| Optical frequency combs(OFC)are comb teeth with equal frequency spacing in the frequency domain and pulsed light with a fixed repetition frequency in the time domain.The applications of optical frequency combs have been expanded from frequency measurement to the precise determination of basic physical constants,a new generation of global positioning systems,ultra-precision molecular dynamics,detection of extraterrestrial life,verification of the expansion of the universe,ultra-long distance ultra-high-precision measurement,ultra-wideband optical communications,optical information processing,and laser radar.At present,the types of optical frequency combs mainly include femtosecond optical frequency combs based on mode-locked lasers,micro-cavity optical frequency combs based on cascaded four-wave mixing in the cavity,optical frequency combs based on electro-optic modulators,and optical frequency combs based on travelling four-wave mixing effects in the optical fibers.For the first two types of optical frequency combs,the tuning range of the comb spacing is very limited due to the fixed cavity length of the device.For optical frequency combs based on electro-optic modulators,the tuning range of the comb spacing is limited by the bandwidth of the used modulator.The OFC with tunable frequency spacing has potential applications in many fields such as precision measurement,astronomy,optical clock and optical communication.Therefore,in order to further expand the application range of the optical frequency combs,it is necessary to develop OFC with a wide range of comb teeth spacing.During Ph.D.study,the author systematically studied how to obtain OFC with tunable comb spacing and how to further broaden the spectrum of the optical frequency comb,and achieved the following results:(1)OFC generation with large tunable frequency separation,wide bandwidth and high optical signal to noise ratio in a dual wavelength Brillouin laser cavity isproposed and domonstred.Experimentally,two single wavelength lasers withoperating wavelengths in the C-band,frequency spacings of approximately 65 GHz,and line widths of approximately 150 kHz were used as the pump light for generating dual-wavelength Brillouin lasers in the fiber cavity with 500 m long highly nonlinear fiber.Dual wavelength Brillouin lasers with reduced linewidth of 3 kHz(50 times narrower than the pump CW laser)and improved optical signal to noise ratios are generated in the direction opposite the pump laser.Cavity-enhanced cascaded four-wave mixing between dual wavelength Brillouin lasers occurs in the laser cavity,causing the generation of broadband optical frequency combs with step tunable mode spacing from 40 to 1300 GHz.Compared to the cavity-less case,the number of the comb lines generated in the dual wavelength Brillouin laser cavity is increased by nearly two order magnititude.The amplified spontaneous emission(ASE)from the fiber amplifer is suppressed in the OFC generation.(2)Cavity soliton and OFC generation in a multi-wavelength Brillouin laser cavity are proposed and demonstrated.Flatness and OSNR of the spectrum generated in the multi-wavelength Brillouin laser cavity are improved compared to that generated in the dual wavelength Brillouin laser cavity.Noting that,we observe the corresponding temporal pulse generated in the multi-wavelength Brillouin laser cavity turns to a cavity soliton with the significant feature.The profiles of the temporal pulse and spectrum can be fitted into the hyperbolic secant.The intensity enhanced resonant dispersive wave generated in the blue region of the spectrum is observed.By applying the numerical simulation based on Ikeda map,we understand and analyze the main physical mechnasim and feature of the cavity soliton and OFC generation in the multi-wavelength Brillouin laser cavity.(3)Short length fluorotellurite microstructured fibers with near zero flatted dispersion and high nonlinearity used as the nonlinear media for OFC generation are investigated.We present the design and fabrication of fluorotellurite microstructured fibers(FTMFs)with near zero flattened dispersion profiles in the wavelength range of1500-1600 nm.The fiber with the chromatic dispersion value between-0.5 and 0.3ps/nm/km in the wavelength range of 1500-1600 nm is achieved.Flat top OFC extending from 1500 to 1600 nm(3 dB bandwidth)with tunable mode spacing from25 GHz to 100 GHz can be generated in a 2 meter long fiber by using a 1550 nm laser with a pulse width of 0.85 ps and a peak power of 49 W(average pump power of 3 W)as the pump source through numerical simulations.Total bandwidth of OFC from1300-2000 nm is obtained by pumping the 25-50 GHz repetition pulse laser with average pump power of 3W.(4)Broadband bandwidth 1500-1600 nm multi-wavelength lasers with frequency separation of 9.28 GHz(more than 1000 tooth)are generated in the dual wavelength Brillouin laser cavity.The core to achieve the result is that we keep the dual wavelength pump laser,high order Brillouin lasers operating together with the dual wavelength first order Brillouin laser in the cavity,thus benefits the mechanism of not only cascaded four-wave mixing but also cascaded Brillouin scattering to generate the multi-wavelength lasers with frequency separation of Brillouin frequency shift.(5)Increased red frequency shift in coherent broadband mid-infrared supercontinuum generation in all normal dispersion(ANDi)tellurite microstructured fibers are proposed and investiaged through numerical simulations.The proposed tellurite microstructured fibers(PTMFs)have an inverted L-shaped ANDi profile in the wavelength range of < 5.2 ?m.Interestingly,SPM induced spectral broadening is larger on the Stokes side than the anti-stokes side,since the trailing edge of laser pulse becomes smoother than its leading edge,and the effect of self-steepening(SS)on pulse shape is compensated by the effect of the dispersion in the PTMF.Since the trailing edge of output pulse is steeper than the leading edge of output pulse when SS occurs in previously reported ANDi fibers.As a result,coherent broadband mid-infrared supercontinuum light with long operating wavelength can be generated in the PTMF. |
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