Research On Chirp Characteristics Of Square-wave Fiber Laser

Passive mode-locked fiber lasers are widely used in the field of biomedicine,micromachining,ultrafast spectroscopy and so on due to the characteristics of short pulse output,good temporal stability,high conversion efficiency,compact size,and lower cost.Traditional soliton fiber lasers generally operate in anomalous dispersion region,taking advantage of the balance between nonlinear effect and dispersion to generate soliton pulses.Limited to the soliton area theory,soliton energy is usually around 0.1 nJ,while larger pulse energy will lead to pulse splitting.In order to produce short pulses with higher energy,scientists at home and abroad have done a lot of research.Among of these studies,a type of mode-locked laser generating square-wave pulses was reported.The square-wave pulse possesses steep edges and flat top.Its pulse width can be tuned to hundreds of nanoseconds or even microseconds by increasing pump power,and the single pulse energy can rise to ?J.Generally,it is a large chirp across the square-wave pulse,and the time-bandwidth product can reach to 104.In previously reported square-wave fiber lasers,researchers have paid less attention to the chirp characteristics of square-wave pulses.However,the pulse chirp is of great importance in practice.Therefore,this thesis mainly studies the chirp characteristic of the square-wave fiber laser.This work may not only help to understand the formation mechanism and characteristics of square-wave pulse,but also explore its potential application.The main contents and results of the thesis are as follows:1.The switchable generation of dissipative soliton resonance(DSR)pulse and randomly chirped pulse by adjusting polarization controller in a fiber laser was investigated experimentally.The chirp is nearly linear and low across the central region of DSR pulse,whereas it is nonlinear at the edges of pulse.There is no clear chirp across the randomly chirped pulses,and the spectral shape and width of the pulse hardly vary with the pump power2.A fiber laser to generate high-power high-energy square-wave pulse with wide spectrum was experimentally studied.The square-wave pulse with an average power of 1.25 W and a spectral width of 4.5 nm can be obtained from the laser.It is a kind of random chirp across the square-wave pulse.3.A passively mode-locked fiber laser generating stable and chirp adjustable square-wave pulse was experimentally studied.The chirp of the square-wave pulse can be adjusted just by adjusting polarization controller.In our experiment,three typical chirp states,random chirp,V-shape chirp and linear chirp,were respectively observed.4.Pulsed pumped fiber amplifier with high peak power and short pump duration was numerically simulated.It has been theoretically proven that pulsed pumped fiber amplifier pumped by pulse with shorter duration and higher peak power can reduce the ASE.The innovative work in this thesis are as follows:1.The dissipative soliton resonance(DSR)pulse and randomly chirped pulse are obtained in a fiber laser,which indicates that the initial cavity parameters have significant effect on the chirp of the square-wave pulse2.A chirp adjustable square-wave pulse fiber laser was realized.Three typical chirp states were selectively obtained.3.A kind of NALM containing only a DC-YDF was designed.The gain and nonlinear effects of the fiber laser are combined by the NALM,resulting in the generation of high-energy square-wave pulse.