The Research Of Multi-wavelenth Picosecond Stimulated Raman Laser

In the field of nonlinear optics, the technologies using second-order nonlinearoptical effect are relatively mature, and have already been into Practical application,while the technologies using Raman frequency conversion of third-order nonlinearoptical effect still need further study. In recent years, Russia, Australia, UK, USA,China and other countries have actively done researches on solid Raman lasers. In thefield of crystal growth, China has been placed in the advanced world level, and hasdeveloped a large number of excellent Raman crystals. But our country has laggedbehind others in research of solid Raman lasers. To take advantage of our country’sstrength in terms of crystal growth, as well as the advantages of our laboratory’sresearch strength in picosecond laser, this paper mainly studies multi-wavelengthpicosecond Raman lasers pumped by picosecond laser.Since the dephasing time of most Raman crystals are about a few picoseconds,and this makes our research in quasi steady state which is between steady state andtransient state. In the single-pass Raman scattering experiment and yttrium vanadateRaman amplifier experiment, We have observed low-order Raman scattering lightsproduced by parametric four-wave mixing process, and high-order Raman scatteringlights generated by the non-parametric stimulated Raman process. While it is not easyto tell how to classify each order Raman lights into transient state process or steadystate process, it is certain that, picosecond stimulated Raman scattering processlocated in the transition phase between the nanosecond SRS process and femtosecondSRS process. This means picosecond stimulated Raman scattering process is bound toshow part of the characteristics of the steady state process, but also exhibit part of thecharacteristics of the transient state process.In the single-pass Raman scattering experiments, crystals of YVO4, GdVO4andKGW are used as Raman crystals to study the beam spot and spectral distribution ofnon-axial Raman scattering lights pumped by picosecond laser. And a physical modelis provided to explain the angular distribution of single-pass Raman scattering lights,which improves the previous method of calculating the angle of the Raman scatteringlights, and makes second order SRS cones, first order absorption SRS cones andanti-first order SRS cones to be estimated more accurately. However, parametricfour-wave mixing dominates in the single-pass Raman experiments, causing parts ofconsumption of pump light, and preventing the non-parametric stimulated Ramanprocess. The non-axial Raman scattered lights will not only undermine the possibilityof Raman coaxial output, but also affect the beam quality of Raman lights. Then, inorder to obtain coaxial multi-wavelength picosecond Raman lights, you can’t simplysolve the problem just by increase the pump power. Later, we tried a Raman amplifier combining with a Raman generator in Ramanamplification experiment. The beam quality of low-order Raman light has beensignificantly improved by Raman cleanups in Raman amplifier, and appears inGaussian mode. If the angle between the pump light and Raman seed can beappropriately adjusted, then three Stokes or three anti-Stokes Raman lights can beobtained. However, the pump light and Raman seed must strictly meet thephase-matching condition to produce high-order Raman lights, and, as a result, theyare very sensitive to the angle changes. Furthermore, all of the Raman amplificationlights still not coaxially output, but the beam quality of them are much better than thatin the single-pass Raman experiment. This provides us with a new way of thinking, sothat high-order picosecond Raman lights can be obtained in low energy pump by useof parametric four-wave mixing process.In order to obtain coaxial output of multi-wavelength picosecond Raman lights,we finally choose the scheme of synchronously pumping external Raman cavity,which will ensure the coaxial Raman output by Raman cavity. After the precisesynchronization adjustment between pump pulse train and Raman cavity, first-orderRaman light is obtained coaxially with the pump. Limited by coating technology formirrors in Raman cavity and the low damage threshold of Raman crystal, high-orderRaman lights are not obtained under our current experimental conditions. But as longas the beam shape of the pump can be improved, and the mirrors in cavity Raman canbe optimized, the target of coaxial output of multi-wavelength picosecond Ramanlights will be achieved.