Study On High Efficiency Frequency Doubling And Frequency Tripling Of All-Solid-State Laser

The short wavelength laser generated by nonlinear frequency conversion has the characteristics of high photon energy.It has a wide range of applications in industry and science,such as wafer cutting,grating writing,ultraviolet hypersensitivity,material processing,and so on.This paper mainly studies the frequency doubling and frequency tripling nonlinear optical frequency conversion technology of all-solid-state nanosecond and picosecond lasers.Through the design of new materials and experimental schemes,the conversion efficiency and output mode are improved.On this basis,the prototype lasers are developed.The main contents of this paper are as follows:(1)Frequency doubling of nanosecond pulse laser is realized in monoclinic KDP crystal microstructure.The influence of KDP crystal microstructure parameters on frequency conversion efficiency is analyzed through experiments,and the polarization characteristics of SHG(Second harmonic generation)laser are studied.The type-I phase matching in KDP crystal microstructure is obtained;(2)The nanosecond pulse laser frequency doubling of one-dimensional is studied,and 1.93 mW green light is achieved by 359.26 mW fundamental frequency light;(3)LBO crystal with Brewster angle cutting is used to realize frequency tripling and beam splitting,and high efficiency 355 nm ultraviolet laser is generated in a cavity.When the pump power is 29 W,5.6 W output power of THG(Third harmonic generation)light is obtained;(4)High-efficiency broadband frequency doubling and frequency tripling are realized by using spectral phase matching frequency doubling and crystal cascade frequency tripling;(5)A high-repetition-rate regenerative amplifier is built,and a 10.9 W/10 ps/100 kHz picosecond pulse laser is realized.The frequency doubling and frequency tripling experiments are carried out.The position of the half-wave plate is adjusted by electronic control,which can realize the flexible selection of output wavelength.Potassium dihydrogen phosphate(KH2PO4,KDP)is a kind of crystal with excellent nonlinear optical properties.In order to further study the frequency doubling properties of KDP crystal microstructure,we experimentally studied the effects of the length,diameter of single crystal KDP crystal microstructure and laser polarization on the conversion efficiency of frequency doubling.By using a KDP microstructure with a length of 0.95 mm and a diameter of 25 ?m,532 nm light power of 705?W is obtained at 908 mW fundamental light power,and the conversion efficiency is 8.55×10-4 W-1.The experimental results show that the diameter of KDP crystal microstructure has a great influence on the conversion efficiency on low fundamental frequency light power,while the length of KDP crystal microstructure has a great influence on the conversion efficiency of high fundamental frequency light power.By studying the polarization characteristics of the second-harmonic light,type-I phase matching is conformed in the KDP crystal microstructure.After that,one-dimensional ANDS samples are studied.The nanosecond pulse frequency doubling of one-dimensional ANDS sample is reported for the first time.An 8 mm long and 71 ?m cross-section size sample was used to generate 1.93 mW of 532 nm power at 359.26 mW of 1064 nm power.The experimental results show that,under the same cross-sectional size,longer samples have higher conversion efficiency.For ANDS samples with the same length,the larger the cross-sectional size of the sample,the higher the frequency doubling efficiency.These results are published in IEEE Photonics Technology Letters,2019,Vol.31,No.13,page.1080-1083 and Optical Materials,2020,vol.110,page.110464.The output mirror with high transmittance film in UV band is used to separate the IR,visible and ultraviolet light.However,the optical coating film with UV band usually has a low damage threshold.A new method of third harmonic generation with Brewster angle cutting LBO crystal is introduced,which can realize wavelength separation and resist UV damage.The output surface of LBO crystal and other optical elements in laser system do not need to be coated with UV film.Therefore,this design is very suitable for the frequency tripling.Considering the fundamental frequency astigmatism caused by Brewster angle cutting,we use convex-concave cavity to compensate the astigmatism.By suitable resonator design,the instability of the resonator caused by the thermal lens effect of gain medium is further offset.In order to further increase the output power of the ultraviolet laser,a flat-flat cavity is used.Finally,when the pump power is 29 W and the repetition rate is 50 kHz,the UV laser power of 5.6 W and pulse width of 14.2 ns is obtained.The conversion efficiency of the pump-UV light in the experiment is higher than that of the existing intracavity frequency conversion UV nanosecond laser.The results are published in Journal of Optics,2019,Vol.21,No.11,page.115501.In the broadband third harmonic generation,not only phase matching but also group velocity matching should be satisfied.In order to improve the broadband nonlinear conversion efficiency of picosecond fiber laser,we adopt the design of spectral phase matching frequency doubling and double-crystal cascade frequency tripling.When the fundamental spectral width is 5.5 nm and the input fundamental power is 8.6 W,frequency doubled light with 515 nm central wavelength and 343 nm central wavelength is obtained,and the output power is 4.52 W and 2.39 W,respectively.This is the first time to realize broadband third harmonic picosecond ultraviolet laser generation by combining spectral phase matching frequency doubling and double-crystal cascade frequency tripling.The results are published in Optics and Laser Technology,2021,Vol.141,page.107105.The high repetition rate picosecond regenerative amplifier and frequency doubling and frequency tripling systems are researched.The device of high repetition rate regenerative and single-pass amplifier is built.Using fiber laser as seed source and Nd:YVO4 crystal as gain crystal,and 808 nm laser diode as pump source,10.9 W/10 ps/100 kHz picosecond laser is obtained.Then,the SHG and THG by LBO crystal are realized.Finally,the 6.5 W second harmonic light power is obtained,with the pulse width of 8 ps.The 2.76 W third harmonic light power is obtained,with the pulse width of 7 ps.We develop a laser prototype according to our research.