The Studies Of Intracavity Pumped Anti-Stokes Raman Lasers

Stimulated Raman scattering (SRS) is an important way to generate new laser wavelengths. By using the existing laser resources as the pumping source and using the Stokes scattering effect in Raman crystals, solid-state Raman lasers can achieve frequency conversion to the long laser wavelengths, and largely extend the laser spectral range. Because of their high efficiency, high stability and compactness, the solid-state Raman lasers have gained extensive attention in the areas of communication, transportation, medical care, measurement, national defense, and so on. The researches on high-efficient Stokes conversion in solid Raman media have been deeply investigated. By contrast, there are few reports about the anti-Stokes generation process. The frequency up-conversion in anti-Stokes scattering can be an effective method to expand the spectral range of coherent light sources, which have many potential applications.Anti-stokes Raman scattering is a typical four-wave mixing process. In the Raman media, one anti-Stokes photon can be generated by the three order nonlinear interaction between two fundamental photons and one first Stokes photon. In macroscopic view, the optical intensity of the anti-Stokes wave is proportional to the square of the fundamental intensity multiplied by the Stokes intensity. In contrast to the usual extracavity pumped anti-Stokes Raman lasers, intracavity pumped anti-Stokes systems can make full use of the intracavity high fundamental power density. And the intracavity systems use round trips of the fundamental waves inside the Raman crystals, while in the extracavity pumped structure we can only get one-way anti-Stokes output, which is limited by the direction of the fundamental wave and wave vectors’relation in the four-wave mixing. Besides, the intracavity anti-Stokes lasers have more advantages of high conversion efficiency, low generation threshold, compactness and portability. Until now, there are not relevant reports on either the theoretical model or the experimental research of the intracavity anti-Stokes Raman lasers.In this thesis, we studied the characteristics of intracavity anti-Stokes Raman lasers with a BaWO4crystal or a SrWO4crystal as the Raman medium respectively, and achieved bidirectional outputs of the first anti-Stokes waves. In the theory part, by numerically solving the anti-Stokes rate equations and coupled wave equations of four-wave mixing, we established the theoretical model for intracavity anti-Stokes Raman lasers. This model was used to analyze the generation processes of the fundamental, the first Stokes, and the first anti-Stokes waves. The main contents of this thesis are as follows:1. By using an Nd:YAG rod crystal as the gain medium, an α-cut BaWO4crystal as the Raman medium, an actively Q-switched intracavity pumped anti-Stokes Raman laser was achieved at the wavelength of968nm for the first time. At the pumping voltage of750V, the maximum output first anti-Stokes energy was0.79mJ, and the output fundamental energy was8.75mJ. The intracavity fundamental energy was calculated to be51.47mJ according to the transmission of the output coupler. The corresponding anti-Stokes conversion efficiency was about1.5%.2. By using an Nd:YAG crystal as the gain medium, an α-cut SrWO4crystal as the Raman medium, an actively Q-switched intracavity pumped anti-Stokes Raman laser was achieved at the wavelength of969nm for the first time. At the pumping voltage of775V, bidirectional first anti-Stokes waves were obtained. The maximum forward and backward output energies were0.683mJ and0.667mJ, respectively.3. The phase matching characteristics of the four-wave mixing effect are theoretically and experimentally studied. The phase matching angles of the first anti-Stokes generation process and the second anti-Stokes phase mismatching are calculated by taking the BaWO4and the SrWO4crystals as examples.4. Based on the first anti-Stokes coupled wave equations in the four-wave mixing process, the expressions of the first anti-Stokes intensity and the single pulse energy were deduced. Under the plane-wave approximation, the theoretical model for intracavity anti-Stokes Raman lasers was established by adding additional terms to describe the anti-Stokes generation process. Numerically solving these equations, the temporal pulse shapes were simulated, and the first anti-Stokes output energies as functions of the pumping voltage were calculated. The stimulated results agree with the experimental ones on the whole.