Research On Narrowband Eye-safe Crystalline Raman Laser Technology

The 1.4-1.8?m band belongs to the typical"eye-safe"band and has extremely high transmittance in the atmosphere and optical glass.Therefore,coherent radiation sources in this band are used in optical communications,optical ranging,laser countermeasures,optical remote sensing or other important applications,and it can also be used as a pump source for down-conversion of mid-infrared optical parameters.The current methods for producing eye-safe lasers in the 1.4-1.8?m band mainly include semiconductor disk lasers,erbium-doped solid-state or fiber lasers,optical parametric oscillations and stimulated Raman scattering(SRS).Semiconductor disk lasers can emit average power of several watts for continuous wave(CW)operation and peak power of kilowatt level for pulse operation,but the complexity of growth and fabrication impedes its further practical application.The erbium-doped glass fiber lasers suffer from poor average power capability,and have restrictions on peak power,which makes it difficult to achieve ideal laser output performance.Resonantly pumped erbium-doped crystalline lasers can achieve high average and peak powers.However,high brightness narrowband pump sources are required for efficient operation,which will increase the system complexity and cost.Although OPOs can generate tens to hundreds watts of output in eye-safe wavelength range,the serious thermal problems generated in the nonlinear crystal will cause the onset of the poor beam quality and crystal damage.Moreover,OPOs need to meet phase matching condition,resulting in obstacles for creating robust system.The Stimulated Raman Scattering based on third-order nonlinear effect has the advantages of automatic phase matching,shortening the pulse duration,and cleaning the Raman beam.It is a promising method to generate eye-safe lasers.The narrowband eye-safe laser has the advantages of large coherence length and reduced light scattering loss,so it has important applications in lidar and biological engineering.The main research contents are shown as follows:1.Combined with the pumping technology of low quantum depletion at 880 nm,using Nd:YLF crystal with long upper energy level life and weak thermal lens effect,the 1314 nm active Q-switched fundamental light output is obtained,and the maximum average output power is 7.4 W,the optical-to-optical conversion efficiency is 15.8%,and the beam quality factor M² is about 1.7.2.The narrowband eye-safe intracavity Raman laser was obtained for the first time.By adding an etalon with a thickness of 0.3 mm in the fundamental resonator,then the Np-cut KGW crystal was pumped by the active Q-switched 1314 nm Nd:YLF laser in the cavity,and two groups of eye-safe Raman lasers of 1461 nm and 1490 nm were obtained after rotating the KGW crystal,the highest average power were 3.6 W and 4 W,respectively.The highest peak power were 330 k W and480 k W,respectively,the line width is less than 0.05 nm,and the beam quality factor M² is less than1.4.This peak power is higher than other eye-safe band intracavity Raman lasers one to two orders of magnitude.3.A single longitudinal mode cascaded crystalline Raman laser at 1.7?m was realized for the first time.On the basis of the narrowband Raman laser at 1.5?m,YVO₄ crystal was used to replace KGW crystal,a single longitudinal mode cascade crystalline Raman laser at 1715.2 nm was realized.When the repetition frequency is 4 k Hz,the maximum single longitudinal mode laser output power is 1.8 W,and the spectral linewidth is about 340 MHz.The measured maximum average output power is 2.7 W,and the maximum peak power is 380 k W,which is an order of magnitude higher than other 1.7?m crystal Raman lasers.