Investigation Of Spatial-division Multiplexed Distributed Optical Fiber Sensing Techniques

With the capability to offer tens of kilometres long sensing range and meter-scale spatial resolution,distributed optical fiber sensing techniques are among the most important research fields of optical fiber sensing,and have shown great potential in industry fields.However,limited by the geometric structure and light guiding feature of the conventional single mode fiber(SMF),it can normally just provide a single spatial channel for transmission,and as a result the multiplexing of multiple sensing techniques is not achievable.Therefore the traditional SMF based distributed optical fiber sensor can normally only be able to measure a single unambiguous parameter by single-shot measurement,while multi-parameters and multi-dimensional simultaneous monitoring is unachievable.In addition,the classical SMF based Brillouin distributed optical fiber sensors intrinsically suffer from the cross-sensitivity issue between temperature and strain.Therefore it has practical significance to seek for effective and reliable discriminative measurement solutions.In recent years,space-division multiplexing(SDM)technique has been demonstrated as a breakthrough for transmission capacity scaling,thanks to the successful development of some critical devices for SDM,such as the fan-in/out coupler.On the other hand,we believe SDM technology can also bring new implementation configurations,new solutions and new application fields for distributed optical fiber sensing.Originating from this inspiration,we have systematically investigated multi-core fibers(MCFs)and few-mode fibers(FMFs)based space-division multiplexed distributed optical fiber sensors,including space-division multiplexed Brillouin optical time-domain reflectometry/analysis(BOTDR/A),Raman time-domain reflectometry(ROTDR),and frequency-swept correlation peak searched based phase-sensitive optical time-domain reflectometry(?-OTDR).The achievements include long range and distributed three-dimensional(3D)shape sensing based on Brillouin distributed measurements in MCF,proposals to deal with the cross-sensitivity issue,and high sensitive temperature monitoring and bending sensing based on space-division multiplexed ?-OTDR.The main research contents and the novelties are summarized as following.(1)Characterized the Brillouin scattering in MCF,reported for the first time the highly bending dependence of Brillouin frequency shift(BFS)in off-centre cores of a MCF.We discovered the bending dependence feature of BFS in off-centre cores in experiments,and then established a solid numerical model after careful analysis and evaluating.The linear relationship between BFS and curvature has been verified after experimental calibration.We then provided the theoretical model and numerical calculation method for computing the distributed bending angles and curvatures based on the strain measurement by Brillouin distributed sensing,and the feasibility of the proposed technique was experimentally validated,meanwhile the error of result has also been evaluated by calculating the propagation of uncertainty and analysed thoroughly.Then the 3D shape of the MCF was eventually retrieved with the help of Frenet-Serret formulas,together with the obtained discrete bending angle and curvature information.In comparison with the MCF grating based solution,the Brillouin distributed strain measurement based scheme significantly reduces the technical difficulty and implementation process requirements,and has shown remarkable progress.(2)Proposed and demonstrated a heterogeneous multicore fiber based Brillouin optical time-domain analysis(BOTDA).We investigated the characterization of Brillouin scattering in heterogeneous MCF,and found that the cores made from different preforms show distinct BFSs,and their temperature sensitivities are also different,however their strain sensitivities are almost the same.Based on this feature,we proposed to implement discriminative measurement by solving a coefficient matrix by making use of the difference between sensitivities.In order to eliminate the BFS shift induced by bending in off-centre cores,we proposed to average the BFS of two symmetrical outer cores,and then conduct the matrix calculation by using the averaged BFS of outer cores and the BFS of the central core.The proposed system provided a new perspective for discriminative measurement in Brillouin distributed sensors based on heterogeneous MCF space-division multiplexed solutions.(3)Proposed and demonstrated a MCF space-division multiplexed hybrid Raman-and Brillouin-optical time-domain reflectometry(ROTDR and BOTDR).In order to overcome the inconvenience that requires time-division multiplexed separate interrogation of Raman and Brillouin scattering signals in SMF based hybrid ROTDR and BOTDR system,we proposed a MCF space-division multiplexed hybrid ROTDR and BOTDR system,The two reflectometry share the same pulse generation devices,but are implemented in different cores,therefore efficient management of input pump power is enabled.On one hand,high pump power is launched in the ROTDR path for monitoring the weak spontaneous Raman scattering signal;on the other hand,the injected pump power can be controlled with an attenuator so as to avoid the emergence of nonlinearity(e.g.stimulated Brillouin scattering)in fiber.Simultaneous measurement of ROTDR and BOTDR is achieved through space-division multiplexing instead of time-division multiplexing.The proposed space-division multiplexed hybrid system provides a competitive alternative solution for temperature and strain discriminative measurement.(4)Proposed a few-mode fiber based space-division multiplexed phase-sensitive optical time-domain reflectometry.The technology is a modified system over the traditional?-OTDR,which consists of frequency sweeping and correlation peak searching.High sensitive temperature sensing is obtained and we have preliminarily verified the feasibility to make bending sensing by using FMF.The contrast experiments based on ?-OTDR indicate that the fundamental mode is insensitive to bending while the high order mode is sensitive to bending;on the other hand,both the fundamental mode and the high order mode undergo identical temperature and longitudinal strain,so the measurement of fundamental mode can be used to compensate the effects of temperature and longitudinal strain on high order mode,thus a temperature and longitudinal strain insensitive high sensitive bending sensor is achieved.The proposed system offers a competitive long range distributed bending sensor.In conclusion,space-division multiplexed physical transmission link offers new implementation method for distributed optical fiber sensors;new applications has been carried out,and it also helps to solve the cross-sensitivity problem that is widely existing in SMF based distributed optical fiber sensors.What's more,it is believed that the SDM will also make it possible for the multiplexing of multiple sensing techniques and multi-parameters simultaneous measurements.