Taofei Jiang

High-Spatial-Resolution Fast BOTDA for Dynamic Strain Measurement Based on Differential Double-Pulse and Second-Order Sideband of Modulation

We demonstrate a high-spatial-resolution fast Brillouin optical time-domain analysis (BOTDA) for distributed dynamic strain measurement based on differential double-pulse and second-order sideband of modulation. The frequency-agility probe wave is obtained from the second-order sideband of the modulated light by using the microwave signal from a wideband arbitrary waveform generator (AWG), which reduces the bandwidth requirement of the AWG by half to ~ 5.5 GHz. The differential double-pulse scheme is proposed to improve the spatial resolution while keeping the capability of dynamic measurement. In experiment, a spatial resolution of 20 cm is achieved by using a 52/50 ns differential double-pulse, and the distributed vibration measurement is demonstrated over a 50-m Panda polarization-maintaining fiber observing the vibration frequency of up to 50 Hz. With only five averages, the standard deviation of the strain accuracy is measured to be 14 με.

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 2467.4με 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 5372.9με and the vibration frequency components of 5.58Hz and 11.14Hz is achieved by using FAT to extend the strain range.

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.

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.

1200°C high-temperature distributed Brillouin optical fiber sensing based on photonics crystal fiber

We demonstrate an up to 1200°C high-temperature distributed Brillouin sensing based on a pure-silica photonics crystal fiber. A Brillouin frequency shift (BFS) hopping is observed between 800°C-900°C for the first annealing process and after that the BFS exhibits the stability and repeatability with a measurement accuracy as high as ±2 °C . The BFS dependence on temperature in the range of room temperature to 1200°C agrees well with an exponential function instead of a linear function, which is mainly attributed by the change of the acoustic velocity in a silica fiber.

High-sensitivity distributed static pressure sensor based on Brillouin dynamic grating

Brillouin dynamic gratings (BDGs) has been, in recent years, addressed using a polarization-maintaining fiber with many prominent advantages and proved that the BDGs have a profound high sensitivity to fiber birefringence changes. When the two parallel polarized counter-propagating pump waves with a frequency offset of fiber Brillouin frequency shift, and the orthogonally polarized probe pulse wave satisfied with the phase-matching condition, the BDGs would be excited through stimulated Brillouin scattering and read, while the optical frequency difference between the pump and probe waves is determined by the birefringence. The birefringence-induced frequency shift (BireFS) associated with the impact of external environment may be affected by local static pressure. Though the measurement of the BireFS changes along the fiber, one can realize a distributed fiber static pressure sensing. In our presentation, a temperature-insensitive distributed static pressure sensor based on BDGs is proposed and experimentally demonstrated for the first time, to the best of our knowledge. The measurement principle is to interrogate the static-pressure-induced fiber birefringence changes through generating and mapping the BDGs in the fiber under test (FUT). The experimental setup adopted two pump waves to excite a BDG and a short probe pulse to read the Brillouin grating spectrum associated with the birefringence with a spatial resolution of 20 cm. The sensing technique features a distributed measurement, temperature-insensitivity and high sensitivity to the static pressure. The distributed transverse load measurement experiment is conducted in an temperature-insensitive elliptical-core polarization-maintaining fiber with a measurement accuracy as high as 0.8 × 10-3 N/mm; and the distributed hydrostatic pressure measurement experiment is also performed in a thin-diameter pure silica polarization-maintaining photonic crystal fiber with a measurement accuracy as high as 0.025MPa with a character of temperature compensation.

Temperature-compensated distributed hydrostatic pressure Brillouin sensor using a thin-diameter and high-birefrigent photonics crystal fiber

In this work, a distributed hydrostatic pressure sensor based on Brillouin dynamic gratings (BDGs) was proposed and demonstrated for the first time to the best of our knowledge. The hydrostatic pressure sensing is realized through measuring the pressure-induced birefringence changes through exciting and probing the BDGs; while the temperature can be compensated by measuring the temperature-induced Brillouin frequency shift (BFS) through differential pulsewidth pair Brillouin optical time-domain analysis (DPP-BOTDA). The thin-diameter high-birefringent PM-PCF is used as the fiber under test and its porous-structure cladding is liable to be deformed exhibiting a high sensitivity to pressure. A distributed measurement with a 20-cm spatial resolution and a measurement accuracy as high as 0.025 MPa is reported.

Fatigue Damage Detection of Rib-to-Deck Welded Joints Using 5cm High-Spatial-Resolution DDP-BOTDA System

In this paper, a distributed fiber optic sensor (DFOS) based fatigue crack detection system for rib-to-deck welded joints was developed by using the RP1000 distributed optical fiber sensing system. RP1000 is the latest high performance fiber optic distributed strain and temperature monitoring system recently researched and developed by Harbin Institute of Technology. The RP1000 system, which is based on differential double-pulse Brillouin optical time-domain analysis (DDP-BOTDA), can realize a spatial resolution of 5 cm and a sensing range of 5 km. The rib-to-deck welded joints specimen was a full-scale orthotropic deck specimen with weld meltthrough (WMT) of 12-mm-thick deck plate and 8-mm-thick rib. A piece of polarization-maintaining (PM) optical fiber was bonded along the weld on both deck plate and rib to form the distributed sensing network. A fatigue test was conducted and the specimen was subjected to cyclic tensile loading. The fatigue cracks were visually inspected at the root of the specimen. The strain responses of the sensing fiber near the creaks confirmed the effectiveness of damage detection and characterization of the proposed system. The RP1000 DDP-BOTDA distributed optical fiber sensing system shown good accuracy and stability with a high spatial resolution of 5 cm during the monitoring process. doi: 10.12783/SHM2015/382