results showed that the salinity sensitivity and temperature sensitivity of the sensor could reach 0.3769 nm/‰ and −0.956 nm/°C, respectively. This demonstrated that the two-channel SPR sensor not only showed the possibility in the simultaneous measurement of two parameters with high sensitivity, excellent stability, small size, and high mechanical strength but also shed new light on the multiparameter measurement fields and the development of lab-in-fiber sensors. [ABSTRACT FROM AUTHOR]

A new type of two-channel surface plasmon resonance (SPR) sensor based on hollow-core fiber (HCF) for simultaneous measurement of seawater salinity and temperature was presented in this article. One of the SPR channels that are excited on the outer surface of the HCF with Au film coating was used to measure the seawater salinity, while another SPR channel that is excited on the inner surface of the HCF with Ag film coating and polydimethylsiloxane (PDMS) infiltration was used for seawater temperature measurement at the same time. The experimental results showed that the salinity sensitivity and temperature sensitivity of the sensor could reach 0.3769 nm/‰ and −0.956 nm/°C, respectively. This demonstrated that the two-channel SPR sensor not only showed the possibility in the simultaneous measurement of two parameters with high sensitivity, excellent stability, small size, and high mechanical strength but also shed new light on the multiparameter measurement fields and the development of lab-in-fiber sensors. [ABSTRACT FROM AUTHOR]

A refractive index (RI) fiber sensor with low detection limit but large dynamic range is proposed and demonstrated using an exposed core microstructured optical fiber. The exposed-core fiber is highly birefringent due to its asymmetry and also supports multimode propagation; thus, can be used simultaneously as a Mach-Zehnder and Sagnac interferometer. The Mach-Zehnder interference is significantly more phase sensitive to RI due to a longer effective path length difference. This leads to a lower detection limit compared to that for the Sagnac interferometer, which has a larger free spectral range that allows the dynamic range of the RI measurement to be extended. By combining these two interferometers, the proposed sensor achieves a detection limit of as low as 6.02 × 10−6 refractive index units (RIU) while maintaining a large dynamic range from 1.3320 to 1.3465 RIU. The proposed sensor also has the advantages of biocompatibility, low cost, high stability, small size, ability to operate remotely and to be fabricated. [ABSTRACT FROM AUTHOR]

Simultaneous measurement of hydrogen concentration and temperature with high sensitivity and high precision was realized by integrating a photonic crystal fiber (PCF) modal interferometer and a high-birefringence fiber loop mirror (HBFLM). The PCF was coated with sol-gel Pd/WO3 coating, whose refractive index and volume would be changed along with the variation of surrounding hydrogen concentration or temperature, and then the resonant dip of PCF modal interferometer would shift accordingly. Meanwhile, the resonant dip of the HBFLM has a high sensitivity to surrounding temperature. By integrating PCF modal interferometer and HBFLM together, the two independent resonant dips that can be simultaneously monitored at the output spectrum of the integrated interferometers would all shift with the change of external hydrogen concentration or temperature. Finally, combined with the dual-wavelength matrix method, the hydrogen concentration and temperature could be simultaneously measured. Experimental results showed that the resonant dip of the PCF modal interferometer moved in the short wavelength direction with the increase of hydrogen concentration, and the sensitivity of the hydrogen concentration was −1.12 nm/% within the concentration range from 0% to 1%. In the process of temperature change, the HBFLM realized a higher temperature sensitivity of −1.84 nm/°C. The sensor had high sensitivity, low cost, and simple structure. [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]

A method for simultaneous measurement of magnetic field and temperature with high sensitivity and high precision was realized according to magnetic fluid (MF)-infiltrated photonic crystal cavity, where two different types of MF were, respectively, infiltrated into certain air holes adjacent to a waveguide to form two cascaded cavities. As the refractive index (RI) of MF is dependent on external magnetic field and temperature, the two independent resonant dips of cascaded cavities that can be simultaneously monitored at the output spectrum of the waveguide would all shift with the change of external magnetic field or temperature. Using finite-difference time-domain method, the RI sensitivity of the proposed cavity was firstly analyzed and optimized, and then the linear relationships between the shifts of two resonant wavelengths and external magnetic field/temperature were calculated. Finally, combined with the dual-wavelength matrix method, the magnetic field detection limit could reach to 1.333\times 10^\mathbf -4 T with the uncertainty of \pm 0.22 \times 10^\mathbf -4 T (coverage factor k = 2) and detection range from 0 to 0.06 T. Simultaneously, the temperature detection limit could reach to 0.301 K with the uncertainty of ±0.051 K (k = 2) and detection range from 250 to 340 K. [ABSTRACT FROM PUBLISHER]

Abstract: A novel harmonic detection theory and method for multi-component gas sensing based on photonic crystal waveguide (PCW) slow light is proposed. The PCW is used as gas chamber, and harmonic detection method was adopted for signal processing. This system could real-time and remote monitoring multi-component gases simultaneously with sensitivities increased by 10,278, 8650 and 6282 times respectively compared with system PCW not used. The proposed theory and method possesses powerful practicability and favorable application prospects. It could be also applied to other fluid concentration detection system, thus providing a new idea for expanding applications of slow light in sensing fields. [Copyright &y& Elsevier]

□ A novel demodulating method based on a Gaussian index profile fiber Bragg grating slow-light Mach-Zehnder interferometer to demodulate the frequency shift of a Brillouin optical time-domain reflectometer is proposed and demonstrated. The slow-light was generated with a Gaussian index profile fiber Bragg grating, and transmission spectra and group index spectra characteristics of the grating were obtained through theoretical analysis and numerical simulation. With the slow-light Mach-Zehnder interferometer used, the phase sensitivity and relative power sensitivity can reach up to 0.0132 rad/MHz and 0.0144/MHz, respectively. These results were improved to 133 times and 22.4 times, respectively, which were as large as values based on a double-pass Mach-Zehnder interferometer. [ABSTRACT FROM AUTHOR]

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]

To improve the sensitivity of Brillouin optical time domain reflecting (BOTDR) based on the Mach–Zehnder interferometer (MZI), a novel method was proposed by using fiber grating (FG). The FG was introduced not only as a filter to reduce the noise caused by Fresnel reflection and Rayleigh scattering, but also as the slow-light medium to improve the sensitivity of MZI used in BOTDR. Both uniform and nonuniform FGs were modeled, and then their transmission and group index spectra were calculated. By calculating the sensitivities of BOTDR demodulation, it was found that the power of noise light could be greatly reduced when nonuniform FG was utilized. Meanwhile, the phase and relative output power sensitivities of BOTDR demodulation could be improved to almost 20 times as large as the ones based on double-pass MZI. Besides, the sensitivity of BOTDR distributed sensor could also be improved. [ABSTRACT FROM PUBLISHER]

The main structures of the gas absorption cell, including small graded-index lens type, Bragg fiber type, photonic crystal type, etc., are compared in this article. Also, different principles of the gas absorption cell were discussed in detail in order to improve the sensitivity of the optical gas sensor. The study showed that the structure of gas cells plays a key role in the improvement of the sensitivity of optical gas sensors. The end of the article discusses the future direction of the development of the gas cell. [ABSTRACT FROM PUBLISHER]

Graphene as a novel material has laid a foundation for its applications in optical fiber sensors, due to its unique properties, especially the optical properties. On the other hand, optical fiber sensors have received world-wide attention due to their high sensitivity, small size, good anti-electromagnetism disturbance ability and other potential advantages. In this paper, the developments of graphene in the applications of optical fiber sensors were reviewed from four aspects. Firstly, the common preparation methods of graphene were introduced. Next, the optical properties of graphene have been concluded. And then, some typical optical fiber chemical and biological sensors based on graphene, such as temperature sensors, biological sensors and gas sensors, were reviewed. It was shown that graphene had a great potential in the optical fiber sensing technology. Furthermore, the deficiencies and challenges of the graphene in the applications of optical fiber sensors were analyzed. In a whole, the unique advantages of graphene have present their versatility and importance in the application fields of optical fiber sensors. [ABSTRACT FROM AUTHOR]