Study Of Photonic Generation Of Millimeter-wave Signal And Associated Techniques For Radio-over-fiber System

With speedy development of mobile communications and Internet networks, thecurrent microwave spectrum can not provide enough bandwidth to support increasingmultimedia broadband services. So, millimeter-wave (mm-wave) band is considered to bean effective pass to larger capacity and higher data rate wireless communication. Radioover fiber (ROF) technology is a combination of optical fiber communication technologyand mm-wave communication technology. It can effectively extend the transmissiondistance for the mm-wave signal at atmosphere and realize broadband wirelesscommunications. Therefore, ROF technology has wide practical prospect in multimediamobile communications, fixed broadband wireless access networks, local area networksand intelligent traffic, etc.This thesis focuses on the study ofphotonic generation of millimeter-wave signals andassociated techniques for ROF system. The main contents and conclusions of the thesis areas follows.(1) Starting with the electrical current expression of a mm-wave signal generated byoptical heterodyning, the conditions of photonic generation of the mm-wave signal isdiscussed. The top frequency and restriction factors for the electromagnetic wave signalgenerated by optical heterodyning are analysed, and it is found that the top frequency ismaily restricted by the formation conditons of the optical beat signal and the frequencybandwidth of the photodetector.(2) The characteristics of the longitudinal-mode of the erbium-doped fibre (EDF) ringlaser are analysed, and the generation conditions and measurement methods of thesingle-longitudinal-mode of the laser are explained. On the basis of the interferenceprinciple of the standing wave and rate equation for the EDF, the mathematical expressionfor the absorbing coefficient and the FBG's bandwidth of the EDF saturable absorber arederived, and the principle of selecting the single-longitudinal-mode of the dual-wavelengthlaser is presented.(3) A novel approach to synthesize a narrow dual-transmission-peak filter bycascading a tunable bandpass filter with a bandstop filter is presented and experimentallydemonstrated. By using the synthesized narrow dual-transmission-peak filter, two switchable dual-wavelength fiber ring lasers with the frequency intervals of 49.85 GHz and59.83 GHz, respectively, are achieved. The stable mechanism and output power propertiesof the dual-wavelength lasers are investigated.(4) By cascading a delay interferometer as a comb filter with a tunable bandpass filter,and employing a saturable absorber formed by a unpumped polarization-maintaining EDF(PM-EDF) to select the single-longitudinal-mode of the laser, a stable wavelength-tunabledual-wavelength single-longitudinal-mode fiber laser is demonstrated. Two beating signalsat 20.07 GHz and 39.96GHz, respectively, with the linewidths of less than 25 kHz areobtained by beating the two wavelengths at a photodetector.(5) Aiming at the disadvantages of the wider bandwidths for the filters such as a DI, aproposal for photonic generation of the mm-wave signal is presented by employing a dualwavelength single-longitudinal-mode fiber laser based on a microring resonator.Theoretical simulation shows that the comb filer with the 60 GHz-level frequency spacingand pm-level bandwidth of the resonant peak can be obtained by properly choosing theradius and the coupling coefficient of the microring resonator, which has the possibility ofreducing the longitudinal modes of the fiber laser and improving the stability of the beatingsignals.(6) The micro-/nao-fibers with about 5.8μm diameters is fabricated by using astandard single-mode fiber and a flame-heated taper-drawing technique. The two kontresonators with the different ring diameters are designed and assembled with themicro-/nao-fibers, respectively. The experimental results indicate that the resonators hasobvious comb resonant spectrum, smaller bandwidths of the resonant peaks than the DI. Bycascading the knot resonator with a tunable bandpass filter and employing a PM-EDFsaturable absorber, a wavelength-tunable dual-wavelength single-longitudinal-mode laser isdemonstrated. Two beating signals at 15.57 GHz and 35.57GHz respectively are obtaintedby beating the two wavelengths at a photodetector. This verifies the proposal for realizationof the dual wavelength single-longitudinal-mode fiber lasers and photonic generation ofmm-wave signals by employing the microring resonators.(7) The design of the generator for mm-wave signals in a ROF sysytem is discussed.The way of the connection between the transmitting antenna and photodetector is explained,and the mehtods of the selecting the photodetectors are derived. A generator for a 60 GHz mm-wave signal based on the zero-biased uni-travelling-carrier photodetectors is designed,and its possibility is analysed.