Ultrafast Amplifiers Based On Yb-doped Gain Material And Nonlinear Compression

Ultrafast lasers are at the forefront of many scientific breakthroughs and technological achievements,and advances in their technology continue to open doors to new fields and interdisciplinary areas.With their short pulse width,broad spectrum and high peak power,ultrafast lasers offer unparalleled advantages in frontier scientific research,industrial laser processing,precision metrology and biomedical applications.Among them,ultrashort pulse lasers based on ytterbium-doped materials have been widely studied for their unique advantages.In particular,fiber-solid hybrid amplification technology is an effective means of achieving compact,high-energy,high-peak-power ultrashort pulsed lasers by combining the advantages of large single-pass gain and good stability of fiber lasers with the weak nonlinear effects and high peak power of bulk solid amplifiers.Ultrashort pulse lasers based on ytterbium-doped fiber and bulk materials were carried out in this dissertation.High-power,polarization-maintaining fiber amplifiers and high-energy solid-state regenerative amplifiers had been constructed,while further compression of the pulse width was realized based on self-phase-modulation in bulk media inside Multi pass cell.The main research and innovative achievements are as follows:1.An all-PM-Yb-doped fiber amplifier based on chirped pulse amplification was constructed.The seeding laser pulses were extracted from a nonlinear polarization evolution(NPE)mode-locked oscillator at 41.3 MHz to 1 MHz using AOM.High fidelity pulses were obtained by using matched dispersion pair of chirped fiber Bragg grating(CFBG)as pulse stretcher and transmission grating based pulse compressor.Using rod-type fiber,laser pulse as short as 320 fs with energy of 40μJ was obtained after the compressor.The measured output power fluctuation of root mean square was 0.63%in 24 hours.2.High power fiber amplifiers based on parabolic pulse and nonlinear Cubicon amplification were demonstrated.Spectral filtering was implemented and pulse shaping was introduced by the self-phase modulation in the fiber stretcher.The third-order dispersion was compensated for by the nonlinear phase shift in the amplifier.With optimization,pulses centered at 1038 nm with duration of 382 fs and 20 W average power at 1 MHz repetition rate were obtained.The long-term average power fluctuation was measured to be 0.5%in 24 h,and the beam quality factor(M2)was 1.25.The experiments results prove the feasibility of parabolic pulse shaping and non-linear Cubicon amplification,exploring a low-cost and easily designed all-fiber ultrafast laser amplification system.3.Fiber-solid hybrid ultrashort pulse amplifiers were comprehensively studied.A compact and robust seed source was conducted using fiber oscillator and CFBG pulse stretcher.Thermal lensing effects were deliberately considered,and with thermal lensing effect compensated by cavity design,a thermal insensitive Yb:KGW regenerative amplifier was constructed.The repetition rate of the regenerative amplifier was adjustable between 1 kHz and 10 kHz,pulses with energy of 1 mJ at 1 kHz and 0.4 mJ at 10 kHz in sub-400-fs pulse duration were achieved with good beam quality.A modified Frantz-Nodvik equation was developed to simulate the behavior of regenerative amplification and provide important insights into bifurcation dynamics and gain narrowing effect.For overcoming the negative contribution of gain-narrowing,dual-crystals regenerative amplifier was developed.With two Yb:KGW crystals aligned according to two different optical axes,the different emission spectra were combined,resulting in a reduced pulse width of 222 fs with a pulse energy of 1.2 mJ at 1 kHz.4.Post-compression of ultrashort pulses in the 200fs-10ps range was conducted.Nonlinear pulse compression was achieved using fused silica in Herriott-type multi pass cell.At 40 μJ of input pulse energy,the pulses of a Yb:KGW RA laser system(200 kHz repetition rate,173 fs pulse duration)were spectrally broaden from 13 to 52 nm while maintaining excellent beam quality,allowing compression down to 51 fs with an output pulse energy of 30 μJ.The overall transmission was 75%.With this reliable approach,compression of 580 μJ,10.6 ps pulse down to 388 μJ,1.03 ps was also demonstrated.We achieved a spectral broadening factor of 7.8 while maintaining an efficiency of over 65%.