| This dissertation presents a comprehensive exploration of k Hz high-averagepower femtosecond lasers,which have broad applications in fundamental research,medical applications,industrial processing,and military applications.To promote the development of laser technology,it is essential to optimize output parameters and performance to meet the growing demands for frontier science.In this context,the recent breakthrough in pulse nonlinear compression technology has opened the door for miniaturization and cost reduction,making k Hz high-average-power femtosecond lasers scalable for various applications.The primary research content and innovative research achievements of this dissertation are summarized below:1.The dissertation presents a theoretical investigation of dispersion management in chirped pulse amplification(CPA)technology.The effects of various orders of dispersion on pulse duration and shape are derived,and dispersion in the stretcher,amplifier,and compressor of the titanium-sapphire chirped pulse amplification system is systematically analyzed.The findings provide theoretical guidance for managing dispersion in high-average-power titanium-sapphire amplifiers operating at kilohertz repetition rates.2.The dissertation presents a theoretical investigation of dispersion management in chirped pulse amplification(CPA)technology.The effects of various orders of dispersion on pulse duration and shape are derived,and dispersion in the stretcher,amplifier,and compressor of the titanium-sapphire chirped pulse amplification system is systematically analyzed.The findings provide theoretical guidance for managing dispersion in high-average-power titanium-sapphire amplifiers operating at kilohertz repetition rates.3.To support this dissertation,a high-average power titanium-sapphire laser system was designed and constructed based on chirped pulse amplification(CPA)technology.The system operates at a k Hz level,and a four-stage cascade main amplifier was built based on a commercial regenerative amplifier with an output energy of 11 m J.The thermal effect generated by the pump laser was distributed among the four amplification stages via a staged amplification approach.Finite element analysis was used to model the temperature distribution inside the gain medium of each amplifier stage,facilitating better thermal management of the main amplifier.By simulating the focal length of the thermal lens in each amplifier stage,the seed and pump mode matching in each amplifier stage was better controlled in experiments to obtain higher energy extraction efficiency.An output of 32.5 W at a repetition rate of 1 k Hz was obtained from the amplification stage with a total pump power of 100 W at 532 nm,resulting in an efficiency of 21.5%.Ultimately,by employing dispersion compensation via a compressor,an output characterized by 22.5 W、22.5 m J,and 44.5 fs at a repetition rate of 1 k Hz was attained.This result represents a globally preeminent achievement among comparable technologies.4.This dissertation presents the implementation of pulse compression for highaverage-power picosecond lasers based on the multi-pass cavity(MPC)technique,achieving the generation of high-power femtosecond laser output.The MPC cavity is composed of two 3-inch concave mirrors with R=300 mm,and a 25 mm fused silica block is placed at the center of the cavity as the nonlinear medium.By precisely calculating and controlling mode matching,the injected picosecond laser is sent through the nonlinear medium 111 times,broadening the pulse spectrum from 0.2 nm to 2.75 nm.Finally,by compensating for the compressor dispersion with a transmission grating,the 500 k Hz,100 W,200 μJ,12.5 ps laser pulse is then compressed to 71.3 W,143 μJ,780 fs,achieving an overall efficiency of 71.3% and a pulse compression ratio of 16.5.To further improve the output energy of the MPC device,a spiral half-wave plate was utilized to modulate a linearly polarized beam into radially or azimuthally polarized beam with cylindrical vector polarization.When all polarization states were superimposed coherently and destructively at the center of the beam spot,the Gaussian distribution of the beam spot was thus transformed into a hollow structure with a Laguerre-Gaussian distribution.This feature was leveraged to reduce the power density at the center of the beam spot,thus breaking the energy limitation of the MPC device.The energy limit of the same MPC device was increased from 500 μJ to 1.2 m J.Finally,using the cylindrical vector beam,the injection pulse with 1 k Hz,1.2 m J,and 14 ps was compressed to 1 m J and 475 fs in the same MPC device,compression ratio 30.This currently represents the maximum energy output and highest single-stage compression ratio in solid-state MPC.The polarization properties of the vector beam before and after the MPC device were studied and measured,and it was experimentally demonstrated that the MPC device and the self-phase modulation process are insensitive to the polarization state. |
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