Two-Channel Ultra-Broadband Optical Frequency Comb Generation By A Gain-Switched 850nm-VCSEL Under Orthogonal Optical Injection

Optical frequency comb?OFC?is an impulse train with repetitive frequency in the time domain and an equally spaced spectral line distribution in the frequency domain.OFC has many applications in different fields such as metrology,optical communications,optical arbitrary waveform generation and microwave photonic.There are three main techniques for the OFC generation based on semiconductor lasers?SLs?:mode locking,external modulation and gain switching,where gain switching has attracted specific attention due to its easy implementation,high flexibility and low losses.Compared with edge emitting laser,vertical-cavity surface-emitting lasers?VCSEL?possess some unique virtues such as single-longitudinal-mode output,low threshold,high coupling efficiency to optical fiber and easy-integration into laser arrays.In particular,due to the anisotropy of the medium in VCSEL chamber,it has the potential to output two orthogonal polarization components.Therefore,two-channel broadband OFCs are expected to be obtained based on VCSEL.In this work,we propose a scheme for generating tunable and ultra-broadband two-channel optical frequency comb?OFC?based on a gain-switched 850 nm vertical-cavity surface-emitting laser?850 nm-VCSEL?under orthogonal optical injection,and the influences of some operation parameters on the performances of generated OFC are numerically investigated.For this scheme,a 850 nm-VCSEL is modulated by a large signal and then is driven into a gain-switching state.Under this case,Y polarization component of the laser oscillates and behaves periodic pulse state while X polarization component is suppressed.As a result,one-channel OFC with Y polarization direction?Y-OFC?is generated.Further introducing an optical injection polarizing along X direction?named as orthogonal optical injection?,X polarization component of the gain-switching 850 nm-VCSEL can also be oscillated to be periodic pulse state with a spectral intensity equivalent with that of the X polarization component.Thus,two-channel OFC can be obtained.The simulated results show that,based on a850 nm-VCSEL under current modulation with a modulation depth of m=0.75 and a modulation frequency of f_m=4.2 GHz and an orthogonally optical injection with a frequency detuning of?v=10.0 GHz,dual-channel OFC with bandwidths beyond105.0 GHz within a 10 dB amplitude variation can be obtained under optimized injected field amplitude E_inj.Through examining the optical spectra of X polarization component and Y polarization component output from the gain-switching 850 nm-VCSEL subject to orthogonal optical injection,the range of E_inj and?v have been determined for achieving dual-channel OFC.Finally,through analyzing the OFC performance under f_m taken other values,the tunability of the comb spacing is demonstrated.