The distributed optical fiber sensing (DOFS) technology has been widely applied in pipeline safety monitoring and perimeter defense due to its ability to continuously detect and accurately locate external vibrations along the fiber transmission path. Backscatter-based DOFS systems, such as phase-sensitive optical time domain reflectometers, are commonly associated with long response times. In recent years, many combined structures with various interferometers have emerged, which increase the complexity and cost of DOFS systems. Interferometer-based DOFS systems require isolation protection for the reference fiber or demodulation for localization, and the use of multiple interferometers, which limits their practical applications. Another emerging type of DOFS system, based on the nonlinear dynamics of semiconductor lasers, generally suffers from low fiber utilization. We propose an in-line detection and location system for pipeline leakage based on a semiconductor laser with optical injection. By utilizing the time difference at which the phase changes of forward and backward propagating light reach the laser, the system enables precise location. It offers high fiber utilization, eliminates the need for signal demodulation, and provides a rapid response.

The proposed system (Fig. 1) consists of a master laser, a slave laser, an optical circulator, a sensing fiber, and a mirror. The slave laser, which serves as the injected component, receives the output light from the master laser, thus ensuring frequency and phase locking of its output light to maintain system stability. Additionally, it functions as a modulation conversion unit, receiving phase-modulated light caused by leakages and converting it into corresponding intensity-modulated light, i.e., phase-to-intensity modulation conversion. Due to the mirror reflection at the end of the sensing fiber, two counter-propagating beams exist in the fiber. When a pipeline leakage occurs, both the forward light and backward light in the fiber undergo the same phase modulation simultaneously. However, there is a time difference in the time it takes for the two phase-modulated beams to reach the slave laser, which results in two output signal waveforms with the same time difference. This time difference corresponds to the round-trip time of light from the leakage position to the mirror and can be used for location. Location accuracy is improved by using the empirical wavelet transform (EWT) denoising algorithm.

By modeling and simulating numerically the DOFS system, the operation state of the system is selected. The simulation results (Fig. 2) show that the injection locking state is suitable for sensing. The feasibility of the proposed system is verified experimentally through a simulation of pipeline leakage. A phase modulator driven by a sinc signal with a bandwidth of 60 kHz is placed at different positions along the sensing fiber to simulate the optical phase change caused by a pipeline leakage. In the injection locking state (Fig. 4), the system output signals are collected under no leakage (Fig. 5) and simulated leakages at different positions (Fig. 6). The signal remains stable without significant amplitude fluctuations when there is no leakage. However, two distinct waveform changes emerge in the signals with different time intervals when the leakages occur at different positions. The acquired signals are processed by applying the EWT denoising algorithm (Fig. 7). The Pearson correlation coefficients between the EWT decomposition components and the original signal are calculated (Table 2). High-frequency and mid-frequency noise, weakly correlated with the original signal, is discarded, leaving only the residual components that represent the trend changes and main features of the signal for position determination. Nine leakage positions are simulated separately. At each position, 20 measurements are taken, and the average time difference for each set of data is calculated. The location errors at the nine positions are all less than 10 m (Table 3). Compared with the results before denoising, the location accuracy improves by about 90%. The location errors at all positions in the system remain in a small range, with an average absolute error of approximately 7?10 m, and the standard deviation is approximately 4.0?6.5 m (Fig. 9).

We propose an in-line detection system for pipeline leakage based on a semiconductor laser with optical injection. The optical injection locking effect of the semiconductor laser is utilized to convert phase modulation induced by a leakage into intensity modulation, which enhances the system’s anti-interference, simplifies the signal processing process, avoids complex phase demodulation steps, and improves the real-time response capability of the system. This system achieves bidirectional light transmission through the mirror at the end of the fiber. The single-fiber sensing structure increases fiber utilization and facilitates the use of the time difference between the two waveform changes caused by the same leakage for location. The time difference is obtained more accurately using the EWT denoising algorithm. The system’s location capability at different leakage positions is validated through simulation experiments. All location errors are less than 10 m, and an average location accuracy improvement of 90% is achieved after denoising. The results of the simulation experiments indicate that the system provides accurate location information with high reliability.