| Optical frequency comb(OFC)consist of a set of equally spaced discrete frequency components.OFC with high repetition rates,as well as high degree of coherence have been extensively applied in many fields such as metrology,optical arbitrary waveform generation,spectroscopy,optical communication,THz generation.Many methods for generating OFC have been proposed and investigated,such as mode-locking,external modulation,and current modulation.Compared with other schemes,the approach for generating OFC based on current-modulated semiconductor lasers has the advantages of a relatively simple experimental facility and easily adjusted comb spacing.Therefore,the approach for generating OFC based on currentmodulated semiconductor lasers has attracted much attention.As a typical semiconductor laser,vertical-cavity surface-emitting laser(VCSEL)has some distinct advantages such as low threshold current,reduced manufacturing costs,easy to integrate,etc.Due to the anisotropy in the gain active region or laser cavity of VCSEL,the output light of VCSEL containing two orthogonal polarization components.By means of current modulation,VCSEL can output two OFCs with orthogonal polarization directions simultaneously.At present,the research on generating OFC by optical injection of current-modulated VCSEL mainly focuses on sinusoidal current modulation.However,sinusoidal signal cannot obtain flat and broadband OFC at a low modulation frequency(≤ 1 GHz),which has limitations in application.In this work,we proposed a scheme for generating broadband and tunable OFC based on a 1550 nm VCSEL under pulsed current modulation and optical injection.The scheme includes two parts: numerical simulation and experimental research.Firstly,the performance of OFC generated by pulsed current-modulated VCSEL and sinusoidal current-modulated VCSEL was compared by numerical simulation.Through simulation,it is found that at a low modulation frequency(≤ 1 GHz),the OFC with large bandwidth and high CNR can be obtained by means of pulsed current modulation,but at the same modulation frequency,the OFC bandwidth is significantly smaller,which is obtained by means of sinusoidal current modulation.Then,we experimentally study the effects of injection wavelength,modulation frequency and pulse width on OFC bandwidth and CNR.In the experiment,a pulsed electrical signal is utilized to drive a 1550 nm-VCSEL into the gain-switching state with a broad noisy spectrum.Next,a continuous optical wave is further introduced for generating broadband and tunable OFC.The experimental results show that the change of injection wavelength will affect the relative strength of the two polarization components in VCSEL.Selecting the appropriate injection wavelength can obtain the OFC with large bandwidth and high CNR.The modulation coefficient also has a great influence on the performance of OFC.With the increase of modulation frequency,the bandwidth of OFC increases first and then decreases,while CNR increases rapidly first and then flattens out.With the increase of pulse width,OFC bandwidth also presents a trend of first increasing and then decreasing,but the change of pulse width has little effect on CNR.Therefore,a wideband tunable OFC can be obtained by optimizing the pulse width at different modulation frequencies and matched injection wavelengths.Selecting the appropriate modulation coefficient and the matching injection wavelength,an OFC with a comb spacing of 0.5 GHz,a the bandwidth of 82.5 GHz and CNR of 35 d B is experimentally acquired,and the single sideband phase noise at the 0.5 GHz reaches-123.3 d Bc/Hz@10 k Hz.It should point out that OFC with different comb spacing can be obtained by varying the frequency of pulse electrical signal.For the frequency of pulsed current signal is varied within the range of 0.25GHz-3 GHz,the bandwidths of generated OFC can exceed 60 GHz through selecting optimized injection optical wavelength and width of pulse electrical signal. |
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