| Potassium Niobate (KNbO3) has the largest nonlinear optical coefficient (d21 = 17.9pm/ V, d32 = 20.9pm/V 858nm ; d31 = 15.8pm/V,d32 = 18.3pm/V 1064nm ) and relative large damage threshold (about 350A/W /cm2). Hence, it is widely used for second harmonic generation (SHG) with near infrared lasers (840nm-1100nm) at low and medium power level, such as diode lasers, diode pumped solid state lasers, Tirsapphire lasers and cw Nd:YAG lasers. Moreover, it is possible to realize non-critical phase-matching (type-I) at room temperature around 860nm with large nonlinear optical coefficient d32. So, KNbO3 is found to be one of the most important crystals for the generation of blue light Research on the generation of blue lasers has become a focus because of its important applications and potential commercial value in many fields, such as high-density optical storage, numeric video-frequency technique, color-laser vision, laser typography, laser medicine and material science and so on.In the field of fundamental research, such as quantum optics, blue light can be used as a pump source of optical parametric oscillator (OPO) to generate the neoclassical light fields around wavelength of 860nm, which can be detected with 99.9% of quantum efficiency, the highest efficiency in the measurement of neoclassical light as known yet It is also possible to generate the squeezed vacuum state at 852nm, which is the resonance wavelength of the transition line D2 of Cesium atoms and can be supposed to investigate the properties of atomic radiation in the interaction between atoms and quantum state of light fields.This thesis mainly focuses on the generation of blue light by utilizing the external frequency doubling with KNbO3. Following is about the contents.1. We analyze the generation of external cavity frequency-doubling with KNbO3 crystal theoretically. In order to get the second-order nonlinear conversion from 858nm to 429nm with KNbO3, we use non-critical type-I phase-matching to avoid the walk-off effect The corresponding optimum focusing condition is as followings: double refraction parameter B=0, focus parameter = 2.84 , optimum phase mismatch parameter = 0.574. Based on the theoretical analysis and numerical calculations, we obtain the ideal nonlinear conversion coefficient about 1.45%/W with crystal length of 7mm and 2.07%/W with crystal length of 10mm at the phase-matching temperature around 23.50C.2. We design a configuration of frequency-doubler according to the optimum focus condition and the cavity stability condition |A + D| < 2 and optimize it based on "mode-matching" and "optimum coupling". 99.96% of optical impedance-match efficiency and over 95% of spatial mode-matching efficiency are achieved eventually by using T= 10% of input coupler in our experiment.3. 39% of total conversion efficiency at pump level of 600mW 858nm is obtained experimentaily with our single-resonant, external ring frequency-doubler pumped by Tr.Sapphite laser at room temperature (about 21.50C ). The conversion efficiency is seriously limited by what is called "BLURA" (blue light induced infrared absorption) and thermal effects. Yet, up to 137 mW of CW blue light at 429nm at pump level of 565 mW has been achieved and stabilized over a hour when the cavity is locked via Pound-Drever-Hall(PDH) technique.4. A brief analysis on frequency doubling at 852nm with BBO crystal has been presented. Due to its relative small losses, theoretically it is supposed to get 50% of conversion effi.ciency at a pump level of 600 mW by using a BBO crystal with cutting angle of 27.030 and length of 10mm. But the large wall-off angle (about 3.70) and small acceptance angle (about 0.35 mrad) are the problems in the process of frequency doubling.
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