A method based on equal frequency resampling is proposed to suppress laser nonlinear frequency sweeping for the ultimate spatial resolution in optical frequency domain reflectometry. Estimation inaccuracy of the sweeping frequency distribution caused by the finite sampling rate in the auxiliary interferometer can be efficiently compensated by the equal frequency resampling method. With the sweeping range of 130 nm, a 12.1 µm spatial resolution is experimentally obtained. In addition, the sampling limitation of the auxiliary interferometer-based correction is discussed. With a 200 m optical path delay in the auxiliary interferometer, a 21.3 µm spatial resolution is realised at the 191 m fibre end. By employing the proposed resampling and a drawing tower FBG array to enhance the Rayleigh backscattering, a distributed temperature sensing over a 105 m fibre with a sensing resolution of 1 cm is achieved. The measured temperature uncertainty is limited to ±0.15 °C.

By effectively suppressing the nonlinear sweep noise and random range of wavelength sweep in the optical frequency domain reflectometer, the theoretical spatial resolution and uniform sweep distribution are delivered for high sensing accuracy. A strain accuracy of ±0.51 μϵ is realised with a 5 mm sensing resolution, while the accuracy is ±5.89 μϵ with a 1 mm sensing resolution. Theoretical limitation between the strain accuracy and sensing resolution is further studied for the sub-millimetre resolution sensing. It is found that signal to noise ratio and frequency bandwidth of the calculated cross-correlation are critical factors in measuring accuracy. Increasing the sweep range can provide a better spatial frequency step for a high signal to noise ratio in the cross-correlation. With a 130 nm sweep range, the measurement accuracy is limited to ±19.31 μϵ with a 0.5 mm sensing resolution. Besides, for the long-distance sensing of 104 m, the measurement accuracy is ±8.72 μϵ with a 1 mm sensing resolution.

Phase-sensitive optical frequency domain reflectometer (Φ-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 Φ-OFDR. By efficiently eliminating random hopping in the phase unwrap and suppressing the laser-induced nonlinear sweep for the theoretical spatial resolution, the proposed Φ-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 Φ-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 µm spatial resolution, the proposed Φ-OFDR is able to retain accurate distributed measurement compared to conventional C-OFDR methods. Besides, the computation time of the Φ-OFDR is only 3.2% of the C-OFDR.

Optical frequency domain reflectometry (OFDR) has high spatial resolution and measurement accuracy, driving its popularity in various fields. Integration of OFDR technology has made it accessible, cost-effective and deployable in many applications, including battery management and photonic integrated circuits. An integrated OFDR system based on Rayleigh backscattering and silicon photonics technology on an SOI platform has been developed for the first time. The system's simplified configuration was simulated, fabricated and characterized in detail, achieving an experimental spatial resolution of 8.28 μm, matching the theoretical level. This system shows high potential for sensing, monitoring and detection where precise spatial information is crucial. OFDR's accessibility and high performance in distributed measurements make it a promising technology for future advancements.