Broadband Tunable Microwave Frequency Comb Generation Based on Modulated Optical Injection Semiconductor Laser

Microwave Frequency Comb (MFC) has the unique advantage of simultaneously providing multiple continuous microwave signals and then plays an important role in some application fields such as satellite communication, remote sensing, distance measurement, and anti-interference detection. At present, sustained efforts have been paid to explore novel techniques to generate the MFCs signal with adjustable comb distance, pure comb line, power balance and broad bandwidth. In this work, a system scheme for generating broadband tunable MFC based on a modulated optical injection Semiconductor Laser (SL) is proposed and experimentally investigated. First, via a distributed feedback semiconductor laser (DFB-SL1) strongly modulated by a single-tone electrical signal, a seed Optical Frequency Comb (OFC) including a few comb lines can be obtained. Next, the seed OFC is sent into a phase modulator driven by the same single-tone electrical signal as that loaded on the DFB-SL1, thus a promoted OFC with more comb lines can be obtained. Finally, the promoted OFC is injected into another DFB-SL (DFB-SL2), except the regenerated injection OFC, another sub-OFC can be observed located nearby the redshifted central wavelength of DFB-SL2. In this case, the whole OFC can be regarded as a combination of two sub-OFCs, and then a broadband MFC can be obtained through converting the output of DFB-51.2 into electrical signal by a photo-detector. The experimental results show that, by utilizing a single-tone electrical signal with a frequency of 2.9 GHz, under optimized injection power and frequency detuning between two DFB-SLs, an MFC with 55.1 GHz bandwidth within a +/- 5 dB amplitude variation can be obtained, where the phase noise of each comb line is maintained below - 98.66 dBc/Hz at 10 kHz frequency offset. Through varying the frequency of the single-tone electrical signal and selecting matched operating parameters, the comb spacing of generated broadband MFC can be tuned. Also, the variations of optimized MFC bandwidth with the frequency detuning Delta f and injection power P-inj are analyzed. For - 9.0 GHz <=Delta f <= 3.6 GHz, there is only one comb line within a +/- 5 dB amplitude variation calculated from DC, and the MFC bandwidth maintains at 2.9 GHz. For 4.8 GHz <= Delta f <= 37.4 GHz, with the increase of Delta f, the minimum comb line interval between two sub-CFCs is periodically varied within [0, f(mod)], and meanwhile the intensity of each comb is also varied. Therefore, the bandwidth of generated MFC behaves a complex varied trend. The maximum bandwidth is about 55.1 GHz obtained under Delta f= 37.4 GHz. For Delta f >37.4 GHz, the bandwidth of the generated MFC is relatively small due to too far apart between two sub-CFCs. Considering the red-shift induced by the optical injection, the beat frequency is not located at the frequency detuning between the two DFB-SLs. Besides frequency detuning Delta f, injection power P-inj is another key parameter. With the increase of the injection power, the red-shift is more severe, and higher frequency comb lines will be enhanced, which leads to the variation of the distribution of MFCs. For a fixed Delta f=37.4 GHz, with the increase of P-inj, the MFC bandwidth first increases, after reaches its maximum value of 55.1 GHz, and then decreases.