Sub-millimeter resolution and high-precision φ-OFDR using a complex-domain denoising method.

Phase noise is one of the main obstacles to achieve high spatial resolution, high precision, and large measurement range in φ-OFDR. Here, we proposed a complex-domain denoising method to achieve unwrapping of phase signals. In this method, the wrapped phase was used to construct a complex signal, and then both real and imaginary parts are denoised by using a wavelet packet. The two sets of denoised signals are reconstructed into a complex form, allowing to obtain an unwrapped phase. Additionally, the spatial position correction algorithm addresses the phase decoherence from strain accumulation. Finally, a high numerical aperture optical fiber is used to enhance the Rayleigh scattering intensity by 15 dB. The comprehensive approach yields remarkable results: a sensing resolution of 0.89 mm, a root mean square error of 1.5 µε, and a maximum strain sensing capability of 2050 µε.