Research And Application Of Bandpass Microwave Photonic Filter Based On Excitation And Conversion Of Optical Fiber Mode

Microwave Photonics(MWP)is an interdisciplinary subject that integrates microwave technology and photonic technology,which focuses on the generation,manipulation and transmission of microwave signals in the optical domain.MWP not only has the advantages of large capacity,low loss,and anti-electromagnetic interference of optical fiber communication,but also provides abundant bandwidth resources for the generation,manipulation,transmission and measurement of microwave signals,breaking the "electronic bottleneck" in the microwave domain.MWP has an enormous application range,such as radar,communication,sensor and instrumentation,etc.In this thesis,a bandpass microwave photonics filter(MPF)based on modal interference is analyzed and investigated on the theoretical basis of fiber mode,and its application in sensing field is researched.The purpose is to simplify the system architecture of MPF and provide a high-resolution demodulation solution for optical fiber sensing.On the other hand,a bandpass MPF based on the mode characteristics of few mode fiber is designed on the basis of the fiber dispersion theory in order to improve the tunable flexibility of MPF.The work overview is as follows:(1)A MPF based on a broadband source(BOS)sliced by modal interference-typed single-mode-few-mode-single-mode(SMF-FMF-SMF,SFS)is theoretically analyzed and experimentally studied.The principle of FMF modal interference and the MPF based on BOS sliced by MZI are theoretically analyzed.In the experiment,the SFS was fabricated by the core offset fusion,and the MPF based on BOS sliced by the single SFS and the cascaded SFS was realized,and the frequency responses of the single passband and the four passbands were obtained respectively.The MPF based on a single SFS has a single passband,and the center position of passband is depended on the length and second-order dispersion of SMF delay line,and the free spectral range(FSR)of the Mach Zehnder Interferometer(MZI).The 3d B bandwidth of passband is affected by the spectrum of the BOS and the optical amplifier.The MPF based on cascaded SFS has four passbands,in which two passbands are respectively corresponding to the FSR of single SFS transmission spectrum,and the other two passbands are related to the FSR of both SFS transmission spectrum.(2)A MPF based on a BOS sliced by modal interference-typed multimode-singlemode-multimode(MMF-SMF-MMF,MSM)is investigated and applied in temperature interrogating.In the experiment,the structure of MSM was fabricated,and the MPF based on the BOS sliced by MSM was realized.The center frequency of the passband of the MPF frequency response does not drift significantly with temperature,and the intensity of its peaks and dips changes sharply with temperature.Compared with the method of spectral demodulation,the method by interrogated the peaks and dips of the MPF response improves the temperature interrogating resolution by two orders of magnitude.(3)A tunable bandpass MPF based on hole-assisted graded index few mode fiber(HGI-FMF)is designed.According to the requirement of tunable bandpass MPF for taps,the HGI-FMF is optimally customed,which realizes the differential modal group delay(DMGD)among the four mode groups is not only equal and can be tuned with the wavelength.In the simulation,the performance of the tunable bandpass MPF based on HGI-FMF which carried four mode groups as finite impulse response(FIR)taps and the influence of HGI-FMF manufacturing error on MPF performance are analyzed in detail.The passband response of the MPF is tunable with the wavelength of the optical source,and the MPF performance has excellent tolerance to HGI-FMF manufacturing errors.