Research Of The Key Technologies For Distributed Optical Fiber Sensors

Distributed optical fiber sensors(DOFS)offer the capability to acquire the physical quantities with continuous spatial distribution along an optical fiber.With the features of immune to electromagnetic interference,anti-corrosion and small size,DOFS have been widely employed for the environmental monitoring in industrial applications such as the energy industry,civil engineering,traffic security,etc.Among various DOFS,Brillouin optical time domain analysis(BOTDA)based on stimulated Brillouin scattering(SBS)is one of the most attractive fields owing to its prominent capability to perform high-quality temperature or strain measurements over long-distance fibers beyond 100 km with spatial resolutions down to the meter scale.The temperature or strain information is retrieved by exacting Brillouin frequency shift(BFS)from each measured Brillouin gain spectrum(BGS)through a post-processing algorithm.However,the existing post-processing techniques exploit the fact that the entire measured BGS is expected to be close to a Lorentzian shape,thus resulting a large error in BFS estimation around temperature or strain changing area,where the BGS is evolved to be a double-peak curve.Optical pulse coding and distributed Raman amplification(DRA)is turned out to be intrinsically suited technologies to improve the signal-to-noise ratio(SNR)and extend sensing range.However,all currently existing coding and decoding(CODEC)techniques present fundamental and/or practical limitations,for instance,additional devices are used to alleviate the decoding distortion,operation time are consumed.on decoding and code word switching,coding gain is compromised due to signal-dependent noise.All these issues make the conceptual advantages of the coding technique to be hardly realized in practice to match the vast diversity of field applications.In DRA-assisted systems,the sensor performance is limited by relative intensity noise(RIN)and pump pulse distortion,thus the sensing range is difficult to be further improved.Moreover,the design criterion and proper operation condition to support the best performance of DRA-BOTDA have not been rigorously analyzed yet.In this paper,several innovative technologies have been proposed to overcome the above-mentioned limitations and to improve the performance of the sensors.The main research efforts and achievements are summarized as follows:1)The physical mechanism of double-peak BGS that exists around hotspot area is thoroughly investigated.The presented analysis points out that traditional post-processing techniques would lead to a large error in the BFS estimation of double-peak BGS.The envelop evolution of BGS around hotspot is discussed in detail and a model describing the complex profile of BGS is proposed by solving the instantaneous Brillouin three-wave coupled equations.Instead of using traditional techniques,the proposed model can offer accurate BFS around hotspot,leading to more precise BFS information in the case of complex temperature/strain distribution.2)The fundamental theory and conceptual advantages of the optical pulse coding technique are analyzed in this paper,along with the practical performance and limitation of traditional CODEC techniques applied in DOFS.We proposed a creative CODEC technique based on the concept of deconvolution,offering the possibility of a single-sequence aperiodic code and overcoming all drawbacks inherent to conventional CODEC.This enables,for the first time to the best of our knowledge,substantial performance improvement over optimized single-pulse DOFS without any hardware extension and extra time consumption.All these advantages are experimentally validated using the most standard implementations of distributed sensors based on BOTDA and Raman optical time-domain reflectometry(ROTDR).3)The detrimental impacts of RIN and pump pulse distortion in first-order DRA-BOTDA is investigated.Experimental results demonstrate that balance detection allows perfectly canceling RIN and thus significantly improving SNR.A novel solution based on pulse compensation technique is proposed,which is theoretically and experimentally validated as a robust approach to avoid sensing pulse distortion caused by walk-off effect between Raman pump and Brillouin pump pulse.By designing the injected power of Raman pump,Brillouin pump pulse and probe properly,the optimized DRA-BOTDA system can be realized.After optimizing all conditions for a proper operation of first-order DRA,an ultra-long-distance BOTDA system is constructed with the employment of inline Erbium-doped fiber,achieving a sensing range of 150 km in a 300 km-long fiber-loop,with 2 m spatial resolution and 1.3 MHz frequency uncertainty on the BFS when using 1000 averages per trace.The improvement brought by the proposed method and its implementation is quantified by a figure-of-merit(FoM)of 4×106,which constitutes the highest FoM so far reported in a Brillouin-based DOFS.