Distributed disturbance sensing device and the related demodulation method based on polarization sensitive optical frequency domain reflectometry

A distributed disturbance sensing device based on polarization sensitive optical frequency domain reflectometry (OFDR), wherein the device consists of:

• Tunable laser source:
• the tunable laser source is used to provide light source for the optical frequency domain reflectometry system, which consists of an ultra-narrow linewidth tunable fiber laser or an external cavity semiconductor laser;
• Tunable laser source driving module:
• the tunable laser source driving module connects with tunable laser source and is used to provide tuning driving for the tunable laser source, which is a ultra low-ripple voltage driver and the driver consists of a high-precision D/A convertor, a Field programmable gate array (FPGA) and an low-noise amplifier;
• A optical splitter with splitting ratio of 1:99:
• the optical splitter splits the emergent light of laser at the splitting ratio of 1:99, split the light to laser optical frequency and phase monitoring module and core interference module;
• The laser optical frequency and phase monitoring module:
• this module collects and monitors the laser optical frequency and phase, which configuration is either depolarized Michelson interferometer, or Mach-Zehnder interferometer based on 3×3 coupler;
• Core interference module:
• the core interference module is the core of the optical frequency domain reflectometry (OFDR), which consisting of a tunable polarization-maintaining (PM) attenuator, a PM splitter, a reference arm, a test arm and a PM coupler, wherein the tunable PM attenuator is used to adjust the light intensity;
• the PM splitter splits the lights to the reference arm and the test arm of the interferometer at one of the following ratios, such as 50:50, 20:80, 30:70 or 40:60;
• the reference arm consists of a phase modulator and a 45° angle aligner. The reference arm fiber is PM fiber. The modulation frequency value of the phase modulator is based on the maximum beat frequency, which processes the signals by the high-pass filtering The function of the phase modulator is that reduces Rayleigh scattering coherent noises and ghost peaks. The 45° angle aligner is used to align the axis to 45° between two section of the axis of PM fiber to ensure the fast and slow axis of PM fiber on the reference arm having equal light intensity;
• the test arm of the core interference module is connected to the polarization generator of the polarization generating module. The back scattering light of the sensing optical fibers through the high-speed optical switch and the circulator of the polarization generating module into the second port of the PM coupler, whereas the reference arm light of the core interference module goes into the first port of PM coupler. The back scattering light of the sensing fibers and the reference arm light form beat-frequency interference in the PM coupler, and generate beat-frequency signal that goes through the third and forth port of the PM coupler into the polarization splitting balanced detecting module;
• Polarization generating module:
• the polarization generating module consists of a polarization state generator and a PM circulator, wherein the polarization state generator generate the light of several fixed polarization state, including linear polarized light, 45° linear polarized light, left-handed or right-handed circular polarized light. The polarization generator provides reference polarization state for the following algorithms;
• the circulator is a dual-axis polarization-maintaining circulator, for connecting polarization generating module, high-speed optical switch and the polarization-maintaining coupler of the core interference module. The detailed connecting method is as follows:
• the light travels through the first port of the circulator into the circulator, and goes through the second port of the circulator into the high-speed optical switch. The back scattering light of the sensing fibers connected to the high-speed optical switch returns back through the second port of the circulator, goes through the third port of the circulator into the first and second ports of the polarization generating module of the core interference module;
• High-speed optical switch:
• the high-speed optical switch links the sensing optical fibers, which enables this system to use several optical fibers, strengthens the sensing distance of the system and establish of large-scale sensing network;
• Polarization splitting balanced detecting module:
• the polarization splitting balanced detecting module consists of two polarization splitters and two balanced detectors, wherein the polarization splitters split the beat-frequency signal outputted from the core interference module into two components of the fast and slow axes with orthogonal polarization property. Afterwards, the fast axis component of the two polarization splitters goes into the first balanced detector, while the slow axis component moves into the second balanced detector;
• High-speed data acquisition module:
• the high-speed acquisition module is used for collecting the electric signals from the balanced detectors of the polarization splitting balanced detecting module and the laser optical frequency and phase monitoring module;
• Computer:
• the computer is used to process the digital signal from the high-speed data acquisition module, this module function is that process laser optical frequency and phase from laser optical frequency and phase module, demodulating the beat-frequency signal generated by the core interference module, and providing control signal to the tunable light source driving module;
• Sensing optical fibers:
• the sensing optical fibers are either common communication single-mode fiber or vibrating sensing optical fiber, and the fiber is implemented in fence and large-scale civil structures for disturbance monitoring.