Research On The Optical Fiber Sensing Method Based On Tunable Microwave Photonic Filter

With the development of microwave photonics,as its core component of microwave photonic filter(MPF)has become a hot research area at present.Compared with the traditional electric filter which is limited by the bottleneck of bandwidth,tuning ability and reconfigurable,microwave photonic filter shows the advantages of with large bandwidth,tunability,reconfigurable,resistance to electromagnetic interference and can processe the signal directly in optical domain,so it has been widely used in the field of fiber communication,etc.Actually,since MPF can be able to process signals in optical domain,so if it is applied onto the optical fiber sensing field,it can not only play the advantage of large optical signal bandwidth in optical domain,but also the signals of the high measurement precision in electrical domain.Therefore,optical fiber sensor based on MPF s for high precision optical fiber sensing reveals a new possible way.According to theoretical research and experimental verification,we designed a variety of different implementation plans,and several MPF s was carried out and applied onto the field of optical fiber sensing.The main research work is illustrated as follows:(1)Based on the theoretical analysis of multiple wavelength light source based tunable MPF,we investigated a MPF using the Mach-Zehnder interferometer(MZI)as spectral integral unit.By combining phase modulation and long section of optical fiber as the dispersion medium,and a theoretical simulation and experimental research was carried out.Experiment results shows that by tuning variable optical delay line(VODL)change of MZI in the sensing arm,high sensitivity length measurement is realized.(2)The basic theory and the tuning method of microwave photonic filter based on double cascaded MZI is studied.By using two MZI as a spectrum splicer,combined with phase modulation and time delay using long fiber,a band-pass filter with up to 4 passbands has been achieved.By adjusting the sensing arm length of one of the cascaded MZI,continuous tuning of the passbands in a wide range can be realized.On this basis,a fiber length sensor based on the MPF with cascaded MZI was proposed and demonstrated in the experiment.By measuring the frequency difference between two of the four passbands,a really high sensitivity of up to-5.358 GHz/mm was obtained.(3)According to theoretical study of the MPF based on phase modulation to the intensity modulation conversion,a novel two-passband tunable MPF using a fiber Bragg grating Fabry-Perot(F-P)filter and electrical yttrium iron garnet(YIG)filter has been proposed and experimental investigated.In the experiment,by changing the control voltage of YIG filter or the laser wavelength,the independent tunability of the two passbands was realized.Meanwhile,based on the theory of the fiber Bragg grating sensor,optical fiber axial strain measurement is studied by using this kind of structure,which converts optical spectrum measurement into frequency domain.In this case,a higher sensitivity was achieved.