Research On Compensation For The Impairment Of Modulator Nonlinearity And Dispersion Effect In Optical OFDM System

The orthogonal frequency division multiplexing(OFDM)technology has received a lot of attentions and actively investigated in optical communications due to its high spectrum efficiency,ability of resistance to inter-symbol interference and the good performance of anti dispersion.The OOFDM system based on the direct detection(DDO-OFDM)is very easy to be implemented and has the merits of low cost and stability operation.This paper focuses on how to improve the transmission performance of the OOFDM system based on the direct detection.The main research results are as follows:The technology of pre-distortion is employed to compensate the effect caused by the Electro-optical modulator.We theoretical analyzed the Nonlinear damage of the OFDM signal caused by the Electro-optical modulator,then put forward a kind of pre-emphasis technology to compensate the signal attenuation.By simulation experiments,we observed that at high modulation index,the increase of OSNR is not linear to improve system EVM value,the system is mainly influenced by the nonlinear interference of electro-optic modulator.At higher modulation index,nonlinear effect is relatively weak with the pre-distortion,and the system EVM values are obviously improved.The technology of pre-emphasis is employed to compensate the attenuation of the OFDM signal caused by the frequency selective fading due to the fiber dispersion.We theoretical analyzed the attenuation of the OFDM signal caused by the frequency selective fading due to the fiber dispersion,and then put forward a kind of pre-emphasis technology to compensate the signal attenuation.We obtained the improvement of system performance caused by using digital pre-emphasis technique in the transmitter under different power attenuation factor ? by Monte-Carlo simulation.When ? is large,that is when the power attenuation of high-frequency subcarrier is small,nonlinear interference in MZM limited the system performance.As the value of ? decreases,fiber dispersion makes the power attenuation of high-frequency OFDM signal subcarrier become larger.Compare using both the technology of pre-emphasis and MZM pre-distortion to using none of pre-emphasis and MZM pre-distortion or using one of pre-emphasis and MZM pre-distortion,the transmission performance is significantly improved.