Research On Automatic Bias Control And Distortion Compensation Algorithms For IQ Modulation

Compared with the traditional intensity modulation and direct detection systems,the amplitude,phase,frequency and polarization state of the optical carrier are used to carry information in coherent optical communication systems,improving the spectrum efficiency of communication system,thus they are widely used in high speed optical communication systems.In coherent optical communication systems,IQ(In phase and Quadranture)modulators are usually used at the transmitter to generate various high-order and multi-dimensional modulation formats.The IQ modulator is composed of two sub MZMs(Mach Zehnder modulator)nested in a MZ interferometer(MZI).Due to the influence of environmental temperature change,device aging and manufacture tolerance,there are some problems in transmitter,such as half wave voltage drift,IQ amplitude imbalance and IQ delay skew.These problems will lead to obvious deterioration of IQ modulated optical signal quality.In this paper,the corresponding automatic bias control(ABC)and compensation algorithm based on digital coherent optical receiver are designed for the IQ modulation.Firstly,aiming at the half wave voltage drift of IQ modulator,an ABC scheme based on optical power monitoring of output signal is proposed,which does not need dither signal so that it has the advantages of simpler structure and lower cost.In the proposed scheme,the first-order and second-order derivatives of the average power of the output signal are used to monitor the MZM bias voltage of the IQ channels.For high-order QAM signal,the backward light is used instead of forward light to improve the monitoring accuracy and make the ABC of two branch of MZM transparent to the modulation format.Compared with the traditional monitoring scheme which only uses average optical power,the new scheme eliminates the influence of input power variation,RF signal swing and phase imbalance.Thus,the bias voltage can be adjusted by look-up table operation without any iterative operation,and the response speed of ABC system is improved.On this basis,based on the influence of phase imbalance on the time domain waveform characteristics of the signal,a low bandwidth photodetector is proposed to monitor the instantaneous power and its variance of the modulated optical signal,so as to realize the automatic bias control of the MZI.This paper further analyzes the influence of IQ amplitude imbalance and IQ component delay skew,and proves that the new ABC scheme has good robustness.Aiming at the phenomenon of IQ amplitude imbalance and IQ component delay skew in modulation,an improved adaptive equalization algorithm is used to compensate signal impairment at the receiver.The simulation results show good performance for QPSK(Quadrature Phase Shift Keying)and 16QAM(Quadrature Amplitude Modulation)optical signals.