Application And Generation Of Nyquist Pulses Based On Optical Frequency Comb

In currently deployed optical communication networks,Sinc-shaped Nyquist pulses have been widely applied in Wavelength Division Multiplexed(WDM)communication systems to improve spectrum efficiency and transmission capacity,owing to its property of rectangular spectrum.In recent years,most studies focus on the methods of generating Sincshaped pulses,including using electrical Nyquist filter,liquid crystal spatial modulator,and optical parametric amplification technology.However,the Nyquist pulse spectrum generated by the above method is not ideal and the bandwidth is limited.As a laser carrier applied to a WDM system with high spectrum efficiency,it will cause larger channel crosstalk and reduce the transmission performance of the optical communication network.However,the Nyquist pulse generated based on the optical frequency comb can have an ideal rectangular frequency spectrum,and overcome the limitations of the sampling rate and processor capacity of electronic devices.Flexible structure,ideal waveform and high repetition rate of this scheme show the wide application prospect in WDM optical communication field.The paper takes the Nyquist pulse generation based on the optical frequency comb and its application in optical communication systems as the main research direction,and focuses on the method of generating Nyquist pulses based on the optical frequency comb based on the external modulator.We experimentally demonstrate Nyquist-WDM communication link based on flat electro-optic combs,which reduce inter-channel crosstalk penalty with no guard band.The specific work is divided into the following parts:1)The Nyquist pulse generation method based on optical frequency comb is studied,and the generation of Nyquist pulse based on optical frequency comb using electro-optic modulator is theoretically analyzed and verified experimentally.First,9 optical frequency combs with 3 GHz frequency spacing are generated by cascading two Mach-Zehnder modulators(MZM).The zero-crossing pulse width of the Nyquist pulse is 74 ps,and the pulse repetition period is 334 ps.5 coherent optical frequency combs with frequency space tunable are generated based on the dual-paraller Mach-Zehnder modulator(DP-MZM),which the frequency space tuning range is 1GHz-30 GHz.In addition,11 optical frequency combs with frequency space of 20 GHz are generated based on cascaded DP-MZM and phase modulator.The zero-crossing pulse width of the Nyquist pulses is 9.1 ps,and the repetition period of Nyquist pulses is 50 ps.2)The characteristics of Nyquist pulse laser carrier based on optical frequency comb are studied.The Nyquist pulsed laser carrier based on 3 optical frequency combs with frequency space 9GHz is digitally modulated.At the linewidths are 100 k Hz,15 MHz,and38 MHz,the corresponding signal-to-noise ratio of modulated Nyquist signal are 31.93 d B?28.29 d B and 24.32 d B respectively.The results show that the signal-to-noise ratio can be improved by reducing the linewidth of frequency comb.The spectrum characteristics of Nyquist pulse based on optical frequency combs are explored.After modulation of the Nyquist pulse based on 3 optical frequency combs and 9 optical frequency combs,which the roll-off factors are 0.33 and 0.11,respectively.The roll-off factor of the Nyquist signal spectrum decreases as the number of comb increases.3)We experimentally demonstrate Nyquist WDM communication link based on the electro-optic frequency comb,which completes 45Gbit/s data transmission in 56 km of optical fiber.Five flat electro-optic frequency combs are used as WDM sources.The NyquistWDM channels are generated by cascading a Mach-Zehnder modulator(MZM).The results show that the normalized root mean square error(NRMSE)of the 5 channels can be between1.23% and 2.04%.High flat electro-optic frequency comb can effectively reduce the Nyquist quality difference between channels.After Nyquist pulses are modulated,the transmission performance of 5 Nyquist-WDM channels is measured in 56 km fiber,respectively.The results show that the 5 Nyquist-WDM channels based on a flat electro-optic frequency comb can provide a channel crosstalk loss of less than 0.7d B without guard band.