Research On Harmonic Transformation Technology Of Femtosecond Fiber Laser

With the continuous development of laser technology,traditional infrared and near-infrared laser have been unable to meet people’s needs,visible laser and ultraviolet laser have gradually entered people’s field of vision.The single photon energy of the short-wavelength laser is high,and when it contacts the material surface,it will directly break the chemical bonds of the material molecules,and the thermal effect is small,so as to achieve the effect of "cold processing".laser can be classified into continuous laser and pulsed laser according to their working methods.Compared with continuous laser and long-pulsed laser,femtosecond laser have the advantages of narrower pulse width and higher peak power.Therefore,ultraviolet femtosecond laser comes into being and are widely used in special material processing,medical treatment and military fields.In recent years,there have been many researches on femtosecond solid-state ultraviolet lasers at home and abroad,but femtosecond ultraviolet laser based on fiber lasers are rarely reported.Compared with solid-state lasers,fiber lasers have stable and compact structure,stable heat dissipation,and have broad market prospects.In this paper,a femtosecond fiber laser based on chirped amplification system was used as the fundamental frequency laser source,a suitable nonlinear crystal was selected,and nonlinear frequency conversion technology was used to convert the near-infrared laser into a pulse in the visible and ultraviolet wavelength bands;The walk-off effect in the nonlinear crystal was compensated by the compensation plate;the beam quality of the Sum Frequency Generation was found to be poor in the experiment,and the calculation and simulation of the thermal effect of the nonlinear crystal were carried out.The main research results of this paper were as follows:1.By analyzing and calculating the phase matching conditions of BBO and LBO crystals,and BBO crystal is used for frequency doubling to obtain frequency-doubling green laser.The frequency-doubling efficiency is about 40%,and the beam quality is no different from the fundamental frequency laser.After that,BBO crystal and LBO crystal were used to conduct sum-frequency experiments,respectively,to obtain ultraviolet laser,and the efficiency of green laser to ultraviolet laser was 10% and20%.2.The walk-off effect in nonlinear crystals was analyzed,and the compensation plate was used to compensate the walk-off effect.For the sum-frequency experiment based on BBO crystal,the conversion efficiency of green laser to ultraviolet laser is from 10% increased to 15%.Compared with the traditional time delay line compensation,the scheme was simple in structure,convenient in construction,and more suitable for the integration of optical paths.3.The beam quality of green laser and ultraviolet laser were measured.It was found that the beam quality of green laser was better,and the beam quality of ultraviolet laser was poor.The change was calculated and simulated,the thermal effect caused by the temperature change was analyzed,the optical path difference caused by the thermal effect was calculated,the optical path difference was decomposed by the Zernike polynomial,and the relationship between the beam quality and the optical path difference was deduced.Several solutions to solve the thermal effects are proposed.