•Novel spherical end face zeolite thin film-coated fiber sensor.•Arc-shaped inline fiber Fabry–Perot interferometer.•Enhanced sensitivity 0.91 nm/ppm in the range from 0 to 70 ppm isopropanol concentration.•Expanded absorption surface area resulting in optimized interference extinction ratio.•Highly stable and repeatable sensing performance.A novel zeolite thin film-coated spherical end face fiber sensor for detection of trace organic vapors was experimentally demonstrated. The spherical end-face was fabricated by electrical arc discharge on the end face of a standard single-mode fiber. The proposed sensor comprise of the fiber's spherical end-face covered with a layer of zeolite thin film. The zeolite film and spherical end face constituted an arc-shaped inline Fabry–Perot (F–P) cavity, which improves the interference performance. The trace chemical vapor concentration was measured by monitoring the shift of F–P interference wavelength which induced by the organic vapor molecular adsorption of the zeolite film. The proposed trace organic vapors sensor performed with the enhanced sensitivity 0.91 nm/ppm with the range from 0 to 70 ppm.

•Sensitivity dependence of fiber loop mirror on the length of high birefringence fiber (HBF) are theoretically analyzed and experimentally demonstrated.•A model of fiber loop mirror with the distinct concepts of sensing length and total length is established.•It explains why the contradictory conclusions regarding the sensitivity dependence on HBF length used in the FLM exist in the literatures.•The conclusions in this paper can be used as a guideline for choosing the HBF length in Sagnac loop for sensing applications.A model of fiber loop mirror with the distinct concepts of sensing length and total length is established. The sensitivity dependence of fiber loop mirror on the sensing length and total length is discussed theoretically and experimentally. The sensor sensitivity showed being linearly dependent of the ratio of the sensing length and the total length. The experimental results well support the predications by the model.

•A fiber optic interferometric sensor based on chitosan/poly acrylic acid bilayer for detection of Ni2+ ions is proposed.•The proposed sensor exhibits Ni2+ detection sensitivity of 0.05537 nm/μM with concentration detection limit of 0.1671 μM.•The sensor was observed to be stable and exhibits a fast response over a relatively wide measuring range.An interferometric sensor with a no-core fiber (NCF) spliced between single mode fibers (SMF) and functionalized with chitosan (CS)/poly acrylic acid (PAA) self-assembled polyelectrolyte layers for the detection of heavy metal ion Nickel (II) (Ni2+), is proposed and experimentally demonstrated. The sensing scheme is based on interference of core and cladding modes to detect changes in refractive index induced by Ni2+ adsorption on the functionalized sensor. Wavelength shifts were measured real-time for the continuous monitoring of adsorption of Ni2+ at different concentrations. The proposed sensor exhibits a linear response in the concentration range upto 500 μM, with a Ni2+ detection sensitivity of 0.05537 nm/μM and concentration detection limit of 0.1671 μM.

Simultaneous measurement of relative humidity and temperature has been realized by using an optical fiber sensor formed by cascading a photonic crystal fiber (PCF)-based in-fiber Mach-Zehnder interferometer (MZI) and a fiber Bragg grating (FBG). The PCF-MZI was fabricated by using a short PCF fusion-spliced between two single-mode fibers with its air holes in the cladding area being collapsed in the splicing regions. It was then coated with a layer of polyvinyl alcohol (PVA), whose refractive index is sensitive to humidity. Because the PCF-MZI and FBG have different responses to humidity and temperature, simultaneous measurement has been achieved with resolutions of 0.13% RH and 1.0 °C for humidity and temperature, respectively. The relative humidity measurement range is up to 30%–95% RH.

A miniature refractometer by using a Mach–Zehnder interferometer (MZI) in a standard single mode fiber (SMF) with two waist-enlarged fusion bitapers is proposed and experimentally demonstrated. Such an all-fiber SMF-MZI incorporates an intermodal interference between the core mode and cladding modes. Because the cladding modes are sensitive to the external environment, the SMF-MZI is exploited for refractive index measurement. Experimentally, high sensitivity of 266.5 nm/RIU and resolution of 3.75×10−5 RIU are achieved within a range from 1.346 to 1.4412. The temperature effect of the proposed refractometer is also analyzed.

A temperature-independent refractometer by using a tapered photonic crystal fiber (PCF) based Mach–Zehnder interferometer (MZI) is proposed and experimentally demonstrated. It is fabricated by sandwiching a tapered PCF of 29 mm long between two standard single mode fibers (SMFs) with the fully collapsed air holes of the PCF in the fusion splicing region. It has been found that tapering the PCF greatly enhances the sensitivity of the refractometer. A maximum sensitivity of 1529 nm/RIU (refractive index unit) is achieved within the range from 1.3355 to 1.413. The refractometer is nearly temperature-insensitive due to the ultra low temperature dependence of the used.

We report a compact opto-fluidic platform capable of continuous analyte loading and unloading with a 3 dB optical insertion loss. The customized opto-fluidic manipulator enabled infiltration of photonic crystal fibers (PCFs) at ten times the rate achievable by capillary action. Additionally, it is to our knowledge, the first demonstration of complete and rapid evacuation performed with a syringe pump for extended lengths (>100 mm) of PCF. These properties render the device highly promising for continuous real-time sensing applications. Study was conducted on a PCF under macro-bending, taking advantage of its wavelength-dependent bending losses that red-shifted with the increasing refractive indices of the infiltrated analytes. The flexibility of the platform also facilitated the selection of an optimal bending radius (12.5 mm) for the analysis, based on sensitivity (Δλ/Δn) and signal to noise ratio (Q-factor). The subsequent comparison of experiment with simulation results was noted to show good coherence. Moreover, experimental results showed repeatability throughout the multiple cycles of infiltration and evacuation executed. The further employment of the device in the chemical sensing of ethanol solutions exhibited good consistency with calibrated data for concentrations up to 50% by weight.

A label-free fiber-optic Fabry–Perot interferometric (FPI) immunosensor is proposed by layer-by-layer (LBL) self-assembly of a suspended chitosan and polystyrene sulfonate membrane at the tip of a hollow fiber. Immunoglobulin G (IgG) was immobilized on the polyelectrolyte substrate and the detection of target anti-immunoglobulin G (anti-IgG) was monitored through the measurement of substrate's effective optical thickness. The sensitivity and specificity of the sensor were studied using correlated and non-correlated anti-IgG/IgG pair. Sensitivity of 0.033 μm/(pg/mm2) and limit of detection of 0.005 nM was achieved with the sensor displaying minimal response toward non-correlated anti-IgG. The results demonstrate the feasibility of the sensor for immunosensing.

An alternative all-fiber sensor for simultaneous strain and temperature measurement based on a photonic crystal fiber (PCF) spliced between single-mode fibers cascaded with a long period grating (LPG) is proposed. By collapsing the air holes at two splicing regions along the PCF, a simple but effective modal-interference (MI) is occurred between the core and cladding modes of the PCF. Due to the different responses on the changes of strain and temperature on the MI and the cascaded LPG, the strain and temperature can be measured simultaneously. Experimental results show that the sensing resolution of 9.1 με in strain measurement is experimentally achieved over a range of 2640 με, while the temperature sensing resolution is 0.27 °C within a range of 30–100 °C.

A relative humidity fiber sensor based on Fabry–Perot interferometry configuration is presented. The proposed fiber sensor is functionalized with a thin layer of a moisture-sensitive natural polymer chitosan to form a low fineness Fabry–Perot sensor. The sensing scheme used in this work is based on the swelling effect of chitosan sensing film (degree of swelling varies as a function of relative humidity) which will induce optical path modulation when relative humidity is changed. As observed, the proposed sensor exhibits a sensitivity of 0.13 nm/%RH for relative humidity ranging from 20%RH to 95%RH with a RH uncertainty of ±1.68%RH and fast response time of 380 ms.

A refractive index sensor is demonstrated by integrating a photonic crystal fiber (PCF) Mach–Zehnder interferometer (MZI) into a cavity ringdown loop for improved sensitivity. By splicing a short section of PCF between two single-mode fibers and collapsing air holes over a short region at two splicing point, a MZI will be formed. This is due to the coupling between core and cladding modes in the collapsed region. Through determining the decay constants of cavity ringdown curve under varying external refractive index, a minimum detectable refractive index of 7.8 × 10−5 RIU can be achieved.

The inscription of a long-period grating in a side-hole single-mode fiber for sensing application is demonstrated. The grating is made by pressing a plate with a periodic groove against a short section of length of a side-hole single-mode fiber. Its strength and wavelength of the cladding-mode resonant can be tuned over 15 dB and 6.2 nm, respectively, by varying the ambient temperature of the grating. The refractive index sensitivity and the polarization dependence of the MPLG are also investigated.

An optical fiber curvature sensor with low-birefringence photonic crystal fiber (PCF) based Sagnac loop is demonstrated experimentally. The low-birefringence PCF of about 40 cm long is inserted into Sagnac loop, and a section of it about 155 mm is used as the sensing element. The Sagnac output spectra under different curvatures are measured and analyzed. The results show that the wavelength shift of the transmission dip has a linear relationship with the curvature. The sensitivity of the curvature measurement of − 0.337 nm is achieved in the range of 0–9.92 m− 1. And the temperature effect of the proposed sensor is also analyzed.

A self-organizing network is used to improve the performance of FBG sensors in a WDM network. Experimental results demonstrated that the strain measurement accuracy up to 3 μɛ has been achieved even when the FBG spectrums within the network are partially or completely overlapped.

Genetic algorithm is used to improve the performance of FBG sensors within in a wavelength-division multiplexed sensor network. Simulations and experiments demonstrated wavelength detection accuracy of a few microstrains even when the spectrums of FBGs within the network are partially or completely overlapped.

An intensity-modulated Nickel ions (Ni2+) probe is experimentally demonstrated by using optical fiber Bragg grating (FBG) cascaded by a cleaved fiber end which is functionalized by multilayers of chitosan/poly acrylic acid (PAA). The multilayer film can effectively adsorbed Ni2+ that modulate signal to noise ratio (SNR) of the FBG. The proposed probe exhibits an enhanced sensitivity with detection limitation of 0.01 mM. This kind of relative measurement method contributes to eliminate power fluctuation of the optical source. Temperature can be monitored simultaneously by wavelength shift, which is benefit to minimize temperature cross effect on Ni2+ detection in the future.