Fibre optic fault affects fibre optic communication quality, data transmission rate and leads to network breakdown. Obtaining line fault information in real-time, accurately locating it, and improving fault maintenance efficiency is an important problem faced by the high-quality and rapid development of the fibre optic communication industry. Optical time-domain reflectometer (OTDR) is a common equipment for fault detection and quality analysis of optical fibre communication links. There are two unavoidable problems due to the limitation of its technical principle. First, the pulse modulation technology used for event location makes OTDR susceptible to the influence of the dispersion effect and widens the pulse width. The longer the measurement distance, the lower the spatial resolution. Second, for a spatial resolution of 1 m, the sampling frequency of OTDR should be at least 200 MHz, and the pulse laser with a pulse width of less than 10 ns and peak power of several watts should be emitted. The luminance efficiency of the light source decreases with high power after a long time of application. The performance of the passive optical devices inside the instrument is affected, leading to different degrees of photosensitive surface damage. Further, it reduces the photoelectric conversion efficiency, device life and overall performance of OTDR. Because of coherence and difference frequency detection, the coherent optical frequency domain reflector (COFDR) can allow light detection to have a larger amplitude range. Additionally, the receiver bandwidth can be very low, effectively reducing the noise and improving the dynamic range. However, its detection range is limited by the coherent length, frequency modulation rate and linearity of the tunable light source. Moreover, the development capacity of the ultra-narrow linewidth, single-frequency, coherent laser in China is weak, and the external purchase price is very high. Commercial COFDR costs nearly one million yuan, and the cost performance of engineering applications is not high. Therefore, this study conducts relevant research to solve these problems and realise the precise positioning and real-time online monitoring of optical fibre faults.

Based on the mechanism of incoherent optical frequency domain reflectometry (IOFDR) technique, this study proposes a cost effective, high-precision and distributed fibre quality detection method by employing Rayleigh backscattered light in the fibre as the signal light combined with its light wave conduction equation. The system structure is designed, and the system prototype is developed. The microwave signal source is used to perform step frequency modulation of the incoherent laser source. After the photoelectric conversion of the reflected light through the photoelectric detector, it beats with the local oscillator signal in the electrical domain. The amplitude-frequency and phase-frequency responses of the system at each modulation frequency constitute the frequency domain information of the system. Then, after Fourier inverse transformation, the time domain information of the event point distribution of the optical fibre is obtained. The theoretical mechanism and numerical model are derived in detail. The system is developed, and the key indexes of the system are verified through experiments.

The system can preliminarily realise distributed detection of 10 km (Fig. 4) optical fibre with very low optical power (<10 mW). It can also guarantee the spatial resolution of 0.1 m with no difference along the optical fibre (Figs. 5 and 6). The dynamic range is more than 34.5 dB [Fig. 6(a)], and the event blind area is very small (Fig. 7). Wavelength division multiplexing technology can also ensure the normal operation of the existing optical fibre communication network. It is an online health monitoring system for optical fibre networks with great development potential and popularisation value to realise a high-precision location of optical fibre fault information.

Based on the mechanism of the IOFDR technique, this study proposed a low-cost, high-precision and distributed fibre quality detection method by taking Rayleigh backscattering light in the fibre as the signal light combined with its light wave conduction equation. Additionally, its numerical model is derived in detail. The system structure is designed. A high-precision online health monitoring system for optical fibre networks is developed, which realises the quality and health monitoring of optical fibre networks by frequent-spatial transformation. The experimental verification shows that the system can realise distributed detection of 10 km optical fibre with very low optical power (<10 mW). It can also guarantee the spatial resolution of 0.1 m with no difference along the optical fibre. The dynamic range is better than 34.5 dB, and the event blind area is very low; thus it accurately reflects the position and quality of fibre failure points or joints. In future studies, we will improve the range of frequency modulation and frequency modulation rate, focussing on the suppression and solution of nonlinear problems in frequency modulation. The spatial resolution and measurement distance index of IOFDR will be further improved, which is a low-cost and high-precision optical fibre sensing and detection technology with great development potential and promotion value.