Generation Of Broadband Optical Frequency Comb Based On A Gain-switching 1550 Nm Vertical-cavity Surface-emitting Laser Under Optical Injection

Optical frequency comb(OFC)is composed of a series of equally spaced discrete comb lines,and these discrete comb lines corresponding to different frequency components can be simultaneously provided in a frequency band.OFC has been widely applied in metrology,spectropy,optical communication,and other fields.At present,the commonly used approaches to generate OFC mainly include mode-locking,external modulation and gain-switching.Mode-locked lasers can produce an OFC with broad bandwidth,but the line spacing of the OFC is difficult to tune.By using external modulation of continuous wave light,an OFC with adjustable comb line spacing can be obtained.However,in order to improve the flatness of the comb,multiple external modulators need to be used,which results in a large insertion loss.The method of generating OFCs based on gain-switching semiconductor lasers has received extensive attention due to its easy implementation and flexible adjustment of the comb line spacing.Compared with edge emitting laser(EEL),vertical-cavity surface-emitting laser(VCSEL)has some distinct advantages such as low threshold current,reduced manufacturing costs,easy to integrate,etc.Therefore,OFC generation based on a gain-switching VCSEL has more application prospects.In this work,we experimentally investigated the broadband OFC generation based on a gain-switching 1550 nm VCSEL subject to optical injection.During implementing the experiment,a large modulated current is adopted to drive a 1550 nm VCSEL into the gain-switching state with a broad noisy spectrum.By further introducing an optical injection,a high quality OFC can be produced.The experimental results show that the power and wavelength of the injection light severely affect the performance of the generated OFC.Under proper optical injection parameters,a broadband total-OFC spliced by two sub-combs originating from two orthogonal polarization components of a VCSEL can be obtained.With the increase of the injection power,the bandwidth of total-OFC firstly increases sharply,then maintains at a relatively high level,and finally shows a downward trend.As the injection light wavelength?_i changes,when?_i is closer to Y-PC of the 1550 nm VCSEL,the Y-OFC is the main contributor to the total-OFC.On the contrary,when?_i is closer to X-PC of the 1550 nm VCSEL,the X-OFC becomes the main contributor to the total-OFC.In particularly,if?_i is set in the middle of two polarization modes,X-OFC and Y-OFC are almost equalized.As a result,the bandwidth of the produced OFC is significantly increased.By selecting optimized operation parameters,a high quality total-OFC can be generated,with bandwidth of 70.0 GHz(56.0 GHz)within 10 d B(3 d B)amplitude variation and single sideband phase noise at the fundamental frequency below 120.6d Bc/Hz@10 k Hz.