| With the continuous development of fiber grating technology,embedded technology and computer technology,it’s of great research value and engineering significance for the construction of large-scale distributed fiber-optic sensor networks more powerful and suitable for monitoring of complex engineering application.Hence,in this paper,a time/wavelength division multiplexing(TWDM)fiber optic sensing technology scheme was proposed based on a wavelength tunable optical time domain reflectometer(TOTDR).Through the theoretical analysis and experimental preparation of TOTDR,ultra-weak reflection fiber Bragg grating(UW-FBG),fiber apodization and other core technologies,the application of TWDM fiber-optic sensor systems in a large-capacity distributed fiber grating sensing system and in on-line monitoring of passive optical networks(PONs)has been successfully implemented.To improve the spatial resolution of the above sensing system,a TOTDR technology scheme based on a tunable mode-locked fiber laser was pit forward,and a tunable mode-locked fiber laser based on liquid crystal on silicon(LCoS)was also deeply analyzed theoretically and experimentally.The contents below were focused by this paper:(1)Research on the wavelength tunable optical time domain reflectometer based on distributed feedback laser(DFB)array.The tunable light source module using DFB laser array was designed,and a prototype was developed and tested.Experiments showed that:a)the light source module could achieve pulse width of lOns-lOus;b)The output power was able to be continuously adjusted within the range of-30dBm-8.8dBm;c)The wavelength tuning range was 27nm,with the tuning accuracy of 1.4pm.Then a TOTDR photodetection system with low noise,wide bandwidth and high impulse response as well as adjustable dynamic range was designed and simulated.The single-channel photodetector had a bandwidth of 107.3 MHz,gain of 53dB,and three-gear gain switching was achieved by the total amplifier circuit.On this basis,a TOTDR design scheme based on a DFB laser array was put forward.Finally,the TOTDR prototype was developed.The effects of dark current effect,average times,pulse width,magnification transformation,and wavelength tuning on fiber measurement were discussed.(2)Research on a two-step scanning-mask exposure method for the fabrication of arbitrary apodized fiber gratings.Firstly,the optical characteristics of the apodized grating,the process of making the apodized grating by two-step exposure method,together with the non-ideality of under-compensation and over-compensation of the grating were analyzed in detail using the coupled mode theory and the transmission matrix method.On this basis,a programmable apodized grating two-step exposure fabrication system based on dynamic optical shielding plate was proposed and developed.In this system,a programmable high-accuracy displacement platform was utilized to control the specially designed optical shielding plate movement through the designed control function,thus controlling the refractive index distribution of the apodized fiber grating,with a control accuracy of 0.5 μm.Then,an experimental fabrication platform was set up.Uniform optical gratings,Linear apodized gratings,Gaussian apodized gratings and Nuttall apodized gratings were fabricated.The grating performance was tested and analyzed.It’s found that different gratings exhibited the same optical characteristic as the simulation results.(3)Research on a time-and wavelength-division multiplex distributed sensing interrogation system based on distributed feedback laser array and ultra weak fiber Bragg grating.Firstly,the transmission characteristics of TWDM were proposed and discussed,including the effects of reflection loss,fiber attenuation and grating crosstalk.Furthermore,the TWDM network planning method and system capacity analysis method were put forward.Secondly,UW-FBG was developed at 22 different Bragg wavelengths.The test results showed that the peak wavelength jitter of UW-FBG developed at each wavelength was less than 0.06 nm,and the peak reflectance was less than-30 dB.On this basis,using the developed TOTDR as an optical signal transmission and demodulation unit,a TWDM fiber grating sensing experiment with 22 wavelengths was built and tested,in which the UW-FBG number was no less than 30 at each wavelength.The system could achieve 27 nm wavelength tuning,with the grating wavelength demodulation accuracy of 1.4 pm and the wavelength measurement error of less than 30pm.The minimum interval of UW-FBG with the same wavelength was 12m.When TWDM technology was used to network UW-FBG with different reflection wavelengths,the average minimum distance could be reduced to 0.55m in the experiment.In addition,a temperature test platform was set up with UW-FBG as temperature sensors.According to the experimental results,the wavelength-temperature coefficient of UW-FBG obtained by TOTDR measurement was 11.88 pm/℃ and the temperature resolution was 0.1 ℃.(4)Research on a TOTDR-and fiber grating based PON network on-line monitoring system.Firstly,a TOTDR-based PON network on-line monitoring system architecture was proposed as the monitoring requirements.Secondly,strong reflection grating(Reflectance>98%)was developed at 32 different Bragg wavelengths.The test results showed that the peak wavelength jitter of FBG developed at each wavelength was less than 0.4 nm,the average reflectance was above 98%and the reflectivity fluctuation does not exceed 0.6%.Finally,a PON network on-line monitoring experimental system based on the principle of TWDM optical fiber sensing was established.The PON branch identification capabilities,TDM and WDM multiplexing capabilities were tested.Experimental results show that the system can meet the monitoring requirements of 1 × 64 PON networks.(5)Research on tunable narrow pulse light source for fiber grating sensing system.In order to achieve high spatial resolution of TOTDR,this dissertation has also conducted in-depth theoretical analysis and experimental research on tunable femtosecond pulsed light source,a tunable mode-locked fiber laser based on a large-scale optical integrated chip of LCoS.Firstly,a phase-only liquid crystal spatial light modulator was used as a basic control element,and a phase holographic grating principle was applied to design and implement a function of an LCoS-based wideband tunable filter.Then,a complete theoretical model of a passively mode-locked fiber laser based on.a saturable absorber is established.The iterative symmetric stepwise Fourier method and traveling wave model are used to numerically simulate the pulse formation and cavity within the cavity-mode-locked fiber laser.The influence of cavity parameters on the output pulse characteristics are analyzed.Finally,the LCoS diffraction characteristics was used as a tunable filter and applied to a tunable mode-locked fiber laser system.Experiments showed that the wideband tunable mode-locked fiber laser could achieve continuously adjustable wavelengths(1530～1560 nm).The output pulse width could be controlled by the filter bandwidth as well.When the filter bandwidth changed within the range of 0.8 to 3.2 nm,the output pulse width varied from 7 to 50 ps. |
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