Generation Of Optical Frequency Comb And Its Application In Communication

The optical frequency comb(OFC)is the spectrum which has a series of equally spaced frequency components with a stable and coherent phase relationship.OFC are widely used in laser communication,precision measurement,gas detection and other fields due to their advantages such as large number of carriers,fixed frequency interval and good coherence.With the rapid development of modern science and society,people's demand for various digital services has gradually increased and data traffic grown rapidly.In recent years,with the widespread application of artificial intelligence,big data,cloud computing,the Internet of Things and blockchain technology,it has further accelerated the growth of data traffic.Therefore,it is important to use a simple and efficient multiplexing method to improve the transmission capacity.The OFC has a large number frequency components with adjustable frequency interval,and each frequency component can be used as a communication carrier.As a carrier source of a wavelength division multiplexing(WDM)communication system,the OFC has the advantages of simple structure,low cost,and easy carrier coordination.OFC has shown a broad application prospect in the communication field.This paper focuses on the generation of optical frequency combs and their applications in free-space optical(FSO)communications.Explored the OFC generation method based on laser external modulation and the method based on the cyclic frequency shift structure.The feasibility of the optical frequency comb in FSO communication system with certain turbulence is verified.The specific work is as follows.1?Based on the traditional OFC generation methods,A detailed theoretical analysis of the optical frequency comb generation scheme using single and cascaded modulators was performed,and use the simulation software verify it.Then,an OFC generating device based on a dual-drive Mach-Zehnder modulator was built in the experiment,and a 5 line OFC with a frequency interval of 25 GHz was generated.Then through cascading Mach-Zehnder modulator and phase modulator,a line OFC with a spacing of 25 GHz was generated,and the number of comb teeth was increased to 16 by using the nonlinear effect of the fiber.2?The principle of the cyclic frequency-shifting OFC generation scheme based on the IQ modulator was analyzed,and an experimental structure was built on this basis.Finally,an OFC with a flatness of less than 3d B and an interval of 25 GHz was obtained,and the number of comb teeth reached 37.Then,based on the stimulated Brillouin scattering effect,an OFC generating device with a double Brillouin frequency shift interval was built.Aiming at the problem of unflat gain of the OFC,the gain structure was added to the annular cavity,and the optimal pumping position was found.A double-sided reverse pumped gain cavity structure was proposed,and the OFC with 7 flat comb teeth and flatness of 1.5d B was obtained.3?The OFC with 7 flat comb teeth generated by the cascaded Mach-Zehnder modulator and phase modulator are used as the 7 channels of the FSO-WDM transmission system,and the atmospheric turbulence simulation pool is used to simulate the real environment.The transmission rate of each channel is 3.5Gb/s,and the total transmission rate is 24.5Gb/s.When the temperature difference of the atmospheric turbulence simulation pool is 220?,the BER is less than 10-3.During the change from back-to-back conditions to a temperature difference of 220? in the turbulent pool,the power loss of the edge channel CH1 and the center channel CH4 is 4.99 d B and 4.88 d B respectively,and the sensitivity changes are small.The combination of OFC and WDM can achieve channelization and broad-spectrum FSO signal transmission,which is very important in the construction of communication networks.