Adaptive Bias Control Of Mach-zehnder Electro-optic Modulator Based On FPGA

Lithium niobate Mach-Zehnder electro-optic modulator(MZM)is one of the key devices of microwave photonic link.Different modulation formats can be realized by properly setting the DC bias point of MZM.Therefore,the accuracy and stability of the DC bias point of MZM directly affect the performances of microwave photonic link,such as spurious-free dynamic range(SFDR).In this thesis,the stability of MZM's bias control was studied theoretically,then an FPGA based adaptive bias control system was designed and manufactured.And the MZM's bias stability using the control system were tested and analyzed.In the first part of this thesis,the bias drifting of lithium niobate MZM electro-optical modulator was simulated and analyzed.The control method using the slope of MZM's output power is proposed.The influence of system parameters on control accuracy was investigated,including voltage step and MZM output optical power acquisition precision.The results shows that the control algorithm based on the first partial derivative of MZM output optical power can reach 2° in four typical DC bias points(including the positive quadrature bias point(+quad),the negative quadrature bias point(-Quad),the maximum bias point(Peak),the minimum bias point(Null))The control scheme avoids the introduction of low frequency dither signal in the link.In the second part,the hardware and software of the control system according to the scheme was designed and tested.Firstly,according to the parameters of laser,MZM and fiber coupler,the MZM DC bias point control link was constructed.The parameters of the system hardware were obtained according to simulation results.Then the components of in the system were selected and tested.Finally,the FPGA control algorithm and the host computer control software were designed.In the third part,a bias control system for MZM was constructed and tested.The bias control experiment results show that the bias angle drifting ranges of the +Quad bias point,the-Quad bias point,the Peak bias point and the Null bias point are-0.8298°?0.6718°,-0.2413°?0.2803°,-0.2776°?0.1266° and 0°?1.33°,respectively.In addition,the modulator bias point control system can automatically adjust and relock the bias point in about 3 seconds even with large bias drifting.Since the dither signal is not introduced,the control scheme has the advantage of not affecting the SFDR compared with the control scheme introducing dither signal.