Study On The Stability Of ?-OTDR Distributed Optical Fiber Vibration Sensing System

Distributed optical fiber sensors(DOFS)offer a powerful and cost-effective solution in structural health monitoring.They propose interesting approaches in several applications including long linear assets,such as oil/gas pipelines,hydraulic fracturing,powerlines,railroad tracks,boundaries facilities,bridges,seismic wave detection and biomedical devices.Recently,the phase-sensitive optical time-domain reflectometry(?-OTDR)technique has attracted much more attention for implementation as DOFS due to its advantage of simple configuration,high sensitivity,fast response,easy processing scheme.However,the ?-OTDR system still presents some limitations such as: laser frequency drift(LFD),the finite extinction ratio(ER)of the optical modulator(OM),nonlinear effects,I/Q quadrature imbalances,coherent fading,etc..The LFD determines the minimum detectable vibration frequency by the ?-OTDR system and restricts from applications in the fields where there is need of detection of low frequency events.Both the finite ER of the OM and the nonlinear effects set the maximum measurable distance by the system.The use of optical hybrid to perform the coherent In-phase/Quadrature(I/Q)demodulation in DOFS increased the phase measurement speed but concurrently brings the I/Q imbalances effects.In response to the imperfections caused in ?-OTDR system by the instability of the performance of photonic devices and/or pulse propagation inside the FUT,we proposed in this dissertation several approaches to push further these limitations.The main research contents and achieved results are presented below:(1)The recent progress in different areas of performance metrics of ?-OTDR system such as improvement of the signal-to-noise ratio(SNR),spatial resolution(SR)in the sub-meter range,enlargement of the sensing range,increased frequency response bandwidth over the conventional limits,phase signal demodulation and chirped-pulse ?-OTDR for quantitative measurement have been summarized.(2)We proposed theoretically and demonstrated experimentally that by adapting the probe pulse repetition rate to the frequency of vibration to be measured,consistent intensity change should be always realized in the disturbance region compared to the induced phase-noise change from pulse to pulse.In particular,two different pulse repetition rates were used to demonstrate the effectiveness of the adaptive pulse modulation method(APM)for detection of low-frequency events.Experimentally,the sensing of external vibrations of 5;1;0.5 and 0.1 Hz frequencies with intensity-based ?-OTDR system using a 1 km long optical fiber was achieved with SNR greater than 16 d B.Beyond the efficiency of the proposed method to detect low-frequency vibrations,conjointly the computational cost of the sensing operation was significantly reduced.Moreover,an approach suitable for measuring low-frequency vibrations induced by PZT through direct acquisition of the intensity signal in the disturbance region was established.Various driving voltages applied to the PZT confirmed the consistency of the method.(3)We analyzed the impact of the probe pulse waveform on the visibility of the OTDR trace due to modulation instability(MI)effects.Theoretically,we pointed out the influence of the MI effects phenomenon in the frequency domain basing on the nonlinear Schr?dinger equation.The Fermi-Pasta-Ulam(FPU)recurrence which is the long-term manifestation of the MI effects could be blurred with probe pulse signal whose power level is not constant over a certain time.Experimentally,we demonstrated that a ?-OTDR system probed with triangular or Gaussian pulse is more resistive to nonlinear effects than that of the well-adopted rectangular pulse.(4)The 90° optical/electrical hybrid I/Q demodulator introduced into DOFS system to significantly reduce the phase measurement time concurrently brought the limitation of I/Q quadrature imbalances that might corrupt the accuracy of the measured phase signal.We introduced a novel coherent optical pulse phase rotation reflectometry(COPPRR)system insensitive to I/Q quadrature imbalance.The theoretical basis of the COPPRR system is the optical pulse phase rotation and integration(PRI)technique that stipulates the use of integration of N groups of I/Q signals with regular phase-shift to mutually eliminate the effects of quadrature imbalance carried in the individual pair of signals.Experimental results effectively demonstrated a phase recovery of 100 Hz sinusoidal vibration robust against various types of simulated quadrature imbalance where the I/Q demodulation completely failed to restore the phase signal.Besides,we proposed to adjust the counter-phaser rotation such that it includes the phase decorrelation effects.With this scheme,both the SNR of phase measurement results and intrusion detection were improved.