Pengbai Xu

Characterization of evolution of mode coupling in a graded-index polymer optical fiber by using Brillouin optical time-domain analysis.

A narrow bandwidth (2GHz) π-phase-shift flattop fiber Bragg grating (FBG) is proposed to achieve Brillouin optical time-domain analysis (BOTDA) for perfluorinated graded-index polymer optical fibers (GI-POFs) for the first time to best of our knowledge. Using the technique of BOTDA, we explore the evolution of mode coupling in perfluorinated GI-POFs by analyzing the Brillouin frequency shift (BFS) variation along the whole fiber, and compare them with that of silica graded index multimode fibers (GI-MMFs). The characteristics of mode coupling of GI-POFs and GI-MMFs were also investigated in terms of the speckle patterns at the output face of the two fibers. The results show that compared with silica GI-MMFs, GI-POFs exhibit more efficient mode coupling and the excellent ablility of mode scrambling regardless of alignment conditions.

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.

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.

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 optical fiber sensing using Brillouin optical time domain analysis

An up to 1100°C and 1200°C high-temperature distributed Brillouin sensing based on a GeO2-doped single mode fiber (SMF) and a pure-silica photonics crystal fiber (PCF) are demonstrated, respectively. We found that the BFS dependence on temperature of SMF and PCF 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. A Brillouin frequency shift (BFS) hopping is observed in both kinds of fiber between 800°C-900°C in the first annealing process and after that the BFS exhibits the stability and repeatability with a measurement accuracy as high as ±1.42°C for SMF and ±2.6°C for PCF, respectively. The BFS hopping is a highly temperature-dependent behavior, which means that a high-temperature (> 800°C) would accelerate the process of BFS hopping to reach a stable state and after BFS hopping, both SMF and PCF shows good repeatability from 1000°C to 1100°C and 1000°C to 1200°C without annealing. The process of coating burning of silica fiber not only introduces a loss induced by micro-bending but also imposes a compressive stress on the bare fiber, which contributes to an additional BFS variation at the temperature period of coating burning (272°C-564°C).

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.

Detecting cm-scale hot spot over 24-km-long single-mode fiber by using differential pulse pair BOTDA based on double-peak spectrum

In distributed Brillouin optical fiber sensor when the length of the perturbation to be detected is much smaller than the spatial resolution that is defined by the pulse width, the measured Brillouin gain spectrum (BGS) experiences two or multiple peaks. In this work, we propose and demonstrate a technique using differential pulse pair Brillouin optical time-domain analysis (DPP-BOTDA) based on double-peak BGS to enhance small-scale events detection capability, where two types of single mode fiber (main fiber and secondary fiber) with 116 MHz Brillouin frequency shift (BFS) difference have been used. We have realized detection of a 5-cm hot spot at the far end of 24-km single mode fiber by employing a 50-cm spatial resolution DPP-BOTDA with only 1GS/s sampling rate (corresponding to 10 cm/point). The BFS at the far end of 24-km sensing fiber has been measured with 0.54 MHz standard deviation which corresponds to a 0.5°C temperature accuracy. This technique is simple and cost effective because it is implemented using the similar experimental setup of the standard BOTDA, however, it should be noted that the consecutive small-scale events have to be separated by a minimum length corresponding to the spatial resolution defined by the pulse width difference.

1200°C high-temperature distributed optical fiber sensing by using brillouin optical time domain analysis

An up to 1100°C and 1200°C high-temperature distributed Brillouin sensing based on a GeO 2 -doped single mode fiber (SMF) and a pure-silica photonics crystal fiber (PCF) are demonstrated, respectively. We found that the BFS dependence on temperature of SMF and PCF 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. A Brillouin frequency shift (BFS) hopping is observed in both kinds of fiber between 800°C–900°C in the first annealing process and after that the BFS exhibits the stability and repeatability with a measurement accuracy as high as ±1.42°C for SMF and ±2.6°C for PCF, respectively. The BFS hopping is a highly temperature-dependent behavior, which means that a high-temperature (> 800°C) would accelerate the process of BFS hopping to reach a stable state and after BFS hopping, both SMF and PCF shows good repeatability from 1000°C to 1100°C and 1000°C to 1200°C without annealing. The process of coating burning of silica fiber not only introduces a loss induced by micro-bending but also imposes a compressive stress on the bare fiber, which contributes to an additional BFS variation at the temperature period of coating burning (272°C–564°C).

Characterization of phase-shifted Brillouin dynamic gratings in a polarization maintaining fiber

We numerically calculate and experimentally investigate the characterization of phase-shifted Brillouin dynamic gratings (PS-BDGs) in a polarization maintaining fiber (PMF). A phase-shifted point is induced into the middle of a conventional BDG through phase-modulating one of the two pump pulse, generating a PS-BDG thanks to the stimulated Brillouin scattering (SBS). When the frequency difference between a high frequency pump1 pulse with 1ns and π-1ns and a low frequency pump2 pulse with 100ps is equal to the Brillouin frequency shift of the PMF, a transient PS-BDG with a 3dBbandwidth of 354MHz of the notch spectrum is simulated based on the coupled-wave equations of BDG. By increasing the repetition rate up to 250MHz, an enhanced PS-BDG with a deep notch depth is obtained since the residual acoustic wave of the former SBS process is enhanced by the optical waves of the latter SBS process. Then a proof-of-concept experiment is built to verify the transient PS-BDG and the results show that the notch feature is consistent with the simulation results and the notch frequency of the PS-BDG can be changed by tuning the phase shift Δϕ . The proposed PS-BDGs have important potential applications in optical fiber sensing, microwave photonics, all-optical signal processing and RoF (radio-over-fiber) networks.

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 centrally alignment splicing at the interface between GI-MMF and SMF to excite the fundamental mode only in GI-MMF. The sensing system can resist a minimal bend radius of 1.25mm while maintaining the measurement performance, with which the measured coefficient of strain is 421.6MHz/%. We also demonstrate that the higher-order modes exciting in GI-MMF can be easily influenced by bending, so that the fundamental mode exciting is essential for bend-insensitive distributed sensing.