先進光纖光柵之製作與特性量測

Abstract

隨著光纖光柵在光纖通訊及光纖感測領域被發現有越來越多的應用，致力於改善光纖光柵的製作與特性量測對於製作具有特殊光學特性的濾波元件而言就顯得非常重要。在本論文中，我們利用逆散射之剝層（layer-peeling）法來設計先進的光纖光柵結構，並根據實驗的實際製作架構，使用最小方差逼近（least-square fitting）法來找出最佳的光纖光柵曝照參數。 在製作具有複雜結構之先進光纖光柵方面，我們提出了三個新的製作光纖光柵的方法。第一個方法我們稱之為偏振控制雙光束干涉法，第二個方法稱之為偏振控制相位光罩法，這兩個製作方法目的都是利用曝照光束偏振的特性，在製作光纖光柵逐段曝照過程中既可以任意控制光柵折射率（ac-index）的大小，同時又可以保持光柵不同位置的平均折射率（dc-index）為定值。藉著利用這些方法，具有低損耗及良好頻譜響應的濾波元件可以順利的被完成。第三個方法是干涉式側向繞射光柵位移監控的技術，我們提出這個光學監控方法來製作具有較長長度的光纖光柵元件。我們成功完成一些實驗的例子來證明這些方法的可行性，也利用LabView的自動控制軟體來建立自動化的光柵曝照系統，藉此可以提升光纖光柵製作過程的準確性及重複性。 在光纖光柵特性量測方面，我們發展光纖式麥克森干涉儀（the balanced Michelson interferometer）的方法及側向繞射（side-diffraction）量測技術來分析光纖光柵完整的特性，包括光柵的複數反射係數及複數耦合係數等，這些係數代表著光纖光柵所有的光學和結構參數。當我們再結合逆散射之剝層法反推時，光纖光柵不同位置的平均折射率變化也可以求得。對未來先進光纖光柵製程的改善工作，這些特性的分析方法希望能夠有所幫助。

When the FBG devices begin to find a lot of applications in fiber communication and fiber sensing, it also becomes more important to further improve the FBG fabrication and characterization techniques for achieving more complex optical filter properties. In this thesis, in order to design the advanced fiber grating structures, the layer peeling inverse design method combined with the least square fitting method is developed. Based on these synthesis methods, the best experimental parameters for our sequential writing setup can be found. For the fabrication of advanced fiber grating structures, we have also proposed three new exposure methods. One is the two-beam interferometer method with the polarization control, and another is the phase mask method with the polarization control. Both of them have the same purpose for achieving a controllable ac-index profile in a single scan, with the dc-index profile being kept constant during the scan. The spectral shapes with a steep edge, very low sidelobes, a flat top, and very little ripples can be achieved by the use of these methods. The third method is the interferometric side-diffraction position monitoring technique for writing long fiber Bragg gratings. Some examples are presented to demonstrate the feasibility of these methods. The automation of the exposure system has also been setup for enhancing the accuracy and repetibility of the fabrication process. In order to determine the complete characteristics of the fiber gratings, we develop the balanced Michelson interferometer method and the side-diffraction technique. The characteristics to be determined include the measurement of the complex reflection coefficient and the complex coupling coefficient of the grating. When the discrete layer peeling method is combined with these two measurement methods, we can get all of the information about the grating structure including the dc-index change. In addition, this analysis process may also be helpful for improving the fabrication processes of advanced FBGs.