Brillouin optical time-domain analyzer (BOTDA) has been widely studied and employed in academia and industry due to its capability of providing information about the spatial distribution of targeted quantities over a long optical fiber. Several advanced techniques have been proposed during the last decade to further enhance the signal-to-noise ratio (SNR) beyond the conventional single-pulse BOTDA. As one of the most efficient approaches, optical pulse coding technology has been extensively developed, and it launches one or several trains of pulses into the sensing fiber, with the exploitation of code types such as Golay, Simplex, Cyclic, genetic-optimized codes, and their derivatives. Meanwhile, this technology can assist BOTDA to further achieve considerable SNR improvement without compromising the spatial resolution and measurement time and thus has significant superiority and application prospects. Despite many corresponding advances, there are still a series of challenges in cost reduction and performance improvement. Therefore, it is important and necessary to summarize the existing research to inspire the future development of this field more rationally. Focusing on the BOTDA sensors based on optical pulse coding, we introduce the principle of several mainstream optical pulse coding technologies employed in distributed fiber sensors, and discuss the research progress of BOTDA sensors based on optical pulse coding in recent years.

The fundamentals of conventional optical pulse coding technologies applied in BOTDA sensors are reviewed in detail, including unicolor unipolar codes (Golay, Simplex, Cyclic, and deconvolution-based codes), color codes, and bipolar codes. The theoretical coding gain of such present mainstream coding schemes is also summarized, all of which can be approximately regarded as proportional to the square root of the code length.

It is significant and necessary to improve and optimize the coding system for reducing the influence of system imperfections and get as close as possible to the theoretical coding gain. With the development of BOTDA sensors based on optical pulse coding technology in the past decades, a series of optimization schemes have been proposed in coding parameters, post-processing algorithms, coding system designs, and coding types (Fig. 8). This makes the basic theory of pulse-coded BOTDA more mature and complete, and provides theoretical guidance for realizing the highest possible sensing performance of pulse-coded BOTDA. The performance optimization of pulse-coded BOTDA sensors from the above four aspects is discussed in detail. Additionally, the analysis results of performance and shortcomings of different mainstream unicolor unipolar coding schemes in practical applications are summarized in Table 2, which demonstrates the comparative application advantages of the deconvolution-based optical pulse codes over the other current codes.

In addition to the optimization measures of BOTDA sensors based on optical pulse coding, the combination of optical pulse coding technology and other advanced technologies such as differential pulse-width pair, optical pre-amplification, pulse pre-pump, distributed optical amplification, and digital signal processing has been put forward to further enhance the comprehensive performance of pulse-coded BOTDA sensors. The performance of reported BOTDA sensors based on optical pulse coding is sorted chronologically and several representative results are summarized in Table 3. The bipolar complementary Golay code based on a three-tone probe yields the highest overall performance thanks to its high coding gain, low optical noise, and high robustness.

Optical pulse coding technology has been studied for decades and has fully proven to work well in BOTDA sensors with long sensing distance (> 25 km) and high spatial resolution (<5 m). Compared with the optimized single-pulse BOTDA sensors, the overall performance of pulse-coded BOTDA sensors with a proper code type is greatly improved with the assistance of optimized system parameters and post-processing algorithms. The present mainstream coding schemes adopted in distributed optical fiber sensors include unicolor unipolar coding, color coding, and bipolar coding, whose theoretical coding gain can all be approximately proportional to the square root of the code length. The analysis of unicolor unipolar coding schemes is relatively mature. With the aspects of hardware and software costs, robustness to baseline fluctuations, tolerance to signal-dependent noises, tolerance to non-uniform code envelop, and ability of arbitrary energy boost considered, the deconvolution-based code exhibits performance advantages in nearly all aspects. This provides an economical and effective solution to sensing performance improvement for any given sensing device, without any auxiliary hardware or additional measurement time. Based on the existing research results, how to overcome the optical noises and the higher-order non-local effects to obtain a longer coding length to further improve the SNR has become a current challenge.

As opposed to unicolor unipolar coding, since color coding and bipolar coding are researched at a later stage and are more costly in practical applications, the solution of reducing the cost of coding without compromising its performance is a current challenge. Further in-depth analysis and exploration of color coding and bipolar coding will help evaluate and compare the performance of different coding schemes more comprehensively and rigorously for adapting to different application requirements.

Furthermore, the combination of optical pulse coding technology and other advanced technologies such as distributed amplification technology is a common strategy for performance enhancement. By taking distributed Raman amplification technology as an example, the optimization standards of optical pulse coding technology and distributed Raman amplification technology are different, and the direct superposition of their respective optimization schemes when such two technologies are applied simultaneously may cancel out the benefit. Till now, there is still a lack of specific optimization standards and targeted optimization schemes for the combination of such two technologies. Therefore, it is of significance for further improving the performance of distributed sensors to conduct in-depth investigations on their targeted optimization schemes when the optical pulse coding technology and other advanced technologies are adopted simultaneously.