OPGW positioning and early warning method based on a Brillouin distributed optical fiber sensor and machine learning.

A method of optical fiber composite overhead ground wire (OPGW) positioning based on a Brillouin distributed optical fiber sensor and machine learning is proposed. A distributed Brillouin optical time-domain reflectometry (BOTDR) and Brillouin optical time-domain analyzer (BOTDA) are designed, where the ranges of BOTDR and the BOTDA are 110 km and 125 km, respectively. An unsupervised machine learning method density-based spatial clustering of applications with noise (DBSCAN) is proposed to automatically identify the splicing point based on the Brillouin frequency shift (BFS) difference of adjacent sections. An adaptive parameter selection method based on k-distance is adapted to overcome the parameter sensitivity. The validity of the proposed DBSCAN algorithm is greater than 96%, which is evaluated by three commonly external validation indices with five typical BFS curves. According to the clustering results of different fiber cores and the tower schedule of the OPGW, the connecting towers are distinguished, which is proved as a 100% recognition rate. According to the identification results of different fiber cores of both the OPGW cables and tower schedule, the connecting towers can be distinguished, and the distributed strain information is extracted directly from the BFS to strain. The abnormal region is positioned and warned according to the distributed strain measurements. The method proposed herein significantly improves the efficiency of fault positioning and early warning, which means a higher operational reliability of the OPGW cables.

Online distributed strain measurement of fiber Michelson hydrophones based on DPP-BOTDA with a pulsed-probe wave.

We propose and demonstrate a novel differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) system with a pulsed-probe wave for online distributed strain measurement of fiber Michelson hydrophones (FMHs). Different from the conventional DPP-BOTDA using a continuous probe wave, a pulsed-probe wave is used in our scheme to avoid the interferences between the reflected lights from the sensor arm and reference arm of the FMH, where the probe pulse width should be adjusting precisely equal to the time delay between the two arms. The Brillouin frequency shift (BFS) containing the strain and/or temperature information is measured by sweeping the frequency difference of the probe pulse and the pump pulse. In the experiment, an optimized 8/8.5 ns pump pulse pair is applied to realize a 5-cm spatial resolution, and the probe pulses of 624.5 ns and 1269 ns are applied to measure the strain distribution of the sensor arms of two FMHs. We have successfully measured the temperature-induced strain of a FMH with an arm length difference of 62.45 m as temperature increases from -40°C to 80°C and the distributed strain variation of the other FMH with the arm length difference of 126.91 m as the hydrostatic pressure increases from 0.1 MPa to 10 MPa. The results indicate that the proposed scheme can provide a desirable solution for online distributed strain measurement of the FMHs.

Slope-assisted BOTDA based on vector SBS and frequency-agile technique for wide-strain-range dynamic measurements.

We present a slope-assisted BOTDA system based on the vector stimulated Brillouin scattering (SBS) and frequency-agile technique (FAT) for the wide-strain-range dynamic measurement. A dimensionless coefficient K defined as the ratio of Brillouin phase-shift to gain is employed to demodulate the strain of the fiber, and it is immune to the power fluctuation of pump pulse and has a linear relation of the frequency detuning for the continuous pump and Stokes waves. For a 30ns-square pump pulse, the available frequency span of the K spectrum can reach up to 200MHz, which is larger than fourfold of 48MHz-linewidth of Brillouin gain spectrum. For a single-slope assisted BOTDA, dynamic strain measurement with the maximum strain of 2467.4µÎµ and the vibration frequency components of 10.44Hz and 20.94Hz is obtained. For a multi-slope-assisted BOTDA, dynamic measurement with the strain variation up to 5372.9µÎµ and the vibration frequency components of 5.58Hz and 11.14Hz is achieved by using FAT to extend the strain range.

Temperature-compensated distributed hydrostatic pressure sensor with a thin-diameter polarization-maintaining photonic crystal fiber based on Brillouin dynamic gratings.

A temperature-compensated distributed hydrostatic pressure sensor based on Brillouin dynamic gratings (BDGs) is proposed and demonstrated experimentally for the first time, to the best of our knowledge. The principle is to measure the hydrostatic pressure induced birefringence changes through exciting and probing the BDGs in a thin-diameter pure silica polarization-maintaining photonic crystal fiber. The temperature cross-talk to the hydrostatic pressure sensing can be compensated through measuring the temperature-induced Brillouin frequency shift (BFS) changes using Brillouin optical time-domain analysis. A distributed measurement of hydrostatic pressure is demonstrated experimentally using a 4-m sensing fiber, which has a high sensitivity, with a maximum measurement error less than 0.03 MPa at a 20-cm spatial resolution.

Phase-shifted Brillouin dynamic gratings using single pump phase-modulation: proof of concept.

Two novel phase-shifted Brillouin dynamic gratings (PS-BDGs) are proposed using single pump phase-modulation (SPPM) in a polarization maintaining fiber (PMF) for the first time to our knowledge. Firstly, based on the stimulated Brillouin scattering (SBS), a transient PS-BDG with a 3-dB bandwidth of 354MHz is written by a 2-ns pump1 pulse and a 100-ps pump2 pulse, where the phase of pump1 pulse is shifted with π from its middle point through phase modulation. Then, with a high repetition rate of 250MHz for both pump pulses, an enhanced PS-BDG with a deep notch depth is obtained and its notch frequency can be easily tuned by changing the phase shift. We demonstrate a proof-of-concept experiment of the transient PS-BDG and show the notch frequency changing by tuning the phase shift. The proposed PS-BDGs have important potential applications in microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

High-sensitivity distributed transverse load sensor with an elliptical-core fiber based on Brillouin dynamic gratings.

A high-sensitivity distributed transverse load sensor based on Brillouin dynamic gratings (BDGs) is proposed and demonstrated experimentally for the first time, to the best of our knowledge. The principle is to measure the transverse-load-induced birefringence change through exciting and probing a BDG in an elliptical-core polarization-maintaining fiber. A distributed measurement of transverse load is demonstrated experimentally using a 10 m sensing fiber, which features high sensitivity to a transverse load with a measurement accuracy as high as 0.8×10(-3) N/mm at a 20 cm spatial resolution.

Bend-insensitive distributed sensing in singlemode-multimode-singlemode optical fiber structure by using Brillouin optical time-domain analysis.

We propose a bend-insensitive distributed Brillouin optical fiber sensing by using a singlemode-multimode-singlemode optical fiber structure for the first time to the best of our knowledge. The sensing fiber is a graded-index multimode fiber (GI-MMF) sandwiched by two standard single-mode fibers (SMFs) with central-alignment splicing at the interface between GI-MMF and SMF to excite the fundamental mode in GI-MMF. The sensing system can resist a minimal bend radius of 1.25mm while maintain the measurement performance, with which the measured coefficients of strain and temperature are 421.6MHz/% and 0.826MHz/°C, respectively. We also demonstrate that the higher-order modes excited in GI-MMF can be easily influenced by bending, so that exciting the fundamental mode is essential for bend-insensitive distributed sensing.

Sub-MHz ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings.

We propose and demonstrate an ultrahigh-resolution optical spectrometry based on Brillouin dynamic gratings (BDGs). Taking advantage of creating a long grating in an optical fiber, an ultra-narrow bandwidth optical filter is realized by operating a BDG in a long single-mode fiber (SMF), and the optical spectrometry is performed by sweeping the center wavelength of the BDG-based filter through a swept-tuned laser. The BDG-based optical spectrometry features ultrahigh resolution, large wavelength coverage, and a simple direction-detection scheme. In the experiment, a 4 fm (0.5 MHz) spectral resolution is achieved by operating a BDG in a 400 m SMF, and the wavelength coverage can be readily extended to C+L bands with a commercial tunable laser.