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For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation. [ABSTRACT FROM AUTHOR]

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]

In this paper, a fiber-coupled whispering gallery mode (WGM) microsphere resonator was fabricated with using a simple package scheme. The resonant properties of the WGM resonator are analyzed and their responses to bulk refractive index and temperature are theoretically and experimentally investigated. Experiment results show that the bulk refractive index sensitivity is about 20.49 nm/RIU with detection limit of $4.3\times 10^{-4}$ RIU and very low temperature sensitivity of 7.38 pm/°C. Moreover, the WGM microsphere resonator is firstly used for acoustic intensity level sensing, and the sensitivity is about −1.9 pm/dB, which can enlarge the application field of the WGM sensor and can be used for noise monitoring to enhance environmental protection. [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]

A novel hot wire fiber anemometer was proposed based on surface plasmon resonance (SPR) effect in a dual-defect hole photonic crystal fiber (PCF), and the flow rate sensing characters of high sensitivity, wide measurement range, and strain compensation were proved by the numerical analysis. A gold film was selected as the plasmonic material to stimulate SPR, and diethylene glycol was considered as temperature-sensitive material. By theoretical and simulation analyses, the relationship between flow rate and ambient temperature was discussed, and the variations of structural parameters in the PCF were analyzed along with the changes of temperature and strain. Through further simulation research, the temperature sensitivity of the proposed SPR sensor was calculated as 6.83 nm/°C. Besides, the simulated flow rate sensitivity and resolution were improved to 32.19 nm/(m/s) and $6.21\times 10^{-4}$ m/s, respectively, when the flow rate was around 1 m/s and laser power was 100 mW. The maximum detectable flow rate could be up to 23.4 m/s when the resolutions of flow rate and wavelength were 0.3 m/s and 0.02 nm, respectively. Finally, the cross-sensitivity problem between flow rate and strain could be theoretically solved by detecting the wavelengths of dual SPR. [ABSTRACT FROM AUTHOR]

A composite optical fiber sensor based on an in-line Mach–Zehnder interferometer (MZI) with fiber Bragg grating was proposed. The MZI was fabricated by two standard single-mode fibers and one section of photonic crystal fiber. And the sensing region was coated on hydrogel to measure pH and temperature. When the pH around the sensor changes, the swelling (shrinking) of the hydrogel results in the variations of the refractive index, which causes variations in the transmission spectrum. Due to resonance wavelengths of the two structures that experience different responses to the ambient pH and the temperature, these parameters can be measured simultaneously. The experimental sensor can indicate the pH change at the wide measurement range (2–12), and the pH and temperature sensitivities are −0.27 nm/pH and 0.088 nm/°C, respectively. Because of the wide pH measurement range and excellent stability, the proposed sensor has a broad application of the biochemistry measurements. [ABSTRACT FROM AUTHOR]

A novel, highly sensitive dual-parameter sensor based on tandem-interferometric arrangement of Fabry–Pérot interferometer and Mach–Zehnder interferometer (MZI) has been designed and implemented in this paper. The sensor is composed of a single-mode fiber (SMF), hollow-core fiber, no-core fiber (NCF), and photonic crystal fiber (PCF). Those can simultaneously form a Fabry–Pérot interferometer and a MZI. The both interferometers are able to measure strain. Meanwhile, the PCF was filled with liquid crystal to measure electric field intensity. Hence, simultaneous measurement of electric field and strain can be achieved. The sensitivities of electric field and strain can reach up to 10.5 nm/(kV/mm) and 1.25 pm/ $\mu \varepsilon $ , respectively. [ABSTRACT FROM PUBLISHER]

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]

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]

Magnetic fluid has not only rich optical properties, but also interesting thermal-lens effect. The thermal-lens effect of magnetic fluid was studied by using the Brownian dynamics method. The Theoretical simulation and analysis were taken to research the whole generation process of thermal-lens effect. The thermal-lens effect generation of water-based magnetic fluid irradiated by the laser of different optical powers was studied in microscopic view. Based on the model of the thermal-lens effect, Monte Carlo method and Kirchhoff diffraction theory were adopted to simulate and analyze the far-field diffraction ring pattern generated by a Gauss light beam passing through the magnetic fluid. Then, the influences of different central temperatures of the magnetic fluid to the ring numbers, width, and intensity distribution of far-field diffraction ring pattern were obtained. The results were verified with the published experimental results. [ABSTRACT FROM AUTHOR]

A novel, highly sensitive double-parameter sensor based on the composite interference structure has been designed and implemented in this paper. It can be regarded as a cascade of the traditional single-mode fiber (SMF)–hollow-core fiber (HCF)–photonic crystal fiber (PCF)-SMF structure, which forms a reflected Fabry–Perot interferometer and a transmitted Mach–Zehnder interferometer at the same time. The HCF is filled with ethanol, which is sensitive to temperature. Then the simultaneous measurement of the temperature and the refractive index (RI) can be achieved by monitoring the reflected spectrum and transmitted spectrum. The fact that these two spectrums do not interfere with each other makes it easier to obtain a demodulation method for simultaneous temperature and RI measurement that does not involve complex fast Fourier transform and matrix computation. The sensitivities of the temperature and the RI can reach up to 0.454 nm/°C and 178.7 nm/RIU, respectively. [ABSTRACT FROM AUTHOR]

Slow light in photonic crystal waveguide (PCW) is now being heavily investigated for applications in optical devices. However, slow light with high group index in perfect PCW is usually accompanied by large group velocity dispersion (GVD), which would severely limit the bandwidth of slow light, deform optical pulses, and disturb its practical applications. In this review, various optimization methods that are proposed to overcome these drawbacks are introduced and compared. These methods rely largely on the ability to modify the slow light properties of PCWs with a change in their structural parameters or a change in their effective refractive indexes through external agents. For each optimization method, the corresponding group index, GVD, bandwidth, and normalized delay-bandwidth product are all presented along with the physical parameters, the potential advantages, and the fabrication complexity of PCW that enable them. Finally, the key problems and future development directions of slow light in PCW are discussed. [ABSTRACT FROM PUBLISHER]

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]

A tunable slow light with wide bandwidth, large group index, and low dispersion is realized in polymer-infiltrated slot photonic crystal waveguide (PI-SPCW). The slow-light properties of PI-SPCW are first optimized by shifting the first and second rows of air holes adjacent to the slot. Using the three-dimensional (3-D) plane wave expansion method, the low dispersion slow light with nearly constant group indices of 55, 100, 172.5, and 222 over bandwidths of 9.0, 3.5, 1.1, and 0.8 nm are demonstrated. Then, the dynamic modulation of the optimized slow light in PI-SPCW is investigated. Thanks to the electro-optic effect of the infiltrated polymer, the central wavelength of flatband slow light could be flexibly tuned by the external driving voltage. For the slow light with high group index of 100, the modulation sensitivity up to −0.75 nm/V is observed and the effective bandwidth could be further enlarged to 11 nm. These achievements open the possibility for achieving a wide range of targeted optical functionalities in PI-SPCW that can be dynamically controlled according to the practical requirements and highlight the versatility, flexibility, and tunability offered by electro-optic effect in PI-SPCW. [ABSTRACT FROM PUBLISHER]

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]