Research On The Bias Voltages Control Technology Of Advanced Optical Modulation

With the rapid growth of internet data services in recent years, including onlinevideo, high-definition television and other new business, the demand of terminal userfor increased bandwidth will be more significantly. Core network traffic growsexplosively, and the requirements of optical transmission system capacity are more andmore stringent. Due to its higher spectrum efficiency, better nonlinearity tolerance,better dispersion and polarization mode dispersion (PMD) tolerance, and lower cost,advanced optical modulation formats have been widely used in the next-generation ultrahigh-speed optical fiber communication systems.This paper mainly studies feedback control performance for multiple bias voltagesof QPSK and16-QAM optical transmitters according to IQ optical modulator. Firstly,the existing bias control methods are studied, including monitoring the average outputoptical power and its statistics (mean and variance), analyzing asymmetric biasdithering, back-ward light power, and low frequencies radio frequency (RF) spectralpower. The average output optical power method is used widely. Secondly, based on theaverage output optical power method, the cooperative control of two Mach-ZehnderModulators’(MZMs’) bias voltages and Mach-Zehnder interferometer’s (MZI’s) biasvoltage is analyzed theoretically. Multiple extreme values of the average output opticalpower as the monitoring signal of feedback control for two MZMs’ bias voltages arealso studied by numerical simulations. Lastly, in order to verify the correctness of thetheoretical analysis, we build the QPSK and16-QAM optical signal transmitters, anddesign the experimental verification system. The optical transmitter is composed ofDFB laser (Agilent hp8164A), IQ optical modulation (JDSU10G DPMZ) and arbitraryfunction generator (AFG3102). The bias voltage and the average optical power will beachieved by LabVIEW software, and the experimental results are compared with thetheoretical simulation.The results show that the average optical powers of QPSK and16-QAM opticaltransmitters are both the extreme values when MZMs’ bias voltages are biased at thenull transmission points. This conclusion is unaffected by MZI’s bias voltage, and theredoes not exist the risk of trapping in local extremum if the amplitude of the driver signalis controlled in the proper range. This study is good for ensuring the robustness ofadvanced optical signal generation.