Study Of Novel Dual-wavelength Laser Based On Stimulated Raman Scattering

Stimulated Raman scattering(SRS)is a third-order nonlinear optical effect,which is generated from the interaction of intense laser with the atom,molecule or crystal lattice.The SRS radiation possesses laser characteristics,such as coherence,directionality and high intensity.As one of the most efficient optical frequency conversion technologies,lasers that operate at different wavelengths from ultraviolet to infrared can be obtained by SRS,including yellow laser,eye-safe laser and dual-wavelength laser for many important applications.SRS is also a powerful tool in detecting material structure and imaging surface morphology of materials.The main technologies involve time-resolved ps SRS,impulsive SRS,femtosecond stimulated Raman scattering,SRS imaging,and surface-enhanced SRS.In addition,SRS signal is applicable to detect the formation of laser-induced plasma,filamentation and shockwave in the liquids.Simultaneous dual-wavelength pulsed laser has many important applications in atmospheric detection,medical diagnosis and prognosis,spectral analysis,and difference-frequency generation of terahertz wave.Based on the emitting simultaneously for energy splitting in the active ions or optical frequency conversion in non-linear medium,we can obtain the dual-wavelength laser from ultraviolet to infrared.However,the existing technology still have some deficiencies,such as instability energy,inflexible energy ratio,fixed frequency interval,and so on.In order to solve these problems,this dissertation is devoted to studying novel dual-wavelength laser,including dual-wavelength laser based on two kinds of Raman crystals,dual-wavelength laser based on two kinds of Raman liquids,dual-wavelength laser based on stimulated Raman scattering and second-order nonlinear optical effect.We also study SRS in NaCl solution and analyze the structure of water and NaCl solution.The main research contents are as follows:(1)Based on SRS of two Raman mediums,optical-path-splitting dual-wavelength laser system was developed.Using GdVO4 and BaWO4 crystals as the Raman medium,we systematically investigated the output characteristics of the dual-wavelength laser.We obtained four groups of dual-wavelength laser formed by Stokes and Anti-Stokes lines,including 558.4 nm and 559.7 nm,587.4 nm and 590.2 nm,619.5 nn and 624.4 nm,as well as 508.2 nm and 507.2 nm.The frequency interval between 1st-Stokes lines is 42 cm-1 Our experiment proved that the generated dual-wavelength laser possessed many advantages including good stability,high efficiency,tunable energy ratio of dual wavelength and adjustable frequency interval by replacing Raman laser medium.This kind of dual wavelength laser have promising application potentials in generating coherent THz radiation by difference frequency,differential absorption,fine spectrum,etc.(2)We studied dual-wavelength laser based on two kinds of Raman liquids.Dual-wavelength laser can be obtained by exciting SRS of two constituents with the same laser with different Raman shifts.For the first time,dual-wavelength SRS was achieved in alcohol-water blend.We demonstrated the efficient dual-wavelength SRS at 631 nm and 650 nm with close output energies,which correspond to the stretching vibrations of C-H3 and O-H groups,respectively.The highest overall conversion efficiency reached 44.8%.The numerical simulations based on SRS coupled wave equations agreed well with the experimental results.The energy ratio of two Raman shifts can be flexibly adjusted by changing liquid proportioning,interaction length,or pump condition.Owing to the higher damage threshold,liquid can operate at higher pump power and density than solid mediums,which is favorable for large energy nonlinear optical frequency conversion.This work created a simple,efficient,and low-cost approach to achieve dual-and multi-wavelength laser sources.(3)Cascaded frequency doubling and stimulated Raman scattering are reported for the first time.The focused 1064 nm laser pulses can trigger the stimulated Raman scattering and the frequency doubling process in a single potassium dihydrogen phosphate(KJDP)crystal.The near-infrared light was thus converted into new spectra lasers spanning the green-to-red spectral range.They can serve as several dual-wavelength lasers,such as green-yellow,green-red laser.Because of the superiority of KDP crystal,which can be grown into large size,it can be used to convert near-infrared ultra-high power laser to visible dual-wavelength laser by increasing the crystal length or designing the appropriate coupling resonator.Furthermore,we found that the 3rd-Stokes threshold of the LSRS was close to the laser-induced damage threshold.This research presents a good way to avoid the optical damage of two-order nonlinear optics crystals by monitoring its 3rd-Stokes signal.Therefore,this will make it possible for the applications of KDP frequency doublers of inertial confinement fusion(ICF)equipment.(4)Based on optical composite mechanisms with SRS in liquid and second order nonlinear effect of crystal,we have implemented the tunable ultraviolet output and dual-wavelength ultraviolet laser.Pumped by nanosecond laser at 532 nm,we demonstrated the 1st-Stokes component at 631 nm and the 2nd-Stokes component at 773 nm in pure ethanol.Using nonlinear second harmonic and sum frequency mixing of the Stokes(631 nm and 650 nm)and fundamental laser(532 nm)in ?-barium borate(BBO),tunable ultraviolet output was achieved,including 266 nm,288 nm,315 nm,347 nm and 386 nm.The spectral coverage are wider than the results with Raman crystals.The dual-wavelength ultraviolet laser,which operated at 288 nm and 315 nm,is achieved simultaneously by second harmonic generation and sum-frequency mixing in two BBO crystals.The laser source can apply to differential absorption laser radar for environmental O3 measurements.(5)The SRS of sodium chloride(NaCl)solutions and water was studied with 532 nm pico-second pulsed laser source.We found that the SRS enhancement of water was on account of the dissolving mono-atomic ions(Na-and Cl-).The corresponding characteristics included a descending threshold,spectrum purification,and Raman gain promotion.In the best SRS results,the pump threshold decreased by 45%and the maximum conversion efficiency increased by 10.4%.Moreover,we also revealed the existence of clusters in water and NaCl solutions,and they could be identified in detail by combining the principles of SRS and analysis of the SRS spectra.In water,the O-H stretching band was proved to be the superposition of stretching modes in different water clusters.For the solution,the O-H stretching vibration of water molecules in hydration shells around Cl-ions can be obtained directly and its variation with respect to solute concentration was investigated using the SRS spectra.This work suggests that the SRS is a promising approach to identify structures in water and NaCl solutions.