In the coherent -OTDR system, the phase signal is retrieved based on the reference point and the observation point which are off and closer to the two sides of step of the phase change. In the experiment, the optical pulse with the changed peak power, width or shape is injected into the fiber for interrogating the change of the quantitative characteristic of the measured phase signal. When the pulse width is fixed at 200 ns and its peak power is adjusted from 14 dBm to -23 dBm, the amplitude is slightly increased from 17.3575 rad to 17.4411 rad as long as the Rayleigh backscattering signal can be found in the electrical signal. Changing the pulse width from 260 ns to 80 ns when the peak power is fixed at 14 dBm, the maximum amplitude and the minimum amplitude of the measured phase signal are 17.4625 rad to 17.4509 rad, respectively. When the arbitrary shape of the optical pulse generated from the MZI structure with a changed delay fiber from 3 m to 6 m, the amplitude varies from 17.4558 rad to 17.4819 rad. For every measurement, the change of frequency is also small. And the small value of standard deviation supports the accuracy of the measurement. All the measurements show that the changed pulse nearly has no impact on the quantitative characteristic of the measured phase signal in the coherent -OTDR system.
The absolute difference (phase jumping) between two adjacent phases should be less than pi in order to accurately recover the phase signal with traditional phase unwrapping algorithm, which limits the capability of quantitative measurement for traditional PHI-OTDR. In this article, a new process of phase unwrapping with an auxiliary light in coherent PHI-OTDR is proposed, so the resolvable phase jumping is extended to a larger value beyond pi and the capability of phase unwrapping is improved. In this PHI-OTDR system, wrapped differential phases between a reference position and the subsequent positions of fiber are firstly acquired for all probe light pulses. And wrapped differential phases of interleaving pulse sequence are unwrapped using the traditional phase unwrapping algorithm. Then, positions where the differential phases are correctly unwrapped are obtained from the discontinuously linear profile of the phase change curves of the original probe light. Based on these positions, the information of perturbation can be correctly retrieved. In experiment, the new process of phase unwrapping with the auxiliary for PHI-OTDR is executed and a perturbation signal which induces a maximum phase jumping of 3.7154 rad is well reconstructed. More experimental results all demonstrate the validity of the proposed method.
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