Trace detection of cadmium (II) ions based on an excessively tilted fiber grating.

Cadmium (Cd) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd, and hence forming a complex.

Sub-millimeter resolution and high-precision φ-OFDR using a complex-domain denoising method.

Phase noise is one of the main obstacles to achieve high spatial resolution, high precision, and large measurement range in φ-OFDR. Here, we proposed a complex-domain denoising method to achieve unwrapping of phase signals. In this method, the wrapped phase was used to construct a complex signal, and then both real and imaginary parts are denoised by using a wavelet packet.

Acousto-optic reconfigurable filter based on vector mode fusion in dispersion-compensating fiber.

An acousto-optic reconfigurable filter (AORF) is proposed and demonstrated based on vector mode fusion in dispersion-compensating fiber (DCF). With multiple acoustic driving frequencies, the resonance peaks of different vector modes in the same scalar mode group can be effectively fused into a single peak, which is utilized to obtain arbitrary reconfiguration of the proposed filter. In the experiment, the bandwidth of the AORF can be electrically tuned from 5 nm to 18 nm with superposition of different driving frequencies.

Optical Frequency Domain Reflectometry Based on Multilayer Perceptron.

We proposed an optical frequency domain reflectometry based on a multilayer perceptron. A classification multilayer perceptron was applied to train and grasp the fingerprint features of Rayleigh scattering spectrum in the optical fiber. The training set was constructed by moving the reference spectrum and adding the supplementary spectrum.

Submetric Spatial Resolution ROTDR Temperature Sensor Assisted by Wiener Deconvolution.

A submetric spatial resolution Raman optical time-domain reflectometry (ROTDR) temperature sensor assisted by the Wiener deconvolution postprocessing algorithm has been proposed and experimentally demonstrated. Without modifying the typical configuration of the ROTDR sensor and the adopted pump pulse width, the Wiener demodulation algorithm is able to recover temperature perturbations of a smaller spatial scale by deconvoluting the acquired Stokes and anti-Stokes signals. Numerical simulations have been conducted to analyze the spatial resolution achieved by the algorithm.

Distributed pH sensing based on hydrogel coated single mode fibers and optical frequency domain reflectometry.

We propose a distributed pH sensor based on an optical frequency domain reflectometry using a PEGDA-based pH-sensitive hydrogel coated on a single mode fiber. The volume of hydrogel increased as pH value of the surrounding fluid decreased, which converts the pH value to the axial strain in the fiber. Taking capacity of distributed strain measurement with high spatial resolution in optical frequency domain reflectometry, the pH value of the external medium is distributed measured by the wavelength shifts of the local Rayleigh backscattering spectra.

Ion-Imprinted Chitosan-Based Localized Surface Plasmon Resonance Sensor for Ni Detection.

Heavy metals are important sources of environmental pollution and cause disease in organisms throughout the food chain. A localized surface plasmon resonance sensor was proposed and demonstrated to realize Ni detection by using ion-imprinted chitosan. Au nanoparticles were coated on the multimode fiber to excite the local surface plasmon resonance, and Ni-imprinted chitosan was then functionalized by using the dip coating technique.

Ultra-narrow linewidth vertical-cavity surface-emitting laser based on external-cavity weak distributed feedback.

We demonstrate an ultra-narrow linewidth vertical-cavity surface-emitting laser (VCSEL) based on external-cavity weak distributed feedback from Rayleigh backscattering (RBS). A single longitudinal mode VCSEL with the linewidth as narrow as 435 Hz and a contrast of 55 dB are experimentally achieved by RBS fiber with a feedback level of RBS signal of -27.6 dB.

Tens of hertz ultra-narrow linewidth fiber ring laser based on external weak distributed feedback.

We suggest and demonstrate a single-frequency fiber ring laser with an ultra-narrow linewidth based on an external weak distributed feedback. A π phase-shifted fiber Bragg grating (PSFBG) is used to improve mode selection and enable single-longitudinal mode (SLM) laser operation. The linewidth is then further strongly compressed using a signal generated by a weak distributed feedback structure (WDFS) and injected into the main laser cavity to suppress spontaneous emission.

Enhancing spatial resolution of BOTDR sensors using image deconvolution.

We propose to employ the image deconvolution technique for Brillouin optical time domain reflectometry (BOTDR) systems to achieve a flexible and enhanced spatial resolution with pump pulses longer than phonon lifetime. By taking the measured Brillouin gain spectrum (BGS) distribution as an image blurred by a point spread function (PSF), the image deconvolution algorithm based on the two-dimensional Wiener filtering can mitigate the ambiguity effect on the Brillouin response. The deconvoluted BGS distribution reveals detailed sensing information within shorter fiber segments, improving the inferior spatial resolution and simultaneously maintaining other sensing performance parameters.

Ultra-narrow-linewidth DFB laser array based on dual-cavity feedback.

Herein, we propose a structure to simultaneously compress the distributed feedback (DFB) laser array's linewidth. The proposed structure is meticulously designed to ensure single longitudinal mode operation via the interference phenomenon between the laser's primary cavity and the dual-cavity feedback. Given the weak feedback effect for each wavelength in the laser array, the proposed structure could realize the intense compression of the laser linewidths.

Control of polarization switching in a VCSEL via resonant feedback from a whispering-gallery-mode cavity.

We report a method for flexibly switching the dominant polarization of a vertical-cavity surface-emitting laser (VCSEL) by introducing polarization-resolved resonant optical feedback from a whispering-gallery-mode (WGM) cavity to the lasing cavity. Switching between the originally dominant mode and a side mode is experimentally demonstrated under different bias currents once one of them is locked to the resonance mode of the WGM cavity. In addition to a controllable polarization state, the reported VCSEL also demonstrates a linewidth as narrow as tens of kilohertz, which is highly desirable for many applications, including high-speed data communication, light detection and ranging (lidar), and absorption spectroscopy.

Vector optical-chirp-chain Brillouin optical time-domain analyzer based on complex principal component analysis.

A vector optical-chirp-chain (OCC) Brillouin optical time-domain analyzer (BOTDA) based on complex principal component analysis (CPCA) is proposed and experimentally demonstrated by employing a four-tone OCC probe with two orthogonal polarization states. The polarization-fading-free complex Brillouin spectrum (CBS) of the vector OCC-BOTDA is obtained by combining the amplitude and phase response spectra of the probe wave at both Brillouin gain and loss region. We utilize the CPCA method to determine the Brillouin frequency shift (BFS) directly using the measured CBS, and the sensing accuracy is improved by a factor of up to 1.

Distributed directional torsion sensing based on an optical frequency domain reflectometer and a helical multicore fiber.

We propose and experimentally demonstrate a distributed directional torsion sensor based on an optical frequency domain reflectometer (OFDR) using a helical multicore fiber (MCF). A theoretical model is first established to reveal that the ability of the torsion direction discrimination stems from the fiber design of the central-offset cores with helical structure and the shorter helical pitch holds higher sensitivity. Such a distributed torsion sensor is then experimentally realized by using an OFDR system with an adjacent sensing distance of 9.

Intensity-modulated bend sensor by using a twin core fiber: theoretical and experimental studies.

A new optical fiber bend sensor is proposed and demonstrated based on a sandwich structure created by splicing a segment of twin core fiber (TCF) between two segments of single mode fibers (SMFs). One core of the TCF is aligned with the cores of two segments of SMFs. An incident beam is directed into the TCF by the lead-in SMF.

Nonlinear Hydraulic Pressure Response of an Improved Fiber Tip Interferometric High-Pressure Sensor.

We demonstrate a silica diaphragm-based fiber tip Fabry-Perot interferometer (FPI) for high-pressure (40 MPa) sensing. By using a fiber tip polishing technique, the thickness of the silica diaphragm could be precisely controlled and the pressure sensitivity of the fabricated FPI sensor was enhanced significantly by reducing the diaphragm thickness; however, the relationship between the pressure sensitivity and diaphragm thickness is not linear. A high sensitivity of -1.

A fluorometric optical fiber nanoprobe for copper(II) by using AgInZnS quantum dots.

An optical fiber nanoprobe is presented for fluorometric determination of copper(II). The method based on the use of water-dispersible AgInZnS quantum dots (QDs) deposited at the end of an optical fiber in a poly(vinyl alcohol) matrix. The fluorescnece of the QDs, best measured at excitation/emisssion wavelengths of 365/570 nm, is quenched by Cu(II) due to both static and electron transfer from the QDs to Cu(II).

Orbital angular momentum generation in two-mode fiber, based on the modal interference principle.

In this Letter, we demonstrate, to the best of our knowledge, a novel method to generate an orbital angular momentum (OAM) based on the principle of the modal interference in a two-mode fiber. At the interference dips, the left- or right-handed circular polarized HE modes can be ideally converted into the ±1-order OAM beam. To verify this concept, we employed the femtosecond laser micro-processing technology to write micro-waveguides in the two-mode fiber and hence realized the in-line modal interferometer.

Polarization independent fast BOTDA based on pump frequency modulation and cyclic coding.

We propose and experimentally demonstrate a scheme of polarization independent fast Brillouin optical time domain analysis (F-BOTDA) based on pump frequency modulation and cyclic coding. The Brillouin gain spectrum (BGS) is reconstructed by fast scanning frequency of the pump using an arbitrary waveform generator (AWG). To realize long range distributed dynamic strain sensing, polarization diversity technique and cyclic coding are employed to eliminate polarization fading and enhance the signal-to-noise ratio (SNR).

Intensity-modulated directional torsion sensor based on in-line optical fiber Mach-Zehnder interferometer.

In this Letter, we demonstrated an intensity-modulated directional torsion sensor based on an in-line Mach-Zehnder interferometer in single-mode fiber. A non-circular symmetric perturbation is created to excite non-circular symmetric cladding mode and then interference with the core mode at the second perturbation. An initial rotation angle is designed between two perturbations for the purpose of discriminating the torsion direction.

Distributed fiber sparse-wideband vibration sensing by sub-Nyquist additive random sampling.

The round-trip time of the light pulse limits the maximum detectable vibration frequency response range of phase-sensitive optical time domain reflectometry (ϕ-OTDR). Unlike the uniform laser pulse interval in conventional ϕ-OTDR, we randomly modulate the pulse interval so that an equivalent sub-Nyquist additive random sampling (sNARS) is realized for every sensing point of the long interrogation fiber. For a ϕ-OTDR system with 10 km sensing length, the sNARS method is optimized by theoretical analysis and Monte Carlo simulation, and the experimental results verify that a wideband sparse signal can be identified and reconstructed.

Simultaneous directional curvature and temperature sensor based on a tilted few-mode fiber Bragg grating.

We demonstrate a directional curvature sensor based on tilted few-mode fiber Bragg gratings (FM-FBGs) inscribed by a UV laser. The eigenmodes of LP and LP mode groups are simulated along with the fiber bending. The directional curvature sensor is based on the LP mode resonance in the tilted FM-FBG.