Optical pulse coding (OPC) has caught much attention in optical fiber sensing in recent years, especially when combined with phase-sensitive optical time domain reflectometry (Φ-OTDR).

In the 1970s, optical fiber sensing technology emerged rapidly with the development of optical fiber communication technology, and it employs optical fiber as the sensing medium or optical transmission path to sense changes in the surrounding environment by the characteristic changes of light waves. With the increasing demand for sensors in society and the continuous maturation of optical sensing technology, optical fiber sensing systems have been widely adopted. These systems based on the light scattering principle can carry out long-term monitoring in harsh environments and achieve the measurement of physical quantities with large spatial scales or high spatial densities by continuous sensing points in optical fibers. Meanwhile, they have become a key component of borderline security, firefighting early warning, pipeline monitoring, transportation line supervising, and large-scale structural health monitoring among other fields.

Based on Raman scattering, Brillouin scattering, and Rayleigh scattering, a variety of optical fiber sensing schemes can be implemented. Rayleigh scattering is a kind of elastic scattering caused by refractive index changes in the optical fiber and has a faster response speed compared with the other two scattering methods. Additionally, based on the interference effect, Rayleigh scattering-based optical fiber sensing is more sensitive to the changes in the measured parameters. Φ-OTDR based on Rayleigh scattering is one of the most important applications of distributed acoustic sensing (DAS) and quasi-distributed acoustic sensing (Q-DAS), with fast response and high sensitivity. Despite the sound performance of Φ-OTDR, it is still affected by some of its factors, such as signal-to-noise ratio (SNR), spatial resolution, and transmission distance. The mutual constraints among these factors can limit the Φ-OTDR performance. By coding the probe pulses injected into the fiber, the SNR of the sensing signal can be significantly increased without increasing the peak power of the pulses, thus avoiding nonlinear effects. Meanwhile, the single-pulse response can be obtained after decoding at the receiving end, and the spatial resolution of the system is determined by the length of a single pulse rather than the entire probe pulse sequence, thus maintaining the spatial resolution and receiving a high-SNR sensing signal. In most cases, OPC is a viable solution to meet the demands of high accuracy, long distance, and high sensitivity sensing because it can overcome the limitations among various key parameters.

Regarding the combined applications of OPC technology and Φ-OTDR, the development of optical pulse coding technology in optical fiber sensing is firstly reviewed, and its applications in sensing systems based on Raman scattering, Brillouin scattering, and Rayleigh scattering are introduced. Meanwhile, we present the representative studies of researchers in China and abroad and conduct a comparison of the performance enhancement brought by different coding schemes and traditional schemes. The development of the technique is summarized as shown in Tables 1-4, with the system performance of the different schemes compared. Then the coded Φ-OTDR technical schemes proposed by our group are presented in more detail, including Φ-OTDR based on unipolar and bipolar Golay coding, and the suppression of interference fading and frequency drift therein. Finally, the Φ-OTDR technical route based on orthogonal codes with the same carrier proposed by our group is highlighted.

In recent years, under the joint efforts of several research teams at home and abroad, optical pulse coding technology has been successfully integrated with optical time domain reflection technology in depth, which has led to remarkable development in the direction of optical fiber sensing based on optical time domain reflection technology. By various innovative ways of combining optical pulse coding technology with Φ-OTDR, the constraints among the key performance parameters of Φ-OTDR can be overcome. Optical pulse coding can help Φ-OTDR achieve distributed optical fiber sensing with long distance, high SNR, high spatial resolution, and quasi-distributed optical fiber sensing with long distance, high SNR, and large bandwidth. The acoustic wave sensing technology based on optical pulse coding can still be further extended to engineering applications, such as vehicle positioning, seismic wave detection, and perimeter security. It is worthwhile to deeply explore high-level applications of the technology in engineering fields in the future.