Research On High-capacity FBG Sensing Network And System With Bus Chain Topology

At present fiber Bragg grating (FBG) has so far become one of the mostpromising optical passive devices and has been widely applied in sensingmeasurement field because of its several unique advantages, for example it can beeasily embedded into composite materials to become intelligent composite materialwith sensing property due to its small volume, light quality and flexible bendability;the distributed sensing network can be constructed by making use of variousmultiplexing technologies so as to reduce sensing system costs; it can also be appliedin such severe environments of intense electromagnetic interferences, fire protectionand explosion protections because the FBG sensor is intrinsically safe.With the view of improving FBG sensors multiplexing capabilities and ofenhancing its viability during actual engineering application and futuremaintainability, a high-capacity FBG sensing network and system with bus chaintypology has been successfully designed. The main researching findings in this paperare developed in the following literature:A FBG sensing system with bus topological structure based on the opticalfrequency domain reflectometry (OFDR) has been successfully designed so as tosettle the problem that sensors multiplexing quantities are limited by light sourcebandwidth and sensor measuring range in FBG sensing network based on wavelengthdivision multiplexing (WDM). In this new system8FBGs with strong reflectivity andthe same wavelength are positioned into the branching circuit of FBG sensingnetwork with bus chain typology so that the simultaneous interrogation for FBGsensor wavelength and position are successfully achieved. In addition to that,constant temperature experiment result shows that the wavelength repeatability of thesystem is±5pm. In addition to that, a system for measuring non-uniform strainalong FBG using OFDR is studied and with this system the non-uniform strain alongarea adjacent to structural damage has been experimentally measured and a goodagreement is obtained between measuring result and theoretical simulation.The key modules which are necessary for constructing FBG sensing system have been designed. They are composed of low-noise photoelectric module, broadbandlight source, erbium-doped fiber amplifier, wavelength scanning module based onWDM, wavelength scanning and sequence triggering module based on time divisionmultiplexing (TDM) and pulse modulating module.A Full Duplex Three Terminals Add/Drop Wavelength Division Multiplexer isinvented to build FBG sensing network with bus chain typology based on WDM andwith this new network the sensor viability during actual engineering application andfuture maintainability have been improved. The invented Add/Drop WavelengthDivision Multiplexer is functioning as optical splitter, while unlike the latter one, itwill not result in obvious optical intensity loss. The experiment reveals that thewavelength resolution of FBG sensing network with bus typological structure basedon Add/Drop Wavelength Division Multiplexer is less than1pm and the long-termwavelength stability is1.7pm.A high-capacity FBG sensing network with bus chain typology based on TDMhas been proposed in order to compensate the defect of the relatively short measuringdistance caused by the limited laser coherence length in FBG sensing network basedon OFDR. Furthermore, this newly proposed sensing network can overcome theeffects of multiple reflections and spectrum shadow on sensing capacity andmeasuring accuracy in cascading FBG sensing network based on TDM and the sensorviability during actual engineering application will be immensely enhanced. In thisnew network17FBG sensors with strong reflectivity and same wavelength have beenconnected into the branching sensing circuit of the network and wavelength andposition interrogation for these17FBG sensors have been successfully realized byemploying TDM technology. Subsequently4FBG sensors have been selected fortemperature experiments and the result reveals that a good agreement is obtainedbetween experiment results and theoretical value. At last, this network sensingcapacity can be immensely enhanced by making use of the currently highly matureEDFA to amplify output power of light source which is the major influencing factoron the capacity of FBG sensing network with bus chain typology based on TDM.In order to further improve the capacity of FBG sensing network, a FBG sensingnetwork jointly constructed by four kinds of technologies, SDM, WDM, OFDR andTDM has been researched. In particularly the FBG sensing network built by combined technologies of WDM/TDM has been concretely designed so as to realizesimultaneous interrogation for wavelength and position of34FBG sensors and theexperiment result shows that the FBG sensing network with bus chain typology basedon WDM/TDM can greatly enhance sensor multiplexing capacity and has broadapplication space in large-scale structure monitoring field.