Research On High Performance Optical Nyquist Pulse Generation Technology For High Speed Broadband Optical Network

With the rapid development of various high-definition video and multimedia services,the demand for bandwidth is rapidly increasing.As a result,optical communication networks with Tbit/s transmission rate in single channel become the focus of research.WDM and OTDM technology based on optical Nyquist pulse is the most promising technology to realize super channels and improving spectrum efficiency,while optical Nyquist pulses are also widely used in all-optical signal processing,spectroscopy and optical storage.Therefore,it is of great practical significance to produce high-performance Nyquist pulses.There are many methods for generating Nyquist pulses.The main research in this paper is based on the time-frequency duality of the Fourier transformation.The Nyquist pulses can be obtained directly from the generation of a flat optical frequency comb that satisfies certain conditions.And the spectrum bandwidth of the generated Nyquist pulse is extended by the four-wave mixing(FWM)effect in a highly nonlinear fiber(HNLF).Based on the analysis of the modulation principle of electro-optic modulators,three generation schemes of Nyquist pulse based on optical external modulators are proposed.Scheme 1 is based on a dual polarization Mach-Zehnder modulator(DPol-MZM),which generates a 5-line optical frequency comb by controlling the operating state of the DPol-MZM upper and lower arm modulators,the carrier component contained in the output of lower arm sub-modulators cancels out the excess carrier component in the output of upper arm sub-modulator.Scheme 2 is based on the use of an intensity modulator(IM)in the Sagnac loop,combined with photo-generated microwave signal technology to generate high-frequency signals for driving the RF input port of the IM,thereby obtaining optical Nyquist pulses with high spectral line spacing in the frequency domain.Scheme 3 is based on two dual parallel Mach-Zehnder modulators(DPMZM)cascades.By controlling the relationship between the operating points of the sub-modulator and the respective input signals,a flat optical frequency comb with 2 or 4 comb lines is obtained.Remodulation of the generated optical frequency comb by the cascaded DPMZM,a flat optical frequency comb with linear phase and 4 or 16 comb lines is obtained.The optical frequency comb obtained by this scheme has a comb line spacing that is four times the minimum frequency of the two DPMZM loaded RF signals.Finally,this paper studies the spectrum bandwidth extended of the generated Nyquist pulse by using the FWM effect in HNLF.The performance of the optical frequency comb generated when IM works at the general transmission point and the maximum transmission point respectively is simulated.The optical frequency comb with 3 spectral lines and linear phase generated by IM is send to HNLF,use the FWM effect to perform spectrum expansion,and then use the wavelength selective switch(WSS)to filter out five lines of similar power to obtain optical frequency comb with 5 spectral lines.Then sent it into the cascaded IM for remodulation to obtain a phase-locked optical frequency comb with 15 spectral lines,which increases the bandwidth and comb number of the generated optical frequency comb compared to using two cascaded IM.