Tuning nonlinearity of the laser is the main source of deterioration of the spatial resolution \nin optical frequency -domain reflectometry (OFDR) system. In this paper, we develop methods for \ntuning nonlinearity correction in an OFDR system from the aspect of data acquisition and post -\nprocessing. An external clock based on a zero -crossing detection is researched and implemented \nusing a customized circuit. Equal -spacing frequency sampling is, therefore, achieved in real -time. \nThe zero -crossing detection for the beating frequency of 20 MHz is achieved. The maximum sensing \ndistance can reach the same length of the auxiliary interferometer. Moreover, a nonlinearity \ncorrection method based on the self -reference method is proposed. The auxiliary interferometer is \nno longer necessary in this scheme. The tuning information of the laser is extracted by a strong \nreflectivity point at the end of the measured fiber. The tuning information is then used to resample \nthe raw signal, and the nonlinearity correction can be achieved. The spatial resolution test and the \ndistributed strain measurement test were both performed based on this nonlinearity correction \nmethod. The results validated the feasibility of the proposed method. This method reduces the \nhardware and data burden for the system and has potential value for system integration and \nminiaturization.
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