| The optical parametric oscillators(OPO) are widly used in the field of national defence, environment protection, physic and biochemistry etc. because of its high conversion efficiency and wide tunability. Besides, the optical parametric oscillators are often used in the applications where the required wavelength of laser sources using normal amplifying media can not be reached. Before 1990's, the optical parametric osicallators were normally based on the technique of angle-phase matching, which required a rigorous working condition such as the inputing angle of the beam and the temperature of the environment. Because of the restriction of the accepting angle of the pump beam, the conversion efficiency of the optical parametric oscillators based on the technique of angle-phase matching is poor, and the the tunability scale small. These disadvantages limited the use of OPO. With the development of quasi-phase-matching(QPM) technique, the OPO based on the QPM eliminates the limitation very well, and become one of the attractive focus in the domains of nonlinear optics in recent years.The objective of our research work is to fabricate an OPO system based on the periodically poled MgO doped lithium niobate crystal (PPMgLN) with a good conversion efficiency and a high output power. The PPMgLN wafers were frabricated by means of high voltage pulse trigged domain reversal technique. By using a Q-switched Nd3+:YAG laser with a modulation frequency between 10 kHz to 50 kHz, and a pulse width between 20-30ns, we established a singly resonant optical parametric oscillator(SRO) system. Up to today, we have obtained a power output of 4.8W from the OPO (including the signal and the idler) at an input pump power of 10.8W. The efficiency of the OPO was 44% and the slope efficiency was 47%. The tunability of the OPO was from 1.42μm to 1.73μm and 2.76μm to 4.27μm. To perform the tuning, it is very convenient just to shift the nonlinear crystal so that it could work at different poled area with different reversing periods. When the function of continous tunabilty was required, the temperature-tunability could be used together with the position tuning approach. On account of the benefits of the high conversion efficiency, high output power, wide tunability and convenient operating method, the optical parametric oscillator we fabricated had reached a practically usable extent. Some important parameters of the OPO were measured, such as the beam quality and the spectrum of the beam output. The results obtained showed that the OPO was of good performance.In the last chapter of this thesis, we simulated the characteristic of an OPO based on the technique of QPM by means of numerical analysis with the classical theory of electromagnetism under the condition of slowly-varying-envelope waves. All the simulation was processed with the assumption that all the waves which coupled with each other were all the monochromatic plane waves. The result obtained from simulation showed that if high output power and hight conversion efficiency was intended, the reflection coefficient of the output mirror should be reduced while the waist size of the pump beam be increased. In this chapter, we also simulated the working state of the doubly resonant optical parametric oscillator(DRO). The result showed that due to the storage of the signal and the idler light in the resonant cavity, the pump power was suffocated to transform to the signal and the idler to some extent. These were different from the SRO where the pump, the signal and the idler were transformed to each other as fast as it could be.After a lot of experiments and theoretical analysis, a clear conclusion was obtained that the performance of the QPM tecnique based OPO was much better than the normal angle-phase matching based OPO, especially in its conversion efficiency and its tunability. |
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