Significance The distributed optical fiber sensor based on Brillouin scattering can continuously measure temperature or strain along the optical fiber and has become a research hotspot at home and abroad. Currently, it is widely used in many fields such as modern industries, civil structural health monitoring, and national defense security. There are four main types of Brillouin distributed optical fiber sensing technologies: Brillouin optical time domain reflectometry (BOTDR),Brillouin optical time domain analysis (BOTDA), Brillouin optical correlation domain reflectometry (BOCDR),and Brillouin optical correlation domain analysis (BOCDA). Among them, the BOCDA has the unique advantages of high spatial resolution, high-speed measurement, and random accessibility of measuring position, so it has extremely high potential application value. According to their different operating principles, this paper reviews the research progress of sine-frequency-modulation BOCDA (sine-FM BOCDA), phase-coded BOCDA, and broadband-source-based BOCDA in recent years. Broadband-source-based BOCDA includes amplified-spontaneous-emission-based BOCDA (ASE-based BOCDA) and chaos-based BOCDA,and the latter is proposed by our group. Additionally, in view of their limiting factors of sensing distance, spatial resolution, and measurement speed, the performance improvement of these BOCDA technologies is analyzed, and their future developments are also prospected.

Progress The BOCDA is a novel distributed sensing method based on stimulated Brillouin scattering (SBS). The interference of the pump wave and the counter-propagating probe wave results in stimulated Brillouin acoustic field through electrostriction effect. The two waves are modulated in phase or frequency by the same waveform, and their frequencies are detuned around the Brillouin frequency offset of the fiber. The magnitude of the SBS acoustic field is stimulated at a specific position referred as correlation peak (CP), and the spatial resolution is only determined by the full-width at half-maximum (FWHM) of the CP ( Fig. 1). Consequently, compared to time-domain technology with inherent predicament of 1-m-spatial resolution, the correlation-based method does not suffer from the spatial resolution limitation. For the sine-FM BOCDA, the highest spatial resolution (1.6 mm) is achieved by applying the beat lock-in detection scheme, but the sensing distance is limited to less than 5 m ( Fig. 4). In order to increase the sensing distance, a differential measurement scheme with dual modulation and temporal gating is proposed to achieve a measurement range of 10.5 km, but the measurement time is too long [ Fig. 7 (b)]. Later, time-domain data processing is proposed in the differential measurement BOCDA system, which effectively improves the measurement speed of the system ( Fig. 8). For the phase-coded BOCDA, by using the short optical pulse source modulated by the pseudorandom sequence (PRBS), a highest spatial resolution (0.64 mm) of the current BOCDA system is realized ( Fig. 13). By using Golomb codes to replace PRBS and applying temporal gating, the coding noise is effectively suppressed ( Fig. 14), and then the optimal phase-coded BOCDA system based on temporal gating is proposed, by which the longest sensing distance (17.5 km) and the maximum number of resolution points (beyond 10 ⁶) are achieved. The improvement of the measurement speed of the phase-coded BOCDA system is mainly studied from three aspects: the decrease of the number of averaging (incoherent sequence compression, Fig. 15), the reduction of the position addressing (double-pulse-pair analysis, Fig. 16), and the elimination of frequency scanning (transient SBS gain analysis without spectral scanning, Fig. 17). Therefore, the measurement speed is significantly promoted, and the dynamic monitoring could be further explored. For the broadband-source-based BOCDA, the millimeter-level spatial resolution could be easily achieved because the FWHM of the CP is determined by the source bandwidth. Consequently, spatial resolution of 4 mm is realized with ASE source of 25 GHz despite that the sensing distance is only 5 cm ( Fig. 19). In order to get better performance, the chaos-based BOCDA is proposed by our group, higher resolution of 3.5 mm is achieved with chaotic laser of 10 GHz, and the sensing distance reaches 165 m (Figs. 23--24). Moreover, by suppressing the time delay signature and using the time-gated scheme, the noise background is largely inhibited, and the sensing distance is greatly improved to 10.2 km with a spatial resolution of 9 cm ( Fig. 25).

Conclusion and Prospect The sine-FM BOCDA system is easy to achieve high spatial resolution by adjusting higher modulation amplitude and has preferable signal-to-noise ratio (SNR). However, the direct modulation of semiconductor laser induces frequency modulation superimposed on amplitude modulation through carrier density modulation and temperature change effects, which leads to a predicament that is hardly to set both modulation frequency and modulation amplitude at a higher bandwidth simultaneously. In order to obtain mm-order spatial resolution, a special laser diode (LD), three-electrode LD, is used as light source, and intensity modulation (IM) is used synchronously to compensate for intensity chirp, which leads to complexity and high cost of the system. Phase-coded BOCDA system combines long sensing distance with high spatial resolution, and the measurement speed is also greatly improved. The off-peak Brillouin interactions will introduce coding noise and degrade the sensing performance of the system, although the application of Golomb codes and temporal gating has effectively improved the SNR. In addition, to obtain a higher spatial resolution, a higher modulation rate is required, but high-performance modulation devices are seldom available, which will further increase the cost and the complexity of the system. The spatial resolution of the broadband-source-based BOCDA system can easily reach millimeter level. However, the ASE-based BOCDA system has poor SNR and the sensing distance is largely limited. In the chaos-based BOCDA, the spatial resolution is theoretically determined by the chaotic bandwidth. The chaos of 10 GHz is easily obtained, and the sensing distance is successfully extended to 10.2 km. However, the main weakness of the current chaos-based BOCDA is the location of a single CP scanned by the variable delay line, which results in a time-consuming system and poor practicability. In general, these BOCDA technologies involved in this paper have made a significant progress and they have been used for static temperature or strain measurement. In modern industry, the demand for distributed measurement of dynamic parameter continues to increase. Domestic and foreign researchers have made some progresses in these aspects, including fast frequency sweeping and slope-assisted method, but the dynamic strain range and vibration frequency still have great potential for development. In summary, the BOCDA will develop in the direction of long sensing distance, high spatial resolution, and high-speed real-time measurement, and has broad application prospects in modern industry and civil structural health monitoring.