在此研究中，我們分別利用三硼酸鋰和β相偏硼酸鋇晶體來產生摻鐿光纖主振盪功率放大器 (MOPA) 波長為 532 奈米之二階諧波，以及波長為 266 奈米之四階諧波。為了提高各諧波的轉換效率，我們使用聲光調制器來提高脈衝的尖峰功率，藉由控制聲光調制器的占空比來改變輸入光纖放大器的脈衝數。二階諧波和四階諧波的轉換效率主要被光纖放大器中的非線性效應所限制，其中主要是「自相位調制」及「受激拉曼散射」為造成 MOPA 輸出頻寬大於非線性晶體之相匹配頻寬。藉由改變不同的初始偏振條件和縮短摻鐿光纖的長度，我們成功地將基頻頻寬由6奈米縮小至1奈米。最後，我們分別在一級和二級光纖放大器中使用3公尺和3.5公尺的摻鐿光纖，聲光調制器的占空因數為0.35，輸出平均功率為28瓦的近紅外光的情況下，可以產生超過2.5瓦的綠光 (532奈米)，以及超過200毫瓦的紫外 (266奈米) 皮秒級雷射脈衝。

In this thesis, Lithium Triborate (LBO) and Beta-Barium Borate (β-BBO) crystals were used for generation of second and fourth harmonics of the output of high-power ytterbium-doped dual stage Master-laser-fiber-amplifier (MOFA) at the fundamental wavelength of 1064 nm. The pulse duration of the amplifier output is around 10 ps. An acousto-optic modulator (AOM) was used to modulate the duration of pulse bursts before the fiber amplifier. We found that stimulated Raman scattering (SRS) and self-phase modulation (SPM) are the most important nonlinearities limiting the performance of whole laser system. In order to reduce the nonlinearities in the fiber amplifier and achieve the highest average power at 266nm, several schemes were implemented:
- the lengths of the active fibers in the fiber amplifier stages were optimized;
- the polarized launch conditions of the fiber amplifier were controlled such that its spectral width was minimized;
- the repetition rate and duty circle of pulse bursts were adjusted to maximize the doubling and quadrupling efficiencies;
We demonstrated that the spectral bandwidth of the amplifier output can be narrowed from 6 nm to 1 nm when the shorter active fibers are employed and the polarization launch conditions are varied from linear to circular states. Also, a pulse-burst repetition rate of 52.5 MHz, and burst duty circle of 0.35 are determined to be optimal for extraction of the maximum peak power while keeping amplified spontaneous emission below the acceptable level. This allows us to generate the maximum average power of the UV pulses at 266 nm over 200 mW when the power of the fundamental IR pulse was 28 W.

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