High-Resolution φ-OFDR Using Phase Unwrap and Nonlinearity Suppression

Phase-sensitive optical frequency domain reflectometer (phi-OFDR) is investigated to deliver an accurate distributed measurement with high spatial resolution. It is found that random phase noise and quadrant discrimination during phase calculation are the main reasons for the random hopping in phi-OFDR. By efficiently eliminating random hopping in the phase unwrap and suppressing the laser-induced nonlinear sweep for the theoretical spatial resolution, the proposed phi-OFDR is proved to be able to decouple the limitation between resolution and accuracy in coherent OFDR (C-OFDR). Distributed strain measurement with 20 mm spatial resolution in phi-OFDR is obtained and analysed. Measurement with little deviation and uniform sensitivity between applied strain and phase change both validate the efficient noise suppression for extreme resolution measurement. Then the influence of the initial sweep frequency between two times measurements is studied. With a further reduced 800 mu m spatial resolution, the proposed phi-OFDR is able to retain accurate distributed measurement compared to conventional C-OFDR methods. Besides, the computation time of the phi-OFDR is only 3.2% of the C-OFDR.