Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry （BOTDR） are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser （FL） with 4-kHz linewidth at 3 dB and a distributed feedback （DFB） laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals （backscattered Brillouin light and local oscillator （LO）） is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.

Digital coherent detection research on Brillouin optical time domain reflectometry with simplex pulse codes

The digital coherent detection technique has been investigated without any frequency-scanning device in the Brillouin optical time domain reflectometry （BOTDR）, where the simplex pulse codes are applied in the sensing system. The time domain signal of every code sequence is collected by the data acquisition card （DAQ）. A shift-averaging technique is applied in the frequency domain for the reason that the local oscillator （LO） in the coherent detection is fix-frequency deviated from the primary source. With the 31-bit simplex code, the signal-to-noise ratio （SNR） has 3.5-dB enhancement with the same single pulse traces, accordant with the theoretical analysis. The frequency fluctuation for simplex codes is 14.01 MHz less than that for a single pulse as to 4-m spatial resolution. The results are believed to be beneficial for the BOTDR performance improvement.

Fiber optic monitoring of an anti-slide pile in a retrogressive landslide

Anti-slide piles are one of the most important reinforcement structures against landslides,and evalu-ating the working conditions is of great significance for landslide mitigation.The widely adopted analytical methods of pile internal forces include cantilever beam method and elastic foundation beam method.However,due to many assumptions involved in calculation,the analytical models cannot be fully applicable to complex site situations,e.g.landslides with multi-sliding surfaces and pile-soil interface separation as discussed herein.In view of this,the combination of distributed fiber optic sensing(DFOS)and strain-internal force conversion methods was proposed to evaluate the working conditions of an anti-sliding pile in a typical retrogressive landslide in the Three Gorges reservoir area,China.Brillouin optical time domain reflectometry(BOTDR)was utilized to monitor the strain distri-bution along the pile.Next,by analyzing the relative deformation between the pile and its adjacent inclinometer,the pile-soil interface separation was profiled.Finally,the internal forces of the anti-slide pile were derived based on the strain-internal force conversion method.According to the ratio of calculated internal forces to the design values,the working conditions of the anti-slide pile could be evaluated.The results demonstrated that the proposed method could reveal the deformation pattern of the anti-slide pile system,and can quantitatively evaluate its working conditions.

Applicability of Time Domain Reflectometry for Yuhuangge Landslide Monitoring

Yuhuangge （玉皇阁） landslide in Wushan （巫山）, Chongqing （重庆）, is one of the focal monitoring geological hazards in the Three Gorges Reservoir. Time domain reflectometry （TDR） and in-place inclinometers were arranged to monitor the deep deformation. Time domain reflectometry is based on transmitting an electromagnetic pulse into a coaxial cable grouted in rock or soil mass and watching for reflections of this transmission due to cable deformity induced by the ground deformation. Comparing the monitoring data of No. 5 Station, in the middle profile of the landslide, from June to December of 2008, the depth of slip surface determined by TDR is -33.58 m, which is consistent with the geological condition of the borehole nearby. The deformation curve trend of the TDR and inclinometer is similar, and it is uniform with the deformation caused by the Three Gorges Reservoir 175 m experimental impoundment. Further, TDR can monitor the tiny deformation accurately. Therefore, TDR is applicable to monitor the Yuhuangge landslide deep deformation and reflect the deformation characteristics well. It is significant to promote the application of TDR in landslide monitoring.

Interferometric Distributed Sensing System With Phase Optical Time-Domain Reflectometry

We demonstrate a distributed optical fiber sensing system based on the Michelson interferometer of the phase sensitive optical time domain reflectometer （q0-OTDR） for acoustic measurement. Phase, amplitude, frequency response, and location information can be directly obtained at the same time by using the passive 3 ×3 coupler demodulation. We also set an experiment and successfully restore the acoustic information. Meanwhile, our system has preliminary realized acoustic-phase sensitivity around -150 dB （re rad/μPa） in the experiment.

Complex and thermodynamic properties of polar liquids using time domain reflectometry

The study of complex properties in a binary mixture of 1,2-dichloroethane(DE)and n-methylformamide(NMF)polar liquids has been carried out in the frequency range of 10 MHz to 30 GHz for 11 different concentrations using time domain reflectometry technique at 283,288,293 and 298 K temperatures.Complex property of binary liquids indicates the type of distribution of the dielectric relaxation time.The Bruggeman parameter gives the information about molecular interactions within binary polar liquids.Thermodynamic parameter deals with the passing of a dipole across a potential barrier which separates the minima of energy.

Dielectric relaxation studies of binary mixture of β-picoline and methanol using time domain reflectometry at different temperatures

Complex permittivity spectra of binary mixtures of varying concentrations of β-picoline and Methanol(MeOH)have been obtained using time domain reflectometry(TDR)technique over frequency range 10 MHz to 25 GHz at 283.15,288.15,293.15 and 298.15K temperatures.The dielectric relaxation parameters namely static permittivity(ε),high frequency limit permittivity(ε∞1)and the relaxation time(τ)were determined by fiting complex permittivity data to the single Debye/Cole-Davidson model.Complex nonlinear least square(CNLS)fiting procedure was carried out using LEVMW software.The excess permittivity(ε_(0)^(E))and the excess inverse relaxation time(1/τ)^(E) which contain information regarding molecular structure and interaction between polar-polar liqids were also determined.From the experimental data,parameters such as effective Kirkwood correlation factor(geff),Bruggeman factor(fB)and some thermo dynamical parameters have been calculated.Excess parameters were fitted to the Redlich-Kister polynomial equation.The values of static permittivity and relaxation time increase nonlinearly with increase in the mol-fraction of MeOH at all temperatures.The values of excess static permittivity(ε_(0)^(E))and the excess inverse relaxation time(1/τ)^(E) are negative for the studied β-picoline MeOH system at all temperatures.

Dielectric properties of benzylamine in 1,2,6-hexanetriol mixture using time domain reflectometry technique

The dielectric properties of binary mixtures of benzylamine-1,2,6-hexantriol mixtures at different volume fractions of 1,2,6-hexanetriol have been measured using Time Domain Reflectometry(TDR)technique in the frequency range of 10 MHz to 30 GHz.Complex permittivity spectra were fitted using Havriliak-Negami equation.By using least square fit method the dielectric parameters such as static dielectric constant(ε_(0)),dielectric constant at high frequency(ε_(∞)),relaxation time
τ(ps)and relaxation distribution parameter(β)were extracted from complex permittivity spectra at 25℃.The intramolecular interaction of different molecules has been discussed using the Kirkwood correlation factor,Bruggeman factor.The Kirkwood correlation factor(gf)and effective Kirkwood correlation factor(g^(eff))indicate the dipole ordering of the binary mixtures.

Complex permittivity spectra of binary polar liquids using time domain reflectometry

The study of complex properties in a binary mixture of polar liquids has been carried out in the frequency range of 10 MHz to 30 GHz at 293 K and 298 K temperatures using time domain reflectometry.The complex properties of polar liquids in binary mixture give information about the frequency dispersion in the dielectric permittivity(ε')and dielectric loss(ε").The information regarding the orientation of electric dipoles in a polar liquid mixture is given by Kirkwood parameters.The Bruggeman parameters are used as the indicator of liquid1 and liquid2 interaction.Molar entropy(ΔS)and molar enthalpy(ΔH)are also discussed at the end of the paper.

Experimental study on slope sliding and debris flow evolution with and without barrier

A constitutive model on the evolution of debris flow with and without a barrier was established based on the theory of the Bingham model. A certain area of the Laoshan Mountain in Nanjing, Jiangsu Province, in China was chosen for experimental study, and the slope sliding and debris flow detection system was utilized. The change curve of the soil moisture content was attained, demonstrating that the moisture content of the shallow soil layer increases faster than that of the deep soil layer, and that the growth rate of the soil moisture content of the steep slope is large under the first weak rainfall, and that of the gentle slope is significantly affected by the second heavy rainfall. For the steep slope, slope sliding first occurs on the upper slope surface under heavy rainfall and further develops along the top platform and lower slope surface, while under weak rainfall the soil moisture content at the lower part of the slope first increases because of the high runoff velocity, meaning that failure occurring there is more serious. When a barrier was placed at a high position on a slope, debris flow was separated and distributed early and had less ability to carry solids, and the variation of the greatest depth of erosion pits on soil slopes was not significant.

Health Monitoring of Long-Haul Fiber Communication System Using Chaotic OTDR

A novel chaotic optical time-domain reflectometry(OTDR)-based approach was proposed for monitoring long-haul fiber communication systems with multiple fiber segments. The self-phase modulation and group velocity dispersion effects of the optical cable was considered in demonstrating the proof-of-concept experiment and simulation. In experiments, the correlation peaks are clearly obtained from the correlation trace between the reference and reflected(or scattered) light signals propagating in three optical-fiber segments. The technique affords a high spatial resolution of 2 m, and further long-haul fiber simulations indicate that the sensing distance can be more than 3300 km. Thus, the new proposed technique can be effectively applied for health monitoring of long-haul fiber communication systems.

Development of Fully-Distributed Fiber Sensors Based on Brillouin Scattering

Brillouin scattering based optical fiber sensors(BOFS)have the unique advantages over other sensors such as long distance,fully distributed,and multi-parameter sensing.The progresses on the development of BOFS technology in Nanjing University are reviewed.The key technologies to make BOFS with ultra-long distance,high spatial resolution,and fast measuring speed are discussed and realized.

An Improved Modeling of TDR Signal Propagation for Measuring Complex Dielectric Permittivity

Time domain reflectometry（TDR） is a measurement technique based upon transmission line theory. The solutions of transmission line equations are reformulated in terms of independent physical properties, instead of coupled per-unit-length circuit parameters. The complete TDR response is effectively modeled by a non-uniform transmission line using the non-recursive ABCD matrix approach. Approaches to calibrate line parameters and perform TDR measurements based upon such model are introduced with an example on dielectric spectroscopy. TDR modeling in terms of decoupled physical parameters and non-recursive algorithm allows more convenient calibration of line parameters and facilitates interpretation of TDR measurements.

Study of Ф-OTDR Stability for Dynamic Strain Measurement in Piezoelectric Vibration

In a phase-sensitive optical-time domain reflectometry （Ф-OTDR） system, the challenge for dynamic strain measurement lies in large intensity fluctuations from trace to trace. The intensity fluctuation caused by stochastic characteristics of Rayleigh backscattering sets detection limit for the minimum strength of vibration measurement and causes the large measurement uncertainty. Thus, a trace-to-trace correlation coefficient is introduced to quantify intensity fluctuation of Ф-OTDR traces and stability of the sensor system theoretically and experimentally. A novel approach of measuring dynamic strain induced by various driving voltages of lead zirconate titanate （PZT） in Ф-OTDR is also demonstrated. Piezoelectric vibration signals are evaluated through analyzing peak values of fast Fourier transform spectra at the fundamental frequency and high-order harmonics based on Bessel functions. High trace-to-trace correlation coefficients varying from 0.824 to 0.967 among 100 measurements are obtained in experimental results, showing the good stability of our sensor system, as well as small uncertainty of measured peak values.