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A new frequency coding method is proposed and experimentally demonstrated that employs a phase modulator driven by sequential radio frequency and a synchronously controlled acoustooptic modulator to generate time division multiplexing probe pulses for coherent optical time domain reflectometry (C-OTDR). Each probe pulse contains two probe frequencies that are symmetrical regarding the frequency of local oscillator (LO). Heterodyne between the backscattered Rayleigh light of the dual frequency probe pulse and LO generates the same intermediate frequency. Experimental results show that the new C-OTDR, using ten frequency-coded probe pulses, brings an 8.0-dB dynamic range enhancement, compared with conventional C-OTDR.