The Research Of The Generation Of Optical Millimeter Wave Based On The Polarization Modulator And The Key Technologies On The Radio Over Fiber Transmission System

Wireless communication technology has become an important development direction of the global communication network in the information age,because of its good characteristics.Wireless communication technology is easy to use with flexible user accesses and can exist everywhere.But at present,the available low frequency radio spectra are extremely crowded.And the narrow band communication rate which is available for wireless access cannot meet the needs of people in broadband communication services.Therefore,the millimeter wave broadband wireless communication is the future development direction.The Radio over Fiber(RoF)technology which can use optical fiber to transmit microwave or millimeter wave for a long distance has developed recently.The radio frequency technology which uses atmosphere as a transmission medium has a higher transmission loss and a limited transmission bandwidth.But the RoF technology can make up for this shortcoming and can easily use microwave photonics technology to process the microwave or millimeter wave signals in optical domain.In RoF system,the generation of high quality millimeter wave is the key technology to improve the performance of RoF system.At present,the research focus is how to obtain high quality and high frequency optical millimeter wave signal in this field.Another key technology in RoF system is the generation of frequency comb optical(OFC).OFC has an important application in RoF technology because it can generate arbitrary frequency optical millimeter wave.In addition,OFC is also widely used in the field of optical arbitrary waveform generation,the processing of the photonic microwave signal,the wavelength division multiplexing(WDM)system,the spectral measurement and so on.Because these applications all need ultraflat OFC.Therefore,the generation of OFC with high flatness and multi-comb-lines is another hot spot in the research of communication technology.In this paper,we first introduce the RoF technology and its system structure,characteristics,key technologies and applications.Next,the generation of optical millimeter wave signal in RoF system is introduced in this paper,especially the optical millimeter wave signal generation based on phase modulator(PM).Then,we introduce the frequency multiplication technique for the generation of optical millimeter wave,especially the frequency multiplication technique based on the nonlinearity of MZM,PM and PolM.Finally,the method to generate OFC based on external modulation in RoF system is introduced,especially the OFC generation based on MZM and PolM.On this basis,a scheme for the generation of RoF optical millimeter wave and a scheme for producing OFC are proposed in this paper:(1)On the basis of the study of the optical millimeter wave generation technology in RoF system,we propose a scheme of generating optical millimeter wave using two cascaded PolM based on frequency octupling.This scheme uses two cascaded PolM to suppress the odd order sidebands twice,and only leave the fourth order sidebands by the control of radio frequency driving signal and PolM modulation indexes.In this way,we can achieve the generation of frequency octupling optical millimeter wave.The system structure of this method is simple and easy to use.Since the modulator is PolM which can be use without adjusting the direct current(DC)bias,there is no influence introduced by bias drift.Therefore,the system has a high stability to generate the optical millimeter wave.In addition,there is no need of optical filter which is needed in traditional external modulation methods,so the generated optical millimeter wave has a high frequency tuning speed and a large tunable range.And the generated optical millimeter wave has a good transmission performance.(2)On the basis of the research of OFC generation technology in RoF system,we propose four schemes of generating OFC based on two cascaded PolM.Two PolM are modulated by two radio frequency signal with different frequencies.By adjusting the parameters,9,12,and 16 comb lines can be generated with tunable frequency spacing and high flatness.The scheme requires no DC bias voltage of the modulators and no optical filter,which makes our method simple and stable.