光柵式側向耦光技術於摻鉺釔鋁石榴石晶體光纖雷射之研究

Abstract

近年來，高功率光纖雷射的發展日益增長且應用廣泛。由於半導體雷射光源擁有高電光轉換效率，結構精巧與低成本等優點，目前已被應用於各種不同的幫浦雷射系統架設中。本實驗室於2011年提出半導體雷射陣列之側向式光柵耦光系統，於光纖內纖衣尺寸為400 μm之架構下，將21 W之976 nm雷射光源耦入並達到50%耦光效率。為凸顯及提升側向式耦光系統高光亮度耦合效率的性質，我們針對雷射加熱基座長晶法生長之小尺寸單纖衣摻鉺釔鋁石榴石晶體光纖，以理論設計與實作出晶體光纖之側向式光柵耦光系統。 於光柵實作中，我們克服了玻璃基板N-LaSF41蝕刻的困難，設計一二氧化矽內嵌膜層式光柵結構，考慮理論與實作光柵凹槽之結構差異，繞射效率可優化至89.51%。運用光學模擬系統架構，理論模擬出應用於內纖衣尺寸130 μm之合適的基板與光纖介面尺寸，優化整體之耦合效率。於實驗架構中，藉由高溫熔燒680 ℃與慢速降溫7 ℃/Hr，完成玻璃基板與晶纖之熔接，應用於晶體光纖之側向耦光系統得以實現。此外，於晶體光纖雷射對摻雜濃度為1 at.%之纖心大小為40 μm的模擬中，結果可得以1532 nm之10 W幫浦光源可得1645 nm之7 W功率輸出，斜向效率73%，理想晶體光纖長度為9 cm。

In recent years, high-power fiber lasers have been extensively investigated for various applications. Because of the high electrical-optical efficiency, compact and low cost, laser diode arrays (LDAs) have been used in many high-power fiber laser systems as the pump sources. In 2011, side-coupling schemes, using gold-embedded grating couplers, were investigated to couple the light from a high-power LDA into double-clad glass fibers in our group. A 50% coupling efficiency has been achieved from a 21-W, 976nm LD array into a 400-μm double-clad fiber. To demonstrate the main advantage of this pumping scheme, high brightness conversion efficiency, this research is focused on the gain fibers with small inner-cladding diameters. Our system is a resonantly side-pumped 1645-nm Er:YAG crystal fiber laser utilizing a gold grating coupler. Firstly, we overcome difficulties of dry-etching the high index glass substrate, N-LaSF41, by utilizing a SiO2 thin-film embedded grating structure. Based on the theoretical simulation, an optimized diffraction efficiency of 89.51% could be achieved with the grating structure. Secondly, we optimize the overall coupling efficiency for a 130 μm fiber, by choosing a proper size of the fiber-substrate interface. In addition, the glass substrate and crystal fiber are fused under high-temperature melting at 680 ℃ and low-cooling speed at 7 ℃/Hr to eliminate scattering loss. Finally, we simulated the fiber laser performance for a 1 at.% Er:YAG crystal fiber with 40-μm-core diameter. The result shows a 9-cm Er:YAG crystal fiber is long enough to achieve 7-W 1645 nm laser output with 73% slope efficiency under 10-W pumping at 1532 nm .