Research Based On Saturable Absorber For Passively Q-Switched And Passively Modelocked Fiber Lasers

Passive mode-locking and passive Q-switching are common techniques used to generate short pulses in solid-state,semiconductor,and fiber lasers.The most commonly used non-linear device for achieving passive mode-locking and passive Q-switching is the saturable absorber,which modulates the loss in the cavity according to the intensity of the optical field inside the cavity to achieve a short pulse output.Among all sorts of saturable absorber devices,nanomaterial-based saturable absorbers are currently the most widely studied,which have the advantages of simple preparation techniques,low cost,a large operating bandwidth range,and high integration.In studying the parametric properties of saturable absorbers of nanomaterials,besides the need to search for materials with excellent properties,it is also necessary to fully investigate the synthesis techniques and material properties of saturable absorbers,as well as the influence of device parameters on the light modulation,which is of great value for further research on pulse generation techniques and the realization of controllable output of pulses.In this paper,different techniques are employed to integrate typical saturable absorber materials,carbon nanotubes,and emerging material WO₃ into laser systems,investigating the impact of the saturable absorber’s performance on the pulse output.Furthermore,the structural parameters of the laser system and the preparation techniques of the saturable absorber are further optimized to explore the controllability and switchable output of passive Q-switching and passive mode-locking techniques.The main research content of this thesis is as follows:1.Q-switched and mode-locked pulse output based on typical saturable absorber carbon nanotubes in erbium-doped fiber laser are realized respectively.By integrating saturable absorbers with different parameters into erbium-doped fiber lasers,the influence of nonlinear absorption characteristics of carbon nanotube saturable absorbers on pulse start-up mechanism is studied in detail.In the laser,the insertion linear loss and unsaturated loss of the saturable absorber and the modulation depth are the key factors affecting the intracavity pulse output.Large insertion linear loss and unsaturated loss are not conducive to pulse establishment.2.Q-switched and mode-locked pulse output based on tungsten oxide saturable absorber in erbium-doped fiber laser are realized respectively.Firstly,with the same structure of the laser,WO₃ is used as the saturable absorber to achieve Q-switched pulse output in the laser.However,due to the limited modulation range of the saturated absorber light,the mode-locked pulse cannot be started.By introducing the fiber polarizer element,the modulation depth in the cavity is increased,and the traditional soliton mode-locked pulse output is realized in the cavity.It can be seen from the experimental data that lower saturation light intensity and smaller modulation depth are not conducive to the establishment of mode-locked pulses.3.An all-polarization-maintaining fiber passively Q-switched laser based on tungsten oxide saturable absorber is studied.In order to further explore the stability of Q-switched fiber lasers,saturable absorbers are integrated into fiber lasers with all-polarization-maintaining fiber structure.The WO₃ saturable absorber has a high damage threshold.When the intracavity pump injection power increases from 74 m W to 240 m W,the pulse repetition frequency increases from 16.2 k Hz to 47.6 k Hz,and the pulse width decreases from 6.4μs to 4.4μs.4.The evolution mechanism of passively Q-switched and passively mode-locked pulses based on tungsten oxide saturable absorber is studied.By improving the preparation technology of saturable absorber,the transition state of Q-switching and mode-locking is obtained in the experiment,that is,the state of Q-switching and mode-locking.The complete pulse transition mechanism and the establishment process of mode-locking are observed.This has certain guiding significance for deepening the study of pulse dynamics and obtaining high-quality mode-locked lasers.