Low-Loss Silicon Nitride Reconfigurable Microwave Photonic Filter

The microwave photonic filter is one of the important components in the microwave photonics system.It has the advantages of immunity to electromagnetic interference,wide-band tunability and reconfiguration.Microwave photon filters are not only used in radar systems and aerospace fields that require high-speed signal processing capability,but also widely used in communication systems,especially in millimeter-wave Radio on fiber?ROF?systems.Microwave photonic filters have been developed from the traditional discrete optical fiber devices to integrated chips,and there are reports on SOI,Si₃N₄,InP and polymer platforms.Among them,silicon nitride optical waveguides are widely used for integrated optical devices due to their advantages such as large refractive index difference between core and cladding,small device size and compatibility with the CMOS process.This thesis mainly focuses on low-loss silicon nitride optical waveguides with core thickness of 200nm and reconfigurable microwave photonic filters based on the microring resonators,which is constructed by the single stripe silicon nitride optical waveguide.The first part mainly studies the fabrication processes and optimization methods of low loss single stripe silicon nitride optical waveguides.The parameters of the cross-section,bending radius,coupling pitch,and coupling length of the silicon nitride optical waveguide were designed.On this basis,a 1.5?m wide Si₃N₄ waveguide was fabricated by the standard microfabrication processes,which including photolithography,reactive ion etching?RIE?,Plasma enhanced chemical vapor deposition?PECVD?,annealing etc.And the loss characteristics of the optical waveguide were measured by the cut-back method and the micro-ring method,and the influence of the annealing process on the propagation loss of the waveguide was analyzed.And the measured propagation losses of the unannealed,3 hours annealed and 6 hours annealed silicon nitride waveguides measured by the cut-back method were 3.45 dB/cm,1.61 dB/cm and 1.34 dB/cm,respectively.The end-face coupling losses of small-diameter fibers with a mode size of 4.8?m were 1.73 dB/cm,1.63 dB/cm,and 1.64dB/cm,respectively.At the same time,the propagation losses were also obtained from the measured transmission spectrum of the racetrack microring resonator.The propagation losses of the silicon nitride waveguides of unannealed,3 hours annealed and 6 hours annealed were 3.95 dB/cm,1.64 dB/cm,and 1.66 dB/cm,respectively.The results of the two propagation loss measurements show that the annealing process can significantly reduce the propagation loss of the single-stripe silicon nitride optical waveguide.In the second part,based on the low loss silicon nitride optical waveguides,the reconfigurable microwave photonic filters based on microring resonators were studied.Based on the coupled mode theory and the transmission matrix method,the theoretical model of the MZI coupled microring resonator and the microring coupled asymmetric MZI interferometer is analyzed.The size of the 2×2 multimode interference coupler?MMI?and the metal electrodes for thermo optical tuning are designed and optimized.The reconfigurable photonic filter based on the low loss silicon nitride optical waveguide was fabricated and tested.The experimental results show that for a single MZI coupled microring resonator,the extinction ratio is tuned from the 1.6dB²2dB in the 0¹0.5V range,and about 11.5V is required to achieve frequency shifting of a FSR.In order to get a greater extinction ratio,two MZI coupling ring resonators is cascaded,and the extinction ratio is reached to 30.3dB.The reconfigurable photonic filter based on the microring coupled asymmetrical MZI interferometer can achieve a 3dB bandwidth of about 3.125GHz,which is about 13.9%of the band pass filter'FSR.And by changing the device parameters,the 3dB bandwidth can be increased to 11.2GHz,accounting for 50%of the FSR,so the narrow band pass filter and the broadband pass can be realized.Finally,a reconfigurable microwave photonic band-stop filter is constructed by using the MZI coupled microring resonator.The measured results show a out of band rejection ratio from the 0.2dB⁴4.1dB and 16.4GHz frequency tuning can be achieved.