| Radio frequency signal processing over optical fiber is an emerging cross-research field involving radio frequency technique and photonics,which relates to fiber-optics communications,wireless communications,microwave engineering,analog and digital signal processing,optics and electronics integration,optoelectronic materials and components,radio-over-fiber communication systems and network applications and other related fields.The original intention of the research of radio frequency over optical fiber technology is to introduce powerful photonic technology into the radio frequency system,thereby eliminating the electronic bottleneck while bringing many advantages,such as high speed,low loss,large bandwidth,small size,low power consumption,light weight,high technical advantages such as integration,excellent stability,anti-electromagnetic interference,flat frequency response,and easy hybrid integration.Therefore,by using photonics-assisted radio frequency signal processing technology,functions or tasks that were previously difficult or even impossible to complete in the electrical domain can be realized.Because of this huge advantage,since the beginning of research in this field in the 1990s,radio over fiber communications have been widely used in the fields of signal processing,civil communications,national defense and military,aerospace and medical health,which has attracted scholars' wide attentions at home and abroad.The radio frequency signal processing over optical fiber and the application of radio over fiber(RoF)communication systems are two important research branches of microwave photonics.In recent years,they have attracted great interests from researchers and become the current research hot topics of microwave photonics.This thesis focuses on the needs of civil and national defense military applications such as radio-over-fiber communications,optical fiber and radio frequency hybrid access networks,and microwave photonic radars.Research work are mainly on two aspects of communication system design and applications.The research contents and innovations of this dissertation are as follows:1.A multi-modality coherent radio-over-fiber communication scheme based on optical tandem single-sideband modulation and optical orthogonal single-sideband multiplexing is proposed.In order to solve the needs and challenges faced by multi-purpose radio frequency signal transceiving and transmission,a multi-modality coherent radio-over-fiber communication system using optical tandem single sideband modulation(OTSSBM)and optical orthogonal single sideband multiplexing(OOSSBM)is proposed.Digital signal processing algorithms-assisted coherent detection is used at the receiving end to identify and separate the mixing channels of multiplexed phase modulation format signals,and the multi-rate signals modulation,transmitting,transmission,reception,and demodulation are realized in the coherent radio-over-fiber communication system.(1)The multi-modal coherent RoF system is designed and numerically simulated.The spectrum structure of the radio frequency signal in the optical domain in the RoF system is analyzed,and the transmitted 2 Gbit/s and 5 Gbit/s signals are recovered at the receiving end by digital signal processing algorithms.The comparisons of the time-domain waveform and eye diagram for the BPSK signals before transmitting and after receiving are showed.A multi-channel high-spectrum-efficiency coherent radio over fiber communication experimental platform has been built for the transmitting,transmission,reception and processing of optical-borne radio frequency signals.The experimental results show that for the proposed multiplexing techniques and schemes under different types and conditions(i.e.single-channel and dual-channel;OTSSBM and OOSSBM;40 km single-mode fiber transmission and back-to-back system,etc.),the system performance is satisfying over 40 km single mode fiber transmission,and the bit error rate(BER)is lower than 10-9,and the quality factor(Q-factor)achieves above 6.(2)The following case is analyzed.When OTSSBM and OOSSBM are used to transmit BPSK signals at 2 Gbit/s and 5 Gbit/s,respectively,under the premise of maintaining moderate energy efficiency,the spectral efficiencies of the two multiplexing schemes achieves 4.2 bit/s/Hz and 4.9 bit/s/Hz for OTSSBM and OOSSBM,respectively,and the comprehensive utilization of both OTSSBM and OOSSBM schemes can achieve 7.4 bit/s/Hz for spectral efficiency.While improving the spectral efficiency and channel capacity of the optical single-carrier radio over fiber communication system,coherent detection assisted by digital signal processing algorithms is used for signal demodulation and recovery,without adding additional mixing channel separation hardware or optoelectronic components,which simplifies the system structure and complexity.2.The integrated coherent RoF transmitter and receiver based on silicon photonics are designed.Using the cascaded silicon photonic micro-ring resonator(MRR)structure,a high quality factor(high-Q),ultra-narrow-band,and tunable three-passband optical bandpass filter with wavelength selectivity is designed,and the scheme of optical multi-carrier generation based on silicon MRR is realized.A silicon photonic dual-electrode Mach-Zehnder modulator(DE-MZM)is designed for modulating high-speed radio frequency signals.Utilizing the designed MRR filter and DE-MZM and other silicon photonic components,an integrated RoF transmitter that transmits multi-channel radio frequency signals and provides multi-type radio frequency signal access is designed.In addition,A hybrid integrated digital coherent optical receiver is designed using silicon planar optical waveguides,and the system performance of the designed integrated transmitter and receiver is verified and tested.(1)An ultra-narrow-band and tunable optical band-pass filter(BPF)using silicon add-drop MRR is designed.The designed single-ring resonator BPF has a central wavelength at 1552.52 nm,a full width at half maximum(FWHM,i.e.3 dB bandwidth)at 0.04 nm,and a free spectral range(FSR)at 10 nm.The filter has steep rising and falling edges of window,and the filter passband can be tuned by thermo-optic effect.By cascading three silicon single-rings,a multi-band tunable narrow-band optical bandpass filter with wavelength selectivity and reconfigurability is formed.Each frequency band contains a flat passband,and the channel spacing(i.e.FSR)is up to 10 nm.The absorption loss is less than 3 dB/cm,and the fineness of each MRR filter is 250.The Qtotal reaches 38750,and the cascaded multi-band MRR filter can generate multi-carrier optical comb with size dimension in millimeters.(2)A high-speed silicon dual-electrode Mach-Zehnder modulator(DE-MZM)with 30 GHz bandwidth and data rate close to 10 Gbit/s for BPSK signals is designed.The above three optical frequency bands generated can be used as optical carriers to modulate different radio frequency bands and various types of radio frequency signals.When BPSK modulation format is used to be transmitted,each channel can reach data rate at 10 Gbit/s.A variable optical attenuator(VOA)based on silicon waveguide with sub-micron size is designed and its characteristics are analyzed.A dual-parallel dual-electrode Mach-Zehnder modulator is designed,which can constitute an I/Q modulator.Three frequency bands of MRR filters and three silicon modulators are connected in series and then connected in parallel,which can be modulated on three optical carriers and loaded with various different types of broadband signals(such as WiFi,WiMAX and other radio frequency signals;it can also be combination of digital signal and analog signal).The integrated RoF transmitter has the whole chip size of 7.8 mm2 in millimeter scale.(3)In order to solve the further requirements of the coherent radio over fiber system for the digital coherent receiver in terms of integration,power consumption,operation stability,sensitivity,responsivity amplitude fluctuation,and phase deviation,an integrated digital coherent optical receiver front-end based on silicon planar optical waveguide is designed,and the performance and parameters of the designed integrated coherent receiver front-end module have been tested.In the wide wavelength range from 1520 nm to 1620 nm,the phase drifting is withiną1°,which ensures the excellent phase orthogonality of the corresponding ports.When the temperature is between-5? and 80?,the responsivity amplitude fluctuates withiną0.25 dB,and the responsivity amplitude deviation between adjacent photodetector ports is within 0.4 dB.The receiving performance of signals with PDM-QPSK modulation format at 112 Gbit/s is tested.The clear constellation diagrams on the X and Y channels in the polarization orthogonal direction,which are easy to judge,are obtained.The quality factor(Q-factor)and signal optical power(optical signal-to-noise ratio,OSNR)has an approximate linear relationship.3.A scheme based on DP-DPMZM and SOA-MZI is designed for radio frequency(RF)signal processing in optical domain.In order to implement multiple functions,improve system integration and reduce costs for radio frequency signal processing in a fiber-optics system,a comprehensive scheme is designed for the three significant techniques of radio over fiber signal processing-phase shifting,filtering,and frequency multiplication.(1)Based on the dual-polarization dual-parallel Mach-Zehnder modulator(DP-DPMZM),a broadband RF photonic phase shifter with frequency multiplication function is designed,which not only performs from frequency doubling to frequency sextupling control on the RF signal,but also realizes 360° phase shift of the radio frequency signal in the optical domain.The phase shift range and frequency multiplication effect are verified by simulation.The relationship between the amount of phase shift and the phase control parameter are approximately linear,and the two types of RF photonic signal processing-frequency multiplication and phase shift control are carried out separately and simultaneously.The effects of the change of extinction ratio,the impacts of amplitude and phase imbalance of the 90° mixer on phase drifting,amplitude fluctuation and system stability are analyzed.(2)With the help of single-sideband modulation(used to carry radio frequency signals)of Mach-Zehnder modulator(MZM)and the optical non-linear effects(slow light effect and coherent population oscillation)of the semiconductor optical amplifier(SOA),a 3 dB bandwidth and center wavelength tunable radio frequency photonic filter is designed.The center wavelength of the RF photonic filter is adjusted in the frequency range from 15 GHz to 20 GHz,and its free spectrum range(FSR)exceeds 15 GHz(different center wavelengths have different FSRs,and the lowest FSR also exceeds 15 GHz).By adjusting the injection current of the SOA,the frequency band and 3 dB bandwidth can be adjustable.When the SOA drive current is around 420 mA,FSR=15.44 GHz;the 3 dB bandwidth of the filter passband BW3dB=2.45 MHz,the quality factor Q-factor>6300(for a single-passband filter,the center wavelength is similar to the value of FSR,so Q-factor=Finesse=FSR/BW3dB?6302),and the out-of-band rejection ratio of the filter reaches 41 dB.(3)A polarization beam splitter,a polarization coupler and two SOAs are used to form a Mach-Zehnder interferometer structure(SOA-MZI).Therefore,a broadband RF photonic phase shifter is designed and numerically simulated.The simulation results show that the dynamic range of the phase shift reaches 360°,the adjusting accuracy of the phase shift reaches 0.1°,the phase shift bandwidth is close to 30 GHz,the phase shift amount and the SOA drive current show a satisfactory linear relationship,and the phase shift amount can be continuously adjusted according to the phase shift accuracy.These characteristics mentioned above are superior to traditional solutions.In addition,a preliminary analysis is made on the causes of nonlinear distortion of the designed RF photonic phase shifter.The above three innovation points not only improve the channel capacity,spectrum efficiency and multi-modality application of the radio-over-fiber communication system,enrich the diversity of RoF signal transmitting types and access services,but also improve system integration,lower power consumption,reduce the component size,and enhance the stability and reliability of the system.The phase of the radio frequency signal is continuously and accurately controlled in the optical domain,and the radio-over-fiber signal processing such as frequency multiplication and filtering is performed at the same time,which enhances the comprehensive functions of the radio-over-fiber signal processing system.This dissertation focuses on ultra-wideband wireless access networks based on radio-over-fiber communications,microwave photonic radars,optically-controlled phased array antenna systems,electronic countermeasures systems,and other fields that require high-performance radio-over-fiber signal processing.The research achievements obtained has certain practical value and application prospects in related research areas in the future. |
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