Study Of YVO₄ And SrWO₄ Crystals Actively Q-switched Intracavity Raman Lasers

As an important frequency shifting technology, stimulated Raman scattering (SRS) can shift the laser frequency over a fixed frequency gap to produce new laser lines and considerably extend the scope of laser spectroscopy. The Raman laser wavelength is determined by the wavelength of the fundamental laser and the Raman shift of the Raman-active crystal. SRS possesses many attractive features, i.e. beam clean-up, pulse compression, no need for phase matching, and high conversion efficiency, etc. Compared with the traditional gas and liquid Raman media, Raman crystals offer high gain, high molecule density, good thermal and mechanical properties. A good number of crystals have been found to be Raman-active, such as Ba(NO₃)₂, BaWO₄, SrWO₄, KGd(WO₄)₂, and YVO₄ crystals. The Raman lasers which using crystal media have the advantages of compactness, high efficiency and high stability and have wide applications in such areas as information, communication, medicine, national defense, and so on.There are several types of solid Raman laser cavity, such as single-pass Raman generators, extracavity or intracavity Raman lasers, coupled resonator configurations and self-Raman lasers. Especially for the intracavity Raman laser, the Raman crystal is inside the resonator and it can utilize the high intracavity power densities. And the Raman crystal and the laser crystal are separated. When the pump power goes higher, the output characteristics of the Raman laser are not seriously sensitive to the thermal effect of the laser medium. So the optical-to-optical conversion efficiency can be as high as 60-80 percents respect to the fundamental laser. Therefore, the intracavity solid state Raman lasers have attracted a great deal of attention in recent years.In this paper, by using Nd:YAG crystal as the laser medium, using YVO₄ and SrWO₄ crystal as the Raman media, we achieve efficient actively Q-switched intracavity Raman lasers. We make a systemic research on the characteristics of the Raman lasers with YVO₄ and SrWO₄ crystals, including the factors of cavity length, pulse repetition frequency, reflectivities of output mirrors on the impact of the output characteristics of the Raman laser. The main contents of this dissertation are as follows:New rate equations are introduced to describe the actively Q-switched intracavity Raman laser. In the rate equations, the intracavity fundamental photon density, Raman photon density and the initial population inversion density of the gain medium are assumed to be Gaussian spatial distributions. The rate equations can describe well the performances of the actively Q-switched intracavity Raman lasers.By using Nd:YAG as gain medium, YVO4 as Raman medium, an efficient LD-pumped actively Q-switched Nd:YAG/YV04 intracavity Raman laser is realized. With different cavity lengths, output mirror reflectivities and pulse repetition frequencies, we research the output characteristics of the Raman laser. The characteristics including the average output power, pulse width and pulse energy versus the incident pump power are measured. At a repetition frequency of 20 kHz and an incident pump power of 9.8 W, an average output power of 1.46W at 1176 nm and an optical-to-optical conversion efficiency of 14.9% are obtained. The rate equations are used to describe the performance of the actively Q-switched Nd:YAG/YVO₄ intracavity Raman laser. The theoretical results are in agreement with the experimental results.By using Nd:YAG as gain medium, SrWCO₄ as Raman medium, an efficient LD-pumped actively Q-switched Nd:YAG/SrWO₄ intracavity Raman laser is realized. We also research the output characteristics of the Raman laser including the average output power, pulse width and pulse energy versus the incident pump power. At a repetition frequency of 20 kHz and an incident pump power of 10.2 W, an average output power of 1.7 W at 1179.6 nm and an optical-to-optical conversion efficiency of 16.7% are obtained. The rate equations are used to describe the performance of the actively Q-switched Nd:YAG/YVO₄ intracavity Raman laser. The theoretical results are in agreement with the experimental results.