| Optical frequency combs(OFCs)are frequency components with equal frequency interval in frequency domain and pulse light with fixed repetition rate in time domain.OFCs connect optical frequency with microwave frequency and can be used as a frequency scale,which have important application value in the fields of precision spectroscopy,optical clock,coherent optical communication,optical arbitrary wave generation,ultra-precision ranging and so on.At present,there are several methods to realize OFCs:1)femtosecond OFCs based on mode-locked laser,2)OFCs based on optical microcavity,3)OFCs based on electro-optical modulator,4)OFCs based on traveling wave four-wave mixing effect.As for the first two types of OFCs,it is difficult to achieve a wide range of tuning between the teeth of the generated OFCs due to the limitation of the modulation range of the length of the mode-locked laser and the optical microcavity.The frequency spacing of the OFCs generated based on electro-optic modulator is difficult to reach more than 100 GHz due to the limitation of the current working bandwidth of electro-optic modulator.For the OFCs based on the four-wave mixing effect of traveling wave in optical fiber,although which can achieve a wide range of tooth spacing tuning,there are some problems such as high noise and wide linewidth.In 2019,our research group proposed the idea of using intracavity Brillouin laser as pump light to generate soliton OFCs.By adjusting the frequency interval between Brillouin lasers,we realized the OFCs with the tuning range of the frequency spacing covering GHz～THz.However,it currently suffers from problems such as spectral unevenness and low power conversion efficiency,especially the mechanism of intracavity instability generation needs to be elucidated.During my Ph D studies,I have conducted systematic research on the generation of flat and efficient optical frequency combs in Brillouin laser cavities and the mechanism of intracavity instability generation,and have obtained the following results:(1)A spectral flat soliton OFCs based on dispersion-engineered Brillouin laser cavity is proposed and implemented.By introducing a piece of optical fiber with normal group velocity dispersion(GVD)into the Brillouin laser cavity with all-anomalous GVD,the cavity net dispersion is finely engineered to near zero,enabling shaping of the intracavity soliton pulses,resulting in flat-topped bright soliton pulses and spectrally flat soliton OFCs.Compared with the previous hyperbolic Secant type(sech~2)optical solitons,due to the large overlap in the time domain between the above flat-top bright soliton pulse and the dual-wavelength Brillouin laser,the conversion efficiency of the soliton optical frequency comb was high,and its value was up to 6%.In addition,by varying the frequency spacing between the dual-wavelength Brillouin lasers in the cavity,the flat-spectrum soliton OFCs with comb spacing tuning range of 50 GHz～500 GHz are achieved.(2)The mechanism of instability generation in single-wavelength Brillouin laser cavities has been elucidated,and both Turing and Faraday instabilities have been experimentally observed.The existence of Turing and Faraday instabilities in a single-wavelength Brillouin laser cavity was found by numerically solving the Ikeda map equations and combining it with Floquet analysis,and both instabilities were observed experimentally.Further,the effects of intracavity dispersion,loss and non-linear coefficients on the Faraday instability were investigated using Floquet analysis.The results show that the periodic variation of the intracavity dispersion leads to the occurrence of the intracavity Faraday instability;while the value of intracavity and periodic variation of the intracavity nonlinear coefficients affect the magnitude of the gain of the Faraday instability.(3)The mechanism of instability generation in a dual-wavelength Brillouin laser cavity has been elucidated,the frequency splitting phenomenon due to Faraday instability has been observed for the first time,and the generation of multiple OFCs based on the frequency splitting phenomenon has been achieved.Through theoretical and experimental studies,it was found that a comb-like frequency sideband due to Faraday instability is generated in a two-wavelength Brillouin laser(whose frequency spacing isΔf)cavity with a frequency spacing equal toΔf/n(n is a positive integer),namely,the first observation of frequency splitting due to Faraday instability.When the power of the intracavity dual-wavelength Brillouin laser is high,the above-mentioned comb frequency sidebands interact with the intracavity dual-wavelength Brillouin laser,resulting in the creation of multiple OFCs. |
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