Study On Femtosecond Laser Self-similar Amplification System For High-energy And Short Duration

High-quality laser pulses with sub-100 femtosecond and micro-joule single pulse energy play an irreplaceable role in the fields of organic material processing and scientific research.Compared with the chirped pulse amplification technology typically output pulse over 200 fs width,the self-similar amplification technology can achieve a narrower pulse width.What’s more,the output pulse from self-similar amplifier exhibits linear chirp that can be compensated to near transform limit by ordinary compressor.This thesis focuses on the experimental build-up of self-similar femtosecond pulse amplification systems,and the numerical establishment of a selfsimilar amplification model.The effects of spectral intensity and phase modulation on self-similar amplification systems are simulated from the perspectives of modulation period,modulation depth and arbitrary phase shift.A high-repetition-rate-frequency low-energy self-similar amplification system is built to realize the near transformlimited pulse output.While ensuring compressed pulse with high quality,the system output single pulse energy is increased to microjoule level by using acousto-optic modulator to reduce the repetition-rate frequency,grating filter for spectrum tailoring,and large mode area photonic crystal fiber as gain medium.The work of this thesis is divided into the following points:1.The historical development of femtosecond fiber laser is introduced.Principles and the development of widely-used mode-locking and amplification technologies are covered.Both the advantages and disadvantages of each technology are analyzed.2.From the nonlinear Schr?dinger equation,the asymptotic solution of the selfsimilar amplification theory is derived.Based on the split-step Fourier algorithm and the fourth-order Runge-Kutta method,the numerical model for self-similar amplifier is established.3.The effects of spectral intensity modulation and phase modulation on the selfsimilar amplification system are explored from the perspectives of modulation period,modulation depth and arbitrary phase shift.According to the simulation,large modulation periods make sub-pulses inseparable from main-pulses,disturbing the selfsimilar amplification process.Arbitrary phase shift will significantly change the specific shape of the modulated spectrum,thereby affecting the self-similar amplification.Small modulation periods bring a longer delay for sub-pulses,making the amplification of main-and sub-pulse independently and self-similar amplification will not be affected by arbitrary phase shift in this case.These conclusions are still tenable when modulation depth changes.4.Two sets of femtosecond laser self-similar amplification experimental systems are built.The first one uses a 60 MHz picosecond laser as the seed source.94 fs transform-limited pulses after compression is realized in a 2 m polarization-maintaining double-clad ytterbium-doped fiber by pre-chirping management.The seed source is replaced by a laser mode-locked with saturable absorption mirror.Then a high-energy low-frequency self-similar amplification system is constructed with single-polarization large-mode-area photonic crystal fiber as the gain medium.A pulse selection device is integrated to reduce the repetition-rate frequency to obtain higher energy gain,and a grating filter is added to attenuate the influence of the low-frequency spectral modulation in the seed pulse on the self-similar amplification.Finally,the system generates high-quality pulse train with 1.2 μJ energy and 61 fs pulse duration after compression,and only weak pulse side-band is observed when the pump power is 12W.