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Abstract We propose and investigate a temperature sensor based on Sagnac interferometer with liquid crystal (LC) photonic crystal fiber (PCF) by finite element method (FEM). LC E7 which is anisotropic under electrical field is designed to be infiltrated into the cladding air holes of PCF. The birefringent PCF is then launched into a Sagnac interferometer. The interference spectrum of the Sagnac interferometer is influenced by the thermal effect of LC E7 and thus could be used to measure the temperature. The simulation results show that the temperature measuring sensitivity and resolution reach to 15 nm/K and 6.7E−4 K in the range of 290–320 K. The measuring range and sensitivity could be modified further by optimizing the PCF structure parameters. [ABSTRACT FROM AUTHOR]

Photonic crystal fiber-based polarization filter has important application prospects in optical fiber communications, nonlinear optics, and sensors. In order to achieve polarization filters with optimal performance, some numerical methods have been used to search for optimal results under different geometric structure parameters of fibers. However, it is unrealistic to calculate solutions under every parameter. In addition, the search direction and scope of adjusting fiber geometric structure parameters still depend on the designer's intuition and experience. Therefore, we propose a novel design method for photonic crystal fiber polarization filters to overcome these questions. The optimization design problem of polarization filters is transformed into the single objective function optimization problem. The combination of artificial neural networks and intelligent optimization algorithms realizes the optimal value-solving problem in two-dimensional parameter space. The method efficiently and automatically searches for optimal structures in the parameter space by fine-tuning both variables simultaneously, improving the bandwidth of the polarization filter and achieving higher crosstalk at 1550 nm. The method eliminates computationally expensive wavelength parameters and greatly reduces computation time. This novel design method realizes the reverse design of photonic crystal fiber-based polarization filters. In addition, the new method can also be used to design polarization filters at other objective wavelengths, and it is potentially applicable to the design of other photonic crystal fiber devices. • Wavelength parameters are removed, drastically cutting down on computation time. • The polarization filters optimization design is transformed into function optimization. • Machine learning techniques provided optimal design parameters of polarization filters. [ABSTRACT FROM AUTHOR]

Surface plasmon resonance induced tunable broadband polarization filters based on photonic crystal fibers (PCFs) were designed and investigated. The characteristics of the designed PCF were calculated by using the finite element method (FEM). The air holes in the PCFs which were arranged in hexagonal lattice were of the same size except one big air hole. The big air hole which was located on the right side of the fiber core was designed to be coated with gold film in the inner wall. Because of the arrangement of the big air hole, the surface plasmon resonance inspired on the surface of gold film was quite different for the two orthogonal polarized incident lights. As the edge of the big air hole approaching the adjacent air holes, the plasmon modes were influenced strongly by the adjacent air holes and as a results the cascaded resonances appeared. The index-matching liquid infilling in the big air hole was used to adjust the resonant wavelengths and strength. Numerical simulation results showed that the bandwidth of the polarization filter was 1200 nm covering the wavelengths from 1.0 μm to 2.2 μm with high extinction ratio (ER). [ABSTRACT FROM AUTHOR]

• In this paper, we proposed a strain optical fiber sensor with exceeding free spectral measurement range and high sensitivity. • We designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop. • A high strain sensitivity of about 17.08 pm/µε with a large measurement range of 0–5187 µε was experimentally achieved. • The method used in this paper can also be utilized to expand the sensing range in other interferometers. Sagnac interferometer-based sensors have shown high sensitivity, but limited measurement range due to the free spectral range (FSR) limitation especially the higher the sensitivity. In this paper, we designed an optical fiber Sagnac interferometer with a polarization maintaining fiber (PMF) and a fiber Bragg grating (FBG) in the loop. The strain sensing performance was experimentally conducted. The PMF in the Sagnac interferometer showed a high strain measurement sensitivity with a limited measurement range due to the FSR. On the contrary, FBG in the Sagnac interferometer showed a low strain sensitivity but a large measurement range. By combining the PMF and FBG in a Sagnac interferometer, a high strain sensitivity of about 17.08 pm/µε with a large measurement range of 0–5187 µε was experimentally achieved. The method used in this paper can also be utilized to expand the sensing range in other interferometers. The proposed PMF and FBG constructed Sagnac interferometer shows the merits of simple structure, high sensitivity, and large measurement range, which make it a competitive candidate in strain monitoring. [ABSTRACT FROM AUTHOR]

Compared with traditional optical fiber, photonic crystal fiber (PCF) has many novel optical properties owing to its diversity in cladding distribution. But, it is a problem to measure the optical properties of PCF under variable structural parameters. Artificial neural networks (ANNs) have been used to predict the optical properties of PCF, but ANNs have the multiple local minima problems, whereas support vector machines' solutions are globally unique and will not fall into local minimum values. In this paper, support vector machines (SVMs) based on radial basis functions were used to predict the effective refractive index (n e f f), chromatic dispersion (D), and confinement loss (α c) of PCF. Well-trained SVMs can accurately and quickly predict the optical properties of any geometric parameters in the studied parameter space. A data set similar to Gaussian distribution was formed by two logarithmic transformation methods to avoid the problem that machine learning models cannot be well trained on extremely skewed distribution. Compared to ANNs, SVMs are more accurate and show stable prediction results. [ABSTRACT FROM AUTHOR]

• An optical fiber sensor combining the Sagnac interference and modal interference was fabricated and experimentally investigated for the measurements of liquid level and strain. • A sensitivity of 525 pm/mm for liquid level detection, and 32 pm/με for strain detection were obtained. • This sensor has advantages of simple structure, high sensitivity and good accuracy, which is an ideal candidate of liquid level and strain simultaneous detection. An optical fiber sensor for the detections of strain and liquid level was experimentally investigated in this paper. This sensor was fabricated through cascading the Sagnac interference (SI) and multi-mode interference (MMI). A construction of single mode fiber-no core fiber-single mode fiber (SMF-NCF-SMF) was used to excite the MMI, while a section of 10.0 cm long Panda polarization maintaining fiber (PMF) was utilized to generate the polarization modes interference in the Sagnac loop. Two interference valleys corresponding to the MMI and SI appeared in the transmission spectrum and were utilized to detect the liquid level and strain, respectively. The measurement sensitivity of liquid level is higher than most of the reported sensors. This designed sensor could become a good candidate for the liquid level and strain detection owing to the simple structure, high sensitivity, and low hysteresis effect. [ABSTRACT FROM AUTHOR]

• An ultrahigh-sensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers with a nematic liquid crystal film was proposed and experimentally demonstrated. • The molecules of NLC film changed from isotropic to isotropic just by rotating the two single-mode patch cables without any orientation agent, electric, or magnetic devices. • Experimental results revealed that the temperature sensor based on the cascaded configuration showed an ultrahigh sensitivity of 43.13 nm/°C, which was amplified about 5 times contrast to a single Sagnac interferometer. We demonstrated an ultrasensitive fiber-optic temperature sensor based on cascaded Sagnac interferometers (CSIs). One of the Sagnac interferometers (SIs) consisted of a panda polarization-maintaining fiber (PMF) as the reference arm, while the other one contained two single-mode patch cables in a ferrule matching sleeve as the sensing arm, where a nematic liquid crystal (NLC) film was infiltrated into the micron-scale gap of the sleeve. In the sensing SI, the orientation of NLC film was induced by just rotating the two single-mode patch cables without any orientation agent, electric, or magnetic devices. Vernier effect was acquired by adjusting the thickness of NLC film. Experimental results revealed that the temperature sensitivity of the CSIs was up to 43.13 nm/°C, which was enhanced about 5 times compared to the single NLC SI. The designed optical fiber temperature sensor possessed the merits of compact structure, high stability, ultrahigh sensitivity, low hysteresis effect and high resolution, showing a promising application prospect in some scientific filed requiring accurate temperature measurement, such as biomolecules, medicine and other filed. [ABSTRACT FROM AUTHOR]

• A bubble microcavity in an ultra-thin core optical fiber was fabricated to form a Fabry-Perot interferometer. • The size of the bubble decreased gradually with further arc discharges, which was inverse to the reported literature. • The bubble microcavity in the ultra-thin core optical fiber showed a strain sensitivity of 2.08 pm/με and a gravity sensitivity of 2.908 nm/N in a large measurement range. • The bubble microcavity in the ultra-thin core optial fiber was insensitive to temperature and bending, which was benefit to avoid the cross influences. Recent progress in optical fiber sensors based on Fabry-Perot interferometers (FPIs) has achieved much attention. In this paper, we designed and fabricated an FPI sensor by building a bubble in an ultra-thin core optical fiber. The size of the bubble decreased gradually with further arc discharges, which was inverse to the bubble fabrication in standard single mode fibers and multimode fibers. The bubble microcavity in the ultra-thin core optical fiber demonstrated a strain sensitivity of 2.08 pm/με in the range of 0–5000 με and a gravity sensitivity of 2.908 nm/N in the range of 0–4.89 N with high linear responses. Meanwhile, the bubble microcavity sensor was insensitive to temperature and bending, which was benefit to avoid the cross influences. The FPI sensor based on a bubble microcavity in an ultra-thin core optical fiber possesses the properties of easy to fabricate, small size, robustness, and high sensitivity. [ABSTRACT FROM AUTHOR]

A temperature sensor based on modes coupling effect in a liquid crystal-filled microstructured optical fiber was proposed and studied in this paper. The sensing characteristics were investigated by using a full-vectorial finite-element method (FEM) with perfectly matched layer and scattering boundary condition. For the purpose of temperature sensing, liquid crystal E7 was designed to be infiltrated into one cladding air hole in PCF to form a defect core. As the phase matching condition is satisfied, the core modes couple to several different ordered defect core modes, and finally experience a large increase in the confinement loss spectra. Furthermore, the loss spectra shift toward opposite directions in the two temperature segments (15–29 °C, 29–50 °C) for X-polarized (X-pol) direction, while it experiences a blue shift in the temperature range of 15–50 °C for Y-polarized (Y-pol) direction. Numerical simulation results show that the sensitivity reaches −5.26 nm/°C at 30 °C. The coupling effect between the core fundamental mode and high-order defect core modes utilized in this paper will be beneficial to the high-sensitivity detection. [ABSTRACT FROM AUTHOR]