Dynamic distributed optical fiber sensors based on digital optical communication techniques

Abstract

Dynamic distributed optical fiber sensor (DOFS) appears in the 1970s and has been the subject of remarkable research interest in the last 30 years because of its high sensitivity, long sensing distance, good environmental tolerance, low energy consumption, etc. Signals are extracted as a function of position along the length of the sensing fiber in such dynamic DOFSs, which provides the potential to reduce the cost and the complexity of a dynamic sensing system when a large number of measuring points are required. Dynamic DOFSs are suitable for long-distance and large-scale security tasks, such as leakage detection of oil and gas pipelines, structural health monitoring, perimeter security protection and safety monitoring of communication links. However, the sensing performance of dynamic DOFSs is limited by the bottleneck of the sensing techniques. For example, Rayleigh backscattering noise limits the sensing distance of distributed optical fiber interferometers, and the weak intensity of the backscattering light limits the sensing distance of backscattering light based dynamic DOFSs. Besides, it is difficult to realize wide frequency response and large dynamic range in phase-sensitive optical time-domain reflectometry (φ-OTDR). Moreover, the sensing speed is a problem for dynamic Brillouin optical time-domain analyzer (BOTDA). Inspired by the similarities between the optical fiber communication systems and distributed optical fiber sensing systems, this thesis aims to break the limitations imposed by traditional sensing techniques and thus improve the sensing performance of dynamic DOFSs by applying the advanced digital signal generation and processing techniques used in optical communication systems to distributed optical fiber sensing systems. First, an ultralong haul unidirectional forward transmission-based distributed optical fiber vibration sensor (DOFVS) is proposed. In this sensing scheme, vibration sensing is implemented by using two fibers that are placed close to each other. A frequency shift optical delay line (FS-ODL) at the far ends of these two fibers is used to loop back the sensing wavelength of the unidirectionally forward transmitted light. At the receiver side, a phase and polarization diversity coherent receiver is used to retrieve the phase of the continuous-wave light. The location information is obtained by calculating the time delay between two constructed differential phase signals. Thanks to the use of Erbium-doped fiber amplifiers and the elimination of the Rayleigh backscattering noise, over 600 km sensing distance is achieved in this sensing scheme. A lower bound of 5 Hz vibration frequency is detected while the upper bound of the detectable frequency is only limited by the bandwidth of the photodetector and the sampling rate set by the analog to digital converter. Less than 125 m spatial resolution is demonstrated when the vibration frequency is larger than 1 kHz. Multi-point detection is important for dynamic DOFSs, especially in ultralong haul sensing systems. In order to make the detection of multi-point vibrations easier and reduce the data processing load due to the use of FS-ODL in the above-proposed sensing scheme, a simplified unidirectional forward transmission-based DOFVS is proposed. Loop back configuration is formed by directly splicing the far ends of the two sensing fibers. Location information is obtained by analyzing the null points in the frequency spectrum of the demodulated phase signal. Both Kth null frequency determination method and double fast Fourier transform algorithm can be used to determine the null frequency. Location performance of these two methods is investigated in depth. It is experimentally demonstrated that the location errors of single-point vibrations and multi-point vibrations are less than ±100 m and ±200 m over a 500-km sensing range respectively. Finally, a novel dynamic BOTDA using spectrally efficient frequency division multiplexing (SEFDM) is proposed to overcome the limitation of spatial resolution in orthogonal frequency division multiplexing (OFDM) based BOTDAs due to the requirement of the orthogonality between the symbol duration and subcarrier spacing. In the proposed scheme, the probe signal is composed of SEFDM symbols. The SEFDM symbol is generated by intercepting a time-centralized OFDM symbol, which is mapped by a partial Zadoff Chu sequence. Brillouin gain spectrum is retrieved through channel estimation. Complementary pulse coding is utilized to enhance the signal-to-noise ratio of Brillouin signals. The SEFDM-BOTDA is investigated both theoretically and experimentally. Intersymbol interference (ISI), which has a strong relationship to the symbol length and the cyclic prefix length, is studied in depth. Three SEFDM symbols with different lengths are used to investigate the sensing performance. Since the orthogonality between subcarriers is violated, the spatial resolution can be largely improved, maintaining a good frequency resolution. 1.294 MHz measurement accuracy is reached over a 10 km sensing range when the spatial resolution is 3.1 m. A dynamic measurement with a vibration frequency of 26 Hz is demonstrated. The use of SEFDM provides higher flexibility for BOTDAs to be used in a wider application area.