Property Optimization Of Femtosecond Lasers

Due to the short pulse duration,broadband spectrum and high peak power, femtosecond laser has become an indispensable tool of the exploration of the fields of physics,chemistry,biology,and optical communications over the last two decades. Ti:sapphire lasers which can generate extremely short pulses with broad spectral bandwidth have been widely used in both commercial applications and scientific researches.However,the separation of the stable regions,the high pump threshold and the fixed repetition rate of the laser severely limit widespread use of Ti:sapphire lasers.Meanwhile,the generation of ultraviolet(UV)and extreme ultraviolet(EUV) combs,which would lead to atomic clocks in X-ray region,needs laser pulses with high repetition rate and high energy.So,the optimization of Ti:sapphire lasers is important for both practical and scientific researches.This work is aimed to explore novel schemes and techniques to control the femtosecond lasers in order to build up a stable and multi-functional femtosecond laser system.In this thesis,the properties of the femtosecond Ti:sapphire lasers were optimized.The stability was improved,the pump threshold was lowered and the application domain was extended with the help of harmonic mode locking and high repetition rate pulsed laser amplification.The main results have been summarized as following:1.It was demonstrated for the first time that the stable regions to maintain the mode-locking of a Kerr-lens mode-locked Ti:sapphire laser could be wellcontrolled with an additional nonlinear medium in the laser cavity.The gap between the two stable regions which depended on the cavity asymmetry could be effectively shortened by varying the position of the additional medium nearby the confocal focus to change the intracavity Kerr-effects, while the physical length of the cavity kept constant.When the nonlinearity provided by the additional medium was large enough,the gap disappeared. With the nonlinearity modulation,the stability of the Ti:sapphire laser was much improved,for the total stable range was enlarged and the gap was shortened.Additionally,with the additional medium,the higher the pump power was,the larger the stable range and the shorter the gap was obtained. This filled the gaps of traditional Ti:sapphire lasers,where the gap between stable regions increased with the pump power.2.It was demonstrated that the pump threshold to start mode-locking of a Kerr-lens mode-locked Ti:sapphire laser could be controlled by varying the relative position of the additional medium to the confocal focus,which supplied an effective way to reduce the pump threshold and also the production costs.3.The mechanism of pulse-splitting was deeply studied and a new way was demonstrated to control the harmonic mode-locking and multiple pulsing operation with the pulse duration unchanged of a Kerr-lens mode-locked Ti:sapphire laser.When the effective nonlinear length of the nonlinear medium which was inserted in the Ti:sapphire laser was varied by changing the position of the medium or the pump power of the laser,stable harmonic mode-locking and multiple-pulse operation were observed.The dependence of the pulse splitting on the Kerr-effect(especially self-phase modulation) of the nonlinear medium inside the laser cavity was confirmed by several experiments.4.A passive way of high repetition rate laser pulse amplification was demonstrated.Without any active control,the amplification of the high repetition rate laser pulses from an oscillator was more than 30 dB in an external active cavity.This method was much simpler than the common way using an external passive cavity.It was demonstrated in this thesis that the stable regions,the pump threshold and the harmonic mode locking of femtosecond lasers could all be well-controlled with the help of the additional nonlinear medium in the cavity.A simple and effective new way to amplify high repetition rate laser pulses was also demonstrated.