Research On Orthogonally Polarized Dual-wavelength And Continuous-wave Eye-safe Raman Laser

Raman laser is a kind of laser generated by frequency conversion of Stimulated Raman scattering(SRS),which has the advantages of beam quality improvement and pulse duration shortening.With the wide application of dual-wavelength lasers in the fields of differential lidar,laser interferometer,medical and terahertz,the research hotspot of Raman lasers has also changed to achieve dual-wavelength Raman laser output.Orthogonally polarized dual-wavelength Raman laser with a small wavelength separation can be separated easily and used to produce terahertz laser through difference-frequency generation.Therefore,in this thesis,the effective output of the orthogonally polarized dual-wavelength Raman laser with a small wavelength separation is studied by choosing Nd:YLF crystal as the laser medium and BaWO₄ crystal as the Raman medium.And the output characteristics of the dual-wavelength Raman laser are investigated in detail.In addition,in the existing laser wavelength range,the 1.5 ?m laser is known as eye-safe laser because of less damage to human eyes.At the same time,it is widely used in laser communication,remote sensing,military and other fields.Therefore,another research hotspot in the field of Raman lasers is to achieve 1.5 ?m eye-safe Raman lasers output.However,the eye-safe Raman lasers reported so far are all operated in pulsed mode,continuous-wave(CW)operation has not been successfully realized.This is because the thermal effect of CW laser is more serious,which leads to the decrease of laser gain and higher Raman conversion threshold.Therefore,in this thesis,we use in-band pumping,amplifying pump spot and composite crystal to alleviate the thermal effect,and successfully achieve the high-efficiency output of CW eye-safe Raman laser at 1.5 ?m.The specific research contents are as follows:1.The properties of Nd:YLF laser crystal and BaWO₄ Raman crystal are investigated in detail.The thermal lens focal length of Nd:YLF laser crystal is calculated,and the influence of cavity structure on the stability of dual-wavelength fundamental laser is analyzed.The results show that the thermal lens in Nd:YLF crystal is negative for 1047 nm laser oscillation,while it is positive for 1053 nm laser.When the resonant is a plane-concave configuration,if the radius of curvature of the output mirror is larger,the 1047 nm laser power drops earlier and the threshold for 1053 nm laser oscillation is lower.2.Using Nd:YLF crystal as the laser medium and BaWO₄ crystal as the Raman medium,the orthogonally polarized dual-wavelength Raman laser at 1159 nm and 1167 nm is realized.By adjusting radius of curvature of the output mirror and controlling the direction of the crystal axis of BaWO₄ crystal,the dual-wavelength Raman laser with comparable output powers is obtained.Finally,under an incident pump power of 8.5 W,the maximum output powers obtained at 1159 nm and 1167 nm are 203 mW and 170 mW,respectively.The corresponding optical conversion efficiency is 4.4%.3.Using the composite Nd:YVO₄ crystal as self-Raman medium and a wavelength-locked 878.9 nm laser diode as in-band pumping source,high-efficiency continuous-wave eye-safe Raman laser at 1524 nm is realized for the first time.The influences of the pump spot size,radius of curvature of the output mirror,and the crystal length on the output performance of the laser are systematically studied.Finally,when the pump spot radius is 200 ?m,the radius of curvature of output mirror is 200 mm,and the Nd:YVO₄ crystal length is 14 mm,a 519 mW stable output of continuous-wave Raman laser at 1524 nm is achieved under an incident pump power of 13.6 W.The corresponding optical conversion efficiency and slope efficiency are 3.8%and 5.0%,respectively.The output power stability of Raman laser is measured to be 6.3%.