Abstract
We have demonstrated and characterized the generation of ultra-broadband microwave frequency combs (MFCs) based on a current modulated distributed feedback semiconductor laser (DFB-SL) subject to optical injection, and the comb spacing of MFCs can be tuned easily through adjusting the modulation frequency $\text{f}_{m}$ . For the DFB-SL under only current modulation with modulation frequency $\text{f}_{m }= 1.2$ GHz and modulation power $\text{P}_{m }= 22$ dBm, the generated MFC has a bandwidth of about 15.6 GHz within ±5.0 dB amplitude variation. Further, introducing continuous-wave optical injection into the current modulated DFB-SL with injection power $\text{P}_{i }= 1170~\mu \text{W}$ and no frequency detuning between the injection light frequency and the oscillation frequency of the free-running DFB-SL, the bandwidth of MFC can be increased to 57.6 GHz. Through measuring the single sideband (SSB) phase noises of some representative comb lines, the performances of the MFCs generated before and after introducing optical injection are compared, and the SSB phase noises at 10 kHz offset frequency can be increased by 10 dB for the harmonics with frequencies of more than $40\text {f}_{m}$ after adopting optical injection. Moreover, the influences of operation parameters on the bandwidths of MFCs are analyzed, and the optimized parameter range for generating ultra-broadband MFCs has been determined.
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