Research On Radio Over Fiber And Linearization Technology

With the booming development of multimedia services featuring ultra-wideband and large capacity,Radio over Fiber?RoF?system is becoming one of the hot topics in the communication industry.RoF system transmits high frequency and broadband signals using fiber as medium,which has the advantages of the fiber communication and radio communication,including large bandwidth,high capacity,low loss,strong flexibility,resistance to electromagnetic interference and so on.In the view of these merits,RoF system will become the future development direction in communication.However,the inherent nonlinearity of the key devices in RoF system,for example modulator,seriously affects the performance of RoF system.How to suppress the nonlinear distortion and improve the dynamic range further is one of the research issues in RoF communication field.In this thesis,we mainly study the linear optimization technology in RoF system.Firstly,the thesis summarizes the background and the significance of RoF system,as well as the structure,advantages and typical applications,and then sums up the linearization schemes reported at home and abroad.Next,the key devices in RoF system are introduced respectively,such as laser,electro-optic modulator,photodetector etc.The common modulation patterns based on Mach Zehnder modulator?MZM?are also described.Then,we analyze the main performance indexes of Ro F system,such as link gain,noise figure and dynamic range etc.Among them,the spurious free dynamic range?SFDR?is one of the most key indicators evaluating the Ro F system.Focusing on suppressing the third order intermodulation distortion?IMD3?and improving the SFDR effectively,we propose three novel linearization schemes based on the balanced detection technology.In these configurations,the IMD3 is suppressed.At the same time,even order distortion and common mode noise are reduced,the power fading induced by dispersion and the frequency conversion are also solved respectively.These schemes greatly improve the SFDR of RoF system,and implement the linearized RoF link with broadband and large dynamic range.The linearization scheme based on two double parallel Mach Zehnder modulators?DPMZM?in parallel and the balanced detector?BD?,not only suppresses the IMD3 and the even order distortion,but also decreases part of the noise effectively.The scheme utilizes the DPMZM's sub-modulators working on different statuses to realize the suppression of the IMD3.Then we adopt two parallel DPMZMs to obtain signals which satisfy the requirement of the balanced detection technology.As a result,the even order distortion and common mode noise can be suppressed by the balanced detection.The simulation and experimental result show that the second order and third order distortion are reduced obviously,and the SFDR can reach to 124.5dB Hz^2/3.The linearization scheme based on parallel MZMs and BD guarantees the signal transmits in the fiber for a long distance,while the RoF link is linearized by suppressing IMD3.We use MZMs working on single side band modulation mode to solve the power fading induced by dispersion.Then,two parallel MZMs and BD are adopted,and the IMD3 can be suppressed by controlling the power of two signals.The simulation shows that the IMD3 is reduced more than 35dB,and SFDR is improved more than 20dB compared with single MZM's signal.The SFDR reaches to 125.8dB Hz^2/3,which indicates that the linearization effect of the scheme is very well.The linearization scheme based on double polarization double parallel Mach Zehnder modulator?DP-DPMZM?and BD implements the frequency conversion and linearization simultaneously,which achieves a set of system with multiple functions.This scheme utilizes single DPMZM to realize the frequency conversion by modulating the Radio Frequency?RF?and Local Oscillator?LO?simultaneously.To suppress the IMD3,we take advantage of the character combined DP-DPMZM and BD.The linearized signal is finally got by controlling the power of RF signals and LO signals.The simulation indicates that the IMD3 is reduced by 25dB,and SFDR is promoted by 25d B in comparison to single DPMZM's signal.The dynamic range can reach to more than 120dB Hz^2/3,which achieves a frequency conversion system with large dynamic range.