High-spatial-resolution Distributed Fiber Sensors Based On Backscattered Signals In Optical Fiber

Recently,‘Internet of things'(Io T)has been intensively investigated and widely used in various fields,such as ‘Smart Home',‘Smart City' etc.In Io T,the sensing system is a very important portion since it collects all the useful information,just like the skin and eyes of human beings.Compared with the conventional electrical sensors,optical fiber sensors provide distinctive advantages,such as light weight,compact size,immunity to electromagnetic interference,and large-scale multiplexing capability.In some applications,such as health monitoring of airplane,building or bridge,an ultrahigh spatial resolution of centimeter level or even millimeter level is required.However,there is normally a trade-off relationship between the spatial resolution and other parameters(such as measurement range,measurement speed and measurement accuracy).Therefore,it is of great significance to develop high-performance fiber sensing systems which can simultaneously achieve high spatial resolution,high measurement speed,and relatively long measurement range.In my work,I mainly focus on developing high-performance sensing systems based on optical frequency domain reflectometry(OFDR)and Brillouin optical correlation domain analysis(BOCDA).The novelties and research achievements are listed below:1)Development of long-measurement-range and high-spatial-resolution OFDR system based on wideband ultra-linearly swept optical source(ULSOS).In an OFDR system,the swept optical source is the most important component,which determines the spatial resolution and the measurement range.Optical frequency sweeping can be realized either by inner-cavity tuning of a tunable laser,or by external modulation of a narrow-linewidth laser.To realize ultra-linearly frequency sweeping,the external modulation scheme is employed.To broaden the frequency sweeping range,we propose a high-efficient frequency chirp magnification(FCM)technique based on high-order sideband modulation,injection locking,and cascaded four-wave-mixing(FWM).By using the proposed FCM technique,a wideband ULSOS with sweeping range of >100 GHz and sweeping error of <200 k Hz has been realized,and an OFDR system with a measurement range of 2 km and a spatial resolution of 1.1 mm for break-point monitoring has been demonstrated.Moreover,the effect of the phase noise is also analyzed,and two phase noise compensation methods based on optical fiber delay loop(OFDL)and ultra-fast frequency sweeping are proposed.Based on the proposed OFDR system,health monitoring of fiber network and distributed temperature/strain sensing have been realized.Besides,benefiting from the high performance of the ULSOS,a phase-dispersion spectroscopy has also been proposed,which provides a measurement range of ?100 GHz and a spectral resolution of <200 k Hz.2)High-performance BOCDA based on FCM and convexity extraction algorithm.In a Brillouin-based sensing system,the temperature/strain information can be obtained directly by measuring the Brillouin frequency shift(BFS).Here,we focus on the BOCDA system,which provides high spatial resolution and high measurement accuracy.In BOCDA,the number of effective sensing points is proportional to the spectral bandwidth of the sinusoidally-modulated laser source.To increase the number of effective sensing points,an FCM scheme based on FWM is employed to broaden the spectral bandwidth of the laser source.A measurement range of >1 km along with a spatial resolution of <10 cm has been realized for distributed temperature/strain sensing.Besides,a convexity extraction algorithm is proposed to suppress the background noise.A dynamic range of >800 MHz and an improving factor of 5 for spatial resolution are realized.3)Ultra-fast BOCDA systems based on the injection-locking scheme.In BOCDA,a lock-in amplifier(LIA)is normally employed to remove the intensity noise of the laser source and improve the single-to-noise ratio(SNR).However,due to the limited bandwidth of the LIA,the measurement speed of BOCDA is normally less than 5,000 points/s.To improve the measurement speed,an injection locking scheme instead of the LIA is employed to remove the intensity noise of the laser source.An ultrahigh measurement speed of 1 M points/s along with a cm-level spatial resolution is achieved.In BOCDA,a so-called pump-probe amplification configuration is employed to generate stimulated Brillouin scattering(SBS)interaction.One major drawback is that it needs double-end access to the fiber under test,which tremendously reduces the flexibility in practical application.To improve the flexibility of BOCDA,a single-end-access BOCDA system has also been proposed.Besides,to further improve the measurement speed,a dual-slope-assisted BOCDA(DSA-BOCDA)scheme is also proposed,which provides a maximum repetition rate of 5 k Hz for distributed temperature/strain sensing.Benefiting from the high measurement speed and high spatial resolution of DSA-BOCDA,dynamic tracking of the propagation of mechanical wave is realized.Overall,the main purpose of this thesis is to propose novel techniques to improve the performance of high-spatial-resolution distributed fiber sensors.The sensing systems reported in this paper are promising to further extend the application fields of distributed fiber sensors.