Linearity Of The Microwave Photonic Link Using Direct Modulation And External Modulation

By combining microwave and optical techniques,microwave photonics is able to realize high speed processing and broadband transmission of microwave signal.As the main field of the microwave photonics,microwave photonic link is characterized by significant advantages,such as large bandwidth,small volume,light weight,low loss,immunity to electromagnetic interference and so on.Therefore it has been widely used in the civil communication and national defense.However,the nonlinear characteristics of the microwave photonic link,such as saturation,frequency mixing,frequency multiplication,harmonic and intermodulation,will greatly disturb the communication quality,in particular with high-power input signal.Therefore,the study and optimization on the linearity of the microwave photonic link are of great significance.Firstly,the microwave photonic link is introduced,including transmitter,transmission link and receiver.At the transmitter,the microwave photonic link is categorized into two modes according to different modulation methods,direct modulation or external modulation.The nonlinear distortions under the two modes are analyzed by theoretical modeling.In order to compare the nonlinearity,related performance parameters are introduced under different modulation signals,including link loss,compression dynamic range(CDR)and spurious free dynamic range(SFDR).Then,experiments are carried out.The microwave photonic link under the two modes are set up to obtain key performance parameters through analysis of acquired signals.The microwave photonic link based on direct modulation using directly modulated laser(DML)overperforms the one based on external modulation in terms of parameter comparisons.And the microwave photonic link using direct modulation has been characterized with high linearity for short-distance transmission as its performances drop significantly in the 10 km optical fiber transmission.Concerning the weakness of DML in terms of linewidth and chirp,the frequency responses of the two modes link are measured.The weakness brings complex dispersion distortions for fiber links with different lengths.The experimental results show the microwave photonic link using direct modulation has been characterized with high linearity for short-distance transmission.Finally,two linearization methods for microwave photonic link are proposed and verified by using OptiSystem,based on the theoretical analysis of the external modulated microwave photonic link.The first one is the improved structure using dual-parallel Mach-Zehnder modulator(DPMZM).The carrier interference ratio(CIR)can be increased by controlling the three dc biases of the DPMZM,to meet the relevant conditions to implement the suppression of the third-order intermodulation distortion(IMD3).An increase of 33 dB or 14 dB on CIR is obtained under the back-to-back(BTB)link or 10 km fiber link,when the laser linewidth is set as 2 MHz.As the laser linewidth is reduced to 1 KHz,a 20-dB increase on the CIR is achieved for the 10 km fiber link.The other is the digital signal processing(DSP)linearization using coherent detection.The improved linearization is realized by the digital acquisition of the output signals and the offline processing based on the system transfer function,showing advantages from coherent detection and DSP.