Study On Fiber-bulk Hybrid Femtosecond Fiber Laser Amplification System

Ultrafast lasers have found numerous applications in areas of fundamental research as well as for medical and industrial applications.The fiber femtosecond laser amplification system offers major advantages in terms of compactness,lack of misalignment,and environmental insensitivity.And bulk crystals as gain medium can extract higher output energy.Thus,a hybrid system with a fiber seed source and solid-state amplifier can inherit the advantages of both.In order to obtain femtosecond lasers with high energy,high quality and narrow pulse width,theoretical analysis and experimental research on front-end fiber laser and bulk main amplifier are studied,respectively.The hybrid femtosecond laser system delivers pulses with micro-Joule-level,high pulse quality and high beam quality,providing high quality femtosecond laser sources for ultra-fine“cold”micro-machining and micro-nano processing applications.The thesis is summarized as below:1.A numerical model for hybrid fiber-solid ultrashort pulse amplification system is established.Based on this theoretical model,the period-doubling bifurcation phenomenon of regenerative amplifier is numerically studied,and the effects of seed parameters on nonlinear amplification process are analyzed,providing a theoretical basis for obtaining stable single energy pulse.2.We investigate the stability of a dispersion-managed mode-locked Yb-doped fiber laser of near-zero net cavity dispersion.The instability is primarily due to the filtering effect of the chirped fiber Bragg grating.The size of the unstable region is dependent on the modulation depth of the saturable absorbers.At modulation depth higher than 30%,stable mode-locking can operate throughout the dispersion region.Stable mode-locking around zero cavity dispersion is experimentally viable by a SESAM with a 34%modulation depth.The fiber laser can generate laser pulses with a17-nm spectral bandwidth and a 139-fs dechirped pulse duration.3.We investigate the multi-pulsing mechanism in a dispersion-managed mode-locked Yb-doped fiber laser.The spectral filtering effect,induced by the chirped fiber Bragg grating restricts the spectral broadening induced by self-phase modulation and causes extra loss,leading to a decreased pump power threshold for the multi-pulsing state.The threshold of multiple pulses is related to the spectral filter bandwidth and the net cavity dispersion.On the basis of this conclusion,we devise a strategy to obtain single-pulsing operation with the shortest pulse width and the highest pulse energy.4.A hybrid femtosecond laser system based on a femtosecond Yb-doped fiber laser and an Yb:KGW regenerative amplifier is developed.In order to overcome the gain narrowing effect,the solid-state nonlinear amplifier is used to broaden the spectrum.The effects of different gain spectra on the nonlinear amplification results are investigated when the Yb:KGW crystal operates for the N_p-polarization and N_m-polarization,respectively.With optimization,the system can output 270-fs pulses with21-μJ pulse energy at a 60-k Hz repetition rate.5.We systematically study the optimization scheme of the hybrid femtosecond laser system based on nonlinear amplification.In order to obtain better beam quality and temporal quality,we optimize the system from three aspects:the cavity design,the improvement of seed source and the dynamics of nonlinear amplification process.The nonlinear amplification process is optimized by pre-shaping technique.As a result,high pulse quality is achieved.