Study On Lithium Niobate Asymmetric MZI Tunable Comb Filter

Tunable bandpass filter is a key component of the multiplexing/demultiplexing end of the Coarse-Waveguide-Division-Multiplexing?CWDM?system.The tunability of the bandpass filter is a key function because it can be used to achieve channel selection,facilitate channel center wavelength alignment in the CWDM system,and reduce the difficulty in the process of the device fabrication.At present,the implementation of the tunability in the optical waveguide device is mainly by means of thermo-optic and electro-optic effects.However,the tuning speed via the thermo-optic effect is slow and not suitable for high-speed tuning applications,whereas the electro-optic tuning,which features high speed,is thus the best choice for these applications.To achieve electro-optic tuning efficiently,materials with large electro-optic coefficient should be used to fabricate devices.Among various electro-optic materials,lithium niobate?LiNbO₃,LN?is the best choice due to its mature manufacturing process and stable operation.In the aspect of the configuration of the comb filter,the asymmetric Mach-Zehnder interferometer?MZI?is widely employed due to its simple structure and good performances.However,due to its asymmetric structure and the characteristics of the lithium niobate waveguide,bending loss is inevitably generated,and usually the bending loss is more deterioration at the long wavelength.Therefore,to solve the loss problems above,we measured the loss of our fabricared bent waveguide,and decreased the bending loss by using a wider waveguide.Secondly,the tapered directional coupler was used to increase the bandwidth to a wider level that the conventional directional coupler is hard to achieve because it cannot maintain the same split ratio in a large wavelength range due to dispersion.In this dissertation,the importance of the wavelength division multiplexing?WDM?system in optical fiber communication as well as the applications of the coarse wavelength division multiplexing?CWDM?system are introduced firstly.Then the use of the band-pass filter in the demultiplexing end of the CWDM system is explained.The current research situation and the faced problems are aslo introduced,and the application of our designed tunable comb filter in CWDM system is presented.Then lithium niobate and its electro-optical properties is introduced,and the working principle and key parameters of the MZI comb filter are presented.Next,the design of single-mode waveguide is carried out,and the design parameters of the device are optimized.The effects of the tapered coupler,dispersion,and bending loss on the performances of the sedigned device are discussed.Finally,the designed optimal device was simulated and the design parameters were summarized,which are the foundament of the following experimental works.Finally,the design of the mask and the experimental process are introduced.The bending loss was experimentally measureded,confirming the increase in the bending loss at long wavelengths and the improvement in the loss by increasing the width of waveguide width.Finally,three sets of our fabricated samples were measured.The results confirm that the isolation was very good throughout the C+L band for all three samples,and the isolation was not reduced due to the losses at the long wavelength.The isolations of Sample 1 and Sample 2 are 20 dB and 35 dB respectively without electrodes,which are good agreement with the results of theoretical and experimental analysis.Sample 3,which was successfully formed the electrode,exhibits an isolation of¹6 dB,a tuning range of over 100 nm,a tuning efficiency of 4.5 nm/V,and an insertion loss of⁸ dB with a total device length of 30 mm.