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A miniaturepolymer Bragg grating (PBG) sensor is fabricated and employed for ultrasonic imaging of seismic physical models (SPMs). The sensing Bragg grating is inscribed into an ultraviolet (UV) glue polymer waveguide with a femtosecond laser. The uniform polymer waveguide is fabricated by sealing the UV glue into a capillary fibre through capillary effect. By using line-by-line inscription technique, the laser beam is scanned transversely to periodically modulate the refractive index of the polymer waveguide, leading to the formation of the PBGs with various grating lengths. The sensor response to ultrasonic waves are investigated experimentally. When compared to single-mode fiber Bragg grating and phase-shifted fiber Bragg grating, the PBG with the same reflectivity presents a higher response amplitude due to its lower Young’s modulus. Besides, the sensor has a good spectral stability when transferred from air to water due to the waterproof coating on the sensor end. Finally, the sensor is used to scan a large-scale 3D SPM and the structural features, such as fault, fluctuation, and depositional termination, can be distinctly reconstructed. The proposed PBG sensor provides a new technique with easy fabrication, high sensitivity, and good stability for high-fidelity ultrasonic imaging of seismicphysical models. [ABSTRACT FROM AUTHOR]

A Sagnacinterferometer-based magnetic field sensor is theoretically proposed and experimentally demonstrated by taking advantage of the high birefringence of an asymmetric four-hole fiber (AFHF) as well as the sensibility of magnetic fluid (MF) to magnetic field. A variation in the magnetic field intensity would induce a change in the RI of MF, leading to a birefringence change of MF-filled AFHF and resulting in a shift of the interference spectrum. The magnetic field sensing can be realized by monitoring the spectral shift. The proposed sensor has a high sensitivity of 226.2 pm/Gs (2.96 ∼ 89 Gs) with a measurement accuracy of 0.221 Gs, accompanied with competitive linearity, good repeatability and stability. In addition, it is insensitive to ambient temperature disturbance. The proposed sensor can be used in industry, biomedicine, aerospace and other fields which require a magnetic field measurement with high precision and good stability. [ABSTRACT FROM AUTHOR]

In this paper, an optical fiber sensor based on cholesteric liquid crystal (CLC) is designed to detect the proportion and concentration of methanol and ethanol in alcohol solution. Due to the different polarity of methanol and ethanol molecules, the pitch of CLC will swell in different degrees. Therefore, the reflected light wavelength of CLCs will shift differently when the sensor is put into the mixed solution of two alcohols with different proportions and concentrations. The function of chitosan polymer network is to fix and protect CLC droplets, it also allows small molecules to pass through and interact with CLC. The sensitivities of the two CLCs are 0.296 nm/v% and 0.467 nm/v% to methanol, and 0.764 nm/v% and 1.133 nm/v% to ethanol, respectively. The detection range of the sensor for alcohol solution is 0.5v/v% –30v/v%. The linear relationship between wavelength shift and proportion and total concentration was obtained by least square method, and the related characteristics of the sensor were explored. [ABSTRACT FROM AUTHOR]

Optical orbital angular momentum (OAM), carried by optical vortices, is a fundamental property of light that grabs broad interests. The detection of OAM modes, including fractional OAM is of great importance in diverse fields. Here, we firstly propose and develop the limited integer-component measurement method to determine the fractional topological charge (TC) of OAM states. This method indicates that a 2-integer-mode responsive device could be applied for the detection of fractional TC. An orthogonal long-period fiber grating (OLPFG) element is used as a fractional OAM TC probe to verify the Applicability. We experimentally demonstrate the fractional TC detection, ranging from −1 to 0, and recognize different fractional OAM modes with the minimum TC interval of 0.01. This scheme provides a concise, precise, continuous, and fiber-compatible fractional TC detection method that might have great potential in micro-detection, optical communication, quantum information, and chiral sensing applications. [ABSTRACT FROM AUTHOR]

A novel bending sensor based on an in-fiber Mach–Zehnder interferometer (MZI) has been fabricated by femtosecond laser. The MZI is formed by an off-axial straight waveguide inscribed in a no-core fiber spliced between two standard single mode fibers. The in-fiber MZI is formed due to the interference of a fundamental mode and a higher-order mode which excited in the no-core fiber. Analysis results of the structure agree well with the measured transmission spectra. Experimental results show that the device has low temperature crosstalk and high curvature sensitivity of 14.91 nm/m−1 and 12.86 nm/m−1 for the 0° and 180° orientations in the range of 0−3 m−1, respectively. We can distinguish the bending orientation of the device by the shift direction of the spectrum. The advantages of high sensitivity, low temperature crosstalk, compact structure, and easy fabrication make the device attractive for actual engineering bend sensing. [ABSTRACT FROM AUTHOR]

Tilted Bragg grating (TBG) has been intensively investigated for photonic devices such as fiber sensors and on-chip filters. In this article, we present that TBG can be equivalently realized by designing the tilted sampled Bragg grating (TSBG). As its basic grating is uniform and the sampling structure is in the order of micrometer, the fabrication of TSBG can be realized with one step of holographic exposure combined with micro-lithography. As examples, we designed and simulated the equivalent light responses of the TSBGs in the multimode planar waveguide and single-mode fiber respectively. Moreover, the fabrication tolerance of TSBG has also been analyzed. The proposed technique may pave a new way for different 2D gratings with good design flexibility and easy fabrication in various photonic devices. [ABSTRACT FROM AUTHOR]

Sensors operating at an exceptional point (EP) are sufficiently sensitive to small perturbations. In this article, we demonstrate that an EP enhances sensing in the frame of a fiber ring cavity. An air gap inserted into a fiber ring cavity couples the clockwise and counterclockwise propagating modes, leading to eigenfrequency splitting. The EP was achieved in a nonreciprocity cavity constructed using an isolator. A fiber displacement sensor was realized by monitoring the splitting strength in the spectrum, which depends on the width of the air gap. The experimental results showed that the sensitivity of the sensor at the EP was sufficiently enhanced as compared to that in the reciprocal cavity. [ABSTRACT FROM AUTHOR]

A high-sensitivity Fabry--Perot interferometer (FPI) strain sensor based on a concave-core photonic crystal fiber (CPCF) was proposed and demonstrated. The FPI was formed by directly splicing a CPCF to a standard single-mode fiber. The CPCF was fabricated by cleaving a high-numerical-aperture solidcore photonic crystal fiber under axial tension using a traditional fiber cleaver. An FPI strain sensor with a cavity length of 4.85 μm and a strain sensitivity of 31.58 pm/με was successfully produced. The sensor also demonstrated a low temperature sensitivity of 0.653 pm/°C, reducing the cross-sensitivity between tensile strain and temperature. The benefits of the proposed sensor include simple fabrication, high strain sensitivity, and low temperature crosssensitivity, making it attractive for practical applications. [ABSTRACT FROM AUTHOR]

Ahighly sensitive optical fiber curvature sensor based on cascaded fiber interferometers (CFIs) is demonstrated theoretically and experimentally. Theoretical results show that the scheme of cascaded fiber interferometers can be regarded as an equivalent fiber interferometer with longer interference length and narrower FSR. It serves as a curvature sensor with sensitivity up to 4.362 nm/m-1 within the measurement range of 0-1.134 m-1. Compared to the single fiber modal interferometer, the curvature sensitivity of the CFI is almost doubled. In addition, simultaneous curvature and temperature measurement is achieved using the coefficient matrix method, eliminating the issue of temperature cross sensitivity. [ABSTRACT FROM AUTHOR]

In this paper, a new sensor system for simultaneous and independent multipoint strain and temperature measurements is presented. The interrogation of the sensing heads has been carried out by monitoring their fast Fourier transform phase variations. In particular, two of each microstructured optical fiber cavity interference frequencies were used for the measures. This method is independent of the signal amplitude and also avoids the necessity of tracking the wavelength evolution in the spectrum, which can be a handicap when there are multiple interference frequency components with different sensitivities. The sensing heads present birefringent and multimodal properties, and therefore, both characteristics lead to their own interference with different properties and sensitivities. The multiplexing capability of the sensing heads and the interrogator method has also been tested and validated. Sensors were operated within a range of temperature 30 °C-80 °C and a deformation of ~450 με was applied. Crosstalk between measurements can be corrected through simple math operations leading to independent and crosstalk-free multipoint and multiparameter sensors. [ABSTRACT FROM AUTHOR]

A fiber Bragg grating (FBG) has been used as a sensor head for the measurement of temperature and static strain. However, a standard FBG sensor, which is constructed on a single-mode fiber, cannot work well under conditions where both the temperature and strain change independently since the sensor has cross-sensitivity between them. The cross-sensitivity problem can be solved by using an FBG constructed on a polarization maintaining fiber (PM-FBG) instead of a standard FBG. In this paper, we report improvement on the resolution for the simultaneous measurement of temperature and static strain. A Fabry-Perot interferometer constructed with PM-FBGs (PM-FBG-FPI) is introduced as a sensor head. The fine structure of a PM-FBG-FPI reflection spectrum enables high-resolution detection of wavelength shifts. The resulting high-resolution measurements are demonstrated experimentally. [ABSTRACT FROM PUBLISHER]

A long-standing goal of optical fiber sensors is the development of a miniature and versatile optical fiber device, which is capable of performing multiple sensing functions, and supporting a simple and efficient system. Here, we demonstrate an elegant way of achieving such a device by use of an inner air-cavity in a slightly tapered optical fiber. Owing to the small size of only a few tens microns and inner cavity structure, a spatially precise "point sensing" with high sensitivity and good robustness can be readily achieved. The refractive index, strain, and temperature sensitivities obtained are ∼1060 nm/RIU (refractive index unit), 22.5 pm/με, and 80 pm/°C, respectively. The inner air-cavity-based device is flexible, ultra-compact, versatile, and highly efficient, which provides a promising new way for a wide range of optical fiber sensing applications. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a novel distributed sensor based on coherent interaction of the Brillouin gain and loss via optical differential parametric amplification (ODPA), which provides narrowed parametric Brillouin gain spectrum, strong Brillouin signal via differential gain, and short measurement time simultaneously. In experiment, a 0.5 m spatial resolution with a strain accuracy of 6\ \mu \varepsilon is realized by using 20/15 ns Stokes and anti-Stokes pulse pair in a 15 m polarization-maintaining fiber. The spectrum narrowing phenomenon in ODPA Brillouin sensor is discussed. [ABSTRACT FROM PUBLISHER]

A novel technique for chirp control of a uniform fiber Bragg grating (FBG) based on the special strain function modulation is presented. A cylinder taper cantilever beam (CTCB) is specially designed and capable of providing special position-dependent strain function gradients along an originally uniform FBG, which allows tunable quasi-triangular and quasi-Gauss-gradient FBG filter. Furthermore, the feasibility of the tunable chirped FBG based on the strain function modulation as sensor is validated. This state-of-art technique is expected to be applied in fiber filter, FBG sensor and multiwavelength fiber laser. [ABSTRACT FROM PUBLISHER]

The guiding properties of realistic silica/air hollow-core Bragg fibers have been investigated by calculating the dispersion curves, the confinement loss spectrum, and the field distribution of the guided modes through a full-vector modal solver based on the finite-element method. In particular, the silica bridge influence on the fundamental mode has been analyzed by comparing the properties of an ideal structure, without the silica nanosupports, and of two realistic fibers, with squared off and rounded air-holes. Simulation results have demonstrated the presence of anticrossing points in the dispersion curves, associated to the transition of the fundamental mode into a surface mode. It has been shown that surface modes are responsible for the sharp loss peaks, also experimentally measured, which pollute the loss spectrum of the fundamental mode and of the higher order modes. Then, the influence on the guiding properties of each geometric characteristic in the hollow-core Bragg fiber cross-section has been deeply investigated, thus showing which parameter it is better to change in order to properly modify the loss values or its spectral behavior. Moreover, in order to improve the loss properties of hollow-core Bragg fibers, the number of silica and air layers in the fiber cladding has been increased, and the layer thickness has been modified. Results have shown that the first change is more effective for the loss reduction, while the second is useful for a spectral shift. Finally, among the different possible applications, the feasibility of a DNA biosensor based on a hollow-core Bragg fiber has been demonstrated. [ABSTRACT FROM PUBLISHER]

The cladding-modified long-period-grating (LPG) refractive-index sensors with a high-refractive-index overlay are optimized based on systematical studies on the structure-modified LPG. Dependence of the cladding-mode effective indexes on the parameters of the structure-modified LPG, such as the overlay refractive index (RI), the overlay thickness, the cladding-layer radius, and the ambient RI, as well as the order of cladding mode, are investigated in detail. An optimization procedure based on reducing the cladding radius, keeping the grating period as an adaptive parameter, and employing the HE13 mode notch-wavelength shift is proposed for the LPG RI sensors. By using the proposed optimization method, it is shown that an ambient RI sensitivity as large as 5980 nm/RI can be achieved, representing a threefold sensitivity enhancement, as compared to the best result obtained from the reported structure, in which the high HE17 mode resonate notch wavelength was employed in sensing [ABSTRACT FROM PUBLISHER]

The theoretical modeling of large-mode-area silica holey fiber tapers with collapsed air holes that are fabricated with a nonadiabatic process is reported. The transmission spectra of such tapers exhibit multiple interference peaks due to the beating between two modes of the taper waist. The devices investigated here can be useful for a variety of sensor applications since the interference peaks are sensitive to the external environment. Our theoretical model confirms the experimental results [ABSTRACT FROM PUBLISHER]

In this study, a theoretical model of the lead (delay coil and array buses) sensitivity in Sagnac sensor arrays (SSA) for underwater acoustic detection, which induces a spurious pickup and thus a sensor error, is described. In the worst-case scenario, the pickup in a 20-km coil is predicted to be reasonable in deep waters (quietest) (about 17 dB higher than the hydrophone signal) but fairly high (∼ 52 dB) in shallow waters (noisiest). We show that this pickup can be eliminated for the first few hydrophones in the array by optically folding or double winding the coil, although these approaches are not as effective for the most distant hydrophones in long arrays (≥ 1 km). These figures are confirmed experimentally in an SSA: Folding a 5-km coil decreased the relative pickup from ∼-42 dB for the first hydrophone and by ∼-8 dB for a hydrophone 1 km down the array. These results concur that for most practical arrays, it is imperative to isolate the coil in an acoustically shielded enclosure, which will reduce the coil pickup to an arbitrarily low level for all hydrophones. Theory and experiment also agree that the pickup in the array buses increases in proportion to the distance along the array. In the worst-case scenario (spherical acoustic wave incident normal to the array), the pickup in a 1-km bus is predicted to be ∼-7 dB in quiet waters and ∼-40 dB in noisy waters. However, in the field, the probability of this event is expected to be extremely low, and these values will be typically at least 15 dB weaker. Nevertheless, in long arrays, the bus pickup must be reduced, which can be achieved by desensitizing the bus fiber and/or pickup subtraction. Combined, these two techniques are expected to make the bus pickup under normal conditions negligible in deep-water applications (∼-48 dB) and acceptable in noisier shallow-water applications (∼-15 dB). [ABSTRACT FROM PUBLISHER]

A distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer (φ-OTDR) is described. The sensing element is a cabled single-mode telecommunications fiber buried along the monitored perimeter. Light pulses from a continuous-wave Er:fiber Fabry-Pe´rot laser with a narrow (≍3 kHz) instantaneous linewidth and low (few kilohertz per second) frequency drift are injected into one end of the fiber, and the backscattered light is monitored with a photodetector. The effect of phase changes resulting from the pressure of the intruder on the ground immediately above the buried fiber are sensed by subtracting a φ-OTDR trace from an earlier stored trace. In laboratory tests with fiber on reels, the effects of localized phase perturbations induced by a piezoelectric fiber stretcher on φ-OTDR traces were observed. In field tests, people walking on the ground above a buried fiber cable induced phase shifts of several-π radians. [ABSTRACT FROM PUBLISHER]

This paper discusses a phase-sensitive technique for remote interrogation of passive Bragg grating Fabry-Pe´rot resonators. It is based on Pound-Drever-Hall (PDH) laser frequency locking, using radio-frequency phase modulation sidebands to derive an error signal from the complex optical response, near resonance, of a Fabry-Pe´rot interferometer. We examine how modulation frequency and resonance bandwidth affect this error signal. Experimental results are presented that demonstrate, when the laser is locked, this method detects differential phase shifts in the optical carrier relative to its sidebands, due to minute fiber optical path displacements. [ABSTRACT FROM PUBLISHER]

We report on a method to measure the longitudinal position of an acoustic burst, or spark, using an all-fiber Sagnac interferometer (SI). A focused pulsed ∼10 ns laser is used to ionize air creating the spark. This ionization point is situated near a coil of test fiber connected to the SI but acoustically separated from the bulk of the SI. A fast Fourier transform (FFT) performed on the temporal signal of the SI shows a series of s from which the position of the acoustic signal is determined. Over an SI length of ∼35 km, longitudinal positions are determined to within tens of meters, corresponding to a best run percent uncertainty of 0.07%. [ABSTRACT FROM PUBLISHER]