Research On The Bias Control For External Modulators In RoF System

Combined with the advantages of wireless communication and optical fiber communication technique,radio-over-fiber communication system finds its widely use recent years.But the working point of the electro-optic modulators,which is a key device in Ro F system,may drift away due to the influence of external factor,which will impact the communication quality of the system badly.It calls for an urgent need for bias control for the electro-optic modulators.For this problem,bias control for the electro-optic modulators is studied in this paper,and finally a bias control scheme to stabilize three working points of the DPMZM,which is used in a linearity improving scheme to increase Spurious-free Dynamic Range of the Ro F system,at arbitrary points is proposed.And then the improvement is offered to this scheme,the electric circuit is designed.Based on the physical feature of the material of the electro-optic modulators,the phenomenon and the reason of working points drifting are discussed.Then several kinds of usual modulators and modulation formats are introduced,especially a scheme based on DPMZM to increase Spurious-free Dynamic Range of the system by adjusting the three working points to the optimum positions to suppress the IMD3.Then several kinds of bias control methods for MZM are analyzed in detail,including scheme by monitoring the output optical power,scheme by adding pilot tone,and modified scheme for monitoring optical power——monitoring optical power backward.The principles of schemes above are derivate,the advantages and disadvantages are analyzed.Then a traditional method to control DPMZM working at peak,null and quadrature points by adding pilot tone is introduced and derivate,and the advantages and disadvantages are analyzed.The necessity for bias control at arbitrary points for DPMZM is demonstrated for the linearity scheme introduced,and a new method to control DPMZM working at any points by adding three pilot tones and analyzing harmonics of the three pilot tones in different slot time is proposed.First the principle of the method is derivate,then the method is verified by simulation with VPI.At last a modified scheme is come up with to eliminate the error arising during the simulation.The whole electric circuit for the proposed scheme is designed finally.Then the circuit is realized by modules,including pre-amplification module,filter module,MCU module,pilot tone generating module,and power supply module.Choosing appropriate chips to demonstrate the module function,then adjusting the parameters to simulate each module to show the feasibility,and finally finish the key circuits design.