三維結構光子晶體光纖之研製

Abstract

過去十年來，光子晶體光纖一直是被廣泛研究的課題。這種光纖在纖衣中引入週期性的孔洞排列，造成許多獨特的性質，例如大模態面積，無截止單模等等。近來，開始有人嘗試在傳播方向上做週期性的調變，使得原先二維的週期性成為三維。在文獻中，三維週期性的光子晶體光纖一般被認為有類似光纖光柵的效果，而由於光子晶體光纖的組成材料相對折射率大，效果應較一般光纖光柵更大。 在本論文中，我們使用二氧化碳雷射束對光子晶體光纖在傳播方向上的外徑尺寸作週期性的調變。使用我們的雷射加熱基座生長系統所製作的光子晶體光纖光柵具有以下之優點：首先，和傳統製作方式比較，大多數紫外光製作的光柵會隨時間而退化，且纖心摻雜鍺元素會破壞原本的物性。相較之下，以雷射束製作的成品即使在高溫下也非常穩定，因為其微擾是純結構性的。其次，和其他的雷射加熱系統比較，一般單側或雙側的雷射加熱法容易造成較大的雙折射效應，雷射加熱基座生長系統中的環形加熱方式較任何一種文獻上已提及的雷射加熱法擁有更高的對稱性，應能夠減少局部孔洞的嚴重塌陷，保持其傳輸特徵。 我們利用Rsoft公司的商用模擬軟體下基於波束傳播法(beam propagation method)的相關法(correlation method)求解模態，並配合耦合模理論對三維結構光子晶體光纖光柵之穿透頻譜作一預測。我們嘗試了雷射加熱基座生長系統中的兩種製作模式: 週期性開關光閘及變速度控制步進平台，其中利用週期性開關光閘的方式，我們成功製作出一帶斥濾波器，且其衰減峰值在波長1557.5 nm時可達-16 dB，與模擬結果有良好的吻合。

In the past ten years, photonic crystal fibers (PCFs) have been widely researched. This special type of fibers, with two-dimensional periodic cladding, provides many impressive properties, such as large mode area, endlessly single mode, etc. Recently, PCFs with periodic modulation in the transmission direction are also developed, increasing the periodicity from 2D to 3D. In literature, 3D structure PCFs have been used as fiber gratings and are more efficient than common fibers because of their larger index difference in constitute material. In this thesis, CO2 laser was employed to periodically modulate the diameter of PCF. There are some advantages to fabricate 3D structure PCF by laser heated pedestal growth (LHPG) system: First, many UV-induced gratings will degrade by time, and the Ge doping in core destroys the original physical properties of the fiber. In contrast, the grating fabricated by CO2 laser is stable even in high temperature. Second, the common laser-heated system always heats the fiber in one or two directions, which may induce larger birefringence effect. The ring heating design of our LHPG system has higher symmetry than any other laser-heated system in literature, which may reduce local collapse of the hole structures in PCF and maintain the transmission characteristics. Two methods, periodical control of shutter or motor stages, were used to fabricate 3D structure PCF with about 224.25 micron pitch. The fabricated structure served as a notch filter and the peak attenuation of 16 dB at 1557.5 nm was achieved. Numerical simulations based on a commercially available beam propagation method (BPM) software and coupled-mode theory were used to model the 3D structure PCF. The simulated result agrees well with our fabricated 3D structure PCF.