Significance Distributed fiber sensing and measurement techniques have been given attractive attention in recent decades due to high sensitivity, high resolution, and large capacity. They have found a wide range of applications in the structural health monitoring of civil infrastructures such as bridges and dams, power-transmission line monitoring, oil-gas extraction and pipeline leakage detection, marine geophysical exploration, dynamic measurement, fiber-optic device characterization, fault diagnosis, etc. On the one hand, distributed measurement techniques can be categorized in principle into scattering effects (including Rayleigh backscattering, Brillouin scattering, and Raman scattering) and coupling effects (polarization crosstalk). On the other hand, these techniques can be divided into optical time domain reflectometry (OTDR), optical frequency domain reflectometry (OFDR), and optical coherence domain reflectometry (OCDR). OTDR employs the short and high power light pulse for interrogation, which is an effective tool for long distances. However, the tradeoff between sensing length and spatial resolution restricts the measurements to only meter-level spatial resolutions. OCDR utilizes the low coherence light from a broadband light source. They can offer a micrometer-level spatial resolution, whereas the measurement range is less than a few meters. OFDR is a distributed optical fiber measurement method based on the frequency-modulated continuous wave principle in the optical domain. It obtains the characteristics, such as scattering/reflection/loss and polarization features, along the optical fiber according to the mapping relationship between the Fourier transformation frequency of the interference signal and the characteristic location. In addition, the distribution of external physical fields, such as temperature/stress/strain sensing, can be further acquired. Unlike distributed measurement methods based on time-domain or coherent-domain, OFDR offers superior comprehensive properties, including high spatial resolution, high measurement sensitivity, long measurement distance, broad dynamic range, and high-speed response. However, due to the influence of phase noise, amplitude noise, and environment noise, the performance of OFDR in practice is not satisfactory. In the past few years, various methods have been proposed to compensate for the laser source noise and environment noise to improve the performance of the OFDR. Distributed sensing based on OFDR is also developing towards high performance and multi-parameters. With the continuous expansion and deepening of the application field, OFDR is facing more daunting challenges, which put forward higher requirements for its measurement performance and anti-interference ability. Therefore, it is of great importance and necessary to provide an overview of recent research progress in existing high-performance OFDR tests and sensing techniques to guide the future development direction. Progress We first review the measurement principle of OFDR and summarize key technologies to enhance OFDR system performance, such as the noise sources in distributed measurement (Fig. 1), the degradation mechanisms of the spatial point spread function (Fig. 3), and the error or noise compensation techniques. Then, the measurement limit of distributed sensing based on OFDR is derived, and several methods for improving the sensing accuracy and measurement distance are analyzed (Fig. 14). Subsequently, an outline of the current development status of domestic and foreign OFDR instruments is given (Table 6). Besides, application examples are given in measuring integrated waveguide devices, polarization maintaining fibers, and inside stress sensing of optical fiber coil. Finally, several future research directions of OFDR are prospected. Conclusions and Prospects OFDR systems can provide a good performance of high spatial resolution, high speed, and long measurement and sensing length. This technique can be widely applied to the fields of high-performance fiber optic component measurement and high-precision multi-parameter sensing. In the future, OFDR will continue to develop toward the goal of higher performance, stronger environmental adaptability, and higher measurement cost-effectiveness. The mixed modulation technology such as multi-domain localization (including time, frequency, and coherent domain) and multi-dimensional modulation (including amplitude, phase, and polarization modulation) can provide an effective way to break through the measurement limits and realize the ultra-high performance of OFDR technology. Furthermore, the high-precision OFDR sensing technology should be stepped up to meet the demands of multi-parameter decoupling and anti-interference ability improvement. Correspondingly, for the noise compensation algorithms at present, artificial intelligence and advanced algorithms are all important means for noise suppression capability enhancement and demodulation accuracy improvement. Besides, new requirements are put forward for the small size, low power consumption, and low cost of the core modules in OFDR instruments. With the continuous innovation of OFDR technology theory and the progress of technology development, China's current overall technology level has achieved international parallelism. However, the typical application fields of OFDR technology need to be continuously expanded, and the advantages of the technology need to be continuously emphasized. In this context, the development of domestic OFDR technology should be highly valued and vigorously developed to realize OFDR technology independent control and localization of hardware, including continuous mode-hopping-free tunable laser source, high-speed and high-precision optoelectronic conversion, and data acquisition module. Moreover, the OFDR technology should gradually move towards engineering applications in the field rather than being confined to laboratory measurements. The environmental adaptability of OFDR instruments should be enhanced to ensure that the core technical indicators of distributed testing and sensing are not degraded in different scenarios. Finally, a highperformance distributed specialized measurement and quantitative sensing methodology should be proposed to promote application development in core fields and typical scenarios, which provides a solid foundation and strong support for satisfying the requirements of applications such as testing of military devices, exploration of oil and gas resources, and power and energy monitoring.
