鋸齒狀長週期光纖光柵之新製法及其對光學特性的影響

Abstract

先前我們研究小組提出的鋸齒狀光纖光柵 (corrugated fiber grating)克服了傳統需要先有感光光纖以曝光來製作長週期光纖光柵 (LPFG)的限制。此外，鋸齒狀光纖光柵更表現出對於光共振性質和耦合強度具可調性的特點。雖然鋸齒狀光纖光柵具有優於以傳統紫外曝光產生光纖光柵的一些優點，但在製程上需利用熱蒸鍍金屬膜的方式製作仍然有許多的不便，除了需要鍍多種金屬層來增加緊密度以抵抗氫氟酸的蝕刻之外，一次也只能在一根光纖上鍍膜。因此一種利用微影製程，在光纖上製作鋸齒狀結構的方法，在本論文中我們提出。數根光纖可同時利用微影製程製作的鋸齒狀光纖光柵，不但所需的時間較短，製作的良率也比較高。 此外在此微影製程中所使用的光阻為一種負光阻SU8，此種光阻於固化後可以達到較高的深寬比和化學惰性，因此在本論文中利用以製做週期性結構以及抵抗氫氟酸的蝕刻。 除了證實先前的可調式光學特性之外，以此法製成鋸齒狀光纖光柵還將被放置在於折射率大於空氣的環境下檢驗其光學性質。由於實驗中的共振模態為纖核模態(core mode)和低階纖殼模態(cladding mode)，因此環境折射率對於共振條件的影響並不大，這有助於選擇 適當的材料以封裝鋸齒狀光纖光柵。 此種微影製程的方法可以幫助鋸齒狀光纖光柵的普及和應用，而且以其獨特的光學特性，相信在光纖通信和光纖感測的領域應該都非常的有潛力。

The proposed corrugated fiber gratings by our group do not need fiber photosensitivity to make long period fiber gratings. Besides, the corrugated structure offers flexibility on adjusting coupling strength and resonant conditions. However, making the corrugated structure on a fiber by thermal coating method is not convenient and time consuming. Multiple metal layers need be coated on a fiber for protecting the etching of hydrofluoric acid. Hence, a new method based on photo-lithography is proposed for fabricating corrugated structures on a fiber. The proposed method can increase the production yield and save time. Since SU8 epoxy resin is a negative photoresist used in the process, it can generate a large aspect ratio and is inert to chemical properties. Hence, SU8 can be used to cover and protect a fiber during the etching process. In addition to confirming its being intact of wavelength and loss tunability when mechanical forces such as stress and torsion are applied, the corrugated fiber gratings thus fabricated are also tested in a different environment. Because only first few cladding modes and the core mode are involved for the optical characteristics of the corrugated fiber grating, the refractive index of the surrounding material is found to affect little on its resonant conditions. This may help the selection of an appropriate material to package such fiber gratings. The proposed photo-lithographical method may improve the generalization of the corrugated fiber gratings, which may find useful applications in optical fiber communication and fiber sensing.