Research On Radio Over Fiber And Coherent Optical Communication Based On Structural Optical Multiplexing

With the increasing demand of user traffic for broadband services,the communication rate is constantly put forward higher requirements.As the main technology of data access network,radio over fiber(ROF)communication and coherent optical communication provide technical support for wireless mobile terminals and optical broadband services,respectively.As a long-distance transmission scheme of modulating wireless signal to the optical sideband of optical fiber,ROF system realizes seamless fusion and flexible switching of radio frequency signal between optical fiber and wireless end,solving the high loss problem of long-distance wireless transmission,and greatly improving the communication capacity.The coherent optical communication technology employing the balanced detection signal demodulation method has higher sensitivity and larger communication capacity than ROF system.The classical optical communication system mainly utilizes optical multiplexing technologies to realize the capacity expansion,such as wavelength division multiplexing(WDM)and optical time division multiplexing(OTDM),for which,the development is close to the bottleneck at present.Under the situation of the increasing communication rate demand,exploring and researching a new physical dimension has become the key to further greatly improve the system capacity.As a singular beam carrying various orthogonal modes,structured light doesn't need to occupy extra optical bandwidth and spectrum resources,and can be employed for spatial optical multiplexing in the classic ROF or coherent optical communication system.At present,two structure lights called vortex optical beam(OV)and cylindrical vector beam(CVB)are generally used for mode multiplexing communication,which are supported for transmission in free optical space(FSO)and orbital angular momentum(OAM)fiber or few / multi-mode fiber channels.Structured light can be used in ROF system and coherent optical communication system,which greatly improves the capacity of optical communication system.However,there are few researches on the application of structured light in ROF system,especially the research on the CVB in a high-frequency millimeter-band wireless system.In the FSO communication based on the OAM multiplexing,the spiral phase of the beam is easily disturbed by the atmospheric turbulence,which leads to the decrease of the optical signal-to-noise ratio(OSNR)and the deterioration of the transmission stability of the communication system.Aiming at the above two problems,this paper mainly uses the method of theoretical analysis and experimental verification.Firstly,the transmission of the CVB in ROF communication system is studied.In addition,the channel estimation technology is employed to enhance the resistance of the multi-mode OAM beam to atmospheric turbulence in the coherent optical communication system.The main works and innovations described in this paper can be summarized as follows:(1)The modulation emission,information transmission and signal reception of single carrier QPSK-and multi carrier QAM-OFDM signals in coherent optical communication system are investigated experimentally.In this work,a coherent optical communication experiment system is designed,which is capable of transmitting single carrier and multi carrier signals formats.And the QPSK signal of 25 GBaud and QAM-OFDM signal of 12 GBaud are effectively transmitted.In addition,combined with the experimental results,the off-line digital signal processing flow,such as clock synchronization,frequency offset / channel estimation and phase recovery are all analyzed.And the variation of bit error rate(BER)and error vector amplitude(EVM)value with the optical signal-to-noise ratio(OSNR)is studied.(2)We proposed and experimentally demonstrated a 100-GHz millimeter wave(MMW)ROF communication system based on CVB multiplexing.In this work,the multi-mode CVBs carrying QPSK-OFDM signals with a rate of 6Gbit/s are employed for multiplexing,and two types of ROF communication system composed of "free space to wireless" and "free space to few mode,and then to wireless" are realized,respectively.For this job,the multiplexed CVBs are applied to the classic ROF communication system with high-frequency band,which improves the system capacity linearly and solves the bottleneck problem of capacity improvement for high-frequency MMW-band corresponding to the ROF system.In addition,the dielectric PB-phase meta-surface used to generate and de-multiplex the CVBs with variety vector modes occupies the characteristics of wide operating wavelength ranging from C to L band,a tunable q-parameter and a high efficiency over 85%.Hence,it can be perfectly compatible with the wide optical sideband of high frequency MMW.(3)We proposed and experimentally demonstrated an OAM multiplexing in a free space optical communication system using ISFA technology to mitigate atmospheric turbulence(AT).In this work,the multi-mode OVs carrying different QAM-OFDM signals are employed to realize a free space orbit angular momentum(OAM)multiplexing communication.By loading and switching different phase screens on the spatial light modulator(SLM),the loading and switching of AT phase screens with different intensity and transmission distance are realized.The generation and de-multiplexing of multi-mode OVs are realized by using a dielectric MS with adjustable q-parameter.The frequency domain averaging technology is mainly used to suppress the noise response of random turbulent channel,which greatly improves the sensitivity of communication system.Compared with the optical method to realize the noise compensation scheme brought by turbulence,this work uses the digital frequency domain to directly process the signal damaged by atmospheric turbulence,which is more direct.At the same time,the scheme can be compatible with the optical compensation scheme,so as to further optimize the effect of noise compensation,and has high tolerance for turbulence intensity and transmission distance.