Research On Technology Of Harmonic Mode-Locked Fiber Laser Based On Micro-Nano Fiber

High repetition-rate ultrashort pulses have the advantages of the high repetition frequency and narrow pulse width,which have critical applications in optical frequency comb,optical analog-to-digital conversion technology,and optical communication.The harmonic mode-locked（HML）technique is the most effective method for achieving a high repetition-rate ultrashort pulse output.The choice of a mode-locked mechanism is crucial to achieving high harmonic splitting.Compared with other mode-locking mechanisms,the nonlinear multimode interference（NL-MMI）effect is a new mode-locking mechanism that does not depend on the polarization state,has a high damage resistance threshold,and can achieve nonlinear saturable absorption characteristics by changing the light intensity transmitted into the graded-index multimode fiber（GIMF）.The single-mode fiber-graded-index multimode fiber-single-mode fiber（SMS）structure based on the NL-MMI effect can be used as a saturable absorber for HML fiber lasers.However,the GIMF length in the SMS needs to be precisely controlled if the SMS is to achieve a nonlinear saturable absorption effect.Second,the periodic energy coupling between modes in the GIMF can make the NL-MMI mode-locking unstable.In addition,it isn’t easy to achieve stable HML in SMS-based mode-locked fiber lasers by only increasing the input pump power.Therefore,achieving stable,high repetition-rate HML fiber lasers based on NL-MMI is a critical problem.Thus,in this paper,the following studies are carried out to address the above issues.1.The generation mechanism of passive mode-locking is introduced,and the effects of intracavity dispersion and nonlinear effects on the intracavity pulses of fiber lasers are analyzed.The principle of HML generation and the factors affecting the harmonic order are analyzed.A new method for realizing HML and improving the harmonic order is explored.The preparation method and optical properties of micro-nano fiber（MNF）were investigated.Finally,the MNF was prepared by the arc discharge method,and the method of integrating the MNF into the laser was explored to further improve the repetition rate and harmonic order.2.Based on the NL-MMI mode-locking theory,the NL-MMI mode-locking instability caused by the fluctuation of self-imaging points within the SMS is investigated.The modeling of the saturable absorber of the SMS using simulation software reveals the effect of the periodic energy coupling between multiple modes in the GIMF on the NL-MMI mode-locking stability.The simulation results show that the energy coupling between some higher-order modes and suppressed modes in the GIMF can be filtered out by tapered GIMF in SMS,which helps to form a stable self-imaging point and thus improves the stability of NL-MMI mode-locking.The experiment obtained tapered-SMS（TA-SMS）saturable absorbers by fusing single-mode fibers at both ends of the tapered GIMF.In addition,the addition of MNF helps to enhance the intracavity nonlinear coefficients to improve the harmonic order and suppress the generation of Kelly-sidebands by using MNF’s anomalous waveguide dispersion characteristics.In the experiment,based on the TA-SMS saturable absorber,a stable pulse output with a central wavelength of 1558.17 nm,a repetition frequency of 56.91 MHz,and a signal-to-noise ratio of 70 d B was achieved at a pumping power of 81 m W.The HML pulse output with a spectral center of 1560.15 nm,a harmonic order of 22nd order,a repetition frequency of 1.26 GHz,and a signal-to-noise ratio of 47 d B was achieved at a pumping power of 594 m W.3.Research on HML fiber lasers based on hybrid multimode interference saturable absorbers was conducted.According to the modeling in Chapter 2,it is known that the tapered GIMF produces larger evanescent-wave fields relative to the tapered single-mode fiber for the same tapered waist diameter,which is more beneficial for light-matter interaction.The hybrid multimode interference saturable absorber was prepared by laser pulse deposition method of Bi₂Te₃solution on the TA-SMS.The nonlinear saturable absorber characteristics of the TA-SMS are further improved,and the nonlinearity in the laser cavity is enhanced.In the experiment,based on the hybrid multimode interference saturable absorber,the HML pulse output with the spectral center of 1557.64 nm,the harmonic order of 30th order,the repetition rate of 2.11 GHz,and the signal-to-noise ratio of 38 d B was realized by shortening the cavity length and increasing the pump power.