以基因演算法設計之奈米光柵應用於高功率半導體雷射陣列耦光之研究

Abstract

於高功率雷射領域中，以半導體雷射做為光源所發展的光纖雷射漸漸地主導了此領域的走向。半導體雷射於電光轉換效率、高功率輸出及光束品質等方面均具極佳的優勢，因此於不同雷射幫浦架構中，其被作為可靠的高功率雷射光源。於本實驗室中，採用半導體雷射陣列做為光源之光柵式側向耦光系統於2011年被提出，而此側向耦光系統具兩種會導致耦光效率損耗的機制，其分別為二次繞射損耗及邊緣損耗。於稍後研究成果中，將光源為976-nm輸出波長之半導體雷射陣列耦入400-μm內纖衣尺寸光纖，且將所採用的週期性光柵之週期由680 nm替換為675 nm以有效地改善二次繞射損耗且達到67.63%實驗耦光效率。 於本論文中，我們將著手改善邊緣損耗所造成之影響。我們同樣採用675-nm週期光柵及400-μm內纖衣尺寸光纖，接著利用基因演算法以設計出具顯著±1st階不對等繞射光效率的非對稱性光柵結構以取代週期性光柵之兩端結構而形成非週期性光柵。透過電子束微影製程的參數測試及改良後，我們成功地製作出具40-nm寬極細結構之非對稱性光柵結構，接著採用二次對準技術將非對稱性光柵結構成功地與週期性光柵做整合以製作出非週期性光柵。透過非週期性光柵與週期性光柵之實驗耦光效率分析及比較，由基因演算法所設計之非對稱性光柵結構可有效地改善邊緣損耗且將實驗耦光效率再往上提升至75%，其為目前光柵式側向耦光技術所能達到之最高耦光效率。

In high power laser field, the fiber laser with adopting diode-pumped laser as light source gradually dominates this field. The adoption of diode-pumped laser as a reliable high-power laser light source in various configuration of pumped laser is due to the high electrical-optical conversion efficiency, high output power, and excellent beam quality. In 2011, we demonstrated the grating based side-coupling scheme with using laser diode array (LDA) as light source and there were two loss mechanisms, secondary diffraction loss and edge loss, decreasing the coupling efficiency. In the later research, the secondary diffraction loss could be effectively improved and the 67.63% coupling efficiency was achieved by coupling 976-nm LDA into 400-μm double cladding fiber and substituting periodic grating with 675-nm period for that with 680-nm period. In this study, we start to improve the edge loss. At first, we choose the same grating based side-coupling scheme and we adopt grating with 675-nm period and 400-μm double cladding fiber to prevent the side-coupling scheme from being influenced by secondary diffraction loss. Next, we adopt Genetic Algorithm to design the asymmetric grating structure with significant ±1st order asymmetric diffraction efficiency used to replace the structures at the two edges of periodic grating to form aperiodic grating. Through a series of experimental parameters test and improvement of E-beam lithography process, we successfully fabricate asymmetric grating structure with 40-nm thin structure integrated with periodic grating by secondary alignment to form aperiodic grating. After comparing the experimental coupling efficiency of aperiodic grating and periodic grating, the adoption of asymmetric grating structure designed by Genetic Algorithm could effectively improve the edge loss and make the coupling efficiency of aperiodic gating be up to 75% which is the highest coupling efficiency for side-coupling technology.