Compensation Of Imbalances Of 90° Optical Hybrid Used In Distribute Fiber Sensing System

With the continuous improvement of laser performance,the higher stability of the wavelength,the lower phase noise and the emergence of narrow linewidth lasers,the phase of laser and polarization are stable enough for the application of information transformation.Phase detection has been a routine practice in optical communication and radar system,and also has a promising prospect in distributed optical fiber sensing field(DOFS).The optical phase is hardly detected straightly,so coherent detection as a feasible method for phase detection is employed,through various kinds of mixers,such 2×2 coupler and 90° optical hybrid.Coherent decetion relates amplitude fluctuation to phase fluctuation,which also brings about the enhancement of signal-to-noise ratio(SNR).The influences of mismatches of the hybrid are noticed,and some compensation methods are proposed in optical communication.In DOFS,coherent detection,as a method for phase detection,has been discussed in some papers recently.The hybrid is used in ?-OTDR with coherent detection.However,the existent compensation methods are not suitable for ?-OTDR signals.Accroding to characterizes of heterodyne ?-OTDR signals,a suitable method is proposed in this paper,and real-time compensation of mismatches of the hybrid with heterodyne ?-OTDR signals is achieved.The work of this paper is in the following:1.This paper briefly illustrate the structure of hybrid and analyzes the reasons leading to possible several sorts of mismatches among the outputs,which are defects of internal devices during creating and deviations of assembling.2.In this paper,MATLAB is used to simulate different effects of different kinds of mismatches of hybrid and the different characteristics of them.3.A method,eclipse fitting,has tried in this paper,but it is inadequate.Combined with the characteristics of heterodyne ?-OTDR signals,a more suitable method for the signals is proposed.4.A heterodyne DAS system is set.This system's data is used to calculate and compensate the mismatches.The compensated data is used to demodulate the perturbation in real time and compared with the demodulated perturbation from uncompensated data.