| Optical superlattice is a new nonlinear frequency conversion material which develops rapidly in recent years. It has many advantages such as utilizing the largest nonlinear coeffiecent of crystal, no walk-off effect, and a designable nature which can flexibly meet the requirements of many nonlinear processes, etc. Combined with the all-solid-state laser techniques, it can generate any laser light whose spectra is within a wide range from ultraviolet to mid-and far-infrared. As a nonlinear frequency convertor, optical superlattice can be positioned either outside or inside the laser cavity. The intra-cavity using of nonlinear frequency convertors can enhance the nonlinear conversion efficiency, reduce the system size, and realize functions which can not be realized in extra-cavity scheme such as passive mode locking. Studying the intra-cavity frequency-conversing properties of optical superlattice, exploring its intra-cavity usages, and improving the performance of lasers with intra-cavity optical superlattice are of significance in both science and applications. This article firstly studies the cavity-designing method and thermal lensing effect, and improves the performance of laser systems used in experiments. Based on these techniques, a watt-level, continues-wave (cw), intra-cavity second-harmonic generation green laser of the novel Nd:GdVO4laser crystal is realized in experiment. Further the optimization of an intra-cavity third-harmonic generation process is studied theoretically. Then, based on the results and techniques obtained from intra-cavity frequency conversion, we study the performance of a passively mode-locked, picoseconds’Nd:GdVO4laser with an intra-cavity optical superlattice as the frequency convertor. The content of this thesis is as following:1. Chapter1briefly introduces the history and significance of laser, and the developments and achievements of nonlinear frequency conversion techniques and passive mode-locking techniques. It shows the application potential of optical superlattice in these two areas and the state-of-art results. The end of this chapter introduces the motivation and significance of this work. 2. In Chapter2, a systematic study of the designing of all-solid-state lasers’cavity is performed theoretically and experimentally. Using the propagation character of a Gaussian beam, a method for on-line measurement of the solid-laser’thermal lensing effect is introduced. The thermal focal length of a side-pumped laser module is measured by this method, providing important parameter for the designing of laser cavity. Based on above methods, several solid lasers, either side-pumped or end-pumped, are set up, and the thermal lensing effect and output beam quality are optimized. Good results are obtained, and these make a solid foundation for latter studies.3. In Chapter3, a numerical mode of intra-cavity SHG is set up from the basic principles, and the relations between the system parameters and output power are studied. CW intra-cavity SHG green lasers using KTP or MgO doped period-poled lithium niobate (MgO:PPLN) are built up. Intra-cavity SHG in a Nd:GdVO4laser with optical superlattice is realized experimentally for the first time, and high power green laser is obtained form all these lasers. Experimental results are compared with those given by theoretical calculations. The advantage and disadvantage of optical superlattice compared with former nonlinear materials in intra-cavity usage are concluded.4. In Chapter4a theoretical study of single-resonant third-harmonic generation laser is performed, and the relations between output power and gain coefficient, loss and nonlinear interaction length are obtained. From these relations the optimum parameters for optical superlattice with cascade periods or quasi-period structures are calculated. The numerical results show quasi-period-structure optical superlattice may have a better performance in reducing the length of the nonlinear device than that with a cascade-periods structure.5. Chaper5, starts form the basic principles of the intra-cavity second order nonlinearity mode-locking, and studies the application of optical superlattice in passive mode-locked ultra-short-pulse lasers experimently. A passive mode-locking laser with MgO:PPLN, which uses the technique called nonlinear mirror mode locking (NLM), is built up and the output pulse train is measured to be96.7MHz in repetition frequence and30-45ps in pulse width. The output power, beam profile, and pulse width are measured and using these results, the pulse squeezing ratio is calculated and discussed. The imperfect performance of this laser, which is unstable and has a relative large pulse width, is analyzed, and the main cause is attributed to the relatively large loss and unstable temperature in the optical superlattice. Another technique which is known as casade second-order nonlinearity mode locking (CSM) is also performed. Some suggestions for improvement are provided for further works in this system.6. In the last chapter, a conclusion of the work in the thesis is made. |
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