Study On Triangular-shaped Pulse Photonic Generator And TeraHertz Waveguide-based Gratings

With the development of broadband wireless access technology,the low frequency band can not meet the demand of bandwidth.As a new interdisciplinary subject,microwave photonics combines microwave and photonics,has made significant progress in a number of areas,including microwave/millimeter photonic generator,photonic signal processing techniques and radio over fiber(RoF)system.Microwave photonics has been considered as a prospective subject,who can find many applications in both military and civilian areas,including high-speed broadband wireless communication,radar and satellite systems and so on.With the further exploration of spectrum resources,the research scope of microwave photonics is expanding towards Terahertz(THz).Therefore,it is necessary to study the waveguides and waveguide-based THz devices applied in THz systems.Under the supports of the National Nature Science Foundation of China,this thesis is devoted to investigate new prototype of microwave photonic generator and subwavelength waveguide-based THz gratings,by theory,simulation and experiment.The main innovative research efforts are summarized as follows:1.Two novel structures of periodic triangular-shaped pulse photonic generators based on continuous radio frequency(RF)modulation combined with dispersion effect.In both two proposals,radio frequency power fading effect caused by optical dispersion is utilized to make the expression of ouput optical intensity approximately equals to the Fourier expansion of idea triangular-shaped pulses.Periodic triangular-shaped pulse sequence with repetition rate quadruple of driving frequency can be obtained.One proposal is based on quadrupling RF modulation combined with grating dispersion,triangular-shaped pulse signal is obtained with a high modulation depth(m=4.438).For another proposal,it is based on two cascaded Mach-Zehnder modulators(MZMs)and triangular-shaped pulse signal is obtained with lower modulation depth(1?m1?3,m2=0.606),which make the scheme more practical.2.A novel structure of periodic triangular-shaped pulse sequence generator based on four-wave nixing effect(FWM)effect in a semiconductor optical amplifier(SOA)has been proposed.Firstly,two frequency components are generated after small signal modulation in a dual-parallel Mach-Zehnder modulator(DP-MZM),which can be used as pumping beams of the FWM effect in SOA.After FWM effect,two new frequency components with the same data information are generated.Then the carrier component is recovered,after which there are five frequency components in the signal optical spectrum.After removing the undesired frequency components by an optical band pass filter(OBPF),the triangular pulse signal can be realized by controlling the modulation index of 1.2 and the SOA bias current of 0.145 A.3.A novel proposal of triangular-shaped pulse train generator based on time-domain synthesis in a DP-MZM has been proposed and verified.By using a RF signal to modulate a continuous-wave(CW)via a DP-MZM,signals with square-like envelopes are directly generated firstly.When two identical square-like pulses with harmonics power ratio(9:1)between the 1st-and the 3rd-order components suffer a differential envelope phase shift(7c/2),the superposition of these envelops contributes to a triangular-shaped waveform in intensity.In this scheme,as no the dispersion elements or filters are used,the scheme has good tunability.In addition,this scheme only cantains a DP-MZM and a time delay line(TDL),which makes the cheme easier to be integrated.4.An optical triangular-shaped pulse generation approach which combines the quadrupling RF modulation technique and the active polarization control has been proposed.The generated triangular-shaped optical pulse train has tunable repetition rate which is four times of the driving frequency.By applying a linear polarizer(LP)with higher polarization degree,we generate the target pulse train using a relatively small modulation index.The merit of this proposal is that by changing the LP polarization angle to compensate the modulation depth,the modulation depth can be dynanically adjusted in a certain range(2.5?m?4.438).5.Waveguide-based THz uniform and phase-shifted gratings have been proposed.Inspired from the fiber Bragg gratings,our proposed filters are achieved by incorporating Bragg grating structure(periodic structural perturbations)in the subwavelength polymer waveguide along the THz wave propagation direction.The proposed subwavelength polymer waveguide can be fabricated using the cutting-edge 3D printing technique.In addition,as of the proposed filters are based on a cylinder waveguide,the filters cancel the polarization dependency of the transmission spectrum of the grating.This feature makes these filters more flexible in practiee.Our work illustrates a promising grating structure used in refractive index sensing with resolution of 2 GHz and sensitivity of 0.14 THz/RIU.6.THz gratings based on subwavelength rectangular polymer waveguide aiming at filtering two different polarization states simultaneously have been designed.Such grating can be used to efficiently filter both polarization and moreover,with an addition of a pi-shift,it can operate as an extremely narrow bandpass filter.The most striking feature is the simultaneous filtering of two polarization states.Such a simultaneous filtering has not been reported.This work would stimulate researches in the THz fields.