| The blue-green laser is the optical transmission window in seawater.Because of its high transmittance in seawater,it is widely used in many fields such as underwater laser communication,marine detection radar,and marine environment monitoring.486.134 nm is the dark line of solar radiation in the blue band,called H-βFraunhofer dark line,which is the best working wavelength of spaceborne ocean lidar.In this paper,based on the requirements of H-βdark line blue laser in marine applications,an experimental study of H-βdark line all-solid-state blue laser is carried out by using laser frequency doubling technology,and finally a high repetition rate 486.1 nm blue laser output is obtained.The specific research conducted is as follows:In terms of theory,the coupled wave equations of the frequency doubling process are established from the classical electromagnetic theory,and the main factors affecting the frequency doubling efficiency are analyzed by solving them.Based on the phase matching theory,the optimal phase matching angle of the nonlinear crystal is simulated and calculated.Moreover,the effects of the power density of the fundamental frequency laser,the acceptance angle and the walk-off angle of the crystal on the length of the nonlinear crystal are simulated and analyzed.The processing parameters of the nonlinear crystal are determined.According to the needs of the frequency doubling experiment,the fundamental frequency laser source is optimized and analyzed.Based on the thermal effect theory,the thermal distribution model of the double-ended pumped slab Tm:YAP crystal is established,and the temperature field distribution and the variation of the focal length of the thermal lens inside the Tm:YAP crystal are analyzed in detail.The resonant cavity structure of the laser is optimized.Based on the theory of F-P etalon wavelength tuning,the influence parameters of double F-P etalon tuning are simulated and analyzed,which provides a theoretical basis for the experimental study of the fundamental frequency laser source.In terms of experiments,an H-βdark-line all-solid-state blue laser experimental platform is built to investigate the wavelength-tuned fundamental frequency laser source experiment and the 486.1 nm quadruple frequency experiment.The laser uses a LD double-end pumped slab Tm:YAP crystal,and the pulse laser is modulated by an acousto-optic Q-switch.The 0.5 mm and 0.025 mm F-P etalons are added to the cavity to tune the wavelength.At a repetition rate of 10 k Hz,an average output power of 15.44 W at 1944.4 nm is obtained.The linewidth is 0.43 nm,the pulse width is 41.5 ns,and the single pulse energy is 1.54m J.The second harmonic generation experiment uses the type I phase matching(θ=90°,φ=26.4°)of the XZ plane of the 4×4×15 mm~3 LBO crystal to obtain a 972.2 nm laser output with an average power of 4.62 W,and the frequency doubling efficiency is about 29.9%.The fourth harmonic generation experiment uses 4×4×5 mm~3 BBO crystal type I phase matching(θ=20.7°),and the 486.1 nm blue laser output with the highest average power of 1.18 W is obtained.The fourth harmonic generation efficiency is about 25.5%,the laser line width is 0.18 nm,the pulse width is 36.2 ns,the single pulse energy is 0.12 m J,and the corresponding peak power is 3.26 k W.The experimental results show that the Tm:YAP laser can achieve 486.1 nm blue laser output. |
