Research On Bias Point Control Technology Of LiNbO₃ Electro-optic Modulator Based On Built-in PD

In the past 10 years,the amount of information generated on a daily basis has been increasing day by day.From the very vague video calls that appeared at first,to various 4K live broadcasts today,these have brought new challenges in bandwidth and capacity to today's communication systems.As a key infrastructure of optical communication systems to convert the message from electrical domain to optical domain,electro-optical modulators also need to be improved to meet such a requirement.At present,the usual materials used in commercial electro-optic modulators are inorganic crystal lithium niobate(Li Nb O₃,LN)with a large electro-optic coefficient.In addition to the large electro-optic coefficient,LN has also relatively mature waveguide fabrication processes and industrial lines.However,in practical applications,due to the influence of external environmental factors and the aging of the device,the operating point of the electro-optic modulator is prone to drift off the required one,which leads to the distortion of the output signal.Therefore,it is urgent to develop a scheme that can precisely and stably control the operating point of the electro-optic modulator.Based on the criticality of the bias point control technology,this dissertation studies how to control the operating point of the electro-optic modulator stably.The method using the built-in photo-detector(PD)to detect the leakage light of the electro-optic modulator to feedback control the operating point of the modulator is proposed.The main work of the dissertation is as follows:Firstly,the research background and significance of the bias point control of electro-optical modulator are introduced,and the state of the art relevant to the research of the bias point control technology of the electro-optical modulator at home and abroad is also introduced.Then,the theoretical basis of the bias point control technology is introduced,including the electro-optic effect of LN crystal,the annealed proton exchange waveguide,the analysis of LN single-mode waveguide,and the related principles of Mach-Zehnder electro-optic modulator(MZM).Next,two widely used bias point control schemes and their respective principle are introduced.Subsequently,according to the light field distribution of the device,a surface-mount PD method and an end-face vertical PD method are proposed to realize the bias point control of the modulator.According to the proposed two schemes,the modulator is simulated and designed,and the device parameters are determined.The simulated results show that both methods can collect the leakage light of the modulator efficiently.Further,with the designed parameters the devices based on the two methods are fabricated.Finally,the performances of the device are evaluated.The light transmission test shows that both devices can work normally.The half-wave voltage of the modulator fabricated with the surface-mount PD method is 5.4 V,and the extinction ratio of the leakage light detected by the PD is 5.4 d B.During the test,the working point can be stably controlled,and the automatic control of the bias point is realized.The half-wave voltage of the modulator produced with the end face vertical PD method is 4.5 V,the insertion loss is 32 d B,and when the wavelength is 1550 nm,the extinction ratio of the leakage light received at the end face is 14.8 d B.In theory,the bias point of the modulator can also be automatically controlled.