Study On The Output Couplers In Extreme Ultraviolet Optical Frequency Comb

With the development of laser technology,the control of repetition rate,phase of carrier envelope and power stability of ultrashort laser pulse is becoming more and more precise.It promotes the development of precision spectral measurement,ultrafast dynamics of atomic molecules,optical frequency comb and optical frequency standard.In recent years,attempts have been made to extend the spectral coverage of optical frequency combs from near-infrared/visible to extremely ultraviolet?XUV?for precise spectral measurements of some important atomic/ion.At present,high harmonic generation?HHG?based on the interaction of strong laser and gas-phase atomic molecules is the only feasible way to realize the conversion of optical comb from near-infrared/visible to extreme ultraviolet.Then this scheme still faces the problems of the low conversion efficiency and the difficulty of coupled output.Therefore,it is of great significance to study the output coupling of the XUV comb for its application to other fields.The paper mainly explores the performance of diffraction grating,Brewster plate and hole mirror as the output coupler of XUV comb,which is mainly divided into the following aspects:1.Based on the theory of rigorous coupled-wave analysis?RCWA?,a MATLAB code is written to simulate the output efficiency of gratings with different structures and different materials for special wavelength of XUV comb.It is found that the grating structure has no diffraction loss to the infrared comb,the maximum output efficiency of the grating with SiO₂ at 40.8 nm and 60.8 nm is about 12%.When increase the grating period,and the output efficiency can be increased by about 3 percentage in the acceptable loss range for infrared comb.In the experiment,a enhanced cavity was built using a rectangular structure grating?SiO2 material?.The multiple of enhanced power of intracavity infrared comb was measured to be close to 150 times,and 13th harmonics were successfully output in the vacuum environment.2.On the basis of Fresnel reflection and refraction theory,the theoretical output efficiency of SiO₂?Al₂O₃ and SiC for XUV comb is calculated using MATLAB.The maximum output efficiency of SiO₂ and Al₂O₃ is close to 20%,while the output efficiency of SiC at 60 nm can be close to 70%.Then the SiC plate is also used in the experiment as output coupler to build the enhanced cavity,and the multiple of power enhancement in the cavity is finally measured to be 82 times.3.Based on the Gauss beam theory,the output efficiency of XUV comb is calculated by using MATLAB with different hole diameter of hole mirror.The calculations show that the longer the diameter,the higher the output efficiency of the hole mirror to the XUV comb,and the higher the output efficiency to the XUV comb with shorter wavelength when the aperture is certain.Therefore,it has a great advantage to output the XUV comb with short wavelength by hole mirror.The exploration of using small hole mirror to build femtosecond resonance enhancement cavity was preliminarily carried out.Limited by the multiple of power enhancement in the cavity,the power threshold to produce higher harmonics has not been reached.This identified technical issues to be overcome in the future.