The Theoretical And Experimental Studies Of Passively Q-switched Raman Lasers

Stimulated Raman scattering (SRS) is one of effective methods for frequency conversion, and the wavelengths of the generated Raman scattered light are determined by the pump laser wavelengths and the Raman shifts of the Raman crystals. The laser spectrum with SRS can extend from the ultraviolet to the near infrared by using different pump sources and different Raman-active media. Raman-active media include solids, liquids and gases. Compared with the traditional gas and liquid Raman media, solid-state Raman media have the advantages of high molecule density, small physical size, high Raman gain coefficient, good thermal and mechanical properties, and so on. The solid-state Raman lasers using crystal Raman media have the advantages of compactness, high stability and high efficiency. And they have wide applications in fields of medical treatment, information, communication, measurement, military affairs, and so on.Solid-state Raman media and all-solid-state Raman lasers have attracted much interest in the fields of laser materials and solid-state lasers in recent years. Researchers from Russia, America, Germany, Australia, Taiwan, etc. are actively taking part in the research of the all-solid-state Raman lasers. In the Chinese Mainland, the research groups from Shandong University, Fujian Institute of Research on the Structure of Matter, Shanghai Institute of Optics and Fine Mechanics are engaged in the research for solid-state Raman lasers and have much research output in theory and experiment.The yellow-orange lasers with the spectral region from560nm to600nm have attracted intensive research interests, which have had wide applications in medicine, communication, spectroscopy, laser radar, metrology, remote sensing, information storage, and so on. But they are hardly obtained by frequency doubling Nd-doped lasers. An efficient method for generating the yellow-orange laser is intracavity frequency doubling the first Stokes Raman laser. With the Nd doped1.06μm fundermental laser and the SRS in Raman crystal, the1.18μm first-Stokes wavelength can be generated. And the first-Stokes laser is frequency-doubled afterwards to generate the yellow-orange laser.Because actively Q-switched Raman lasers have the advantages of high output power and high efficiency, most reported Raman lasers are actively Q-switched. Compared with actively Q-switched Raman lasers, passively Q-switched Raman lasers have the advantages of compactness, low cost and easiness of operation although their output powers and efficiencies are relatively low. So research on passively Q-switched frequency-doubled Raman lasers is also important. Nevertheless, the reports on LD-pumped passively Q-switched Raman lasers are relatively few. So in this dissertation, we mainly studied passively Q-switched Raman lasers.In this dissertation, we firstly studied a new method of solid-state Raman gain coefficient mearement. The Raman gain coefficient gR could be obtained by analyzing the relation between the ratio of Raman laser intensities of the two Raman crystals and the pump laser intensity. The Raman gain coefficient of YVO4crystal was measured to be4.5cm/GW by this method. Secondly, by using Nd:YAG as the gain medium, using Cr4+:YAG as saturable absorber, we studied the output laser characteristics of LD-pumped passively Q-switched intracavity SrWO4, KLu(WO4)2and BaWO4Raman lasers, respectively. Thirdly, by using the KTP intracavity frequency doubling of the LD-pumped passively Q-switched Raman laser, the efficient yellow lasers were obtained. And a theoretical model for the passively Q-switched intracavity frequency-doubled Raman laser was built.The main contents of this dissertation are as follows:1. A new method for measuring Raman gain coefficients of solid-state materials was presented. Two crystals with the same material and different lengths were used. The pump source was a picosecond pulse laser. The Raman gain coefficient gR could be obtained by analyzing the relation between the ratio of Raman laser intensities of the two crystals and the pump laser intensity. The gain coefficient of YVO4crystal was measured to be4.5cm/GW.2. By using an Nd:YAG crystal gain medium, a Cr4+:YAG saturable absorber and an a-cut SrWO4Raman medium, an LD end-pumped passively Q-switched Nd:YAG/SrWO4intracavity Raman laser was demonstrated for the first time. A1.78W1180nm laser was obtained at an incident pump power of14.9W and a saturable absorber’s initial transmission of81%. The highest pulse energy was88.1μJ.3. By using an Nd:YAG ceramic gain medium, a Cr+:YAG saturable absorber and an c-cut KLu(WO4)2Raman medium, an LD end-pumped passively Q-switched Nd:YAG/KLu(WO4)2intracavity Raman laser was demonstrated for the first time. A1.89W1178nm laser was obtained at an incident pump power of15.7W and a saturable absorber’s initial transmission of81%. The highest pulse energy was84.0 μJ.4. The rate equations for the passively Q-switched intracavity frequency-doubled Raman lasers were obtained by considering the spatial distributions of the intracavity photon density and the population inversion density, and the photon density relation between the frequency-doubled Raman laser and the Raman laser. These rate equations were normalized by introducing some synthetic parameters. By solving the normalized rate equation numerically, a group of general curves were generated. These curves could give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters. They could also be used to estimate the laser pulse characteristics of any passively Q-switched intracavity frequency-doubled Raman laser.5. By using an Nd:YAG ceramic gain medium, a Cr4+:YAG saturable absorber, an a-cut SrWO4Raman medium and a KTP frequency doubling medium, an LD end-pumped passively Q-switched Nd:YAG/SrW04/KTP intracavity frequency-doubled Raman laser was demonstrated for the first time. With a coupled cavity, a1.02W590nm laser was obtained at an incident pump power of14.0W and a saturable absorber’s initial transmission of81%, the corresponding conversion efficiency was7.29%. The highest pulse energy was56.2μJ.6. By using an Nd:YAG crystal gain medium, a Cr4+:YAG saturable absorber, an a-cut BaWO4Raman medium and a KTP frequency doubling medium, an LD end-pumped passively Q-switched Nd:YAG/BaWO4/KTP intracavity frequency-doubled Raman laser was demonstrated for the first time. With a coupled cavity, al.21W590nm laser was obtained at an incident pump power of14.1W and a saturable absorber’s initial transmission of81%. The highest conversion efficiency was9.06%. The highest pulse energy was68.5μJ. The main innovations of this thesis are as follows:1. A new method for measuring Raman gain coefficients of solid-state materials was presented. Two crystals with the same material and different lengths were used. The Raman gain coefficient gR could be obtained by analyzing the relation between the ratio of Raman laser intensities of the two crystals and the pump laser intensity. The gain coefficient of YVO4crystal was measured to be4.5cm/GW.2. By using an Nd:YAG crystal gain medium, a Cr4+:YAG saturable absorber and an a-cut SrWO4Raman medium, an LD end-pumped passively Q-switched Nd:YAG/SrWO4intracavity Raman laser was demonstrated for the first time.3. By using an Nd:YAG ceramic gain medium, a Cr4+:YAG saturable absorber and an c-cut KLu(WO4)2Raman medium, an LD end-pumped passively Q-switched Nd:YAG/KLu(WO4)2intracavity Raman laser was demonstrated for the first time. A1.89W1178nm laser was obtained at an incident pump power of15.7W and a saturable absorber’s initial transmission of81%. The obtained maximum average output power was much higher than those of the previously reported diode-pumped passively Q-switched intracavity Raman lasers.4. A theoretical model for the passively Q-switched intracavity frequency-doubled Raman lasers were built for the first time, by considering the spatial distributions of the intracavity photon density and the population inversion density, and the photon density relation between the frequency-doubled Raman laser and the Raman laser.5. By using an Nd:YAG ceramic gain medium, a Cr4+:YAG saturable absorber, an a-cut SrWO4Raman medium and a KTP frequency doubling medium, an LD end-pumped passively Q-switched Nd:YAG/SrWO4/KTP intracavity frequency-doubled Raman laser was demonstrated for the first time.6. By using an Nd:YAG crystal gain medium, a Cr4+:YAG saturable absorber, an a-cut BaWO4Raman medium and a KTP frequency doubling medium, an LD end-pumped passively Q-switched Nd:YAG/BaWO4/KTP intracavity frequency-doubled Raman laser was demonstrated for the first time. With a coupled cavity, a1.21W590nm laser was obtained at an incident pump power of14.1W and a saturable absorber’s initial transmission of81%. The highest conversion efficiency was9.06%. The highest pulse energy was68.5μJ. To the best of our knowledge, the obtained average output power, conversion efficiency and pulse energy were much higher than those of the previously reported LD-pumped passively Q-switched frequency-doubled Raman lasers.