Remote phase-sensitive optical frequency domain reflectometry

We propose a common-path, self-referenced phase-sensitive optical frequency domain reflectometry (φ-OFDR) system capable of creating a remote sensing window at the far end of an ultra-long fiber. By utilizing the reflection from the fiber's far end as the local oscillator, the system inherently compensates for laser phase noise accumulated over the lead fiber, effectively decoupling the sensing performance within the window from the constraints of the total transmission distance. To further enhance robustness, what we believe to be a novel single-ended polarization diversity receiving scheme is developed to mitigate polarization fading while preserving the common-path stability. Experimental validation was performed using a standard DFB laser with a large linewidth of 1.3 MHz (corresponding to a coherence length of ∼50 m) and a 75-km single-mode lead fiber. A spatial resolution of 2 cm and a vibration frequency response of up to 2 kHz were successfully demonstrated within the remote sensing window located at the far end, which consisted of a 1-m fiber segment containing weak reflectors inscribed by a femtosecond laser. These results establish a practical and cost-effective methodology for localized, high-precision quasi-distributed sensing in scenarios with long lead-in distances, such as deep-well geophysical exploration and structural health monitoring.