Research On LD-pumped Yb:YAG Crystal All-solid-state Ultrafast Laser

The femtosecond lasers have numerous applications in ultraprecision machining,biomedical,military defense,frontier scientific exploration and so on due to the characteristics of ultra-short pulse,extremely wide spectral width and ultra-high peak power.At present,yb-doped all-solid-state ultrafast lasers pumped by Laser Diode（LD）are the most widely used in all-solid-state femtosecond lasers due to the advantages of high efficiency,low cost,miniaturization and compact structure.However,the output power of the current all-solid femtosecond lasers based on Yb:YAG crystal is generally low,and the high output power of femtosecond lasers are mainly realized by the amplification stage,which greatly limits the further expansion of its application.In this paper,LD pumped Yb:YAG crystals combined with semiconductor saturable absorption mirror（SESAM）mode-locked systems were studied,focusing on thermal lens effect suppression,beam quality optimization,and output coupler optimization.Then the resonator has been designed detailedly,and the average power of 2.16 W and the pulse width of 665 fs was achieved.As far as we know,the output power of the laser in this paper is the highest in the femtosecond lasers with a bulk single crystal Yb:YAG as the gain medium.Furthermore,to overcome the limitations of low depth modulation of the SESAM and poor beam quality of the LD,the systematic research of double-focused Kerr-lens mode-lock（KLM）was carried out.The research contents are as follows:1.The SESAM mode-locked resonator and the double-focus Kerr lens mode-locked resonator were designed by the resonator simulation software re Zonator.In order to obtain a stable and insensitive resonator,the thermal lens effect was fully considered.According to the resonator parameters,the crystal heat sink and SESAM heat sink were designed and processed to further reduce the influence of the thermal effect.2.By using a"W"folded cavity structure,Yb:YAG crystal doped with 5 at.%concentration as the gain medium and a multimode 940 nm LD as the pump source,the continuous wave（CW）output power of 3.55 W was obtained.To further explore the influence of different output couplers on mode-locking,the SESAM mode-locked mode was carried out by output couplers of T=4%and T=16%,respectively.The average power of 307 m W and 1.47 W picosecond lasers were obtained,corresponding to the laser center wavelength of 1032 nm,the spectral width of 0.8 nm and 0.7 nm,and the pulse repetition frequency of 97 MHz.3.In order to obtain higher power and narrower pulse width output,some improvements were made based on the above experiments.In order to enhance the self-absorption effect of Yb:YAG crystal at 1030 nm,the doping concentration was increased from 5 at.%to 10 at.%.By optimizing the mode-to-pump ratio,the spot size on SESAM,and selecting the coupling output mirror reasonably,the output power of 5.4 W can be obtained by using the coupling output mirror T=5%under the CW operation.The average power of 2.16 W,the pulse width of 665 fs,the spectral width of 2.2 nm,and the repetition rate of 95 MHz were obtained under the pump power of 16.14W by SESAM mode-locking and appropriate dispersion compensation by prisms.And the laser center wavelength shifted from 1032 nm to 1050 nm.4.The Kerr-lens mode-lock method was adopted because the SESAM limited the shorter pulse and higher power.And due to the poor beam quality of the pump source,the Yb:YAG laser medium was separated from Ca F₂（calcium fluoride）Kerr medium by the characteristic double-focused resonant cavity structure to overcome the limitation of the traditional KLM resonator on the pump source.The maximum CW power of 1.38 W was obtained when we adapted the T=1.5%output coupler.The mode-locked pulse trains can be obtained by adjusting the resonator and pushing the end mirror slightly,but the pulse trains were difficult to be sustained and stable,then we failed to achieve stable mode-locked operation.The generation of mode-locked pulse trains in the experiment showed that the scheme was viable to some degree,so the causes,solutions,and subsequent research work were analyzed.