| Frequency combs include microwave frequency comb(MFC)and optical frequency comb(OFC).MFC is a set of microwave components that can simultaneously observe microwave signals of multiple frequencies within a continuous frequency range when observed in the frequency spectrum.Due to the ability of broadband MFC signals to simultaneously cover the C,Ku,Ka and other frequency bands,it has broad application prospects in remote sensing,satellite communication,radar guidance,medical treatment,and other fields.OFC refers to a series of optical pulse sequences with equal intervals and stable phase relationships in the optical spectrum.Since OFC can simultaneously provide optical signals of multiple frequencies with balanced frequency spacing and wide coverage,it can replace multiple single mode lasers to emit dozens or even hundreds of optical longitudinal modes within a frequency band.Therefore,OFC can be used as an ideal multi-carrier optical source and has been widely used in optical wavelength division multiplexing systems,arbitrary waveform generation,MFC generation,optical atomic clocks,precision metrology,and other fields.Semiconductor lasers(SL)have the advantages of small volume,long service life,light weight,easy integration,and they can output rich nonlinear dynamic states under external disturbances such as current modulation,optical injection,optical feedback,and photoelectric feedback.These complex dynamic states have important applications in the fields of MFC and OFC acquisition.Therefore,this thesis will focus on the novel methods of generating broadband high quality MFC and OFC signals based on the nonlinear dynamics of SL,in order to obtain broadband frequency comb signals with flexible tunable comb spacing and balanced power.The main work and research results are as follows:(1)A theoretical model for obtaining MFC based on optical injection current modulated distributed feedback laser(DFB)is proposed.The MFC performance of current modulated DFB output before and after the introduction of optical injection was numerically studied.The current modulated DFB can output a narrow bandwidth seed MFC;After introducing optical injection into current modulated DFB,a broadband MFC with better performance can be generated.The simulation results indicate that driven by current modulation with modulation depth m=0.92 and modulation frequency f_m=1.2 GHz,directly modulated DFB can output a seed MFC with a bandwidth of 21.6 GHz within a range of 10 d B in amplitude variation,and the comb spacing of the MFC can be tuned by changing the modulation frequency f_m.Further introducing optical injection with appropriate parameters,the bandwidth and comb lines signal to noise ratio(SNR)of the seed MFC are significantly improved,with the bandwidth increasing to 37.2 GHz,while the SNR of each comb line is increased by at least 20 d B.In addition,this paper also theoretically analyzes the effects of modulation depth m and injection coefficient k on the generated MFC bandwidth.Simulation results show that a tunable ultra bandwidth MFC signal can be obtained with appropriate system parameters.(2)A scheme for obtaining broadband tunable OFC based on current modulated vertical cavity surface emitting laser(VCSEL)under optical injection is proposed and experimentally investigated.Compared to the commonly used sine wave modulation,this scheme uses a square wave signal with more spectral components as the modulation signal source.Firstly,a 1550 nm VCSEL is modulated by square wave to present gain switching state,at which time the optical spectrum output from the VCSEL presents a wide noise spectrum without significant comb lines;Further introducing optical injection into the gain switching VCSEL to allow the VCSEL output high quality OFC.The experimental results indicate that the modulation frequency f_mand the injected optical wavelengthλ_ihave a significant impact on the bandwidth of OFC.As the modulation frequency f_mincreases,the bandwidth of obtained OFC first increases,then remains at a high level above 70 GHz,and finally decreases.As for injected optical wavelengthλ_i,Whenλ_ilocated in the middle of the two polarization components,the generated Y-OFC and X-OFC which originating from two polarization modes of the VCSEL have almost the same power,and then a flat broadband OFC can be obtained.Under preferred parameters,using square wave modulation can produce a flat OFC with comb lines amplitude variation within 10 d B and a bandwidth of about 77 GHz;At the same time,the OFC has a pure spectral characteristic,and its corresponding beat signal has a single sideband(SSB)phase noise at the fundamental frequency as low as-115.7d Bc/Hz@10 KHz.In addition,by changing the modulation frequency f_mof the square wave signal,high quality OFC with a bandwidth exceeding 70 GHz and comb spacing continuously tunable in the range of 1.5 GHz to 2.8 GHz can be obtained. |
