Study On Effects Of Saturable Absorption Onpassively Mode-locked Fiber Lasers

Passively mode-locked fiber lasers have found great potentials in the applications of industrial processing,medicine and scientific researches because of the advantages of high peak power,ultrashort pulse width,compact structure and low cost.To achieve passively mode-locked fiber lasers,saturable absorber(SA)is the key element.Therefore,finding new ideal SAs with high quality and low cost is essential in the field of ultrafast fiber lasers.In recent years,several kinds of low dimensional materials were successively exploited for passive mode-locking fiber lasers since they have excellent nonlinear saturable absorption properties,such as wide operation wavelength,fast recovery time,easy-to-integration with fiber and cost-efficiency.It should be noted that all of these nanomaterials based SAs show very different characteristics of saturable absorption.When one faces a wide variety of nanomaterial-based SAs,a possible trouble is how to choose a proper SA for his/her practical application.Therefore,it will be important to well understand how the characteristics of these nanomaterial-based SAs impact on passively mode-locked fiber lasers.Focusing on this frontier area,the research works presented in this dissertation are conducted with theoretical and experimental methods,in time domain and frequency domain as well.The major achievements of this thesis are summarized as follows:1)SAs based on different low dimensional materials were prepared and their saturable absorption properties including modulation depth,saturation intensity and non-saturable transmission were measured by the home-made balanced twin-detector measurement system.2)In the anomalous dispersion regime where mode-locked fiber lasers can generate ultrafast pulses.By solving the Ginzburg-Landau equation,we theoretically analyzed the performances(e.g.pulse duration,peak power and spectral bandwidth)of mode-locked fiber lasers as the SA's modulation depth or saturation intensity.In the experiments,SAs with different saturable absorption properties were incorporated in the Er3+-doped fiber lasers for mode-locking.Both the theoretical and experimental results suggest that larger modulation depth is favor to the narrower pulse duration and higher peak power,while the mode-locking performance is almost unchanged as the change of S A's saturation intensity.3)Compared to the that working in anomalous dispersion regime,mode-locked fiber lasers in normal dispersion regime emitting dissipative solitons have larger pulse energy.Thus,we also investigate the effects of nanomaterial saturable absorption(e.g.modulation depth,saturation intensity)on passively mode-locked fiber laser with similar methods.The results reveal that the pulse duration narrows and the pulse energy increases as the increasing of modulation depth.Besides this,the better performance can be improved by reducing the saturable intensity,which is differ from the results obtained in anomalous regime.In addition,we note that dissipative solitons can not initiate in the cavity if the S A used has too low modulation depth.