Sub-wavelength Grating Coupling of Laser Diode Stack for Side-pumped High Power Fiber Laser

近年來，利用半導體雷射激發之光纖雷射逐漸成為高功率雷射的主流，由於它擁有極佳的光束品質、窄線寬以及很高的電光轉換效率等優點。此外，半導體雷射陣列及陣列堆疊是一種很可靠的高功率激發光源，並廣泛地被使用在各種應用上。當光纖雷射的激發光源來到此種擁有更多的發光區域之堆疊式雷射陣列，其在慢軸的多模態發光特性往往使得能被使用的功率受到限制，以致於到目前為止，堆疊式雷射陣列只能用來當成塊狀雷射的激發光源，因而仍然有許多技術問題需要被改善與克服。 基於上述，本論文提出了一種更精巧且更有效率的光柵側向式耦光架構，利用特殊設計的光柵耦合器，可以直接將半導體雷射陣列及陣列堆疊的光經由側面耦合進雙纖衣光纖的內層纖衣。在實驗上，我們利用電子束微影系統來製做此種次波長光柵。此外，我們也考慮到回返繞射所產生的損耗、製程上所產生的光柵側壁垂直度誤差，這些都可經由最佳化光柵的周期、線寬以及深度來達到最高的耦光效率。實驗結果顯示黃金鑲嵌二氧化矽之光柵耦合器的耦光效果遠比單純的黃金光柵耦合器還要好，這是因為此種二元結構擁有較高的熱膨脹抵抗能力以及較佳的熱傳導路徑。 對於一維半導體雷射陣列側向耦光架構，實驗結果顯示黃金鑲嵌二氧化矽之光柵耦合器能將21瓦，波長976 奈米的連續輸出光，耦合進直徑400 微米之雙纖衣光纖的內層纖衣中，耦光效率為50 %。更進一步，藉由此光柵耦合器，側向激發之摻鐿雙纖衣光纖雷射也被實現。在雙向側向激發的架構中，雷射功率可達10瓦，斜率效率約為61 %。此外，對於半導體雷射陣列堆疊側向耦光架構，亦可藉由此光柵耦合器，實現了只使用單一一組半導體雷射陣列堆疊，即達成分佈式側向耦光之架構。據我們所知，這是世界上首次如此精巧地實現了分佈式側向耦光之架構。當304.4瓦的半導體雷射陣列堆疊輸出，約124.6瓦的光可被耦合進入雙纖衣光纖之內層纖衣，耦光效率為41 %。更進一步，產生出58瓦的光纖雷射，斜率效率約為50 %，雷射中心波長1090奈米，線寬約為0.11奈米，此窄線寬亦代表了此種分佈式側向激發雷射的優勢。 最後，此種光柵側向耦光技術不只能有效率地實現高功率且高品質的雷射光源，同時光柵還能藉由奈米壓印技術來大量生產以降低成本，十分具有商業競爭潛力。In recent years, diode-pumped solid-state fiber laser technology is gradually becoming main-stream of high-power laser field, owing to its superior beam quality, narrow linewidth, and high electrical-optical efficiency. Besides, as the pump source, the laser diode array/stack (LDA/S) is a reliable high-output power source and has been widely used in different applications. When the pump source shifts to an even higher power LDS with many more elements and emitters, the pump power is limited by the complexity of the source itself. Presently, LDS can only be used as a bulk gain medium pumping source combined with spatial multiplexing technique due to the distributed multi-mode emission on the slow-axis. This shows that improved pump sources and methods are still needed for better performance. Therefore, in this work, a more compact and highly efficient grating side-pumped configuration has been implemented. Through the use of a specially designed grating coupler, the emission from LDA/LDS was directly side-launched into the clad of a double-clad fiber through a set of brightness-preserved focusing/collimating optics. In the experiments, an electron-beam was used to fabricate the sub-wavelength grating. In addition, with the consideration of the backward diffraction loss and the groove wall non-verticality due to fabrication distortion, the grating pitch and groove width were optimized for the highest coupling efficiency. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability. For the LDA side-coupled scheme, the experimental results show that, the gold-embedded silica grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave LDA into the inner cladding of a 400-μm-diameter double-clad fiber with an overall coupling efficiency of 50%. Furthermore, a side-pumped ytterbium-doped double-clad fiber laser by using the grating coupler was demonstrated. The output power of 10 W with a slope efficiency of 61% was demonstrated for the bi-directional side-pumped fiber laser. Besides, for the LDS side-coupled scheme, a single LDS distributed side-coupled scheme using grating couplers is presented. To the best of our knowledge, this is the first demonstration of this compact distributed side-pumped configuration in the world. The launched power was 124.6 W at 304.4 W LDS emission with a coupling efficiency of 41%. Furthermore, the output power of 58 W with a slope efficiency of 50% was demonstrated for the distributed side-pumped fiber laser. The central wavelength is at 1090 nm, and the narrow linewidth shows the advantage of the distributed side-pumped scheme, which is nearly 0.11 nm. Finally, this technique would not only be an efficient way to achieve a high-power and high-quality light source but also has the advantage for mass production of the grating couplers by using nano-imprint technique, which has the potential in business competition